Carbon and Nitrogen Mineralization in Tallgrass Prairie and Agricultural Soil Profiles

In situ mineralization of N may contribute significantly to total inorganic‐N pools deep in the soil profile. We conducted long‐term laboratory incubation experiments to evaluate the net C and N mineralization in soils collected from various depths in tallgrass prairie and agricultural fields of the same geological materials and soil type. Samples were packed to a bulk density of 1.4 g cm ‐3 in 5‐cm‐diameter by 10‐cm‐long cores. The cores were incubated at 35°C for 40 wk in sealed containers. Net C mineralization was measured by evolved CO 2 , and N mineralized was measured by periodic leaching with NH + 4 and NO ‐ 3 measured in the leachate. Carbon and N mineralization in the surface horizon were greater in the tallgrass prairie than in the agricultural soil. In both the tallgrass prairie and agricultural soil profiles, C mineralization was least at the water‐table depth. Carbon mineralization was described by a first‐order kinetic model, but N mineralization was described better by a consecutive (sigmoidal) reaction model. At most depths, the ratios of potentially mineralizable organic C (C 0 ) to total organic C (C 0 /TOC) and potentially mineralizable organic N (N 0 ) to total N (N 0 /TN) were greater in the agricultural soil profile than in the tallgrass prairie soil profile. The C 0 and N 0 in the surface soil (0–0.2 m) represented 11.6 and 12.2% of the total organic C and N pools for the tallgrass prairie soil profile, respectively, and 21.0 and 10.2% of the total organic C and N pools for the agricultural soil profile. Management practices affected the mineralization potentials and rates of both the surface and subsurface soils.