Potential nitrogen immobilization as influenced by available carbon in Japanese arable and forest soils

Abstract

Microbial nitrogen (N) immobilization following the addition of organic materials to soils regulates soil N availability, which affects plant growth and N leaching from soils. In this study, the potential for microbial N immobilization was evaluated by short-term incubation experiments following the addition of available carbon (C) under non-limiting conditions of N and phosphorus (P) to seven Japanese arable and forest soils. Glucose was added as a model substrate at concentrations close to microbial biomass C. The forest soils had lower pH and smaller increases in respiration rates after the glucose addition, and higher organic and biomass C compared to the arable soils. Microbial N immobilization, estimated by net decreases in extractable N, was significantly correlated with the concentrations of added glucose and was on average 43 mg N g−1 glucose C during 3- and 7-day incubation for all soils. Net increases in biomass N measured by the chloroform fumigation-extraction method using the common conversion factor of 0.54 at 3 and 7 days after the glucose addition were lower than the microbial N immobilization for all soils, and the biomass N accounted for a smaller portion of immobilized N in the arable soils than in the forest soils. Therefore, the present study suggests that microbial N immobilization would be dependent on the concentrations of available C in organic materials and higher than the increases in biomass N, especially for arable soils when organic materials are added to soils under non-limiting conditions of N and P.