Phosphorus Sorption Dynamics in Andisols from Lake Atitlan, Guatemala.

Soils are known for their capacity for phosphorus (P) retention, volcanic soils even more so.  This characteristic can be problematic for agriculture, as soil can be deficient in P levels necessary for crop yield.  P from subsequent fertilizer additions can then be occluded as well, prompting growers to use higher quantities of fertilizer to overcome the P retention, or sorption.  Fertilizer overuse can be detrimental to local waterways, increasing nutrient loads and leading to eutrophication.  In addition, agricultural soils can change from a sink to a source of P once introduced into an aquatic ecosystem.  The P sorption properties, however, can be used in tertiary wastewater treatment, allowing the soil to remove P from aqueous waste.  Lake Atitlan, an endorheic caldera located in the western highlands of Guatemala, is facing eutrophication as the input of nutrients from raw sewage, fertilizer overuse, and general human development has increased over time.  The purpose of this study was to determine the extent of P sorption and the fate of sorbed P from soils collected from the near shore watershed of Lake Atitlan.  Soil samples collected from various land uses, stratified by the age of volcanic deposits around the lake, were subjected to P sorption batch assays: samples were flooded with a solution containing phosphate along a concentration gradient from 0 to 1250 mg PO₄-P/kg soil.  After 24 hours this solution was removed and soluble reactive phosphate was measured; the difference between the final and the initial concentrations is the sorbed fraction.  The same samples were then dried and reflooded with a solution containing no phosphate; after 24 hours they were reanalyzed for phosphate and the level of desorption was determined.  The data was used to calculate the Kash: the Freundlich coefficient normalized for soil ash content, the equilibrium phosphate concentration (EPC₀), the phosphate saturation ratio (PSR), and the soil phosphorus storage capacity (SPSC).  These values show the diversity of phosphorus retention in soils from the Atitlan basin- many soils initially desorbed phosphate at low levels, while others sorbed up to 90% of phosphate at the 1250 mg P/kg soil level.  The desorption test showed that all analyzed  soils were efficient in retaining the sorbed fraction, and even continued sorbing additional phosphate.  Sorption was correlated to the concentration of oxalic acid extractable aluminum, but no correlation to oxalic acid extractable iron was found.  The EPC₀- the concentration at which phosphate is neither sorbed nor desorbed from the soil, ranged from 0 to 6 mg P/kg soil.  Many soils had a 0 EPC₀; these soils have a high affinity for phosphate and do not readily desorb.  The PSR and SPSC, both measures of sorption capacity remaining in the soil under ambient conditions followed the same trends as the sorption and EPC₀.  These various measures of soil phosphorus retention depend more on age (tertiary or quaternary ash deposit) and location of soil then the use of the land itself.   Because sorption is correlated to aluminum and not iron concentrations, release of phosphate due to reduction of iron in anaerobic environments is not a substantial risk.  Therefore, these findings can be used to ascertain suitability of selected locations for the creation of wastewater treatment ponds or wetland buffer areas.  The results of this study can also be used to better inform local farmers of best management practices in the application of fertilizer to both increase yield and protect Lake Atitlan from further eutrophication.