Organic matter stabilization in soil microaggregates: implications from spatial heterogeneity of organic carbon contents and carbon forms

Abstract

This study investigates the spatial distribution of organic carbon (C) in free stable microaggregates (20-250 m; not encapsulated within macroaggregates) from one Inceptisol and two Oxisols in relation to current theories of the mechanisms of their formation. Two-dimensional micro-and nano-scale observations using synchrotron-based Fourier-transform infrared (FTIR) and near-edge X-ray absorption Wne structure (NEXAFS) spectroscopy yielded maps of the distribution of C amounts and chemical forms. Carbon deposits were unevenly distributed within microaggregates and did not show any discernable gradients between interior and exterior of aggregates. Rather, C deposits appeared to be patchy within the microaggregates. In contrast to the random location of C, there were micronscale patterns in the spatial distribution of aliphatic C-H (2922 cm ). Aliphatic C forms and the ratio of aliphatic C/ aromatic C were positively correlated (r 2 of 0.66-0.75 and 0.27-0.59, respectively) to the amount of O-H on kaolinite surfaces (3695 cm ¡1 ), pointing at a strong role for organomineral interactions in C stabilization within microaggregates and at a possible role for molecules containing aliphatic C-H groups in such interactions. This empirical relationship was supported by nanometer-scale observations using NEXAFS which showed that the organic matter in coatings on mineral surfaces had more aliphatic and carboxylic C with spectral characteristics resembling microbial metabolites than the organic matter of the entire microaggregate. Our observations thus support models of C stabilization in which the initially dominant process is adsorption of organics on mineral surfaces rather than occlusion of organic debris by adhering clay particles.

Synchrotron‐Based Near‐Edge X‐Ray Spectroscopy of Natural Organic Matter in Soils and Sediments

“…This confirms earlier results for three forest soils from the United States, Kenya, and Brazil (Lehmann et al, 2007). A detailed observation of a portion of the microaggregate showed an even greater variation in NOM forms which was visualized by principal component and cluster analysis (Figure 17.18D).…”

“…The first five eigenvalues (marked by larger symbols) are significantly higher than the rest. This suggests reducing our data set to five principal components (J. Lehmann, unpublished data 2006, measured as described in Lehmann et al, 2007). …”

“…Data Figure 17.18. Further reduction in the number of clusters will reduce redundancy, but can mask minor features such as those in cluster 19 (J. Lehmann, unpublished data 2006, measured as described in Lehmann et al, 2007). stack collection is impractical because individual fluorescence images can require 8 or more hours to collect. One approach is to conduct an overview scan at moderate resolution (e.g., 1 or 4 µm 2 pixels) followed by detailed small area scans and spectra collected at individual points in the image.…”

Three-dimensional soil organic matter distribution, accessibility and microbial respiration in macroaggregates using osmium staining and synchrotron X-ray computed tomography

“…This is in part because scientists have lacked methods for fine (< 10 µm) scale 3-D discrimination between minerals and SOM within aggregates. Approaches to date have generally been limited to mapping SOM in two dimensions (Lehmann et al, 2007) or in 3-D within smaller regions of aggregates (Yu et al, 2016). In a recent study, Hapca et al (2015) used a combination of Xray computed tomography (CT) and scanning electron microscope images to map soil chemical composition, including soil carbon, at a resolution of 15.8 µm in a small block of soil (side length of 1 cm).…”

Design of Decision Support System for Mechanical Conservation Tillage

“…Mechanical conservation tillage is a new agricultural production mode [1][2], and the development of this tillage is becoming the national economy and the people's livelihood issues stage by stage. How to spread mechanical conservation tillage effectively in China is a difficulty.…”