Relational analysis of soil enzyme activities, nutrients and microbes inRobinia pseudoacacia plantations in the Yellow River Dalta withdifferent degradation degrees

A great deal of degradation of plantations across the globe are caused by natural and artificial factors. The Yellow River Delta (YRD) is one of China's three estuarine deltas where plenty protection forests were established in the 1980s to improve local ecological environment. Among the forest plants is Robinia pseudoacacia, covering the largest area as main sand/windbreaker in this region. In recent years, however, much of the R. pseudoacacia plantation has dry-tops and some even wither to death. This has caused significant loss to shelter forest construction and management of salinization in the region. In this paper, R. pseudoacacia plantations with different degradation degrees in the YRD were selected for relational analysis of soil enzymes, nutrients, microbes as a means of exploring the causes of degradation of the plants. The results showed soil urease, polyphenol oxidase and peroxidase activities decreased with the aggravation of R. pseudoacacia plantations degradation. However, hydrogen peroxidase activity presented first increasing and then decreasing tendency. Urease was significantly positively correlated with polyphenol oxidase and peroxidase. Peroxidase was also significantly positively correlated with polyphenol oxidase, but not with the other enzymes. Soil nutrients such as organic matter, total nitrogen, available nitrogen and available phosphorus decreased with increasing degradation degree of R. pseudoacacia plantations. The reverse was the case for soil pH and soil salts. Soil enzymes, especially urease, were significantly positively correlated with soil nutrients and negatively correlated with soil pH and soil salts. Bacteria count was highest in R. pseudoacacia plantation soils with different degradation degrees. With increasing degradation degree of R. pseudoacacia plantation, average soil bacteria count was in the following order: no degradation > slight degradation > moderate degradation > severe degradation. The order for average fungi count was as follows: slight degradation > no degradation > moderate degradation > severe degradation. Then that for actinomycetes was: moderate degradation > slight degradation > no degradation > severe degradation. Urease was highly positively correlated with bacteria and actinomycetes. Bacteria count was highly positively correlated with enzymes, except hydrogen peroxidase. The other enzymes showed no significant correlations with microbial count.