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

To assess the influence of the pore space structure and organic matter on d enitrification, a comparative study was performed on clods in a soil under cropped and pasture managements. For each management, the potential denitri fication rate was estimated. Denitrification under oxic conditions was also measured on 100 clods, which were saturated with KNO3 solution (4 g l(-1)) . Size and density fractions of the soil were separated, and the C and N co ntents of their particulate organic matter were determined. Clod porosities were measured and the distributions of distances of any point within the c lod to the nearest air-filled pore were estimated on 20 thin sections for e ach soil management. Potential denitrification rates were similar (105 x 10 (-11) and 98 x 10(-11) mol N2O kg(-1) dry soil s(-1) for pasture and croppe d 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. Th e quantity or the quality of organic matter fractions did not explain the d ifference in denitrification activities. Even if macroporosity represented a small fraction of the total porosity in both soils, the differences in ma cropore distribution induced by soil management practices led to significan tly different maximal distances between any clod point and the nearest air- filled pore (8 and 14 mm for pasture and cropped soils, respectively). Cons equently, we demonstrated that the pore space structure appears to be the m ajor factor explaining the difference in mean denitrification rates between pasture and cropped soil clods, while the distribution of particulate orga nic matter is suspected to be involved in the differences in denitrifying a ctivity distribution between the clods of the two soils. (C) 1999 Elsevier Science Ltd. All rights reserved.