Effects of biochar amendment on nitrogen mineralization in black soil with different moisture contents under freeze-thaw cycles

Freeze-thaw cycles (FTCs) affect the nitrogen cycling process in the non-growing season. To explore the effects of biochar application combined with soil moisture on nitrogen mineralization under FTCs, a laboratory FTC test of simulated soil columns was conducted. In this laboratory study, black soil from the Songnen Plain, China, was subjected to different biochar application rates (0%, 2% and 4%), moisture contents (15%, 20% and 25%) and numbers of FTCs (each FTC consisted of freezing at −20 °C for 48 h and thawing at 10 °C for 48 h). We explored the changes in soil moisture and aggregates as well as the effects of biochar application and different moisture contents on nitrogen availability under FTCs. The results showed that FTCs promoted the fragmentation and decomposition of soil aggregates, processes that are also related to the moisture content and biochar application rate. Moreover, the application of biochar increased the soil water holding capacity. The ammonium nitrogen and nitrate nitrogen contents in the soil first increased and then decreased during the FTCs and peaked after the third FTC. However, influenced by transfer and transformation processes, the inorganic nitrogen contents were higher than the initial levels after freezing and thawing. In addition, the content of ammonium nitrogen increased with increasing initial soil moisture content but decreased with increasing biochar application rate. However, the content of nitrate nitrogen increased with increasing initial soil moisture content and biochar application rate. Analysis of variance showed that FTCs and moisture content are the major driving factors that affect nitrogen mineralization (p < 0.05). This study provides guidance for the regulation of farmland soil nitrogen and the efficient utilization of agricultural resources in areas with seasonally frozen soil.