Ecoenzymatic stoichiometry and nutrient dynamics along a revegetation chronosequence in the soils of abandoned land and Robinia pseudoacacia plantation on the Loess Plateau, China

The effects of vegetation restoration on plant characteristics and soil physicochemical properties have been widely documented; however, knowledge of the variation in soil ecoenzymatic activity and stoichiometry remains limited, particularly with respect to their relationship with plant and soil variables in restored ecosystems. Here, the vegetation and soil from one farmland and two restored land-use types (Robinia pseudoacacia plantations: RP and abandoned lands: AL) of four age classes (10, 18, 28, and 43-years) were investigated and sampled on the Loess Plateau, China. The activity of C, N, and P-acquiring enzymes (β-1,4-glucosidase, BG; β-1,4-N-acetylglucosaminidase, NAG; leucine aminopeptidase, LAP; and alkaline phosphatase, AP) and other major influencing factors (soil physical properties, soil nutrient contents, and microbial biomass) were determined. It was found that both restoration time and vegetation types significantly affect plant characteristics, soil physical properties, soil nutrient content, and microbial biomass. Soil BG, NAG + LAP, and AP enzyme activities were higher in RP and AL than in farmland, and increased with restoration time in AL. For RP, the activity of these enzymes increased from year 10 to year 28, then decreased from year 28 to year 43. Soil ecoenzymatic C:P and N:P activity ratios were ordered: farmland > AL > RP, and decreased with restoration time in AL and RP; thus, P limitation was stronger in RP than in AL and increased with restoration time in both AL and RP. Soil ecoenzymatic C:N:P acquisition ratios of farmland, AL, and RP deviated from the 1:1:1 ratio and depended on the availability of environmental nutrients and demand for microbial nutrients. The vegetation characteristics and soil physical properties were closely related to the nutrient acquisitions of microbes and, ultimately, contributed towards shaping soil ecoenzymatic activity and stoichiometry (particularly vegetation coverage, belowground biomass, soil water content, soil bulk density, and pH). Furthermore, variation in soil ecoenzymatic activity and stoichiometry was better accounted by dissolved nutrients in the soil (particularly C and N) and microbial biomass (particularly N and P) than by plant characteristics and soil properties. Overall, this study demonstrates that the C:N:P stoichiometry of soil microbes and ecoenzymatic activity is nutrient dependent, rather homeostatic, with the potential to influence nutrient cycling on the Loess Plateau.