Simple N Balance Assessment for Optimizing the Biochar Amendment Level in Paddy Soils

Excessive biochar amendment may lead to soil nitrogen (N) surplus and ammonium (NH4-N) deficiency. An incubation experiment was conducted to investigate impacts of six biochars on N balance and retention in paddy soils. Nitrogen balance is the changes of soil N contents before and after experiment, whereas N retention is the difference of soil N losses with and without biochar. Results showed total N and nitrate (NO3-N) contents in soils were enhanced with the increase of biochar levels, whereas NH4-N decreased. The biochar-scaled N retention potential (N retention divided by biochar level) was reduced with increase of biochar level. Given that soil total N and NO3-N balance was zero and NH4-N balance decreased by 30%, the optimum levels of biochar were 5 g kg1 for LB500 and BCB600, 10 g kg(-1) for LB600, BCB500, and RSB500, and 30 g kg(-1) for RSB600, respectively. This research highlights a simple N balance assessment can optimize biochar amendment.