Chicken manure (CM) carbonization is an efficient waste control method that converts manure into an agriculturally useful amendment. However, the recommended temperature range of 300–500 °C has led to confounding results on the maintenance of the nitrogen (N) supply ability of CM after carbonization, and thus requires further investigation. The objective of this study was to identify an upper limit (threshold) temperature to guide the carbonization of CM in order to ensure that the good N supply ability of the manure after carbonization is preserved. CM was carbonized at 350, 375, 400, 425, 450, and 475 °C. Afterwards, chemical properties, surface functional groups, N speciation, N supply ability, and their relationships were investigated. The results indicated that the N supply ability of CM carbonized at 350–375 °C was comparable to the N supply ability of CM, which is possibly due to the minute alteration of the chemical properties, surface functionality, and N speciation that occurred in CM at these temperatures. At ≥400 °C, the N supply ability of CM was significantly reduced due to the increasing alterations of the chemical makeup as heating temperature increased. For sustainable maintenance of the good N supply ability of CM after carbonization, the manure should not be carbonized above 375 °C.

Carbonization of cage layer chicken manure (CLCM) can improve its bio-recalcitrance which might improve nitrogen (N) bioavailability in soil. However, temperature(s) to exhibit appropriate variations in the chemical makeup of the manure during carbonization in order to achieve this objective is unknown. In this study, we investigated the alterations in chemical compositions, surface functionalities, and N speciation initiated by different carbonization temperatures (350, 500, and 650 °C) and the effects of these alterations on N bioavailability in soil. The objective was to identify suitable temperature condition(s) for the conversion of CLCM into a carbonized product of appropriate bio-recalcitrance that is capable of improving N bioavailability in soil more than the un-carbonized CLCM. The results showed an increased bio-recalcitrance of the manure with increasing carbonization temperatures due to drastic changes in the chemical makeup and accumulation of heterocyclic aromatic N structures. Subsequently, these alterations in the chemical makeup and state of the organic N species in the manure affected N bioavailability in soil. Notably, N bioavailability of CLCM and benefits on plants were improved when soil was supplied with CLCM that was altered at 350 °C. With these observations, we concluded that alterations in chemical and surface structural compositions and N speciation at 350 °C are optimum for instituting the required bio-recalcitrance to CLCM in order to improve N bioavailability in soil for plants.