Characterization of active nitrogen pools in soils under different cropping systems

Abstract

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;No) was more sensitive to changes in the size of the microbial biomass N (Nmic) than to changes in organic N. One unit change in organic N did not lead to substantial changes in No, but 1 unit change in Nmic resulted in three or more units change in No. The active N pools and turnover rate were more sensitive to changes in organic C than to changes in microbial biomass C (Cmic). A unit change in organic C resulted in 10.6 units change in No, but a unit change in Cmic resulted in only 0.8 unit change in No. Cmic or Nmic are better indexes than organic C or N for the estimation of No 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).