Do degree and rate of silicate weathering depend on plant productivity?

Abstract

Abstract. Plants and their associated below-ground microbiota possess the tools for rock weathering. Yet the quantitative evaluation of the impact of these biogenic weathering drivers relative to abiogenic parameters, such as the supply of primary minerals, water, and acids, is an open question in Critical Zone research. Here we present a novel strategy to decipher the relative impact of these drivers. We quantified the degree and rate of weathering and compared these to nutrient uptake along the “EarthShape” transect in the Chilean Coastal Cordillera. These sites define a major north–south gradient in precipitation and primary productivity but overlie granitoid rock throughout. We present a dataset of the chemistry of Critical Zone compartments (bedrock, regolith, soil, and vegetation) to quantify the relative loss of soluble elements (the “degree of weathering”) and the inventory of bioavailable elements. We use 87Sr∕86Sr isotope ratios to identify the sources of mineral nutrients to plants. With rates from cosmogenic nuclides and biomass growth we determined fluxes (“weathering rates”), meaning the rate of loss of elements out of the ecosystems, averaged over weathering timescales (millennia), and quantified mineral nutrient recycling between the bulk weathering zone and the bulk vegetation cover. We found that neither the degree of weathering nor the weathering rates increase systematically with precipitation from north to south along the climate and vegetation gradient. Instead, the increase in biomass nutrient demand is accommodated by faster nutrient recycling. In the absence of an increase in weathering rate despite a five-fold increase in precipitation and net primary productivity (NPP), we hypothesize that plant growth might in fact dampen weathering rates. Because plants are thought to be key players in the global silicate weathering–carbon feedback, this hypothesis merits further evaluation.

Rock weathering and nutrient cycling along an erodosequence

“…More generally, this recycling hypothesis has relevance for the suggestion that plants and associated microbiota impact weathering fluxes (Moulton and others, 2000;Berner and others, 2003;Chaudhuri and others, 2007;Cornelis and others, 2009;Riotte and others, 2014). In agreement with a recent study along a vegetation gradient in Chile (Oeser and von Blanckenburg, 2020a) we find that the large range of nutrient recycling factors, inversely related to erosion rate, buffers ecosystem nutrition making it virtually independent from rock weathering.…”

“…Regarding weathering and the operation of the global thermostat via CO 2 drawdown, an important consequence is that ecosystems have the ability to buffer mineral nutrient uptake via recycling, and thus weathering rate is not a straightforward function of primary production (Uhlig and von Blanckenburg, 2019;Oeser and von Blanckenburg, 2020a). More specifically, plant growth may not necessarily accelerate rock weathering as suggested in models and syntheses of weathering throughout Earth's history others, 2003, Pagani andothers, 2009).…”

Modelling the effect of catena position and hydrology on soil chemical weathering

“…6), due to the higher water availability on the NFS (García-Gamero et al, 2021). Oeser & von Blanckenburg (2020) pointed out that the presence of denser vegetation might counteract a potential weathering increase in their study along the climate and vegetation gradient in the Chilean Coastal Cordillera. This could explain why SC8 is not an outlier, similar to SC5.…”

“…Similarly, Dixon et al (2016) in their study for chemical weathering in postglacial soils of New Zealand found an important pedogenic threshold coincident at mean annual precipitation of ~800 mm-yr-1, very similar to the threshold value that was identified in this sensitivity analysis. Oeser & von Blanckenburg (2020) on the other hand, in their study in the EarthShape Critical Zone located along the Chilean Coastal Cordillera, found no correlation between the degree of weathering and mean annual precipitation. Therefore, they pointed out that a competitive effect seems to offset the expected increase in the rate of weathering with precipitation.…”

Comment on soil-2021-78

“…Zealand found an important pedogenic threshold coincident at mean annual precipitation of ~800 mm-yr-1, very similar to the threshold value that was identified in this sensitivity analysis. Oeser & von Blanckenburg (2020) on the other hand, in their study in the EarthShape Critical Zone located along the Chilean Coastal Cordillera, found no correlation between the degree of weathering and mean annual precipitation. Therefore, they pointed out that a competitive effect seems to offset the expected increase in the rate of weathering with precipitation.…”