Desorption kinetics of legacy soil phosphorus: Implications for non-point transport and plant uptake

Authors: Penn, Chad; Williams, Mark; CAMBERATO, JAMES - Purdue University; WENOS, NICHOLAS - Purdue University; WASON, HOPE - Purdue University

Submitted to: Soil Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2022
Publication Date: 1/8/2022
Citation: Penn, C.J., Williams, M.R., Camberato, J., Wenos, N., Wason, H. 2022. Desorption kinetics of legacy soil phosphorus: Implications for non-point transport and plant uptake. Soil Systems. 6:6. https://doi.org/10.3390/soilsystems6010006.
DOI: https://doi.org/10.3390/soilsystems6010006

Interpretive Summary: The quantity and speed in which soils can supply phosphorus (P) to solution has a direct impact on transport to surface waters where excess P causes pollution, and also for plant uptake regarding efficient crop production. These experiments were designed using flow-through cells to examine how the manner in which water interacts with soil can influence how much P can desorb, and how fast. The interaction between chemical and physical processes controlled how P desorption would be manifested; although P desorption is a chemical process, it occurs through a physical environment. Depending on the dynamic between physical and chemical processes, P desorption may be enhanced or depressed. The result was that for the slow flow rate, the net desorption rate was limiting due to lack of dilution, and for the fast flow rate, what little P was able to desorb, desorbed fast, yet the amount of P released was limited. Quantification of the physio-chemical processes controlling P desorption would improve the modelling of non-point transport of dissolved P and plant uptake. An immediate application would be through improving our understanding of how hydrology partly dictates P desorption.

Technical Abstract: 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.