Denitrification in north temperate forest soils: Spatial and temporal patterns at the landscape and seasonal scales

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Abstract Spatial and temporal patterns of denitrification were studied in nine forest soils of different texture and drainage classes in Michigan. Denitrification was measured using intact soil cores amended with acetylene. Sampling was done weekly during spring, and monthly at other times. To determine the factors limiting denitrification, selected cores were amended with either water or water plus nitrate. We found that activity was highest in spring and fall and was lowest during summer. Soils showed a characteristic relationship between denitrification rate and soil texture and drainage classes. Over 80% of the annual N loss to denitrification occurred during brief (3–6 week) periods of high activity in the spring and fall. Rates of denitrification during these periods exceeded 0.5kg N ha −1 d −1 in some soils. Estimates of annual N loss to denitrification ranged from −1 yr −1 in a well-drained sand soil to over 40kg N ha −1 yr −1 in a poorly-drained clay loam soil. Lack of available NO −1 1 was the primary factor limiting denitrification in summer, but available carbon was probably occasionally limiting, especially in the well-drained soils. Marked seasonal changes were observed in the variability and frequency distributions of denitrification rates. Denitrification rates showed lower variance in spring and fall compared to summer, regardless of the magnitude of the rates. These patterns may be related to seasonal changes in soil structure, carbon availability or aeration, which create a more uniform environment in the spring an fall. The characteristic relationship between denitrification rates and soil texture and drainage over two seasons and multiple sites suggests that geomorphologically-based models using these parameters may be feasible for producing landscape- and regional-scale estimates of denitrification.