Kinetic Properties of Microbial Exoenzymes Vary With Soil Depth but Have Similar Temperature Sensitivities Through the Soil Profile

The vertical distribution and control factor of microbial biomass and bacterial community at macroecological scales

Microorganisms exist throughout the soil profile and those microorganisms living in deeper soil horizons likely play key roles in regulating biogeochemical processes. However, the vertical differentiations of microbes along soil depth and their global biogeographical patterns remain poorly understood. Herein, we conducted a global meta-analysis to clarify the vertical changes of microbial biomass, diversity, and microbial relative abundance across the soil profiles. Data was collected from 43 peer-reviewed articles of 110 soil profiles (467 observations in total) from around the world. We found soil microbial biomass and bacterial diversity decreased with depth in soils. Among examined edaphic factors, the depth variation in soil pH exhibited significant negative associations with the depth change in microbial biomass and bacterial Shannon index, while soil total organic carbon (TOC) and total nitrogen (TN) exhibited significant positive associations. For the major bacteria phyla, the relative abundances of Proteobacteria and Bacteroidetes decreased with soil depth, while Chloroflexi, Gemmatimonadetes, and Nitrospirae increased. We found both parallels and differences in the biogeographical patterns of microbial attribute of topsoil vs. subsoil. Microbial biomass was significantly controlled by the soil nutrient concentrations in both topsoil and subsoil compared with climatic factors, while bacterial Shannon index was significantly controlled by the edaphic factors and across latitudes or climatic factors. Moreover, mean annual precipitation can also be used as a predictor of microbial biomass in subsoil which is different from topsoil. Collectively, our results provide a novel integrative view of how microbial biomass and bacterial community response to soil depth change and clarify the controlling factors of the global distribution patterns of microbial biomass and diversity, which are critical to enhance ecosystem simulation models and for formulating sustainable ecosystem management and conservation policies.

The carbon-quality temperature hypothesis: Fact or artefact?

Climate warming can reduce global soil carbon stocks by enhancing microbial decomposition. However, the magnitude of this loss remains uncertain because the temperature sensitivity of the decomposition of the major fraction of soil carbon, namely resistant carbon, is not fully known. It is now believed that the resistance of soil carbon mostly depends on microbial accessibility of soil carbon with physical protection being the primary control of the decomposition of protected carbon, which is insensitive to temperature changes. However, it is still unclear whether the temperature sensitivity of the decomposition of unprotected carbon, for example, carbon that is not protected by the soil mineral matrix, may depend on the chemical recalcitrance of carbon compounds. In particular, the carbon-quality temperature (CQT) hypothesis asserts that recalcitrant low-quality carbon is more temperature-sensitive to decomposition than labile high-quality carbon. If the hypothesis is correct, climate warming could amplify the loss of unprotected, but chemically recalcitrant, carbon and the resultant CO₂ release from soils to the atmosphere. Previous research has supported this hypothesis based on reported negative relationships between temperature sensitivity and carbon quality, defined as the decomposition rate at a reference temperature. Here we show that negative relationships can arise simply from the arbitrary choice of reference temperature, inherently invalidating those tests. To avoid this artefact, we defined the carbon quality of different compounds as their uncatalysed reaction rates in the absence of enzymes. Taking the uncatalysed rate as the carbon quality index, we found that the CQT hypothesis is not supported for enzyme-catalysed reactions, which showed no relationship between carbon quality and temperature sensitivity. The lack of correlation in enzyme-catalysed reactions implies similar temperature sensitivity for microbial decomposition of soil carbon, regardless of its quality, thereby allaying concerns of acceleration of warming-induced decomposition of recalcitrant carbon.

Successive walnut plantations alter soil carbon quantity and quality by modifying microbial communities and enzyme activities

Knowledge of the spatial–temporal variations of soil organic carbon (SOC) quantity and quality and its microbial regulation mechanisms is essential for long-term SOC sequestration in agroecosystems; nevertheless, this information is lacking in the process of walnut plantations. Here, we used the modified Walkley-Black method, phospholipid fatty acid analysis, and micro-plate enzyme technique to analyze the evolution of SOC stocks and quality/lability as well as microbial communities and enzyme activities at different soil depths in walnut plantations with a chronosequence of 0-, 7-, 14-, and 21-years in the Eastern Taihang Mountains, China. The results indicated that long-term walnut plantations (14-and 21-years) enhanced SOC stocks, improved SOC quality/lability (as indicated by the lability index), and promoted microbial growth and activities (i.e., hydrolase and oxidase activities) in the 0–40 cm soil layers. Besides, these above-mentioned SOC-and microbial-related indices (except for oxidase activities) decreased with increasing soil depths, while oxidase activities were higher in deeper soils (40–60 cm) than in other soils (0–40 cm). The partial least squares path model also revealed that walnut plantation ages and soil depths had positive and negative effects on microbial attributes (e.g., enzyme activities, fungal and bacterial communities), respectively. Meanwhile, the SOC stocks were closely related to the fungal community; meanwhile, the bacterial community affected SOC quality/liability by regulating enzyme activities. Comprehensively, long-term walnut plantations were conducive to increasing SOC stocks and quality through altering microbial communities and activities in the East Taihang Mountains in Hebei, China.