In soils with a fragipan or poor permeability, water may remain in a soil profile long enough to make it anoxic and reductive. The reductive dissolution of iron (Fe)- and manganese (Mn)-oxides can release associated phosphorus (P). Therefore, the dissolved P would be vulnerable to subsurface flow and could contaminate nearby streams. It was hypothesized that single rainfall events could cause subsurface P concentrations to increase via reductive dissolution in wet winter-spring conditions. Also, dissolution—being microbially mediated—would be buffered by the presence of nitrate (NO₃^–), which is preferred as an electron acceptor over Fe and Mn in microbial reactions. Unsaturated zone monitoring occurred from May to September in 2017 and 2019, using Teflon suction cups below the surface of a grassland soil in New Zealand. Events in July and August in 2017 and 2019 resulted in reducing conditions [Fe(III)/sulfate-reducing] and up to 77 and 96% greater P and Fe release, respectively. In an additional experiment in 2019, 100 mm of flood irrigation was applied, and 10 mg NO₃^––N + carbon was injected into half the cups at the site. The other cups received no N. Cups treated with N yielded up to 45% total dissolved P and 21% less Fe than the no-N cups. A laboratory incubation of soils from the site confirmed that NO₃^– inhibited P release. This effect may act to decrease the amount of P lost in subsurface flow in systems regularly fertilized with N but should not be relied on as a method to mitigate P losses.

中文翻译：

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在农业土壤剖面饱和期间与氧化铁相关的磷的还原溶解

在易碎或渗透性差的土壤中，水可能会在土壤剖面中停留足够长的时间，使其缺氧和还原。铁 (Fe) 和锰 (Mn) 氧化物的还原溶解会释放出相关的磷 (P)。因此，溶解的 P 很容易受到地下流动的影响，并可能污染附近的溪流。假设单次降雨事件会导致地下 P 浓度通过潮湿的冬春季条件下的还原溶解增加。此外，微生物介导的溶解会因硝酸盐（NO ₃ ^-)，在微生物反应中作为电子受体优于 Fe 和 Mn。不饱和区监测发生在 2017 年和 2019 年的 5 月至 9 月，使用的是新西兰草地土壤表面以下的特氟龙吸盘。2017 年和 2019 年 7 月和 8 月的事件导致还原条件 [Fe(III)/硫酸盐还原]，P 和 Fe 释放量分别增加了 77% 和 96%。在 2019 年的另一项实验中，采用了 100 毫米的洪水灌溉，并在现场的一半杯子中注入了10 毫克 NO ₃ ^– –N + 碳。其他杯子没有接受 N。用 N 处理的杯子比没有 N 的杯子产生高达 45% 的总溶解 P 和 21% 的铁。现场土壤的实验室孵化证实，NO ₃ ^–抑制 P 释放。在定期施氮的系统中，这种效应可能会减少地下流中的 P 损失量，但不应依赖于作为减轻 P 损失的方法。