Background and Aims More attention is being paid to utilization of the ecological resources in the lowlands of the Yellow River Delta. However, most proposed measures emphasize breeding, irrigation and cultivation. Soil enzymes play an important role in soil biochemical processes, such as litter decomposition, nutrient fixation and nutrient release. Insufficient attention has been paid to the relationships among soil enzymes, soil microbes and soil physical-chemical quality, yet such study can indicate the dynamic process of soil nutrients and soil health. Our objectives were to determine: 1) the distribution of soil enzymes of different plantations at different soil depths, 2) the correlations among microbes, soil nutrients and soil physical and chemical characteristics and 3) soil enzymes that are key factors in maintaining soil fertility.

Methods We analyzed soil enzymes, soil chemical characteristics and soil microbes for six kinds of plantations (Tamarix chinensis, Salix matsudana, Robinia pseudoacacia, Fraxinus chinensis, Ziziphus jujube and Morus alba) in Kenli County, Dongying, Shandong Province. Soil samples were collected at 0-20 cm and 20-50 cm depths. Established methods were used to analyze soil enzymes and soil physical and chemical characteristics. Analysis was done using an integrative method combining correlation and component analyses in SPSS.

Key Results Soil urease and soil peroxidase decrease with increasing soil depth. There are positive relationships between soil peroxidase and soil urease, and soil peroxidase and soil polyphenol oxidase, while there is negative correlation between soil urease and soil polyphenol oxidase. The kind of plantation clearly affects soil urease and soil polyphenol oxidase, but not soil catalase and soil peroxidase. Soil urease activity of different plantations is higher than that of the Tamarix chinensis forest; the soil polyphenol oxidase activity is lower than that of the Tamarix chinensis forest, except for the Robinia pseudoacacia forest. There are close correlations among soil enzymes and soil nutrients, and few correlations with pH values and soil microbes. Among them, soil urease shows positive relationships with total N, organic C and available C. Soil polyphenol oxidase shows a positive relationship with available K and negative relationships with soil catalase, total N, available P, organic C, peroxidase and soil urease.

Conclusions This study suggests that plantation type affects soil enzyme activities. The activities of soil urease and soil polyphenol oxidase can be regarded as indexes to assess soil quality in the lowlands.