Crop residue biochars – A way towards sustainability as a potent climate change mitigator and fertility enhancer

Air pollution is one of the major leading causes of death in India, with 7 out of 10 most polluted cities located in Indo-Gangetic Plain region. With more than 70% residue generated being burnt, crop residue burning is one of the major contributor. Along with pollutants (black carbon, carbon dioxide, carbon monoxide) emissions, residue burning also depletes soil nutrients. Hence, this study aims to provide a sustainable alternative for residue burning, by converting the residues into value added product (biochar). This study demonstrated that biochar not only reduces air pollution by sequestering carbon but can also increase crop productivity in a degraded land (saline). In this study, rice husk and wheat straw crop residues were converted to biochar, in an indigenous reactor. For comparative assessment, both biomass and biochar [rice husk biomass (RHBM), rice husk biochar (RHBC), wheat straw biomass (WSBM), wheat straw biochar (WSBC)] were applied at 1%, 2.5% and 5% (w/w) on two saline soils (Samchana : EC - 4.97 dS/m and Dobh : EC - 5.33 dS/m). It was observed that, in both Samchana and Dobh soil, CO₂ emissions were significantly lowered with RHBC (46–53%, 41–56%) and WSBC (66–80%, 61–74%) treatments as compared to RHBM and WSBM and hence increased carbon squestration. Additionally, okra was grown for two seasons (Samchana season 1 - Samc1, Samchana season 2 - Samc2, Dobh season 1 - Dobh1, Dobh season 2 -Dobh2). Salinity reduction was observed, more so using RHBC and WSBC. In all the analyzed soil parameters (pH, EC, soil organic carbon (SOC), soil available phosphorus (SAP), mineral nutrients and nutrients rations), both RHBC and WSBC provide better enhancements as compared to their biomass counterpart as well as unamended soil. These improvements were reflected by plant growth parameter enhancements. Apart from improving various soil and plant growth parameters, biochar amendments also enhance salt leaching via improvements in saturated hydraulic conductivity (Ks). RHBC and WSBC improved Ks by 9–95% and 64–112%, respectively. This was reflected in higher desalination ratios with (DR) with RHBC and WSBC w.r.t their biomass counterpart and control. In Dobh soil, DR values were negative for RHBM and WSBM treatments, indicating that no reclamation occurred, which is correlated with lower soil and plant growth parameters observed. Overall, we concluded (1) biochar significantly reduces emissions of air pollutant (CO₂) by capturing and sequestering in soil (2) biochar provides better salinity reduction and improvement in plant growth parameters and salt tolerance. Therefore, biochar can be considered as a sustainable air pollution mitigation strategy along with enhancing crop productivity in a degraded saline soil. This study presents biochar as a potent means to achieve sustainable development goals (SDG) – SDG 2 (by enhancing productivity), SDG 3 and SDG 13 (by reducing CO₂ emission).