Desorption Kinetics of Legacy Soil Phosphorus: Implications for Non-Point Transport and Plant Uptake

Soil phosphorus (P) solubility and kinetics partly control dissolved P losses to surface water and uptake by plants. While previous studies have focused on batch techniques for measuring soil P desorption kinetics, flow-through techniques are more realistic because they simulate P removal from the system, akin to runoff, leaching, and plant uptake. The objectives were to measure soil P desorption by a flow-through technique at two flow rates and several batch methods, and utilize both for understanding how flow rate impacts the thermodynamics and kinetics of soil P desorption. Desorption obeyed first-order kinetics in two different phases: an initial rapid desorption phase followed by a gradual release. Desorption was limited by equilibrium and the kinetics of physical processes as demonstrated by an interruption test. Dilution-promoted desorption occurred with increasing cumulative volume, which increased desorption rate via first-order kinetics. The batch tests that simulated cumulative solution volume and time of flow-through were similar to the flow-through results; however, the batch methods overestimated the desorption rates due to less limitations to diffusion. Fast flow rates desorbed less P, but at a greater speed than slow flow rates. The differences were due to contact time, cumulative time, and solution volume, which ultimately controlled the potential for chemical reactions to be realized through physical processes. The interaction between these processes will control the quantity and rate of desorption that buffer P in non-point drainage losses and plant uptake.

The Extended Effect of Top-Dressed Recovered Struvite Fertiliser on Residual Irish Grassland Soil Phosphorus Levels Compared to Commercial Phosphorus Fertiliser

Phosphorus (P) is an essential growth-limiting nutrient that cannot be synthetically produced. Mineral P fertilisers are typically applied to crops to sustain modern farming practice and food production. These fertilisers are generally derived from finite phosphate ore, for which there is much concern over long-term sustainability. To address these concerns, various technologies have been developed to recover P from municipal wastewater treatment plants. One product recovered from these processes is struvite, which is a precipitate formed of magnesium, ammonium, and phosphate. To analyse the fertilisation value of the recovered struvite, field trials were conducted on perennial ryegrass (Lolium perenne) over three growing seasons, analysing the dry matter yield of recovered struvite fertiliser. The trial was based on a three-crop silage system designed to mimic typical Irish agricultural practice. This research highlights that recovered struvite as a fertiliser provides additional benefits including increasing the soil P levels. The struvite test case produced a statistically significantly increased soil P level from the baseline of Morgan’s extractable P content of 6.4 mg/L to the optimum Morgan’s soil P level of 11.13 mg/L. The findings of this research provide insight into the added benefits of recovered struvite fertiliser as a sustainable renewable P fertiliser.