Mineral Nutrients Sourced in Deep Regolith Sustain Long‐Term Nutrition of Mountainous Temperate Forest Ecosystems

D Uhlig,F Von Blanckenburg,W Amelung

doi:10.1029/2019gb006513

Abstract

AbstractPrimary productivity of forest ecosystems depends on the availability of plant‐essential mineral nutrients. Because nutrient demand of trees often exceeds nutrient supply from rock, tree nutrition is sustained by efficient reutilization of organic‐bound nutrients. These nutrients are continuously returned from trees to the forest floor in litterfall. However, over millennia nutrient limitation may develop in landscapes from which nutrients are permanently lost by drainage and erosion. Such a deficit is prevented if advection of unweathered bedrock toward the surface as driven by erosion continuously supplies fresh nutrients. Yet the mechanisms and the depth range over which this deep nutrient resource is accessed are poorly known. We show that in two montane temperate forest ecosystems in the Black Forest and Bavarian Forest the geogenic source of nutrients was found within a depth zone of several meters. This deep zone contains a large pool of biologically available nutrients. We applied isotope ratios as proxies for nutrient uptake depth, and we tracked the regolith depth at which the isotope ratios of 87Sr/86Sr and 10Be(meteoric)/9Be match the respective values in plant tissue. We mapped the depth distribution of the biologically available calcium‐bound form of the most plant‐essential mineral nutrient phosphorus and found that the depth of phosphorus availability is as deep or even deeper as the range defined by the isotope ratios. We conclude that nutrient supply from a regolith depth of several meters is critical for forest ecosystem function in landscapes of moderate hillslopes and rainfall that are affected by permanent nutrient loss.

Highlights

A key regulator of primary productivity in forest ecosystems is the availability of nutrients (Fernández‐ Martínez et al, 2014)
We conclude that nutrient supply from a regolith depth of several meters is critical for forest ecosystem function in landscapes of moderate hillslopes and rainfall that are affected by permanent nutrient loss
Due to the tight binding of P to organic matter and soil minerals, 90% of total P loss may occur via particulate export (Tiessen, 2008), where fine particulate export amounts to losses of up to 800 mg P m−2 year−1 (Meyer & Likens, 1979)

Summary

Introduction

A key regulator of primary productivity in forest ecosystems is the availability of nutrients (Fernández‐ Martínez et al, 2014). The magnitude of this organic mineral nutrient turnover exceeds geochemical release fluxes from primary minerals by a factor of between 5 and 100 (e.g., Riotte et al, 2014; Schuessler et al, 2018; Uhlig et al, 2017; Wilcke et al, 2017). This cycle has been called the “organic nutrient pathway” (Uhlig & von Blanckenburg, 2019a).

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Conclusion