Assessment of soil carbon sequestration and climate change mitigation potential under conservation agriculture (CA) practices in the Eastern Gangetic Plains

Conservation agriculture (CA) cropping is based on the principles of minimum soil disturbance, permanent soil cover with crop residue retention and crop rotations with diverse crops. The CA cropping performs well in improving soil health, increasing yield and increasing crop profit in the intensive rice-based, triple–cropping systems on the Eastern Gangetic Plain (EGP), but its effects on the greenhouse gas (GHG) emissions and the dynamics of carbon (C) and nitrogen (N) in the soil has not been studied properly. Two experiments lasting 5 years have examined soil C, N and life cycle GHG emissions in the EGP plains’ intensive rice (Oryza sativa L.)–based cropping soils of Bangladesh. The present study employed a streamlined life cycle assessment (LCA) approach to assess implications of GHGs from CA cropping in comparison with conventional cropping. Minimum disturbance of soil and increased residue retention were assessed at both long-term studies involving rice-based triple cropping systems at Durgapur and Godagari in the EGP since 2010. Component crops of the rice-based systems (lentil (Lens culinaris Medik), mustard (Brassica campestris L.), chickpea (Cicer arietinum L.), jute (Corchorus olitorius L.), early wet season rice & mustard) were established by strip planting (SP) and bed planting (BP), or following 3-4 tillage operations by 2-wheel tractor followed by hand-broadcast seeding and fertilizing (CT). All practices were compared with the conventional low residue retention or increased retention. In case of irrigated and rainfed rice, non-puddled (NP) transplanting were adopted in SP and BP; while soil puddling was used for CT. The life cycle GHG t-1 crop or rice equivalent yield (REY) were assessed under four practices of cropping a) traditional crop establishment practices (CT) with farmers’ practice of residue return (LR), b) CT with return of increased residues (HR); c) strip planting (SP for upland crop)/ transplanting on non-puddled soils (NP for rice) with LR or; d) SP/NP with HR. The cropping systems studied in the long-term trials were mustard-irrigated rice-monsoon rice at Alipur and wheat-jute-monsoon rice at Digram sites. The SP/NP of soils with HR sequestered carbon in soils after five years of cropping at both the locations, relative to current practices of cropping by farmers (CTLR). The increased soil C was associated with reduced CO2eq emissions (13 to 59 % lower than those under CT and BP with LR and HR, respectively, relative to SOC), reduced water soluble carbon (WSC, by 15-23 mg kg-1, relative to CT with LR and HR) contents in soils and increased potentially mineralizable C (PMC) and lower decay rate constant (e.g. 50 % in rice soils). Similarly, at each location (0–10 cm soil depth), SP, including NP, together with HR increased total N by 9 and 32 % relative to BPHR and CTHR and by 62 % relative to the current practice (CTLR), respectively. The increased total N in soil resulted from the increased potentially mineralisable N (PMN) with its low decay rate in soil under all crops with SPHR, relative to other tillage and residue retention practices. The total mineralisation of N in soils under SPHR was statistically equal to (in wheat and jute cropping) or was lower (in mustard and rice cropping) than those under CT with HR. However, soils under SP with residue retention practices had synchronized release of N with crop demand, while CT with LR or HR had increased mineralization during 0–45 days of crop establishment. Conservation agriculture involving SP, and NP of rice, together with HR, has altered the C and N cycling. The alterations were occurred by slowing the early mineralisation of N, reducing the level of mineral N available to plants in the early growing season (low N requirement) but increasing soil total N and plant N uptake by enhancing the synchrony between crop demand and available N supply. In case of C cycling, SP/BP with HR at both the locations modified the C cycle by slowing the in-season turnover of C and by increasing the levels of total organic C in the soil. For all crops in the mustard-irrigated rice-monsoon rice cropping system, SP/NP with LR and HR were the best actual life cycle GHG mitigation option. With the considerable accumulation of SOC (3.8 - 4.2 t CO2eq ha-1) in SP/NP at 0 10 cm soil depth after 5 years in comparison with CT, the life cycle GHG savings with the best mitigation practice (SP/NP with LR) for 1 t of rice-equivalent yield were 46 % relative to CT with LR. Production of 1 t of REY in the rice–based system caused 0.73, 0.74, 0.98 and 1.12 t of CO2eq LCA GHG emission (actual). Production of 1 t of irrigated rice in the EGP after accounting for C sequestered in soils accounted for 0.91, 0.95, 1.25 and 1.41 t CO2eq for NPLR, NPHR, CTLR and CTHR, respectively, whereas the LCA GHGs for the production of 1 t of monsoon rice were 1.10, 1.21, 1.4 and 1.65 t, respectively. For each unit RE mustard production, NPLR, NPHR, CTLR and CTHR were responsible for 0.09, 0.18, 0.31 and 0.29 t CO2eq, respectively. Overall, methane (CH4) released during the on-farm stage of the LCA represented the dominant contributor to LCA GHG in the cropping system. The GHG emitted by machinery usage at on-farm stage (irrigated rice), CO2 emission from soil respiration (monsoon rice), and GHG related to inputs manufacture (REY of mustard) were secondary sources in that order of magnitude. The NPLR and NPHR were the most effective GHG mitigation options when sequestered C was taken into account in footprints of component crops of rice-based rice-upland cropping system. The NPLR and NPHR practices avoided 51 % and 35 % of the actual LCA footprints compared with CTHR and current farmers’ practice, respectively. By not including soil C sequestration in the carbon footprint equation, the life cycle GHG estimates were over-estimated by 9 to 26 %. When soil C sequestration estimated by subtracting C losses from net primary production (NPP) was accounted for in the LCA GHG, the largest decrease in LCA GHG by 20 % was recorded in NPHR but LCA GHG increased by 12 % in CTLR. Overall, the NPLR and NPHR were the most effective GHG mitigation options in production of crops of mustard-irrigated rice-monsoon rice system but NPHR offered yield benefit and its higher CH4 emission was offset by the extra soil organic carbon (SOC) sequestration. The emerging CA approaches being developed for the EGP involving strip planting or NP have the potential to mitigate GWP of intensive rice-based triple cropping systems but further study is needed for a more diverse range of rice-dominant and rice-based triple cropping systems.