Forest ecosystems persist over thousands of years. The prevailing paradigm describes how a tight nutrient re‐utilisation loop (recycling) from organic matter stored in the forest floor into trees ensures forest ecosystem nutrition. However, these nutrients are constantly lost by drainage during storage on the forest floor. This nutrient loss must be balanced on the long‐term, because otherwise ecosystems would run into a deficit of mineral nutrients. In supply‐limited weathering regimes, which are often found in non‐eroding, well‐drained settings, chemical weathering has run to completion and the nutrient loss from the forest floor is counterbalanced by external atmospheric wet and dry deposition.
In kinetically limited weathering regimes, which are often found in eroding, temperate settings, the regolith (comprising soil, saprolite and weathered rock) still contains mineral nutrients, as mineral dissolution kinetics are slower than the advection of minerals from deep, unweathered rock to the Earth’s surface. At the surface, minerals and plant litter are removed by erosion. The mechanisms by which forest ecosystem nutrition is maintained in the face of these losses is the topic of this thesis. I explore the role of the deeper regolith and the mechanisms and fluxes by which this deep reservoir serves to sustain ecosystems over the long‐term. I use geochemical mass balances and innovative metal isotope proxies to derive fluxes and sources from rock weathering into forest trees in montane, eroding and temperate forest ecosystems.
How montane, temperate forest ecosystems are nourished was explored at two study sites in the Schwarzwald (site CON) and the Bayerischer Wald (site MIT). Both sites are underlain by paragneiss of contrasting mineralogy, mantled by Cambisols developed on periglacial slope deposits and covered by Fagus sylvatica and Picea abies. At both sites I quantified nutrient availability, nutrient accessibility, fluxes of nutrient supply by chemical weathering, and nutrient uptake by forest trees. The regolith at site CON experienced substantially more nutrient loss through chemical weathering than at site MIT.
Nevertheless, nutrient uptake fluxes from forest trees are virtually identical at both sites. Considering a forest ecosystem in a mass balance comprising a shallow `organic nutrient cycle´ and a belowground `geogenic nutrient pathway´ shows that the nutrient inventory in the forest floor is of finite size that lasts only for decades, because persistent nutrient loss through plant litter drainage and erosion occurs from the forest floor. This permanent nutrient loss is balanced by fluxes from a reservoir consisting of the biologically available fraction (water‐soluble and easily exchangeable fractions for the metal elements, and exchangeable and calcium‐bound phosphorus fractions for phosphorus) from the upper regolith (<3 m), and even more significantly from the deep regolith (>3 m). This reservoir of nutrients ensures forest ecosystem nutrition over millennia, because it can replace the nutrient loss from the forest floor and is continually replenished through chemical weathering. Supply from this deep reservoir is linked to the organic nutrient cycle which is regulated by uptake into forest trees through the adjustment of the number of nutrient re‐utilisation cycles from plant litter depending on nutrient supply fluxes. Thus, uniform uptake of mineral nutrients emerges despite large differences in their release through rock weathering.
With this new concept in mind I constrained the uptake depth of the most plant‐essential mineral nutrient phosphorus (P). While biologically accessible P forms, namely exchangeable P, are negligible throughout the regolith, the Ca‐bound P, biologically available as PO4 3‐ through mineral dissolution, increases with depth and dominates in the lower part of the regolith and in unweathered rock. To test whether this deep P and other mineral nutrients from this depth are utilised by trees, I applied isotopic tracing methods. To track the depth of nutrient uptake radiogenic Strontium (Sr), namely the 87Sr/86Sr ratio, an established isotope proxy for source tracing was used together with the first application of the meteoric cosmogenic Beryllium (Be) isotope system, namely 10Bemeteoric/9Bestable.
From the agreement in these isotope ratios between plant tissue and the forest floor´s biologically accessible fraction I demonstrate that these elements, and by inference also P and other mineral nutrients, are turned over between the forest floor and trees. I also show that these elements initially originate from the lower part of the regolith. Because this depth lies beyond reported rooting depths of the prevalent tree species it is speculated that nutrient uplift occurs through a combination of root‐mycorrhiza symbiosis, dimorphic root systems and capillary rise of pore water.
Nutrient export after rock weathering was explored in the montane, temperate forest ecosystems of the Southern Sierra Critical Zone Observatory, California, underlain by granodiorite bedrock, mantled by Entisols and Inceptisols and covered mainly by Pinus ponderosa. Magnesium (Mg) stable isotopes are sensitive indicators of Mg utilisation by biota. Mg utilisation takes place from up to 6 m depth, as evidenced by the light Mg isotopic composition of the easily exchangeable fraction. It was further found that trees, in particular wood, are isotopically heavy, whereas stream water is isotopically light. Converting this difference into a mass balance shows that 50‐100 % of the Mg released by chemical weathering is utilised by trees. From the comparison between the river dissolved fluxes of other plant‐essential and plant‐beneficial elements (K, Ca, P and Si) with their weathering release fluxes a deficit is found in the river dissolved fluxes that is attributed to nutrient uptake by forest trees. Thus, either the mineral nutrients are accumulating today in re‐growing forest biomass after clear cutting, or they are exported in plant litter and coarse woody debris, rather than appearing in drainage.
The two major outcomes of this thesis are that the permanent nutrient loss at the Earth surface by plant litter drainage and erosion is balanced by nutrient uptake from the deep regolith, and that erosion and weathering are coupled through nutrient uptake at depth and erosion of these nutrients in plant debris at the surface.
Uhlig, D. (2018): The deep Critical Zone as a source of mineral nutrients to montane, temperate forest ecosystems: PhD Dissertation. PhD Dissertation for Freien Universität Berlin.