High precipitation causes large fluxes of dissolved organic carbon and nitrogen in a subtropical montane Chamaecyparis forest in Taiwan

Fluxes of dissolved organic carbon (DOC) and nitrogen (DON) may play an important role for losses of C and N from the soils of forest ecosystems,especially under conditions of high precipitation. We studied DOC and DON fluxes and concentrations in
relation to precipitation intensity in a subtropical montane Chamaecyparis obtusa var. formosana forest in Taiwan. Our objective was, to quantify DOC and DON fluxes and to understand the role of high precipitation for DOC and DON export in this ecosystem. From 2005 to 2008 we sampled bulk precipitation, throughfall, forest floor percolates and seepage (60 cm) and analyzed DOC, DON andmineral N concentrations. Average DOC fluxes in the soil were extremely high (962 and 478 kg C ha-1
year-1 in forest floor percolates and seepage, respectively) while DON fluxes were similar to other (sub)tropical ecosystems (16 and 8 kg N ha-1 year-1, respectively). Total N fluxes in the soil were dominated by DON. Dissolved organic C and N concentrations in forest floor percolates were independent of the water flux. No dilution effect was visible. Instead, the pool size of potentially soluble DOC and DON was variable as indicated by different DOC and DON concentrations in forest floor percolates at similar precipitation amounts. Therefore, we hypothesized, that these pools are not likely to be depleted in the long
term. The relationship between water fluxes in bulk precipitation and DOC and DON fluxes in forest floor percolates was positive (DOC r = 0.908, DON r = 0.842, respectively, Spearman rank correlation).We concluded, that precipitation is an important driver for DOC and DON losses from this subtropical montane forest and that these DOC losses play an
important role in the soil C cycle of this ecosystem.Moreover, we found that the linear relationship between bulk precipitation and DOC and DON fluxes in forest floor percolates of temperate ecosystems does not hold when incorporating additional data on these fluxes from (subtropical) ecosystems.

溶解性有机碳（dissolved organic carbon, DOC）与溶解性有机氮（dissolved organic nitrogen, DON）的通量，对森林生态系统土壤碳、氮流失具有重要调控作用，尤其在高降水条件下。本研究以中国台湾地区的亚热带山地台湾扁柏（Chamaecyparis obtusa var. formosana）林为研究对象，探讨了降水强度与DOC、DON通量及浓度的关联。本研究旨在量化DOC与DON通量，并明确高降水在该生态系统DOC、DON输出过程中的作用。
2005年至2008年间，我们采集了大气降水（bulk precipitation）、穿透雨（throughfall）、枯落物层渗滤液（forest floor percolates）以及60cm深度的渗流水（seepage）样品，并分析了其中的DOC、DON与矿质氮（mineral N）浓度。土壤中DOC的平均通量极高：枯落物层渗滤液与渗流水的DOC通量分别达962与478 kg C ha⁻¹ year⁻¹；而DON通量则与其他（亚）热带生态系统相近，分别为16与8 kg N ha⁻¹ year⁻¹。土壤总氮通量以DON为主导。
枯落物层渗滤液中的溶解性有机碳、氮浓度与水通量无关，未观测到稀释效应。相反，在相似降水量条件下，枯落物层渗滤液的DOC、DON浓度存在差异，这表明潜在可溶性DOC与DON的库大小存在变异。据此我们提出假说：这些碳、氮库在长期尺度上不太可能被耗竭。
大气降水的水通量与枯落物层渗滤液中的DOC、DON通量呈显著正相关，斯皮尔曼等级相关系数（Spearman rank correlation）结果显示：DOC的r=0.908，DON的r=0.842。
我们由此得出结论：降水是该亚热带山地森林DOC与DON流失的重要驱动因子，且此类DOC流失在该生态系统的土壤碳循环中扮演重要角色。此外，我们发现，若纳入（亚）热带生态系统的相关通量数据，温带生态系统中大气降水与枯落物层渗滤液DOC、DON通量间的线性关系并不成立。