Abstract
Leaching is one major pathway of phosphorus (P) and nitrogen (N) losses from forest ecosystems. Using a full factorial N×P fertilization and irrigation experiment, we investigated the leaching of dissolved organic and inorganic P (DOP and DIP) and N (DON and DIN) from organic layers (litter, Oe/Oa horizons) and mineral A horizons at two European beech sites of contrasting P status. Leachates showed highest DIP and DIN concentrations in summer and lowest in winter, while dissolved organic forms remained rather constant throughout seasons. During the dry and hot summer 2018, DOC : DOP and DOC : DON ratios in leachates were particularly narrow, suggesting a release of microbial P due to cell lysis by drying and rewetting. This effect was stronger at the low-P site. The estimated annual mean fluxes from the Oe/Oa horizons in the non-fertilized treatment were 60 and 30 mg m−2 yr−1 for total dissolved P and 730 and 650 mg m−2 yr−1 for total dissolved N at the high-P and the low-P site, respectively. Fluxes of P were highest in the organic layers and decreased towards the A horizon, likely due to sorption by minerals. Fertilization effects were additive at the high-P, but antagonistic at the low-P site: At the high-P site, fertilization with +N, +P, and +N+P increased total P fluxes from the Oe/Oa horizon by +33, +51, and +75 %, while the respective increases were +198, +156, and +10 % at the low-P site. The positive N-effect on DIP leaching possibly results from a removed N limitation of phosphatase activity at the low-P site. Fluxes of DOP remained unaffected by fertilization. Fluxes of DIN and DON from the Oe/Oa horizons increased upon +N and +N+P, but not upon +P fertilization. In conclusion, the estimated P fluxes from the A horizons were comparable in magnitude to reported atmospheric P inputs, suggesting that these systems do not deplete in P due to leaching. However, a particularly high sensitivity of DIP leaching to hotter and drier conditions suggests accelerated P losses under the expected more extreme future climate conditions. Increases of P leaching due to fertilization and drying-rewetting were higher in the low-P system, implying that the low-P system is more susceptible to environmental future changes.
Highlights
Leaching is one major pathway of phosphorus (P) and nitrogen (N) loss from forest ecosystems (e.g. Bol et al, 2016; Hedin et al, 1995)
During the dry and hot summer 2018, dissolved organic carbon (DOC):DOP and DOC:DON ratios in leachates were narrow, suggesting a release of microbial P due to cell lysis by drying and rewetting. This effect was stronger at the low-P site
The C:N:P stoichiometry of soil organic matter (SOM) has been identified as a key parameter: While critical C-to-nutrient ratios – above which nutrients that are mineralized during decomposition become immobilized by soil microorganisms and below which they are released in excess of biological demand 65 – are well established for N, but remain uncertain for P (e.g. Davies et al, 2016; Mooshammer et al, 2014)
Summary
Leaching is one major pathway of phosphorus (P) and nitrogen (N) loss from forest ecosystems (e.g. Bol et al, 2016; Hedin et al, 1995). Phosphorus and N are leached in dissolved inorganic and organic forms, or sorbed to colloids, with concentrations and fluxes varying strongly among soils and ecosystem types (Bol et al, 2016; Kaiser et al, 2003; Qualls and Haines, 1991a). We studied the leaching of dissolved organic and inorganic P and N in temperate beech forest soils as affected by seasons, nutrient status, and fertilization. We hypothesized: (i) Leaching of DIP and DIN from organic forest floor layers will show stronger seasonal variations and stronger fertilization effects than DOP and DON, as inorganic forms are more strongly controlled by mineralization and biotic uptake; in the mineral soil, seasonal and fertilization effects on leaching will be superimposed by sorption processes. We hypothesized: (i) Leaching of DIP and DIN from organic forest floor layers will show stronger seasonal variations and stronger fertilization effects than DOP and DON, as inorganic forms are more strongly controlled by mineralization and biotic uptake; in the mineral soil, seasonal and fertilization effects on leaching will be superimposed by sorption processes. (ii) The contribution of dissolved organic forms to total P and N leaching from organic layers will be greater at the low-P site due to stronger biotic uptake of 85 inorganic nutrients. (iii) Fertilization with N and P removes nutrient limitations, and will enhance P and N leaching, with effect sizes depending on the site, due to differences in nutrient status and mineral assemblage; effect sizes will differ between P and N, due to differences in sorption and uptake affinities of the two nutrients. (iv) We expect synergistic effects of the combined N and P fertilization since N addition will remove N limitation for biologically mediated P mobilization processes
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