Development of SOM and aggregation in an agriculturally managed re-cultivated loess

Soil organic matter (SOM) and organic glues from biological processes are considered to be major contributors inaggregate formation. But there is limited knowledge on soil structural formation during initial soil development –the step when SOM content is low and soil properties are mostly controlled by the parent material. In our study we used a chronosequence approach in the re-cultivated open-cast mining area near Cologne, Germany to elucidate thedevelopment of soil structure and soil organic matter during initial soil formation in a loess material. We selectedsix plots with different ages of agricultural management after re-cultivation (0, 1, 3, 6, 12, and 24 years after firstseeding). In each plot 12 spatially independent locations were sampled with stainless steel cylinders (100 cm3)at three depths representing the topsoil (1-5 cm), the plowing layer (16-20 cm), and the management-unaffectedparent material (41-45 cm). All samples were analysed for bulk density, organic and inorganic carbon and totalnitrogen content, and aggregate size distribution (≥20 mm, 20-6.3 mm, 3.6-2 mm, and≤2 mm). We calculated soil organic carbon stocks during this early phase of soil formation and assessed the development of aggregationby determining the aggregate stability and their organic carbon content. The re-cultivated soils in the area were alkaline and no differentiation was determined along the chronosequence with having an average pHCaCl2of 7.5. The reclamation was established with freshly excavated loess, thus CaCO3content in the soil was relatively high having concentrations in management-unaffected parent material layer of73.7 mg g−1in 0 year and 124.5 mg g−1in 24 years. Bulk density and soil organic carbon content showed different temporal developments. In just one year bulk density increased to an average of 1.6 g cm−3and changed after first plowing with remaining stable after 12 years to 1.5 g cm−3from topsoil to parent material. Soil organic carbon content increased during first three years only in topsoil (1-5 cm). After ploughing fresh OM input was detected in0-30 cm, where OC stocks increased from 1.23 kg OC m−2in 3 years to 4.06 kg OC m−2in 6 years. Although wedetected OM input and an increase of OC concentrations in aggregates along the chronosequence, we did not see significant differences in aggregate size distribution. Due to high carbonate content in re-cultivated soils, CaCO3 was dominating as a cementing agent and had a strong influence on aggregation in loess soil

Hotspots of soil organic carbon storage revealed by laboratory hyperspectral imaging

Subsoil organic carbon (OC) is generally lower in content and more heterogeneous than topsoil OC, rendering it difficult to detect significant differences in subsoil OC storage. We tested the application of laboratory hyperspectral imaging with a variety of machine learning approaches to predict OC distribution in undisturbed soil cores. Using a bias-corrected random forest we were able to reproduce the OC distribution in the soil cores with very good to excellent model goodness-of-fit, enabling us to map the spatial distribution of OC in the soil cores at very high resolution (~53 × 53 µm). Despite a large increase in variance and reduction in OC content with increasing depth, the high resolution of the images enabled statistically powerful analysis in spatial distribution of OC in the soil cores. In contrast to the relatively homogeneous distribution of OC in the plough horizon, the subsoil was characterized by distinct regions of OC enrichment and depletion, including biopores which contained ~2–10 times higher SOC contents than the soil matrix in close proximity. Laboratory hyperspectral imaging enables powerful, fine-scale investigations of the vertical distribution of soil OC as well as hotspots of OC storage in undisturbed samples, overcoming limitations of traditional soil sampling campaigns

Combination of Imaging Infrared Spectroscopy and X-ray Computed Microtomography for the Investigation of Bio-and Physicochemical processes in Structured Soils

Soil is a heterogeneous mixture of various organic and inorganic parent materials. Major soil functions are driven by their quality, quantity and spatial arrangement, resulting in soil structure. Physical protection of organic matter (OM) in this soil structure is considered as a vital mechanism for stabilizing organic carbon turnover, an important soil function in times of climate change. Herein, we present a technique for the correlative analysis of 2D imaging visible light near-infrared spectroscopy and 3D X-ray computed microtomography (mCT) to investigate the interplay of biogeochemical properties and soil structure in undisturbed soil samples. Samples from the same substrate but different soil management and depth (no-tilled topsoil, tilled topsoil and subsoil) were compared in order to evaluate this method in a diversely structured soil. Imaging spectroscopy is generally used to qualitatively and quantitatively identify OM with high spatial resolution, whereas 3D X-ray mCT provides high resolution information on pore characteristics. The unique combination of these techniques revealed that, in undisturbed samples, OM can be found mainly at greater distances from macropores and close to biopores. However, alterations were observed because of disturbances by tillage. The correlative application of imaging infrared spectroscopic and X-ray mCT analysis provided new insights into the biochemical processes affected by soil structural changes

The role of SOM and CaCO3 on soil aggregate development in reclaimed soils

Soil organic matter (SOM) and extracellular polymeric substances (EPS) from biological processesare considered to be major contributors in aggregate formation. But there is limited knowledge onsoil structural formation after reclamation – the step when SOM content is low and soil propertiesare mostly controlled by the parent material. In our study we used a chronosequence approach in the reclaimed open-cast mining area near Cologne, Germany to elucidate the development of soilstructure and soil organic matter during initial soil formation in a loess material. We selected sixplots with different ages of agricultural management after reclamation (0, 1, 3, 6, 12, and 24 yearsafter first seeding). In each reclaimed field 12 spatially independent locations were sampled with stainless steel cylinders (100 cm3) at two depths in the topsoil (1-5 cm and 16-20 cm). Sampleswere wet sieved into four aggregate size classes of <63 μm, 63-200 μm, 200-630 μm and 630-2000μm. Each aggregate size class was characterized by organic carbon (OC), total nitrogen (TN) andCaCO3 concentration. The chemical composition of the SOM of selected samples was characterized using solid-state 13C NMR spectroscopy. Wet sieving into aggregate size classes showed different trends along the chronosequence. Contradicting relation between CaCO3 and OC contribution to aggregate size classes display two different mechanisms on soil aggregate formation in young loess derived soils. CaCO3 influence daggregation predominantly in finer aggregate size classes, where the highest concentration and contribution was measured. SOM, on the other hand, played an important role on formation oflarge macro-aggregates after organic manure application in year 4. Furthermore, the loss of totalOC after year 12 was connected with the loss of OC contributing to the largest aggregate size class. Our findings reveal that SOM and CaCO3 role on stabilizing aggregates is not equally distributedand is aggregate size class dependent

Structures and processes of the initial ecosystem development phase in an artificial water catchment (Final report CRC/TR 38)

Objective of the Transregional Collaborative Research Centre (CRC/TR) 38 was the study of structures and processes of the initial ecosystem development. It was assumed that the initial phase is characterized by less structured and therefore less heterogeneous ecosystems. Thus, analysis of young ecosystems in their initial stages should provide better insights into ecosystem functioning. Following this basic concept, the idea of the CRC/TR 38 was to analyze the establishment of new structures and processes which lead to a growing structuring and in consequence to a growing complexity and heterogeneity in an artificially created watershed. Further, with the help of this step-by-step development of the ecosystem it was aimed to learn from occurring feedbacks, which appear between old and newly emerging structures and patterns in order to better understand also the behavior of more mature systems. Special emphasis was placed on the spatial and temporal dynamics of both evolving structures and related processes and their interactions. In summary, the CRC/TR 38 was able to identify a number of structures and processes that are considered to be relevant and specific for young systems.Ziel des Sonderforschungsbereichs/Transregio (SFB/TRR) 38 war die Untersuchung von Strukturen und Prozessen der initialen Ökosystemgenese. Es wurde angenommen, dass die initiale Entwicklungsphase durch geringere Strukturierung und damit durch eine geringere Heterogenität der Ökosysteme gekennzeichnet ist. Entsprechend sollte die Untersuchung von Ökosystemen in ihrer initialen Entwicklungsphase verbesserte Erkenntnisse zur Funktion von Ökosystemen bieten. Diesem Konzept folgend untersuchte der SFB/TRR 38 in einem künstlichen Wassereinzugsgebiet die Entwicklung von neuen Strukturen und Prozessen, die zu einer zunehmenden Strukturierung und damit zu einer zunehmenden Heterogenität führte. Weiterhin war beabsichtigt, mit Hilfe der schrittweisen Entwicklung des Ökosystems auftretende Rückkopplungsprozesse zwischen alten und sich neu etablierenden Strukturen und Mustern zu erkennen und damit auch das Verständnis der Funktionsweise gereifter Ökosysteme zu verbessern. Besonderes Augenmerk wurde auf die zeitliche und räumliche Dynamik sowohl der sich entwickelnden Strukturen als auch der damit verbundenen Prozesse und ihrer Interaktionen gelegt. Zusammenfassend kann gesagt werden, dass der SFB/TRR 38 in der Lage war, zahlreiche Strukturen und Prozesse zu identifizieren, die als relevant und spezifisch für junge Ökosysteme betrachtet werden können

Root Exudates Induce Soil Macroaggregation Facilitated by Fungi in Subsoil

Subsoils are known to harbor large amounts of soil organic carbon (SOC) and may represent key global carbon (C) sinks given appropriate management. Although rhizodeposition is a major input pathway of organic matter to subsoils, little knowledge exists on C dynamics, particularly stabilization mechanisms, such as soil aggregation, in the rhizosphere of different soil depths. The aim of this study was to investigate the influence of natural and elevated root exudation on C allocation and aggregation in the topsoil and subsoil of a mature European beech (Fagus sylvatica L.) forest. We experimentally added model root exudates to soil at two different concentrations using artificial roots and analyzed how these affect SOC, nitrogen, microbial community composition, and size distribution of water-stable aggregates. Based on the experimental data, a mathematical model was developed to describe the spatial distribution of the formation of soil aggregates and their binding strength. Our results demonstrate that greater exudate additions affect the microbial community composition in favor of fungi which promote the formation of macroaggregates. This effect was most pronounced in the C-poor subsoil, where macroaggregation increased by 86% and SOC content by 10%. Our modeling exercise reproduced the observed increase in subsoil SOC at high exudate additions. We conclude that elevated root exudation has the potential to increase biotic macroaggregation and thus the C sink strength in the rhizosphere of forest subsoils

Few recurring types of microdomains define smallest units of soilfunctioning

Soil aggregation is a key factor for a number of important biogeochemical processes (e.g. soil organic matter stabilization and nutrient and pollutant sorption) in soils. Although there is a large number of studies on the factors controlling such soil processes, it is still challenging to study these processes in-situ. However, it can be assumedthat the spatial arrangement of organic and mineral soil constituents in soil aggregates, and thus the aggregate structure determine the processes happening at the aggregate scale. Using nanoscale secondary ion mass spectroscopy and a novel digital image processing approach, we extensively analyzed the spatial distribution of ions characteristic for mineral and organic soil components on the micrometer-scale in an intact soil aggregate. We were surprised that 40 spatially independent measurements could be statistically clustered in just two complimentary types of micrometer-sized domains. Each domain is characterized by a micro-architecture built of a definitemineral assemblage with various organic matter forms and a specific pore system. Each of these microdomainsfulfil different functions in soil. Our results demonstrate that the manifold mineral and organic soil components arrange in a limited number of micro-architectures because of self-organization and feedback mechanisms. Thesemicrodomains are the smallest units in soil that fulfill specific functionalities

Complementary effects of sorption and biochemical processing of dissolved organic matter for emerging structure formation controlled by soil texture

Background: Percolating dissolved organic matter (DOM) from the topsoil is considered the main source of subsoil organic carbon (OC) in temperate soils, but knowledge about its influence on OC storage and structure-forming processes is limited. Aims: We conducted a 30-day incubation experiment with artificial soils to study the effects of percolating DOM and soil texture on OC turnover and initial structure formation. Methods: Artificial soils with contrasting texture, but identical mineral composition, were used to mimic subsoil conditions, where mineral surfaces free of OM come into contact with percolating DOM. After the incubation, we assessed the solution exchange, OM covers on minerals, microbial community and OC turnover, and aggregate formation and stability. Results: A higher sand content caused a lower porosity, accompanied by a lower moisture content. In contrast, the OC retention (21% of the OC input), microbial activity, and community size were unaffected by soil texture. The OM covered 10% of the mineral surfaces within an otherwise OC-free mineral matrix. The formation of large, water-stable aggregates occurred in all soils, but was pronounced in the clay-rich soils (58% mass contribution), which also supported a higher mechanical stability of the aggregates. Conclusions: The initial retention and microbial mineralization of DOM are decoupled from pore sizes and soil solution exchange but are driven by the mineral composition and OC input. The biochemical processing of the percolating DOM can induce large aggregates. Here, the presence of fine mineral particles enhances the formation and mechanical stability of the aggregates, irrespective of their surface charge or sorptive properties