Determination of Agricultural Impact on Soil Microbial Activity Using δ18OPHCl and Respiration Experiments

Improved understanding of microbial activity and associated nutrient cycling in agricultural systems is required to maximize production while maintaining and improving soil fertility. We compared past and current microbial activity using the stable δ¹⁸O_{P HCl} pool and respiration incubations under crop and native systems to a depth of 1 m. Contrary to current understanding, agricultural practices have not decreased microbial activity in our crop system. Differences in average δ¹⁸O_{P HCl} signatures between land use systems, indicated higher past microbial activity in the entire soil profile under the crop system (13.7‰) compared to the native system soil profile (11.0‰), while current microbial activity was double under the crop system, especially between 15 and 100 cm. Evenly distributed microbial activity in the top and subsoils of the crop system, as well as an increase of biomass in the native system subsoil, highlight the importance of investigating microbial dynamics beyond the top 0-30 cm of the soil profile. In these relatively dry, carbon and nutrient poor Australian soils, the influence of water is perhaps the key to explaining our results.