Rare earth elements of a 1000-year paddy soil chronosequence: implications for sediment provenances, parent material uniformity and pedological changes.

摘要

A series of paddy soil profiles with approximately 50, 300, 700 and 1000 years of paddy cultivation history and an uncultivated mud beach profile under nearly identical landscape and climate conditions were studied. The signatures of rare earth elements (REEs) were used to identify sediment provenances and parent material uniformity of the studied profiles and to assess long-term paddy management effects on REE mobilization and fractionation. The distribution patterns of samples on a delta Eu_N- Sigma REEs plot indicate that the parent materials of paddy soils and the uncultivated soil mainly originate from Yangtze River sediments. The REE chondrite-normalized curve could be used to adequately evaluate parent material uniformity, thus allowing further studies of soil REE changes along a time sequence of paddy cultivation. To understand anthro-pedogenic effects on REE mobilization and fractionation, the REE concentrations of paddy soils were normalized to those of the uncultivated soil. Paddy management resulted in accumulation of all REEs within the upper 100 cm, possibly due to anthropogenic inputs such as irrigation water and phosphate fertilizers. In particular, a distinct positive Ce anomaly was observed. This was likely due to periodic reduction and oxidation processes caused by artificial submergence and drainage. These REE signatures are especially marked in the anthrostagnic epipedon (Ap), as compared with the hydragric horizon (Bg). The positive Ce anomaly increased gradually with increasing paddy cultivation age. This demonstrates the utility of Ce anomaly as a trace of the frequency and intensity of redox conditions. The relative enrichment of all REEs and gradual accumulation with paddy cultivation time in the Ap horizons imply low mobility of REEs in the investigated paddy soils. In conclusion, our study demonstrates the effectiveness of various REE proxies as tracers of sediment provenances and parent material uniformity in present paddy soils. It also reveals the effects of long-term paddy management on REE enrichment and positive Ce anomalies.