EFFECT OF NITROGEN ADDITIONS IN THE MICROBIAL CONTRIBUTION TO AGRICULTURAL SOIL ORGANIC MATTER

Abstract:

Recent evidence demonstrates that the concentration dynamics of soil biomarkers that comprise microbial cell walls, such as amino compounds, are closely relate to carbon and nitrogen turnover in soil. On average, 60% of soil organic nitrogen comes from amino compounds. Amino sugars, in particular, originate from bacterial and fungal living biomass and necromass and are estimated to represent up to 5% and 12% of soil organic carbon and soil organic nitrogen, respectively. In this context, they can be used to track microbial and fungal contribution and potentially cycling of soil C and N. In non-fertilized terrestrial ecosystems, carbon and nitrogen processing are tightly coupled during photosynthesis and microbial decay, but in agricultural soils under heavy nitrogen management these elements may not be completely coupled. To evaluate the effect of nitrogen additions in the microbial cell wall residue contribution to soil organic matter, amino sugars (mannosamine, galactosamine, glucosamine, and muramic acid) were quantified within agricultural soils under continuous corn management that were amended with three different nitrogen rates (230 lb N ha^-1, 170 lb N ha^-1, and zero nitrogen) and two nitrogen mixtures (aqua ammonia and UAN and aqua ammonia, respectively) over a period of 2 years. Soil C trended with increased N input, but crop productivity seems to be more related to the kind of nitrogen input supplemented and soil moisture. Additionally, our results indicate that higher nitrogen rates lead to an increased accumulation of microbial residues (total amino sugars) in mass of soil. The relative contribution of microbial derived carbon and nitrogen as a proportion of soil organic carbon and nitrogen, respectively are significantly higher in plots with limited nitrogen sources, mainly associated to fungal-derived amino sugars.