Showing papers by "Karsten Kalbitz published in 2016"

Gone or just out of sight? The apparent disappearance of aromatic litter components in soils

Abstract: . Uncertainties concerning stabilization of organic compounds in soil limit our basic understanding on soil organic matter (SOM) formation and our ability to model and manage effects of global change on SOM stocks. One controversially debated aspect is the contribution of aromatic litter components, such as lignin and tannins, to stable SOM forms. In the present opinion paper, we summarize and discuss the inconsistencies and propose research options to clear them. Lignin degradation takes place stepwise, starting with (i) depolymerization and followed by (ii) transformation of the water-soluble depolymerization products. The long-term fate of the depolymerization products and other soluble aromatics, e.g., tannins, in the mineral soils is still a mystery. Research on dissolved organic matter (DOM) composition and fluxes indicates dissolved aromatics are important precursors of stable SOM attached to mineral surfaces and persist in soils for centuries to millennia. Evidence comes from flux analyses in soil profiles, biodegradation assays, and sorption experiments. In contrast, studies on composition of mineral-associated SOM indicate the prevalence of non-aromatic microbial-derived compounds. Other studies suggest the turnover of lignin in soil can be faster than the turnover of bulk SOM. Mechanisms that can explain the apparent fast disappearance of lignin in mineral soils are, however, not yet identified. The contradictions might be explained by analytical problems. Commonly used methods probably detect only a fraction of the aromatics stored in the mineral soil. Careful data interpretation, critical assessment of analytical limitations, and combined studies on DOM and solid-phase SOM could thus be ways to unveil the issues.

Small scale variability of vertical water and dissolved organic matter fluxes in sandy Cambisol subsoils as revealed by segmented suction plates

Abstract: Dissolved organic matter (DOM) is considered as a major carbon source in subsoils. As soil water fluxes are highly variable at small scale, and transport versus sorptive retention of DOM is related to water flux and associated contact time with minerals, knowledge of the small scale spatial variability of the dissolved organic carbon (DOC) concentrations and fluxes into the subsoil is decisive for a solid estimation of organic carbon (OC) translocation into the subsoil. Here, we made advantage of novel segmented suction plates (4 × 4 segments, each 36 cm2) to analyze the small scale spatial and temporal variability of DOC transport at 10, 50 and 150 cm depth of three subsoil observatories (approximately 50 m apart) in a sandy Dystric Cambisol under beech in the Grinderwald, 40 km northwest from Hannover, Germany. Water fluxes, DOC concentrations and fluxes as well as the specific UV absorbance (SUVA) at 280 nm were determined in weekly samples from August 2014 to November 2015 for each individual segment. The DOC fluxes decreased with depth (19.6 g C m−2 year−1, 10 cm; 1.2 g C m−2 year−1, 150 cm) and were strongly related to the water fluxes. The SUVA at 280 nm also decreased with depth (0.03 L mg C−1 cm−1, 10 cm; 0.01 L mg C−1 cm−1, 150 cm), indicating a selective retention of aromatic moieties, that was eased with increasing water flux at least in the subsoil. The proportion of temporal fluctuations and small scale variability on the total variance of each parameter where determined by the calculation of intra class correlations. The seasonal heterogeneity and the small scale spatial heterogeneity were identified to be of major importance. The importance of the small scale spatial heterogeneity strongly increased with depth, pointing towards the stability of flow paths and suggesting that at a given substrate hydrological processes rather than physicochemical processes are decisive for the sorptive retention of DOM and the variability of OC accumulation in the subsoil. Our results clearly show the demand of small scale sampling for the identification of processes regarding carbon cycling in the subsoil.

A novel tool for stable nitrogen isotope analysis in aqueous samples.

Abstract: Rationale Bulk stable isotope analysis (BSIA) of dissolved matter (e.g. dissolved organic carbon, total nitrogen bound (TNb), etc.) is of particular importance since this pool is a prime conduit in the cycling of N and C. Studying the two elemental pools is of importance, as transformation and transport processes of N and C are inextricably linked in all biologically mediated systems. No system able to analyze natural abundance stable carbon and nitrogen isotope composition in aqueous samples (without offline sample preparation) and simultaneously has been reported so far. Extension of the high-temperature combustion (HTC) system, to be capable of measuring TNb stable nitrogen isotope composition, is described in this study. Methods To extend the TOC analyzer to be capable of measuring TNb, modifications from the HTC high-performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS) interface were implemented and expanded. A reduction reactor for conversion of NOx into N2 was implemented into the new developed system. The extension addresses mainly the development of the focusing unit for nitrogen and a degassing device for online separation of TNb from molecular nitrogen (N2) prior to injection. Results The proof of principle of the system was demonstrated with different compound solutions. In this initial testing, the δ15NAIR-N2 values of the tested compounds were determined with precision and trueness of typically ≤0.5‰. Good results (u ≤ 0.5‰) could be achieved down to a TNb concentration of 40 mgN/L and acceptable results (u ≤ 1.0‰) down to 5 mgN/L. In addition, the development resulted in the first system reported to be suitable for simultaneous and direct δ13C and δ15N BSIA of aqueous samples. Conclusions The development resulted in the first system shown to be suitable for both δ13C and δ15N direct BSIA in aqueous samples. This system could open up new possibilities in SIA-based research fields. Copyright © 2016 John Wiley & Sons, Ltd.