Implementing sustainable agricultural management is important for long term food security under a changing climate; for example, using organic management strategies and/or cover crops reduce the need for harmful inorganic fertilizers. These strategies may also affect the microbial processes that underpin soil N retention, an often overlooked pathway of reducing potential N losses. Nitrogen can be stabilized in multiple organic soil organic matter (SOM) pools, however the rate and retention time that nitrogen is stabilized within these various SOM pools remains unclear. To address this knowledge gap, our work used novel approaches to explore the drivers of nitrogen distribution and retention across multiple SOM pools.
To test potential drivers of nitrogen movement and distribution across SOM pools, we conducted a laboratory incubation in which combinations of N15-labeled cover crop residues were added to soils collected from long-term conventional and organic systems at the Kellogg Biological Station LTER. At four times following residue addition, (24 hours, one week, one month, three months), we measured N15in microbial biomass, inorganic N, particulate organic matter, and mineral-associated organic matter pools. To understand the role of microbial communities in N dynamics, we also measured soil respiration, enzyme activities, and microbial community structure via PFLA.
Results from the enzyme assays show that the C:N acquisition ratios decreased by two-fold between harvests, while differences in enzymatic activities between microbial communities decreased through time. Furthermore, we have detected treatment level variation in both C:N acquisition ratios as well as the ratio of β-D-cellobiosidase:N-acetyl-β-glucosaminidase, reflecting changes in microbial community structure and function. In addition, we observed strong effects of agricultural management systems on soil organic matter pools over time. Taken together, these results indicate that microbial communities have shifted similarly in response to cover crop residue inputs; these shifts in the microbial community have subsequently altered the rate of deposition and retention of nitrogen into various SOM pools.