The Earth’s Critical Zone ranges from deep, unweathered rock to the canopy of the highest trees and is the zone, where rock meets life. Physical, chemical, and biological processes shape this zone. Over timescales that can range from minutes to millennia, they form habitats for myriads of organisms. Weathering converts pristine rock into regolith (comprised of weathered rock, saprolite and soil) by means of inorganic and biogenic disintegration. These processes include physical and chemical alteration of rock and provide nutrients which are necessary to fulfill plants’ physiological needs. Terrestrial plants act as a gigantic geochemical pump that continuously passes water and mineral-derived nutrients from the rooting zone towards the soil and the top of the canopy. In doing so, they directly and indirectly modulate the shape and architecture of the Critical Zone. One widely accepted paradigm is that plants enhance weathering to constantly liberate nutrients by (1) mechanically weakening rock and regolith, (2) directing photosynthetic energy to soil microbiota, (3) increasing the solubility of minerals through acidification, and by (4) regulating surface runoff and water residence times at depth. Regardless of whether the resulting weathering serves to fulfill plant-physiological needs or is merely a side effect of the presence of plants, the question arises whether we can identify the geochemical imprints of plant nutrition. This is the central topic of this thesis. I focus on the below-ground geochemical characterization of four different ecosystems and of the associated above ground vegetation. I further use radiogenic and stable Sr isotopes as a proxy for mineral nutrient sources and geochemical mass balances to quantify the fluxes and sources from rock weathering into plants. How the architecture and chemistry of the Critical Zone varies along a gradient of mean annual precipitation (MAP; 10–1100mm yr−1) and net primary productivity (NPP; 30– 500 gC m−2 yr−1) was explored in the four “EarthShape: Earth Surface Shaping by Biota” (DFG-SPP 1803) study sites. These are situated along the Chilean Coastal Cordillera in arid (Pan de Azúcar), semi-arid (Santa Gracia), mediterranean (La Campana), and humid- temperate (Nahuelbuta) climate conditions. These sites were chosen to isolate the effect mean annual precipitation (MAP) and net primary productivity (NPP) and to minimize variabil- ity in confounding variables on weathering. Accordingly, all sites are underlain by granitoid rock and with roughly uniform rock uplift rates. In contrast, mantling soils range from poorly developed Regosol in the arid site to mature umbric Podzol in the humid-temperate site. Vegetation cover increases along this gradient from 5% to 100% ground cover and the prevailing plant species change from desert shrubs to deciduous and evergreen species. At all sites, I quantified the rate and the degree of chemical weathering, and hence the release and availability of mineral-derived nutrients to terrestrial ecosystems. Weathering rates are lowest in the arid and sparsely vegetated site and highest in the mediterranean site with almost complete vegetation cover. In the semi-arid and humid-temperate site however, weathering rates are similar despite massive differences in MAP, NPP, and vegetation cover. A trend similar to that observed for weathering rate applies to the relative degree of chemical weathering among the four sites: It is lowest in the arid site and highest in the mediterranean site. Due to differences in mineralogy, however, the degree of chemical weathering in the semi-arid site is higher than in the humid-temperate site. Yet, at none of the sites the regolith is entirely depleted in its stock of primary minerals, such that mineral nutrients remain available for plant nutrition. The absence of statistically significant differences in the degree and rate of chemical weathering between the sites (excluding the arid Pan de Azúcar) does not allow for distinguishing the abiotic (purely climate-related) from the biotic (climate- and bio-related) drivers of weathering. After quantifying the nutrient availability in these sites, I quantified their accessibility to plants and the fluxes into and out of the different ecosystems. These fluxes characterize the geogenic nutrient pathway (i.e. long-term nutrient supply through weathering) and the organic nutrient cycle (i.e. nutrient uptake by plants and subsequent recycling of leaf litter). Key to this characterization is knowledge of the weathering release fluxes, the size of the nutrient-reservoir available to plants, and a representative chemical composition of the ecosystem dominant plant taxa. The latter information is nearly impossible to acquire purely by means of sampling and chemical analysis of plants. Thus, I applied an allometric model which allocates relative growth rates between stems/ twigs and leaves to estimate bulk plants’ representative chemical composition. Bio-availability of most elements in soil, bar a few exceptions, increase from the arid to mediterranean site. However, despite featuring the thickest soils, element bio-availability is lowest in the humid-temperate site. The pattern of elemental release is rather uniform across the different climate regimes and levels of NPP, and so does not correlate with the size of the bio-available pool. In contrast, plants’ nutrient uptake rates increase in spite of the rather uniform weathering release rates from the semi-arid to the humid-temperate site. Hence, recycling rates (RecX) increase along this gradient and the ecosystems with high NPP maintain their nutrient supply by increasing recycling rather than increasing weathering. The organic nutrient pathway thus intensifies, whereas the geogenic nutrient pathway stays steady despite increasing MAP and NPP. Weathering and ecosystem nutrition are intimately linked through the supply of fresh mineral nutrients from regolith and bedrock (i.e. the “geogenic nutrient pathway”) that must replace any nutrient leakage. This link is muted if re-utilization of nutrients from plant litter during re-mineralization is efficient (i.e. the “organic nutrient cycle”). The shifting balance between these two cycles along the EarthShape climate and vegetation gradient was quantified by applying Sr isotope geochemistry in combination with mass balance calculations. Radiogenic and stable Sr isotope ratios as well as molar Ca/Sr ratios in the different compartments of the Critical Zone (bedrock, bulk regolith, the bio-available fraction being representative for regolith fluid, and vegetation) were measured to define the Sr sources and to quantify its fluxes. Sr is released through weathering over the entire depth of the regolith profiles. This release is isotopically congruent and no shift in isotope ratios of the released Sr occurs during secondary mineral formation and its initial transfer into the bio-available pool. However, Sr contained in the bio-available fraction is isotopically heavier than in rock and regolith. The cause of this offset was found in the plants ability to fractionate Sr. Although 88Sr/86Sr in organs of plants at all four study sites systematically increases from roots towards their leaves, bulk plants preferentially take up Sr with low 88Sr/86Sr. The shift in 88Sr/86Sr in the bio-available pool occurs after preferential uptake of isotopically light Sr into plants and its subsequent export in the form of fractionated organic solids (i.e. leaf litter) from the ecosystems. This export effectively diminishes the litter pool that is available for nutrient re-utilization and decreases the “net” nutrient recycling factor (RecX). Accordingly, RecX is essentially unaltered in the arid and semi-arid site where Sr export fluxes are low but shifts from 1 to almost zero recycling in the mediterranean site, and roughly halves recycling from 5 to 2 – 3 in the humid-temperate site. The major outcomes of this thesis are that (1) Ecosystems exert a substantial control over weathering by both directly and indirectly modulating processes which either enhance or reduce weathering fluxes. The silicate-weathering fluxes become effectively decoupled from the ultimate nutrient demands of biota by intensifying the organic nutrient cycle; and (2) Organic solids proving a significant export path of elements released during weathering. This export potentially impairs the ability for direct nutrient (re-) acquisition from leaf litter and thus reduces the recycling factor.
