Carbon to nitrogen stoichiometry of organic amendments influences the improvement of aggregate stability of a cropping vertisol

Abstract

Abstract Soil aggregation is one of the key processes controlling air, water and gas transport in soil. Long-term cropping without returning organic matter in the soil can reduce the water stability of the aggregates. Microbial decomposition of organic matter plays a significant role in aggregate formation and hence can reverse the decline in the water stability of the aggregates. The inoculation of soil with beneficial microbes can improve the aggregate stability of cropping soil, potentially restoring its condition to healthy soil. However, the restoration of the aggregate stability may also be dependent on the C:N ratio of added organic matter. We hypothesize that a higher C:N ratio of added organic matter and microbial inoculation can trigger a more persistent improvement in aggregates. We treated pasture (aggregates were water-stable) and cropping (aggregates were unstable in water) vertisols with sugarcane (C:N = 104) and lucerne (C:N = 23) residues with and without microbial inoculant that had both bacteria and fungi. After 4 months of incubation, we found that the slaking index dropped by 46% in sugarcane-treated cropping soils, whereas the reduction was 27% in lucerne treatment. A similar reduction in the slaking index was also observed in the pasture soil but the magnitude of the reduction was lower than in the cropping soils. However, microbial inoculation did not show a statistically significant influence on reducing the slaking of cropping or pasture soils in this study. The reduction of slaking in both soils was supported by an increase in mean weight diameter (MWD), macro-aggregates and the aggregate-associated soil organic carbon. Our results demonstrated that organic carbon input with a high C:N ratio facilitates the restoration of water stability of the structurally unstable cropping soils.