Elucidating microbial carbon utilization and nitrous oxide dynamics with C-13-substrates and N2O isotopomers in contrasting horticultural soils

Quantifying how cropping systems influence soil C and nutrient turnover is important not only for better understanding the processes that regulate soil functioning, but also for long-term food production goals. We conducted an incubation study on soils collected from adjacent sites with a long-term legacy of annual vegetable, and perennial asparagus, haskap, and apple production. Soils were amended with C-13-labelled vanillin or glucose and incubated for 42 d. Gas and soil samples were collected to measure CO2, N2O, their isotopic signatures, and soil inorganic N and microbial biomass carbon (MBC) dynamics. Throughout the incubation, soil NO3- -N remained highest from vegetable soils, followed by soils from asparagus, haskap, and apple production. In contrast, soils from apple production supported the highest levels of MBC, whereas soil MBC from vegetable production was lowest. In general, glucose-C was utilized more readily than vanillin-C, but differences in glucosederived CO2-C were more distinct among soils than vanillin-derived CO2-C. Soils from a history of vegetable production had the highest metabolic quotients - indicating poor substrate use efficiency, whereas substrate-C was used most efficiently from soils under apple production. Substrate-C use efficiency inversely corresponded to patterns in denitrifier-N2O production based on (N2O)-N-15 isotopomers. In general, soils from the intensive vegetable cropping legacy were most susceptible to gaseous losses of both C and N-CO2 by way of soil organic matter priming, and N2O via increased denitrification with additions of labile substrates. Hence, the legacy of past cropping system management had consequences for how C and N was processed in soil.