Enhancing Soil Health, Sequestering Carbon and Reducing Greenhouse Gas Emissions In Tropical Soils Using Empty Oil Palm Fruit Bunch Biochar and Compost

Abstract

The efficacy of the easily accessible empty oil palm fruit bunch (EFB) biochar and compost in improving soil quality, carbon sequestration, crop production and mitigating greenhouse gas emissions, following their application to soil is yet to be adequately investigated in Ghana. On this basis, the study was conducted to analyse the effect of pyrolysis and composting of EFB on the chemical properties and content of potential toxicant element of the derived biochars and composts, the EFB biochar loading capacity and potential phytotoxicity on four different tropical soils through incubation studies and pot experiments, and the effect of different application rates of sole EFB biochar, sole compost and biochar-compost combinations on soil properties, okra yield and nutrient use efficiency, carbon storage and greenhouse gas emssions. In the field experiments, there was a one-time application of EFB biochar (10 and 20 t ha–1) [B10, B20], compost (20 t ha–1) [CP20], biochar-compost combinations [B10CP20, B20CP20], an unamended control [B0], and mineral fertiliser (100 kg N ha–1, 60 kg P2O5 ha–1 and 60 kg K2O ha–1) [NPK] across two cropping cycles in a randomised complete block design with four replications. Notably, pyrolysis increased nutrient content in EFB biochar but also elevated polycyclic aromatic hydrocarbon (PAH) levels, surpassing safety thresholds. However, EFB composts showed low C/N ratios, high nutrient content, and negligible PAH and heavy metal levels. Furthermore, based on integrating soil chemical properties, phytotoxicity and nutrient uptake parameters,(amelioration score), the biochar application rate of 2.0% emerged as the ideal rate for Acrisol, Red Ferralsol and Vertisol soil types, whilst 1% was best suited for Brown Ferralsol. These rates enhanced the soils’ chemical properties and increased nutrient uptake without exhibiting any inhibitory effect on maize germination. In the field studies, EFB amendments boosted okra pod yield by up to 283% over the unamended control. CP20 and B20CP20 increased yields by 58% and 100%, respectively, compared to mineral fertiliser in the first cycle. However, only B20CP20 maintained higher yields in the second cycle. All EFB treatments increased the soil’s pH by 0.6 to 1.6 points, enhanced phosphorus uptake and recovery efficiency, while B20CP20 notably increased soil cation exchange capacity and available phosphorus and zinc content, relative to the unamended control. CP20 also increased arbuscular mycorrhizal fungi and gram-positive bacterial biomass than NPK and B20. Additionally, soil carbon management index was increased by the B10CP20 treatment, whiles B20 increased total organic carbon content and stock. Also, the combined treatments improved gas diffusion and convection by 32–89% and 28–30%, respectively, indicating enhanced soil pore organization. Despite the improved soil properties, the single compost and combined applications of biochar and compost led to significantly lower emissions of nitrous oxide (N2O) (~74% decrease) and carbon dioxide (CO2) (~50% decrease) compared to the unamended treatment. However, methane (CH4) emissions were notably higher (87 ̶ 685%) for all biochar and compost treatments compared to the unamended treatment. The increased soil pH, AMF biomass, air permeability and volumetric water content emerged as the major regulators of the decreased emissions of N2O and CO2 in the amended soils. In conclusion, the study demonstrated the potential of EFB biochar and compost in improving soil health, enhancing crop yields, and mitigating GHG emissions in tropical soils, offering promising solutions for sustainable agriculture in tropical regions. However, further research is needed to address the increase in methane emissions and to assess the long-term impacts on crop yields and soil health.