SP3

Formation and composition of soil microaggregates as explored by their elemental and isotopic label composition

We elucidate processes and mechanisms for the formation of soil microaggregates by analyzing the microspatial elemental/isotopic composition and the three‐dimensional association of organic and mineral soil components at submicron resolution. We specifically focus on the analytical power of NanoSIMS (secondary ion mass spectrometry) to co‐localize the sequestration of organic matter and minerals and quantify how it influences the heterogeneous soil microarchitecture. These spatially explicit data is supplemented by bulk scale measurements, such as 13C‐NMR spectroscopy to characterize the chemical composition of organic matter. The data enables us to identify and quantify the contribution of distinct parts of the particle surfaces for the functions of soils for binding organic carbon, their water storage and transport function, and as habitats for microorganisms. The identified mechanistic relationships form an integral part of the quantitative model developed in the research
unit.

Preparatory work using cutting-edge methods for the evaluation of
microaggregates

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.

Advancing the spatial analysis of microaggregates

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.

Coverage and composition of mineral-associated organic maĴer over time

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.

Linking microaggregate structure with soil functions

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.

Ingrid Kögel-Knaber, Prof. Dr
Technische Universität München
Emil-Ramann-Str. 2
85354 Freising Google Maps site planExternal link