Developing diversity indicators from organic matter and microbe to depict their changes across different soil-landscapes in a subtropical hilly area

Soil organic matter (SOM) molecular diversity and microbial functional diversity are critical for regulating soil organic carbon (SOC) persistence and maintaining soil ecological functions. Heterogeneous soil-landscapes critically regulate SOC sequestration processes and microbial community dynamics through distinct topographic conditions and anthropogenic influence in the area of Earth surface. Understanding how SOM diversity and microbial diversity interact and influence soil ecological functions across soil-landscapes within a heterogeneous area is key to developing targeted land management strategies, thereby achieving sustainable development goals for the region. In this study, topsoil samples (0–15 cm) were collected from four distinct soil-landscapes within a small watershed in a hilly area, including natural forestland (FL) on hillslopes, agricultural orchards (OR) and upland (UL) on downlands, and paddy fields (PF) within the basin. We used organic carbon (OC) molecular assays and microbial assays to investigate SOM molecular composition, and microbial activity and community structure in bulk soil and aggregate fractions, correlating SOM molecular composition with microbes. Our results indicate that the contribution of lignin to SOC decreased (except in PF), amino sugars increased, while glomalin-related soil proteins (GRSPs) remained unchanged, with the SOC loss in agricultural landscapes. Compared to FL, agricultural landscapes of PF, UL and OR showed significantly reduced mass proportions of larger-sized aggregates and decreased lignin contributions in macroaggregates and microaggregates, indicating that plant-derived lignin phenols play a major role in SOC accumulation. Based on SOM composition, microbial community and enzyme activities, we developed synthesis diversity indicators that effectively depict SOM-microbial links and soil ecological function across soil-landscapes, and these indicators showed significant positive correlations with SOC and microbial abundance. Collectively, our findings demonstrated the importance of lignin phenols in SOC accumulation and highlighted the differential roles of microbial-derived C (GRSPs and amino sugars) in SOC dynamics. By integrating investigations of SOM composition with microbial activity and community structure, along with using diversity indicators to describe the changes of SOM and microbes in different soil-landscapes, we can better understand SOM-microbial interactions and soil ecological function maintenance in diverse terrestrial soil-landscapes.