How Slow Rock Weathering Balances Nutrient Loss During Fast Forest Floor Turnover in Montane, Temperate Forest Ecosystems

David Uhlig,Friedhelm Von Blanckenburg

doi:10.3389/feart.2019.00159

Abstract

Mineral nutrient cycling between trees and the forest floor is key to forest ecosystem nutrition. However, in sloping, well-drained landscapes the forest floor experiences permanent nutrient loss in particulate form by plant litter erosion and as solute after plant litter decomposition, solubilisation and export. To prevent nutrient deficit, a replenishing mechanism must be in operation that we suggest to be sourced in the subsoil and the weathering zone beneath it, provided that atmospheric input is insufficient. To explore such a mechanism, we quantified deep (up to 20 m depth) weathering and mineral nutrient cycling in two montane, temperate forest ecosystems in Southern Germany: Black Forest (CON) and Bavarian Forest (MIT). From measurements of the inventories, turnover times, and fluxes of macronutrients (K, Ca, Mg, P) we found evidence for a fast, shallow `organic nutrient cycle´, and a slow, deep `geogenic nutrient pathway´. We found that the finite nutrient pool size of the forest floor persists for a few years only. Despite this loss, foliar nutrient concentrations in Picea abies and Fagus sylvatica do not indicate deficiency. We infer that ultimately the biologically available fraction in the deep regolith (CON: 3 - 7 m, MIT: 3 - 17 m) balances nutrient loss from the forest floor and is also decisive for the level of the forest trees’ mineral nutrient stoichiometry. Intriguingly, although the nutrient supply fluxes from chemical weathering at CON are twice those of MIT, nutrient uptake fluxes into trees do not differ. The organic nutrient cycle apparently regulates its efficiency to cater for differences in its replenishment by the deep geogenic nutrient pathway, and thereby ensures long-term forest ecosystem nutrition.

Highlights

A fundamental feature of forest ecosystems is that for most rock-derived nutritive elements demand by forest trees vastly exceeds mineral nutrient supply from chemical weathering and atmospheric deposition
We provide quantitative means to assess the importance of the slow “geogenic nutrient pathway” through which fresh material is uplifted into soil, and that is coupled to the fast “organic nutrient cycle” on long-term forest ecosystem nutrition similar to the conceptual model of Buendía et al (2010)
As we suggest that the replenishment of this forest floor compartment originates from chemical weathering, we speculate that the number of nutrient recycling loops between forest trees and organic matter in the forest floor depends on the size of the geogenic-derived nutrient supply fluxes, namely chemical weathering

Summary

Introduction

A fundamental feature of forest ecosystems is that for most rock-derived nutritive elements demand by forest trees vastly exceeds mineral nutrient supply from chemical weathering and atmospheric deposition. Maintaining Forest Nutrition From Subsoil (Aerts and Chapin, 1999) that all contribute to satisfy nutrient demand through a cycle, that we call here the “organic nutrient cycle”. The most common strategy is efficient re-utilization of organic-bound nutrients from plant litter in the forest floor (e.g., Jobbágy and Jackson, 2004; Lang et al, 2016). The fast turnover of the forest floor ensures forest ecosystem nutrition. Because organic-bound nutrients are not directly utilizable by plants (George et al, 2011; Jansa et al, 2011), microbial activity is central in ensuring the fast turnover (e.g., Lang et al, 2016). Mycorrhizal fungi promote the breakdown of organic matter into plant-available inorganic forms that are subsequently taken up by tree roots (e.g., Chapin et al, 2012)

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Conclusion