Forms of Soil Phosphorus Along a Nutrient Enrichment Gradient in the Northern Everglades

About 60 MT y−1 of P from agricultural runoff have flowed into an area of the northern Everglades marshes of Florida since the late 1960s, creating a nutrient enrichment gradient. The objectives of this study were to determine (i) in what forms the added and native P have been stored over the past 30 years and (ii) whether forms that are resistant to recycling are the main forms of P storage. The peat soils were sampled at 18 stations along a gradient from the source of nutrient inflow in February, May, August, and December and were then subjected to sequential extraction of P. In general, the P concentrations along the gradient decreased to relatively low concentrations about 8 to 10 km into the interior of the marsh. Concentrations of humic organic P, residual insoluble organic P, Ca-bound P, exchangeable inorganic P, and Fe/Al-bound inorganic P (only at the 20 to 25-cm depth) were all correlated negatively with distance from the nutrient input, i.e., they were 2 to 4 times higher in the enriched area. In contrast, concentrations of microbial biomass P in the surface soil were not correlated with distance from the nutrient input although variability was great. Because more peat is accreting in the enriched area, all forms of P are accreting faster (1.8 to 8 times faster, depending on the form) in the enriched area. Deposition of organic P in the peat was the most important process storing excess P from the nutrient inflows. Formation of Cabound P in these neutral to slightly alkaline (pH 7.2 to 7.9) peat soils was also an important mechanism of P deposition at the stations closest to the phosphorus input. The excess P enrichment has permeated most components of the marsh system, including organic and inorganic forms as well as rapidly and slowly cycling forms.