Understanding and quantification of phosphorus (P) fluxes are key requirements for predictions of future forest ecosystems changes as well as for transferring lessons learned from natural ecosystems to croplands and plantations. This review summarizes and evaluates the recent knowledge on mechanisms, magnitude, and relevance by which dissolved and colloidal inorganic and organic P forms can be translocated within or exported from forest ecosystems. Attention is paid to hydrological pathways of P losses at the soil profile and landscape scales, and the subsequent influence of P on aquatic ecosystems. New (unpublished) data from the German Priority Program 1685 ''Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources'' were added to provide up-to-date flux-based information. Nitrogen (N) additions increase the release of water-transportable P forms. Most P found in percolates and pore waters belongs to the so-called dissolved organic P (DOP) fractions, rich in orthophosphate-monoesters and also containing some orthophosphate-diesters. Total solution P concentrations range from ca. 1 to 400 μg P L -1 , with large variations among forest stands. Recent sophisticated analyses revealed that large portions of the DOP in forest stream water can comprise natural nanoparticles and fine colloids which under extreme conditions may account for 40-100% of the P losses. Their translocation within preferential flow passes may be rapid, mediated by storm events. The potential total P loss through leaching into subsoils and with streams was found to be less than 50 mg P m -2 a -1 , suggesting effects on ecosystems at centennial to millennium scale. All current data are based on selected snapshots only. Quantitative measurements of P fluxes in temperate forest systems are nearly absent in the literature, probably due to main research focus on the C and N cycles. Therefore, we lack complete ecosystem-based assessments of dissolved and colloidal P fluxes within and from temperate forest systems. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Forest phosphorus cycle during ecosystem developmentForests are complex biogeochemical systems in which nutrient cycles readily change and become re-adjusted upon interactions with biotic and abiotic controls over diurnal, annual, decadal, centennial, and longer timescales (Hedin et al., 2003). Phosphorus (P) is an essential element for all living organisms. Modern agriculture avoids P limitation of primary production by continuous application of fertilizers, while forest ecosystems have developed efficient strategies for adapting to low P supply (Elser et al., 2007;Ilg et al., 2009;Rennenberg and Schmidt, 2010;Hinsinger et al., 2011). Increasing production of forests biomass in response to high atmospheric nitrogen (N) input and climate c...

Abstract. Plants and soil microbiota play an active role in rock weathering and potentially couple weathering at depth with erosion at the soil surface. The nature of this coupling is still unresolved because we lacked means to quantify the passage of chemical elements from rock through higher plants. In a temperate forested landscape characterised by relatively fast ( ∼ 220 t km −2 yr −1 ) denudation and a kinetically limited weathering regime of the Southern Sierra Critical Zone Observatory (SSCZO), California, we measured magnesium (Mg) stable isotopes that are sensitive indicators of Mg utilisation by biota. We find that Mg is highly bio-utilised: 50-100 % of the Mg released by chemical weathering is taken up by forest trees. To estimate the tree uptake of other bioutilised elements (K, Ca, P and Si) we compared the dissolved fluxes of these elements and Mg in rivers with their solubilisation fluxes from rock (rock dissolution flux minus secondary mineral formation flux). We find a deficit in the dissolved fluxes throughout, which we attribute to the nutrient uptake by forest trees. Therefore both the Mg isotopes and the flux comparison suggest that a substantial part of the major element weathering flux is consumed by the tree biomass. The enrichment of 26 Mg over 24 Mg in tree trunks relative to leaves suggests that tree trunks account for a substantial fraction of the net uptake of Mg. This isotopic and elemental compartment separation is prevented from obliteration (which would occur by Mg redissolution) by two potential effects. Either the mineral nutrients accumulate today in regrowing forest biomass after clear cutting, or they are exported in litter and coarse woody debris (CWD) such that they remain in "solid" biomass. Over pre-forest-management weathering timescales, this removal flux might have been in operation in the form of natural erosion of CWD. Regardless of the removal mechanism, our approach provides entirely novel means towards the direct quantification of biogenic uptake following weathering. We find that Mg and other nutrients and the plant-beneficial element Si ("bio-elements") are taken up by trees at up to 6 m depth, and surface recycling of all bio-elements but P is minimal. Thus, in the watersheds of the SSCZO, the coupling between erosion and weathering might be established by bio-elements that are taken up by trees, are not recycled and are missing in the dissolved river flux due to erosion as CWD and as leaf-derived bio-opal for Si. We suggest that the partitioning of a biogenic weathering flux into eroded plant debris might represent a significant global contribution to element export after weathering in eroding mountain catchments that are characterised by a continuous supply of fresh mineral nutrients.

Mineral nutrient cycling between trees and the forest floor is key to forest ecosystem nutrition. However, in sloping, well-drained landscapes the forest floor experiences permanent nutrient loss in particulate form by plant litter erosion and as solute after plant litter decomposition, solubilisation, and export. To prevent nutrient deficit, a replenishing mechanism must be in operation that we suggest to be sourced in the subsoil and the weathering zone beneath it, provided that atmospheric input is insufficient. To explore such a mechanism, we quantified deep (up to 20 m depth) weathering and mineral nutrient cycling in two montane, temperate forest ecosystems in Southern Germany: Black Forest (CON) and Bavarian Forest (MIT). From measurements of the inventories, turnover times, and fluxes of macronutrients (K, Ca, Mg, P) we found evidence for a fast, shallow "organic nutrient cycle", and a slow, deep "geogenic nutrient pathway". We found that the finite nutrient pool size of the forest floor persists for a few years only. Despite this loss, foliar nutrient concentrations in Picea abies and Fagus sylvatica do not indicate deficiency. We infer that ultimately the biologically available fraction in the deep regolith (CON: 3-7 m, MIT: 3-17 m) balances nutrient loss from the forest floor and is also decisive for the level of the forest trees' mineral nutrient stoichiometry. Intriguingly, although the nutrient supply fluxes from chemical weathering at CON are twice those of MIT, nutrient uptake fluxes into trees do not differ. The organic nutrient cycle apparently regulates the efficiency of nutrient re-utilization from organic matter to cater for differences in its replenishment by the deep geogenic nutrient pathway, and thereby ensures long-term forest ecosystem nutrition.

<p><strong>Abstract.</strong> Plants and soil microbiota play an active role in rock weathering and potentially couple weathering at depth with erosion at the soil surface. The nature of this coupling is still unresolved because we lacked means to quantify the passage of chemical elements from rock through higher plants. In a temperate forested landscape of the Southern Sierra Critical Zone Observatory (SSCZO), California, we measured magnesium (Mg) stable isotopes that are sensitive indicators of Mg utilisation by biota. We find that Mg is highly bio-utilised: 50–100 % of the Mg released by chemical weathering is taken up by forest trees. To estimate the tree uptake of other bio-utilised elements (K, Ca, P and Si) we compared the dissolved fluxes of these elements and Mg in rivers with their solubilisation fluxes from rock (rock dissolution flux minus secondary mineral formation flux). We find a deficit in the dissolved fluxes throughout, that we attribute to the nutrient uptake by forest trees. Therefore both the Mg isotopes and the flux comparison suggests that a substantial part of the major element weathering flux is consumed by the tree biomass. This isotopic and elemental compartment separation is preserved only if the mineral nutrients contained in biomass are prevented from re-dissolution after litter fall, showing that these nutrients have been removed as "solid" biomass. The enrichment of <sup>26</sup>Mg over <sup>24</sup>Mg in tree trunks relative to leaf litter suggests that this removal occurs mainly in coarse woody debris (CWD). Today, CWD is exported from the ecosystem by tree logging. Over pre-anthropogenic weathering time scales, a similar removal flux might have been in operation in the form of natural erosion of CWD. Regardless of the removal mechanism, our data provides the first direct quantification of biogenic uptake following weathering. We find that Mg and other bio-elements are taken up by trees at up to 7 m depth, and surface recycling of all bio-elements but P is minimal. Thus, in the watersheds of the SSCZO in which weathering is fast and kinetically-limited, the coupling between erosion and weathering might be established by bio-elements that are taken up by trees, not recycled and missing in the dissolved river flux due to erosion as CWD and as leaf-derived bio-opal for Si. We suggest that the partitioning of a biogenic weathering flux into eroded plant debris might represent a significant global contribution to element export after weathering in eroding mountain catchments that are characterised by a continuous supply of fresh mineral nutrients.</p>

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