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

Abstract

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.