Nitrogen immobilization/remineralization in legume‐amended soils as influenced by texture and compaction

Abstract

The role of intrinsic soil properties and management induced changes in bulk density on legume shoot biomass‐nitrogen (N) turnover to soil mineral N nitrate (NO₃) plus ammonium (NH₄)], SMN, through soil microorganisms is poorly understood. In this study, the influence of intrinsic soil properties and changes in bulk density in soils amended with red clover (Trifolium pratense L.) on N immobilization/remineralization was investigated. Time in incubation, soil type, bulk density, and legume amendment had significant influence on the amounts of microbial biomass carbon (C) (MBC), N (MBN), and the SMN measured during incubation. During the first 32 days in incubation, MBC and MBN in the legume‐amended soils were higher than the control whereas an opposite trend existed for SMN. The SMN measured at the end of incubation, i.e., 70 days after incubation, was significantly higher than the unamended control. The ratio of SMN to MBN (SMN:MBN) was < 1.0, in general, during the first 32 days in incubation in legume amended soils, indicating N immobilization in microbial biomass during this period. Forty‐two days after incubation, the SMN:MBN ratios in the legume amended soils were >1.0, indicating remineralization of the immobilized N, derived, at least partially, from the legume. In the unamended control, these ratios were > 1.0 throughout the incubation. Over time, 63% to 76% of the variability in N‐immobilization/remineralization (SMN:MBN) was accounted for clay content, water (WFP) and air (AFP) filled porosities, volume fraction of pores (VFP) <1.5 μm, total N, C to N ratios in soils, bulk density, and legume amendment. The results indicate the influence of intrinsic soil properties and bulk density on microbially mediated legume N turnover to SMN changed over time.