Soil carbon dynamics in Indian Himalayan intensified organic rice-based cropping sequences

The contribution of soil to supporting, regulating, provisioning and cultural functions as well as its role in the ecosystem services is well-known in the international literature. However, in the domain of organic agriculture, the impact of cropping systems shifts from cereal-cereal to high-frequency diversified cropping sequences with legume as a component crop on soil carbon dynamics is not widely known. In order to identify an alternative cropping system to widely prevalent rice-fallow production system in Himalayan region of India, seven cropping sequences viz., rice -fenugreek (green vegetable) - maize (R-F-M); rice -vegetable pea - maize (R-Vp-M); rice-coriander (leaves)-cowpea (R-C-Cp); rice - fenugreek (green vegetable) - baby corn (R-F-Bc); rice - broccoli - Sesbania (green manuring) (R-B-S); rice - buckwheat (R-Bw) and rice - maize (R-M) were assessed for five consecutive years from 2013 to 2018 for their productivity and resource conservation values. Results revealed that the inclusion of legumes in rice-based sequences increased the rice grain yield by 13.4 to 24.6% over R-M (3.13 Mg ha−1) sequence. The R-B-S sequence had the highest very labile carbon (VLC) (4.6 g kg−1 soil) followed by the R-Vp-M. Relative proportion of various organic carbon fractions in the top 10 cm soil followed the order of VLC (30.2%) > non labile carbon (NLC, 27.6%) > labile carbon (LC, 23.4%) > less labile carbon (LLC, 18.9%). The carbon management index (CMI) was the highest (100.9%) in the R-B-S sequence followed by R-C-Cp (98.0%). The addition of a third crop in the sequence increased the active carbon (AC) pool by 1.1 to 5.8%. The passive carbon (PC) pool was highest in soil under the R-C-Cp sequence (9.15 Mg ha−1) at 0–10 cm soil depth. The carbon retention efficiency under the R-C-Cp cropping sequence was the highest (15.1%) followed by the R-B-S (14.9%). R-B-S and R-C-Cp sequences had 12.5% and 10.6% higher soil microbial biomass carbon (SMBC) over the R-M sequence, respectively. Similarly, R-B-S and R-C-Cp increased the FDA by 49.6 and 41.8%, and DHA by 135.0% and 103.9%, respectively over R-M sequence. In conclusion, the management of crops from organic agriculture aimed at improving soil ecosystem services, in contrasting degradation of soil health and the decline of SOC, can also have positive effects on crop productivity in the eastern Himalayan region of India as well as all over the world.