Compost Combined with Peanut Shells Biochar: Enhanced Formation and Structural Characteristics of Soil Humic Acids and Yield of Wheat Crop in a Clay Loam Soil

Application of compost alone could not enhance stability of soil humic acids (HA) particularly in short-term application in arid and sime-arid condtions due to the rapid loss of soil organic matter via oxidation. Therefore, mixing of compost with recalcitrant carbon (e.g. biochar) may enhance the stability of soil HA and improves soil characteristics. In this research, influence of peanut shells biochar combined with compost on the formation and structural characteristics of soil HA (as the most important fraction of soil organic matter), and the yield of wheat plant was investigated under a clay loam soil conditions. Five treatments were: 1) no biochar or compost was added (Ck); 2) peanut shells biochar with 2.5 t ha-1 (BC); 3) 12.5 t ha-1 compost (compost); 4) 2.5 t ha-1 of biochar +12.5 t ha-1 of compost (BCL1+compost); 5) 5 t ha-1 of the biochar+12.5 t ha-1 compost (BCL2+compost). The results showed that, the BCL2+compost amended soil had the highest grain yield, straw yield, biological yield, and harvest index. Although there was no significant difference between Ck and both BC and compost treatments, however the mixing of compost with biochar together significantly reduced the soil pH. In addition, the BCL2+compost amended soil had the highest HS, HAc, FAc and Hum carbon content as well as the highest humification index (HAc/FAc). Concerning the elemental composition, although the HA of the BC-treated soil had the highest O content, but the combination of BC and compost treatments raised the concentrations of C, and N in the soil HA structure, and they increased with the increase of biochar mixing level. This result supports the lingo-protin theory of HS formation. The single application of peanut shells biochar and the combined treatments decreased the H/C molar ratio in the extracted soil HA, which indicated the high degree of aromatisity in the formed HA compared to the CK and the single application of compost. Unlike the compost, the FT-IR spectrum of the HA for the BC-treated soil showed a lowest band intensity of aliphatic functional groups. While the BCL1+compost-treated soil had a HA with a highest intensity of bands at 1544 and 1656cm-1 which confirmed the contribution of the biochar in the formed HA and increasing the aromaticity. This study concluded that the combining application of biochar (low level) and compost can be an effective practice for soil HA formation and soil quality enhancement in clay loam soil.