Inorganic Nitrogen Dynamics in a Soil Applied with Biochars Pyrolysed at Different Temperatures.

Biochar is a pyrolysed organic material and has shown potentials to function as a soil conditioner for improved soil environments and amendment for enhanced plant growth. However, it is still not clear how biochar affects particularly soil nitrogen (N) dynamics when applied to soil. There have been inconsistent studies showing increases, decreases, or no changes on mineralization and immobilization with biochars applied to soil. N mineralization and immobilization are mediated by soil microbes that are affected by particularly volatile matters (VM) contained in the biochar. Therefore, in this study, an incubation study using woodchip biochars (WB) pyrolysed at 300°C (WB300), 550°C (WB500) and 800°C (WB800) was conducted to evaluate the effect of biochar application to soil on N mineralization and immobilization. A soil used in this study was a Japanese Andisol (Tokyo), oven-dried, and sieved by 2 mm. Each WB was applied at 2.5% (w/w) without or with 100 mg NH₄⁺-N kg^-1 to soil. Soil TC, TN, mineralizable N, NH₄⁺-N, NO₃^–-N, and pH were analyzed on 0, 1, 3, 7, 14, 21, and 28 d of the incubation. N mineralization was stimulated in all WB-applied treatments without N addition during 1–3 to 7 d. This may be because VM of the biochar increased soil microbial growth which decomposed soil organic matter to release NH₄⁺-N. The amounts of mineralized N during 1–3 to 7 d were higher in the soils with higher amounts of VM (or lower pyrolysis temperature) of the biochars. When N was applied, N immobilization during 1 to 3 d followed by mineralization during 3 to 7 d were observed only in WB300- and WB550-applied soils. This may be due to the fact that WB300 and WB550 contained enough amounts of VM to boost microbial growth first, then to decompose soil organic matter later. The WB800-applied soil did not follow the same trend possibly because of lack of VM for microbial growth. It was concluded that N mineralization upon biochar addition depended on the amount of VM in the biochar.