Integrated Crop and Livestock Systems for Enhanced Soil Carbon Sequestration and Microbial Diversity in the Semiarid Texas High Plains

Integrated Crop and Livestock Systems for Enhanced Soil Carbon Sequestration and Microbial Diversity in the Semiarid Texas High Plains

Summary

Five agroecosystems (three integrated crop-livestock and two continuous cotton systems) were investigated to measure soil C sequestration potential, greenhouse gas emissions and bacterial diversity. The greatest total organic matter content was measured in the Old World Bluestem (OWB)-Row Crop system compared to the other four systems. Paddocks containing Bermuda grass (component of the OWB-BER system) typically had the greatest CO2 and N2O fluxes compared to all other systems but these differences were masked on a whole-systems-level basis. C storage within water-stable aggregates as well as bacterial community structure within these aggregates was influenced by system and microenvironment (SOM fraction).

• Edge of millet paddock with cattle grazing the Buffalograss/Blue grama paddock in background
• Cattle in Bermudagrass paddock with ungrazed exclosure (cage) showing.
• Table describing the five agroecosystems studied.

Objectives/Performance Targets

1. Measure the amount of C stored in active, intermediate and passive SOM pools using a detailed physical fractionation method, which provides eight different soil aggregate fractions.

2. Evaluate greenhouse gas fluxes (CO2, CH4, and N2O) to calculate global warming potential within each system.

3. Characterize soil microbial community structure using fatty acid methyl ester (FAME) profiling and microbial biomass C (MBC) including bacterial diversity in whole soil and different soil aggregate fractions using a novel molecular biological tool (i.e., pyrosequencing).

4. Translate results from Objectives 1 through 3 into practices incorporated in agriculture in the THP and similar ecosystems. Specifically, we will increase producer and consultant awareness regarding the direct and indirect positive effects for managing agricultural lands to achieve enhanced soil functioning.

Our long-term objective is to use the data acquired under the proposed research to guide policy-makers and agricultural and land managers to make the best ecological and socio-economical-supported decisions possible. In doing so, they will meet the increased agricultural demands for food and fiber while sustaining natural resources to preserve and enhance local communities. Information we acquire will supplement past and concurrent data, including an additional Ph.D. research project (in progress) that is evaluating energy use, efficiency and economics in the proposed sites. These established long-term replicated integrated research sites as well as access to grower-managed farms with known histories and similar systems, provides an unparalleled opportunity enabling us to achieve such an important yet complex goal. Subsequent funding will enable us to provide detailed assessment of C sequestration potential and soil microbial biodiversity estimates in these semiarid agroecosystems; systems which may serve as models for sustainable agricultural production across this agriculturally, socially, and economically important eco-region.

Accomplishments/Milestones

• Soil samples from year 1 were collected in July 2010. Samples were collected in grazed and ungrazed areas when applicable. Analyses completed include:

  Bulk density completed on 4/5 systems (last one pending rain to soften ground)

  Whole soils: complete chemical analysis, microbial biomass C, fatty acid methyl ester profiling of microbial community, and complete fractionation of SOME samples to determine eight different pools of soil C.

  Modified fractionation for DNA extraction and pyrosequencing of bacterial community.

  Objective #1 PENDING analyses: complete fractionation of samples to determine eight different pools of soil C (approximately 60% complete)

  Gas collars were installed in Fall 2009 for soil respiration (CO2, N2O, and CH4 analyses):

  Gas samples conducted weekly throughout active growing season and monthly thereafter.

  CO2, N2O initial analyses are complete. CH4 analysis is proving challenging due to instrumentation limitations. We are currently working on modeling CH4 from these sites and converting all gas data to global warming potential units on a systems level basis. Currently, CO2 and N2O analyses are complete on a paddock level basis.

Gas flux measurement is an continuing process with sampling frequency decreased over the less productive season. Flux measurement frequency of measurement after harvest and the removal of steers was decreased to every 2 weeks for CO2 while N2O and CH4 flux measurement will be done once every 4 weeks with additional measurements taken following precipitation events.

In the fall of 2009 PVC collars were installed in each replicate of the dryland native range, cotton, and millet as well as the irrigated W.W. B. Dahl Old World Bluestem and Bermuda grass pasture. These collars are used in the analysis of soil respiration and green house gases. Soil CO2 flux is determined over a two minute period using a LI-8100 gas analyzer. Carbon dioxide flux measurement is repeated weekly throughout the growing season. A long-term flux analysis system was also installed into one of the millet replicates in June 2010 before being transferred to the adjoining cotton replicate in July 2010. They system was then transferred to the Native Range pasture prior to the cotton harvest. The long-term system has minimal impact within the paddock consisting of two solar panels, batteries, a LI-8400 multiplexer and 4 long-term chambers. Carbon dioxide flux is measured in each of the four collars every two hours which enables us to examine the effects of daily temperature changes and changes in soil moisture due to precipitation events. Nitrous oxide and methane flux is measured weekly throughout the growing season with additional sampling occurring following precipitation events when possible. As N2O and CH4 flux cannot be determine using the LI-8100 system additional field days and student workers are required to collect this data. Gas samples are collected using a closed cap system (constructed from a PVC cap made to fit the PVC collars) every 15 minutes over a 45 minute period using polyvinyl syringes and nylon stopcocks. Under the current sampling strategy a minimum of three individuals are required to collect samples over a 5 hour period. Samples are transported to the lab and analyzed using a Shimadzu GC-2014 gas chromatograph. Sampling is attempted on the same day each week; however occasional site management practices require the sampling schedule be adjusted.

To date, analysis of CO2 flux has shown that at the first sampling in July 2010, fluxes ranged from 0.012 mg m-2 hr-1 in the bermuda grass pastures to 0.0019 mg m-2 hr-1 in the dryland cotton. Throughout the season the bermuda grass produced the highest flux rates with only one exception occurring during a precipitation event. Moving into the winter flux rates in all pastures decreased to near zero and have remained consistent throughout the winter months.

Nitrous oxide fluxes followed a similar trend with bermuda grass producing the greatest flux rates throughout the growing season. By late August 2010 had decreased to near zero levels and little variation between pastures has been measured. Organic matter within the five systems ranged 1.5 %LOI in the TAWC continuous cotton system to 2.6 %LOI in the TAWC integrated crop-livestock system. Percent total carbon of the whole soils produced similar results with the continuous cotton system having the lowest percent total carbon compared to the integrated crop-livestock systems.

3. Characterize soil microbial community structure using fatty acid methyl ester (FAME) profiling and microbial biomass C (MBC) including bacterial diversity in whole soil and different soil aggregate fractions using a novel molecular biological tool (i.e., pyrosequencing).

This component of our study used the SARE-2 systems and two TAWC sites to evaluate bacterial diversity of different aggregate size classes of soil samples using bacterial tag?encoded FLX amplicon pyrosequencing of the 16S rDNA gene. Soil samples collected from the 0?5cm depth and were separated into macroaggregates (>250 um), microaggregates (53?250 um) and silt+clay (< 53 um). Distance?based redundancy analysis (db?RDA) using relative abundances of the predominant 29 phyla/ subphyla and also using 1000+ species was used to test for differences in overall bacterial community composition.

Based on phyla level data, the soil bacterial community composition was significantly impacted by the system treatment and by location within the soil microenvironment (i.e., soil fraction). No significant interaction between the different treatments was observed, signifying the uniqueness of each of the SARE-2 systems with respect to microbial diversity. For both phyla and species level information, differences between aggregate fractions were significant, indicating that soil aggregates represent a distinctive microenvironment capable of selecting for specific microbial lineages. Non-Irrigated system was distinguished by higher abundance of Rubrobacteriales and Tenericutes phyla, while Irrigated system was characterized by an array of different phyla, including Bacteroidetes, Verrucomicrobia, TM7, as well as Beta- and Gamma-proteobacteria. Actinobacteria, had superior occurrence in microaggregate fractions and was equally presented in both systems. To our knowledge, this objective of the SARE-2 study is the first study that investigates bacterial community composition within the soil microenvironment using pyrosequencing technology, which revealed differences in bacterial diversity across aggregates that may affect C sequestration.

• Biplot of the results of db-RDA which modeled the assemblage of bacterial community as a function of soil fraction. Model accounted for 75% of the variability in inter-sample distances (Unifrac).  Individual points represent samples, and the ellipses represent the bivariate 95% confidence interval of mean scores based on group affiliation.  Arrows reflect the correlation of individual sub-phyla with ordination axes.
• Collecting soil gas samples in Bermuda grass paddock
• Total soil carbon content for each system at 0-5 and 5-20cm depths.
• Automatic soil CO2 chambers installed in Buffalograss/ Blue grama paddock

Impacts and Contributions/Outcomes

We expect this research to benefit producers in the Southern Region by providing a detailed and accurate C budget which can be fed into more general models to use in calculating (for example) C credits, C sequestration potential based on management choices, and conservation of microbial biomass. Specifically, we will acquire actual greenhouse gas flux rates over a 3-year period. Our results will provide researchers and land managers with information that can lead to modified management decisions to target C sequestration potential and microbial diversity in addition to on-going SARE-funded research targeting the economic and water budget decision tools in similar study sites.

In the first year of this project, we were able to acquire a large database of information on the status, content, and composition of soil organic matter in soils under different crop and integrated crop and livestock agroecosystems. In October, 2010, Dr. Jennifer Moore-Kucera was awarded a travel grant to attend and participate in the 2nd annual Argonne National Labs Soils Workshop. Dr. Moore-Kucera presented preliminary data on our pyrosequencing results within soil aggregrate fractions. This poster presentation was well received.

Results of this project are also incorporated into undergraduate and graduate level course-work taught by Dr. Moore-Kucera. In this effort, agricultural and natural resource students are made aware of research conducted specifically on integrated crop and livestock systems and the important contribution agricultural practices can have on C storage and microbial functioning.

Collaborators: