Effects Of Land-Use Changes On Soil Organic Carbon Of Reclaimed Alluvial Soils In The Yangtze Estuary

Climate warming is a severe environmental problem which has caused extensive concern. Soil organic carbon (SOC) is a large reservoir that can act as a sink or source for atmospheric CO2, which strongly affects C cycle and global climate. Soil C storage and dynamics is strongly affected by land-use and coverage changes, which is a key research subject in the scientific community. The C reserve in wetlands takes up 20-25 percents of the total carbon in the earth and it makes a critical difference with the carbon balance. Reclamation is common disturbance to alluvial lands of estuaries, impacting the carbon recycle fiercely. So it is urgent to study the dynamics and variations of SOC in reclaimed estuarine soils. In the present study, we tried to clarify the changes of SOC and total nitrogen (TN) content under different land use types and durations in East End of Chongming Island at the Yangtze Estuary, the largest alluvial island in the world, which has a chronosequence of reclaimed lands and provides a good and special case to study the effects on SOC after reclamation. Three long-term land-use types, i.e. paddy field, vegetable field and woodland, were involved.1. The soil moisture decreased significantly during the period of reclamation due to land use changes. It accelerated the mineralization of the organic sediments and caused the SOC and TN to decrease prominently in the soils with the depth of 0-40cm by varying degrees, with no distinguishable changes observed below 40cm, reflecting the impact of cultivations. The significant positive correlation of SOC with TN content(R=0.944, P<0.001) is revealed and the same order of magnitude in the terms of SOC and TN was expressed:tidal flat～paddy field< vegetable field< woodland.2. The SOC descended dramatically during the soil development for the first 15 years after reclamation and, which released CO2 to the atmosphere until touching the lowest point. Then the SOC ascended dramatically owing to accumulation of organic matters from farming residue and fallen leaves. 3. Our results show that the SOC in paddy field had no remarkable change with the tidal flat after reclamation, suggesting that it is recommended pattern to cultivate paddy during the first period after reclamation for lessening carbon emission as well as removing salts from the soils.