Rapid and distinct responses of particulate and mineral-associated organic nitrogen to conservation tillage and cover crops

Authors: JILLING, ANDREA - University Of New Hampshire; KANE, DANIEL - Michigan State University; WILLIAMS, ALWYN - University Of Queensland; YANNARELL, ANTHONY - University Of Illinois; DAVIS, ADAM - University Of Illinois; JORDAN, NICHOLAS - University Of Minnesota; KOIDE, ROGER - Brigham Young University; MORTENSEN, DAVE - University Of New Hampshire; SMITH, RICHARD - University Of New Hampshire; SNAPP, SIEGLINDE - Michigan State University; Spokas, Kurt; GRANDY, STUART - University Of New Hampshire

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2019
Publication Date: 11/1/2019
Citation: Jilling, A., Kane, D., Williams, A., Yannarell, A., Davis, A., Jordan, N., Koide, R., Mortensen, D., Smith, R., Snapp, S., Spokas, K.A., Grandy, S. 2019. Rapid and distinct responses of particulate and mineral-associated organic nitrogen to conservation tillage and cover crops. Geoderma. 359. https://doi.org/10.1016/j.geoderma.2019.114001.
DOI: https://doi.org/10.1016/j.geoderma.2019.114001

Interpretive Summary: We need improved methods to assess changes to soil C/N content to gain insight into the impacts of different agricultural management practices. This study investigated changes in the distribution of carbon and nitrogen in different particle size fractions following two different tillage managements (ridge tillage or chisel plow) as well as the use of a cover crop on agricultural soils from three locations. The soil was sampled three years following the start of the different management treatments. Soils were analyzed for the carbon and nitrogen content, which was analyzed in different particle size fractions. We also compared these changes in soil organic matter distribution to alterations in crop yield. From this data, there is the suggestion that the mineral-associated organic pool (MAOM) is an important player in the short-term increases in SOM C and N that was observed following the use of cover crops and ridge tillage. The response was site specific, with locations having lower SOM levels showing the highest levels of improvement with ridge tillage and cover crop use. These results highlight the need to determine the C/N content of particle size classes, versus assessment of solely changes in total soil C/N content to properly assess impacts of management practices on soil processes. These results are significant to farmers and policy makers and will assist scientists and engineers in understanding the potential pathways for mechanisms to improve carbon sequestration and soil fertility.

Technical Abstract: Particulate organic matter (POM) is considered the most active fraction of organic matter in cultivated soils; POM responds readily to management and is more physically accessible to decomposers compared to more decomposed or mineral-associated forms of organic matter. However, there is increasing evidence that mineral-associated organic matter (MAOM) can also play a dynamic role in short-term fertility. In this study, we investigated the short-term response of soil organic matter fractions to tillage and cover cropping. We collected soils from an ongoing experiment replicated across three growing sites in north central and mid-Atlantic United States: a high organic matter site in Illinois (IL) and two low-organic matter sites, located in Michigan (MI) and Pennsylvania (PA). Management treatments included two levels of tillage (chisel plow and ridge tillage) and two levels of cover crop (with or without rye cover crop). Using a particle size-based technique, we isolated free particulate organic matter, occluded POM, a coarse silt fraction, and MAOM. We assessed the management effect on N and C in soil organic matter (SOM) fractions. In a follow-up exploratory analysis, we examined how observed changes in SOM fractions and bulk soil characteristics corresponded to N mineralization and crop performance. After three years of management, tillage and cover cropping significantly influenced the behavior of N and C across all SOM fractions, however responses were site-specific. In the OM-depleted MI and PA soils, ridge tillage and cover cropping increased C and N significantly within POM fractions. The response to ridge tillage was most pronounced: a 76% and 24% increase in occluded POM N content in MI and PA, respectively. In contrast, in the OM-rich soil at the IL site, POM fractions were less responsive to tillage. Here, the inclusion of cover crops led to higher C and N, specifically within the fine fractions (coarse Silt and MAOM), where cover cropping caused, most notably, a 24% increase in MAOM N content in IL. Using partial least squares regression, we explored broad cross-site relationships between SOM fractions, soil N availability, and crop performance. When analyzing all sites together, both particulate and fine fractions were positively associated with potential N mineralization and measures of crop performance. Overall, properties associated with fine fractions contributed most significantly to the final predictive model. This suggests that along with POM, silt and clay-sized SOM fractions may also be potential sources of N for crops. However, understanding this potential relationship between fine fractions and soil N supply will require further investigation.