Geomorphic control on the δ15N of mountain forests

[摘要] Mountain forests are subject to high rates of physical erosion which canexport particulate nitrogen from ecosystems. However, the impact ofgeomorphic processes on nitrogen budgets remains poorly constrained. We haveused the elemental and isotopic composition of soil and plant organic matterto investigate nitrogen cycling in the mountain forest of Taiwan, from 24sites with distinct geomorphic (topographic slope) and climatic(precipitation, temperature) characteristics. The organic carbon to nitrogenratio of soil organic matter decreased with soil ¹⁴C age, providingconstraint on average rates of nitrogen loss using a mass balance model.Model predictions suggest that present day estimates of nitrogen depositionexceed contemporary and historic nitrogen losses. We found ∼6‰ variability in the stable isotopic composition(δ¹⁵N) of soil and plants which was not related to soil¹⁴C age or climatic conditions. Instead, δ¹⁵N wassignificantly, negatively correlated with topographic slope. Using the massbalance model, we demonstrate that the correlation can be explained by anincrease in nitrogen loss by non-fractioning pathways on steeper slopes,where physical erosion most effectively removes particulate nitrogen.Published data from forests on steep slopes are consistent with thecorrelation. Based on our dataset and these observations, we hypothesisethat variable physical erosion rates can significantly influence soilδ¹⁵N, and suggest particulate nitrogen export is a major, yetunderappreciated, loss term in the nitrogen budget of mountain forests.

[学科分类] 地球化学与岩石