In previous studies, we assessed sulfur (S) uptake by crops from elemental S (ES) and sulfate-S (SO₄-S) in S-fortified monoammonium phosphate fertilizers over two years. The recovery by the crop ranged from 16 to 28% for ES and from 9 to 86% for SO₄-S. Here, we used a model which takes into account organic S cycling, SO₄-S leaching and ES oxidation to explain the observed recoveries. Higher recoveries of ES than SO₄-S in two of the four sites could be explained by partial leaching of SO₄-S and relatively fast oxidation of ES, due to a warm climate and high S-oxidizing soils. The same model was used for longer-term (5-year) predictions, and a sensitivity analysis was carried out. The size of the labile soil S pool and total S uptake strongly affected the recovery of both SO₄-S and ES. Predicted recoveries after 5 years were over threefold higher for a small than for a large labile organic S pool and for a high-uptake than for a low-uptake scenario. Leaching mainly affected SO₄-S, with predicted recoveries halved under a high-leaching scenario. Slow oxidation resulted in recoveries in the first year being fourfold lower for ES than for SO₄-S or even lower in case of a long lag-time. However, it is predicted that total recoveries of ES will eventually reach those of SO₄-S or exceed them if there is SO₄-S leaching. Our model demonstrates that long-term trials are needed to evaluate the true effectiveness of a slow-release fertilizer source such as ES.

在以前的研究中，我们评估了两年来，S强化磷酸一铵肥料中作物从元素S（ES）和硫酸盐S（SO ₄ -S）吸收的硫（S）。ES的作物收成率在16％到28％之间，SO ₄ -S的收成率在9％到86％之间。在这里，我们使用了考虑有机S循环，SO ₄ -S浸出和ES氧化的模型来解释观察到的回收率。在四个位置中的两个位置，ES的回收率高于SO ₄ -S，这可以通过SO _4的部分浸出来解释。-S和相对较快的ES氧化，这归因于温暖的气候和高S氧化土壤。长期（5年）预测使用相同的模型，并进行了敏感性分析。不稳定土壤S库的大小和总S吸收量强烈影响SO ₄ -S和ES的恢复。5年后的预测回收率，对于小而不稳定的有机S池和高摄入量，其回收率要比低摄入量高三倍以上。浸出主要影响SO ₄ -S，在高浸出情况下，预计的采收率减半。缓慢的氧化导致第一年的回收率比ES ₄低四倍-S或更长的滞后时间甚至更低。但是，据预测，如果有SO ₄ -S浸出，ES的总回收率最终将达到SO ₄ -S的回收率或超过它们。我们的模型表明，需要长期试验来评估缓释肥料来源（例如ES）的真实有效性。