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The biogeochemical functioning of soils (e.g., soil carbon stabilization and nutrient cycling) is determined at the interfaces of specific soil structures (e.g., aggregates, particulate organic matter (POM) and organo‐mineral associations). With the growing accessibility of spectromicroscopic techniques, there is an increase in nano‐ to microscale analyses of biogeochemical interfaces at the process scale, reaching from the distribution of elements and isotopes to the localization of microorganisms. A widely used approach to study intact soil structures is the fixation and embedding of intact soil samples in resin and the subsequent analyses of soil cross‐sections using spectromicroscopic techniques. However, it is still challenging to link such microscale approaches to larger scales at which normally bulk soil analyses are conducted. Here we report on the use of laboratory imaging Vis–NIR spectroscopy on resin embedded soil sections and a procedure for supervised image classification to determine the microscale soil structure arrangement, including the quantification of soil organic matter fractions. This approach will help to upscale from microscale spectromicroscopic techniques to the centimetre and possibly pedon scale. Thus, we demonstrate a new approach to integrate microscale soil analyses into pedon‐scale conceptual and experimental approaches. Highlights: Quantification of soil constituents using Vis‐NIR spectroscopy.New approach to use resin embedded soil core sections with intact structure.Reproducible quantification of soil constituents important for soil carbon storage.Vis‐NIR as promising tool for upscaling from microscale to pdeon scale. [ABSTRACT FROM AUTHOR]

Permafrost-affected soils of the Arctic account for 70 % or 727 Pg of the soil organic carbon (C) stored in the northern circumpolar permafrost region and therefore play a major role in the global C cycle. Most studies on the budgeting of C storage and the quality of soil organic matter (OM; SOM) in the northern circumpolar region focus on bulk soils. Thus, although there is a plethora of assumptions regarding differences in terms of C turnover or stability, little knowledge is available on the mechanisms stabilizing organic C in Arctic soils besides impaired decomposition due to low temperatures. To gain such knowledge, we investigated soils from Samoylov Island in the Lena River delta with respect to the composition and distribution of organic C among differently stabilized SOM fractions. The soils were fractionated according to density and particle size to obtain differently stabilized SOM fractions differing in chemical composition and thus bioavailability. To better understand the chemical alterations from plant-derived organic particles in these soils rich in fibrous plant residues to mineral-associated SOM, we analyzed the elemental, isotopic and chemical composition of particulate OM (POM) and clay-sized mineral-associated OM (MAOM). We demonstrate that the SOM fractions that contribute with about 17 kgCm-3 for more than 60 % of the C stock are highly bioavailable and that most of this labile C can be assumed to be prone to mineralization under warming conditions. Thus, the amount of relatively stable, small occluded POM and clay-sized MAOM that currently accounts with about 10 kgCm-3 for about 40 % of the C stock will most probably be crucial for the quantity of C protected from mineralization in these Arctic soils in a warmer future. Using δ15 N as a proxy for nitrogen (N) balances indicated an important role of N inputs by biological N fixation, while gaseous N losses appeared less important. However, this could change, as with about 0.4 kgNm-3 one third of the N is present in bioavailable SOM fractions, which could lead to increases in mineral N cycling and associated N losses under global warming. Our results highlight the vulnerability of SOM in Arctic permafrost-affected soils under rising temperatures, potentially leading to unparalleled greenhouse gas emissions from these soils. [ABSTRACT FROM AUTHOR]

Dark, that is, nonphototrophic, microbial CO2 fixation occurs in a large range of soils. However, it is still not known whether dark microbial CO2 fixation substantially contributes to the C balance of soils and what factors control this process. Therefore, the objective of this study was to quantitate dark microbial CO2 fixation in temperate forest soils, to determine the relationship between the soil CO2 concentration and dark microbial CO2 fixation, and to estimate the relative contribution of different microbial groups to dark CO2 fixation. For this purpose, we conducted a 13C‐CO2 labeling experiment. We found that the rates of dark microbial CO2 fixation were positively correlated with the CO2 concentration in all soils. Dark microbial CO2 fixation amounted to up to 320 µg C kg−1 soil day−1 in the Ah horizon. The fixation rates were 2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of dark microbial fixation were small compared to the respiration rate (1.2%–3.9% of the respiration rate), our findings suggest that organic matter formed by microorganisms from CO2 contributes to the soil organic matter pool, especially given that microbial detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses indicated that CO2 was mostly fixed by gram‐positive bacteria, and not by fungi. In conclusion, our study shows that the dark microbial CO2 fixation rate in temperate forest soils increases in periods of high CO2 concentrations, that dark microbial CO2 fixation is mostly accomplished by gram‐positive bacteria, and that dark microbial CO2 fixation contributes to the formation of soil organic matter. [ABSTRACT FROM AUTHOR]

The high aggregate stability of Andosols and the direct effects of sample drying led to several inconsistencies during physical soil organic matter fractionation. We have determined that NaCl addition displayed little influence on clay dispersion. At the microscale, we observed the re‐aggregation of the clay fraction caused by freeze‐drying. This issue was avoided by analyzing aliquots of soil suspension. Thus, we recommend reserving a small soil liquid aliquot to be subjected to microscopy analysis. [ABSTRACT FROM AUTHOR]

We examined the potential of stable-isotope Raman microspectroscopy (SIRM) for the evaluation of differently enriched C-labeled humic acids as model substances for soil organic matter (SOM). The SOM itself can be linked to the soil water holding capacity. Therefore, artificial humic acids (HA) with known isotopic compositions were synthesized and analyzed by means of SIRM. By performing a pregraphitization, a suitable analysis method was developed to cope with the high fluorescence background. Results were verified against isotope ratio mass spectrometry (IRMS). The limit of quantification was 2.1 × 10C/ C for the total region and 3.2 × 10C/ C for a linear correlation up to 0.25 C/ C . Complementary nanoscale secondary ion mass spectrometry (NanoSIMS) analysis indicated small-scale heterogeneity within the dry sample material, even though-owing to sample topography and occurring matrix effects-obtained values deviated in magnitude from those of IRMS and SIRM. Our study shows that SIRM is well-suited for the analysis of stable isotope-labeled HA. This method requires no specific sample preparation and can provide information with a spatial resolution in the micrometer range. [ABSTRACT FROM AUTHOR]

Differentiation of biochar and native soil organic matter (SOM) is required to assess the effect of biochar amendment on in-situ changes of SOM. Therefore, we used C 4 biochar produced at high temperature (1200 °C) by gasification (BC GS ) and measured the 13 C abundance of density and particle size fractions. We quantified the BC GS effects on distinct native C 3 -SOM pools of a grassland topsoil one year after BC GS amendment. The chemical composition was analyzed with solid-state 13 C CPMAS NMR, whereas information on the nanostructure of BC GS were obtained by Raman microspectroscopy measurements. Our aim was to assess BC GS induced chemical changes of SOM and physical fractions and to validate the accuracy of BC GS detection by 13 C NMR spectroscopy. Quantification by isotopic measurements and 13 C NMR spectroscopy for aromatic C yielded similar estimates of BC in soils. Of the total BC GS , 52% were recovered as free particulate organic matter (POM) and 33% were located in aggregated soil structures isolated as occluded POM particles. Around 4% of the total BC GS was detected in the clay fraction. After one year of field exposure, the surface of the BC GS particles decreased in unordered graphitic-like structures. The higher ordered BC residue is supposed to be more recalcitrant. The native SOC stock increase ( p = 0.06, n = 4) in the clay fraction indicated increased sequestration of organic matter as mineral-bound SOM due to BC GS amendment. With respect to soil functionality, the BC GS amendment induced a tremendous shift from a soil system dominated by organo-mineral associations to POM−dominated OC storage, resulting in increased soil air capacity. [ABSTRACT FROM AUTHOR]

Organic carbon (OC) stored in permafrost affected soils of the higher northern latitudes is known to be highly vulnerable to ongoing climatic change. Although the ways to quantify soil OC and to study connected C dynamics from ecosystem to global scale in the Arctic has improved substantially over the last years, the basic mechanisms of OC sequestration are still not well understood. Here we demonstrate a first approach to directly study micro scale soil structures mainly responsible for soil OC (SOC) stabilization using nano scale secondary ion mass spectrometry (NanoSIMS). A cross section from a permafrost layer of a Cryosol from Northern Alaska was analysed using a cascade of imaging techniques from reflectance light microscopy (RLM) to scanning electron microscopy (SEM) to NanoSIMS. This allowed for the direct evaluation of micro scale soil structures known to be hot spots for microbial activity and SOC stabilization in temperate soils. The imaging techniques were supported by classical soil analyses. Using this unique set of techniques we are able to evidence the formation of micro-aggregate structures in the vicinity of plant residues in permafrost soils. This clearly indicates biogeochemical interfaces at plant surfaces as important spheres for the formation of more complex soil structures in permafrost soils. Organo-mineral associations from these hot spots of microbial activity were recovered from plant residues (free particulate organic matter, fPOM) as fine grained mineral fraction with a typically low C/N ratio. This nicely illustrates the link between classical bulk analysis and state of the art spectromicroscopic techniques. [ABSTRACT FROM AUTHOR]

Earthworms are important drivers for the formation of soil structure and play a key role in soil organic matter (SOM) dynamics. Our previous long-term (126 weeks) laboratory experiment showed that carbon (C) loss declined through time in soil when litter was mixed and consumed by earthworms ( Lumbricus rubellus ). Eventually, the C loss was lower than in treatments where litter was mechanically mixed into soil with exclusion of earthworms. However, it is not clear if the solely physical manipulation of soil or biological activity of earthworms lead to different SOM quality, which would result in a distinction in C loss and consequently C sequestration. Thus, we differentiated between physical (mechanical mixing) and earthworm effects on SOM composition. Two types of soil were used in the experiment: clay and sand, and these were incubated with alder ( Alnus glutinosa ) and willow ( Salix caprea ) litter, respectively. The combination of soils and litter types corresponds to the natural combinations at the sampling sites. To explain underlying mechanisms of a lower C loss in the earthworm vs. mechanically mixed treatment, we separated SOM fractions in order to gain pools defined in the Rothamsted model. Chemical differences between initial litter and the active and slow pool of SOM obtained by fractionation were studied. No significant differences between the earthworm and mechanically mixed treatment were found in C, nitrogen (N), and phenol contents, composition of major chemical groups of litter studied by solid-state 13 C NMR spectroscopy, and composition of aromatic components of SOM studied by analytical pyrolysis (Py GC/MS). This lack of differences in chemical composition suggests that greater SOM sequestration in the earthworm treatment is likely to be connected with physical protection of SOM inside cast aggregates rather than with chemical changes in SOM mediated by earthworms. [ABSTRACT FROM AUTHOR]

Physical separation of soil into different soil organic matter (SOM) fractions is widely used to identify organic carbon pools that are differently stabilized and have distinct chemical composition. However, the mechanisms underlying these differences in stability and chemical composition are only partly understood. To provide new insights into the stabilization of different chemical compound classes in physically-separated SOM fractions, we assessed shifts in the biomolecular composition of bulk soils and individual particle size fractions that were incubated in the laboratory for 345 days. After the incubation, also the incubated bulk soil was fractionated. The chemical composition of organic matter in bulk soils and fractions was characterized by C-CPMAS nuclear magnetic resonance spectroscopy and sequential chemical extraction followed by GC/MS measurements. Plant-derived lipids and lignin were abundant in particulate organic matter (POM) fractions of sand-, silt-, and clay-size and the mineral-bound, clay-sized organic matter. These results indicate that recent conceptualizations of SOM stabilization probably understate the contribution of plant-derived organic matter to stable SOM pools. Although our data indicate that inherent recalcitrance could be important in soils with limited aggregation, organo-mineral interactions and aggregation were responsible for long-term SOM stabilization. In particular, we observed consistently higher concentrations of plant-derived lipids in POM fractions that were incubated individually, where aggregates were disrupted, as compared to those incubated as bulk soil, where aggregates stayed intact. This finding emphasizes the importance of aggregation for the stabilization of less 'recalcitrant' biomolecules in the POM fractions. Because also the abundance of lipids and lignin in clay-sized, mineral-associated SOM was substantially influenced by aggregation, the bioavailability of mineral-associated SOM likely increases after the destruction of intact soil structures. [ABSTRACT FROM AUTHOR]

Organic matter (OM) amendments, originating from waste materials, could be used to enhance soil organic carbon (SOC) storage and fertility of cropland soils. However, there is a limited understanding about the long-term effect of different urban waste compost amendments on soil organic matter (SOM) formation processes and their impact on SOC storage. Accordingly, the long-term effects of different OM amendments on the amount and composition of particulate and mineral associated SOM were investigated. Surface soils were sampled from a Luvisol under cropping rotation, which received biannually 0.4 kg organic carbon m −2 in form of three different urban composts or cattle manure for a period of 15 years. Despite similar C input, different urban waste compost amendments resulted in contrasting C storage. While there were no C stock changes for municipal solid waste compost amended soils, composts from organic waste and green waste and sewage sludge increased SOC stocks in a similar range as conventional farmyard manure. In bulk soils, SOC stocks were increased by approximately 30%, in occluded particulate OM <20 μm by 155% (organic waste compost) and 71% (green waste and sewage sludge compost). Carbon storage in clay fractions showed approximately 20% higher values in all treatments. Organic matter amendments result in C–N coupling as for N proportional stock increases were recorded. The high variability of the composition of different amendment types was reflected in the particulate OM fractions but not in the highly uniform composition of the mineral-associated OM. Bulk soil wettability was not affected by the amendments, as only POM fractions showed increased hydrophobicity, but not clay fractions. Clay fractions showed high alkyl/O-alkyl and low C/N ratios, characteristic for microbial material. The OM composition in the clay fraction is determined by the microbial residues that are independent in their composition from the input material. The increased C storage in the clay fractions of the soil amended with organic wastes and green waste and sewage sludge compost and farmyard manure might be promoted by a better microbial use efficiency leading to C sequestration as microbial compounds. [ABSTRACT FROM AUTHOR]

Permafrost-affected soils of the northern circumpolar region represent 50% of the terrestrial soil organic carbon ( SOC) reservoir and are most strongly affected by climatic change. There is growing concern that this vast SOC pool could transition from a net C sink to a source. But so far little is known on how the organic matter ( OM) in permafrost soils will respond in a warming future, which is governed by OM composition and possible stabilization mechanisms. To investigate if and how SOC in the active layer and adjacent permafrost is protected against degradation, we employed density fractionation to separate differently stabilized SOM fractions. We studied the quantity and quality of OM in different compartments using elemental analysis, 13 C solid-phase nuclear magnetic resonance (13 C- NMR) spectroscopy, and 14 C analyses. The soil samples were derived from 16 cores from drained thaw lake basins, ranging from 0 to 5500 years of age, representing a unique series of developing Arctic soils over time. The normalized SOC stocks ranged between 35.5 and 86.2 kg SOC m−3, with the major amount of SOC located in the active layers. The SOC stock is dominated by large amounts of particulate organic matter ( POM), whereas mineral-associated OM especially in older soils is of minor importance on a mass basis. We show that tremendous amounts of over 25 kg OC per square meter are stored as presumably easily degradable OM rich in carbohydrates. Only about 10 kg OC per square meter is present as presumably more stable, mineral-associated OC. Significant amounts of the easily degradable, carbohydrate-rich OM are preserved in the yet permanently frozen soil below the permafrost table. Forced by global warming, this vast labile OM pool could soon become available for microbial degradation due to the continuous deepening of the annually thawing active layer. [ABSTRACT FROM AUTHOR]

Buried soils contain large reservoirs of organic carbon at depths that are not typically included in regional and global soil carbon inventories. One such palaeosol, the Brady soil of southwestern Nebraska, USA, is buried under six metres of loess. The Brady soil developed at the land surface on the late-Pleistocene-aged Peoria Loess in a period of warmth and wetness during which dunefields and dust sources across the region were stabilized. Abrupt climate change in the early Holocene led to increased loess deposition that buried the soil. Here, we used spectroscopic and isotopic analyses to determine the composition and stability of organic carbon in the Brady soil. We identify high levels of black carbon, indicating extensive biomass burning. In addition, we found intact vascular plant lipids in soil organic matter with radiocarbon ages ranging from 10,500 to 12,400 cal yr BP, indicating decomposition was slowed by rapid burial at the start of the Holocene. We conclude that landscape disturbance caused by abrupt climate change, fire and the loss of vegetative cover contributed to deep carbon sequestration as the soil was quickly buried under accumulating loess. We suggest that terrestrial soil carbon storage in arid and semi-arid environments could undergo landscape-scale shifts in response to rising temperatures, increased fire activity or drought. [ABSTRACT FROM AUTHOR]

The stabilization of soil organic matter (SOM) is triggered by three main mechanisms: (i) low bioavailability due to aggregation, (ii) recalcitrance due to the chemical structure, and (iii) association of the SOM with mineral surfaces. In the present study we used particle size SOM fractions (sand, silt and clay), derived from the Ah soil horizon from a Norway spruce forest in Southern Germany, to study the effects of different stabilization mechanisms on the bioavailability of soil organic carbon (SOC) in a one year incubation experiment. The respired CO2 was hourly recorded, additionally 13CO2 was analysed 20 times and 14CO2 three times during the incubation experiment. To better differentiate between particulate OM (POM) and mineral associated OM (MIN), the incubated fractions and bulk soil were separated according to density (1.8 g cm−3) after the incubation experiment. 13C-CPMAS NMR spectroscopy was used to study the chemical composition of the incubated samples. We demonstrate a clear increase in SOM bioavailability due to aggregate disruption, as the calculated theoretical CO2 evolution of the SOM fractions recombined by calculation was 43.8% higher in relation to the intact bulk soil. The incubated sand fraction, dominated by POM rich in O/N-alkyl C, showed a prolonged bioavailability of SOC moieties with mean residence times (MRT) of 78 years. Interestingly, the silt fraction, dominated by highly aliphatic, more recalcitrant POM, showed low mineralization rates and slow MRT's (192 years) close to values for the clay fraction (171 years), which contained a large amount of mineral-associated SOM. The recorded 13/12CO2 signatures showed a high depletion in 13C during the initial stage of the incubation, but an enrichment of the respired 13CO2 of up to 3.4‰ relative to the incubated SOM was observed over longer time periods (after 3 and 4 days for bulk soil and sand, respectively, and after 14 days for silt and clay). Therefore, we found no evidence for a 13C enrichment of SOM as driven by metabolic isotopic fractionation during microbial SOM mineralization, but an indication of a change in the isotopic composition of the C-source over time. [ABSTRACT FROM AUTHOR]

Ultrasonication is widely used in soil organic matter (SOM) fractionation studies to break up soil aggregates to disperse the soil into its primary particles. An increasing number of studies also aim to incubate SOM fractions obtained by physical soil fractionation to study the bioavailability of different soil organic C (SOC) pools. To evaluate possible influences of ultrasonic soil disruption on short-term SOM bioavailability as well as the content and composition of water-extractable organic matter (WEOM) and salt-extractable (10 mM K2S04) organic matter (SEOM), we conducted a laboratory incubation experiment with aggregated soil material from an Ap horizon of a cropland soil. Bulk soil and subsamples in which aggregates had been disrupted by ultrasonication were incubated in triplicate for 18 d at 20°C. During the incubation, C02 production was measured continuously. At the beginning and at the end of the experiment, the chemical composition of SEOM was analyzed for the different experimental variants by ultraviolet (UV)-absorbance and solid state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C-CPMAS NMR) spectroscopy. Additionally, we studied the effects of WEOM removal on heterotrophic soil respiration to assess artifacts by unavoidable WEOM removal during soil wet sieving, which is an important step in soil fractionation. After repeated leaching of unsonicated and sonicated soil samples, 0.5 to 1.0% of the total organic C (OC) pool was associated with the WEOM fraction. Ultrasonic disruption of soil aggregates resulted in increased amounts and changed composition of SEOM as well as in a 27% increase in C02 production compared with intact soils. Incubation of ultrasonicated samples resulted in decreased O/N alkyl C and increased aromatic C contents in SEOM, indicating mineralization of potentially mobile organic matter (OM). Ultrasonic treatment during soil fractionation leads to enhanced dissolved OM (DOM) leaching and increased SOM bioavailability, which may bias results from incubation experiments on separated SOM fractions. [ABSTRACT FROM AUTHOR]

Land-use change affects not only the amount of soil organic matter (SOM) but also its composition. We performed cross-polarization magic angle spinning (CPMAS) 13C and 15N nuclear magnetic resonance (NMR) spectroscopy to investigate the chemical composition of bulk SOM in topsoils (0–15 cm) under different land use, namely native forest (NF), 27-year cropland with wheat/soybean rotation (CL) and 27-year rangelands with guineagrass (RG) and with bahiagrass (RB), in south-east Bolivia. The findings of this study showed only a subtle alteration of composition of bulk SOM despite the large changes in carbon (C) content. Nevertheless, NF and RB showed a slightly lower abundance of aromatic C but a higher proportion of alkyl C compared to CL and RG where the loss of organic matter was substantial. This suggests that relatively stable components dominated by aromatic structures had relatively enriched during SOM decomposition under agricultural practices. A slight disparity of SOM composition observed between RG and RB (less O-alkyl C but more aromatic C in RG than RB) suggests that grass species influenced SOM quality even under the same land use, namely the rangeland. On the other hand, organic N composition was less affected by land use or management practice than C forms. [ABSTRACT FROM AUTHOR]

Reclamation of post-mining sites commonly results in rapid accrual of carbon (C) and nitrogen (N) contents due to increasing plant inputs over time. However, little information is available on the distribution of C and N contents with respect to differently stabilized soil organic matter (SOM) fractions during succession or as a result of different reclamation practice. Hence, it remains widely unknown how stable or labile these newly formed C and N pools are. Gaining a deeper understanding of the state of these pools may provide important implications for reclamation practices with respect to C sequestration. We thus investigated C, N, and plant-derived compounds in bulk soil and SOM fractions during succession in post-mining chronosequences (reclaimed with overburden or salvaged topsoil) located along a northwest to southeast transect across the USA. Our results indicate that current reclamation practices perform well with respect to rapid recovery of soil aggregates and the partitioning of C and N to different SOM fractions, these measures being similar to those of natural climax vegetation sites already 2–5 years after reclamation. A general applicability of our results to other post-mining sites with similar reclamation practices may be inferred from the fact that the observed patterns were consistent along the investigated transect, covering different climates and vegetation across the USA. However, regarding SOM stability, the use of salvaged topsoil may be beneficial as compared to that of overburden material because C and N in the fraction regarded as most stable was by 26 and 35% lower at sites restored with overburden as compared to those restored with salvaged topsoil. Plant-derived compounds appeared to be mainly related to bio-available particulate organic matter and particulate organic matter partly stabilized within aggregates, challenging the long-term persistence of plant input C in post-mining soils. [ABSTRACT FROM AUTHOR]

Content and quality of organic matter (OM) may strongly affect the denitrification potential of soils. In particular, the impact of soil OM fractions of differing bioavailability (soluble, particulate, and mineral-associated OM) on denitrification remains unresolved. We determined the potential N 2 O and N 2 as well as CO 2 production for samples of a Haplic Chernozem from six treatment plots (control, mineral N and NP, farmyard manure - FYM, and FYM + mineral N or NP) of the Static Fertilization Experiment Bad Lauchstädt (Germany) as related to OM properties and denitrifier gene abundances. Soil OM was analyzed for bulk chemical composition (13C-CPMAS NMR spectroscopy) as well as water-extractable, particulate, and mineral-associated fractions. Soils receiving FYM had more total OM and larger portions of labile fractions such as particulate and water-extractable OM. Incubations were run under anoxic conditions without nitrate limitation for seven days at 25 °C in the dark to determine the denitrification potential (N 2 O and N 2) using the acetylene inhibition technique. Abundances of nirS , nirK, and nosZ (I + II) genes were analyzed before and after incubation. The denitrification potential, defined as the combined amount of N released as N 2 O + N 2 over the experimental period, was larger for plots receiving FYM (25.9–27.2 mg N kg−1) than pure mineral fertilization (17.1–19.2 mg N kg−1) or no fertilization (12.6 mg N kg−1). The CO 2 and N 2 O production were well related and up to three-fold larger for FYM-receiving soils than under pure mineral fertilization. The N 2 production differed significantly only between all manured and non-manured soils. Nitrogenous gas emissions related most closely to water-extractable organic carbon (WEOC), which again related well to free particulate OM. The larger contribution of N 2 production in soils without FYM application, and thus, with less readily decomposable OM, coincided with decreasing abundances of nirS genes (NO 2 − reductase) and increasing abundances of genes indicating complete denitrifying organisms (nosZ I) during anoxic conditions. Limited OM sources, thus, favored a microbial community more efficient in resource use. This study suggests that WEOC, representing readily bioavailable OM, is a straightforward indicator of the denitrification potential of soils. • Largest N 2 O and CO 2 emissions from manured soils rich in labile organic matter • N 2 production was less affected by changes in soil organic matter than N 2 O production. • Denitrification potential and share of N 2 O related closely to water-extractable OC. • Water-extractable OC likely derived from N-rich free particulate organic matter. • Limited OC availability favored abundances of complete denitrifiers possessing nosZ I. [ABSTRACT FROM AUTHOR]

During the last decade, a paradigmatic shift regarding which processes determine the persistence of soil organic matter (SOM) took place. The interaction between microbial decomposition and association of organic matter with the soil mineral matrix has been identified as a focal point for understanding the formation of stable SOM. Using an improved version of the vertically resolved SOM model COMISSION (Ahrens et al., 2015), this paper investigates the effect of a maximum sorption capacity (Q max) for mineral-associated organic matter (MAOM) formation and its interaction with microbial processes, such as microbial decomposition and microbial necromass production. We define and estimate the maximum sorption capacity Q max with quantile regressions between mineral-associated organic carbon (MAOC) and the clay plus silt (<20 μm) content. In the COMISSION v2.0 model, plant- and microbial-derived dissolved organic matter (DOM) and dead microbial cell walls can sorb to mineral surfaces up to Q max. MAOC can only be decomposed by microorganisms after desorption. We calibrated the COMISSION v2.0 model with data from ten different sites with widely varying textures and Q max values. COMISSION v2.0 was able to fit the MAOC and SOC depth profiles, as well as the respective 14C gradients with soil depth across these sites. Using the generic set of parameters retrieved in the multi-site calibration, we conducted model experiments to isolate the effects of varying Q max , point-of-entry of litter inputs, and soil temperature. Across the ten sites, the combination of depolymerization limitation of microorganisms due to substrate scarcity in the subsoil and the size of Q max explain 14C depth gradients in OC. • The vertically explicit SOC model COMISSION was used to study 14C depth gradients. • A texture-based maximum sorption capacity was introduced to the COMISSION model. • One universal parameter set was calibrated to 14C and SOC data from ten sites. • Energy limitation and sorption capacity explain millennial 14C ages in the subsoil. [ABSTRACT FROM AUTHOR]

The loss of organic matter (OM) from soil during long-term agricultural cropping results in a decrease in the inherent fertility of the soil as well as releasing greenhouse gases. Despite the importance of organo-mineral associations in the stabilization of OM within soils, much remains unknown about these organo-mineral associations. We used nano-scale secondary ion mass spectrometry (NanoSIMS) to investigate the incorporation and stabilization of 13C and 15N labelled residues of lucerne (Medicago sativa) and buffel grass (Cenchrus ciliaris) when incubated in a Vertisol from temperate Australia for up to 365 d. We show that newly-added OM forms organo-mineral associations through two mechanisms. Firstly, it was observed that the newly-added OM forms associations with the existing mineral-bound OM. However, this apparent stabilization of newly-added OM by associating with existing mineral-bound OM was not influenced by the C:N ratio of the plant residues, with the lucerne residues (C:N ratio of 11) being incorporated to a similar extent as the buffel grass (C:N ratio of 35). Secondly, we observed that N-rich microbial metabolites attached directly to mineral particle surfaces that did not contain existing OM patches, thereby creating new organo-mineral associations through which additional stabilization of OM would be possible. The information obtained in this study is valuable in understanding the stabilization of OM through organo-mineral associations, and raises the possibility of using cover crops or catch crops with narrow C:N ratios to allow for formation of new organo-mineral associations for increased stabilization of OM in soil. • Soil organic matter (OM) is stabilized through organo-mineral associations. • We show that organo-mineral associations form through two mechanisms. • Some OM is stabilized by associating with the existing mineral-bound OM. • Nitrogen-rich compounds can attach directly to mineral particle surfaces. [ABSTRACT FROM AUTHOR]

As soils have a recognized capacity of carbon storage, the understanding of the mechanisms responsible for soil organic matter (SOM) stabilization are of extreme importance. Yet, our knowledge about the factors and processes controlling the amounts of stabilized versus mineralized SOM is still remarkably limited. We examined the decomposition of different plant inputs in a top- (0 – 0.2 m) and subsoil (0.8 – 0.9 m) of an Andosol. We performed an incubation experiment during 60 days using as amendments 13C isotope labelled plant leaves (Salix spp.) and dissolved organic carbon (DOC) extracted after incubation of leaves (pelleted – 1 mm or liquid - < 0.7 µm). Soil respiration was measured with a δ13C CO2 Isotope Analyzer. After the incubation period, the soils were fractionated by density using a sodium polytungstate solution of 1.8 g cm-3 into two fractions: heavy (> 1.8 g cm-3) and light (< 1.8 g cm-3). The added 13C amount was then measured by Isotope-ratio mass spectrometry (IRMS). This experiment allowed us to observe distinctive SOM stabilization and mineralization patterns in both soil layers depending on the added material. The DOC, whether pelleted or liquid, was significantly more respired than the leaves in both soil layers. In the subsoil but not the topsoil, we observed a significant difference between the respiration of the different DOC forms (pelleted and liquid). Since both DOC forms vary by location (distributed vs point source) and size (pelleted vs liquid), the mineralization results point at several explanations: (i) smaller particles are more efficiently stabilized in subsoil layers by mineral interactions than larger structures due to their greater surface area, (ii) more dilute material is more efficiently adsorbed as concentrated organic matter exceeds sorption capacity (saturation concept), or (iii) more concentrated organic matter is more easily mineralized than more dilute because microorganisms are able to specialize to metabolize the substrate. As the leaves and pelleted DOC were both point sources of SOM, the differences in their mineralization rates indicates a significant influence of material solubility. We also observed distinct patterns of SOM stabilization in top- and subsoil. In the subsoil, a greater proportion of the added DOC (pelleted and liquid) than the leaves was found in the heavy fraction, which is commonly interpreted as stabilized C on clay surfaces and less accessible to microorganisms. This may indicate that mineral association explained differences in CO2 evolution. In the topsoil, added C remained in light fractions typically interpreted as largely unassociated with minerals. However, presence in such light fraction in the topsoil must still have reduced mineralization. Higher proportions of added leaves and pelleted DOC remained in the topsoil mineral fraction than of the liquid DOC, which indicates that point sources of C provided higher stabilization into soil minerals. These differences clearly illustrate distinct mechanisms for SOM protection depending on the evaluated soil layer and amendment added. We demonstrated that SOM stabilization in top and subsoil is significantly affected by the spatial distribution and the solubility of the amendment. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 concentrations have increased interest in the potential for forestecosystems and soils to act as carbon (C) sinks. While many studies provide evidence for aneffect of tree species identity on soil C stocks, little is known about if and how tree speciesinfluence the stability of C in soil. We thus investigated the effects of eleven different tree species and their traits (includingtree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover,descriptors of the soil microbial community, and soil physico-chemical properties) on fiveindices of soil organic matter (SOM) stability (including heterotrophic respiration, C inaggregate-occluded particulate organic matter (POM) and mineral-associated SOM, and bulksoil δ15N and Δ14C). Our results show that the stability of SOM varied independently of the amount of C in soiland that this stability was regulated by tree species and their traits, mainly via thecomposition of roots. Stability of SOM appeared to be greater (as indicated byhigher δ15N and reduced respiration) beneath species with higher concentrationsof nitrogen and lower amounts of acid-insoluble compounds in their roots, whileSOM stability appeared to be lower (as indicated by higher respiration and lowerproportions of C in aggregate-occluded POM) beneath species with higher tissue calciumcontents. Some of our stability indices (C in mineral-associated SOM and bulk soilΔ14C), though, were negligibly dependent on tree species traits, likely reflectingan insensitivity of some SOM pools to decadal-scale shifts in ecological factors.Strategies aiming to increase soil C stocks may thus focus on particulate C pools,which can more easily be manipulated and are most sensitive to climate change. [ABSTRACT FROM AUTHOR]

The soil carbon accrual in Maritime Antarctica is a unique process due to the harsh environmental conditions and very low carbon input, that is locally restricted (bird colonies and few vegetation patches, without roots). Especially ornithogenic soils of this region receive high doses of bioavailable C, N and P, which affects vegetation distribution and productivity. A higher vegetation productivity, together with nutrient availability directly affect soil pedogenesis due to increased mineral weathering and soil organic matter contents. The aim of this work was to study broad soil biogeochemical processes of sites differently affected by the nutrient and organic carbon input from seabird rookeries. With ongoing climate change it is assumed that the underlying processes will be altered substantially. By studying soil transects on Deception Island (South Shetland Islands) we aimed to gain a better knowledge of the feedbacks between climate, nutrient cycling, vegetation patterns and soil biogeochemical processes in Maritime Antarctica. To this end we sampled three spatially separated sites on Deception Island (Penguin Colony, Fumarole Bay, Lago Escondido) that are distributed along a transect from higher to lower ornithogenic influence, respectively. At the sampling sites patches below living and dead moss were sampled. Bulk soil samples from 4 replicated subplots per site were taken from three layers at 0 – 2 cm (including dead mosses); 2 – 3 cm, and 3 – 10 cm depth at corresponding spots under living and dead Polytrichastrum alpinum, accounting for a total of 72 bulk soil samples. Besides C and N contents, we also fractionated the topsoils according to density and particle size in order to differentiate soil organic matter accrual processes between the sites. Using a principal component analysis, we could clearly discriminate between the three sites, with the Penguin Colony having a major correlation with C and N, and both the Fumarole Bay and Lago Escondido having a high correlation with pH. These results are in accordance with Location being the main effect and the Location x Vegetation interaction in an ANOVA, where significant C and N differences were demonstrated in relation to the intensity of seabird influence. Beside the clear local effect of the input of C and N by seabirds, we were able to also demonstrate distinct effects of dead moss cover on soil organic matter stocks and composition. With respect to the ongoing climate change and alterations in vegetation distribution in Antarctica our data highlight the intricate connection between vegetation status and soil chemical and biological development and function. [ABSTRACT FROM AUTHOR]

Decaying plant material is the major source of soil organic carbon (SOC). Nevertheless, it still remains unclear how the biochemical composition of litter is affecting the formation of SOM. Thus, we aimed to disentangle the effect of plant litter composition on C transference into differently stabilized soil fractions. We did this by individually incubating 13C enriched Eucalyptus spp. organs (bark, leaves, twigs and roots) in topsoil (0–20 cm) of a sandy-clay loam (Haplic Ferralsol) from Paula Candido – Brazil (20º 52' S, 42º 58' E). Additionally, a soil sample without plant residue addition was incubated and used as a control. The samples were incubated at 80% of their water-holding capacity at 25 ºC. The molecular composition of the incubated plant organ material was characterized by solid-state 13C-CP/MAS-NMR spectroscopy. At the end of the incubation experiment (after 200 days), aliquots were taken and the SOM was physically fractionated using a combined density-particle size separation method. The total C content and the relative abundance of 13C (δ 13C) of each soil organic matter fraction were measured by IRMS, whereas the litter-C contribution for each SOM fraction was assessed using a two-end member isotope mixing model. No significant differences were observed for total SOC contents among the different treatments. Conversely, lower total SOC contents were observed for the control treatment without residue addition, indicating mineralization of inherited SOM along the incubation period. The partitioning of litter-derived C into SOM fractions and CO2 indicates that leaf had higher proportion of C transferred to the total SOC, followed by roots, bark and twigs. The amount of carbon transferred to the mineral associated organic matter (MAOM) was as follows: leaves > bark > twigs > roots. Contrastingly, for the particulate organic matter (POM) the order was as follows: roots > bark > twigs = leaves. Contrasting NMR spectra demonstrate the significant effect of the biochemical composition of each plant organ in the fate of C in soil. Carbon from leaves was preferentially allocated to the MAOM fraction, and was more enriched in carboxyl and alkyl groups. Twigs, bark and roots, which were more enriched in O-Alkyl group, were preferentially respired or allocated into the POM fraction. [ABSTRACT FROM AUTHOR]