Nitrate-induced hydroxyl radical releases deep soil organic carbon by opening the ‘enzyme latch’ under micro-aerobic conditions

Abstract

Deep soils (defined here as soil layers >1 m beneath the ground) contain a significant amount of soil organic carbon (SOC) with a relatively slow turnover rate. In oxygen (O2) limit conditions, terms here as “micro-aerobic environment”, a high concentration of phenol usually suppresses SOC decomposition by inhibiting hydrolase activities, a mechanism known as the ‘enzyme latch’. Here, we showed that the addition of nitrate (NO3−) into deep soil significantly decreased the soil phenol content. This reduction alleviated the inhibitory effect of phenol on hydrolase activities, resulting in an increased rate of deep soil SOC decomposition. NO3− addition enhanced deep soil SOC decomposition by opening the ‘enzyme latch’ under micro-aerobic conditions. Under micro-aerobic conditions, NO3− addition increased the concentration of hydroxyl radicals (•OH), with the extent of increase negatively correlated with the soil phenol content. Quenching •OH using terephthalic acid removed the suppressing effect of NO3− on phenol content, indicating that NO3− addition opened the ‘enzyme latch’ by promoting •OH generation, which oxidized phenol. Additionally, the addition of NO3− promoted the production of nicotinamide adenine dinucleotide (NADH) and hydrogen peroxide (H2O2) in deep soil. The H2O2 was significantly positively correlated with •OH, suggesting that the enhanced •OH generation under NO3− addition was caused by the Fenton reaction. Overall, our results shed light on a new pathway through which NO3− leaching affected SOC stability in micro-aerobic deep soils. Reducing nitrate leaching is beneficial for the conservation of deep soil SOC.