Soil C/N Ratio as Affected by Climate: An Ecological Factor of Forest NO3 ? Leaching

The soil nitrogen cycle was investigated for several watershed forests of which stream waters have distinctively different nitrate (NO₃ ^?) concentrations. In a watershed with stream water NO₃ ^? of more than 100 µM, soil NO₃ ^? content increased even beneath the rooting zone, revealing "nitrogen saturation" status. A laboratory soil experiment demonstrated that the proportion of net NO₃ ^? production to CO₂ production was largely regulated by a soil C/N ratio, suggesting a key parameter for NO₃ ^? abundance. In the respective watershed soils, little nitrogen was actually present as NO₃ ^? above a soil C/N ratio of 20. The annual mean soil temperature recorded at the sites was correlated with a shift in the C/N ratio in watershed soils (a soil C/N ratio increase of 0.5 per 1°C decrease) along the stream NO₃ ^? gradient of 30 fold. The results suggest that soil microbial metabolisms affected by C/N ratio may be a direct agent regulating NO₃ ^? leaching from watersheds under the influence of an atmospheric nitrogen load and climate.