Soil nitrification and nitrogen mineralization responded non-linearly to the addition of wood biochar produced under different pyrolysis temperatures

Purpose Nitrogen (N) cycle is one of the key biogeochemical cycles in terrestrial ecosystems. Global climate change and soil management practices have disrupted the soil N cycling processes due to increased water and N limitations. Biochar is a soil amendment and improves soil-plant water and N retentions. However, it is uncertain to what extent biochar pyrolysis temperature would affect soil N transformations under two soil moisture regimes. This study aimed to explore how pyrolysis temperature would affect biochar properties and subsequently soil N transformations through a short-term laboratory incubation study at two moisture levels. Materials and methods An incubation study was carried out for 5 days. Biochar added to the soil at a rate of 5% (w/w) was produced under six different pyrolysis temperatures (e.g., 500, 600, 650, 700, 750, and 850 degrees C). We used N-15 natural abundance (delta N-15) of inorganic N (NH4+-N and NO3--N) to assess the potential of biochar materials in facilitating forest soil N transformations at two different soil moisture levels of 50% and 65% water holding capacity (WHC). Results and discussion Pyrolysis temperature significantly increased cumulative nitrification and N mineralization initially, peaked between 600 and 700 degrees C and decreased thereafter. However, both cumulative nitrification and N mineralization were significantly lower in the biochar-amended soils than those of the control soil, with significantly lower delta N-15 of NH4+-N and delta N-15 of NO3--N. The 65% WHC had higher cumulative nitrification and N mineralization compared with those in 50% WHC. This study highlights that application of biochar would reduce N losses and improve soil N retention particularly for forest soil. Conclusions The present study highlights the importance of biochar pyrolysis temperatures for their use as soil conditioner to increase soil N retention. An optimum pyrolysis temperature range of 600-700 degrees C was identified for improving soil nitrification and N mineralization under the laboratory incubation conditions.