Mineralization of Soil Carbon and Nitrogen Following Physical Disturbance: A Laboratory Assessment

Uncertainty persists regarding the influence of physical disturbance on mineralization of soil organic matter. This study examined how disturbance affects mineralization in soils with different management histories and textures. Results from a 100-d incubation (20 degrees C, -10 kPa) using cores (0-15 cm deep; 5-cm diameter) from a field trial at Lincoln, New Zealand with different agronomic treatments in the previous 5 yr (pasture, arable cropping, and chemical fallow) confirmed that mineralization is strongly influenced by management history (C mineralized ranged from 390 mg kg(-1) in fallow soil to 1570 mg kg(-1) under pasture). However, there was no difference in C or N mineralization between disturbed (sieved <4 mm) and intact cores. In another experiment, comparisons of mineralization in intact and disturbed (<4 mm) cores from 14 arable and pasture fields with either silt loam or clay loam texture also showed no effect of disturbance. In a final experiment, large, air-dry aggregates (19-40 mm) from two soil types (silt loam and clay loam) were fragmented using a compressive force and the resulting sub-aggregates separated into size classes (<0.25, 0.25-1, 1-2, 2-4, 4-9.5, and 9.5-13.2 mm) by dry sieving. Mineralization increased only when aggregate size was below a certain threshold value (similar to 3 mm diameter); mineralization was similar to 25-50% greater in fine (<= 1 mm) vs. large (4-40 mm) aggregates, likely due to exposure of previously-occluded organic matter. Unless a substantial quantity of fine aggregates is generated, the influence of physical disturbance may be small.