Long-term fate of fertilizer sulfate- and elemental S in co-granulated fertilizers
Fien Degryse, Roslyn Baird, Ivan Anđelković, Michael J. McLaughlin
Abstract
Abstract In previous studies, we assessed sulfur (S) uptake by crops from elemental S (ES) and sulfate-S (SO 4 -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 SO 4 -S. Here, we used a model which takes into account organic S cycling, SO 4 -S leaching and ES oxidation to explain the observed recoveries. Higher recoveries of ES than SO 4 -S in two of the four sites could be explained by partial leaching of SO 4 -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 SO 4 -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 SO 4 -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 SO 4 -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 SO 4 -S or exceed them if there is SO 4 -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.