Perennial Grass Covers Affect Long-Term Soil Quality

Perennial Grass Covers Affect Long-Term Soil Quality

Summary

We are undertaking research comparing soil organic matter and carbon sequestration potentials in riparian grass filters dominated by native, warm-season (C4) prairie grasses to those dominated by non-native, cool-season (C3) forage grasses. We are further investigating soil organisms present in the two grassland types, to understand how grass type influences carbon and nutrient cycling in the soil. Data on soil respiration rates is being used to identify seasonal and annual differences in total soil biological activity. Extensive extension activities organized by our research team emphasize distributing information on opportunities and best management practices for riparian zones in agricultural landscapes..

Objectives/Performance Targets

We identified four principal objectives for our research:
Objective 1: To determine if cool-season and warm-season grasses differ in their soil organic carbon (SOC) sequestration potentials.
Objective 2: To compare the soil food webs present beneath cool-season and warm-season grasses.
Objective 3: To compare overall soil quality, as quantified by total soil respiration rate, in cool-season and warm-season grasses.
Objective 4: To develop best management practices for perennial cropping systems in reserved lands, filter strips, and riparian buffers.

Objective 1 was addressed by sampling and analyzing soils from a sequence of planted, warm-season (C4) grass filters that ranged in age from 0-10 years. This will allow the comparison of soil carbon pools in sites varying in age, and will provide preliminary estimates of soil C sequestration rates in planted C4-grass filters. We also sampled C3-dominated riparian filters to compare carbon pools in C3 and C4 grass filters. Our data will also provide baseline information on soil C contents in a number of specific sites that can be resampled in 5-10 years, to quantify long-term trends.
We have undertaken all proposed soil sampling required to complete this project. We also georeferenced each individual sample location, so that the same locations can be resampled in future years. We also overlaid each sample point on a georeferenced soil map to be certain that all sample points were located within the same map unit (soil series). This necessitated a good deal of additional sampling because, in fact, some of our samples were collected from different soil map units. Additionally, careful evaluation of past aerial photographs indicated that some of our C4-grass filter plots were C3-dominated grassland soils prior to be planted to C4 species. We have therefore restricted our analysis to grass filters planted onto former row-crop lands. Table 1, below, summarizes the samples collected to date.
Although not included in our proposal, we also sampled each of the cropfields adjacent to our C4 grass filter samples. Specifically, we collected a single cropfield soil core paired to each C4 core. This paired sampling may allow a more robust analysis of soil C sequestration than does the chronosequence approach we proposed.
We sampled soil at times that allowed the collection of intact volumetric cores, and were typically able to sample >1-m deep. Each core is processed in the laboratory according to a well-defined standard operating procedure. We carefully dissect the cores into 9 depth increments (0-15, 15-25, 25-35, 35-50, 50-60, 60-75, 75-85, 85-100, and >100 cm). Gravel and roots are collected from each sample, gravel volume per sample is measured, and soils are dried and sieved in preparation for elemental analysis. Soil bulk density is determined from the volume of the sample less the gravel volume, and total soil mass after drying at 105°C. Subsamples (oven-dried at 65°C) are finely ground prior to analysis of total C, organic C, and total N determinations. It is these latter (post-collection core processing) steps that remain to be completed for fulfillment of Objective 1.

Table 1. Summary of intact soil cores collected to complete Objective 1. “Year planted” refers to C4 grass filters only; all C3 sites are of unknown age (but probably >25 years). All cores are from the same soil type, and all C4 cores are from formerly row-cropped lands planted to grass filters.

Farm Year Planted # of C4 Plots Total # C4 Cores # of C3 Plots Total # of C3 Cores
I. Larson 2001 3 12 0 0
J. Risdal 1999 3 12 0 0
L. Tesdal 1997 3 12 2 8
L. Strum 1994 3 12 2 8
R. Risdal 1990 2 8 6 24

All Farms 14 56 10 40

Objective 2 requires seasonal sampling of the soil food webs beneath C3 and C4 grass filters. The structure and activity of the soil microbial community can impact the flow of nutrients and energy in belowground ecosystems. Temporal patterns in the flow of nutrients can impact nutrient-use efficiency which in turn, effect plant productivity and nutrient loss in plant-soil systems. We devised a sampling protocol to investigate soil microbial community structure, activity and diversity in cool- and warm-season grass plots.
Surface soils (0-15 cm) were collected in 2001 on May 29, July 24 and October 22 and shipped cold overnight to Soil Food Web Inc. (http://www.soilfoodweb.com/index.html) for analysis of total and active bacterial biomass, total and active fungal biomass, nematode community structure, and protozoan population densities and community structure.
Results indicate that bacterial and fungal biomass and activity change across the growing season in these grass-dominated ecosystems. Active fungal biomass was low in the late spring and mid-summer but active bacterial biomass was low only in late spring. Total bacterial and fungal biomass were generally low in the fall but adequate in the late spring and mid-summer. Nematode numbers were consistently low and nematode community structure was dominated by root-feeders for all three sample dates. Total protozoan numbers were relatively low in late spring and mid-summer but only flagellates were low in the fall. To date, seasonal differences within sites exceed differences observed between C3 and C4 plots. Continued sampling through Year 2 (5 more total sampling periods proposed) will enable us to complete our objectives.
Objective 3 was to compare rates of soil biological activity, as measured by in situ soil respiration, in C3 and C4 riparian grass filters. We purchased an automated field soil respiration system (LiCor 6400) and spent some time developing standard sampling protocols. In May of 2001 we initiated regular measurements of soil respiration in the same sites used for soil sampling (Table 1), with one exception (the Tesdal farm). At least monthly, we measure soil respiration rates in at least three plots of C3 grasses and three plots each of C4 grasses planted in years 1990, 1994, 1999, and 2001. The data to date suggest very real differences in the seasonality of soil biological activity in C3 and C4 grasslands, which we hope to verify during our continued sampling in Year 2.
A second component of Objective 3 was to compare soil respiration rates as measured with soda lime and LiCor techniques. We have undertaken 5 such comparisons to date, and they suggest that the soda lime technique may underestimate soil respiration rates by 50% during summer months. We need to continue these comparisons into colder months, to determine if soda lime overestimates soil respiration rates when soil temperatures are low, as has been previously hypothesized.
The graduate student supported on this project, Mathew Dornbush, has also measured aboveground standing live and dead vegetation each month in each grass plot in which we measure soil respiration, and has collected root samples for determination of end-of-season belowground biomass. With soil, vegetation biomass, and soil respiration data from a series of sites, we will have an excellent opportunity to develop soil carbon budgets, which will help us to evaluate mechanisms controlling rates of soil C sequestration in our various grasslands.
Objective 4 focuses on extension activities. The following table summarizes technology transfer activities undertaken by our team in partial fulfillment of Objective 4. Each of these activities involves many people; Farm field days, for instance, typically attract >100 individuals, most of whom come from surrounding communities.

Table 2. Technology Transfer Activities for Grass Filters and Riparian Buffers, SARE, September 15, 2000 – Sept 15, 2001. All activities listed with the exception of “Planning Meetings” refer to public activities involving land owners, students, community groups, etc. “Planning Meetings” involve extension and USDA personnel, in preparation for public events and planning for extension publications.

Activity Number

Invited Speaking Presentations 29
Tours of Bear Creek Watershed 17
Workshops (1 day or more each) 6
Field Days/Site Visits 11
Planning Meetings 14

In addition we have revised one extension bulletin (Maintenance of Riparian Buffers, Iowa State Extension Bulletin #PM1626C), which currently is in press at the printers. We also have improved and expanded our web site (http://www.buffer.forestry.iastate.edu/), which is devoted to riparian buffers and grass filters. This has turned out to be a very effective way to share information with the public (see, for example, our FAQ section).

Accomplishments/Milestones

Objective 1 was addressed by sampling and analyzing soils from a sequence of planted, warm-season (C4) grass filters that ranged in age from 0-10 years. This will allow the comparison of soil carbon pools in sites varying in age, and will provide preliminary estimates of soil C sequestration rates in planted C4-grass filters. We also sampled C3-dominated riparian filters to compare carbon pools in C3 and C4 grass filters. Our data will also provide baseline information on soil C contents in a number of specific sites that can be resampled in 5-10 years, to quantify long-term trends.
We have undertaken all proposed soil sampling required to complete this project. We also georeferenced each individual sample location, so that the same locations can be resampled in future years. We also overlaid each sample point on a georeferenced soil map to be certain that all sample points were located within the same map unit (soil series). This necessitated a good deal of additional sampling because, in fact, some of our samples were collected from different soil map units. Additionally, careful evaluation of past aerial photographs indicated that some of our C4-grass filter plots were C3-dominated grassland soils prior to be planted to C4 species. We have therefore restricted our analysis to grass filters planted onto former row-crop lands. Table 1, below, summarizes the samples collected to date.
Although not included in our proposal, we also sampled each of the cropfields adjacent to our C4 grass filter samples. Specifically, we collected a single cropfield soil core paired to each C4 core. This paired sampling may allow a more robust analysis of soil C sequestration than does the chronosequence approach we proposed.
We sampled soil at times that allowed the collection of intact volumetric cores, and were typically able to sample >1-m deep. Each core is processed in the laboratory according to a well-defined standard operating procedure. We carefully dissect the cores into 9 depth increments (0-15, 15-25, 25-35, 35-50, 50-60, 60-75, 75-85, 85-100, and >100 cm). Gravel and roots are collected from each sample, gravel volume per sample is measured, and soils are dried and sieved in preparation for elemental analysis. Soil bulk density is determined from the volume of the sample less the gravel volume, and total soil mass after drying at 105°C. Subsamples (oven-dried at 65°C) are finely ground prior to analysis of total C, organic C, and total N determinations. It is these latter (post-collection core processing) steps that remain to be completed for fulfillment of Objective 1.

Table 1. Summary of intact soil cores collected to complete Objective 1. “Year planted” refers to C4 grass filters only; all C3 sites are of unknown age (but probably >25 years). All cores are from the same soil type, and all C4 cores are from formerly row-cropped lands planted to grass filters.

Farm Year Planted # of C4 Plots Total # C4 Cores # of C3 Plots Total # C3 Cores
I. Larson 2001 3 12 0 0
J. Risdal 1999 3 12 0 0
L. Tesdal 1997 3 12 2 8
L. Strum 1994 3 12 2 8
R. Risdal 1990 2 8 6 24

All Farms 14 56 10 40

Objective 2 requires seasonal sampling of the soil food webs beneath C3 and C4 grass filters. The structure and activity of the soil microbial community can impact the flow of nutrients and energy in belowground ecosystems. Temporal patterns in the flow of nutrients can impact nutrient-use efficiency which in turn, effect plant productivity and nutrient loss in plant-soil systems. We devised a sampling protocol to investigate soil microbial community structure, activity and diversity in cool- and warm-season grass plots.
Surface soils (0-15 cm) were collected in 2001 on May 29, July 24 and October 22 and shipped cold overnight to Soil Food Web Inc. (http://www.soilfoodweb.com/index.html) for analysis of total and active bacterial biomass, total and active fungal biomass, nematode community structure, and protozoan population densities and community structure.
Results indicate that bacterial and fungal biomass and activity change across the growing season in these grass-dominated ecosystems. Active fungal biomass was low in the late spring and mid-summer but active bacterial biomass was low only in late spring. Total bacterial and fungal biomass were generally low in the fall but adequate in the late spring and mid-summer. Nematode numbers were consistently low and nematode community structure was dominated by root-feeders for all three sample dates. Total protozoan numbers were relatively low in late spring and mid-summer but only flagellates were low in the fall. To date, seasonal differences within sites exceed differences observed between C3 and C4 plots. Continued sampling through Year 2 (5 more total sampling periods proposed) will enable us to complete our objectives.
Objective 3 was to compare rates of soil biological activity, as measured by in situ soil respiration, in C3 and C4 riparian grass filters. We purchased an automated field soil respiration system (LiCor 6400) and spent some time developing standard sampling protocols. In May of 2001 we initiated regular measurements of soil respiration in the same sites used for soil sampling (Table 1), with one exception (the Tesdal farm). At least monthly, we measure soil respiration rates in at least three plots of C3 grasses and three plots each of C4 grasses planted in years 1990, 1994, 1999, and 2001. The data to date suggest very real differences in the seasonality of soil biological activity in C3 and C4 grasslands, which we hope to verify during our continued sampling in Year 2.
A second component of Objective 3 was to compare soil respiration rates as measured with soda lime and LiCor techniques. We have undertaken 5 such comparisons to date, and they suggest that the soda lime technique may underestimate soil respiration rates by 50% during summer months. We need to continue these comparisons into colder months, to determine if soda lime overestimates soil respiration rates when soil temperatures are low, as has been previously hypothesized.
The graduate student supported on this project, Mathew Dornbush, has also measured aboveground standing live and dead vegetation each month in each grass plot in which we measure soil respiration, and has collected root samples for determination of end-of-season belowground biomass. With soil, vegetation biomass, and soil respiration data from a series of sites, we will have an excellent opportunity to develop soil carbon budgets, which will help us to evaluate mechanisms controlling rates of soil C sequestration in our various grasslands.
Objective 4 focuses on extension activities. The following table summarizes technology transfer activities undertaken by our team in partial fulfillment of Objective 4. Each of these activities involves many people; Farm field days, for instance, typically attract >100 individuals, most of whom come from surrounding communities.

Table 2. Technology Transfer Activities for Grass Filters and Riparian Buffers, SARE, September 15, 2000 – Sept 15, 2001. All activities listed with the exception of “Planning Meetings” refer to public activities involving land owners, students, community groups, etc. “Planning Meetings” involve extension and USDA personnel, in preparation for public events and planning for extension publications.

Activity Number

Invited Speaking Presentations 29
Tours of Bear Creek Watershed 17
Workshops (1 day or more each) 6
Field Days/Site Visits 11
Planning Meetings 14

In addition we have revised one extension bulletin (Maintenance of Riparian Buffers, Iowa State Extension Bulletin #PM1626C), which currently is in press at the printers. We also have improved and expanded our web site (http://www.buffer.forestry.iastate.edu/), which is devoted to riparian buffers and grass filters. This has turned out to be a very effective way to share information with the public (see, for example, our FAQ section).

Impacts and Contributions/Outcomes

– We have made presentations to over 1000 landowners, farmers, students, and other persons interested in enhancing soil and water quality through riparian vegetation management.
– We have fully revised an extension bulletin (Maintenance of Riparian Buffers, Iowa State Extension Bulletin #PM1626C) designed for landowners and farmers interested in planting and maintaining riparian buffers on their lands.
– We have expanded our web site (http://www.buffer.forestry.iastate.edu/) of information our Agroforestry Issue Team, the riparian buffer management system, and grass filters.
– We have continued to witness the restoration of native vegetation to streamsides throughout Iowa. In 2001, for the first time ever, planting of native (C4) grasses on CRP lands outstripped the planting of exotic C3 grasses.
– We supported a graduate student, Matt Dornbush, as he finished his M.S. degree, and he has elected to stay on to seek a Ph.D.
– We have trained and supported a host of undergraduate laboratory assistants and technicians.
– We have obtained, based on preliminary data from this study, additional research funds ($16,104) from the Iowa Center for Global and Environmental Research, to investigate “Grass-type controls over carbon fluxes from grasslands.”
– We have overseen the publication of two peer-reviewed journal articles in 2001, with a thrid article accepted for publication:
Tufekcioglu, A., J.W. Raich, T.M. Isenhart and R.C. Schultz. 2001. Soil respiration within riparian buffers and adjacent crop fields. Plant and Soil 229:117-124.
Bharati, L. K-H Lee, T.M. Isenhart, and R. C. Schultz. 2001 Riparian zone soil-water infiltration under crops, pasture and established buffers. Agroforestry Systems: in press.
Raich, J. W., C. S. Potter and D. Bhagawati. 2002. Interannual variability in global soil respiration, 1980-1994. Global Change Biology: accepted for publication pending minor revisions.
– We have participated actively in regional -and national-level professional meetings, including:
Two poster presentations at the Agriculture and the Environment Conference, March 5, 2001, Ames, IA.
Nine posters and one oral presentation at the Annual Meeting of the American Society of Agronomy/ Crop Science Society of America/Soil Science Society of America, Nov 5-9, 2000, Minneapolis, MN.

Collaborators: