Modeling Of Priming Effect As Affected By Physical Protection And Microbial Turnover Following Crop Straw Incorporation

Plant litter incorporation is a common practice to replenish soil organic carbon?SOC?in agricultural lands.Plant litters incorporated in soils are transformed and physically protected from microbial access through occlusion and mineral-organic association.Meanwhile they may stimulate the turnover of microbial biomass and the decomposition of native SOC and hence counteract SOC replenishment.This stimulation,known as priming effect,is commonly regard as a couple effects of microbial co-metabolism and physicial protection.However,it remains unclear how these physical protection processes affect the priming effect.The priming effect after litter addition is divided into the apparent priming caused by microbial turnover and the real priming produced by soil organic carbon turnover which are not yet distinguish.Here,we present a novel model?PROCAS model?that can predict the dynamics and the sinks and sources of protected and primed SOC by considering the changes in local accessibility between litter-derived and native SOC pools during aggregate turnover and SOC transformation and a novel model that can distinguish apparent and real priming effect under constant moisture and drying and wetting conditions.The first model is established on the hierarchical concept of soil aggregates and two new hypotheses we proposed.The first hypothesis is that the straw-derived carbon can stimulate the original soil organic carbon,and the second hypothesis is that the activity of microorganisms controls the turnover of soil aggregates and soil organic carbon.The model was well calibrated using the dynamic datasets in the order of aggregate size distribution,SOC contents in aggregates,soil respiration and the priming effect monitored over 80 days in a unique incubation experiment with and without ¹³C labeled litter.The simulation results explained 95-96%,70-100%,78-80%,and 47%variations of the measured aggregate mass distribution,aggregate carbon content,soil respiration,and priming effect,respectively.In the soil with straw addition,unaggregated fractions and micro-aggregates promoted the formation of macroaggregates;the original soil organic carbon domained in the mineral-associated carbon pool and the straw-derived carbon domained in the occluded carbon pool in all soil fractions;the turnover time of soil organic carbon is much higher than that of aggregates;the turnover time of aggregates increases with time,and the turnover time of macroaggregates is faster than that of microaggregates?1-106 days versus 1-122days?;in macro-and micro-aggregates,the turnover time of occluded carbon is higher than that of the mineral-associated carbon.It predicted that the priming effect was predominantly originated from mineral-associated SOC in macroaggregates and,to a less extent,from occluded SOC in micro-aggregates.The priming effect peaks should be attributed to increased accessibility between litter-derived and native SOC during the formation of macroaggregates and microaggregates within or external to macroaggregates.By parameterizing drying and wetting cycles,the model was used to simulate the turnover of aggregates and soil organic matter.The simulation results explained 64-90%,76-99%,48-86%,and 14%variations of the aggregate mass distribution,aggregate carbon content,soil respiration,and priming effect and showed that the drying and wetting cycles promoted the breakup of large aggregates and the decomposition of microaggregated carbon.The drying and wetting cycles resulted in the"Birch"effect in different aggregates,and this promotion of soil organic carbon decomposition decreased with the increase of drying and wetting cycles.Under drying and wetting conditions,the occluded carbon pools in the microaggregates were the main source of the priming effect,while the occluded carbon pools in the large aggregates were the main sinks of soil organic carbon compensation.Our study provides mechanistic insights into the role of physical protection controlled local accessibility between new and old SOC in regulating the dynamics of the priming effect.Our proof-of-concept model can form the base for incorporating biophysical mechanisms and processes into future ecosystem models.Based on previous studies,we assumed that the decomposition of soil organic matter was divided into two stages.Soil organic matter of different sources was decomposed by specific microorganisms,and the activity and moisture conditions of the microorganisms were parameterized to establish the apparent-real priming effect model?LIRAPE model?.The simulation results of the newly established LIRAPE model showed that the R² values of cumulative respiration under constant water and drying and wetting conditions are both greater than 0.95,and the R² values of the cumulative priming effect are 0.95 and 0.94,respectively.The cumulative NSD of the constant moisture and drying and wetting cycles is greater than 1,while the NSD of the respiratory rate is less than 1.The cumulative respiration rate from the native soil organic carbon is always higher than the total cumulative respiration rate without straw addition.With or without straw treatment,the decomposition rate constants of soil organic carbon and straw carbon are less than 0.0314 day^-1,and the decomposition efficiency constants are less than 0.0835 mg C?mg C?^-1.And the decomposition efficiency and decomposition efficiency of straw carbon are higher than that of soil organic carbon.After the straw incorporation,the apparent-priming effect decreased rapidly and the real-priming effect gradually increased.The simulation results overestimated the cumulative respiration and underestimated the respiration rate.The addition of straw changed the mineralization rate of soil organic carbon,that is,priming effect.The decomposition rate of soil organic carbon and straw carbon is the rate-limiting step of carbon decomposition.Microorganisms decomposing soil organic carbon and straw carbon belong to different microbial structures.Moisture stress conditions such as drying-wetting cycles,etc.,inhibitted the apparent priming effect and promoted the real priming,that was,the decomposition of soil organic carbon was increased and the decomposition of straw carbon was decreased.The microbial community structure was specific for different carbon pools.Both soil organic carbon and straw carbon may not be used as direct substrates for microbial growth,but substances in a soluble state instead.By assuming two specific microbial communities and dividing organic carbon into two decomposition stages,this model successfully simulated the priming effect as well as the real and apparent priming effects.It provided a good basis for exploring the mechanisms of different types of priming effects in the future.