Assessment of long-term soil carbon (C) dynamics under a gradient of management intensity in subtropical grasslands, indicated that tree-grass integrated silvopasture is more beneficial for C sequestration, compared to baseline native rangeland or intensively managed sown pastures. Although the mixed C3-C4 composition of silvopasture limited full elucidation of C sources, the loss of relic stable C fraction and the increase in recent (C4-derived) C in sown pasture suggests that C inputs from the sown grass species (bahiagrass) contributed to stable C sequestration. Data also indicated that conversion from native rangeland to sown pasture elevated C losses through respiration.
Terrestrial ecosystems are sensitive to human-induced and nature-driven changes, and this impacts their overall integrity and function. Soil C plays a critical role in grassland sustainability and productivity. Its key functions include i.) serving as a source for plant nutrients, ii.) improvement of water-holding capacity of the soil, iii.) formation and stabilization of soil aggregates, iv.) provision of habitable condition for soil microbial diversity, v.) reduction of greenhouse gases by serving as sink for CO2 (Weil and Magdoff, 2004). However, C stored in grassland vegetation and soil can change in response to a wide array of management and environmental factors. In Florida, grassland ecosystems cover ~2.5 million ha (~17.5% of total land) and supports 1.7 million cattle and calves (USDA-NASS, 2009). There are growing concerns over current and predicted land-use intensification in Florida due to population-driven demand for increased productivity per unit land area, including grasslands (White et al., 2000; Mulkey, 2007). Hence, management strategies that can enhance soil C sequestration and preserve soil resources in the long-term are essential to ensure ecological balance and sustainability while optimizing land productivity. This is especially important under subtropical conditions like Florida, where the characteristic high moisture, flat topography, and sandy composition of the soils favor rapid decomposition of soil organic matter and leaching of organic compounds (NRCS Websoil Survey, 2013). Limited knowledge exists on the dynamics of soil C under such subtropical conditions in southeastern USA. It is uncertain if intensive management which favors higher above-ground production and animal production will have significant long-term effect on soil C dynamics, including CO2 efflux, compared to low-input management systems such as native rangelands.
An understanding of the effect of adopted management system on soil C in this ecosystem is indispensable for further development of management strategies that will preserve ecological function and promote optimal soil C sequestration. Moreover, this can be instrumental for informing management and policy decisions on wider grassland ecosystems.
This research was conducted to
i. Quantify soil total organic C and particulate organic C under 3 distinct management systems by sampling and analyzing soil samples from each >20 year old fields.
ii. Assess for differences in the contribution of above- and below-ground components under the 3 distinct management systems by quantifying the above- and below-ground biomass.
iii. Determine the effect of management intensification on long-term rate of C loss through respiration (CO2 efflux).
This research was conducted at the University of Florida Range Cattle Research and Education Center in Ona, Florida (27°23’76”N, 82°56’11”W). The experimental site is characterized by relatively homogenous slope (<5%), and subtropical climate with 10-year average annual precipitation of ~1206 mm and temperature of ~21.5°C. The study was carried out on adjacent fields of three grassland management systems that were consistently maintained for over 20 years. The three management systems constituted a gradient of management intensities ranging from native rangeland (lowest), silvopasture (intermediate), to sown pasture (highest). Prior to the establishment of the adjacent silvopasture and sown pasture, the entire site was native rangeland for livestock grazing. Each collocated ecological unit was replicated twice (6 ha each). The dominant soil series was the same across the sites and consisted of Ona and Immokalee fine sand (sandy siliceous, hyperthermic Typic Alaquods). This soil was developed on parent material of sandy marine deposits (Soil Survey Staff, 1999; NRCS Websoil Survey, 2013).
The native rangeland ecosystem consisted primarily of saw palmetto (Serenoa repens Bartr.) and a wide variety of grass genera including Andropogon, Panicum, Aristida, and Schizachyrium spp. (Kalmbacher et al., 1984). This ecosystem was never fertilized, but it had been subjected to periodic burning (every 3 years), occasional livestock grazing activities (<60 days per year), and herbivory by wildlife, all typical features of rangeland in this region. Each experimental unit was grazed only during winter at rate of 125 animal unit days ha-1 yr-1 (a 500-kg animal grazing for 1 day equals 1 animal unit day). Animals were fed a daily supplement of warm-season grass hay and sugarcane molasses at 1.5 to 1.9 kg cow-1 day-1 and 0.7 kg cow-1 day-1 , respectively, throughout the grazing period.
The silvopasture system was managed for 22 years and consisted of slash pine (Pinus elliottii) trees planted in double-rows (1.2 m along ×2.4 m between rows), and bahiagrass (Paspalum notatum) planted in alleys (12.2 m wide). The vegetation of the silvopasture was established in order of bahiagrass first, followed by slash pine, on a previously native rangeland. Native vegetation was suppressed by burning followed by plowing (~ 45 cm deep) and disking with a dual tandem disk harrow until there was no vegetation on the soil surface. These experimental units received periodic applications of 67 kg N ha-1 yr-1 as ammonium nitrate. No fertilizer was applied in the years of 1993-1997, 2000, 2002, 2008, 2009, and 2011. Grazing of the silvopasture began in March 1993, 18 months after planting the trees, and has continued from March-September every year. Each experimental unit was rotationally stocked for 7 months each year, with a 2-week grazing period followed by a 5-week resting period. Stocking rate was 207 animal unit days ha-1 yr-1. Animals were supplemented in the pasture with warm-season grass hay and sugarcane molasses at 1.5 to 1.9 kg cow-1 day-1 and 0.7 kg cow-1 day-1, respectively, from January to April. The management conditions that were applied represent limited inputs of fertilizer and moderate levels of biomass removal through grazing.
The sown pasture system consisted of 32-year-old bahiagrass stand, which was managed to provide forage for grazing livestock. Each experimental unit was stocked at a rate of 360 animal unit days ha-1 yr-1. Grazing occurred on a rotational basis for 7 months, with 1-week residence period followed by 1-week rest period between each grazing event. Animal were fed a daily supplement of warm-season grass hay and sugarcane molasses at 1.5 to 1.9 kg cow-1 day-1 and 0.7 kg cow-1 day-1 , respectively, from January to April. Ammonium nitrate fertilizer was applied at an annual rate of 67 kg N ha-1 yr-1 and dolomite was applied in 2001 and 2008 at a rate of 730 and 550 kg ha-1 respectively. These management conditions are typical of the beef cow-calf production system in Florida.
The study was based on a comparative-mensurative experimental design (Hurlbert, 1984; Arevalo et al., 2009) because the two replicate fields (6 ha each) of each management ecosystems are collocated; in other words, the treatment replicates were not spatially dispersed. This experimental design has an underlying assumption that the soil properties of the three ecosystems (native rangeland, silvopasture, and sown pasture) were similar prior to the conversion and designation of each ecological unit (Hurlbert, 1984; Arevalo et al., 2009). Factors such as the uniformity of initial land-use, the flat terrain, and the limited potential for variation in the sandy-textural class of the soil which were all formed from the same parent material (Kalmbacher et al., 1984; Kalmbacher et al., 1993) lends credence to the assumption that the soils were similar before the management changes were imposed.
Below-ground C sampling and analysis: Soil sampling was conducted in June/July 2012 which corresponds to the period of maximum ecosystem productivity and biochemical cycling rates (decomposition/mineralization). An initial offset (30 m) was established from the edge of each field in order to avoid potential edge effects, and five quadrats (20 m × 20 m), spaced ~75 m apart, were marked out along a diagonal transect within the inner boundary. Four randomly located soil cores (diameter of 2.2 cm) were sampled from each quadrat at soil depths of 0-10, 10-20, and 20-30 cm and composited within a depth for C and N analysis, while one random soil core was sampled in each quadrat for bulk density determination.
Soil samples were air-dried and sieved (2-mm sized sieve), and the modified version of the procedure described by Cambardella and Elliott (1992) was adopted to separate SOC and N into two particle-size fractions: particulate organic C (POC) which corresponded to the > 53µm-sized particles, and mineral-associated C (Cmin) which corresponded to the < 53µm-sized particles. Carbon and N concentrations were determined by dry combustion using a ThermoFlash EA 1112 elemental analyzer. The natural abundance isotope ratios (δ13C) of the POC and Cmin fractions were determined on a Thermo-Finnigan MAT DeltaPlus XL Isotope Ratio Mass Spectrometer (IRMS) interfaced via a Conflo-III device to a Costech ECS 4010 elemental analyzer (Costech, Valencia, CA). The results are presented relative to δ13CPDB standard which expresses whether a sample has a higher or lower 13C /12C isotopic ratio compared to the calcium carbonate standard known as Vienna Pee Dee Belemnite (V-PDB) (Coplen, 1994, Equation 1; Boutton et al., 1998). Carbon derived from initial native rangeland vegetation (C3-derived C) and from newer sown bahiagrass (C4-derived C) were calculated based on mass balance equations described by Follett et al. (2009). The soil samples collected at each soil depth interval were dried at 105°C until constant weight and bulk density was computed by dividing the dry weight by the soil core volume. Soil C and N stocks in each ecological unit were calculated based on the C and N concentration and the bulk density at each depth.
Below-ground root biomass samples were collected using the AMS hydraulic powerprobe (Arts Manufacturing and Supply Inc., American Falls, ID) equipped with soil coring drills (diameter of 5 cm). Three soil cores were randomly sampled in each quadrat at the 0-10, 10-20, and 20-30 cm depths. Samples were air-dried and subsequently dispersed in water to separate roots from soil particles through sieving (sieve size = 250µm). Root samples were dried to constant weight at 65°C, and final weight was recorded after 3 days. The dried root samples were combusted at 550°C for 5 hours to determine ash concentration. Carbon and N concentrations were determined by dry combustion using a ThermoFlash EA 1112 elemental analyzer. Root biomass C presented was expressed on ash-free basis.
Above-ground biomass sampling: The above-ground biomass measurement was conducted in mid-summer, in order to estimate the biomass during the period of peak annual primary production. Sampling was strategically conducted to capture the peak production under each management system such that: 1) sown pasture was sampled 5 days after the cows had been moved out of the pastures, i.e. two days before the next herd moved in; 2) silvopasture was sampled ~ 4 weeks after the cattle had been moved out, and 3) native rangeland was sampled after ~3 months of not being grazed by cattle. Different sampling strategies were employed across the ecological units because of the different vegetation composition.
Soil respiration: A modified box exclusion method (Hanson et al., 2000; Luo and Zhou, 2006) was used to partition heterotrophic and autotrophic respiration under each grassland ecosystem. In each sampling location, open-ended fabricated aluminum boxes (30 x 30cm) were installed by carefully digging trenches (with minimal disturbance to the formed soil column) to sever the roots up to 30cm soil depth. The boxes totally excluded roots from growing laterally into the soil column within this soil depth range where over 85% of roots are concentrated. In June 2012, the vegetation within the boxes was clipped to ground level in order to truncate photosynthetic production and avoid allocation of carbohydrate to the roots from aboveground production, thereby leading to eventual death and decay of existing roots. Measurement of respiration was delayed for 6 months to ensure the re-stabilization of soil conditions within the exclusion box, and reduce disturbance effect accruing to the installation.
The weekly in-situ measurements of soil respiration (RS and RH) were conducted at two random locations (~10m apart) within each field. Soil CO2 measurements were conducted weekly (between 8 and 11 am) during the winter (Jan 1 to March 31) and summer (May 15 – August 15) season in 2013. RS and RH were measured in-situ with environmental gas monitor (EGM-2) portable infra-red gas analyzer (PP Systems, Amesbury, MA), equipped with a soil respiration chamber (SRC-1). The EGM-2 was recalibrated by the manufacturer to match the operational efficiency and accuracy of EGM-4. Before each weekly measurement event, two (2) polyvinyl chloride (PVC) collars (height = 5cm) were inserted on the soil surface (up to 2.5cm) at each sampling location (one within and one outside the exclusion boxes) to ensure a snug fit of the SRC-1 chamber during measurement, and avoid errors related to potential leakage of respired C. The first PVC (PVC-M) was placed within the exclusion box and designated for measurement of RH. Given that the vegetation is non-existent and the roots are dead, the CO2 efflux from this collar is attributable to microbial organisms that are metabolizing the available and accessible soil C within the excluded soil column. The second PVC (PVC-RM) was placed ~0.4m away from each exclusion box to capture the total soil respiration (RS). The vegetation within the PVC-RM collar was consistently clipped to ground level before each measurement to avoid any influence of shoot respiration. Autotrophic respiration was calculated as the difference between soil respiration and heterotrophic respiration. At each instance of soil respiration measurement, STemp (°C) and SMoist (% volumetric water content) at surface 10cm soil depth were also recorded, using HI 98331 temperature probe (Hanna Instruments, Carrolton, TX) and VG-METER-200 soil moisture meter (Vegetronix Inc, Riverton, UT), respectively.
The assessment of C stocks indicated that management intensification has affected the quantity and dynamics of both ecosystem and soil C stocks. It was evident that ecosystem and soil C sequestered under native rangeland (baseline management condition) was lower in comparison to silvopastures and sown pastures, which is characterized by higher intensity of management practices such as fertilizer application, stocking rate, grazing frequency, and sowing of more productive grass species (bahiagrass). For instance, soil C was 41 Mg C ha-1 in native rangeland, 69 Mg C ha-1 in silvopasture, and 62 Mg C ha-1 in sown pasture. Similar trend was observed for ecosystem C, however, the silvopastoral ecosystem sequestered far higher amounts of C in the above-ground biomass (59 Mg C ha-1), compared to native rangeland and sown pasture (4.2 Mg C ha-1 and 2.1 Mg C ha-1, respectively). Furthermore, results indicated that although silvopasture and sown pasture ecosystem contained comparable amount of soil C, there were major contrasts in the allocation of C into different particle size fractions. The relatively stable mineral-associated C (Cmin) fraction constitutes about ~60% (42 Mg C ha-1) of soil C in silvopasture, while it constitutes about ~45% (28 Mg C ha-1) in sown pasture (within 0-30cm soil depth), therefore suggesting that silvopastures is more beneficial for accretion of stable soil C fraction, compared to sown pastures. This trend was reversed for the labile particulate organic C (POC) fraction, which constitutes notably high proportion of C sequestered in the sown pasture ecosystem.
The δ13 isotopic analysis of soil fractions showed that both POC and Cmin became less-depleted in 13C (less negative) as management intensity increased, but most striking changes were observed in POC fraction, with δ13C values ranging from -23.7‰ in native rangelands to -17.5‰ in sown pasture. This is consistent with the potential for increased contribution of the introduced bahiagrass component to soil C accretion under this subtropical condition. Although the percent contribution of C3-derived C generally decreased as management intensity increased, it was quite striking to note that under the sown pasture, C4-derived C accounted for 76% of the stable Cmin fraction. This contradicts the hypothesis that introduction of more productive grass species results in depletion of the stable C fraction. Rather, it is likely that the sown C4 grass promotes occlusion (and stabilization) of soil C and formation of soil microaggregates as the grass-derived litter becomes readily decomposable under the warm-wet climatic condition.
During the winter and summer, management intensification (especially to sown pasture) increased the magnitude of RS and its components. For instance, soil C loss through RH increased under the sown pasture by ~19% during winter and ~35% during summer in comparison to the native rangeland ecosystems. Furthermore, higher magnitude of increase (~100%) was observed in RS due to the elevated contribution of roots under the sown pasture, compared to native rangelands. The effect of management intensification on relationship between RS and abiotic factors were not straightforward. The variability of RH (and RS) explained by abiotic factors (including interaction) declined in sown pastures during both seasons, but the response was seasonally-contrasting in silvopastures. The temperature sensitivity quotient (Q10) also declined from 1.69 in native rangeland to ~1.58 in both silvopasture and sown pasture, during the winter. However, during the summer, Q10 of RH and RS increased with management intensity from 1.09 in native rangelands to 2.29 in sown pasture. Hence, turnover of soil C to the atmosphere through respiration is likely to be accelerated with warming temperature under more intensively managed grassland ecosystems.
Educational & Outreach Activities
- Adewopo J, Silveira ML, Gerber S, Martin T, Sollenberger L. 2014. Effect management intensification on autotrophic and heterotrophic soil respiration in subtropical grassland ecosystems. Ecological Indicators (In preparation).
- Adewopo J, Silveira ML, Gerber S, Martin T, Sollenberger L. 2014. Impact of management intensification on particle-size soil carbon fractions in subtropical grasslands: Evidence from 13C natural abundance. Agriculture, Ecosystems and Environmental (In preparation).
- Adewopo J, Silveira ML, Gerber S, Martin T, Sollenberger L, Xu S. 2014. Impact of land-use conversion on ecosystem carbon stocks in subtropical grasslands. . Soil Sci. Soc. of Am. J. doi:10.2136/sssaj2013.12.0523.
- Adewopo J. 2014. Assessment long-term management intensification impacts on soil carbon dynamics in subtropical grasslands. Successfully Defended Dissertation Submitted to the University of Florida, Gainesville, FL. 139 p.
Posters and Oral Presentation
- Adewopo B.J., M.L. Silveira , S. Gerber, S. Xu. (Oral): Impacts of management intensification on soil carbon stocks in subtropical grasslands. ASA-SSSA-CSSA International conference, Tampa Convention Center, FL. Nov. 2-6th, 2013.
- Adewopo B.J. (Oral – Invited): Impacts of management intensification on soil carbon stocks in subtropical grasslands. Soil and Water Science Annual Research Forum, J.Wayne Rietz Union, University of Florida, Gainesville, FL. Sept. 9th, 2013.
- Adewopo B.J., M.L. Silveira , S. Gerber, S. Xu (Oral): Long-term soil carbon sequestration under subtopical management systems. International Union of Soil Scientists (IUSS) Global Soil Carbon Conference, Madison, WI. U.S.A. June 2-6, 2013.
- Adewopo B.J., M.L. Silveira , S. Gerber. (Poster): Assessment of long-term soil carbon sequestration under three subtropical grassland systems. ETH-Zurich Critical Zone Intersoil Conference – Integrating life across disciplines, Ascona Monte-Verita, Switzerland. April 14th– 16th, 2013.
- Adewopo B.J., M.L. Silveira , S. Gerber. (Poster): Assessment of long-term soil carbon sequestration under three subtropical grassland systems. UF Climate Institute Conference, Sustaining Economies and Natural Resources in a Changing World: Key Role of Land Grant Universities. Gainesville, Fl. April 2nd-3rd, 2013.
- Adewopo B.J., M.L. Silveira , S. Gerber, S. Xu (Oral): Long-term soil carbon sequestration under three subtropical grassland management systems. Southern Association of Agricultural Scientists (ASA-SSSA-CSSA Section). Wyndham Hotel, Orlando Fl. Feb 2nd – 6th, 2013.
This field-scale research under a unique setting grassland management intensity gradient, without the influence of potential confounding factors (such as elevation, climate, and soil type) offered an invaluable means to assess the long-term impact of management intensification on soil C dynamics within a subtropical ecoregion. The analysis of soil C stocks suggests that management intensification is beneficial for improved C sequestration in this biome, while isotopic analysis of soil C sources reveals that introduced sown grass species does not only contribute to the increase in the labile C pool, but also contribute towards increasing the relatively stable C pool. Data also suggested that there are major differences in the stability and allocation of soil C at different depth intervals among the grassland management systems. Further evidence of changes in soil C dynamics with management intensification was revealed by changes in magnitude of soil C loss (through heterotrophic respiration), which increased in sown pasture, possibly due to increase in productivity and accretion of labile soil C fraction. Also, the changes in sensitivity of respiration variables to abiotic control factors constitutes an important implication for assessing the impacts of current and future global warming trends. For instance, the increase of Q10 values with management intensification during the summer suggests the potential for a faster turnover and release soil C into atmospheric C pool (in form of CO2) with increase in global temperatures, especially during the summer (Luo et al., 2001; Smith and Johnson, 2004; Luo and Zhou, 2006; IPCC, 2007; Bloom, 2010)
These findings are not only important for improved national C accounting and accurate global C assessment, but they are expected to inform management decisions that would support sustainable grassland and livestock production within this (and similar) subtropical ecoregion. The presentation of this research at international conferences attracted the attention and commendation of researchers, and the student investigator (Julius Adewopo) received top-three oral student presentation awards at both regional and international conference of ASA-CSSA-SSSA (held at Orlando and Tampa, respectively).
Areas needing additional study
The vegetation composition of the studied ecosystems presents important opportunities and challenges. The C3 and C4 plant species composition of the native rangeland limited the possibility of fully elucidating the changes in C with management intensification, especially under the C3-C4 composed silvopasture. More powerful techniques such as radiocarbon dating (14C) may be very relevant for determining relic and recent composition of soil C based on age rather than photosynthetic pathways. Furthermore, although grass component significantly influences soil C fractions in silvopastures (Archer et al., 2001; Hibbard et al., 2001), information on the biochemistry of tree-based silvopastoral ecosystems is still insufficient to explain the interactions and roles of grass in co-regulating soil C dynamics with the tree component. Additional research into NPP, transformation, and turnover of tree and grass derived input in silvopastures will be relevant to fully quantify the contributions of each vegetation component.
We quantified root biomass C under each management system but the results are likely influenced by the limitation associated with sampling large-diameter tree roots in silvopastures. Additional consideration of potential differences in root structural and chemical composition as they influence root C turnover may foster better understanding of potential changes in root C dynamics and how this may in turn modify long-term soil C responses to management intensification under subtropical conditions. Given that CO2 efflux may also be influenced by the morphology of dominant roots which modulates the rate of intercellular diffusion and exchange of gases within the root zone (Hendricks et al., 1993; Graham et al., 2012), further exploration of spatial variability and relationship of root properties with soil respiration (and its components) would be important for scaling up the impacts of management intensification on soil C.