University Program in Ecology, Duke University, Durham, North Carolina 27708, USA.
Ecology. 2011 Mar;92(3):720-32. doi: 10.1890/10-0899.1.
Although regional and global models of nitrogen (N) cycling typically focus on nitrate, dissolved organic nitrogen (DON) is the dominant form of nitrogen export from many watersheds and thus the dominant form of dissolved N in many streams. Our understanding of the processes controlling DON export from temperate forests is poor. In pristine systems, where biological N limitation is common, N contained in recalcitrant organic matter (OM) can dominate watershed N losses. This recalcitrant OM often has moderately constrained carbon:nitrogen (C:N) molar ratios (approximately 25-55) and therefore, greater DON losses should be observed in sites where there is greater total dissolved organic carbon (DOC) loss. In regions where anthropogenic N pollution is high, it has been suggested that increased inorganic N availability can reduce biological demand for organic N and therefore increase watershed DON losses. This would result in a positive correlation between inorganic and organic N concentrations across sites with varying N availability. In four repeated synoptic surveys of stream water chemistry from forested watersheds along an N loading gradient in the southern Appalachians, we found surprisingly little correlation between DON and DOC concentrations. Further, we found that DON concentrations were always significantly correlated with watershed N loading and stream water [NO3-] but that the direction of this relationship was negative in three of the four surveys. The C:N molar ratio of dissolved organic matter (DOM) in streams draining watersheds with high N deposition was very high relative to other freshwaters. This finding, together with results from bioavailability assays in which we directly manipulated C and N availabilities, suggests that heterotrophic demand for labile C can increase as a result of dissolved inorganic N (DIN) loading, and that heterotrophs can preferentially remove N-rich molecules from DOM. These results are inconsistent with the two prevailing hypotheses that dominate interpretations of watershed DON loss. Therefore, we propose a new hypothesis, the indirect carbon control hypothesis, which recognizes that heterotrophic demand for N-rich DOM can keep stream water DON concentrations low when N is not limiting and heterotrophic demand for labile C is high.
尽管氮（N）循环的区域和全球模型通常侧重于硝酸盐，但溶解有机氮（DON）是许多流域氮输出的主要形式，因此也是许多溪流中溶解 N 的主要形式。我们对温带森林中控制 DON 输出的过程的理解很差。在原始系统中，生物 N 限制很常见，含在难降解有机物质（OM）中的 N 可以主导流域的 N 损失。这种难降解的 OM 通常具有中等程度的碳：氮（C：N）摩尔比（约 25-55），因此，在总溶解有机碳（DOC）损失较大的地方，应该观察到更大的 DON 损失。在人为 N 污染较高的地区，有人认为增加无机 N 的可用性可以减少对有机 N 的生物需求，从而增加流域 DON 的损失。这将导致在具有不同 N 可用性的站点之间，无机 N 和有机 N 浓度之间存在正相关。在对南部阿巴拉契亚山脉 N 加载梯度上的森林流域的溪流水质进行的四次重复同步调查中，我们发现 DON 和 DOC 浓度之间几乎没有相关性。此外，我们发现 DON 浓度始终与流域 N 负荷和溪流水中的 [NO3-] 显著相关，但在四次调查中的三次中，这种关系的方向为负。高 N 沉积的流域排水口的 DOM 中的 C：N 摩尔比与其他淡水相比非常高。这一发现，以及我们直接操纵 C 和 N 可利用性的生物有效性测定的结果表明，由于溶解无机 N（DIN）负荷，异养对易降解 C 的需求可能会增加，并且异养生物可以优先从 DOM 中去除富 N 分子。这些结果与主导对流域 DON 损失的解释的两种主流假说不一致。因此，我们提出了一个新的假说，即间接碳控制假说，该假说认识到当 N 不受到限制且异养对易降解 C 的需求较高时，异养对富 N 的 DOM 的需求可以保持溪流水中 DON 浓度较低。
