Divergent responses of soil glomalin and microbial necromass to precipitation reduction: New perspectives from soil aggregates and multi-trophic networks

Global warming and increased drought are predicted to alter soil aggregation, biota composition, and carbon (C) balance. Microbial-derived C, such as microbial necromass C (MNC) and glomalin-related soil proteins (GRSP), are critical for soil organic carbon (SOC) stability. However, little is known about how climate change affects microbial-derived C within soil aggregates and its contribution to SOC. Here, we investigated the effects of 4-year warming (ca. 0.68 °C) and precipitation reduction (ca. −50% and −25%) on soil GRSP and MNC concentrations in semi-arid secondary grasslands and combined these results with a meta-analysis for GRSP. Results showed that warming increased MNC and its contribution to SOC, while precipitation reduction decreased MNC concentrations. Surprisingly, precipitation reduction increased GRSP concentrations and their contribution to SOC. Field experiments and meta-analysis also revealed that SOC and total nitrogen were negatively correlated with the C contribution of GRSP. Given the chemical recalcitrance of GRSP, this result may imply that the decrease in C and N content under precipitation reduction stimulates the formation of GRSP to enhance its subsequent protection of the SOC pool. Mechanistically, soil biota composition and its interactions dominated the variation in MNC between aggregates and climate change scenarios. The highest MNC concentrations in microaggregates may be attributed to higher fungal diversity, more stable multi-trophic networks, and weaker negative interactions across trophic levels. In addition, precipitation reduction significantly increased the abundance of modules in the multi-trophic network associated with SOC and MNC degradation, which were positively correlated with GRSP accumulation. These results suggest that climate change may regulate SOC dynamics by altering micro-food web structure in soil aggregates. Our study has direct implications for predicting the dynamics and stability of SOC fractions under future climate scenarios.