Forest expansion on cropland on China’s Loess Plateau facilitates C sequestration by increasing microbial-derived but not plant-derived carbohydrates

Abstract

Conversion of cropland to forest affects the microbial transformation of plant-derived organic substances and the accumulation of microbially synthesized products in soil. Labile litter represents the major source for soil microbial products, which promote soil aggregation and long-term carbon (C) stabilization. The present study investigated the changes in carbohydrate monosaccharides and their origins (i.e., microbial-derived or plant-derived carbohydrates) following the afforestation of cropland in the Loess Plateau region by Robinia pseudoacacia. Carbohydrate monosaccharides were determined in the soil fractions and plant organs in cropland and four R. pseudoacacia forests aged 5, 12, 25, and 32 years. The distributions of the soil aggregates and their stability were also measured. Forest expansion on cropland significantly increased the total organic C in the 0–10 cm soil layer by 31.8%. Glucose was the dominant monosaccharide in the plants and all soil fractions from all R. pseudoacacia forests, followed by galactose, xylose, and mannose. Following 32 years of afforestation, the ratios of microbial-derived carbohydrates relative to plant-derived carbohydrates (such as R(Rhamnose + Fucose)/(Arabinose + Xylose)) significantly increased by 131%, 149%, and 86% in the particulate organic matter, stable aggregate, and silt and clay fractions, respectively. Significant and positive correlations were found between the aggregate stability indexes and microbial-derived monosaccharides, thereby indicating that microbial-derived monosaccharides may play key roles in the stabilization of soil aggregates. These results suggest that following forest expansion on cropland, the increasing amounts of microbially-derived compounds corresponding to the increased allocation of soil C to stable aggregates may promote soil aggregation.