Effects of soil structure disturbance on mineralization of organic soil nitrogen.

Disturbance of soil structure by tillage operations is thought to make soil organic N accessible for mineralization which was otherwise protected from degradation. The origin of N released by disturbance of soil structure is, however, poorly understood and needs to be related to microbial activity. This study was performed to investigate the effect of soil structure disturbance on the release of active or protected organic N pools in surface soils (0-2 cm) under plow- (PT) or no-tillage (NT) management. Active soil N was defined as the pool participating in mineralization-immobilization turnover during short-term incubation (6 d) while protected pools were considered inactive during this period. The active pool of soil N was labelled with 15 N in intact samples of PT and NT soils. The samples were either kept intact or sieved and repacked, and then leached weekly during a 35-d incubation period. The disturbance of soil structure increased mineral N release from 6 to 15 mg kg -1 in the NT soil within the first week after disturbance. This release was found to originate from both active and physically protected N pools as could be assessed by the relative differences in 15 N content of mineralized N by intact and disturbed soil samples. In contrast, the release from the PT soil was 7 to 9 mg N kg -1 after disturbance, with only a minor contribution from protected N pools. These results support the theory that disturbance of soil structure by tillage may destabilize and release protected pools of soil N. Over the entire period of incubation, protected N accounted for 27% of total N release in the NT soil and 12% in that of PT. The calculation of availability ratios, defined as the ratio between the 15 N enrichment of mineralized N and that of total soil N, showed that recently added 15 N was less available for mineralization in the NT soil as compared to that of PT. The probable cause for this difference was the higher C/N ratio of organic matter in NT surface soil indicating more nonhumified organic matter when compared to PT organic matter.