Faster cycling but lower efficiency: A microbial metabolic perspective on carbon loss after wetland conversion to cropland

Abstract

Natural wetlands store 20–30% of soil organic carbon (C) in terrestrial ecosystems. Wetlands conversion to croplands has caused a massive loss of soil organic C; however, the microbial contribution to this C loss remains elusive. In this study, we took 24 soil cores from a natural wetland and a 23-year cultivated cropland over four seasons to compare the abundance of functional genes encoding 138 enzymes for ten C-cycling pathways classified as organic C degradation, catabolic and anabolic processes. We found that land use-induced C loss was primarily driven by enhanced organic C degradation (∼24% increase) but suppressed microbial C assimilation efficiency, represented by shifted catabolism and declined anabolism (∼22% decrease). The enhanced organic C degradation was advocated by the enriched genes encoding organic C degradation enzymes; the suppressed microbial C assimilation efficiency was upheld by the shifted catabolic processes and inhibited anabolic processes. Compared with natural wetlands, the croplands showed substantial shifts in catabolic processes - 22% suppression of the Embden-Meyerhof-Parnas pathway, 27% decline in the Citrate Cycle, 22% increase in Entner-Doudoroff, and 25% stimulation in pentose phosphate pathways. Meanwhile, the anabolic process, especially the Wood-Ljungdahl pathway, was significantly suppressed (∼55%) in the cropland. The shifts in organic C degradation, catabolism, and anabolism were stronger in spring and summer than in winter and autumn. At the ecosystem level, microbial metabolic quotient was suppressed by 44%, while microbial biomass carbon was boosted by 5% after wetland conversion to cropland, indicating a faster microbial growth rate but lower C use efficiency. Differentiating microbial metabolic processes explained the C loss after wetland cultivation, indicating an urgency to represent microbial metabolism in soil C models.