Sucrose application,soil microbial respiration and evolved carbon dioxide isotope enrichment under contrasting land uses

Heterotrophic decomposition of organic matter dictates that substrate supply rate, including energy and nutrients, can limit soil microbial activity. In New Zealand, soils are naturally deficient in nitrogen and phosphorus. Fertiliser application is a part of pastoral agriculture, the countrys most widespread land use. We postulated that organic soils under grazed pasture and pristine forest would be at the extremes of substrate quality and supply rate, and thus potential microbial response to food opportunities. Soil microbial responses to the addition of fresh energy (sucrose) were determined by laboratory experiments with root-free samples and intact cores including roots. Responses were quantified by respiration and respired carbon (C) isotope (¹³C) enrichment measurements. A supra-trace sucrose dose (0.002 mol kg^–1 (soil)) caused the forest soils microbial respiration rate to nearly double within 2 h. The peak response took 20 h, and saturation occurred beyond a sucrose dose of 0.05 mol kg^–1 (soil). Intact soil cores from the forest had similar respiration rates and responses. For root-free soil samples from the grazed pasture, respiration response to sucrose was nearly immediate, dose dependent, and there was up to a 9-fold increase in the rate. Intact cores from the pasture had much higher respiration rates, but a similar response to sucrose. For both soils, the similarity of sucrose application effects on respiration and relative ¹³C enrichment of the respired carbon was striking.