Microbial community responses determine how soil-atmosphere exchange of carbonyl sulfide, carbon monoxide and nitric oxide respond to soil moisture

Abstract. Carbonyl sulfide (OCS) plays an important role in the global sulfur cycle and is relevant for climate change due to its role as a greenhouse gas, in aerosol formation and atmospheric chemistry. The similarities of the carbon dioxide (CO2) and OCS molecules within chemical and plant metabolic pathways have led to the use of OCS as a proxy for global gross CO2 fixation by plants (GPP). However, unknowns such as the OCS exchange from soils, where simultaneous OCS production (POCS) and consumption (UOCS) occur, currently limits the use of OCS as a GPP proxy. We estimated POCS and UOCS by measuring net fluxes of OCS, carbon monoxide (CO) and nitric oxide (NO) in a dynamic chamber system fumigated with air containing different [OCS]. Several different soils were rewetted and soil-air exchange monitored as soils dried out to investigate responses to changing moisture levels. A major control of OCS exchange is the total amount of available S in the soil. POCS production rates were highest for soils at WFPS > 60 % and rates were negatively related to thiosulfate concentrations. These soils flipped from being net sources to net sinks of OCS at moderate moisture levels (WFPS 15 to 37 %). By measuring CO and NO while fumigating soils at different levels of OCS, we could show that CO consumption and NO exchange are linked to UOCS under moderate soil moisture. Based on the OCS : CO flux ratio two different UOCS processes could be separated. For one of the investigated soils, we demonstrated changes in microbial activity and red-like cbbL and amoA genes that suggested shifts in the UOCS processes with moisture and OCS concentration. This supports the view that Ribulose-1,5-Bisphosphate-Carboxylase (RubisCO) plays an important role for UOCS and demonstrates a link to the nitrogen cycle.