Hou Jianfeng, Dijkstra Feike A, Zhang Xiuwei, Wang Chao, Lü Xiaotao, Wang Peng, Han Xingguo, Cheng Weixin
Erguna Forest-Steppe Ecotone Research Station, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, Liaoning, China.
University of Chinese Academy of Sciences, Beijing, Beijing, China.
PeerJ. 2019 Apr 9;7:e6712. doi: 10.7717/peerj.6712. eCollection 2019.
Soil microbial processes are crucial for understanding the ecological functions of arid and semi-arid lands which occupy approximately 40% of the global terrestrial ecosystems. However, how soil microbial metabolic activities may change across a wide aridity gradient in drylands remains unclear. Here, we investigated three soil microbial metabolic indices (soil organic carbon (SOC)-based microbial respiration, metabolic quotient, and microbial biomass as a proportion of total SOC) and the degree of carbon limitation for microbial respiration along a 3,200 km transect with a wide aridity gradient. The aridity gradient was customarily expressed using the aridity index (AI) which was calculated as the ratio of mean annual precipitation to mean annual evaporation, therefore, a lower AI value indicated a higher degree of aridity. Our results showed non-linear relationships between AI values and the metabolic indices with a clear aridity threshold for each of the three metabolic indices along the aridity gradient, respectively (AI = 0.13 for basal respiration, AI = 0.17 for metabolic quotient, and AI = 0.17 for MBC:SOC ratio). These metabolic indices linearly declined when AI was above the thresholds, but did not show any clear patterns when AI was below the thresholds. We also found that soil microbial respiration was highly limited by available carbon substrates at locations with higher primary production and relatively lower level of water limitation when AI was above the threshold, a counter-intuitive pattern that microbes were more starved in ecosystems with more substrate input. However, the increasing level of carbon limitation did correspond to the declining trend of the three metabolic indices along the AI gradient, which indicates that the carbon limitation influences microbial metabolism. We also found that the ratio of microbial biomass carbon to SOC in arid regions (AI < 0.2) with extremely low precipitation and primary production were not quantitatively related to SOC content. Overall, our results imply that microbial metabolism is distinctively different in arid lands than in non-arid lands.
土壤微生物过程对于理解占全球陆地生态系统约40%的干旱和半干旱土地的生态功能至关重要。然而，在旱地广泛的干旱梯度上，土壤微生物代谢活动如何变化仍不清楚。在此，我们沿着一条具有广泛干旱梯度的3200公里样带，研究了三种土壤微生物代谢指标（基于土壤有机碳（SOC）的微生物呼吸、代谢商以及微生物生物量占总SOC的比例）以及微生物呼吸的碳限制程度。干旱梯度通常用干旱指数（AI）表示，它被计算为年平均降水量与年平均蒸发量的比值，因此，较低的AI值表明干旱程度较高。我们的结果表明，AI值与代谢指标之间存在非线性关系，沿着干旱梯度，三种代谢指标各自都有一个明确的干旱阈值（基础呼吸的AI = 0.13，代谢商的AI = 0.17，MBC:SOC比值的AI = 0.17）。当AI高于阈值时，这些代谢指标呈线性下降，但当AI低于阈值时，没有显示出任何明显的模式。我们还发现，当AI高于阈值时，在初级生产力较高且水分限制相对较低的地点，土壤微生物呼吸受到可用碳底物的高度限制，这是一种与直觉相反的模式，即在有更多底物输入的生态系统中微生物更饥饿。然而，碳限制程度的增加确实与沿着AI梯度的三种代谢指标的下降趋势相对应，这表明碳限制影响微生物代谢。我们还发现，在降水和初级生产力极低的干旱地区（AI < 0.2），微生物生物量碳与SOC的比值与SOC含量没有定量关系。总体而言，我们的结果表明，干旱地区的微生物代谢与非干旱地区明显不同。
