土壤中磷的形态、含量和有效性随成土过程而发生显著变化。过去的研究表明，在自然成土过程中，磷的含量与有效性随成土时间逐渐降低；在自然生态系统发展的后期，由于不断耗竭，磷成为养分限制因子，从而导致生物产量下降，并影响碳、氮循环，最终导致生态系统的退化。然而受到人为活动强烈影响的农业生态系统，如水稻土长期演变过程中不同形态磷如何转化目前还不清楚。
我室张甘霖研究员课题组通过建立我国亚热带地区不同母质水稻土从十年到千年尺度的时间序列，采用连续分级提取法，系统研究了受人为活动强烈影响、具有外源磷不断输入的水稻土长期演变过程中不同形态磷的转化和分异模型。结果表明，人为活动改变了成土过程中不同形态磷转化的方向和速率(Fig. 1)。与起源土壤相比，在水稻种植初期(< 300 yrs)，由于人为施肥添加，不同形态磷在表层土壤中富集；而老水稻土(> 300 yrs)由于磷吸附容量降低，尽管有外源磷的不断输入，土壤中磷仍然表现为亏缺，表明长期耕作导致磷素固定组分的转化与流失，降低了其磷素固定的能力，从而增加水稻土中磷流失的风险。起源母质影响水稻土长期演变过程中不同形态磷的转化，发育于石灰性母质的水稻土全磷(T-P)的变化80%来源于钙磷(Ca-P)，而发育于酸性第四纪红黏土的水稻土时间序列全磷(T-P)的变化43%来源于铁磷(Fe-P)和铝磷(Al-P)。该成果最近发表在美国土壤学会会志上（Soil Sci. Soc. Am. J. doi:10.2136-sssaj2013.01.0033）。
上述研究结果揭示了千年尺度上水稻土演变过程中不同形态磷的转化，为水稻土的合理施肥和持续利用与管理提供了理论依据和科学参考。
Fig.1 (a) Walker and Syers’ (1976) model of soil P transformations with pedogenesis; (b) and (c) changes in soil P fractions during paddy soil development at Cixi within 0–50cm and 50–120 cm respectively; (d) changes in soil P fractions during paddy soil development at Jinxian within 0–50cm (upper panel) and 50–120 cm (lower panel). Note: the assumed linear change in individual P pools in this study may not reflect actual rates over the course of paddy soil development. PCa = calcium phosphates; Po = phosphorus bound to organic matter.
Lai-Ming Huang Gan-Lin Zhang* Aaron Thompson David G. Rossiterd. Pedogenic Transformation of Phosphorus during Paddy Soil Development on Calcareous and Acid Parent Materials.SSSAJ 2013 doi:10.2136-sssaj2013.01.0033
Abstract
Walker and Syers’ model of gradual phosphorus (P) depletion and decreasing bioavailabity during pedogenesis is well accepted for natural ecosystems but untested beyond decadal scales in agricultural ecosystems. We investigated changes in P speciation and distribution using sequential extraction in soil profiles from two contrasting paddy chronosequences developed on calcareous marine sediments at Cixi and acidic quaternary red clays at Jinxian respectively in subtropical China. Our results demonstrate shifts in P abundance and speciation with depth during paddy soil development. Total P (PT) accumulated in surface and subsurface horizons in the young paddy soils (< 300-yrs) at both sites due to P additions by paddy cultivation. However in the old paddy soils (700-and 1000-yrs) at Cixi we observed depletion of PT despite continuous P additions which we attribute to the decline in soil P sorption capacity coincident with the near complete removal of CaCO3 and substantial loss of Fe oxides. Compared to the systematic decline of PT below the plow pan in the lowland Cixi chronosequence PT increased in the upland Jinxian subsoils suggesting that terraced paddy cultivation resulted in significant translocation of P to the lower horizons. In calcareous paddy soils 80% of changes in PT can be attributed to Ca-bound P while for acid soils 43% was explained by Fe- and Al-associated P. Our study demonstrates distinct patterns of P transformation in agro-ecosystems with human activities altering both the rate and trajectory of P transformations during the early stage of paddy soil development after which P becomes rapidly depleted and less biological available.
