Abstract

A central question in carbon research is how stabilization mechanisms in soil change over time with soil development and how this is reflected in qualitative changes of soil organic matter (SOM). To address this matter, we assessed the influence of soil geochemistry on bulk SOM composition along a soil chronosequence in California, USA spanning 3 million years. This was done by combining data on soil mineralogy and texture from previous studies with additional measurements on total carbon (C), stable isotope values (δ13C and δ15N), and spectral information derived from Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS). To assess qualitative shifts in bulk SOM, we analysed the peak areas of simple plant-derived (S-POM), complex plant-derived (C-POM), and predominantly microbially derived OM (MOM) and their changes in abundance across soils varying several millennia to millions of years in weathering and soil development. We observed that SOM became increasingly stabilized and microbially-derived (lower C : N ratio, increasing δ13C and δ15N) as soil weathering progresses. Peak areas of S-POM (i.e. aliphatic root exudates) did not change over time, while peak areas of C-POM (lignin) and MOM (components of microbial cell walls (amides, quinones, and ketones)) increased over time and depth and were closely related to clay content and pedogenic iron oxides. Hence, our study suggests that with progressing soil development, SOM composition co-varies with changes in the mineral matrix. Our study indicates that a discrimination in favour of structurally more complex OM compounds (C-POM, MOM) gains importance as the mineral soil matrix becomes increasingly weathered.

Highlights

  • We observed that soil organic matter (SOM) became increasingly stabilized and microbially-derived as soil weathering progresses

  • Weathering and leaching of cations led to a reduction of the cation exchange capacity (CEC) of 70 % (difference between youngest (0.1 kyrs) and oldest (3000 kyrs) terrace) in 0-10 cm and 85 % in 10-30 cm depth, which was accompanied by a steady decrease in soil pH along the chronosequence

  • SOM was 225 increasingly processed (C:N ratio, δ13C, δ15N) which went along with increasing peak areas related to complex plant-derived OM (C-POM) and microbial derived OM (MOM), and constant peak areas related to simple plant-derived OM (S-POM)

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Summary

Introduction

Soils harbour the largest and most active terrestrial carbon (C) pool on earth (Jobbágy and Jackson, 2000; Lal, 2008). Many of the detailed mechanistic processes determining the fate of soil organic matter (SOM) in soils have been well studied (Schmidt et al, 2011; Lehmann and Kleber, 2015; Kleber et al, 2021). Differences regarding pedogenic (i.e. soil type, mineralogy, soil microbiome) and environmental (i.e. climate, vegetation cover) properties determine the sizes of SOM stocks, and govern the abundance of diverse biochemical compounds (e.g. lignin, polysaccharides, lipids) found in the terrestrial C pool (Paul, 2016; Hall et al, 2020). The peak areas related to biochemical SOM compounds has been successfully used to parameterize fast and slow-cycling SOM pools (Todd-Brown et al, 2013; Bailey et al, 2018; Laub et al, 2020; Baldock et al, 2021)

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