Evaluation of organic horizon C : N ratio as an indicator of nitrate leaching in conifer forests across Europe

We evaluate the relationship between the carbon-to-nitrogen ratio (C:N) of the soil organic horizon and nitrate leaching in runoff or seepage water fr om 33 conifer forests across Europe. The sites span a geographical range co vering 11 countries from Ireland to western Russia and Finland to the south ern Alps, and encompass a wide range in throughfall nitrogen deposition. Th e aim of the study is to evaluate the hypothesis that the C:N ratio of the organic (OH) horizon can be used to estimate the level of leaching of nitra te from a forest ecosystem. The analysis suggests that C:N ratio can be an indicator of nitrate leaching for conifer forests across Europe if these ec osystems are grouped into broad categories of throughfall nitrogen depositi on. At low levels of N deposition (<10 kg N ha(-1) year(-1)), nitrate leach ing is low regardless of the OH C:N ratio. At intermediate (10-20 kg N ha(- 1) year(-1)) and high (>20 kg N ha(-1) year(-1)) N deposition, nitrate leac hing increases with decreasing C:N ratio. In addition, for any given value of C:N, the level of nitrate leaching is higher at high N-deposition sites than at intermediate N-deposition sites. From the current data, OH horizon C:N ratio can give a reasonable estimate of the annual export flux of nitra te (95% confidence interval ca +/-5 kg N ha(-1) year(-1)) for sites receivi ng throughfall-N up to about 30 kg N ha(-1) year(-1). Above this level, the variability in the data increases, suggesting other factors may need consi deration to refine estimates of nitrate leaching.