Geoderma (Jan 2024)
Phosphorus release from rewetted agricultural peat soils varies strongly in dependence of the phosphorus resorption capacity
Abstract
Rewetting of drained agricultural lowland peat soils is followed by the risk of increased phosphorus (P) release to downstream systems, thereby challenging their restoration as nutrient sinks for years to decades. While extensive knowledge is available on P mobilization under anaerobic conditions caused by reductive (Fe(III))-oxide dissolution, the net P release to the aqueous phase (PSol) is only poorly understood due to unknown significance of P re-sorption to aluminum (Al) oxides and non-reduced Fe(III)-oxides. We therefore hypothesize that PSol is a function of the sorption capacity and P saturation of Al-oxides and non-reduced Fe(III)-oxides. A comprehensive set of 47 Danish topsoil and subsoil samples from agricultural lowlands were incubated for up to 148 days in the laboratory under anoxic and water-saturated conditions at room temperature. Oxalate-extractable Fe, Al and P (Feox, Alox and Pox) varied by three orders of magnitude, with Feox ranging between 1.8 and 1590 mmol kg−1; Alox and Pox also showed high variation with maximum contents of 883 and 153 mmol kg−1, respectively. Bicarbonate-dithionite generally extracted 2 times less Fe, Al and P (FeBD, AlBD, PBD) than oxalate. Oxalate extraction data were used to calculate the degree of P saturation (DPS) and P sorption capacity (PSC). The extent of Fe(III)-oxide reduction measured as 0.1 M HCl extractable Fe(II)HCl was well described by first-order kinetics with rate constants ranging between 0.01 and 0.3 d-1.The estimated maximum Fe(II)HCl produced (Fe(II)max) ranged between 3 and 1490 mmol Fe(II) kg−1, with Feox corresponding to Fe(II)max values very closely. For most soils almost full dissolution of the entire Feox pool was achieved within 21 days of incubation. PSol concentrations, measured in the soil solution extract, ranged between 0.05 and 5.05 mg L-1, increasing with incubation time for most of the soils. While PSol was not correlated with FeBD:PBD and Feox:Pox, DPS, total P (PT) or total Fe (FeT):PT ratio, the investigations revealed a strong reciprocal relationship between the residual sorption capacity (RSC) of the soils, i.e., PSC subtracted the Fe(III)-oxides reduced, and the PSol/Pox ratio. Moreover, the results showed low or lack of P release to the aqueous phase, even for the soil samples where all Feox pools were reduced, if the RSC of the soil was above 100 mmol kg−1. This highlights the importance of redox-stable Al-oxides to capture mobilized P from rewetted lowland peat soils.
