Data from: Aridity Modulates N Availability in Arid and Semiarid Mediterranean Grasslands

While much is known about the factors that control each component of the terrestrial nitrogen (N) cycle, it is less clear how these factors affect total N availability, the sum of organic and inorganic forms potentially available to microorganisms and plants. This is particularly true for N-poor ecosystems such as drylands, which are highly sensitive to climate change and desertification processes that can lead to the loss of soil nutrients such as N. We evaluated how different climatic, abiotic, plant and nutrient related factors correlate with N availability in semiarid Stipa tenacissima grasslands along a broad aridity gradient from Spain to Tunisia. Aridity had the strongest relationship with N availability, suggesting the importance of abiotic controls on the N cycle in drylands. Aridity appeared to modulate the effects of pH, plant cover and organic C (OC) on N availability. Our results suggest that N transformation rates, which are largely driven by variations in soil moisture, are not the direct drivers of N availability in the studied grasslands. Rather, the strong relationship between aridity and N availability could be driven by indirect effects that operate over long time scales (decades to millennia), including both biotic (e.g. plant cover) and abiotic (e.g. soil OC and pH). If these factors are in fact more important than short-term effects of precipitation on N transformation rates, then we might expect to observe a lagged decrease in N availability in response to increasing aridity. Nevertheless, our results suggest that the increase in aridity predicted with ongoing climate change will reduce N availability in the Mediterranean basin, impacting plant nutrient uptake and net primary production in semiarid grasslands throughout this region.

尽管学界已明确调控陆地氮（N）循环各组分的诸多影响因子，但对于这些因子如何作用于总氮有效性——即可供微生物与植物利用的有机态与无机态氮的总和——目前仍不甚明晰。这一情况在氮匮乏生态系统（如干旱区）中尤为突出：干旱区对气候变化与荒漠化过程高度敏感，而后者可引发土壤氮等养分的流失。本研究针对从西班牙至突尼斯的宽幅干旱梯度带内的半干旱韧针茅（Stipa tenacissima）草原，评估了气候、非生物、植物及养分相关因子与氮有效性的相关性。结果显示，干旱度与氮有效性的相关性最为显著，这表明非生物因子对干旱区氮循环的调控作用至关重要。干旱度似乎可调控pH值、植物盖度与有机碳（OC）对氮有效性的影响。本研究结果显示，主要受土壤水分变化驱动的氮转化速率，并非所研究草原中氮有效性的直接调控因子。反之，干旱度与氮有效性间的强相关性，可能由长期尺度（数十年至数千年）下的间接效应驱动，这类效应涵盖生物因子（如植物盖度）与非生物因子（如土壤有机碳、pH值）两类。若上述因子的作用确实强于降水对氮转化速率的短期效应，则可预期，随着干旱度升高，氮有效性将呈现滞后性下降。尽管如此，本研究结果表明，持续气候变化下预测的干旱度升高，将降低地中海盆地的氮有效性，进而影响该区域内所有半干旱草原的植物养分吸收与净初级生产力。