Biochar-induced reductions in the rhizosphere priming effect are weaker under elevated CO2

Abstract

Biochar can store carbon in soils for decades to centuries, and is considered to have some potential to contribute to mitigating climate change. However, both biochar and elevated atmospheric CO2 (eCO2) can influence the rhizosphere priming effect (RPE) on pre-existing soil organic carbon (SOC) decomposition, leading to uncertainty in comprehensively evaluating the soil carbon sequestration potential of biochar under future climate change. In the present study, we investigated the RPE on pre-existing SOC decomposition of paddy rice in a soil-plant-biochar system under eCO2 (700 ppm). Plant-derived sources of carbon were separated from soil-derived sources using a continuous 13C-labelling method. We found that the RPEs decreased significantly with increasing biochar addition rates (1%, 2%, 3%, 4%, and 5% by weight) with mean reductions of 12% and 40% on day 31 and 54 after sowing, respectively, suggesting that biochar can enhance the stability of pre-existing SOC by decreasing the RPE. Although compared to ambient CO2 (aCO2, 400 ppm), eCO2 significantly decreased RPEs by 33% and 37% on day 31 and 54 after sowing, respectively, the decline in the RPE in response to biochar was weaker under eCO2. Decreases in soil enzyme activities and microbial biomass carbon could explain the decline in the RPE in response to biochar addition. Our findings highlight that biochar can inhibit the decomposition of pre-existing SOC by reducing the RPE, while this effect will be weakened in the face of atmospheric CO2 increase in the future.