Effects of land use types on particle size distribution of reclaimed alluvial soils of the Yangtze Estuary

Reclamation is a very common disturbance to the alluvial lands of estuaries. In China, the reclaimed alluvial lands are generally used for agricultural or industrial purposes. However, the physical and chemical properties of these reclaimed lands had been rarely studied. Particle size distribution (PSD) has been proven to be a useful indicator of land use. In the present study, we tried to clarify the changes of PSD under different land use types and durations. East End of Chongming Island at the Yangtze Estuary, the largest alluvial island in the world, which has a chronosequence of reclaimed lands, provides a good and special case to study the changes of PSD after reclamation. Three long-term land use types, i. e. paddy field, vegetable field and woodland, were involved. Soil samples of different land use types and different durations were collected. The soil particle size distribution was determined by using a Beckman-Coulter LS 13320 laser particle size analyzer, and the particle size fractions were performed. The changes of particle size fractions (i. e. , >63 μm, 63-32 μm, 32-16 μm, 16-4 μm, and < 4 μm representing sand-, coarse silt-, medium silt-, fine silt-, and clay-sized fractions , respectively) were analyzed. An order of magnitude was revealed in the terms of the soil mean particle size; tidal flat < paddy field < vegetable field < woodland. Moreover, compared to the tidal flat, fractions of the fine particle size of the other three land use types were significantly decreased, probably due to the wind erosions (anova, post hoc). Soils of vegetable fields and woodlands were more susceptible to wind erosion than the paddy field owing to their dryness and bareness. On the other hand, the mean particle size increased significantly during the soil development for the first 15 years after reclamation (one-way anova) , result from the severe wind erosion while soil organic matters increased during this period. However, at the duration of 38 years, particle sizes decreased again, due to the accumulation of organic matters and the improved forest coverage on those fields. Significant changes were found in the soils with the depth of 0-40 cm, with no distinguishable changes were observed below 40cm, reflecting the impacts of wind erosions and cultivations. Our results show that the distinct changes in soil particle size distribution can occur after reclamation, which sheds light on differential impacts of land uses on soil properties.