Depth Profiles of Soil Carbon Isotopes as Influenced by Crop Residues

Abstract

Soil organic carbon (SOC) is an important component for the sustainable maintenance of soil fertility. SOC supply to the cultivated soil may vary in each cover crop species which should be taken into account in the plant residues management. Studies have documented a strong relationship between decreased SOC and increased δ¹³C with soil depth. Therefore, the present study investigated whether SOC is influenced by different crop residue amendment by determining δ¹³C in soil profiles. Soil samples were collected during 2016-2018 from banana crop land, rice paddy with straw retention, and rice paddy with straw burning prior to crop rotation practice of Sunnhemp (Crotalaria juncea L). Depth profiles of 0-3, 3-9, 9-12, 12-15, 15-18, 18-21, 21-24, 24-27, and 27-30 cm were determined on δ¹³C for each sampling site.The δ¹³C values ranged as follows -27.11‰ to -23.27‰ for banana plantation, -29.52‰ to -26.37‰ for rice straw retention, and -24.63‰ to -16.81‰ for rice straw burning prior toharvest of Sunnhemp, a legume rotation crop. These results indicate a strong relationshipbetween the types of plant residues, including charred rice straw and its isotopic values. It isthus explained that δ¹³C values of SOC will shift towards the δ¹³C values of plant residues.Results revealed that δ¹³C increased with soil depth due to the age of the biomass while SOCdecreased. Further research should be conducted to determine the effect of crop residueincorporation with different soil textures, water contents, and climate conditions.