US Geological Survey, Southwest Biological Science Center, Moab, Utah, USA.
USDA-ARS Rangeland Resource and Systems Research Unit, Crops Research Laboratory, Fort Collins, Colorado, USA.
Ecology. 2024 Sep;105(9):e4393. doi: 10.1002/ecy.4393. Epub 2024 Aug 5.
For many drylands, both long- and short-term drought conditions can accentuate landscape heterogeneity at both temporal (e.g., role of seasonal patterns) and spatial (e.g., patchy plant cover) scales. Furthermore, short-term drought conditions occurring over one season can exacerbate long-term, multidecadal droughts or aridification, by limiting soil water recharge, decreasing plant growth, and altering biogeochemical cycles. Here, we examine how experimentally altered seasonal precipitation regimes in a mixed shrub grassland on the Colorado Plateau impact soil moisture, vegetation, and carbon and nitrogen cycling. The experiment was conducted from 2015 to 2019, during a regional multidecadal drought event, and consisted of three precipitation treatments, which were implemented with removable drought shelters intercepting ~66% of incoming precipitation including: control (ambient precipitation conditions, no shelter), warm season drought (sheltered April-October), and cool season drought (sheltered November-March). To track changes in vegetation, we measured biomass of the dominant shrub, Ephedra viridis, and estimated perennial plant and ground cover in the spring and the fall. Soil moisture dynamics suggested that warm season experimental drought had longer and more consistent drought legacy effects (occurring two out of the four drought cycles) than either cool season drought or ambient conditions, even during the driest years. We also found that E. viridis biomass remained consistent across treatments, while bunchgrass cover declined by 25% by 2019 across all treatments, with the earliest declines noticeable in the warm season drought plots. Extractable dissolved inorganic nitrogen and microbial biomass nitrogen concentrations appeared sensitive to seasonal drought conditions, with dissolved inorganic nitrogen increasing and microbial biomass nitrogen decreasing with reduced soil volumetric water content. Carbon stocks were not sensitive to drought but were greater under E. viridis patches. Additionally, we found that under E. viridis, there was a negative relationship between dissolved inorganic nitrogen and microbial biomass nitrogen, suggesting that drought-induced increases in dissolved inorganic nitrogen may be due to declines in nitrogen uptake from microbes and plants alike. This work suggests that perennial grass plant-soil feedbacks are more vulnerable to both short-term (seasonal) and long-term (multiyear) drought events than shrubs, which can impact the future trajectory of dryland mixed shrub grassland ecosystems as drought frequency and intensity will likely continue to increase with ongoing climate change.
对于许多旱地而言，无论是长期还是短期的干旱条件，都会在时间（例如季节性模式的作用）和空间（例如斑块状植物覆盖）尺度上加剧景观异质性。此外，一个季节内发生的短期干旱条件可能会通过限制土壤水分补给、减少植物生长以及改变生物地球化学循环，从而加剧长期的、数十年的干旱或干旱化。在这里，我们研究了科罗拉多高原上混合灌丛草原中季节性降水变化如何影响土壤水分、植被以及碳氮循环。该实验于 2015 年至 2019 年进行，正值区域多年干旱事件期间，包括三个降水处理，使用可移动的干旱避难所来拦截约 66%的入射降水，这些避难所包括：对照（环境降水条件，无避难所）、暖季干旱（4 月至 10 月避难所）和冷季干旱（11 月至 3 月避难所）。为了跟踪植被的变化，我们测量了优势灌木麻黄的生物量，并在春季和秋季估计了多年生植物和地面覆盖率。土壤水分动态表明，与冷季干旱或环境条件相比，暖季实验干旱具有更长且更一致的干旱遗留效应（在四个干旱周期中有两个发生），即使在最干旱的年份也是如此。我们还发现，麻黄的生物量在整个处理过程中保持不变，而丛生草的覆盖率在 2019 年所有处理中下降了 25%，在暖季干旱处理区最早出现下降。可提取的溶解无机氮和微生物生物量氮浓度似乎对季节性干旱条件敏感，随着土壤体积含水量的降低，溶解无机氮增加，微生物生物量氮减少。碳储量对干旱不敏感，但在麻黄斑块下更大。此外，我们发现，在麻黄下，溶解无机氮和微生物生物量氮之间存在负相关关系，这表明干旱引起的溶解无机氮增加可能是由于微生物和植物对氮的吸收减少所致。这项工作表明，多年生草本植物-土壤反馈比灌木更容易受到短期（季节性）和长期（多年）干旱事件的影响，这可能会影响旱地混合灌丛草原生态系统的未来轨迹，因为随着气候变化的持续，干旱的频率和强度可能会继续增加。
