Biomass, biochemical composition and decomposition behavior of roots and shoots of major rainfed crops

A Study was conducted during 2012–14 to quantify root and shoot biomass of 2 cultivars each of 8 rainfed crops (sorghum, greengram, sunflower, maize, castor, pigeonpea, cowpea and horsegram), to determine their biochemical composition and to examine their decomposition behavior in soil. Root biomass of all the crops and cultivars was lower than the respective shoot biomass. Roots accounted for 12.07% (Horsegram, CRHG 4) to 35.26% (Maize, DHM 117) of the total plant biomass. Root biomass (averaged over cultivars) varied widely with crops, ranging from as low as 5.24 g/plant (Horsegram) to as high as 158.23 g/plant (Pigeonpea) and was in the order pigeonpea > sorghum > maize > castor > sunflower > cowpea > greengram > horsegram. Shoot: root ratios ranged from 1.84 (Maize, DHM 117) to 7.29 (Horsegram, CRHG 4). There were marked differences in shoot: root ratios among crops and even cultivars within crops. Biochemical analysis revealed that cell wall was the dominant fraction of the plant tissue accounting for up to 3/4^th of the tissue. Regardless of crop or cultivar, roots had lower soluble cell contents and higher cell wall contents than shoots. Averaged across crops and cultivars, lignin content of roots was 13.76% as against 8.38% for shoots. Crops differed significantly in the lignin content of their roots, which ranged from 8.25% in maize to 19.15% in pigeonpea. The dicots with taproot systems (castor, sunflower, greengram, cowpea, horsegram, pigeonpea) had higher lignin content than the monocots with fibrous root systems (maize, sorghum). Lignin/N ratios of roots were 2–3 times higher than those of shoots. Patterns of carbon mineralization of roots and shoots were exponential in nature, being faster in the initial stages and slowing down over time. Regardless of crops and cultivars, roots exhibited distinctly slower carbon mineralization than corresponding shoots. Averaged across crops and cultivars, per cent C mineralized in 120 days was 37.35% in roots as against 50.22% in shoots. Lignin content (r =-0.684**) and lignin/N ratio (r =-0.636**) had a highly significant negative relationship with % C mineralized.