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 from 33 conifer forests across Europe. The sites span a geographical range covering 11 countries from Ireland to western Russia and Finland to the southern Alps, and encompass a wide range in throughfall nitrogen deposition. The 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 nitrate 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 ecosystems are grouped into broad categories of throughfall nitrogen deposition. At low levels of N deposition (<10 kg N ha(-1) year(-1)), nitrate leaching 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 leaching 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 nitrate (95% confidence interval ca +/-5 kg N ha(-1) year(-1)) for sites receiving 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 consideration to refine estimates of nitrate leaching.