Laura Summerauer

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Summerauer

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Laura

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0000-0002-3836-653X

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Publications 1 - 10 of 17

Soil geochemistry as a driver of soil organic matter composition: Insights from a soil chronosequence
Mainka, Moritz; Summerauer, Laura; Wasner, Daniel; et al. (2022)
Biogeosciences
A central question in carbon research is how stabilization mechanisms in soil change over time with soil development and how this is reflected in qualitative changes in soil organic matter (SOM). To address this matter, we assessed the influence of soil geochemistry on bulk SOM composition along a soil chronosequence in California, USA, spanning 3 million years. This was done by combining data on soil mineralogy and texture from previous studies with additional measurements on total carbon (C), stable isotope values (δ13C and δ15N), and spectral information derived from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). To assess qualitative shifts in bulk SOM, we analysed the peak areas of simple plant-derived (S-POM), complex plant-derived (C-POM), and predominantly microbial-derived organic matter (OM; MOM) and their changes in abundance across soils with several millennia to millions of years of weathering and soil development. We observed that SOM became increasingly stabilized and microbial-derived (lower Cg:gN ratio, increasing δ13C and δ15N) as soil weathering progressed. Peak areas of S-POM (i.e. aliphatic root exudates) did not change over time, while peak areas of C-POM (lignin) and MOM (components of microbial cell walls (amides, quinones, and ketones)) increased over time and depth and were closely related to clay content and pedogenic iron oxides. Hence, our study suggests that with progressing soil development, SOM composition co-varied with changes in the mineral matrix. Our study indicates that structurally more complex OM compounds (C-POM, MOM) play an increasingly important role in soil carbon stabilization mechanisms as the mineral soil matrix becomes increasingly weathered.
The central African soil spectral library: a new soil infrared repository and a geographical prediction analysis
Summerauer, Laura; Baumann, Philipp; Barthel, Matti; et al. (2021)
Soil
Information on soil properties is crucial for soil preservation, the improvement of food security, and the provision of ecosystem services. In particular, for the African continent, spatially explicit information on soils and their ability to sustain these services is still scarce. To address data gaps, infrared spectroscopy has achieved great success as a cost-effective solution to quantify soil properties in recent decades. Here, we present a mid-infrared soil spectral library (SSL) for central Africa (CSSL) that can predict key soil properties, allowing for future soil estimates with a minimal need for expensive and time-consuming wet chemistry. Currently, our CSSL contains over 1800 soil samples from 10 distinct geoclimatic regions throughout the Congo Basin and along the Albertine Rift. For the analysis, we selected six regions from the CSSL, for which we built predictive models for total carbon (TC) and total nitrogen (TN) using an existing continental SSL (African Soil Information Service, AfSIS SSL; n=1902) that does not include central African soils. Using memory-based learning (MBL), we explored three different strategies at decreasing degrees of geographic extrapolation, using models built with (1) the AfSIS SSL only, (2) AfSIS SSL combined with the five remaining central African regions, and (3) a combination of AfSIS SSL, the remaining five regions, and selected samples from the target region (spiking). For this last strategy we introduce a method for spiking MBL models. We found that when using the AfSIS SSL only to predict the six central African regions, the root mean square error of the predictions (RMSEpred) was between 3.85–8.74 and 0.40–1.66 g kg−1 for TC and TN, respectively. The ratio of performance to the interquartile distance (RPIQpred) ranged between 0.96–3.95 for TC and 0.59–2.86 for TN. While the effect of the second strategy compared to the first strategy was mixed, the third strategy, spiking with samples from the target regions, could clearly reduce the RMSEpred to 3.19–7.32 g kg−1 for TC and 0.24–0.89 g kg−1 for TN. RPIQpred values were increased to ranges of 1.43–5.48 and 1.62–4.45 for TC and TN, respectively. In general, predicted TC and TN for soils of each of the six regions were accurate; the effect of spiking and avoiding geographical extrapolation was noticeably large. We conclude that our CSSL adds valuable soil diversity that can improve predictions for the Congo Basin region compared to using the continental AfSIS SSL alone; thus, analyses of other soils in central Africa will be able to profit from a more diverse spectral feature space. Given these promising results, the library comprises an important tool to facilitate economical soil analyses and predict soil properties in an understudied yet critical region of Africa. Our SSL is openly available for application and for enlargement with more spectral and reference data to further improve soil diagnostic accuracy and cost-effectiveness.
Seasonality, drivers, and isotopic composition of soil CO2 fluxes from tropical forests of the Congo Basin
Baumgartner, Simon; Barthel, Matti; Drake, Travis William; et al. (2020)
Biogeosciences
Soil respiration is an important carbon flux and key process determining the net ecosystem production of terrestrial ecosystems. To address the lack of quantification and understanding of seasonality in soil respiration of tropical forests in the Congo Basin, soil CO2 fluxes and potential controlling factors were measured annually in two dominant forest types (lowland and montane) of the Congo Basin over 2 years at varying temporal resolution. Soil CO2 fluxes from the Congo Basin resulted in 3.45 ± 1.14 and 3.13 ± 1.22 µmol CO2 m−2 s−1 for lowland and montane forests, respectively. Soil CO2 fluxes in montane forest soils showed a clear seasonality with decreasing flux rates during the dry season. Montane forest soil CO2 fluxes were positively correlated with soil moisture, while CO2 fluxes in the lowland forest were not. Smaller differences of δ13C values of leaf litter, soil organic carbon (SOC), and soil CO2 indicated that SOC in lowland forests is more decomposed than in montane forests, suggesting that respiration is controlled by C availability rather than environmental factors. In general, C in montane forests was more enriched in 13C throughout the whole cascade of carbon intake via photosynthesis, litterfall, SOC, and soil CO2 compared to lowland forests, pointing to a more open system. Even though soil CO2 fluxes are similarly high in lowland and montane forests of the Congo Basin, the drivers of them seem to be different, i.e., soil moisture for montane forest and C availability for lowland forest.
Parent material modulated effects of soil degradation on fertility and organic carbon of tropical cropland soils in Eastern Africa
Summerauer, Laura; Bamba, Fernando; Akoraebirungi, Bendicto; et al. (2024)
EGUsphere
Deforestation for cropland expansion in the sloping landscapes along the East African Rift system causes severe soil erosion and thus the loss of fertile, organic rich topsoil. However, the varying effect of land degradation in the region on soils developed from different parent material - which may influence soil fertility and carbon stabilization - are still largely unknown. To examine these factors, we compared soil organic carbon (SOC) and soil fertility indicators in undisturbed forest topsoils with cropland hillslope topsoils along a chronosequence after deforestation (2–7, 10–20, 20–40, > 60 years of cropping, land abandonment) on mafic (South Kivu, Democratic Republic of Congo) and felsic parent material (western Uganda). From previous studies, we expected higher soil fertility and SOC contents and therefore slower degradation on mafic soils due to the higher amounts of clay and pedogenic metal phases which stabilize SOM and thus further maintain soil fertility. However, we found similar SOC contents on both parent materials and a consistent decrease with time after deforestation. SOC values were significantly lower in soils that were cleared more than 60 years ago, compared to cropland which was cleared 2–7 years ago and nearby undisturbed forest topsoils (0–10 cm soil depth). While the effective cation exchange capacity (ECEC) positively correlated with SOC in soils on felsic parent material, this was not observed in soils with mafic parent material, where it correlated with mineralogical proxies (total reserves in bases). In both regions, SOC did not correlate with clay content. Mid-Holocene carbonate volcanism appears to have offset soil degradation in the felsic region, contributing to higher pH and ECEC and impeding land abandonment due to the maintenance of acceptable soil fertility levels. Surprisingly, abandoned cropland sites in the mafic region still had an average SOC content of 14–29 g kg-1 in topsoils, likely due to strong fixation of SOC with reactive metal phases; however, they were characterized by extremely low pH values and high Al3+ mobility, combined with low available nutrient status. Our results emphasize that soil fertility and carbon stabilization are reliant on the mineral composition of the underlying parent material, even in deeply weathered soils of the humid tropics. Soil organic matter in degraded tropical cropland soils does not appear to be a reliable indicator of soil fertility.
Rapid soil degradation following deforestation in Eastern Africa
Summerauer, Laura; Bamba, Fernando; Akoraebirungi, Bendicto; et al. (2025)
EGUsphere
Deforestation for cropland expansion in tropical sloping landscapes causes severe soil erosion and thus the loss of fertile, organic rich topsoil. The effects of land degradation on tropical soils developed from different parent materials, which may influence soil fertility and soil organic carbon (SOC) content, are still largely unknown. Here, we compared SOC and other soil fertility indicators in undisturbed tropical forest topsoils with cleared hillslope topsoils (cropland, abandoned, cropland, and reforestation with Eucalyptus monocultures) along the East African rift system using soil chronosequences after deforestation on both mafic and felsic parent material. In the mafic region, we found a consistent decrease of SOC, nitrogen, and phosphorus content with time after deforestation (relative changes of contents up to −69 % SOC, −72 % nitrogen, and −92 % phosphorus). SOC was strongly stabilized by reactive metal phases with little to no benefits to general soil fertility. Consequently, cropland was frequently abandoned by farmers due to the combination of low pH, high Al3+ mobility, and low available nutrient status at a relatively high average SOC content of 14–29 g kg−1 in topsoils. In the felsic region, mid-Holocene carbonate volcanism mitigated soil degradation to some extent. In both geochemical regions, SOC content did not or only weakly positively correlate with clay content and cation exchange capacity. These results emphasize that soil organic matter, as well as clay content, appear to be unreliable indicators for soil fertility in degraded tropical cropland soils. Additionally, no significant improvement of soil fertility or SOC stocks was observed after replanting degraded fields with Eucalyptus monocultures. The estimated lifespan of croplands on hillslopes in our study area, approximately 100–170 years, underscores the severity of soil degradation for food production and forest protection in the upcoming decades, especially considering that many soils are already approaching this age.
Organic matter cycling along geochemical, geomorphic, and disturbance gradients in forest and cropland of the African Tropics - project TropSOC database version 1.0
Doetterl, Sebastian; Asifiwe, Rodrigue K.; Baert, Geert; et al. (2021)
Earth System Science Data
The African Tropics are hotspots of modern-day land use change and are, at the same time, of great relevance for the cycling of carbon (C) and nutrients between plants, soils, and the atmosphere. However, the consequences of land conversion on biogeochemical cycles are still largely unknown as they are not studied in a landscape context that defines the geomorphic, geochemical, and pedological framework in which biological processes take place. Thus, the response of tropical soils to disturbance by erosion and land conversion is one of the great uncertainties in assessing the carrying capacity of tropical landscapes to grow food for future generations and in predicting greenhouse gas fluxes from soils to the atmosphere and, hence, future earth system dynamics. Here we describe version 1.0 of an open-access database created as part of the project “Tropical soil organic carbon dynamics along erosional disturbance gradients in relation to variability in soil geochemistry and land use” (TropSOC). TropSOC v1.0 (Doetterl et al., 2021, https://doi.org/10.5880/fidgeo.2021.009) contains spatially and temporally explicit data on soil, vegetation, environmental properties, and land management collected from 136 pristine tropical forest and cropland plots between 2017 and 2020 as part of monitoring and sampling campaigns in the eastern Congo Basin and the East African Rift Valley system. The results of several laboratory experiments focusing on soil microbial activity, C cycling, and C stabilization in soils complement the dataset to deliver one of the first landscape-scale datasets to study the linkages and feedbacks between geology, geomorphology, and pedogenesis as controls on biogeochemical cycles in a variety of natural and managed systems in the African Tropics. The hierarchical and interdisciplinary structure of the TropSOC database allows linking of a wide range of parameters and observations on soil and vegetation dynamics along with other supporting information that may also be measured at one or more levels of the hierarchy. TropSOC's data mark a significant contribution to improve our understanding of the fate of biogeochemical cycles in dynamic and diverse tropical African (agro-)ecosystems. TropSOC v1.0 can be accessed through the Supplement provided as part of this paper or as a separate download via the websites of the Congo Biogeochemistry Observatory and GFZ Data Services where version updates to the database will be provided as the project develops.
Filling a key gap: a soil infrared library for central Africa
Summerauer, Laura; Baumann, Philipp; Ramirez-Lopez, Leonardo; et al. (2021)
Soil Discussions
Information on soil properties is crucial for soil preservation, improving food security, and the provision of ecosystem services. Especially, for the African continent, spatially explicit information on soils and their ability to sustain these services is still scarce. To address data gaps, infrared spectroscopy has gained great success as a cost-effective solution to quantify soil properties in recent decades. Here, we present a mid-infrared soil spectral library (SSL) for central Africa (CSSL) that can predict key soil properties allowing for future soil estimates with a minimal need for expensive and time-consuming wet chemistry. Currently, our CSSL contains over 1,800 soils from ten distinct geo-climatic regions throughout the Congo Basin and wider African Great Lakes region. We selected six hold-out core regions from our SSL, augmented them with the continental AfSIS SSL, which does not cover central African soils. We present three levels of geographical extrapolation, deploying Memory-based learning (MBL) to accurately predict carbon (TC) and nitrogen (TN) contents in the selected regions. The Root Mean Square Error of the predictions (RMSEpred) values were between 0.38–0.86 % and 0.04–0.17 % for TC and TN, respectively, when using the AfSIS SSL only to predict the six regions. Prediction accuracy could be improved for four out of six regions when adding central African soils to the AfSIS SSL. This reduction of extrapolation resulted in RMSEpred ranges of 0.41–0.89 % for TC and 0.03–0.12 % for TN. In general, MBL leveraged spectral similarity and thereby predicted the soils in each of the six regions accurately; the effect of avoiding geographical extrapolation and forcing regional samples in the local neighborhood (MBL-spiking) was small. We conclude that our CSSL adds valuable soil diversity that can improve predictions for the regions compared to using the continental scale AfSIS SSL alone; thus, analyses of other soils in central Africa will be able to profit from a more diverse spectral feature space. Given these promising results, the library comprises an important tool to facilitate economical soil analyses and predict soil properties in an understudied yet critical region of Africa. Our SSL is openly available for application and for enlargement with more spectral and reference data to further improve soil diagnostic accuracy and cost-effectiveness.
Low N2O and variable CH4 fluxes from tropical forest soils of the Congo Basin
Barthel, Matti; Bauters, Marijn; Baumgartner, Simon; et al. (2022)
Nature Communications
Globally, tropical forests are assumed to be an important source of atmospheric nitrous oxide (N2O) and sink for methane (CH4). Yet, although the Congo Basin comprises the second largest tropical forest and is considered the most pristine large basin left on Earth, in situ N2O and CH4 flux measurements are scarce. Here, we provide multi-year data derived from on-ground soil flux (n = 1558) and riverine dissolved gas concentration (n = 332) measurements spanning montane, swamp, and lowland forests. Each forest type core monitoring site was sampled at least for one hydrological year between 2016 - 2020 at a frequency of 7-14 days. We estimate a terrestrial CH4 uptake (in kg CH4-C ha−1 yr−1) for montane (−4.28) and lowland forests (−3.52) and a massive CH4 release from swamp forests (non-inundated 2.68; inundated 341). All investigated forest types were a N2O source (except for inundated swamp forest) with 0.93, 1.56, 3.5, and −0.19 kg N2O-N ha−1 yr−1 for montane, lowland, non-inundated swamp, and inundated swamp forests, respectively.
Soil geochemistry as a driver of soil organic matter composition: insights from a soil chronosequence
Mainka, Moritz; Summerauer, Laura; Wasner, Daniel; et al. (2021)
Biogeosciences Discussions
A central question in carbon research is how stabilization mechanisms in soil change over time with soil development and how this is reflected in qualitative changes in soil organic matter (SOM). To address this matter, we assessed the influence of soil geochemistry on bulk SOM composition along a soil chronosequence in California, USA, spanning 3 million years. This was done by combining data on soil mineralogy and texture from previous studies with additional measurements on total carbon (C), stable isotope values (δ13C and δ15N), and spectral information derived from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). To assess qualitative shifts in bulk SOM, we analysed the peak areas of simple plant-derived (S-POM), complex plant-derived (C-POM), and predominantly microbial-derived organic matter (OM; MOM) and their changes in abundance across soils with several millennia to millions of years of weathering and soil development. We observed that SOM became increasingly stabilized and microbial-derived (lower C : N ratio, increasing δ13C and δ15N) as soil weathering progressed. Peak areas of S-POM (i.e. aliphatic root exudates) did not change over time, while peak areas of C-POM (lignin) and MOM (components of microbial cell walls (amides, quinones, and ketones)) increased over time and depth and were closely related to clay content and pedogenic iron oxides. Hence, our study suggests that with progressing soil development, SOM composition co-varied with changes in the mineral matrix. Our study indicates that structurally more complex OM compounds (C-POM, MOM) play an increasingly important role in soil carbon stabilization mechanisms as the mineral soil matrix becomes increasingly weathered.
An Interlaboratory Comparison of Mid-Infrared Spectra Acquisition: Instruments and Procedures Matter (preprint)
Safanelli, José L.; Sanderman, Jonathan; Bloom, Dellena; et al. (2023)
SSRN
Diffuse reflectance spectroscopy has been extensively employed to deliver timely and cost-effective predictions of a number of soil properties. However, although several soil spectral laboratories have been established worldwide, the distinct characteristics of instruments and operations still hamper further integration and interoperability across mid-infrared (MIR) soil spectral libraries. In this study, we conducted a large-scale ring trial experiment to understand the lab-to-lab variability of multiple MIR instruments. By developing a systematic evaluation of different mathematical treatments with modeling algorithms, including regular preprocessing and spectral standardization, we quantified and evaluated instruments' dissimilarity and how this impacts internal and shared model performance. We found that all instruments delivered good predictions when calibrated internally using the same instruments' characteristics and standard operating procedures by solely relying on regular spectral preprocessing that accounts for light scattering and multiplicative/additive effects, e.g., using standard normal variate (SNV). When performing model transfer from a large public library (the USDA NSSC-KSSL MIR library) to secondary instruments, good performance was also achieved by regular preprocessing (e.g., SNV) if both instruments shared the same manufacturer. However, significant differences between the KSSL MIR library and contrasting ring trial instruments responses were evident and confirmed by a semi-unsupervised spectral clustering. For heavily contrasting setups, spectral standardization was necessary before transferring prediction models. Non-linear model types like Cubist and memory-based learning delivered more precise estimates because they seemed to be less sensitive to spectral variations than global partial least square regression. In summary, the results from this study can assist new laboratories in building spectroscopy capacity utilizing existing MIR spectral libraries and support the recent global efforts to make soil spectroscopy universally accessible with centralized or shared operating procedures.

Publications 1 - 10 of 17

Laura Summerauer

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