In situ roots decompose faster than shoots left on the soil surface under subtropical no-till conditions

Abstract

Better understanding of soil C dynamics under actual agricultural conditions is important for predicting impacts of land use and climate change on soil C sequestration. Few field studies have compared simultaneous decomposition of intact roots and surface shoot residues under no-till (NT) field conditions. Therefore, we estimated the actual decomposition of intact roots and shoot residues simultaneously under NT field conditions. Wheat (Triticum aestivum L.), pea (Pisum sativum L.), and vetch (Vicia sativa L.) plants were grown inside polyvinyl chloride (PVC) cylinders hydraulically forced into a field of sandy loam-textured Typic Paleudalf. After 20 days of emergence, the crops were pulse labeled weekly with 13CO2 until flowering. At harvest, the treatments were designed by combining 13C-labeled shoots with unlabeled roots and unlabeled shoots with 13C-labeled roots, resulting in six treatments (two combinations × three species), plus a non-amended control treatment. Soil CO2 emissions were measured continuously by the alkaline trap method during 180 days. Apparent C mineralization was similar for the three species in paired treatments: 54 ± 8.8 % added C for wheat, 54 ± 3.4 % for pea and 51 ± 3.4 % for vetch. However, actual mineralization of the roots 13C was higher than that of the 13C shoots, for the three species (73 vs. 45 % initial C for wheat, 76 vs. 48 % for pea, and 73 vs. 51 % for vetch). These findings emphasize that the environmental drivers of decomposition, i.e., the crop residue location, their contact with soil, and the soil moisture and temperature, are important factors that significantly promote root decomposition in situ compared to shoots, negating the consequences of their different initial chemical composition on their kinetics of decomposition.