Soil Phosphorus Pools for Histosols Under Sugarcane and Pasture.

Land use changes in the Everglades Agricultural Area (EAA) in southern Florida may influence the distribution and availability of P. Cultivated soils in the EAA are being converted back to their historic use as seasonally-flooded prairies as part of Everglades restoration projects. The objectives of this study were to determine the distribution of P in soil chemical fractions in relation to long-term land management to predict P cycling and transformations for future land uses. Soil under pasture (100 yr) and planted to sugarcane (Saccharum sp.) for 50 yr were amended with P (0, 10, 50, 150 kg P ha-1), and its distribution in labile, Fe-Al bound P, Ca bound P, humic-fulvic acid P, and residual P pools determined for surface soil (0-15 cm). Most P fertilizer entered Fe-Al and Ca-bound fractions. Cultivation contributed to higher pH and increased the Ca content in soil compared to pasture due to incorporation of bedrock limestone into soil by tillage. The land uses were differentiated by P storage in different pools. Subsequently, long-term fertilization increased soil total P for cultivated soil relative to pasture, but plant-available P constituted less than 1% of the total P. Labile P increased with increasing P application rate, ranging from 1.3 to 7.2 mg kg-1 for cultivated soil and 1.4 to 10.7 mg kg-1 for pasture. Most of the applied P was recovered in the Fe-Al fraction for pasture and the Ca-bound P fraction for cultivated soil. The Ca-bound P fraction represented the greatest proportion of total P for sugarcane (41%), but only 12% for pasture. The majority of P in the pasture was present in the humic-fulvic acid fraction (45%), compared to only 23% for sugarcane. The higher pH of the cultivated soil (6.8) favored retention in Ca fractions while the lower pH of pasture (5.3) favored P retention in the humic-fulvic acid fraction. The proportion of total P as organic P was greater for pasture (78%) than cultivated soil (52%). Higher P levels in more recalcitrant fractions for cultivated soils indicated that more of the applied fertilizer P was sequestered in stable fractions, which decreased P availability to crops and may subsequently increase P fertilizer requirements necessary to maintain optimal plant-available nutrient levels. Subsequently, continuation of current farming practices and tillage regimes promote the redistribution of Ca from subsurface to surface soil, which leads to greater P sequestration in the Ca-bound fraction. However, P in inorganic fractions may be released upon onset of changes in land use. Thus, conversion to seasonally-flooded prairies may have a more dramatic effect on P release from cultivated than pasture soils since cultivated soils have more P in inorganic pools.