Effect of activated carbon on greenhouse gas emissions, seed yield, soil chemical properties and isoflavone content of soybean genotypes with varying nodulation capacities under sandy soil conditions

Activated carbon (AC) is a known adsorbent for organic compounds including root exudates but could have an influence on crop yield and greenhouse gas (GHG) emissions. A 2-year pot experiment was conducted to assess the effect of AC on GHG emissions, seed yield, soil chemical properties and isoflavone content of soybean genotypes with varying nodulation capacities under sandy soil conditions. We also evaluated the nodulation and seed protein content after harvest. The soybean genotypes were TnVRSN4, Tachinagaha and TnVRNN4 with high, normal and low nodulation capacities respectively. AC was applied at rates equivalent to 0, 2.4, 4.8, and 9.6 t ha⁻¹ in combination with inorganic fertilizers. The results showed that AC tended to reduce soil N₂O emissions in the high nodulating genotype due to the significant reduction in nodulation but did not significantly affect CO₂ and CH₄ emissions. Highest CO₂ emissions and seed yield were observed in the high nodulating genotype and lowest in the low nodulating genotype. AC did not significantly affect seed yield of the high nodulating genotype but significantly reduced seed yield of the low and normal nodulation genotypes in 2017 and 2018 respectively. Although AC generally increased soil total N, total C and C/N ratio, its effect on soil pH, available P and exchangeable cations significantly varied with the soybean genotype. AC did not significantly affect root isoflavone, seed protein and total isoflavone content but significantly reduced the concentration of daidzein and daidzin which were exuded from soybean roots in soil. The high nodulating genotype can perform better in sandy soils with a low nutrient status but further studies using pyrogenic carbonaceous soil amendments are needed to evaluate and compare the three genotypes in terms of productivity and GHG emissions under field conditions, as well as assessing the feasible chemical N fertilizer application rates for the low nodulating genotype.