Nutrient Release and Microbial Activity After Flooding of Everglades Soils.

Cultivated Histosols in the Everglades Agricultural Area (EAA) are being converted to wetlands as part of Everglades restoration projects. Elemental S is used in the EAA to decrease pH and to increase nutrient availability to crops. The microbial oxidation of elemental S to sulfate may alter the utilization patterns of electronic acceptors when soils become flooded. The objectives of this study were to determine short-term changes in P dynamics and greenhouse gas production under different hydrologic conditions: drained soil, flooded soil, flooded soil+SO₄.  The distribution of P in labile, Fe-Al bound, Ca bound, humic-fulvic acid, and residual pools under drained, flooded, and sulfate-reducing conditions were measured for fields under sugarcane cropping, vegetable cropping, and uncultivated soil. The land uses were differentiated by P storage in different pools as cultivation contributed to higher pH and increased P sequestration in Ca-bound fractions more for sugarcane (50%) and vegetable (49%) than uncultivated soils (17%). Uncultivated soils sequestered more P in organic pools, as storage in residual fractions (70%) was higher than for sugarcane (38%) and vegetable (43%) soils.  But the distribution of P in all fractions did not differ in response to flooding.  For all three land uses, mineralizable N ranged from 20-30 mg/(kg*d), while inorganic N consumption averaged 10-20 mg/(kg*d) under both flooding treatments since NO₃ was used as an electronic acceptor.  Microbial CO₂ production rates under flooded conditions were 50%, 62, and 72% of the aerobic rate for sugarcane, vegetable, and uncultivated soils, respectively.  Methane production was minimal in the short-term under flooded conditions due to predominance of denitrification resulting from prior soil oxidation and organic N mineralization, or N fertilization, which contributed to maintenance of high enough levels of NO₃ over the 10 d period to support denitrification.  Methane production was minimal in S-amended soils due to stimulation of SO₄ reduction.  Short-term responses to flooding showed that CO₂ is the primary endproduct of organic matter decomposition, with pathways of decomposition and P dynamics similar among land uses.