Integrated Crop and Livestock Systems for Enhanced Soil Carbon Sequestration and Microbial Diversity in the Semiarid Texas High Plains
We determined the effects of integrated crop-livestock (ICL) systems on soil microbial community composition, soil organic matter pools, C sequestration and greenhouse gas emissions in the semiarid Southern High Plains (SHP). Evaluation of global warming potential (GWP) of a dryland ICL and a deficit-irrigated ICL revealed a near-neutral GWP for the dryland ICL, whereas the deficit-irrigated ICL was a net C-source. Compared to continuous cotton, ICLs stored more C in protected pools (up to 112%), increased aggregate stability (up to 6x greater mean weight diameter), had greater fungal diversity and overall nutrient cycling (as assessed by soil enzyme activity).
- 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.
Two Ph.D. students funded in part on this project successfully completed their degrees and graduated in Summer and Fall 2012. Titles of their dissertations were:
1. Dynamics of Soil Aggregation, Organic Carbon Pools, and Greenhouse Gases in Integrated Crop-Livestock Agroecosystems in the Texas High Plains by Lisa Fultz; graduated Summer 2012.
2. Soil Microbial Community Diversity and Functionality as Affected by Integrated Cropping-Livestock Systems in the Southern High Plains by Marko Davinic; graduated Fall 2012.
There were 2 significant findings related to objective #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.
First, we found significant differences in soil C dynamics within ICLs compared to continuous cotton systems. We measured soil C dynamics in a total of four ICL systems and three continuous cotton systems (Table 1). Soil C dynamics for the TTEF-SARE1 systems were analyzed separately from the other five systems because baseline archived soil samples existed which enabled us to compare data over time. Although this temporal evaluation was not originally proposed, we took advantage of access to archived samples to strengthen our study approach. Thus, both archived soil samples and soil samples collected in 2010 were fractionated into water-stable aggregate fractions and inter-aggregate fractions. This approach revealed a gain of 22% organic carbon content in the OWB-Rotation (an ICL) from 1997 to 2010 whereas soil organic C content in the continuous cotton system remained the same. The distribution of this C, however, varied across both agroecosystems. Specifically, large (>2 mm) water-stable macroaggregates were only isolated in the OWB component of the ICL suggesting that stable conditions existed long enough to support this aggregate fraction. This is of ecological significance because large macroaggregates play an important role in resisting the erosive forces of wind and water. Furthermore, the C protected inside of these macroaggregates (both large and small) is an indication of stable C sequestration. The largest size fraction of the aggregates protected within the macroaggregates is called the intra-aggregate particulate organic matter fraction or iPOM and is considered to be the most sensitive indicator of C sequestration. Evaluation of the 2010 sampling, representing 13 years of management, revealed this ICL system stored 112% more C in the iPOM fraction compared to the continuous cotton system.
Second, three additional ICLs and two continuous cotton systems also enhanced C sequestration potential and ecosystem stability.
At the systems level, soil organic C content was only higher in the Forage_Row crop (6478 kg ha-1) and OWB_BER (6438 kg ha-1) systems compared to one of the continuous cotton systems (Ctn3; 3026 kg ha-1) in samples collected from 0 to 5 cm. No differences in whole soil organic C were measured in samples collected from 5-20 cm. When soil samples were isolated into their aggregate fractions, the Forage_Row crop system resulted in 180% more macroaggregate soil organic C compared to the two continuous cotton systems (Ctn2 and Ctn3) and the Forage_Ctn system. In addition to the iPOM fraction isolated from the macroaggregates, intra-aggregate microaggregates can be an indication to the C sequestration potential of a particular system as this fraction is typically physically protected and contains primarily recalcitrant forms of soil organic C. Soil organic C of the three ICLs was greater (157% Forage_Row crop and OWB_Ber; 117% Forage_Ctn) than that measured in the two continuous cotton systems.
Evaluation of the individual vegetation components of each ICL or cotton system, showed that the bluestem components of the Forage_Row crop system had a greater content of soil organic C (average 9654 kg ha-1) when compared to the annual components (cotton-foxtail millet rotation and continuous cotton average 4069 kg ha-1). The pool of protected (recalcitrant) C was greatest in the bluestem components of the ICLs studied and in general this C pool was higher in perennial vegetative components than annual components. Overall, soil organic C content of the individual fractions was greatest under the two bluestem components of the Forage_Row crop ICL.
Based on data collected from the five agroecosystems (three ICLs and two continuous cotton), we report a ‘critical soil organic matter’ level above which soil stability (as inferred by measurement of mean weight diameter) was enhanced (Fig. 1). We found that when soil organic matter (0-5 cm) exceeds approximately 20 g kg-1 soil (2.0%), mean weight diameter increases. For our data, all perennial vegetative components within the ICLs exceeded this value, whereas the annual components except the corn component (i.e., cotton and foxtail millet) did not.
Soil CO2 and N2O fluxes from each representative vegetative component of the Forage_Ctn and OWB_BER ICLs were measured over a 15-month period (June 2010-September 2011). Severe drought in 2011-2012 forced these analyses to be discontinued due to lack of moisture for both plant and microbial activity. We coupled our data on CO2 and N2O fluxes and soil organic C content with energy data (equivalent CO2 released from fossil fuel consumption and steers) compiled by another Ph.D. student (Cody Zilverberg; graduated Spring 2012 under Dr. Vivien Allen) working in the same systems. With these data, we calculated that the Forage_Ctn ICL (all dryland production) had nearly a net zero global warming potential (2.5 kg CO2 equivalent ha-1 yr-1) whereas the OWB_BER ICL (deficit subsurface drip irrigated system) contributed approximately 3566 kg CO2 equivalent ha-1 yr-1 (Table 2). These results must be viewed with caution because of the limited soil gas flux data (only 1 year in 2010, which was an unseasonably wet year). Regardless, our results suggest that dryland ICL agriculture in the semi-arid Texas High Plains is potentially a net zero contributor to global warming potential.
“Characterize …..(soil microbial communities) including bacterial diversity in whole soil and different soil aggregate fractions using a novel molecular biological tool (i.e., pyrosequencing).” In the original objective, we had only planned on characterization of the bacterial soil community. The lack of sampling in year 2 enabled us to also characterize the fungal community using pyrosequencing analyses as well as the C utilization profiles of the saprophytic community.
In the third scheduled year of this project (2012-2013), soil microbial community assessments were completed (i.e., fatty acid methyl ester profiling or FAMEs and fungal and bacterial pyrosequencing). Adding the fungal component is unique and represents an understudied component important in nutrient dynamics, C storage, and soil stabilization. In addition to being an important factor in nutrient cycling, saprophytic fungi have synergistic effects on mycorrhizal spore germination and successive plant root colonization. The soil fungal community is known for its sensitivity to disturbance, pollution and environmental changes and has been labeled as an exceptional indicator group for determining changes in ecosystem functioning.
Among fungal groups, we focused on the relative abundance of arbuscular mycorrhizal fungi (AMF) and saprophytic fungi. The AMF are ecologically significant because of their role in facilitating increased water and nutrient uptake in plants. Soil fungal hyphae in general also enhance aggregate stability by enmeshing soil particles together. This aggregation increases the resistance of the soil against erosive forces. Given the low water availability, high soil pH, and inherently low soil organic matter content of these soils, AMF likely play critical roles in the overall stability, resistance and resilience of these agroecosystems. We found that the relative abundance of AMF FAME biomarkers in ICLs averaged 11% (ranged from 7 to 13% of the microbial community) and averaged only 1.8% in the continuous cotton systems (Ctn2 and Ctn3). The same trend between systems was found with fungal pyrosequencing data; the relative abundance of Glomerales (dominant AMF order) was 2.1% in the ICLs compared to 0.06% in the continuous cotton. Another significant finding was a strong correlation (r = 0.57; P< 0.001) between the abundance of the AMF FAME biomarker (nmol g-1 soil) and mean weight diameter. Mean weight diameter measurements provide information regarding the stability of soils. In general, the greater the mean weigh diameter, the more stable the soil.
The diversity of bacterial and fungal communities was also greater in the ICL systems (Forage_Ctn and OWB_BER) compared to the continuous cotton system (Ctn2). The relative abundance of the Glomerales (fungal order) or Glomeromycota (fungal phylum) was also significantly correlated with enzymatic activity involved with C, N, S, and P cycling. The relative abundance of Glomeromycota was also correlated (P < 0.001) with the bacterial sub-phylum, beta-proteobacteria which are known for their diverse metabolic functioning.
We have made significant progress regarding scientific awareness of our findings from the previous 3 objectives showing strong evidence linking the diversity and structure of the microbial community with important soil functions such as nutrient cycling and aggregate stability important for agroecosystem sustainability. In 2012, two peer-reviewed journal articles were accepted and five presentations were done at international scientific meetings including, ASA-CSSA-SSSA and Ecological Society of America Annual Meeting. Specifically, PI Jennifer Moore-Kucera and former Ph.D. student Lisa Fultz organized a special symposium at the ASA-CSSA-SSSA meeting, Cincinnati, OH. 21-24 October 2012, entitled, “Integrating Livestock Into Cropping Systems: Ecosystem Responses From Long-Term Studies.” Dr. Vivien Allen (Co-PI) was the keynote speaker of this full-day symposium which highlighted 10 years of research related to Integrated Crop-Livestock Agroecosystem research. PIs Moore-Kucera, Acosta-Martinez, and Vivien Allen and their graduate students were co-authors on six of the eleven presentations highlighted during this session. Additional speakers included Drs. Ted Zobeck, Alan Franzluebbers, Matt Sanderson, Francisco Calderon and Nithya Rajan, This session was attended by SSARE PR coordinator, Candace Pollock and a summary with photo was uploaded on the SSARE Facebook site. A weblink to all presentation abstracts is http://scisoc.confex.com/scisoc/2012am/webprogram/Session10354.html.
Our primary goal during the final 6 months of this project, will be to focus on increasing producer awareness. We intend on accomplishing this goal during summer 2012. We have been invited to present our results during the Texas Alliance for Water Conservation summer 2012 field days. In this fashion, we can better reach producers who have been involved with this study for over 10 years on specific information related to soil carbon dynamics and microbial diversity. This will supplement our successful special symposium held at the 2012 ASA-CSSA-SSSA to highlight our research findings as well as integrate other leading scientists in integrated-crop livestock systems. We have also been invited to give a presentation at the 2013 ASA-CSSA-SSSA special symposium entitled, “Belowground Processes in Grazinglands: Linking Grassland Management and Ecological Research.” This meeting will be held in November 2013 in Tampa, FL and is often well attended by extension agents and researchers. Lastly, we will finalize our results for the scientific community by publishing three additional peer-reviewed manuscripts.
Impacts and Contributions/Outcomes
Davinic, M.*, Moore-Kucera, J., Acosta-Martínez, V., Zak, J. and Allen, V. 2012. Soil fungal distribution and functionality as affected by grazing and vegetation components of integrated crop-livestock agroecosystems. Applied Soil Ecology 66:61-70. http://www.sciencedirect.com/science/article/pii/S0929139313000401
Fultz, L.*, Moore-Kucera, J., Zobeck, T., Acosta-Martínez, V. and Allen, V.G. 2013. Soil aggregate-carbon pools after 13 years under a semi-arid integrated crop-livestock agroecosystem. Soil Science Society of America Journal. In press; accepted with revisions on 3/6/2013.
- Acosta-Martinez, V., Zobeck, T. and Allen, V. 2012. Soil microbial community composition and functionality in an integrated livestock-crop system compared to continuous cotton. Abstracts, Annual Meeting of the American Society of Agronomy, October 2012, Cincinnati, OH.
Calderon, F.J., Fultz, L.M.*, Allen, V. and Moore-Kucera, J. 2012. Soil Carbon and Soil Organic Matter Quality In Soil Size Fractions From Crop and Livestock Systems In Texas. Abstracts, Annual Meeting of the American Society of Agronomy, October 2012, Cincinnati, OH. http://scisoc.confex.com/scisoc/2012am/webprogram/Paper71012.html
Davinic, M.*, Acosta-Martínez, V., Allen, V., Zak, J. and Moore-Kucera, J. 2012. Soil Fungal Dynamics in Perennial and Annual Crops of Integrated Crop and Livestock Systems. Abstracts, Annual Meeting of the American Society of Agronomy, October 2012, Cincinnati, OH. http://scisoc.confex.com/scisoc/2012am/webprogram/Paper71013.html
Fultz, L.M. *, Moore-Kucera, J. and Allen, V. 2012. Significant management impacts on intra-aggregate soil fractions. Abstracts, Annual Meeting of the American Society of Agronomy, October 2012, Cincinnati, OH. http://scisoc.confex.com/scisoc/2012am/webprogram/Paper71011.html
Fultz, L.M.*, Moore-Kucera, J. and Allen, V. 2012. Increases in protected soil organic carbon found in perennial grassland vegetation as part of integrated crop-livestock systems. Annual Meeting of the Ecological Society of American, 5-10 August 2012, Portland, OR.
Zilverberg, C.J.*, Fultz, L.M., Michael, G., Moore-Kucera, J. and Allen, V.G. 2012. Carbon release from steers and fossil fuels in integrated crop-livestock systems. Abstracts, Annual Meeting of the American Society of Agronomy, October 2012, Cincinnati, OH. http://scisoc.confex.com/scisoc/2012am/webprogram/Paper71018.html