Long-term warming increased microbial carbon use efficiency and turnover rate under conservation tillage system

Microbial carbon use efficiency (CUE), a combination of respiration and growth, plays a central role in the flow of carbon through soil. Carbon-depleted agricultural soils are promoted as a global sink for atmospheric carbon in a warming climate. There is an urgent need to understand whether increased soil respiration from warmed cropland soils reduces CUE, potentially creating positive feedback to atmospheric CO2 concentrations. We investigated microbial CUE and turnover rates in response to experimental warming in topsoils under traditional tillage and conservation tillage management using a substrate-independent H218O labeling method. Eleven years of warming increased soil organic carbon (SOC) by 3.9% in topsoil under conservation tillage, while there was no change in traditional tilled soils. Soil microbial respiration was increased by 108% and 11.6% under traditional tillage and conservation tillage, respectively. Warming decreased microbial growth by 60.3% in traditional tilled topsoils, but increased it by 50.8% under conservation tillage. Therefore, CUE and microbial turnover rates decreased by 77% and 22.2%, respectively, in response to warming in traditional tilled topsoils and increased by 29.1% and 19.3%, respectively, under conservation tillage. Soil hydrolase (β-glucosidase, cellobiohydrolase and N-acetyl-glucosaminidase) activities were increased by warming under both management types, but oxidase (peroxidase and phenol oxidase) activities only increased in traditional tilled topsoils. Hydrolase:oxidase ratios were 4.1 times less in warmed traditional tilled soils compared to unwarmed traditional tilled soils but were 0.63 times more in warmed conservation tillage soils compared with unwarmed conservation tillage soils. Microbial CUE was positively correlated with dissolved organic carbon (DOC), DOC:SOC and hydrolase:oxidase, suggesting that substrate quality and availability were important factors driving changes in microbial physiology. The SOC increased with microbial growth, CUE and turnover rate. Our findings suggest that microorganisms in warmed topsoils under conservation tillage use a greater abundance of high-quality substrates more efficiently to build biomass, thereby reducing the positive feedback effect of increased soil respiration caused by soil warming.