Effect Of Freshwater Restoration On The Soil Nutrient And Vegetation In The Degraded Yellow River Delta Wetlands

The Yellow River Delta wetland is the largest wetland ecosystem in the warm temperate zone of China. To restore the degraded wetlands in the Yellow River Delta, freshwater was supplied during the flood season from June to July. In order to explore the effect of wetlands restoration on the soil nutrient and vegetation, the changes of pH value, electricity conductivity, soil organic carbon (SOC), total nitrogen (TN), NH4+-N, NO3--N, total phosphorus (TP) and available phosphorus (AP) in the degraded and restored wetlands of Yellow River Delta were analysed.The results showed that: (1) Plant communities and species continually increased with recovery time in the Yellow River Delta. Restoring plants, such as Ploygonum hydropiper and Cattails have been found after seven years artificial restoration. The height of plant was increased from 45cm to 99.25cm in the course of restoration. (2) The electrical conductivity significantly declined (p<0.01) with the increasing practice time. The horizontal distribution of soil electricity conductivity in upper soil layer (0-20cm) was 8.975ds/m (before artificial restoration)→0.935 ds/m (after three years artificial restoration)→0.696 ds/m (after five years artificial restoration)→0.327ds/m (after seven years artificial restoration). The horizontal distribution of soil electricity conductivity in the lower soil layer (20-40cm) was 4.688ds/m (before artificial restoration)→1.144ds/m (after three years artificial restoration)→1.027ds/m (after five years artificial restoration)→0.364ds/m (after seven years artificial restoration). Soil electricity conductivity in the lower soil layer (20-40cm) was lower than upper soil layer (0-20cm) in restored area. But it is the opposite trend in degraded sites comparing with the restored sites. It indicated that providing freshwater reduced the salt obviously. pH values showed unobvious variations with the increasing practice time. (3) The SOC and TN in the 0-20cm soil layer increased with recovery time (p< 0.01). The SOC and TN were 7.710±0.756g/kg, 0.66±0.021g/kg and 16.96±0.213g/kg, 1.277±0.027g/kg before and after seven years artificial restoration, respectively. The SOC in the 20-40cm soil layer presented the trend of first decrease then it increase with recovery time. The upper soil layer (0-20cm) had more SOC and TN than the lower soil layer (20-40cm) in the restored sites. But the concentrations of SOC and TN had no significant change (p>0.05) in the two soil depth in degraded sites. The NH4+-N was higher than NO3--N in all sample sites. (4) The ratios of C/N were between 4 and 8, suggesting that N concentration was not limiting soil decomposition rates. The ratio of C/N in the 0-20cm soil layer declined with recovery time (p<0.01), was 7.704 (before artificial restoration)→6.776 (after three years artificial restoration)→6.179 (after five years artificial restoration)→5.037 (after seven years artificial restoration). The ratio of C/N in the 20-40cm soil layer presented the trend of first decrease then increase with recovery time. (5) TP in restored sites was higher than in degraded area. The horizontal distribution of soil electricity conductivity in upper soil layer (0-20cm) was 0.4596g/kg (before artificial restoration)→0.4868g/kg (after three years artificial restoration)→0.4419g/kg (after five years artificial restoration)→0.4098g/kg (after seven years artificial restoration). TP in the 20-40cm soil layer showed unobvious variations with the increasing practice time. (6) Available phosphorus in the 0-20cm soil layer increased after five years artificial restoration. But there were unobvious variations with the increasing practice time in the 20-40cm soil layer. The lower soil layer (20-40cm) had more available phosphorus than the upper soil layer (0-20cm) in the restored sites, while the available phosphorus reversed to the above in restored sites. (7) SOC had significantly positive correlation with TN (r=0.947, p<0.01). It suggested that the N was from the soil organic matter (SOM). The significantly negative correlations between pH and SOC, TN were also found (r=-0.564, -0.574, p<0.01). The positive correlations between NH4+-N and SOC, TN were also found (r=-0.550, -0.522, p<0.05), while NO3--N and NH4+-N having significantly negative correlations (r=-0.226, p<0.01). Available phosphorus had positive correlation with SOC and TN( r=0.747, p<0.05, r=0.895, p< 0.01).