The real and apparent priming effects following litter incorporation as modeled under different moisture regimes

Abstract

The addition of litter (maize straw) to soil accelerates activity of microorganisms and significantly changes the mineralization of soil organic carbon (SOC), which are referred to as “apparent” and “real” priming effects (PEs). In this study, the mechanisms of PEs under different moisture conditions were explored using a simple mechanistic model that was dependent on the size of the SOC pool (%C), microbial biomass, and microbial activity (the proportion of actively growing microbial biomass relative to the total soil biomass). In an incubation experiment, soil cores were incubated with 13C-labeled maize straw (δ13C = 600.9 ± 2.53‰) under two moisture regimes: constant moisture (60% of water holding capacity throughout incubation) and drying and rewetting treatments (7-day long drying and rewetting cycles in a 76-day laboratory incubation). Microcosms were sampled after 1, 7, 10, 15, 21, 60, and 76 days, and cumulative soil respiration, microbial biomass and cumulative PEs were measured during the incubation period. The model was parameterized with native SOC and 13C-labeled litter carbon (C) data and simulated microbial metabolic processes, microbial turnover, soil organic matter (SOM) dynamics, and apparent and real PEs. The model contained two special features. The first feature was the assumption of sequential utilization of native SOC and litter-derived C with distinctly different stability and properties, where the organic carbon (OC) must first be solubilized into dissolved organic carbon (DOC) before it can be consumed by microorganisms. The second feature was the first attempt to evaluate the continuous turnover of native SOC, native SOM-derived and litter-derived microbial biomass C under varying moisture conditions. The optimized model explained 93% of the observed cumulative priming effect (CPE) under constant moisture and 94% of the CPE under drying and rewetting treatments, respectively. The simulation results showed that the conversion of native SOC and litter C to DOC was rate-limited, and positive apparent PE dominated the priming process. Although more C was allocated to respiration rather than growth, the compensation by a longer turnover time of microbial C eventually led to a higher sequestration of soil C under the drying and rewetting treatment compared to those under constant moisture. The turnover time of native SOC was significantly affected by the turnover of native-SOM derived and litter-derived microbial biomass C, and the turnover of litter-derived microorganisms may play a key role in regulating real PE. This study demonstrates that the combination of mathematical modeling and 13C-labeled litter varying in solubility and recalcitrance can effectively distinguish C sources and elucidate the mechanisms of PEs in soils under different moisture regimes.