Pedogenic transformation of phosphorus during paddy soil development on calcarious and acid parent materials

Walker and Syers' model of gradual P depletion and decreasing bioavailability 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 yr) at both sites due to P additions by paddy cultivation. However, in the old paddy soils (700 and 1000 yr) 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 agroecosystems 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.