Comparing microbial and chemical approaches for modelling soil organic carbon decomposition using the DecoChem v1.0 and DecoBio v1.0 models

Soil organic matter is a vast store of carbon, with a critical role in the global carbon cycle. Despite its importance the dynamics of soil organic carbon decomposition, under the impact of climate change or changing litter inputs, are poorly understood. Current biogeochemical models usually lack microbial processes and thus miss an important feedback when considering the fate of carbon. Here we use a series of modelling experiments to evaluate two different model structures, one with a standard first order kinetic representation of soil decomposition (DecoChem v1.0, hearafter chemical model) and one with control of soil decomposition through microbial activity (DecoBio v1.0, hereafter biological model). We tested two hypotheses. First, that increased litter inputs and glucose addition prime microbial activity and reduce soil carbon stocks in the biological model, but increase C stocks in the chemical model. Experiments provided some support for this hypothesis, with soil C stocks increasing in the chemical model in response to litter increases. In the biological model, responses to changed litter quantity were more rapid, but with the residence time of soil C altering such that soil C stocks were buffered. However, in the biological model there was a strong response to increased glucose additions (i.e., changes in litter quality), with significant losses to soil C stocks over time, driven by priming. Secondly, we hypothesised that warming will stimulate decomposition in the chemical model, and loss of C, but in the biological model soil C will be less sensitive to warming, due to complex microbial feedbacks. The experiments supported this hypothesis, with the chemical model soil C residence times and steady state C stocks adjusting strongly with temperature changes, extending over decades. On the other hand, the biological model showed a rapid response to temperature that subsided after a few years, with total soil C stocks largely unchanged. The microbial model shows qualitative agreement with experimental warming studies, that found transient increases in soil respiration that decline within a few years. In conclusion, the biological model is largely buffered against bulk changes in litter inputs and climate, unlike the chemical model, while the biological model displays a strong priming response to additions of labile litter. Our result have therefore highlighted significantly different sensitivities between chemical and biological modelling approaches for soil decomposition.