Subsoil application of compost improved sugarcane yield through enhanced supply and cycling of soil labile organic carbon and nitrogen in an acidic soil at tropical Australia

Organic amendments are mostly carried out on the soil surface layer to mitigate the decline in sugarcane soil health, however, the effects of subsoil application of composts on soil carbon (C) and nutrient dynamics and sugarcane yield are largely unknown. A 4 years field trial was conducted at Maryborough, Australia, to investigate the effects of subsoil application of compost on soil C and N cycling, associated biological processes and sugarcane productivity. The trial included four subsoil (ca. 25 cm) amendment treatments of control (CK, without amendment); gypsum (GP); compost (CP); and mineral fertilizer (FE). Overall, the compost treatment increased concentrations of soil NH4+-N by 30% and NO3−-N by 40% at subsoil (10-25 cm) amendment layer. Soil microbial biomass C and N at the 0-10 cm depth were also significantly higher in the compost treatment than the other treatments. The CO2 respiration in the compost and fertilizer treatments, were significantly higher than in the control and gypsum treatments at the subsoil amendment depth. The compost treatment had greater β-glucosidase activities than other treatments at the 10-25 cm soil depth. In addition, subsoil application of compost increased inputs of hot water extractable organic C (HWEOC) by 30% and N (HWETN) by 70% at the application depth, but did not affect total soil C and N contents. The HWEOC and HWETN pools were positively related to the cumulative sugarcane yield during the 4 years cropping cycle. The subsoil application of compost increased the supply of organic C and N for microbial community, enhanced nutrient cycling processes, improved soil environmental conditions and soil health for sugarcane growth and thus increased sugarcane productivity.