Soil organic matter (SOM) content is a key indicator of riparian soil functioning and in the provision of ecosystem services such as water retention, flood alleviation, pollutant attenuation and carbon (C) sequestration for climate change mitigation. Here, we studied the importance of microbial biomass and nutrient availability in regulating SOM turnover rates. C stabilisation in soil is expected to vary both vertically, down the soil profile and laterally across the riparian zone. In this study, we evaluated the influence of five factors on C mineralisation (Cmin): (i) substrate quantity, (ii) substrate quality, (iii) nutrient (C, N and P) stoichiometry, (iv) soil microbial activity with proximity to the river (2 to 75 m) and (v) as a function of soil depth (0–3 m). Substrate quality, quantity and nutrient stoichiometry were evaluated using high and low molecular weight 14C-labelled dissolved organic (DOC) along with different nutrient additions. Differences in soil microbial activity with proximity to the river and soil depth were assessed by comparing initial (immediate) Cmin rates and cumulative C mineralised at the end of the incubation period. Overall, microbial biomass C (MBC), organic matter (OM) and soil moisture content (MC) proved to be the major factors controlling rates of Cmin at depth. Differences in the immediate and medium-term response (42 days) of Cmin suggested that microbial growth increased and carbon use efficiency (CUE) decreased down the soil profile. Inorganic N and/or P availability had little or no effect on Cmin suggesting that microbial community growth and activity is predominantly C limited. Similarly, proximity to the watercourse also had relatively little effect on Cmin. This work challenges current theories suggesting that areas adjacent to watercourse process C differently from upslope areas. In contrast, our results suggest that substrate quality and microbial biomass are more important in regulating C processing rates rather than proximity to a river.

中文翻译：

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沿河岸山坡土壤深度对碳微生物加工的化学计量约束

土壤有机质 (SOM) 含量是河岸土壤功能和提供生态系统服务的关键指标，如保水、减轻洪水、污染物衰减和碳 (C) 封存以减缓气候变化。在这里，我们研究了微生物生物量和养分可用性在调节 SOM 周转率方面的重要性。土壤中的碳稳定预计会在垂直方向、沿土壤剖面向下和横向穿过河岸区发生变化。在本研究中，我们评估了五个因素对 C 矿化 (Cmin) 的影响：(i) 底物数量，(ii) 底物质量，(iii) 养分（C、N 和 P）化学计量，（iv）土壤微生物活性与靠近河流（2 至 75 m）和（v）作为土壤深度（0-3 m）的函数。基材质量，使用高和低分子量 14C 标记的溶解有机物 (DOC) 以及不同的营养添加物来评估数量和营养化学计量。通过比较初始（即时）Cmin 速率和潜伏期结束时矿化的累积 C，评估靠近河流和土壤深度的土壤微生物活动的差异。总体而言，微生物生物量 C (MBC)、有机质 (OM) 和土壤水分含量 (MC) 被证明是控制深度 Cmin 速率的主要因素。Cmin 的即时和中期响应（42 天）的差异表明微生物生长增加，碳利用效率 (CUE) 沿着土壤剖面下降。无机氮和/或磷的有效性对 Cmin 几乎没有影响，这表明微生物群落的生长和活动主要受 C 限制。同样，靠近水道对 Cmin 的影响也相对较小。这项工作挑战了当前的理论，这些理论认为靠近水道的区域处理 C 与上坡区域不同。相比之下，我们的结果表明底物质量和微生物生物量在调节碳处理速率方面比靠近河流更重要。