Inorganic Nitrogen Supply and Dissolved Organic Nitrogen Abundance across the US Great Plains

Across US Great Plains grasslands, a gradient of increasing mean annual precipitation from west to east corresponds to increasing aboveground net primary productivity (ANPP) and increasing N-limitation. Previous work has shown that there is no increase in net N mineralization rates across this gradient, leading to the question of where eastern prairie grasses obtain the nitrogen to support production. One as-yet unexamined source is soil organic N, despite abundant literature from other ecosystems showing that plants take up dissolved soil organic N. This study measured KCl-extractable dissolved organic N (DON) in surface soils across the grassland productivity gradient. We found that KCl-extractable DON pools increased from west to east. If available to and used by plants, this DON may help explain the high ANPP in the eastern Great Plains. These results suggest a need for future research to determine whether, in what quantities, and in what forms prairie grasses use organic N to support primary production.在美国大平原草原生态系统中，年平均降水量自西向东递增的梯度，与地上净初级生产力（aboveground net primary productivity, ANPP）提升以及氮限制程度加剧呈显著对应关系。已有研究表明，该梯度沿线的净氮矿化速率并未出现升高，这引出了一个关键科学问题：东部草原草本植物获取氮素以维持生产力的来源究竟是什么？尽管已有大量其他生态系统的研究文献证实植物可吸收土壤溶解性有机氮，但土壤有机氮这一潜在来源迄今尚未在该梯度研究中得到考察。本研究针对该草原生产力梯度沿线的表层土壤，测定了氯化钾（KCl）提取态溶解性有机氮（dissolved organic nitrogen, DON）的储量。结果发现，氯化钾提取态DON库的储量自西向东逐渐增加。若该DON可被植物获取并利用，则有望解释美国大平原东部较高的ANPP水平。上述研究结果提示，未来需开展相关研究以明确草原草本植物是否利用有机氮维持初级生产力、其利用量以及具体利用形态。