In a previous study, it was possible to follow the fate of added organic matter at the microscale using isotopic labeling under the NanoSIMS. Vogel et al. (2014) observed a preferential retention of new organic matter at already existing organo‐mineral‐associated organic matter coatings.
We systematically enhanced the spatial analysis of intact aggregate architectures based on NanoSIMS. To quantify the spatial arrangement of soil minerals and organic matter coatings at the microscale of isolated intact microaggregates or fine particles we developed novel methods using supervised and unsupervised classification algorithms. We segmented the data into projected mineral surface and organic coatings through a multichannel machine learning segmentation (Schweizer et al., 2018). With this approach it is possible to determine the degree of organic matter coverage, its connectivity, and the CN:C ratio of individual organic coatings of individual isolated microaggregate particles.
We found a development in successive spatial patterns of patchy‐distributed organic matter coatings in small microaggregates over time from 15 to >700 years (Schweizer et al., 2018). Organic matter accrual governed the formation of soil structures in the proglacial environment of the Damma glacier: Over time after glacial retreat, we observed increased organic matter coverages at mineral surfaces and a development of organic coatings from patches to connected soil structures.
The formation of microaggregates did not lead to a complete masking of the mineral surfaces by organic matter. Instead, the organic matter sequestration in soils was decoupled from mineral surfaces sustaining their functionality with respect to their mineral surface properties, e.g. as sites for ion exchange.
We analyzed an intact, resin‐embedded aggregate from Scheyern (Germany) using NanoSIMS measurements in a supervised classification approach adapted from remote sensing (Steffens et al. 2017). This allowed identifying microdomains that represent two different, recurring microaggregate building units: One microdomain is mineral grains glued together by thin layers of clay minerals whereas the other microdomain is OM surrounded by clay minerals resembling organic nuclei for microaggregate formation. All domains are characterized by a definite arrangement of mineral and various organic components leading to a specific pore system and fulfilling different functions in soil.