Climate warming accelerates soil carbon (C) cycling, with microbial C use efficiency (CUE) and the dynamic nature of physical protection of soil organic matter (SOM) via organo-mineral interactions are likely important regulators for microbial feedbacks to the climate system. Warming reduces soil aggregation, which would decrease physical constraints on microbial activity and increase microbial substrate availability. Yet, long-term warming can also reduce substrate availability (e.g., C, nutrient) and microbial biomass, and increase microbial maintenance cost, thus reducing CUE. We hypothesize that warming-altered physical protection may fundamentally influence soil-climate feedbacks. To test the response of microbial activity and CUE to climate factors, soils were collected from an experimental warming study at the Harvard Forest in Massachusetts, USA, where soils have been warmed 5 °C above ambient for 27 years. Macroaggregates and microaggregates (250 – 2000 and < 250 µm) were separately incubated with substrates (glucose, cellobiose and cellulose) or as intact or crushed aggregates, at 15 °C and 25 °C for 24 h. Samples were 18O-water labeled (20 atm%) to calculate CUE. Microbial biomass C (MBC), soil organic C (SOC), total nitrogen (N), dissolved organic C (DOC), respiration, and soil DNA were analyzed.
Results/Conclusions
Long-term warming decreased soil nutrient content, substrate availability, microbial biomass and activity based on reduced soil N, MBC, SOC, DOC, respiration, and DNA content. Long-term warming also increased metabolic quotient at 15 °C, meaning that microbes respired more C per unit of MBC, indicating a lower microbial CUE. This is consistent with observed trends in increased CO2 due to long-term warming. Microaggregates have increased soil nutrient content and C storage, with greater soil N, MBC, DOC, DNA content, and decreased metabolic quotient, suggesting a higher CUE. Higher incubation temperature increased soil DNA content, respiration, and metabolic quotient, indicating more C respired per unit of MBC and a lower CUE. Finally, substrate quality did not affect soil C loss or microbial activity. Adding substrates showed little effect on respiration and metabolic quotient in the 24 h incubation. However, crushed aggregates showed a higher metabolic quotient in heated compared to control, suggesting that physically protected SOM has a lower CUE and is made more vulnerable to microbial degradation due to long-term warming. Our findings show that aggregate size and integrity are controlling microbially available C, with future experiments aimed at defining the microbial actors and mechanisms responsible for this instability.