Different microbial responses in top‐ and sub‐soils to elevated temperature and substrate addition in a semiarid grassland on the Loess Plateau

Abstract

The Loess Plateau soil in northwest China originated from wind sediments and is characterized by deep soil profiles and large organic carbon (C) content. Severe soil erosion constantly exposes deep soils to the surface, making the organic C vulnerable to microbial decomposition. Few, however, have so far examined how soil microbial activity and community composition in the deep loess soil respond to perturbations. We examined microbial responses in three layers of a clay‐loam loess (topsoil, 0–20 cm; midsoil, 40–60 cm; subsoil, 80–100 cm) to substrate additions (0.8 g glucose‐C kg−1 soil) under two temperature regimes (25 and 35°C). Soil C:N ratio was significantly larger in the subsoil (20.3) than topsoil (7.4). Glucose addition significantly increased CO2 efflux during a 30‐day incubation period and the relative magnitude of the increase was four times larger in the subsoil than topsoil. The temperature sensitivity (Q10) of soil CO2 efflux increased significantly with soil depth in the absence of glucose addition (i.e., ambient soil), but it decreased under glucose addition. Also, glucose addition significantly increased phenol oxidase and peroxidase activities in the subsoil, which might contribute to the stimulation of microbial CO2 efflux. Composition of the microbial community was more affected by temperature increase in the topsoil, but more responsive to labile C addition in the subsoil. Together, these results indicated that the composition of soil communities and microbial activities in the topsoil and deep soil responded differently to warming and labile C input. Our findings suggest that organic C in deep loess soils can be highly sensitive to environmental changes, emphasizing the need for more long‐term monitoring and quantitative assessment of organic C release from this important C pool. Highlights Microbial responses to labile C and warming were examined along a Loess Plateau soil profile. Microbial respiration was more responsive to C addition and warming in deep soil than topsoil. Microbial composition and activity were sensitive to temperature in the topsoil but to labile C in the subsoil. Climate change may facilitate CO2 efflux from deep Loess Plateau soils.

Trends in Microbial Community Composition and Function by Soil Depth

“…In addition to diversity metrics, changes in the soil microbiome over depth are reflected in the abundance patterns of broad groups of microbes, with one example being the ratio of Gram-positive to Gram-negative bacteria. While actinomycetes and other Gram-positive bacteria have been shown to be common in deeper soils [ 15 ], Gram-negative bacteria typically decline with depth [ 5 , 13 , 17 , 81 , 82 ]. These trends are likely linked to nutritional preferences.…”

“…It should be noted that lower overall activity may simply be a consequence of reduced microbial biomass, and that normalized levels (e.g., normalizing activity against total microbial biomass carbon) in subsoils can be comparable to or even higher than those for surface soils [ 17 , 18 , 156 ]. For example, C mineralization may occur at similar rates [ 2 ] or even more rapidly for subsurface microbes upon substrate addition [ 3 , 81 , 164 ], which is likely related to how chronically C-starved they are. At six soil depths on a riparian hillslope, normalized activity rates were higher for deeper soils than surface soils, and peaked around 50–100 cm [ 156 ].…”

Unaltered soil microbial community composition, but decreased metabolic activity in a semiarid grassland after two years of passive experimental warming

“…For example, the warming treatment directly led to rapid shifts in the structure of the soil microbial community with significantly increased abundance of actinomycete biomarkers and decreased abundance of fungi (Xiong et al., 2016). Warming may alter soil pH, soil resource availability, plant community composition, and plant production (Bai et al., 2019; Fang et al., 2018; Guan et al., 2018; Li et al., 2017; Xu et al., 2013; Zi et al., 2018), which can all affect the soil microbial community (Feng & Simpson, 2009; C. Wang et al., 2017; Xiong et al., 2016; Xue et al., 2016). For example, soil pH positively correlated with fungi:bacteria ratio (Xiong et al., 2016).…”

Whole-soil warming decreases abundance and modifies the community structure of microorganisms in the subsoil but not in surface soil

“…In addition to SOC availability, temperature (Bradford, 2013) and changes in soil moisture associated with warming (Manzoni et al, 2012) likely affect microbial abundance. We did observe a similar respiration response at all depths (Hicks-Pries et al 2017), indicating that the microbial response might be similar despite the difference in warming magnitude between the top 20 cm and the lower horizons.…”