Effects of biochar, compost, and biochar-compost on soil total nitrogen and available phosphorus concentrations in a corn field in Papua New Guinea

Replenishing soil nutrient particularly total nitrogen (TN) and available phosphorus (P) is important to sustain soil health for food production. Organic amendments such as compost and biochar are commonly applied to improve soil nutrient retention especially N and P. In farms, biochar is usually applied once followed by applying other organic amendment applied in their full rates. Both form and rate of organic amendments can affect soil nutrient concentrations particularly in short term. This study aimed to examine the effects of compost and mixture of compost with biochar (both at full rates) on soil nutrient concentrations in short term. A randomised complete block filed experiment with eight replicates was used for this study. The effects of biochar (5 t/ha) only, compost (at the rates of 10 t/ha, 25 t/ha and 35 t/ha) and biochar mixed with compost (5 t/ha and 10 t/ha, respectively) on soil nutrient concentrations compared with control were explored in a corn field. Compost treatment at the rate of 35 t/ha had significantly higher TN, available P, calcium (Ca) and iron (Fe) compared with other treatments and control. Soil potassium (K) levels remained unchanged among all treatments. Biochar only treatment had significantly higher available P and Ca concentrations compared with biochar mixed with compost treatment. Compost application at higher rate (35 t/ha) proved best practice to significantly increase TN and available P concentrations in short term. Significantly higher available P concentration in biochar only treatment compared with the biochar mixed with compost treatment could have been associated with stimulation of P immobilisation when biochar was mixed with compost. Our results indicated that the form and rate of organic amendments in short term cropping systems are important to be considered while applying to a volcanic soil to ensure N and P availability for plants are not compromised.