Impact of no-till and mulching on soil carbon sequestration under rice (Oryza sativa L.)-rapeseed (Brassica campestris L. var. rapeseed) cropping system in hilly agro-ecosystem of the Eastern Himalayas, India

Decline in soil organic carbon (SOC) and low biomass production in sloping uplands are of growing concern for sustainable agriculture worldwide. This concern is in general in the Eastern Himalayan regions (EHR) of India in particular. A field experiment was conducted with the objectives to generate additional biomass and sequester more C in coarse-textured sloping lands. This experiment is done for four consecutive years in the EHR, India. The rice (Oryza sativa L.)-rapeseed (Brassica campestris L. var. rapeseed) cropping system was practiced during the first two years (2012-13 to 2013-14) and rice-rapeseed-cowpea (Vigna unguiculata L.) system during the two following years (2014-15 to 2015-16) of the study under different tillage and mulch systems. The tillage system included: 1) CT-RI: conventional tillage (CT) with 100% residue incorporation (RI) and 2) NT-RR: no-till (NT) with 100% residue retention (RR). The mulches included 1) rice straw mulch (SM), 2) Gliricidia sp. mulch (GM), 3) brown manuring mulch (BM)-cowpea grown as intercrop with rice up to 40 days after sowing (DAS), killed with 2,4-D and 4) no mulch (NM). The cowpea, as a cover crop was introduced during 2014 and 2015 as pre-rainy season crop before the sowing of rice to generate additional biomass in the system. The four year total above ground biomass yield of rice and rapeseed didn't vary significantly between CT-RI (31.93 and 17.40 Mg ha(-1)) and NT-RR (31.86 and 17.46 Mg ha(-1)), respectively. However, the total above ground biomass yield of cowpea was more under NT-RR (10.75 Mg ha(-1)) when compared to that under CT-RI (9.79 Mg ha(-1)). The amount of total biomass (above + below ground) and C added into the soil was more under NT-RR than that under the CT-RI. After 4 cropping cycles, the NT-RR had higher SOC concentration, pool (29.9 vs. 29.1 Mg ha(-1)), sequestration rate (450 vs. 265 kg ha(-1) yr(-1)) and C retention efficiency (7.7 vs. 4.6%) than those under the CT-RI at 0-30 cm depth. The mulched plots produced more crop biomass (both above and below ground), recycled more C in soils with a trend of relatively more SOC pool (29.7-29.8 vs. 29.0 Mg ha(-1)), sequestration rate (391-428 vs. 221 kg ha(-1) yr(-1)) and C retention efficiency (6.64-6.94% vs. 4.66%) than those of NM treatment. These results were seen despite difference among the mulch treatments (SM, GM and BM) which were statistically non-significant after 4 cropping cycles. Inclusion of cowpea as cover crop during pre-rainy season in the system doubled the rate of C sequestration (478 kg C ha(-1) yr(-1)). Therefore, the data supports the recommendation of cultivation of the rice -rapeseed system under NT-RR along with cowpea intercropping up to 40 DAS (BM) in rice. Besides this, the inclusion of pre-rainy season cowpea before rice could generate additional biomass and enhance SOC sequestration on upland and sloping hills in the EHR of India and in similar conditions elsewhere.