Interactions of local climatic, biotic and hydrogeochemical processes facilitate phosphorus dynamics along an Everglades forest-marsh gradient

Abstract

Ecosystem nutrient cycling is often complex because nutrient dynamics within and between systems are mediated by the interaction of biological and geochemical conditions operating at different temporal and spatial scales. Vegetated patches in semiarid and wetland landscapes have been shown to exemplify some of these patterns and processes. We investigated biological and geochemical factors suggested to contribute to phosphorus (P) movement and availability along a forest-marsh gradient in an Everglades tree island. Our study illustrated processes that are consistent with the chemohydrodynamic nutrient (CHNT) hypothesis and the trigger-transfer, pulse-reserve (TTPR) model developed for semiarid systems. Comparison with the TTRP model was constructive as it elaborated several significant patterns and processes of the tree island ecosystem including: (1) concentration of the limiting resource (P) in the source patch [High Head which constitutes the reserve] compared with the resource-poor landscape, (2) soil zone calcite precipitation requiring strong seasonality for evapotranspiration to promote conditions for secondary soil development and calcium phosphate reprecipitation, (3) rewetting of previously dry soils by early wet season precipitation events, and (4) antecedent conditions of the source patch including landscape position that modulated the effect of the precipitation trigger. Thus, our study showed how water availability drives soil water P dynamics and potentially stability of mineral soil P in this tree island ecosystem. In landscapes with extensive water management, these processes can be asynchronous with the seasonality of hydrologic dynamics, tipping the balance between a sink and source of a limiting nutrient

Tree islands: the bellwether of Everglades ecosystem function and restoration success

“…Tree transpiration and the resulting ion exclusion through root water uptake during the dry season concentrates carbonate minerals under the tree island head (Sullivan et al ; Wetzel et al ; Sullivan et al ; Troxler et al ) and, as Sullivan et al () recently reported on a tree island in the ENP, in areas immediately downstream of the head of the island. The strong dry–wet seasonality in the Everglades, where evapotranspiration exceeds precipitation during the dry season, results in the formation of calcrete layers, which may contribute to local topographic differences between tree islands and the surrounding marsh (Graf et al ; Ross & Sah ; Sullivan et al , ; Troxler et al ).…”

“…A similar local gradient exists for TP in soil pore water and vegetation foliage (Ross et al ). Within an island, soil TP decreases downstream from the head to its lowest level in the tail (Wetzel et al ; Wetzel et al ; Espinar et al ; Troxler et al ). Total nitrogen (TN) levels in the soil, pore water, and foliage did not follow the same pattern and were slightly lower or similar on the tree island head compared with the marsh (Ross et al ; Wetzel et al ).…”

“…Similar to the positive feedback mechanisms believed to create ridges, the focused redistribution of phosphorus on tree islands causes the islands to accumulate biomass at a greater rate than the surrounding sloughs, through the production of tree roots and organic matter accumulation on the island surface (Givnish et al ). The presence of trees concentrates nutrients on the island, some of which may move downstream off the island head by leaching into shallow groundwater (Espinar et al ; Wetzel et al ; Troxler et al ; Fig. ).…”

“…Transpiration of the tree patches is also a possible mechanism of nutrient concentration on tree islands (Wetzel et al ). Trees on tree islands transpire tremendous amounts of groundwater especially during the late dry season when daily evapotranspiration ranges from 3.4 to 9.9 mm/day (Wetzel et al ; daily mean = 5.3 ± 0.5 mm from Troxler et al ). This is enough water to lower the water level in the root zone between 1.0 and 3.0 cm each day during the dry season (Ross et al ; Wetzel et al ), resulting in a depressed groundwater table under the tree patch (Sullivan et al ).…”

“…The dry–wet seasonality of the Everglades plays an important role in moving P concentrated on the tree island head laterally to other parts of the island. Troxler et al () found that the rewetting of tree island head soils during precipitation events mobilized the largest fluxes of P, and that 67% of the annual vertical P flux occurred during the wet season between April and August. The mobilized and reprecipitated P is moved laterally by the gradient between the island head and lower, downstream elevations and surface water flow during the wet season.…”