COS 15-7
Roots and arbuscular mycorrhizal fungi influence nitrogen cycling in agricultural soils under contrasting management
Mineralization of soil organic matter (SOM) by microbes supplies the majority of plant nitrogen (N) in agroecosystems; however, the plant-microbe controls on N mineralization are not well-understood. Roots and associated arbuscular mycorrhizal fungi (AMF) could alter N mineralization dynamics by influencing rhizosphere and hyphosphere microbial community structure and function. This project seeks to determine the extent to which roots and AMF can alter N mineralization by soil microbes, and whether the functional response to root and AMF presence differs in soils with contrasting background microbial communities due to differences in agricultural management. Soil cores were constructed from mesh allowing in-growth of roots and AMF, AMF only, or no in-growth, and installed between maize plants under two agricultural management regimes of contrasting intensity with four replicate plots per treatment. Soil cores were destructively harvested after 42 and 84 d of in-growth and analyzed for N pool size and distribution; potential net N mineralization and nitrification; activity of extracellular enzymes including proteases (azocasein, production of amines), peptidases (NAG, alanine production from Z-phe-ala), aminidases (LAP, TAP), arginine deaminase, and urease; microbial biomass; PLFA; and microbial community structure and function by Illumina sequencing of the soil metagenome.
Results/Conclusions
Agricultural management influenced extracellular enzyme activity, with lower-intensity management associated with higher NAG, TAP, LAP, and arginine deaminase activities, and lower protease activity as measured by azocasein degradation. Management also influenced microbial community structure: less-intense agricultural management was associated with more PLFA biomarkers for bacteria, fungi, and AMF, but fewer for actinomycetes. Cores with no in-growth had fewer bacteria and actinomycetes than cores that allowed AMF in-growth; no in-growth cores also had a more bacteria-dominated community (lower F:B) than root-inclusion cores. N-acquiring enzyme activity differed between harvest dates, with higher LAP, TAP, and urease activities at 42 d, and higher NAG activity at 84 d. We relate these microbial and enzymatic observations to the size and composition of soil N pools, and describe the extent to which roots, AMF, management, and seasonality influenced the soil N cycling pathways responsible for N mineralization. We discuss the bioinformatics approach we will use to analyze our replicated metagenomic data for information about the phylogenetic structure of rhizosphere and hyposphere microbes, as well as their genetic capacity to influence N mineralization. This work contributes to our understanding of the plant-microbe controls on soil N cycling in the context of agricultural management.