Land use effects on soil microbial carbon and nitrogen in riparian zones of Northeast Missouri

Haake, Danelle
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Agricultural and Biosystems Engineering

In the agricultural centers of the world, soil quality is a widely studied topic. This study focuses on two microbial aspects of soil quality within the riparian zone as affected by land use: soil microbial biomass (SMB) and the mineralization/immobilization of nitrogen. The soils in question were collected on six different dates over a two-year period from riparian forests, pastures, and crop fields along three streams in the Mark Twain Reservoir watershed in northeastern Missouri (Monroe County). SMB was determined in soils up to 110 cm depth using chloroform fumigation-extraction. Surface soil (0-15 cm) under pasture and forest contained 35% and 28% more SMB, respectively, than was found under crop. SMB carbon was at its lowest during the summer, most likely due to a combination of climatic variables. Depth had a highly significant effect (P<0.001), with 50-75% of the total SMB at a site found in the top 35 cm of soil. Aerobic soil incubations were used to determine changes in soil inorganic nitrogen over two-week periods. Land use had little effect on levels of inorganic nitrogen in the soil or on rates of N mineralization. As soil depth increased, soil N concentrations and mineralization rates dropped. Mineralization rates in surface soil were approximately 1.8 mg N wk−1 kg−1 dry soil while about 0.2 mg N wk−1 kg−1 dry soil was immobilized in the deepest depth sampled (65-80 cm). Additions of labile C or N during the incubations seem to show that N immobilization is limited by lack of C for these soils, particularly in the surface soils.