Distribution characteristics of soil carbon and nitrogen under different afforestation modes of saline land in the Yellow River Delta

The Yellow River Delta is the largest estuarine wetland ecosystem in the warm temperate zone of in China and the most active land-ocean interacion regions among the large river deltas in the word. However,here rainfall was less and the mineral content in underground water was higher. These conditions caused soil salinization and alkalization. Soil affected by saline and alkaline constitute nearly 50 percent of total and surface. Therefore,the development of management strategies designed to reduce soil salinity has become a high priority in this region. However,a better understanding of ecological effects of different afforestation is necessary before targeted management strategies can be conceived. Thus,the objective of this study is to assess the effects of agro-forestry and pure forest on the organic carbon and nitrogen of saline soil in the Yellow River Delta. Total soluble carbon( TOC),dissolved organic carbon( DOC),total soluble nitrogen( TSN),solubleorganic nitrogen( SON),NH+4-N,NO3-N and total nigrogen( TN) in soil taken from different afforest measures were analyzed. Five afforestation models were Fraxinus chinensis + Gossypium hirsutum( FC),Robinia pseudoacacia cv. Idaho +Gossypium hirsutum( RC),Salix babylonica + Gossypium hirsutum( SC),Fraxinus chinensis( F) and Salix babylonica( S). Compared with bare land,soil TOC and DOC contents in the combination of SC were highest among all treatments,followed by FC,RC,Soil DOC content in all systems tended to be higher in surface layer( 0-20 cm) than in deeper layer( 20-40 cm). Regardless of soil layers,the contents of TSN and SON were higher in agro-forestry than in pure forest,SC had the highest level of soil TSN and SON. The contents of SON in 0-20 cm soil layers were significantly higher than 20-40 cm soil layers under FC,SC and S,respectively was 1.4,1.5 and 2.7 times of them. Conversely,the contents of soil SON in 20-40 cm were significantly higher than in 0-20 cm under RC. Among five afforestation measures,the contents of all kinds of soil nitrogen were significantly higher than bare land,excepted for the lower NO-3-N contents under F. There was a positive correlation between the DOC,and TN and NH+4-N( P0.01). The conclusion indicates that the agro-forestry could significantly improve the contents of soil effective carbon and nitrogen,in particular,SC was the best afforestation model,while the pure forest was poor.