Characterization of active nitrogen pools in soils under different cropping systems

Studies were conducted to evaluate the relationships among different active N pools of organic matter in soils at two long-term cropping systems in Iowa. Results indicated that multi-cropping systems, particularly meadow-based systems, enhanced bioactivities of soils. Mono-cropping systems, particularly soybean, reduced soil microbial biomass and enzyme activities. The mineralizable N pool (potential N mineralization; N-o) was more sensitive to changes in the size of the microbial biomass N (N-mic) than to changes in organic N. One unit change in organic N did not lead to substantial changes in N-o, but 1 unit change in N-mic resulted in three or more units change in N-o. The active N pools and turnover rate were more sensitive to changes in organic C than to changes in microbial biomass C (C-mic). A unit change in organic C resulted in 10.6 units change in N-o, but a unit change in C-mic resulted in only 0.8 unit change in N-o. C-mic or N-mic are better indexes than organic C or N for the estimation of N-o or N availability, because biomass values are more highly correlated with cumulative N mineralized during 24 weeks of incubation, with r values ranging from 0.57 (P < 0.001) to 0.88 (P < 0.001).