Fungi are more important than bacteria for soil carbon loss through priming effects and carbon protection through aggregation

Abstract

Plant inputs from root exudates and litter can improve soil aggregation and enhance soil organic carbon (SOC) content, but they can also increase SOC decomposition referred to as the priming effect. While bacteria and fungi are considered as key microbes for organic matter decomposition, their role in decomposing plant inputs and SOC, and their contribution to forming aggregates and SOC protection is still not fully understood. We conducted a 16-day incubation study to investigate the role of bacteria and fungi on decomposition and protection of C derived from glucose (labile C as a surrogate for root exudates) and wheat root biomass added to soil. Bronopol and captan were used to suppress bacterial and fungal activity, respectively, in the soil. Because glucose and wheat root biomass were enriched in 13C, we were able to estimate their decomposition and priming effect, microbial C use efficiency (CUE), and recovery in different aggregate size classes. As expected, the decomposition of both substrates was reduced significantly when biocides were added, and the decomposition rate of glucose was significantly higher than of wheat root biomass. Fungi were more important than bacteria in causing a priming effect, as indicated by the reduced SOC decomposition in the presence of substrates when captan was added. The mean weight diameter of aggregates at the end of the incubation was significantly reduced when captan was added, either alone or in combination with bronopol, independent of substrate type, suggesting that fungi also played an important role in soil aggregation. However, recovery of C derived from wheat root biomass in macroaggregates was highest when both biocides were added, suggesting that decomposition by both bacteria and fungi may have outweighed the protection of wheat C in new macroaggregates. While we observed variation in the microbial CUE among substrate and biocide treatments, we found no relationship with soil aggregation. We conclude that both bacteria and fungi were important for decomposition of added substrates to the soil, but that fungi can play a larger role in both soil C loss through priming effects and C protection through aggregation.