Alterations in litter chemical traits and soil environmental properties limit the litter decomposition of near-mature Robinia pseudoacacia plantations

Increases in stand age can significantly change litter and soil microenvironmental properties of plantations. However, how these factors change and by which pathways they act together to affect litter decomposition are not well understood. In this study, litters were collected from four Robinia pseudoacacia (Rp) plantations from a 10 similar to 43-year sequence in the Loess Plateau (China) and used to conduct a 592-day in-situ decomposition experiment using litterbag method. The changes in litter chemical traits, soil physiochemical environment, and the litter and soil microbial community were detected. Then, the pathways of these changes affecting litter decomposition were analyzed based on partial least squares structural equation modeling. The results indicated that with increasing stand age, the increased litter substrate quality stimulated litter lignocellulase activity by affecting the fungal community structure in early and late stages of decomposition and the bacterial community structure in early decomposition, which tended to accelerate litter decomposition. However, the decrease in litter chemical diversity weakened the synergistic effects of mixed litter decomposition, and thus directly hindered the overall decomposition. On the other hand, increasing stand age caused excessive soil temperature and low soil moisture in early decomposition stage, which indirectly affected the litter microbial communities through changes in the soil fungal community, leading to decreases in litter lignocellulase activity and litter decomposition. Overall, changes in litter chemical traits with increasing stand age tended to cause dual effects, while those in the soil microenvironment tended to cause increasing negative effects. Together these changes finally affected litter decomposition by litter microbes and their enzymatic activities, leading to depressed litter decomposition and a risk of weakening material cycling in near-mature (33-year) Rp plantations.