Synergistic effects of microbial networks, glomalin-related soil protein, and humic substances jointly enhance the stability of soil aggregates: Evidence from converting pure Pinus massoniana plantations into uneven-aged mixed forests

Abstract

The stability of soil aggregates (SSA) serves as a sensitive indicator of soil fertility and plays a crucial role in determining resistance to erosion. However, the synergistic mechanisms of microorganisms, glomalin-related soil protein (GRSP), and humic substances (HS) by which SSA is altered through the conversion of Punus massoniana monoculture plantations into mixed broadleaf-conifer plantations remain unclear. We applied three different soil aggregate analysis methods, combined with high-throughput sequencing regions, to examine the impact of plantation diversification on SSA and microbial community structure. Specifically, we investigated a mixed plantation (MP) composed of two uneven-aged tree species (P. massoniana and Castanopsis hystrix) and a multiple-species mixed plantation (MMP) composed of several uneven-aged tree species (P. massoniana, Castanopsis hystrix, Michelia hedyosperma, Erythrophleum fordii, and Quercus griffithii). These were compared to a pure plantation (PP) of P. massoniana as the control. We also analyzed HS components and GRSP contents. Our results showed that the mean weight diameter (MWD) index and the proportion of soil macroaggregates (>0.25 mm) were significantly higher, while the new aggregate stability index (AS) was significantly lower in MP and MMP than in PP. Chao1 index values for bacteria (0–10 and 10–20 cm soil layers) and fungi (0–10 cm soil layer) were significantly higher in the MMP than in MP and PP. Moreover, the MP and MMP had greater HS component and GRSP levels than the PP plantations. The relative abundance of Chloroflexi, Gemmatimonadota, Ascomycota, and Kickxellomycota primary affected the particle size distribution of aggregates. PLS-PM revealed that higher litterfall mass (LF) and fine root biomass (FR) in mixed broadleaf-conifer plantations enhanced microbial diversity and network complexity via soil C and N accumulation, which directly stimulates GRSP secretion and HS formation, thereby enhancing SSA. Therefore, the introduction of broadleaf species into pure P. massoniana plantations is an effective strategy for preventing land degradation by preserving the complexity of belowground communities. Taken as a whole, this study provides new insights into the intricate interplay of plant–microbe-soil that will contribute to developing plantation management strategies to enhance stability of soil structure.