Collaborative Research: Influence of the Riparian Zone on Nitrogen Cycling in Streams of Puerto Rico: Role of Nitrification and Denitrification

该项目将研究可能影响热带雨林溪流中氮素行为的基本过程。关于溪水和土壤水之间的水文相互作用或这种相互作用可能对热带生态系统中的氮循环产生的潜在影响，人们知之甚少。在波多黎各的山地热带雨林流域收集的初步数据表明，土壤水将大量的溶解氮以铵的形式带入河岸地带。然而，在溪水中发现的铵或硝酸盐很少，这表明氮在进入大宗溪流之前从溶液中被去除。重要的是要确定从溶液中去除的氮是通过硝化或反硝化作用以气体形式从生态系统输出的，还是通过植物吸收在内部循环的。假设在这些热带溪流中：1)有效的溪流侧无机氮支持活跃的硝化-反硝化作用对，2)这种作用对溪流中的氮化学有重要影响。此外，还假设流经流侧区域的流动路径将对氮气动力学产生可预测的影响。为了研究这些假设，将进行以下工作：1)在不同河侧水力特性的河流河段上建立观测井断面，并测量这些站点的无机氮和有机氮的初始浓度；2)测试这些站点的氮富集对氮化学的影响；3)在实验室土壤和沉积物系统中评估硝化和反硝化的潜力，然后用N-15示踪技术量化氮素损失的具体机制。这种野外测量、田间操作和实验室实验的结合将使我们能够研究典型热带河流生态系统中硝化、反硝化和植物吸收的相对重要性和空间特征。

This project will investigate fundamental processes that are likely to influence the behavior of nitrogen in tropical rain forest streams. Very little is known about hydrologic interactions between stream water and soil water or the potential influences such interactions could have on the nitrogen cycle in tropical ecosystems. Preliminary data collected in montane rain forest watersheds in Puerto Rico indicae that soil water carries large amounts of dissolved nitrogen as ammonium into the riparian zone. Very little ammonium or nitrate is found in stream water, however, suggesting that nitrogen is removed from solution before it enters bulk stream flow. It is important to determine whether the nitrogen removed from solution is exported from the ecosystem in a gaseous form by nitrification or denitrification or whether it is recycled internally by plant uptake. It is hypothesized that in these tropical streams: 1) available stream-side inorganic nitrogen supports an active nitrification- denitrification couple and that 2) this couple has a major influence on in-stream nitrogen chemistry. Furthermore, it is hypothesized that flow paths through the stream-side zone will have a predictable influence on nitrogen dynamics. To investigate these hypotheses work will be performed: 1) to established transect of observation wells on stream reaches with differing stream-side hydraulic characteristics and measure initial concentrations of inorganic and organic nitrogen at these sites; 2) test the effects of nitrogen enrichments on nitrogen chemistry at these sites; and 3) estimate the potential for nitrification and denitrification in laboratory soil and sediment systems, then quantify specific mechanisms of nitrogen loss with nitrogen-15 tracer techniques. This combination of field measurements, field manipulations, and laboratory experiments will enable us to examine the relative importance and spatial characteristics of nitrification, denitrification, and plant uptake in typical tropical stream ecosystems.