Carbon and nitrogen mineralization in different upland soils of the subtropics treated with organic materials

Use and management of organic waste/residues is currently an important global issue for attaining sustainability in agricultural production. However, knowledge about the decomposition characteristics and nutrient release pattern of added organic materials in subtropical soils and their interaction with inherent soil properties are lacking. Thus, laboratory incubation studies were carried out under aerobic conditions with crop residues and chicken manure (1%) applied to six contrasting soils. In all cases, CO2-C effluxes peaked by day 9 and the active C release phase persisted until day 25, indicating that priming effects might have occurred. The high pH and non-calcareous soils had higher CO2 effluxes than the acidic soils. The relative loss of added C differed between soils and its magnitude depended on the decomposable characteristics of the added organic materials, with chicken manure>mungbean residue>wheat residue. Rapid ammonification with presumed immobilization occurred up to day 15. Thereafter, NH4+ oxidation took place in the high pH and non-calcareous soils, with chicken manure exhibiting the greatest nitrification. By contrast, the acidic soils predominantly accumulated NH4+, thereby showing higher net N mineralization. In the acidic soils, nitrification was either small or stable, with the process being limited by the addition of organic materials with a high C/N ratio. Some disappearances of NO3− also indicated that N immobilization and/or denitrification had taken place. As such, the decomposition rate constant (k) correlated well either with pH alone (R2=0.59***) or coupled with C/N ratio (R2=0.61***) of the organic materials. The net N mineralization and nitrification showed a similar trend (R2=0.26-0.42*), although these processes were mostly regulated by the different soil factors. Our results reveal that the parameters pH and C/N ratio of organic materials should be included in equations to calculate the quality of added organic matter. Our newly proposed equations, incorporated as the organic matter quality index (OMQI), can predict k, net N mineralization and nitrification in different soil types under aerobic conditions, and it could be further improved by also considering inherent soil factors.