Pulses of Biogenic Nitrogen Cycling Lead to Atmospheric-Based Nutrient Spiraling in Southern California

在干旱地区、沙漠和水资源有限、高温频繁的地区，氮的循环研究具有挑战性，而且与目前对森林或草原等其他地区的理解不一致。在干旱地区，氮浓度普遍较低，许多生物过程受到湿润期氮供应的限制。然而，许多干旱土地生态系统也表现出高氮损失率，这是氮丰度地区的特征。在潮湿时期，土壤可能释放出高浓度的氮。由于这些氮脉冲，干旱地区可能在大气化学中发挥重要作用，尽管排放的幅度或命运尚不清楚。土壤氮脉冲返回土壤可能是一个以前未被考虑的营养源，有助于顺风氮沉积。针对不同景观特征的干旱区氮动态的新研究可能会解决这些明显的矛盾，有助于更全面的认识，并提高对氮富集的广泛影响的认识。这些发现将被传达给直接受大气氮影响的联邦和州机构，包括约书亚树国家公园。培训机会针对代表性不足的研究生和本科生，包括微量气体、同位素、基因组和建模工具。研究结果将通过加州大学合作推广和针对整个地区K-12教育工作者的年度公共讲座系列传达给当地社区。该项目将开发和测试脉冲诱导的营养螺旋假说，以解释南加州氮浓度和来源的景观分布。中心假设借鉴了河流系统中的营养动力学，其中下游运动导致生物地球化学螺旋？，因为营养物质从溶解形式和生物形式中循环。在干旱地区，假设气体氮排放的土壤脉冲与大气运输和沉积相结合，形成了这样一个螺旋，并在整个景观中传播氮富集。这一假设将在从加州河滨延伸到约书亚树国家公园东部边缘的氮沉积减少的样带上进行评估，在那里将进行重复实验，评估土壤氮脉冲，并通过同位素和基因组工具确定其来源。这些实验将验证土壤氮排放脉冲具有特征同位素特征的预测，并日益主导城市源下风的土壤氮含量。实验结果将用于区域大气运输和化学模型，该模型将模拟整个南加州的氮动力学。模型模拟将进一步验证土壤脉冲排放可以影响大气氮化学和随后的氮沉积的预测。

The cycling of nitrogen in arid lands, deserts, and areas with limited water and frequent high temperatures is challenging to study and does not fit within the current understanding of other areas such as forests or grasslands. In arid lands, nitrogen concentrations are generally low and many biological processes are limited by nitrogen availability during wet periods.  However, many arid land ecosystems also exhibit high rates of nitrogen loss more characteristic of areas with a nitrogen abundance.  During wet periods soils may emit high nitrogen concentrations. Because of these nitrogen pulses, arid lands may have an important role in atmospheric chemistry although the magnitudes or fate of emissions are not well known.  Return of the soil nitrogen pulses to the soil may be a previously unaccounted nutrient source contributing to downwind nitrogen deposition.  New research directed to characterizing arid land nitrogen dynamics across landscapes may resolve these apparent contradictions, contribute to more comprehensive understanding, and improve understanding of the broad-scale effects of nitrogen enrichment. The findings will be communicated to the federal and state agencies that are directly affected by atmospheric nitrogen including Joshua Tree National Park.  Training opportunities, targeted for underrepresented graduate and undergraduate students, include tools for trace gases, isotopes, genomes, and modeling.  The results will be communicated to the local community through UC Cooperative Extension and an annual public lecture series that targets K-12 educators throughout the region.  This project will develop and test a hypothesis of pulse-induced nutrient spiraling to explain the landscape distribution of nitrogen concentrations and sources in southern California.  The central hypothesis borrows from nutrient dynamics in stream systems where movement downstream leads to biogeochemical ?spirals?, as nutrients cycle from dissolved and biotic forms.  In arid lands, it is hypothesized that soil pulses of gaseous nitrogen emissions coupled with atmospheric transport and deposition forms such a spiral and propagates nitrogen enrichment across a landscape.   This hypothesis will be evaluated across a transect of decreasing nitrogen deposition extending from Riverside, CA to the eastern edge of Joshua Tree National Park where replicated experiments will be conducted that assess soil nitrogen pulses and identifies their sources through isotopic and genomic tools. These experiments will test predictions that pulses of soil nitrogen emissions have characteristic isotopic signatures and increasingly dominate soil nitrogen content downwind from urban sources.  The experimental results will be used to inform a regional atmospheric transport and chemistry model that will simulate nitrogen dynamics throughout southern California.  The model simulations will further test predictions that soil pulse emissions can influence atmospheric nitrogen chemistry and subsequent nitrogen deposition.