What Is the Threshold Carbonization Temperature for Sustainable Preservation of the Good Nitrogen Supply Ability of Chicken Manure?

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.