Author:

Keywords:

Science & Technology, Life Sciences & Biomedicine, Soil Science, Agriculture, ageing, availability, desorption, long-term, organic matter, phosphorus, FARMYARD MANURE, PHOSPHATE SORPTION, P AVAILABILITY, USE EFFICIENCY, IRON-OXIDES, CROP YIELD, DESORPTION, GOETHITE, ACID, ADSORPTION, C16/15/042#53326572, 0503 Soil Sciences, 0607 Plant Biology, 0703 Crop and Pasture Production, Agronomy & Agriculture, 4106 Soil sciences

Abstract:

The stocks of phosphorus (P) in soil resulting from decades of over-fertilisation can be used as a long-term source of P provided that crop P bioavailability is ensured. This study was set up to identify to what extent soil organic matter (SOM) affects the long-term availability of these stocks, the premise being that OM may limit irreversible P fixation in soil by blocking P sorption sites on sesquioxides. An ensemble of 42 agricultural soils, composed from experimentally amended soils (field and incubation trials) and soils with contrasting properties, was subjected to 288 days of P depletion with anion exchange membranes as a P sink; this method was previously shown to yield P pools with agronomic significance. Cumulative P desorption data were fitted with a two-pool kinetic desorption model, yielding estimates for a fast (labile) and total desorbable P pool. On average, 42% of oxalate extractable P (Pox) associated with poorly crystalline iron (Fe) and aluminium (Al) (oxy)hydroxides (Feox and Alox) were desorbable and 25% of that fraction (i.e. 11% of Pox) was labile. That labile P pool matched well with the 24 h isotopically exchangeable P (E value) in these soils (R2 = 0.74). Both the fast and total desorbable fraction of Pox were larger at higher degrees of phosphorus saturation (DPS). In soils with a low DPS (<0.30), the labile fraction of Pox increased as the ratio of soil organic carbon to Feox + Alox increased (R2 = 0.70; p < 0.001), but soils with a higher DPS did not exhibit that trend. These results adhere to soil chemical views that enhanced SOM contents reduce fixation of P by competitive sorption and prevention of P diffusion into micropores. No such effects occur in more P saturated soils, probably because orthophosphate sorption and electrostatic effects outcompete effective SOM sorption. The findings suggest that simultaneous application of OM with P fertilisers could keep P better available in the long term, but that this OM application does not affect P fixation when soils are excessively dosed with P.