Abstract
Microbial assimilation and stabilization of soil organic carbon (SOC) is an important process in global carbon cycling. For an improved understanding of climate-induced changes in ecosystem C dynamics, it is important to know the group-specific turnover of microbial C. Consequently, we wanted to answer the questions if fungi store newly assimilated C longer than bacteria and if climatic and edaphic properties of different regions affect microbial C assimilation and its subsequent release. This study presents results from a 112-day long field experiment where endogenous soil microorganisms were labelled with 13C labelled glucose to follow the dynamics of newly assimilated C in two study regions (Kraichgau and Swabian Alb). Whereas microbial assimilation of newly added C was higher in Kraichgau than in Swabian Alb, the opposite result was obtained for the mean residence time (MRT) of microbial biomass C (76 days in Kraichgau and 93 days in Swabian Alb). The accelerated turnover rates of microbial C in the warmer soils of Kraichgau with lower clay content might be an important mechanism explaining the differences in SOC content between both regions. Gram-positive bacteria assimilated more 13C-glucose into their biomass than fungi and the MRT of C was higher in bacteria as compared to fungi in both regions. Beside these dynamic substrate utilization strategies, we could proof cross feeding by gram-negative bacteria. Carbon MRT in fungi was region specific and was best represented by a two-pool model; the initial MRT ranged between 5 to 39 days and was, in the end, higher than 4 years. This provides evidence of a possible shift in fungal community composition; fast growing fungi dominant in the early phase and internal redistribution of C in the second phase of decomposition. Our study identified microbial group and region specific MRT of freshly assimilated C as an important parameter, which might help to explain the commonly found variation in soil respiration and SOC stabilization.
Highlights
Microbial assimilation of soil organic carbon and its subsequent turnover are important processes in global carbon cycling (Bardgett et al, 2008; Xu et al, 2014), since they determine the magnitude of microbial biomass in soils and control processes leading to soil carbon stabilization
The single exponential decay model resulted in 76 days mean residence time (MRT) for 13C in microbial biomass in Kraichgau, which was lower than the MRT of 93 days in Swabian Alb (Table 2)
Our study provides important information on the turnover times of individual microbial groups representing different pathways of in situ soil organic carbon (SOC) decomposition and stabilization, which might improve our conceptual understanding of SOC
Summary
Microbial assimilation of soil organic carbon and its subsequent turnover are important processes in global carbon cycling (Bardgett et al, 2008; Xu et al, 2014), since they determine the magnitude of microbial biomass in soils and control processes leading to soil carbon stabilization. Mechanisms and controls in microbial carbon assimilation and turnover play a fundamental role in Microbial Functions in Carbon Cycling regulating land-atmosphere interactions (Bardgett et al, 2008; Trivedi et al, 2013). Microbial-derived C may play an important role in SOC stabilization (Sokol et al, 2019), the factors regulating the release of C from the microbial biomass are still poorly understood (Throckmorton et al, 2012; Wieder et al, 2013). Conceptual models that describe soil C turnover dynamics often consider only pool sizes and their average turnover rates (Schweigert et al, 2015); turnover times of individual microbial groups representing different pathways of in situ SOC decomposition and stabilization might improve our conceptual understanding of SOC dynamics
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