Process sequence of soil aggregate formation disentangled through multi-isotope labelling

authored by
Wulf Amelung, Nele Meyer, Andrey Rodionov, Claudia Knief, Michaela Aehnelt, Sara L. Bauke, Danh Biesgen, Stefan Dultz, Georg Guggenberger, Maguy Jaber, Erwin Klumpp, Ingrid Kögel-Knabner, Volker Nischwitz, Steffen A. Schweizer, Bei Wu, Kai U. Totsche, Eva Lehndorff
Abstract

Microaggregates (<250 µm) are key structural subunits of soils. However, their formation processes, rates, and transformation with time are poorly understood. We took advantage of multiple isotope labelling of potential organic gluing agents and inorganic building units to unravel their role in soil aggregation processes being initiated with and without plant growth. We added 13C-labelled extracellular polymeric substances (EPS), 15N-labelled bacteria, 57Fe-labelled goethite, and 29Si-labelled montmorillonite to fine soil <250 µm of an Ap horizon from a Stagnic Luvisol, which was planted with Festuca heteromalla or kept bare in a climate chamber. Samples were taken after 4, 12, and 30 weeks, and separated into free (f) and occluded (o) microaggregates of different sizes (<20 µm, 53–20 µm, 250–53 µm), and in stable macroaggregates (>250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (<20 µm) when plants were absent. The excess of bacterial 15N closely followed the pattern of EPS-derived 13C (R2 = 0.72). In contrast to the organic gluing agents, the goethite-57Fe and montmorillonite-29Si were relatively equally distributed across all size fractions. Overall, microaggregates were formed within weeks. Roots enforced this process by stabilizing microaggregates within stable macroaggregates. As time proceeded the labelled organic components decomposed, while the labelled secondary oxides and clay minerals increasingly contributed to aggregate stabilization and turnover at the scale of months and beyond. Consequently, the well-known hierarchical organization of aggregation follows a clear chronological sequence of stabilization and turnover processes.

Organisation(s)
Institute of Soil Science
Section Soil Chemistry
External Organisation(s)
University of Bonn
Forschungszentrum Jülich
University of Bayreuth
Friedrich Schiller University Jena
Sorbonne Université
Technical University of Munich (TUM)
Type
Article
Journal
GEODERMA
Volume
429
ISSN
0016-7061
Publication date
01.01.2023
Publication status
Published
Peer reviewed
Yes
ASJC Scopus subject areas
Soil Science
Sustainable Development Goals
SDG 13 - Climate Action
Electronic version(s)
https://doi.org/10.1016/j.geoderma.2022.116226 (Access: Open)