Effects of mixing maize straw with soil and placement depths on decomposition rates and products at two cold sites in the mollisol region of China

Crop straw is often retained on the ground as mulch or incorporated into soil through tillage practices. These straw management practices may affect straw decomposition rates and subsequently decomposition products due to mixing straw with soil, straw placement depths and experimental sites. Using litterbags containing maize (Zea mays. L) straw only (STO) or straw-soil mixture (SSM), this study aimed to determine the influence of mixing straw with soil and straw placement depths on decomposition rates and decomposition products, and to understand the relation of chemical composition of straw residues with straw decomposition rates. The litterbags were placed at three soil depths (0, 15 and 30 cm) in the fields at two cold sites (Hailun and Harbin), Northeast China, which had similar precipitations but different temperatures. During the 17-month experiment, the decomposition rate constants were higher by 60 %–160 % at a comparable depth in SSM than in STO. The mixing straw with soil was a more important factor in controlling straw decomposition rates than placement depths and experimental sites at the studied region. The relative abundances of O–alkyl C groups in straw residues decreased during the experimental period, even in the winter, while those of aromatic groups increased profoundly. The relative abundances of O–alkyl C groups in straw residues were lower by 0.4 %–6.9 % and those of aromatic groups were higher by 0 %–4.6 % in SSM than in STO. These findings suggest that the conventional litterbag method underestimated straw decomposition rates. Incorporating straw into soil could enhance maize straw decomposition and the formation of more stable soil organic matter (SOM) in the cold high-latitude studied region.