Intra- versus inter-site macroscale variation in biogeochemical properties along a paddy soil chronosequence

[摘要] In order to assess the intrinsic heterogeneity of paddy soils, a set ofbiogeochemical soil parameters was investigated in five field replicates ofseven paddy fields (50, 100, 300, 500, 700, 1000, and 2000 yr of wetlandrice cultivation), one flooded paddy nursery, one tidal wetland (TW), andone freshwater site (FW) from a coastal area at Hangzhou Bay, ZhejiangProvince, China. All soils evolved from a marine tidal flat substrate due toland reclamation. The biogeochemical parameters based on their propertieswere differentiated into (i) a group behaving conservatively (TC, TOC, TN,TS, magnetic susceptibility, soil lightness and colour parameters, δ¹³C,δ¹⁵N, lipids and n-alkanes) and (ii) one encompassing more labileproperties or fast cycling components (N_mic, C_mic, nitrate,ammonium, DON and DOC). The macroscale heterogeneity in paddy soils wasassessed by evaluating intra- versus inter-site spatial variability ofbiogeochemical properties using statistical data analysis (descriptive,explorative and non-parametric). Results show that the intrinsicheterogeneity of paddy soil organic and minerogenic components per field issmaller than between study sites. The coefficient of variation (CV) valuesof conservative parameters varied in a low range (10% to 20%),decreasing from younger towards older paddy soils. This indicates adeclining variability of soil biogeochemical properties in longer usedcropping sites according to progress in soil evolution. A generally highervariation of CV values (>20–40%) observed for labile parameters impliesa need for substantially higher sampling frequency when investigating theseas compared to more conservative parameters. Since the representativeness ofthe sampling strategy could be sufficiently demonstrated, an investigationof long-term carbon accumulation/sequestration trends in topsoils of the2000 yr paddy chronosequence under wetland rice cultivation restricted wasconducted. Observations cannot be extrapolated to global scale but withcoastal paddy fields developed on marine tidal flat substrates after landreclamation in the Zhejiang Province represent a small fraction (<1%) ofthe total rice cropping area. The evolutionary trend showed that thebiogeochemical signatures characteristic for paddy soils were fullydeveloped in less than 300 yr since onset of wetland rice cultivation. Asix-fold increase of topsoil TOC suggests a substantial gain in CO₂sequestration potential when marine tidal wetland substrate developed to2000 yr old paddy soil.

[学科分类] 地球化学与岩石