Afforestation inhibited soil microbial activities along the riparian zone of the upper Yangtze River of China

Abstract

Afforestation strongly influences soil microbial traits, especially microbial biomass and enzyme activity. However, the magnitude and direction of soil microbial traits and their controls following afforestation remain unclear. Here, we examined microbial carbon (C), nitrogen (N), and phosphorus (P) biomass and activities of the most common enzymes involved in C (α-1,4-glucosidase, β-1,4-glucosidase, β-D-1,4-cellobiohydrolase, and β-1,4-xylosidase), N (β-1,4-N-acetyl-glucosaminidase and l-leucine aminopeptidase), and P (alkaline phosphatase) cycling under a 15-year-old forest stands (Morus alba, Salix bablyonica, and a mix of the two species) and adjacent croplands along the riparian zone of the upper Yangtze River of China. Unexpectedly, we found that the 15-year afforestation significantly lowered the contents of total C and N, available P, and NO3– compared to the adjacent cropland soils. The microbial biomass C, N, and P contents were also approximately 14–60%, 34–88%, and 9–89% lower, respectively, in the afforested soils than in the adjacent cropland soils across afforestation types and soil depths. Correspondingly, enzymatic C and N activities were 71–90% and 54–75% lower, respectively, under the S. bablyonica stands across soil depths, whereas enzymatic P activity was 39–72% lower in the afforested soils than in the adjacent cropland soils across afforestation types and depths. Reduced microbial biomass and enzymatic activities after afforestation were attributed to the decreased total C and nutrients in the afforested soils than in the adjacent cropland soils, likely due to high plant uptake nutrients and low soil organic matter inputs during the early stages of forest development. Overall, afforestation with plantations of Morus alba or Salix bablyonica species may reduce soil functions such as C storage and N fertility, suggesting that large-scale afforestation using these species should not be prioritized to restore soil functions within a short period.