Bioenergy sorghum as balancing feedback loop for intensification of cropping system in salt-affected soils of the semi–arid region: Energetics, biomass quality and soil properties

Abstract

Soil salinity and limited freshwater availability are the challenges for intensification of the cropping system in semi–arid regions with monsoonal climate. Augmenting freshwater with saline groundwater could be an alternative strategy for increasing cropping intensity. This experiment was conducted to test the impact of deficit saline water irrigation (DSI) with mulching and tillage on root zone salinity and performance of rainfed bioenergy sorghum [Sorghum bicolor (L.)] followed by saline water irrigated wheat. Three tillage treatments viz. zero–reduced (RT), conventional–conventional (CT), and zero–zero (ZT) were practiced in the main plot and irrigated wheat [comprising of saline water (ECiw 8.0 dS m–1)] with 60%, 80% and 100% of water requirement (WR) – rainfed sorghum and mulch (no mulch and 5.0 Mg rice straw ha–1) in subplots. Deficit saline irrigation, RT, and mulching reduced the salinity by 1.5 times in the second rotation. Ca2+ and Mg2+ and K+ content in soil solution were increased under RT and ZT, respectively compared to CT. The rice straw mulching increased the CO32–, HCO3– and K+ concentration (P < 0.005). Irrigation with 100WR saline water increased Ca2+ and Na+ content in soil solution compared to DSI. The DSI at 60WR with mulch in reduced tillage produced greater biomass. The saline irrigation increased the lower heating value of the sorghum biomass compared to pond water irrigation. The energy output of the RT and ZT was 16.8% higher than CT. Mulching also caused a 7.9% increase in energy output. RT with 60WR and mulch produced higher energy. Energy input associated with production systems involving different tillage and mulching practices was determined at 12.0 – 28.3 GJ ha–1. The bulk of the energy input was through crop residue (50.2–54.2%) followed by mineral fertilizers (28.0–30.2%) and seeds (8.4–9.1%). Mulching reduced the energy efficiency ratio. RT produced a similar amount of holo-cellulose while the greater value of lignin compared to CT. Different soil variables explained ~35% variability in biomass quality. This study concludes that integration of reduced tillage, deficit irrigation, and mulching is effective in the intensification of the cropping system and bioenergy sorghum can be a candidate crop in rotation with saline water irrigated wheat in a semi-arid region. These results also showed the potential of crop diversification in the rainfed region for increased feedstock biomass production and efficient use of saline water.