Does Organic Farming Promote Soil Microbial Communities with High Nitrogen Use Efficiency in Acidic Southeastern US Soils?
Project Director
Shuijin Hu
Year Funded
2018
Award Number
2018-51106-28773
Funded Institution
North Carolina State University
Grant Program
ORG (Organic Transitions)
USDA NIFA Report (alternate)
Project Overview
Studies were undertaken at the Center for Environmental Farming Systems (CEFS) and on four organic farms to characterize nitrogen (N) cycling microbial communities in southeastern US soils and to identify environmental drivers modulating N transformations and nitrous oxide (N2O) emissions. The goal was to identify management strategies that optimize N use efficiency and minimize N2O emissions in organic operations. Researchers tracked ammonia-oxidizing (nitrifying) and nitrate-reducing (denitrifying) microbes that govern N2O formation and the mitigating role of N uptake by plant roots and symbiotic arbuscular mycorrhizal fungi (AMF).
N2O emissions increased with available N and with high N inputs from soluble or organic sources. AMF mitigated N2O by enhancing plant N uptake and regulating the denitrifying microbial community. The N2O emissions factor (EF, % of N inputs lost as N2O) peaked in moderately acidic soils (pH 5.6-6.0) and decreased at neutral (7) or strongly acid pH (≤5.0). A hairy vetch cover crop (low C:N) boosted soil mineralizable N while crimson clover (higher biomass and C:N) boosted microbial biomass after cover crop termination.
At CEFS, the tilled system with conventional inputs had the lowest microbial biomass N (MBN) and highest cumulative N2O emissions, which decreased with reduced tillage or organic inputs. Organic systems had more N-cycling microbes, MBN, and mineralizable soil organic carbon, yet they enhanced microbial N immobilization and emitted less N2O. Biochar slightly mitigated N2O across systems.
Farmer Takeaways
(1) Organic farming systems that reduce tillage and avoid excessive N inputs improve N use efficiency and significantly reduce N2O emissions compared to conventional systems.
(2) Root colonization by arbuscular mycorrhizal fungi (AMF) enhances N uptake and reduces N2O formation.
(3) Correcting moderate soil acidity can reduce N2O emissions and improve N use efficiency.
(4) High biomass cover crops with moderate C:N ratio offer the greatest benefits to soil microbial communities and soil health.
Project Outputs
Qiu, Y., Guo, L., Xu, X., Zhang, L., Zhang, K., Chen, M., Zhao, Y., Burkey, K.O., Shew, H.D., Zobel, R.W. and Zhang, Y., 2021. Warming and elevated ozone induce tradeoffs between fine roots and mycorrhizal fungi and stimulate organic carbon decomposition. Science Advances, 7(28), p.eabe9256.
Zhang X, Qiu Y, Gilliam FS, Gillespie CJ, Tu C, Reberg-Horton SC, Hu S. 2022. Arbuscular Mycorrhizae Shift Community Composition of N-Cycling Microbes and Suppress Soil N2O Emission. Environmental Science & Technology. 2022 Aug 30;56(18):13461-72.
Bloszies, S.A., Reberg-Horton, S.C., Heitman, J.L., Woodley, A.L., Grossman, J.M. and Hu, S., 2022. Legume cover crop type and termination method effects on labile soil carbon and nitrogen and aggregation. Agronomy Journal, 114(3), pp.1817-1832.
Qiu, Y., Zhang, Y., Zhang, K., Xu, X., Zhao, Y., Bai, T., Zhao, Y., Wang, H., Sheng, X., Bloszies, B., Gillespie, C.J., He, T., Wang, Y., Chen, H., Guo, L., Song, H., Ye, C., Wang, Y., Woodley, A., Guo, J., Cheng, L., Bai, Y., Zhu, Y., Hallin, S., Firestone, M.K., Hu, S. 2024. Intermediate soil acidification induces highest nitrous oxide emissions. Nature Communications 15: 2695.