Effects of substrate quality on carbon partitioning in a temperate grassland soil as influenced by microbial activity

Abstract

Using organic amendments, including livestock slurry, in intensively managed agricultural grasslands may potential lead to multiple benefits, including plant nutrient supply to soils and long-term accumulation of soil organic matter. There is increasing interest in the extent to which biological slurry additives (mixtures of selected living or latent microorganisms added to slurry) are able to enhance soil fertility through mobilisation of key elements in the slurry and in the soil. However, little is known about the impacts of slurries amended with biological additives on carbon (C) partitioning within three pools: respired CO2, microbial biomass C (Cmic) and C retained in the soil. We report data from incubation experiments in which soils received livestock slurry treated with a commercial biological additive, alongside 14C-labelled carbohydrates of different lability (14C-glucose, 14C-glucose 6-phosphate (G6P), and 14C-cellulose). The aim of this experiment was to understand how slurry that has received a biological additive ultimately affects the partitioning of C within these pools, alongside how slurry additives influence soil microbial biomass and the priming effect (PE) in soil. The addition of slurry alongside carbohydrates to soil resulted in lower 14C biomass uptake and greater residual 14C activities in soil, as well as to lower cumulative respiration and PE, compared to the corresponding carbohydrate-only treatments. Competition in the soil between the indigenous microbial community and the microbial community associated with slurry is proposed to account for the apparent suppression of biomass uptake of added C-substrate and of cumulative respiration. Our findings also seem to indicate that the addition of a biological additive to slurry resulted in higher cumulative CO2 efflux from soil compared to the unamended slurry, both for glucose and G6P treatments. This suggests that biological slurry additives may have the potential to reduce the suppressive effect of slurry microorganisms on native soil microorganisms.