Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 15, 1999
Publication Date: N/A
Interpretive Summary: Cultivation of native soils has caused large decreases in organic matter content of agricultural soils. This has raised concerns about the sustainable use of agricultural lands under management involving intensive tillage operations. Soil management practices that minimize soil disturbance has been introduced and gained acceptance in crop production with the purpose of reduction of soil erosion and conservation of soil organic matter. The concept of protected organic matter has been developed to account for the stabilization of organic C and N by aggregation. This work investigated the effect of soil structure disturbance on the release of active or protected organic N pools in soils under plow- or no-tillage management. It was found that disturbance of soil structure caused a significant increase in mineral N release in the no-tillage soil within the first week after disturbance with release from both active and physically protected N pools. This was contrasted to smaller release of soil N from the plow-tillage soil after disturbance with only minor releases of protected N.
This work investigated the effect of soil structure disturbance on the release of active or protected organic N pools in soils under plow- (PT) or no-tillage (NT) management. An active pool of soil N was labeled by adding 15N to intact surface layer (0-2 cm) samples of PT and NT soils. The samples were either kept intact or sieved and re-packed, and then leached weekly during a 35 day period of incubation. The disturbance of soil structure caused an increase in mineral N release from 6 to 15 mg/kg in the NT soil within the first week after disturbance with release from both active and physically protected N pools. This was contrasted by the release from the PT soil that remained at 7-9 mg N/kg after disturbance with only a minor release of protected N. Over the entire period of incubation, protected N could account for 27% of total N release in the NT soil and 12% in that of PT. The calculation of availability ratios showed that recently added 15N was less available for mineralization in the NT soil as compared to that of PT. A possible cause of this difference was the higher C/N ratio of organic matter in the NT soil indicating a lower degree of decomposition when compared to PT organic matter. This study found the combination of 15N labeling techniques and incubations of intact/disturbed soil samples to be useful to distinguish the release of N from difference organic soil N pools. The results obtained support the theory that disturbance of soil structure by tillage may destabilize and release protected pools of soil N.