Litter placement effects on microbial and organic matter dynamics in an agroecosystem

Two different agricultural tillage practices were used to study how changes in the structure of the soil—litter system affected litter decomposition rates, microbial community composition, and soil organic matter dynamics. Surface straw placement results in spatial separation of carbon—rich litter (C:N ratio 80:1) and mineralized soil nitrogen. In contrast, when the litter is plowed into the soil, straw carbon and soil nitrogen are in intimate contact. Our field studies in Colorado showed that fungal biomass in surface—straw treatments was 144% of that in the incorporated—straw treatments, probably because fungi, with their extensive hyphal networks, are able to utilize both the surface straw carbon and the available soil nitrogen. Field studies using 14C—labeled wheat straw showed that a greater proportion of added 14C was retained in the surface—straw treatment than in the incorporated—straw treatment. Maximum net N immobilization was higher and litter decomposition was slower in the surface straw than in the incorporated straw placements both with and without experimental nitrogen addition. Slower litter decomposition of the surface litter may contribute to reduced soil organic matter losses. Soil organic matter losses may also be reduced in no—till systems as a result of the increase in the ratio of fungal to bacterial activity because of the greater growth efficiency of fungi and the accumulation of carbon in the less decomposable fungal biomass. The surface placement of straw in no—till agriculture allowed management of microclimate and microbial populations so that losses of soil organic matter and nutrients were minimized.