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 (P_ox) associated with poorly crystalline iron (Fe) and aluminium (Al) (oxy)hydroxides (Fe_ox and Al_ox) were desorbable and 25% of that fraction (i.e. 11% of P_ox) 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 P_ox were larger at higher degrees of phosphorus saturation (DPS). In soils with a low DPS (<0.30), the labile fraction of P_ox increased as the ratio of soil organic carbon to Fe_ox + Al_ox 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.

中文翻译：

---

增加土壤有机碳含量可以提高农业土壤中磷的长期有效性

只要确保作物 P 的生物有效性，几十年过度施肥产生的土壤中的磷 (P) 储量可用作 P 的长期来源。本研究旨在确定土壤有机质 (SOM) 在多大程度上影响这些种群的长期可用性，前提是 OM 可能通过阻断倍半氧化物上的 P 吸附位点来限制土壤中不可逆的 P 固定。由经过实验改良的土壤（田间和孵化试验）和具有对比特性的土壤组成的 42 种农业土壤的集合，以阴离子交换膜作为 P 吸收器进行了 288 天的 P 消耗；这种方法以前被证明可以产生具有农艺意义的磷池。累积 P 解吸数据符合两池动力学解吸模型，产生快速（不稳定）和总可解吸 P 池的估计值。平均而言，42% 的草酸盐可萃取磷 (P_ox ) 与结晶差的铁 (Fe) 和铝 (Al) (氧) 氢氧化物 (Fe _ox和 Al _ox ) 是可解吸的，并且 25% 的部分（即 11% 的 P _ox）是不稳定的。不稳定的 P 池与这些土壤中的 24 小时同位素可交换 P ( E值) ( R ²  = 0.74) 非常匹配。在磷饱和度 (DPS) 越高时，Pox 的快速解吸部分和总解吸部分都_较大。在低 DPS（<0.30）的土壤中，随着土壤有机碳与 Fe _ox  + Al _{ox比例的增加，P ox}的不稳定部分增加（R ²  = 0.70；p  < 0.001)，但 DPS 较高的土壤没有表现出这种趋势。这些结果与土壤化学观点一致，即增加的 SOM 含量通过竞争性吸附和防止 P 扩散到微孔中来减少 P 的固定。在 P 饱和度更高的土壤中不会出现这种影响，这可能是因为正磷酸盐吸附和静电效应超过了有效的 SOM 吸附。研究结果表明，OM 与 P 肥料同时施用可以使 P 在长期内更好地利用，但这种 OM 施用不会在土壤中过量施用 P 时影响 P 的固定。