Mineralogical control on physically protected soil organic matter in a neotropical moist forest
Molly E. Huber A , Joseph B. Yavitt
A * and S. Joseph Wright B
+ Author Affiliations
- Author Affiliations
A Department of Natural Resources & the Environment, Cornell University, Ithaca, NY 14853-3001, USA.
B Smithsonian Tropical Research Institute, Balboa 0843-03092, Panama.
Soil Research 61(6) 569-581 https://doi.org/10.1071/SR22265
Submitted: 20 December 2022 Accepted: 6 April 2023 Published: 9 May 2023
© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing
Abstract
Context: Minerals and organic matter physically associate in many soils, yet the precise nature, either via a hierarchy of aggregate particles or by build-up of organo–mineral associations, remains obscure, especially in tropical forest environments.
Aims: We investigated physically protected organic matter by comparing soils with contrasting parent material, topography, and pedogenesis, but with similar tropical moist forest on Barro Colorado Island in the lowlands of Panama.
Methods: Bulk soil from 10 sites was separated by size into free-floating particles, macroaggregates (>250 μm), microaggregates (53–250 μm), and <53-μm particles. A subsample of macroaggregates was disintegrated and separated into coarse particles (>250 μm), occluded microaggregates, and occluded <53-μm particles. Concentrations and natural abundance of stable isotopes for carbon (C) and nitrogen (N) were determined for each fraction, and ratios (C:N, stable isotopes) were used to characterise organic matter for each fraction.
Key results: Macroaggregates were the largest fraction of bulk soil (71%) and were 15% greater in kaolinite- than smectite-dominated soils. Macroaggregates were composed of coarse particles (14%), occluded microaggregates (62%), and occluded <53-μm particles (24%). Concentrations of C and N widely varied among fractions but the variation was not related to clay mineralogy. The C:N ratio and stable N isotope ratio indicated more decomposed organic matter in kaolinite- than smectite-dominated soils.
Conclusions and implications: Macroaggregates composed of plant detritus and microaggregates in the Barro Colorado Island soils imply that the aggregate hierarchy route ultimately protects soil organic matter in this tropical forest environment.
Keywords: aluminum oxide, carbon:nitrogen ratio, delta 15N, macroaggregates, microaggregates, parent material, pseudo-sand structures, soil type, tropical forest.
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