Emission of the greenhouse gas nitrous oxide (N2O) from riparian forest buffers, warm-season and cool-season grass filters and crop fields.

Final Report for GNC06-061

Project Type: Graduate Student
Funds awarded in 2006: $9,990.00
Projected End Date: 12/31/2008
Grant Recipient: Iowa State University
Region: North Central
State: Iowa
Graduate Student:
Faculty Advisor:
Richard Schultz
Iowa State University
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Project Information

Summary:

We measured nitrous oxide (N2O) fluxes and nitrogen (N) input in riparian buffer and a crop field soils located in the Bear Creek watershed in central Iowa. The results indicate that N2O emissions from soils in the riparian buffers were significantly less than those in the crop field and the ratio of N2O emission to N inputs in the riparian buffer soils was smaller than those in the crop field soils. This study suggests that riparian buffers should not be considered a major source of N2O emission in the watershed.

Introduction:

Non-point source (NPS) pollutants such as sediment, N, phosphorus (P) and pesticides are major causes of water quality problems worldwide (Duda, 1993; Tonderski, 1996; Carpenter et al., 1998). Shortly after the Waikato Valley Authority in New Zealand (1973) first discussed the use of riparian buffers for the prevention of water pollution, a number of research projects were initiated to quantify the ability of riparian buffers to control NPS pollution (e.g. Lowrance et al., 1983; Peterjohn and Correll, 1984). Based on these and other studies, riparian buffers have been recommended as one of the most effective tools for coping with NPS pollution (e.g. Mitsch et al., 2001; Sabater et al., 2003; Hubbard et al., 2004).

Important functions of riparian buffers related to NPS pollution control are filtering and retaining sediment, and immobilizing, storing, and transforming chemical inputs from uplands (Schultz et al., 2000). Many studies have shown that riparian buffers can reduce eroded sediment delivery to surface waters by 70 to 95% (e.g. Lee et al., 2000, 2003), N fluxes by 5 to more than 90% (e.g. Kuusemets et al., 2001; Dukes et al., 2002) and P losses by 27 to 97% (e.g. Uusi-Kamppa et al., 2000; Kuusemets et al., 2001). Denitrification is recognized as the major mechanism for reducing NO3- within riparian systems, with removal generally ranging from 2–7 g N m-2 y-1 (e.g.; Groffman and Hanson, 1997; Watts and Seitzinger, 2000).

It recently has been hypothesized that increased denitrification within riparian areas may trade a water quality benefit for an atmospheric problem (Groffman et al., 1998) resulting from the greenhouse effect of N2O produced during nitrification and denitrification (Wang et al., 1976) and ozone depletion (Crutzen, 1970; Liu et al., 1977). The global warming potential of N2O is 298 times that of carbon dioxide (CO2) and 25 times that of methane (CH4) in a 100-year time horizon (Forster et al., 2007). Some studies (Groffman et al., 1998, 2000; Hefting et al., 2003, 2006) conclude that N transformation by riparian buffers with high NO3- loads results in a significant increase of greenhouse gas emission. Groffman et al. (2002) suggested that the Intergovernmental Panel on Climate Change (IPCC) inventory might be improved by including more measurements of riparian N2O fluxes.

Numerous studies have emphasized the role of vegetation in soil processes within riparian buffers. However, there are conflicting results regarding the relationship between vegetation type and denitrification rate in riparian buffers. While some studies (e.g. Hubbard and Lowrance, 1997; Verchot et al., 1997) have found higher groundwater nitrate removal or denitrification rates in forested riparian zones, other studies (Groffman et al., 1991; Schnabel et al., 1996) have found higher removal in grass dominated riparian sites. Still other studies (e.g. Hefting et al., 2003; Dhondt et al., 2004) have found no significant difference in groundwater nitrate removal or denitrification rate between forested and grass-dominated riparian sites. This variability suggests that there are questions about the relationship between vegetation types in riparian buffers and the emission of N2O from their soils and illustrates the need for additional studies in various regions of the country, in different landscape settings, and under different vegetation communities to quantify the emission of N2O from soils in riparian buffers (Walker et al., 2002).

Numerous studies have observed increased soil N2O emission following the wetting of dry soil in tropical areas (Nobre et al., 2001), semiarid areas (e.g. Wulf et al., 1999; Saetre and Stark, 2005), Mediterranean areas (Fierer and Schimel, 2002), dry tropical forests (García-Méndez et al., 1991; Davidson et al., 1993), savanna (Scholes et al., 1997), agricultural lands (e.g. Kusa et al., 2002; Mikha et al., 2005) and in laboratory studies (e.g. Appel, 1998; Hütsch et al., 1999). The increase rates ranged from 5-fold up to 1,000-fold (e.g. Prieme and Christensen, 2001; Saetre and Stark, 2005) and magnitudes of the episodic N2O emission increase varied depending on soil texture (Appel, 1998; Austin et al., 2004), soil water content (Appel, 1998), root responses (Cui and Caldwell, 1997), amount of added water (Ruser et al., 2006) and the characteristics and availability of substrates (e.g. Van Gestel et al., 1993; Schaeffer et al., 2003 ). Based on these studies, it is apparent that even a single wetting event could account for a large proportion of the annual emission of N2O (e.g. Prieme and Christensen, 2001; Nobre et al., 2001). Thawing frozen soils can also lead to increased N2O emissions (e.g. Herrmann and Witter, 2002; Müller et al., 2003). Although the duration of such elevated emission is limited mostly to a few days, they have been found to be an important source of the total annual emissions from agricultural land (e.g. Wagner-Riddle and Thurtell, 1998; Teepe et al., 2004), forests (e.g. Papen and Butterbach-Bahl, 1999; Teepe et al., 2000), and grasslands (Kammann et al., 1998). Matzner and Borken (2008) observed that the emissions of N2O after thawing frozen soils were in some cases significantly larger from arable soils than from forest soils. Such events usually occurred when soil temperature is near 0oC (e.g. Chen et al., 1995; Müller et al., 2003). Matzner and Borken (2008) stated that the increase in N2O emission after thawing increases with colder temperatures of frozen soil. In temperate regions, observed N2O emissions during freezing-thawing periods in spring may account for up to 70% of the total yearly N2O losses (e.g. Teepe et al., 2000; Regina et al., 2004). From these results, it is summarized that short-term N2O peak emissions following rewetting dry soils and thawing frozen soils contributes substantially to annual N2O emissions. Intergovernmental Panel on Climate Change Tier 1 methodology (2006) estimates soil N2O emission by multiplying N inputs by an emission factor in crop fields since N inputs are a source of N2O emission. However, the N input-based IPCC methodology for estimating N2O emissions may underestimate fluxes in the regions where frequent rewetting of dry soils and thawing of frozen soils occurs.

Therefore, studies assessing the contribution of peak emissions to annual N2O emissions and evaluating the current IPCC methodology are clearly needed to better understand annual N2O fluxes and the N cycle within these systems.

Project Objectives:

The objectives of this study were:
1) to compare N2O emissions from riparian buffer systems comprised of forest, warm-season grasses, and cool-season grasses and an adjacent crop field, and
2) to compare the measured N2O emissions with estimated ones using IPCC methodology.

Research

Materials and methods:

Study Sites

The study area consisted of three forest buffers, three warm-season grass filters, one cool-season grass filter, and one crop field, located in the Bear Creek watershed, Story County and Hamilton County, Iowa, United States of America (42o 11’ N, 93o 30’ W). Bear Creek (total length 56,473 m) is a third order stream with typical discharges of 0.3 to 1.4 m3 sec-1. The watershed drains 6,810 ha of farmland, with nearly 90% of these acres in a corn-soybean rotation. Located within the Des Moines Lobe subregion of the Western Corn Belt Plains ecoregion (Griffiths et al., 1994), the study area was once a tallgrass prairie ecosystem containing wet prairie marshes and pothole wetlands in topographically low areas and forests along higher order streams. An ongoing objective of the Bear Creek watershed project has been to establish riparian buffers along the upper portions of the watershed as willing landowners and cost-share are identified (Schultz et al., 2004). This has provided a variety of sites of different streamside vegetation and buffer age to utilize in assessing the spatial and temporal variability of riparian buffers in reducing NPS pollution. Forest buffers and warm-season grass filters were previously under row-crop cultivation and the cool-season grass filter was previously under livestock grazing.

Tree species included:
– silver maple (Acer saccharinum L.),
– green ash (Fraxinus pennsylvanica Marsh.),
– black walnut (Juglans nigra L.),
– willow (Salix spp.),
– cottonwood hybrids (Populus spp.),
– red oak (Quercus rubra L.), and
– bur oak (Quercus bicolor Willd).

Shrub species included:
– chokecherry (Prunus virginiana L.),
– Nanking cherry (Prunus tomentosa Thunb),
– wild plum (Prunus americana Marsh),
– red osier dogwood (Cornus stolonifera Michx), and
– ninebark (Physocarpus opulifolius Max.).

Warm-season grasses included:
– native grasses such as Indian grass (Sorghastrum nutans),
– Big Bluestem (Andropogon gerardi), and
– Little Bluestem (Andropogon scoparius).

Numerous forb species were present, including:
– purple prairie clover (Petalostemum purpureum),
– Black-eyed Susan (Rudbeckia hirta),
– yellow coneflower (Ratibida pinnata),
– stiff goldenrod (Solidago rigida),
– prairie blazing star (Liatris pycnostachya), and others.

The cool-season grass buffer was dominated by non-native forage grasses (Bromus inermis Leysser., Phleum pratense L., and Poa pratensis L). Details of the riparian buffer design, placement, and plant species are given in Schultz et al. (1995). The crop field was planted to a corn (Zea mays L.) and soybean (Glycine max L. Merr.) rotation, with corn in 2006 and soybeans in 2005 and 2007. Pelletized urea (133.4 kg N ha-1) was applied to the crop field in April 2006, and fall chisel plowing (15-20 cm depth) was conducted in November 2006. Harvested crop yield was 3,934.1 kg dry matter (d.m.) ha -1 (soybeans) in 2005 and 10,419.8 kg d.m. ha-1 (corn) in 2006. The major soil association in the watershed is the Clarion-Webster-Nicolett association with minor areas of Clarion-Storden-Coland, and Canisteo-Okoboji-Nicolett (Dewitt, 1984). The areas used in this study are all located on the same soil mapping unit (Coland) and have similar topography.

Measurement of nitrous oxide flux and environmental factors
Nitrous oxide flux from soils under riparian forest buffers, warm-season and cool-season grass filters, and the crop field were measured weekly from October 2005 through December 2007 (no measurements in mid April to mid May, August, and September to October 2006 in the crop field). Five points were randomly selected in each of the sites for N2O gas collection and soil sampling. Nitrous oxide flux measurements were conducted at mid-morning using static vented chambers (PVC, 30-cm diameter × 15 cm tall with vent). Chambers were equipped with a thermometer to measure air temperature within the chambers at the time of sampling. Ten ml of air were collected from each chamber with a polypropylene syringe at 15 min intervals for 45 min and the gas stored in evacuated glass vials (6 ml, fitted with butyl rubber stoppers) until analysis. Glass vials were prepared by alternately evacuating the vial headspace and flushing with helium to remove air (five cycles of evacuation and flushing). Nitrous oxide concentrations were determined with a gas chromatograph (Model GC17A; Shimadzu, Kyoto, Japan) equipped with a 63Ni electron capture detector and a stainless steel column (0.3175 cm diameter × 74.54 cm long) packed with Porapak Q (80–100 mesh). Samples were introduced into the chromatograph using an autosampler described by Arnold et al. (2001). Details of the chamber design and GC analysis are given in Parkin and Kaspar (2006). Nitrous oxide flux was calculated from the linear slope of N2O concentration change over time (Holland et al., 1999). Our estimated minimum detectable flux was 0.175 g N2O-N ha-1 h-1 (Parkin and Kaspar, 2006). Some of the fluxes measured from the individual chambers were smaller than our detection limit. The measured values of these “nondetects” were included in computing mean fluxes (Gilbert, 1987; Chan and Parkin, 2001).

Soil temperature and soil moisture near the chambers were measured simultaneously with N2O gas collection at a 5 cm depth using a digital thermocouple and a digital soil moisture meter (HydroSense®, Campbell Scientifc, Inc., Logan, Utah, USA). Air temperature was measured inside and outside the gas chamber simultaneously with N2O gas collection. Continuous measurements of soil temperature, air temperature, and soil moisture at the 5 cm soil depth were collected using a data logger (HOBO® Micro station data logger with sensors, Oneset Computer Corporation, Bourne, MA USA) at one site per vegetation type. Daily rainfall and snow data were provided by the nearest weather station (Colo, IA, 42o 01’ N, 93o 19’ W) (Herzmann, 2004).

Diel variation of N2O flux and Q10 relationship
In addition to the regular measurements described above, the diel variation in N2O flux was measured during 21-22 Nov. 2005, 18-19 May 2006, and 16-17 July 2007. For this assessment, three locations were randomly selected for flux measurements within each of the forest buffer, warm-season and cool-season grass filter, and the crop field. Nitrous oxide flux and soil temperature were measured every three hours for 24 hours at all sites. To examine soil temperature sensitivity of N2O flux during the three diel measurement periods, we conducted nonlinear regression analyses using N2O flux = a × Q10 (soil temperature/10) (Q10 represents the increase in activity of N2O flux for every 10ºC increase in soil temperature) (Parkin and Kaspar, 2006).

Cumulative N2O Flux Calculations

Because fluxes were measured during the day time when soil temperatures were generally higher than the daily average soil temperatures, cumulative N2O fluxes were calculated using soil-temperature-corrected daily flux measurements (Parkin and Kaspar, 2003; Parkin and Kaspar, 2006). Temperature corrections were done with a Q10 relationship, using the 5-cm soil temperature at the time each flux was measured, along with the daily average soil temperature for that day. The Q10 factor used in these corrections was computed from diel N2O fluxes measured using the equation:

Daily Average N2O Flux = N2Omeasured × Q (DAT-T)/10

where N2O measured is measured N2O flux at a specific hour, T is the soil temperature at the time the flux was measured, DAT is the daily average soil temperature, Q is the Q10 factor, and Daily Average N2O Flux is the resulting estimated daily average flux based on the single hourly measured N2O flux. Cumulative N2O fluxes were calculated by linear interpolation and numerical integration of daily N2O fluxes between sampling times.

Soil Sampling and Analysis

Six intact soil cores (5.3 cm diameter) were collected to a depth of 15 cm in each of the forest buffer, a warm-season grass filter, a cool-season grass filter, and an adjacent crop field in October 2006 and September 2007. A plastic sleeve liner was placed inside the metal core tube and the liner with the intact soil core removed from the tube and capped for transport to the laboratory. Soils samples were stored at 4oC until analysis. Soil pH was determined using a pH meter (Accument 910, Fisher Scientific Ltd., Pittsburgh, PA, USA) on a 1:1 diluted soil solution. Gravimetric moisture content was determined by oven drying a subsample at 105oC for 24 h and bulk density was determined by the core method (Grossman and Reinsch, 2002). For C and N analysis, soils were air dried at room temperature, sieved (2 mm) and then gravimetric moisture content of the soils was determined. Total C (TC) and total N (TN) were measured using a Flash EA 2000 (ThermoFinnigan, Milan, Italy) direct combustion instrument. Soil inorganic N was extracted with 2M potassium chloride (KCl) and stored at 4oC until filtration (within 4 h of field collection of the soil cores) (Van Miegroet, 1995). Filtrates were frozen and stored until further analysis. Nitrate and ammonium (NH4+) contents were analyzed by colorimetric method (Mulvaney, 1996) with an auto analyzer (Quikchem 8000 FIA+, Lachat Instruments, Milwaukee, WI, USA).

Nitrogen Inputs to Sites and Ratio of N2O Emission to N Inputs

Nitrogen inputs as direct sources of N2O were estimated in a warm-season and a cool-season grass filter, a forest buffer and the adjacent crop field. Pelletized urea (133.4 kg N ha-1) was applied in the crop field (corn) in April 2006. Annual dry and wet deposition was 7.7 kg N ha−1 y−1 on the Iowa State University campus (19 km south of the study site) in January 2003-January 2004 (Anderson and Downing, 2006) and the value was used for N input from deposition in 2006 and 2007. Nitrogen inputs from soybean residue was estimated from samples collected in five randomly located plots (50 cm × 50 cm) in the crop field after the harvest of soybeans in 2005. To estimate corn residues (Yr) in 2006, we used harvest index (HI, 0.53 from Johnson et al., 2006) and harvested corn yields (Ygr, 10,419.8 kg ha-1 y-1) as:

Yr = Ygr [(1/HI) – 1]

where Yr is corn residues (kg ha-1), and Ygr is harvested corn grain and HI is harvest index (Johnson et al., 2006).

N inputs from dead roots in the crop field were calculated from the previous studies conducted in the same sites (Tufekcioglu et al., 1999 and 2003). Biological N fixation was not included as a direct source of N2O because of the lack of evidence of significant emissions arising from the fixation process itself (Rochette and Janzen, 2005; IPCC, 2006).

N inputs from forest buffer litter-fall were estimated from monthly samples collected within five litter-fall collecting baskets (50 cm × 50 cm) placed at random locations in a forest buffer starting in September 2005. In addition above-ground biomass was harvested within five randomly located plots (50 cm × 50 cm) in a warm-season and a cool-season grass filter, and a forest buffer in early November of 2005 and 2006. Collected samples were dried (70oC, 48 h), weighed, and stored for TN analysis. Total N was measured by direct combustion using a Flash EA 2000 (ThermoFinnigan, Milan, Italy). N inputs from dead roots in a warm-season and a cool-season grass filter, and a forest buffer were calculated from previous studies conducted in the same sites (Tufekcioglu et al., 1999, 2003). In these same sites, Lee et al. (2003) also estimated that 0.5 kg N transported from crop fields in run-off was retained in the riparian buffers per rainfall event (> 20 mm rainfall) and there were 13 events exceeding this threshold during 2006-2007. These data were used to estimate N input from runoff to riparian buffers for 2006 and 2007, respectively. Nitrogen input to the riparian buffers from groundwater moving from below crop fields was estimated by averaging N load reduction in groundwater measured in wells under two of the riparian buffers (Kim et al., 2008) . Using the determined cumulative annual N2O emission and N inputs in the sites, the ratio of N2O emission to N inputs (N2O emission factor, EF) in the crop field and riparian buffers was determined.

Estimation of N2O Emissions from Whole Crop Fields and Hypothetical Riparian Buffers in the Watershed

Nitrous oxide emission from all crop fields in the Bear Creek watershed was estimated by multiplying the determined N input and N2O emission factor (EFCF) in the crop field, by the area of the crop fields (6,810 ha). The equation for estimating N2O emission from whole crop fields is:

N2O−N CF = (Fertilizer N + Crop residues N + N deposition) × EFCF × Area CF

where N2O-N CF is the annual direct N2O–N emissions from N inputs to crop fields (kg N2O–N y-1); Fertilizer N is the annual amount of synthetic fertilizer N applied to soils (kg N ha-1 y-1); Crop residues N is the amount of N in crop residues (above- and below-ground), including N-fixing crops returned to soils (kg N ha-1 y-1); N deposition is the N in dry and wet deposition; EFCF is the emission factor for N2O emissions from N inputs in crop fields (kg N2O–N (kg N input)-1); and Area CF is the area of crop fields in the Bear Creek watershed (6,810ha).

To estimate N2O emission from riparian buffers which are ideally installed to maximize the benefits of riparian buffers throughout the watershed, it was hypothesized that 30 m wide buffers existed along both sides of the entire length of the creek (56,473 m). This width was used because buffers 30 m or wider are recommended to obtain fully effective nutrient reduction (Mayer et al., 2006). Nitrous oxide emission from the hypothetical riparian buffers in the watershed was estimated by multiplying the determined N input and N2O emission factor (EFRB) in the riparian buffers of this study, by total area of the hypothetical riparian buffers. The equation for estimating N2O emission from hypothetical riparian buffers is:

N2O–N RB = (Litter and roots N + Run off N + Groundwater N + N deposition) × EFRB × Area RB

where N2O–N RB is the annual direct N2O-N emissions from N inputs to riparian buffers (kg N2O–N y-1); Litter and roots N are the annual amount of N in litter-fall and dead roots (kg N ha-1 y-1); Run off N is the amount of N in runoff from crop fields (kg N ha-1 y-1); Groundwater N is the N in groundwater exported to riparian buffers from crop fields (kg N ha-1 y-1); N deposition is the N in dry and wet deposition (kg N ha-1 y-1); EFRB is the emission factor for N2O emissions from N inputs in riparian buffers (kg N2O–N (kg N input)-1); and Area RB is the area of riparian buffers.

Intergovernmental Panel on Climate Change N2O Flux Calculations

The Intergovernmental Panel on Climate Change (IPCC) Tier 1 methodology (2006) separately estimates direct N2O emission (i.e. directly from the soils to which N is added/released) and indirect N2O emission resulting from offsite N movement (i.e. volatilization of NH3 and NOX, and leaching and runoff of N) from managed soils. The method then estimates direct N2O emission from crop fields by multiplying N inputs by an emission factor. For this study, N inputs from synthetic fertilizer (FSN) and crop residues (FCR), estimated as described above, were summed and multiplied by an emission factor (EF1). The equation for estimating direct N2O emission is:

N2ODirect −N = N2O−NN inputs = (FSN + FCR) EF1

where N2ODirect–N is the annual direct N2O–N emissions produced from managed soils (kg N2O–N y-1); N2O–N N inputs is the annual direct N2O–N emissions from N inputs to managed soils (kg N2O–N y-1); FSN is the annual amount of synthetic fertilizer N applied to soils (kg N y-1); FCR is the amount of N in crop residues (above- and below-ground), including N-fixing crops returned to soils (kg N y-1); and EF1 is the emission factor for N2O emissions from N inputs (kg N2O–N (kg N input)-1). The IPCC default value for EF1 is 0.01. Details of calculating FCR is given in IPCC (1997, 2006).

The IPCC (2006) Tier 1 estimates N2O emission from atmospheric deposition of N volatilized from crop fields (indirect N2O emission) by multiplying N inputs (FSN) by a fraction factor (EF4) for volatilized N. Because synthetic fertilizer was an N input which can be volatilized in the crop fields, the equation for estimating N2O emission is as following:

N2O(ATD)−N = (FSN × FracGASF) × EF4

where N2O(ATD)–N is the annual amount of N2O–N produced from atmospheric deposition of N volatilized from managed soils (kg N2O–N y-1 ); FSN is the annual amount of synthetic fertilizer N applied to soils (kg N y-1); FracGASF is the fraction of synthetic fertilizer N that volatilizes as NH3 and NOx [kg N volatilized (kg of N applied)-1, IPCC is the default value 0.10 for FracGASF]; and EF4 is the emission factor for N2O emissions from atmospheric deposition of N on soils and water surfaces [kg N–N2O (kg NH3–N + NOx–N volatilized)-1, IPCC default value for EF4 is 0.010].

Statistical Analyses

For analyzing normality of the distribution of the data, the Shapiro-Wilk normality test was performed. One-way analysis of variance (ANOVA) was used to evaluate the differences in soil properties, and diel and seasonal N2O flux by site. When the standard assumptions of normality were violated, non-parametric Kruskal-Wallis one-way ANOVA on ranks was used. Differences were considered significant at the P < 0.05 level. To determine the relationship between soil properties and N2O flux, correlation analysis using the GLM procedure was applied and NONLIN procedure was utilized for deriving the best fit of N2O flux models developed by the relationship between soil temperature and N2O flux. These statistical analyses were conducted by SAS version 8.1 (SAS institute, 1999).

Research results and discussion:

Soil Properties and Lengths of Periods of Dried and Frozen Soil

Soil texture was loam at all sites (Marquez et al., 2004). Soils in forest buffer and warm and cool-season grass filters had significantly (one-way ANOVA) lower bulk density, higher pH, TC, TN, and NH4+ than crop fields, while soil NO3- was not significantly different among the sites.

Soils had longer dry (soil moisture < 15%) and frozen (soil temperature < 0oC) periods in 2007 than in 2006 . From 15 June to 15 August 2006 (93 d), soils (5-cm depth) were extremely dry (< 15%) within crop fields for 12 days, within forest buffers 0 days, and within grass filters 51 days. In comparison, from 15 June to 15 August 2007 (93 d), soils were extremely dry (< 15%) within crop fields for 78 days, within forest buffers for 32 days, and within grass filters for 24 days. From January to March 2006 (90 days), soils (5-cm depth) were frozen (< 0oC) within the crop field for 47 days, within forest buffers for 17 days, and within grass filters for 49 days. In comparison, from January to March 2007 (90 days), soils were frozen (< 0oC) within the crop field for 82 days, within forest buffers for 46 days, and within grass filters for 62 days. Diel Variation of N2O Flux and Cumulative Diel N2O Emission Diel variation of N2O flux and soil temperature in the crop field and riparian buffers are shown in Fig. 1. During the 21-22 November 2005, there was no significant difference in N2O flux between the crop field and riparian buffers (one-way ANOVA P = 0.395) and also no significant correlation between soil temperature (5-cm depth, 2-5oC) and N2O flux in the crop field and riparian buffers during this late fall period (all P > 0.05). In contrast, N2O flux in the crop field was significantly higher than in the riparian buffers on both 18-19 May 2006 (7 to 13 times, Kruskal-Wallis one-way ANOVA P < 0.001) and 16-17 July (12 to 18 times, Kruskal-Wallis one-way ANOVA P < 0.001), but there were no differences among vegetation types in riparian buffers during these two periods (Tukey’s Studentized Range Test). Significant correlations between soil temperature (5-cm depth) and N2O flux were only found within the crop field during 18-19 May 2006 (Pearson coefficient r = 0.77 P = 0.02) and 16-17 July 2007 (Pearson coefficient r = 0.48 P = 0.02). The resulting Q10 models (N2O flux = a × Q10 (soil temperature/10)) and Q10 factors were: May 2006 (soil temperature 11-17oC, crop field):
N2O flux (mg N2O-N ha-1 h-1) = 28.9 × 12.28 (soil temperature/10) (R2= 0.67)
Q10 factor 12.78

July 2007 (soil temperature 23-27oC, crop field):
N2O flux (mg N2O-N ha-1 h-1) = 411.0 × 2.27 (soil temperature/10) (R2= 0.87)
Q10 factor 2.27

The cumulative diel N2O emission in the crop field, forest buffer, warm-season and cool-season grass filter was 5.9 g N2O-N ha-1 h-1, 2.2 g N2O-N ha-1 h-1, 3.0 g N2O-N ha-1 h-1, and 1.0 g N2O-N ha-1 h-1, respectively, during 21-22 November 2005, 43.2 g N2O-N ha-1 h-1, 3.9 g N2O-N ha-1 h-1, 6.0 g N2O-N ha-1 h-1, and 5.1 g N2O-N ha-1 h-1, respectively during18-19 May 2006, and 130.3 g N2O-N ha-1 h-1, 7.1 g N2O-N ha-1 h-1, 10.5 g N2O-N ha-1 h-1, and 7.7 g N2O-N ha-1 h-1, respectively during 16-17 July 2007. These indicate that N2O emission from the crop field was 2 to 5-fold higher than from the riparian buffers during 21-22 November 2005, 7 to 11-fold higher during 18-19 May 2006, and 12 to 14-fold higher during 16-17 July 2007

Seasonal Variation of N2O Flux and Cumulative N2O Emission

When assessed over a season, N2O flux in the crop field was significantly correlated with air temperature (Pearson coefficient r = 0.38 P = 0.0001), soil temperature (5 cm depth) (r = 0.42 P < 0.0001) and soil moisture (5 cm depth) (r = 0.35 P = 0.005). In all riparian buffers, N2O flux was significantly correlated with air temperature (Pearson coefficient r = 0.1-0.5 P < 0.01) and soil temperature (5 cm depth) (r = 0.3-0.6 P < 0.0001) during this same period. The average of observed N2O fluxes in the crop field (39.4 ± 7.1 kg N2O-N ha-1 d-1, n = 76) was significantly higher than in the riparian buffers (2.8-11.0 kg N2O-N ha-1 d-1, n = 72-93) (P < 0.0001), but there were no differences among vegetation types in the riparian buffers (Tukey’s Studentized Range Test). Q10 factors used for correcting daily average N2O flux in the crop field were distinguished for three different field soil temperature ranges (< 10oC, 10-20oC, > 20oC) as follows:
1) Soil temperature < 10oC condition; no valid Q10 factor, Measured N2O Flux = Diel average N2O Flux
2) Soil temperature 10-20oC condition; Q10 factor 12.78 was applied
3) Soil temperature > 20oC condition; Q10 factor 2.27 was applied

Since there was no significant effect of soil temperature on diel N2O flux (no valid Q10 factor) in the forest buffers, and warm-season and cool-season grass filters, measured N2O flux was used as a diel average N2O flux.

In both 2006 and 2007, annual cumulative N2O emission was significantly greater in the crop field (7.2 kg N2O-N ha-1 in 2006 and 16.8 kg N2O-N ha-1 in 2007) than in the forest buffers (1.8 kg N2O-N ha-1 in 2006 and 4.5 kg N2O-N ha-1 in 2007) and grass filters (1.8 kg N2O-N ha-1 in 2006 and 3.4 kg N2O-N ha-1 in 2007). The annual cumulative N2O emission in the crop field, forest buffers, and grass filters in 2007 were 2 to 2.5-fold larger than 2006.

N2O Peak Emission and Negative N2O Flux

Several periods of peak N2O emissions contributed significantly to annual N2O emissions in both the crop field and riparian buffers. In 2006 in the crop field, two large peak emissions followed the thawing of frozen soil (13-fold increase, February) and rewetting of dry soil (37-fold increase, November) contributing 33.8% of the annual N2O emission. In 2007 in the crop field, a peak emission followed the thawing of frozen soil (28-fold increase, March) and three peak emissions followed rewetting of dry soil (5 to 70-fold increase, July to October). These four peak emissions contributed 70.3% of annual N2O emission. All of the peak emissions returned to lower levels within a week. In 2006 the warm-season and cool-season grass filters had two peak emissions (July and December) following the rewetting of dry soil that contributed 17.0% of the annual N2O emission. In 2007 grass filters had a peak emission after the thawing of frozen soil (March) and two peak emissions after rewetting of dry soil (June and December) contributing 31.1% of the annual N2O emission. In 2006 the forest buffers had a peak emission after the rewetting of dry soil (July) which contributed 10.8% of annual N2O emission, and in 2007, a peak emission after the thawing of frozen soil (March) and two peak emissions after rewetting of dry soil (June and December) contributed 70.5% of annual N2O emission. Across all vegetation types, N2O peak emissions were 3 to 10-fold greater than base-line levels after the thawing of frozen soil or rewetting of dry soil and the peaks returned to lower levels within a week. Soils within the crop field showed higher peak rates of N2O emission than soils in the riparian buffers in both 2006 and 2007. As a result, the contribution of peak emissions to annual N2O emission was larger in the crop field than in riparian buffers during both years, with the contribution higher in 2007 than 2006.

A few negative N2O fluxes were observed in the crop field and riparian buffers. There were no significant differences among sites (P = 0.99) and the negative fluxes showed no significant relation to soil or air temperature or soil moisture (P > 0.05). The negative N2O fluxes were most frequently observed (81%) in the less than 5oC soil temperature range, and the observed maximum negative N2O flux was -0.64 g N2O-N ha-1 h-1 (-64.0 µg N2O-N m-2 h-1).

Nitrogen Inputs and Ratio of N2O Emission to N Inputs

In 2006, N fertilizer (133.4 kg N ha-1) was applied in the crop field (corn) resulting in a larger N input to the crop field than the riparian buffers. However, in 2007, N input to the crop field was less than into the riparian buffers, mainly due to no fertilizer application. Nitrogen input from crop residues and dead roots in the crop field was 82.1 and 92.2 kg N ha-1 in 2006 and 2007, respectively. Annual dry and wet deposition was 7.7 kg N ha-1 in the crop field and riparian buffers. Total N inputs in the crop field were 323.1 kg N ha-1 through 2006 and 2007.

Nitrogen input from litter and dead roots in the riparian buffers was estimated at 83.6 and 69.0 kg N ha-1 in 2006 and 2007, respectively. N input from runoff in the riparian buffers was estimated at 0.5 and 6.0 kg N ha-1 in 2006 and 2007, respectively. Nitrogen input to the riparian buffers from groundwater discharged from the crop field was 36.1 kg N ha-1 in 2006 and 2007. Total N inputs to the riparian buffer was 246.7 kg N ha-1 through 2006 and 2007 so that N inputs into the riparian buffers was 23.6% less than into the crop field.

The ratio of measured N2O emission to N inputs to soils in the crop field in 2006 (0.03) was 3-fold higher than the ratio for the riparian buffers soils in 2006 (0.01). In 2007, the ratio of measured N2O emission to N inputs to soils in the crop field (0.17) was more than 5-fold higher than to soils in the riparian buffers (0.03) . Overall, the ratio of measured N2O emission to N inputs to soils in the crop field (0.07) was more than 3-fold higher than the ratio to soils in the riparian buffers (0.02).

Estimated N2O emissions from all crop fields and hypothetical riparian buffers in the watershed
The estimated total N2O emission from all the crop fields and hypothetical riparian buffers in the watershed was 77,010.9 kg N y-1 and 835.9 kg N y-1, respectively. This indicates the ratio of N2O emission in all the crop fields to N2O emission in the riparian buffers in the watershed is 0.01.

Comparison of measured N inputs and N2O emission with estimated values by the IPCC method
Estimated N input from crop residues and dead roots in the crop field by the IPCC method (2006) was 56.4 and 118.3 kg N ha-1 in 2006 and 2007, respectively. Compared to the measured N input values, the IPCC method underestimated inputs by 31% in 2006 and overestimated them by 28% in 2007 in the crop fields. In the crop field, estimated N2O emission (by IPCC 2006) was 2.0 kg N ha-1 and 1.2 kg N ha-1 in 2006 and 2007, respectively. The ratio of measured N2O emission to estimated N2O emission in the crop field was 3.5 and 14.2 in 2006 and 2007, respectively; the overall ratio was 7.5 through 2006 and 2007 and this indicates the IPCC method underestimated N2O emission by about 87% in the crop field.

Discussion

Peak N2O emissions and the implications
Peak emissions following rewetting dry soils and thawing frozen soils contributed substantially to annual N2O emissions, especially in the crop fields. The existence of short-term peaks in N2O emissions has significant implications for flux-measurement protocols (Parkin, 2008).

In the crop (soybeans) field in 2007, even though N inputs were less than those added to the crop (corn) field in 2006 because N fertilizer was not applied, both annual N2O emission (16.8 kg N2O-N ha-1 y-1) and the EF (0.17) were larger than for the crop field in 2006 (annual N2O emission: 7.2 kg N2O-N ha-1 y-1, EF: 0.03). In the same region (central Iowa), Parkin and Kasper (2006) observed annual N2O emission from soybean and corn fields (N fertilizer 215 kg N ha-1) at 2.2-2.7 N2O-N ha-1 y-1 and 7.6-10.2 N2O-N ha-1 y-1, respectively. Our N2O emission estimate from the crop field in 2006 is similar to these authors’ observations under corn; however, our emission estimate from the crop field in 2007 when soybeans were present is 6 to 7- fold higher than those observed by Parkin and Kasper’s (2006). The N2O emission from the crop field in 2007 was also larger than the average N2O emission observed in the crop fields throughout the temperate region (3.6 ± 0.5 kg N2O-N ha-1 y-1, Stehfest and Bouwman, 2006). The emission factor in the crop field in 2007 was also larger than other reports (Bouwman et al., 2002; Stehfest and Bouwman, 2006; Novoa and Tejeda, 2006) and the IPCC (2006)’s default value (0.01, uncertainty range 0.003-0.03). A similar pattern was also observed in riparian buffer soils in 2007. These results indicate that N2O emission from soils in the crop field and riparian buffers were caused by additional factors beyond N inputs. One such factor may be the peak N2O emissions observed within the crop field and riparian buffers during each year. There were several peak emissions following rewetting of dry soils and thawing of frozen soils in both sites, and the peak emissions significantly contributed (30-70%) to the total amount of annual N2O emission. This result is consistent with other studies (e.g. Teepe et al., 2000; Prieme and Christensen, 2001; Nobre et al., 2001; Regina et al., 2004) reporting the contributions of peak N2O emissions to annual N2O emissions following rewetting dry soils and thawing frozen soils. In our sites, we observed that the crop fields had N2O peak emissions of greater magnitude than riparian buffers. This result is similar to studies reviewed by Matzner and Borken (2008) in that the emissions of N2O after thawing frozen soils were sometimes significantly larger from arable soils than from forest soils. In our observations, soils within the crop field had lower soil temperatures in winter and higher soil temperature and longer dry periods in summer compared with soils within riparian buffers. This may explain why peak emissions during periods of rewetting and thawing were higher in the crop field than in the riparian buffers. Riparian buffer vegetations provides more shade, preventing high temperature increases during the summer months and provides insulation, preventing severe temperature deceases during winter months. In contrast, soils in the crop field are exposed to direct sunlight during the summer months and cold wind during the winter months. Undisturbed perennial riparian vegetation also contributes to lower soil bulk density and higher organic matter (Marquez et al., 1999; Tufekcioglu et al., 2001; Bharati et al., 2002), resulting in higher soil moisture. In contrast, soils in the crop field exposed to direct sunlight, with higher bulk density, and lower soil organic matter will tend to hold less soil moisture compared with riparian buffer soils. We observed that the contribution of peak emissions to annual N2O emission was larger in 2007 than 2006 in both the crop field and riparian buffers. The period that soils were frozen during the winter months and the period that soils were dry during summer months were longer in 2007 compared with 2006, and this may explain the higher peak emissions during periods of rewetting and thawing observed in 2007.

Since the N2O flux was not measured in the crop field from mid-April to mid-May 2006, and fertilizer was applied and it rained during this period, we might have missed peak N2O fluxes in response to rainfall after fertilizer application (Parkin and Kaspar, 2006; Baggs et al., 2003; Sehy et al., 2003). Also since the N2O flux was not measured in the crop field in August and September to October in 2006, and there were several rewetting events during those periods, we might have missed additional peak emissions. It is suspected that these missed peak emissions may been the reason for the lower annual N2O emission in the crop field 2006.

It has been reported that there will be more severe droughts associated with summer drying and intense precipitation in a future warmer climate (Easterling et al., 2000; Wang, 2005; Burke et al., 2006; Meehl et al., 2006; Rowell and Jones, 2006 Alexander et al., 2006; Sillmann and Roeckner, 2008). Also the increase in freeze and thaw frequency (Gu et al., 2008) and the increased impacts on the area and depth of permafrost regions (Lawrence and Slater, 2005) are predicted in a future warmer climate. The observed peak N2O emissions during the thawing of frozen soils and rewetting of dry soils in the crop field 2007 have important implications for greenhouse gas emissions in a changing climate. This should not be viewed as an unusual event. Rather, it represents a consequence of predicted climate change on greenhouse gas emissions. Also the observed large difference between measured N2O emission and estimated N2O emission by the IPCC method (2006) (87% underestimation by IPCC method) suggests that the current IPCC (2006) N2O emission estimation methodology, based on N input information, may underestimate emissions in the regions where soil rewetting and thawing are common or potentially may be increased by future climate change. Additional studies are warranted to clarify the relationships between antecedent soil moisture/soil temperature and the frequency of dry-wet/frozen-thawed cycles and their subsequent effect on soil N2O flux.

The resulting improvements in N2O emission models would improve the accuracy of the N balance of terrestrial ecosystems and improve predictions of the probable impacts of anthropogenic climate change on such factors as:
1) the increase of summer drying in a future warmer climate with associated increased risk of drought (e.g. Alexander et al., 2006; Sillmann and Roeckner, 2008), and
2) the increase in freeze and thaw frequency (Gu et al., 2008).

N2O Emissions in the Crop Field and Riparian Buffers

N2O emissions from riparian buffers were significantly lower than those from the crop field. At the watershed scale, the contribution of N2O emissions from riparian buffers to total emissions was very small.

In our studies, measured N2O emissions from all riparian buffer soils (1.8-4.0 kg N2O-N ha-1 y-1) were significantly lower than within the crop fields (7.2-16.8 kg N2O-N ha-1 y-1) and there were no observed differences in N2O emissions among the different riparian buffer vegetation types. Studies (Weller et al., 1994; Groffman et al., 1998; Machefert et al., 2004) have measured 0.1-5.3 kg N ha−1 y−1 of N2O emissions from riparian buffer soils, similar to observations in this study. In similar studies in the temperate regions, mean N2O emissions measured in fertilizer-applied grasslands were 8.0 ±1.4 kg N2O-N ha-1 y-1, in grasslands without fertilizer addition were 1.4 ± 0.4 kg N2O-N ha-1 y-1, and in forests were 0.7 ± 0.3 kg N2O-N ha-1 y-1 (Stehfest and Bouwman, 2006). Nitrous oxide emissions from riparian buffer soils in 2006 (1.8 kg N2O-N ha-1 y-1) in our studies were similar to N2O emission from unfertilized grassland and forest soils in temperate regions. This suggests that N2O emissions from riparian buffer soils were similar to those from natural ecosystems. The ratio of N2O emission from crop fields to N2O emission from the hypothetical riparian buffers in the watershed is 0.01. Since dissolved N2O emission in groundwater leached from the crop fields was negligible in comparison to soil N2O emission in the crop fields (ratio between dissolved N2O emission and soil N2O emission, 0.0003) (Kim et al., 2008), this suggests that the contribution of N2O emission from riparian buffers to total N2O emission in the watershed may be around 1% even if riparian buffers are installed ideally to maximize the benefits in the watershed. Weller et al. (1994) estimated 0.35 kg N ha-1 and 0.04 kg N ha-1 of annual N2O loss in soil emission and groundwater (< 1% of the intercepted N) in riparian buffers and concluded that N2O production in the riparian buffers is neither an important fate of N removed from cropland discharges nor an important source of atmospheric N2O pollution. Dhondt et al. (2004) observed -0.6 to 2.5 mg N2O-N m-2 d-1 N2O emission in three NO3- loaded riparian sites and they concluded that the observed N2O emission did not lead to an important pollution swapping from water pollution to greenhouse gas emission. Teiter and Mander (2005) reported that N2O emissions from a riparian gray alder stand varied from -0.4 to 58 μg N2O-N m−2 h−1 and concluded that the global warming potential of the riparian alder forest from N2O was relatively low. Our results, along with those of past studies, suggest that the riparian buffers should not be considered a major source of N2O emission in a watershed. Some studies (Walker et al., 2002; Hefting et al., 2003) have shown much higher N2O emissions from riparian soils. Walker et al. (2002) observed that N2O emission in a recovering riparian zone and a grazed riparian zone was 24.19 kg N ha−1 y−1 and 24.50 kg N ha−1 y−1, respectively. Hefting et al. (2003) observed that N2O emissions were significantly higher in the forested buffer system (20 kg N ha-1 y-1) than within the grassland buffer zone (2-4 kg N ha-1 y-1). They suggested that the higher rates of N2O emissions within the forested buffer zone were associated with higher NO3- concentration in the groundwater, and N transformation by buffer zones with high NO3- loading resulted in a significant increase of N2O emission. This is consistent with the work of Ullah and Zinati (2006) who reported that prolonged N loading resulted in higher N2O emissions in riparian forest soils compared to emission rates from non-exposed forest soils. Hefting et al. (2006) reported that locations with high NO3- removal efficiency also contribute significantly to increased N2O emission from riparian zones.
Considering all of these results, it is likely that N2O emission from riparian buffers are highly site specific and may vary with site characteristics such as soil type, magnitude and speciation of N input, and hydrologic characteristics (Walker et al., 2002). In this study, low N inputs and less N2O peak emissions in riparian buffers might cause less N2O emission than from adjacent crop fields than found in other studies (Walker et al., 2002; Hefting et al., 2003).

It was found that there were uncertainties in the results from the previous studies (Walker et al., 2002; Hefting et al., 2003, 2006; Dhondt et al., 2004; Teiter and Mander, 2005). The studies may have a common problem in N2O measurements frequency. The N2O emission value reported by Walker et al. (2002) was determined with every three month measurements for 14 months. Hefting et al. (2003) and Dhondt et al. (2004) measured N2O emission in February, May, August, and November representing winter, spring, summer and fall seasons. Teiter and Mander (2005) measured N2O emission once a month for 15 months through three years and Hefting et al. (2006) measured N2O emission once in winter and once in summer to obtain the difference of N2O emission in high and low NO3- removal transects. Since it is well known that N2O flux is significantly increased by episodic events such as rewetting of dry soil and thawing of frozen soil as well as N input (e.g. Müller et al., 2003; Mikha et al., 2005) and the peak N2O emission may substantially contribute to total N2O emissions (e.g. Prieme and Christensen, 2001; Nobre et al., 2001), their less frequent and short-term N2O measurements might result in uncertainties in the determined N2O emissions (Parkin, 2008) and the results made based on the determined values might also include uncertainties. Therefore, it is suggested that the results should be applied carefully and further study should consider the issue for an experimental design that clearly samples during potential times of high emissions.

Conclusions

Annual N2O emissions from all riparian buffer soils (1.8 kg N2O-N ha-1 in 2006 and 3.4-4.5 kg N2O-N ha-1 in 2007) were significantly lower than from crop field soils (7.2 kg N2O-N ha-1 in 2006 and 16.8 kg N2O-N ha-1 in 2007) and no differences were observed among the different kinds of riparian buffer vegetation. While N2O peak emissions following the rewetting of dry soils and thawing of frozen soils contributed significantly to annual N2O emission in the crop field, soils in riparian buffers were less sensitive to these events. Over a 2-year period, the EF of soils in riparian buffers (0.02) was about one third that of the crop field (0.07) with N inputs lower in the riparian buffer soils than in the crop field soils. Even if riparian buffers are enlarged to maximize the benefits in the watershed the ratio of N2O emission from crop fields to N2O emission from the hypothetical riparian buffers in the watershed is 0.01. These results indicate that N2O emissions from soils in the riparian buffers were significantly less than those in the crop field and suggest that the riparian buffers should not be considered a major source of N2O emission in the watershed.

In addition, this study also suggests:
1) N input cannot always explain N2O flux,
2) the N input-based IPCC methodology for estimating N2O emissions may underestimate fluxes in regions where frequent rewetting of dry soils and thawing of frozen soils occurs, and
3) additional studies characterizing N2O peak emissions are needed to better understand annual N2O fluxes and the N cycle within these systems, and to improve prediction of the impacts of future climate change.

Table 1. Soil properties (mean ± standard error) (n = 6-9 except bulk density (n =27)) of the sites. Soil samples (depth 0-15 cm) were collected in a forest buffer, a warm-season grass filter, a cool-season grass filter, and an adjacent crop field in Oct. 2006 and Sept. 2007.

Table 2. Nitrogen inputs from crop residues (n = 5), dead roots (n = 5), and plant litter (n = 5) of the previous year in the crop field and riparian buffers in 2006 and 2007 and estimated N inputs (IPCC 2006) from crop residues and dead roots of the previous year in the crop fields in 2006 and 2007.

Table 3. Measured (Mea.) N inputs and N2O emission, ratio of measured (Mea.) N2O emission to N inputs, estimated (Est.) N2O emission by IPCC 2006 method, and the ratio of measured (Mea.) N2O emission to estimated (Est.) N2O emission in the crop field and riparian buffers. Units of all N input and measured (Mea.) and estimated (Est.) N2O-N is kg N ha-1.

Table 4. The estimated N2O emission from hypothetical riparain buffers (N2O RB) and crop fields (N2O CF) in the Bear Creek watershed and the ratio between them (N2O RB/ N2O CF). A 30 m width of riparian buffers was applied on the both sides of the creek along the whole length of the creek.

Figure 1. Diel variation of N2O flux and soil temperature (5-cm dept) in crop field, forest buffer, warm-season and cool-season grass filter in 21-22 Nov. 2005 (A and B), 18-19 May 2006 (C and D), and 16-17 July 2007 (E and F). Observations are mean values with standard errors of the mean.

Figure 2. Cumulative diel N2O emission in crop field, forest buffer, warm-season and cool-season grass filter in 21-22 Nov. 2005, 18-19 May 2006, and 16-17 July 2007.

Figure 3. Daily N2O flux from soils within the crop field and riparian buffers in 2006 and 2007(n = 72-93). I, II, and III indicate replicates. The lower boundary of the box indicates the 25th percentile, the line within the box marks the median, and the upper boundary of the box indicates the 75th percentile. Error bars indicate the 90th and 10th percentiles. Solid circles indicate outliers.

Figure 4. Nitrous oxide emissions (A, B), precipitation (C), and daily average of soil moisture (D) and soil temperature (E) in forest buffers (n = 3), grass filters (n = 4), and adjacent crop field (n = 1) during 2006 and 2007. Observations are mean values with standard errors of the mean in (A) and (B).

Figure 5. Observed negative N2O flux (< -0.175 g N2O-N ha-1 h-1, minimum detectable flux; significance was satisfied with 95% confidence limits) of the slope was tested and on-site soil temperature (5-cm depth) in forest buffers, grass filters, and adjacent crop field during 2006 and 2007. (To view hard copies of the tables and figures from this report, please contact the NCR-SARE office at ncrsare@umn.edu)

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Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Refereed Journal

Kim, D., T.M. Isenhart, T.B. Parkin, R.C. Schultz, T.E. Loynachan, and J.W. Raich. In Review. Nitrous oxide emissions from riparian forest buffers, warm-season and cool-season grass filters, and crop fields. Biogeosciences.
Kim, D., T.M. Isenhart, T.B. Parkin, R.C. Schultz, and T.E. Loynachan. In Review. Nitrate and dissolved nitrous oxide in groundwater within cropped fields and riparian buffers. Biogeosciences.

Kim, D., T.M. Isenhart, T.B. Parkin, R.C. Schultz, and T.E. Loynachan. In Review. Methane flux in riparian forest buffer, warm-season and cool-season grass filter, and adjacent crop fields soils. Journal of Environmental Quality.
Kim. D., 2008. Impact of increased precipitation and temperature extremes by climate change on greenhouse gases emission: A review (in preparation)

Presentations and Abstracts

Isenhart, T.M., Kim, D., Schultz, R.C., Parkin, T.B., Raich, J.W., Loynachan, T.E. 2008. Emission of the Greenhouse Gas Nitrous Oxide (N2O) from Riparian Forest Buffers, Warm-Season and Cool-Season Grass Filters and Crop Fields. In: American Water Resources Association, 2008 summer special conference Riparian Ecosystems and Buffers: Working at the Water’s Edge, June 30-July 2, 2008, Virginia Beach, VA.

Kim, D., Isenhart, T.M., Schultz, R.C., Parkin, T.B., Raich, J.W., Loynachan, T.E. 2008. Biomass Yields and the Effect of Harvesting Trees, Shrubs and Native Grasses on Greenhouse Gas (CO2, N2O and CH4) Flux in Riparian Buffers Designed to Provide Biomass for Biofuel Production. In: American Water Resources Association, 2008 summer special conference Riparian Ecosystems and Buffers: Working at the Water’s Edge, June 30-July 2, 2008, Virginia Beach, VA (Poster presentation).

Kim, D., Isenhart, T.M., Schultz, R.C., Parkin, T.B., Raich, J.W., Loynachan, T.E. 2008. Production and Consumption of the Greenhouse Gas Methane (CH4) from Riparian Forest Buffers, Warm-Season and Cool-Season Grass Filters and Crop Fields. In: American Water Resources Association, 2008 summer special conference Riparian Ecosystems and Buffers: Working at the Water’s Edge, June 30-July 2, 2008, Virginia Beach, VA (Poster presentation).

Kim, D., Schultz, R.C., Isenhart, T.M., Simpkins, W., Parkin, T.B., Raich, J.W., Loynachan, T.E. 2007. Emission of the Greenhouse Gas Nitrous Oxide (N2O) from Riparian Forest Buffers, Warm-Season and Cool-Season Grass Filters and Crop Fields. In: ASA-CSSA-SSSA Annual Meeting, Nov. 4-8, 2007, New Orleans, LA.

Kim, D., Schultz, R.C., Isenhart, T.M., Parkin, T.B., Raich, J.W., Loynachan, T.E. 2007. Emission of the Greenhouse Gas Nitrous Oxide (N2O) from Riparian Forest Buffers, Warm-Season and Cool-Season Grass Filters and Crop Fields. In: 4th USDA Greenhouse Conference, Feb. 5-8, 2007, Baltimore, MD.

Kim, D., Schultz, R.C., Isenhart, T.M., Simpkins, W., Parkin, T.B., Raich, J.W., Loynachan, T.E. 2006. Emission of the Greenhouse Gas Nitrous Oxide (N2O) from Riparian Forest Buffers, Warm-Season and Cool-Season Grass Filters and Crop Fields. In: ASA-CSSA-SSSA Annual Meeting, Nov. 12-16, 2006, Indianapolis, IN (Poster presentation).

Kim, D., Schultz, R.C., Isenhart, T.M., Parkin, T.B., Raich, J.W., Loynachan, T.E. 2006. Flux of dissolved N2O in groundwater from crop field to riparian buffers. In: Soil and Water Conservation Society workshop, Managing Agricultural Landscapes for Environmental Quality: Strengthening the Science Base, October 11-13, 2006. Kansas City, MO.

Dissertation

Kim, D. 2008. Nitrous oxide and methane fluxes in riparian buffers and adjacent crop fields. Ph.D. dissertation, Iowa State University, Ames. IA, pp.119.

Project Outcomes

Project outcomes:

It recently has been hypothesized that increased denitrification within riparian areas may trade a water quality benefit for an atmospheric pollution problem (Groffman et al., 1998) resulting from the greenhouse effect of N2O produced during nitrification and denitrification (Wang et al., 1976) and ozone depletion (Crutzen, 1970; Liu et al., 1977).

Our study demonstrated that N2O emissions from riparian buffer soils were significantly less than those in the crop field soils and the ratio of N2O emission to N inputs and N inputs in riparian buffers were smaller than those in crop fields. This study suggests that the riparian buffers should not be considered a major source of N2O emission in a watershed.

In addition, our study demonstrated that peak N2O emissions following rewetting of dry soils and thawing of frozen soils contributed substantially to annual N2O emissions, especially in the crop field. This study also suggests that the N input-based IPCC methodology for estimating N2O emissions may underestimate fluxes in the regions where frequent rewetting of dry soils and thawing of frozen soils occurs.

Results of this work have been presented at scientific meetings such as the American Society of Agronomy/Crop Science Society of America/Soil Science Society of America, the Soil and Water Conservation Society, the USDA Greenhouse Conference, and the American Water Resources Association. The audience at these meetings came from a broad national base of scientists and professionals who are interested in sustaining and improving the environmental quality of the agroecosystem. Informing these audiences of the results, stimulated further research in other regions of the country. Three scientific papers been submitted for publication in Journal of Environmental Quality and Biogeosciences. These publications will reach even larger audiences of scientists and professionals. We will also prepare a new or modify an existing extension bulletin on buffer design to incorporate the new information. We will officially register the study sites with ‘AmeriFlux’ (http://public.ornl.gov/ameriflux) or ‘United States Trace Gas Network’ (http://www.nrel.colostate.edu/projects/tragnet/) and share previous results of soil CO2 flux and C sequestration and the newly obtained CO2, N2O and CH4 flux data, as well as soil, vegetation and microclimate data from these sites with other scientists. This will provide greenhouse gas modelers with another critical data set of field measures on the process rates and the quantities of CO2, N2O and CH4 gas emitted from conservation buffers and will help refine the emission predictions for these gasses in the Midwest.

Because the work was conducted in the Bear Creek National Research and Demonstration Watershed, results from this study support discussion of results from numerous other on-going and past studies on the effectiveness of riparian buffer phytoremediation of non-point source pollutants and for providing habitat for wildlife. Results from this project should be applicable over much of the Corn Belt Region. Finally, the results provide additional supporting information to the public and to policy makers on the benefits of riparian buffers as one of the most effective tools for coping with non point source pollution caused by agricultural activities.

Economic Analysis

This was a research project, not designed to address economic issues, and we did not.

Farmer Adoption

While results of this study will not be directly used by farmers they are another piece of information that indicates that riparian buffer and grass filter conservation practices are a wise management tool for addressing environmental problems in need of remediation. This information can be used to expand on the ecosystem services information associated with these conservation practices.

Recommendations:

Areas needing additional study

The following research is suggested to build on results from the present study:

This study demonstrated that peak emissions following rewetting dry soils and thawing frozen soils contributed substantially to annual N2O emissions, especially in the crop field. Characterizing N2O peak emissions are needed to better understand annual N2O fluxes and the N cycle within these systems, and to improve prediction of the impacts of future climate change.

Biofuel feedstock is considered a promising renewable energy source and a viable carbon (C) sequestering option. However, there are concerns about the negative effects of annual crop biofuel feedstock production on the environment. Establishing perennial plant biofuel plantations on riparian areas may help meet the projected demands for biomass. However, frequent harvesting in short-rotation perennial systems may have negative effects on soil C sequestration and greenhouse gas flux which are significantly related to the environmental benefits of riparian buffers.

It is therefore important to quantify the biomass produced by a short-rotation management scheme in riparian buffers and to measure the effects of harvesting on soil C sequestration and CO2, N2O and CH4 flux. The results of this study will develop management procedures for harvesting and regenerating biofuel feedstocks in riparian buffers while maintaining their phytoremediation potential.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.