Effects of biochar, urea and their co-application on nitrogen mineralization in soil and growth of Chinese cabbage

Experiments were conducted to study the effect of soil applications of kunai grass (Imperata cylindrica) biochar (0 and 10 t/ha) and laboratory grade urea (0, 200 and 500 kg N/ha) and their co-application on nitrogen (N) mineralization in an acid soil. The results of an incubation study showed that the biochar only treatment and co-application with urea at 200 kg N/ha could impede transformation of urea to ammonium-N (NH4+-N). Soil application of biochar together with urea at 500 kg N/ha produced the highest nitrate-N (NO3--N) and mineral N concentrations in the soil over 90 days. Co-application of urea N with biochar improved soil N mineralization parameters such as mineralization potential (N-A) and coefficient of mineralization rate (k) compared to biochar alone. In a parallel study performed under greenhouse conditions, Chinese cabbage (Brassica rapa L. ssp. chinensis L.) showed significantly greater (P < 0.05) marketable fresh weight, dry matter production and N uptake in soil receiving urea N at 500 kg/ha or co-application of biochar with urea N compared to the control. Application of biochar only or urea only at 200 kg N/ha did not offer any short-term agronomic advantages. The N use efficiency of the crop remained unaffected by the fertilizer regimes. Applications of biochar only at 10 t/ha did not offer benefits in this tropical acid soil unless co-applied with sufficient urea N.