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Research Project: Strategies to Support Resilient Agricultural Systems of the Southeastern U.S.

Location: Plant Science Research

Title: Conservation of soil organic carbon and nitrogen fractions in a tallgrass prairie in Oklahoma

Author
item Franzluebbers, Alan
item Starks, Patrick - Pat
item Steiner, Jean

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2019
Publication Date: 4/20/2019
Citation: Franzluebbers, A.J., Starks, P.J., Steiner, J.L. 2019. Conservation of soil organic carbon and nitrogen fractions in a tallgrass prairie in Oklahoma. Agronomy. Vol 9. https://doi.org/10.3390/agronomy9040204.
DOI: https://doi.org/10.3390/agronomy9040204

Interpretive Summary: Rotational stocking of pastures has received renewed attention as a potential strategy to support a more ecologically based grazing management approach. Soil health data to support the approach has been lacking. Therefore, a team of USDA Agricultural Research Service scientists in El Reno OK collaborated with a scientist in Raleigh NC to determine how continuous and rotational stocking affected soil organic matter and soil biological activity over the course of 8 years in a field-scale study at the Grazinglands Research Laboratory in El Reno OK. Soil carbon and nitrogen fractions were not affected by stocking method – neither positively nor negatively. All properties were highly stratified with depth, meaning that soil organic matter was most concentrated near the surface and declined with greater depth in the profile. The combination of abundant available nitrogen from high mineralization potential and lack of nitrification to nitrate led us to postulate that nitrogen could be effectively conserved in this grazed remnant prairie ecosystem. This information will be valuable for farmers and extension agents to design robust and resilient grazing management systems.

Technical Abstract: Native grasslands in the Great Plains of North America have mostly disappeared in the past century due to agricultural expansion. A long-term grazing study was established on Paleustolls and Argiustolls supporting a remnant, but historically grazed tallgrass prairie in central Oklahoma. Stocking method of beef cattle was differentiated into continuous and rotational treatments (10 sub-paddocks) in 2009 and these treatments continued until present. Soil was sampled in 2009 and 2012 at depths of 0-6, 6-12, 12-20, and 20-30 cm and in 2017 at depths of 0-15 and 15-30 cm. Total, particulate, microbial biomass, and mineralizable C and N fractions were highly stratified with depth, having 2-10 times greater concentration at depth of 0-6 cm as that at 20-30 cm. Strong associations existed among most of these soil organic C and N fractions, given the large range that resulted from sampling at multiple depths. No discernable differences in soil organic C and N fractions occurred due to stocking method at any sampling time or depth. Evidence for biological nitrification inhibition suggested a mechanism for conservation of available N with less opportunity for loss. In addition, strong association of available N with biologically active C indicated slow, but sustained release of N that was strongly couple to C cycling. We conclude that stocking method had a neutral effect on conservation of already high antecedent conditions of soil organic C and N fractions.