Lignin and cellulose dynamics with straw incorporation in two contrasting cropping soils

Incorporation of crop residues is essential to enhance soil organic matter in arable ecosystems. Here, we monitored the dynamics of cellulose and lignin, the most abundant constituents of plant residues, and their relationships with enzyme activities, microbial gene abundances and soil properties after 13-year long-term and one-year short-term crop straw incorporation into upland and upland-paddy soils in a field-based experiment. Lignin, rather than cellulose, accumulated in both soils following straw incorporation. Cellulose was almost completely converted into non-cellulose forms within 6 and 3 months after straw incorporation into upland and upland-paddy rotation soils, respectively. Whereas, lignin accumulated at the rate of 129 and 137 mg kg^-1 yr^-1 within 13 years' straw incorporation in upland and upland-paddy rotation, respectively. The predominance of recalcitrant vanillyl monomers in upland-paddy rotation indicated a high stability of lignin. Structural equation models revealed that the key factor driving cellulose and lignin dynamics was available nitrogen, followed by enzymes activities (cellobiohydrolases and laccases) and functional genes abundances (cbhI and laccase-like) as mediated by soil pH. Our findings highlighted that upland might have higher carbon sequestration rate, whereas upland-paddy rotation system was more beneficial for accumulation of recalcitrant organic fractions under crop residue incorporation.