| There are large areas of tidal flats in China, which were widely reclaimed in the pass decades in order to meat the need of land supply. As an essential element for plant growing, the content and forms of phosphorus in soils have crucial impacts on fertility of soils. There were scarce reserches focused on P species of soils from reclaimed land, especially study of P evolution after reclamation in the initial 100 years was even less.In this study, the tidal flats and reclaimed area in the eastern of Dafeng city was chosen as study area. In order to apply "space substitute for time", study area was divided into five zones according to the historical maps and interpretation results from remote sensing. Namely reclaimed for 90 years (E zone), reclaimed for 60 years (D zone), reclaimed for 30 years (C zone), reclaimed for 10 years (B zone) and tidal flat (A zone), respectively. The area of study area was about 1600 km2, with length of 56 in north and width of 19 to 38 km in west.241 surface samples (0-20 cm) were collected, 60 of which were from tidal flat sediments, and 20 soil and sediment profiles were sampled as well. Sample sites were well-distributed and there were no less than 40 in each zone. Samples included farmland soil, fishfarm sediment, tidal flat sedimet and wasteland soil. pH, salt content, carbon and other physico-chemical characteristics of the soils were tested in the laboratory, and SEDEX P fraction method, which separated Ca bound phosphorus into authigenic CaP and detrital apatite, was applied to soil since the soil and sediment were rich in Ca content. Soil components and forms of P were compared under different ages of reclamation and land-use to reveal the evolution of soil components and P forms transportation and transformation along the reclamation process. Correlation analysis, factor analysis, path analysis were applied to explore the relationship between P forms and basic soil characteristic.The conclusions were listed below:1. Grain size distribution of tidal flat sediments was controlled by hydrodynamic condition, which fluctuated violently. The composition of sediments was different among different part of tidal flat. Grain size distribution of tidal flat sediments changed after reclamation that contents of sand in the samples decayed exponentially while contents of silt increased exponentially along the reclamation time. Spatial distribution of silt showed the gradient of sea-land trend.2. pH and salt contents of tidal flat sediments decreased after reclamation. pH fell from 8.74 to 8.42 while salt contents fell from 7.57 g kg-1 to 1.88 g kg-1. pH decayed exponentially along reclamation time, and kept stable after 30 years. The trends of salts decay with reclamation time fitted rational function. pH and salts content increased along the soil depth, which indicted it was still in the process of desalination. The main ions were Na+ and Cl-, but the ratio of Ca2+, Mg2+, SO42- increased along reclamation process.3. The main carbon was CO3-C in the tidal flat sediments, and CO3-C decayed linearly while TOC increased exponentially along the reclamation then stabilized. In soil profile, TOC was enriched in the surface soil while CO3-C decayed, due to the recycle of plants and leaching process in the soil.4. As the FA revealed, main factors of reclamation included grain size effect, enrichment factor of biology, decalcification factor and desalination factor. The distribution of factors scores was controlled by gradient of land-sea, and was impacted by land-use and human activities such as application of chemical fertilizers.5. The content of TP in tidal flat sediments was close to that of old Yellow river sediments. The content of TP was significantly higher in the reclaimed area than that in tidal flats, and increased along the reclamation time. Distribution of TP in space show obvious land-sea trends, which was higher in land samples and lower in sea parts. The content of TP was also impacted by land-use. For example, the content of TP was lower in paddy field than that in dry land, and TP was enriched in the vegetable land soils which received plenty of fertilizers. Geostatistics revealed that Nugget coefficient was less than 25%, which mean high self-correlation in space. The variation of space was anisotropy and showed an angle of 160°, which was parallel to the coastline.In the soil profile, TP was enriched in the surface (0-20 cm), and the content drop markedly in the deep sample, however, there was leaching phenomena in some profile. TOC:TP increased and CO3-C:TP decreased along reclamation time, which indicted the enrichment of TP was accompanied by accumulation of OM and decay of CaCO3. That Ca:P decreased along reclamation indicted the transformation from stable CaP mineral to secondary CaP.6. The mean contents of ExP, FeP, CaP, DeP and OP were 16.3,10.6,58.7,477.8 and 55.1 mg kg-1 in tidal flat sediments, respectively..The mean contents of ExP, FeP, CaP, DeP and OP were 48.3,82.5,315.9,284.9 and 117.5 mg kg"1 in reclaimed area samples, respectively. The mean content of ExP, FeP, CaP, DeP and OP increased 2.0 fold,6.8 fold,4.4 fold and 1.1 fold, respectively, while DeP decreased 40% after reclamation. CaP increased exponentially while DeP decayed exponentially along reclamation time. The content of OP increased along reclamation time in 60 years, however, it decreased thereafter. ExP and FeP increased after reclamation, and they were impacted by land-use. ExP and FeP enriched in fish farm sediments and vegetable soils which received plenty of fertilizers. The content of CaP was higher in upland soils while the content of FeP was lower in paddy soil.The contents of P species in tidal flat sediments was controlled by hydrodynamic condition and grain size effect of P species, and as for CaP and OP:higher intertidal zone> middle intertidal zone>lower intertidal zone, but on the contrary as for DeP: higher intertidal zone< middle intertidal zone ≤lower intertidal zone. In the soil profile, ExP and OP showed enrichment in the surface, and the content of DeP decreased as depth increased. The trends of FeP and CaP in the profile were not obvious. The contents of CaP and OP in soil profiles increased and DeP decreased significantly after reclamation.7. Path analysis showed that the enrichment of TP was directly due to the increase of CaP, and the transformation from FeP to CaP and ExP was also revealed by the path analysis. Factor analysis showed that TP, ExP and FeP were reckoned as the results of fertilizer applicant while OP was thought to be mainly controlled by natural nutrient enrichment. CaP increased along reclaimed time, which were reckoned as both fertilizers and natural transform from DeP. P species were negatively correlated with pH and salinity except DeP. And P species had grain size effect as well. The content of DeP in coarse particles was higher than that in fine particles while other P species were enriched in fine particles. Except DeP, other P species were significant positively correlated with OM, and negatively correlated with Cl, Sr, and CaCO3, which indicted the enrichment of P species in the soil was accompanied by enrichment of OM and desalinate and decalcification at the same time.8. ExP was significant positively correlated with Olsen phosphorus, which can indict the short-term available phosphorus for crops. CaP formed by Ca2+ and P in alkaline condition, and Ca came from decalcification and P came from fertilizers and other P forms. Correlations between Ca:P, TOC:CO3-C and CaP indicted the formation of CaP was accompanied by the enrichment of OM and decalcification. CaP can be an effective index to indict the extent of soil genesis especially for calcareous soil. The separation of authigenic CaP from mineral apatite in SEDEX revealed the difference performance in the process of soil genesis. More studied are needed to reveal the mechanism of CaP formation and behavior of CaP under human activities. |
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