Currently, the biomass of an invasive and obnoxious weed, kunai grass (Imperata cylindrica), is uncontrollably burnt in Papua New Guinea in subsistence farming systems resulting in unwarranted negative environmental consequences. We explored the possibility of sustainable utilization of biochar produced from the weed biomass along with a standard feedstock-rice husk (Oryza sativa). Biochars were produced with lab-scale pyrolysis at 550°C, characterized for chemical properties and plant nutrient composition. Further, agronomic efficacy of soil incorporation of biochars (5 t ha -1 ) or co-applied with mineral fertilizers (100, 11, and 62 kg ha -1 N, P, K, respectively) was tested for sweet potato (Ipomoea batatas L. Lam) in a field experiment. The two biochars differed significantly (P < 5%) with respect to recovery from the feedstocks, chemical characters and nutrient composition. Kunai grass biochar was poorer in nutrients (< 1%) with distinctly alkaline pH and higher electrical conductivity. Biochar amendment to soil showed significant (P < 5%) improvement of soil moisture, while co-application of biochars along with mineral fertilizers showed soil moisture decrease. Biochar amendment improved the growth parameters and total tuber yield of sweet potato by about 20%, while co-application with mineral fertilizers augmented total tuber yield by 100% and above-ground biomass yields by > 75%. Besides, improving agronomic performance of sweet potato crop, co-application of biochars with mineral fertilizers enhanced uptake of N, P, K, Ca, Mg, and S. Production and utilization of biochar in sweet-potato production could offer an efficient means of disposing biomass of kunai grass with concomitant productivity improvement in Papua New Guinea.

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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 coapplication 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 (NH 4 + -N). Soil application of biochar together with urea at 500 kg N/ha produced the highest nitrate-N (NO 3 À -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.