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.
Tracing organic matter associated with mineral surfaces (left) and new organic matter produced during incubation of a soil with labelled plant litter (right) as revealed by NanoSIMS. (from Vogel et al., Nature Comm., 2014)
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.
Differentiation of mineral surfaces and N-rich and N-poor organic matter (OM) coatings of microaggregates (data from Schweizer et al., Glob. Change Biol., 2018).
Coverage and composition of mineral-associated organic matter 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.
How soil organic matter accrual governs the initial formation of microaggregates in an area where climate change lead to the pronounced retreat of glaciers (from Schweizer et al., Glob. Change Biol., 2018).
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.
An intact soil aggregate from a topsoil (21 % clay) was clustered in two microdomains using 40 spatially independent measurements derived from NanoSIMS. Our results unraveled the arrangement of surfaces with specific functionalities for soil organic carbon storage in these micrometer-sized domains. (from Steffens et al., Environ. Sci. Technol., 2017)
We currently evaluate further data to better understand governing mechanisms for the formation of soil microaggregates. We focus on the microspatial arrangement of organic matter patches in soils with a large clay content gradient, intact structures from the common multi-stable-isotope labeling microcosm experiment of the research unit, and analyses that link aggregate composition with their organic matter concentrations and compositions.