Atmospheric nitrogen (N) deposition represents an important input of N into natural ecosystems such as semiarid shrublands of Southern California, which can receive up to 45 kg N ha⁻¹ y⁻¹. These N inputs will presumably alter soil microbial abundance and composition and impact ecosystem N and carbon (C) cycles. We used a 16S rRNA sequencing-based approach to characterize shifts in soil bacterial communities in chaparral and coastal sage scrub (CSS) shrublands that received annual inputs of 50 kg N ha⁻¹ over a period of 14 years. Experimental N addition caused shifts in bacterial taxonomic composition in these shrublands. CSS exposed to N had an increase in Proteobacteria and Bacteriodetes, while N inputs to chaparral caused an increase in Bacteriodetes and Firmicutes and a decrease in Acidobacteria. Canonical correspondence analysis (CCA) indicated that extractable NH₄ and/or NO₃ concentrations were a strong predictor of Proteobacteria and Firmicutes (positive) and Acidobacteria and Verrucomicrobia (negative) abundance. Increases in soil pH were coincident with declines in Proteobacteria but increases in Acidobacteria, while increases in total C were positively correlated with Acidobacteria abundance. These results support the hypothesis that long-term N inputs in semi-arid shrublands promote the growth of copiotrophic taxa, such as Proteobacteria, Bacteroidetes, and Firmicutes, and inhibit the growth of oligotrophic taxa like Acidobacteria. Nitrogen addition failed to affect α-diversity at the phylum level but significantly increased α-diversity of bacterial genera, and indicator species analyses revealed more genera associated with N treatment plots (125) than control plots (91). These results imply that future increases in N deposition will alter soil microbial abundance and community composition, and in turn, affect ecosystem C and nutrient cycling in these semi-arid shrublands.

大气氮 (N) 沉降代表了 N 进入自然生态系统的重要输入，例如南加州半干旱灌木丛，可接收高达 45 kg N ha ^-1  y ^-1。这些氮输入可能会改变土壤微生物的丰度和组成，并影响生态系统的氮和碳 (C) 循环。我们使用基于 16S rRNA 测序的方法来表征灌木丛和沿海鼠尾草灌丛 (CSS) 灌木丛中土壤细菌群落的变化，这些灌木丛每年接受 50 kg N ha ^-1在 14 年的时间里。实验性的氮添加引起了这些灌木丛中细菌分类组成的变化。暴露于 N 的 CSS 增加了变形菌门和拟杆菌门，而向灌木丛输入 N 导致拟杆菌门和厚壁菌门增加，而酸杆菌门减少。典型对应分析 (CCA) 表明可萃取的 NH ₄和/或 NO ₃浓度是变形菌门和厚壁菌门（阳性）和酸杆菌门和疣微菌（阴性）丰度的强预测因子。土壤 pH 值的增加与 Proteobacteria 的下降同时发生，但 Acidobacteria 的增加，而总 C 的增加与 Acidobacteria 的丰度呈正相关。这些结果支持这样的假设，即半干旱灌丛中的长期氮输入促进了变形菌门、拟杆菌门和厚壁菌门等富营养类群的生长，并抑制了酸杆菌门等贫营养类群的生长。氮添加未能影响门水平的 α 多样性，但显着增加了细菌属的 α 多样性，指示物种分析显示与 N 处理地块 (125) 相关的属比对照地块 (91) 更多。