Addition of isotopically labeled dissolved forms of nitrogen (N) and carbon (C) in streams has revealed that microbes associated with leaf litter can assimilate solutes from the water column. However, whether this assimilation varies among leaf litter species and at different decomposition stages is still largely unknown. Here, we explore differences in the uptake of NH₄‐N and acetate from water column among leaf litter of five riparian tree species at five different times of decomposition. To do so, we conducted a 24 h addition of ¹⁵N‐NH₄ and ¹³C‐acetate at constant rate to estimate the microbial uptake of these solutes from the water column. We measured microbial cellobiohydrolase activity and decomposition rates (k) associated with each leaf litter type to examine their influence on NH₄‐N and acetate uptake. We found that mass specific N‐uptake (μg N mg N⁻¹ d⁻¹) was relatively constant during leaf litter decomposition whereas mass specific C‐uptake (mg C g C⁻¹ d⁻¹) increased over time. Among leaf litter species, cumulative values of N‐uptake during the study period were negatively related to leaf litter k whereas cumulative values of C‐uptake were positively related to k. Our study indicates that leaf litter species and their decomposition stage are important for understanding the reliance of leaf‐associated microbes on dissolved inorganic nitrogen and dissolved organic carbon from the water column.

中文翻译：

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垫料相关微生物对水柱中氮和碳的微生物吸收量在垫料物种之间有所不同

在流中添加同位素标记的溶解态氮（N）和碳（C）的方法表明，与枯枝落叶相关的微生物可以吸收水柱中的溶质。但是，这种同化是否在不同的凋落物物种之间以及在不同的分解阶段有所不同，仍然是一个未知数。在这里，我们探讨了在五个不同的分解时间下，五种河岸树种的凋落物从水柱中吸收NH ₄ ‐N和乙酸盐的差异。为此，我们以恒定速率进行了24小时的¹⁵ N-NH ₄和¹³ C-乙酸盐的添加，以估算水柱中这些溶质的微生物吸收量。我们测量了微生物纤维二糖水解酶的活性和分解率（k）与每种凋落物类型相关联，以检查它们对NH ₄ -N和乙酸盐吸收的影响。我们发现，在凋落物分解过程中，特定质量氮的吸收量（μg N mg N ^-1 d ^-1）相对恒定，而特定质量C吸收量（mg C g C ^-1 d ^-1）随时间增加。在叶片凋落物物种中，研究期间氮素吸收的累积值与叶片凋落物k负相关，而叶片碳素吸收的累积值与k负相关。。我们的研究表明，落叶凋落物的种类及其分解阶段对于了解叶片相关微生物对水柱中溶解的无机氮和溶解的有机碳的依赖性很重要。