Abstract
Declining foliar phosphorus (P) levels call increasing attention to the cycling of this element in temperate forests. We explored the fluxes of P in a temperate mixed deciduous forest ecosystem in six distinct hydrological compartments: Bulk precipitation and throughfall, soil water draining laterally from three different soil depths (0-15, 15-150, 150-320 cm below soil surface), groundwater, creek and spring discharge, which were sampled at daily to bi-weekly resolution from March 2015 to February 2016. Atmospheric P fluxes into the ecosystem were equally partitioned between wet and dry deposition. Approximately 10% of the foliar P stock was lost annually by foliar leaching during late summer. The concentrations of dissolved P in soil water from the forest floor and upper mineral topsoil followed a pronounced seasonal cycle with higher concentrations during the vegetation period. The concentrations of P dissolved in soil water decreased with increasing soil depth. Using an end member mixing analysis (EMMA) we found that P sources feeding the spring water were both soil water from greater depths or groundwater with season specific contributions. Atmospheric P fluxes into the ecosystem determined in this study and P-release from weathering reported for the research site were large enough to compensate P losses with runoff. This suggests that declining foliar P levels of forests are unlikely the result of a dwindling total P supply, but rather caused by tree nutrition imbalances or alternative stressors.
Highlights
The primary productivity of forest ecosystems is often limited or co-limited by P availability (Elser et al, 2000, 2007; Vitousek et al, 2010; Achat et al, 2016)
We focus on the P canopy balance, the relative contribution of P from soil water and groundwater to discharge in a spring and a first order headwater, and the respective P fluxes
The common observation, that percolating water becomes strongly enriched in dissolved P during the passage through the canopy and the forest floor, where the highest P concentrations are recorded, was confirmed by this study
Summary
The primary productivity of forest ecosystems is often limited or co-limited by P availability (Elser et al, 2000, 2007; Vitousek et al, 2010; Achat et al, 2016). The P cycle in temperate forests depends mainly on the general boundary conditions of the past and present, which are the drivers of the local ecosystem development (Walker and Syers, 1976). Important boundary conditions include lithology as the original source of most ecosystem P as well as climate and topography as controls of the water cycle, erosion-sedimentation, weathering processes and soil development, through which the bio-availability of P is determined (Laliberté et al, 2013). Fluxes of P across the ecosystem boundaries occur through runoff and atmospheric transport, which may result in either net gains or losses (Newman, 1995; Mahowald et al, 2008; Buendia et al, 2010; Tipping et al, 2014). Average estimates for above canopy atmospheric deposition mostly range from 10 to 100 mg P m−2 a−1; discharge losses are typically in the range of 1–10 mg P m−2 a−1 (Cole and Rapp, 1981; Sohrt et al, 2017)
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