Foxnut (makhana): a productive and economic crop providing good niche for soil carbon storage and stability in lower Gangetic wetlands

Commercial crop Foxnut (makhana) cultivation is being popularized for the productive utilization of wetlands in eastern India. Here, the impact of two different cropping on organic C (OC) stock, stability, exogenous C input by crops, system yield, and economics has been studied in soils of makhana and rice–rice–fallow collected from at least five– year–old cultivation fields at Malda district, West Bengal State, India. Six geo–referenced soil samples (three from each) after harvesting of the winter (boro) rice at 0–25, 25–50, 50–75, and 75–100 cm depth was considered to capture the variability of OC stock and stability in each depth of soil. Soil inorganic C (TIC) and organic (SOC) fraction, total organic C (TOC), simple oxidizable Walkley and Black C (CWBOC), very labile, labile, less labile, non–labile, active, passive pools, and several SOC indices [recalcitrant, lability, and stratification ratio (SR)] were captured based on oxidation with several strengths of chromic acid. Soils of makhana contained a larger amount of sand, less clay, silt, and more compacted, contained more available macro and micronutrients. Conversely, in most of cases, TOC stock and its different pools were higher in rice–rice–fallow than makhana cropping (P < 0.05) with a decreasing trend along the depth. Recalcitrant indices (RI1 and RI2) of the SOC were larger for soils under makhana than rice–rice–fallow in surface soil, and contrary to the values of the lability index. However, SR was higher in soil under rice–rice–fallow than makhana (P < 0.05), indicating a better soil quality in the former than the latter. The system makhana rice equivalent yield (SMREY) was calculated and these values and benefit to cost (B: C) for makhana cultivation was greater than rice–rice–fallow (P < 0.05). This was again verified with larger carbon management index for rice–rice–fallow than makhana. The soil properties, larger exogenous C–input, and management practice prevailed over the effect of rice and its ecology on the stock and stability of SOC than makhana. Therefore, double rice–based systems, practised in the lower Ganges–basin showed better C economy, but makhana showed high economic return.