Denitrification in pasture and cropped soil clods as affected by pore space structure

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To assess the influence of the pore space structure and organic matter on denitrification, a comparative study was performed on clods in a soil under cropped and pasture managements. For each management, the potential denitrification rate was estimated. Denitrification under oxic conditions was also measured on 100 clods, which were saturated with KNO 3 solution (4 g l −1). Size and density fractions of the soil were separated, and the C and N contents of their particulate organic matter were determined. Clod porosities were measured and the distributions of distances of any point within the clod to the nearest air-filled pore were estimated on 20 thin sections for each soil management. Potential denitrification rates were similar (105×10 −11 and 98×10 −11 mol N 2O kg −1 dry soil s −1 for pasture and cropped soil, respectively). The mean denitrification rate under oxic conditions was only equal to 0.14% of the potential denitrification rate for pasture, whereas it was about 2.1% for cropped soil. The total and soluble organic C content was significantly higher in pasture than in cropped soil clods. The quantity or the quality of organic matter fractions did not explain the difference in denitrification activities. Even if macroporosity represented a small fraction of the total porosity in both soils, the differences in macropore distribution induced by soil management practices led to significantly different maximal distances between any clod point and the nearest air-filled pore (8 and 14 mm for pasture and cropped soils, respectively). Consequently, we demonstrated that the pore space structure appears to be the major factor explaining the difference in mean denitrification rates between pasture and cropped soil clods, while the distribution of particulate organic matter is suspected to be involved in the differences in denitrifying activity distribution between the clods of the two soils.