Physical impact mechanism of large macroaggregate fragmentation on short-term soil microbial respiration after rainfall

The uncertainty of soil respiration under the influence of rainfall significantly affects the carbon flux in forest ecosystems. Thus, this study assessed the impact mechanism of aggregate changes on short-term forest soil microbial respiration under rainfall simulation. The response of short-term soil microbial respiration to the variation in aggregate distribution, carbon concentration in the aggregate particles, dissolved organic carbon (DOC), dry bulk density, volumetric water content, capillary porosity, and soil aeration in the forest soil was investigated by a self-developed rainfall simulation system under 50 and 80 mm/h rainfall treatments. The system ensured the controllability, continuity, and integrity of the analysis from soil erosion to soil respiration. A proportional decrease in large macroaggregates (> 2 mm) and an increase in small macroaggregates (0.25-2 mm) were detected due to rainfall, leading to the release of coarse particulate organic matter in large macroaggregates and an enrichment in microaggregates within small macroaggregates. The splitting of macroaggregates increased the DOC content and decreased the porosity and aeration of the soil. The CO2 flux decreased immediately after rainfall treatment, and it increased at 12 h after the end of 80 mm/h rainfall. Moreover, the soil immediately emitted CH4 at the end of 80 mm/h rainfall. Large macroaggregates and DOC had positive conditional effects on respective CO2 and CH4 fluxes at the end of rainfall. The results indicate that short-term soil microbial aerobic respiration is mainly regulated by the remaining macroaggregate distribution immediately after rainfall. Moreover, short-term soil microbial anaerobic respiration is mainly mediated by the DOC and microaggregates released from large macroaggregates under soil water saturation. The study concludes that the uncertainty of short-term soil microbial respiration is mainly derived from the random fragmentation and redistribution of macroaggregates in the soil immediately after rainfall treatments.