Microbial growth kinetics under deeply vs. shallow rooted plants along soil depths

Abstract:

Climate smart land management practices that replace shallow-rooted annual crop systems with deeply-rooted perennial plants can contribute to soil carbon sequestration. However, the mechanisms that underlie deep soil carbon accrual remain unknown. Our key question is how a change in crop type affects the activity and growth kinetics of soil microorganisms that use root exudates and decompose soil organic carbon in deep soils when compared to surface soils. Here, we used a growth kinetics model with a modified lab incubation experiment to estimate how microbial growth parameters vary along 240 cm of soil depth in profiles under shallow- (soybean) and deeply- rooted plants (switchgrass) decades after land conversion. We also assessed resource availability (total carbon and nitrogen) along the soil profiles to infer mechanisms that control the observed patterns in microbial growth parameters. In general, microbial growth parameters and resource availability were significantly influenced by depth, but very little by vegetation type. For example, total and relative active microbial biomass decreased as resource availability declined with soil depth. Decreases in the relative active microbial biomass increased lag time (response time to external carbon inputs) along the soil profiles. In contrast, maximum specific growth rate did not differ across vegetation systems and depth gradients. Our results suggest that -apart from shifts in lag time- microbial growth potential is similar regardless of soil depth and resource availability, and that dormancy becomes more common as soil depth increases. This study implies that > 10 years of change in vegetation and rooting depth may not be long enough to alter maximum microbial growth rates, and also suggests that a component of the microbial community in deep soils is capable of growing rapidly in response to added resources.