Anthropogenic nitrogen (N) deposition is significantly altering both community structure and ecosystem processes in terrestrial ecosystems across the globe. However, our understanding of the consequences of N deposition in dryland systems remains relatively poor, despite evidence that drylands may be particularly vulnerable to increasing N inputs. In this study, we investigated the influence of 7 years of multiple levels of simulated N deposition (0, 2, 5, and 8 kg N ha⁻¹ year⁻¹) on plant community structure and biological soil crust (biocrust) cover at three semi-arid grassland sites spanning a soil texture gradient. Biocrusts are a surface community of mosses, lichens, cyanobacteria, and/or algae, and have been shown to be sensitive to N inputs. We hypothesized that N additions would decrease plant diversity, increase abundance of the invasive annual grass Bromus tectorum, and decrease biocrust cover. Contrary to our expectations, we found that N additions did not affect plant diversity or B. tectorum abundance. In partial support of our hypotheses, N additions negatively affected biocrust cover in some years, perhaps driven in part by inter-annual differences in precipitation. Soil inorganic N concentrations showed rapid but ephemeral responses to N additions and plant foliar N concentrations showed no response, indicating that the magnitude of plant and biocrust responses to N fertilization may be buffered by endogenous N cycling. More work is needed to determine N critical load thresholds for plant community and biocrust dynamics in semi-arid systems and the factors that determine the fate of N inputs.

人为氮（N）沉积正在显着改变全球陆地生态系统中的群落结构和生态系统过程。然而，尽管有证据表明干旱地区可能特别容易受到氮素输入的增加，但我们对干旱地区系统中氮素沉积后果的理解仍然相对较差。在这项研究中，我们调查了7年多水平的模拟N沉积（0、2、5和8 kg N ha - ¹ 年^{-1的影响）}）在跨越土壤质地梯度的三个半干旱草原站点的植物群落结构和生物土壤地壳（biocrust）覆盖上。生物结皮是苔藓，地衣，蓝细菌和/或藻类的表面群落，并且已显示对氮输入敏感。我们假设添加N会减少植物多样性，增加一年生入侵草Bromus tectorum的丰度，并减少生物结皮。与我们的预期相反，我们发现添加N不会影响植物多样性或B. tectorum丰富。在部分支持我们的假设的情况下，N的添加在几年中对生物结皮层产生了负面影响，这可能部分是由于年际降水差异造成的。土壤无机氮浓度显示出快速但短暂的响应，对氮的添加和植物叶面氮浓度没有显示响应，表明植物和生物结皮对氮肥的响应幅度可以通过内源氮循环来缓冲。为确定半干旱系统中植物群落和生物结壳动力学的N个临界负荷阈值以及决定N个输入命运的因素，还需要做更多的工作。