Quantifying nutrient uptake as driver of rock weathering in forest ecosystems by magnesium stable isotopes

Abstract

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.

Digging deeper: what the critical zone perspective adds to the study of plant ecophysiology

“…Their findings imply that plant roots facilitate rock weathering and production of ‘fracture fill’ (i.e. loose, soil‐like material in fractures) at depth, and lend support to the findings of Uhlig et al () that plant roots also take up nutrients from fracture environments (Fig. ).…”

“…Alongside a focus on nutrient inputs to terrestrial ecosystems, CZ research is beginning to reveal deep subsurface elemental cycling driven by plants, within bedrock fractures. Through an innovative application of Mg stable isotopes and elemental mass balance, Uhlig et al () demonstrated that nutrients (P, K, Mg, Ca and Si) are directly incorporated into plant biomass from weathering of parent material within the rock moisture zone, providing evidence of plant acquisition of nutrients at depth. Taking a different approach, Hasenmueller et al () described the physical and biogeochemical environment of tree roots extending from surface soils into fractured bedrock using a series of pits to assess plant–rock interactions and the potential of plant‐mediated weathering of rock to regolith.…”

Mg Isotope Interlaboratory Comparison of Reference Materials from Earth‐Surface Low‐Temperature Environments

“…Procedures for Mg purification and isotope ratio measurements are similar to those described previously (Uhlig et al . ). A brief description of the method applied during this study is given below.…”

Exploring the Effect of Aspect to Inform Future Earthcasts of Climate‐Driven Changes in Weathering of Shale

“…The basic idea of the cycling module, described more fully in section , was to account for the well‐known phenomenon of biolifting and nutrient cycling of base cations and Si through vegetation. Specifically, Ca, K, Mg, and Si cycle through vegetation approximately 10–40 times before exiting soils and entering rivers (Uhlig et al, ). The cycling of soluble nutrient cations is expected to increase concentrations in the upper soil layers to a much greater extent than could be achieved through the chemical weathering of clay minerals alone in those upper layers.…”