Sediment and soil organic matter source assessment as revealed by the molecular distribution and carbon isotopic composition of n-alkanes

The assessment of organic matter (OM) sources in sediments and soils is a key to better understand the biogeochemical cycling of carbon in aquatic environments. While traditional molecular marker-based methods have provided such information for typical two end member (allochthonous/terrestrial vs. autochthonous/microbial)-dominated systems, more detailed, biomass-specific assessments are needed for ecosystems with complex OM inputs such as tropical and sub-tropical wetlands and estuaries where aquatic macrophytes and macroalgae may play an important role as OM sources. The aim of this study was to assess the utility of a combined approach using compound specific stable carbon isotope analysis and an n-alkane based proxy (Paq) to differentiate submerged and emergent/terrestrial vegetation OM inputs to soils/sediments from a sub-tropical wetland and estuarine system, the Florida Coastal Everglades. Results show that Paq values (0.13–0.51) for the emergent/terrestrial plants were generally lower than those for freshwater/marine submerged vegetation (0.45–1.00) and that compound specific δ13C values for the n-alkanes (C23 to C31) were distinctively different for terrestrial/emergent and freshwater/marine submerged plants. While crossplots of the Paq and n-alkane stable isotope values for the C23n-alkane suggest that OM inputs are controlled by vegetation changes along the freshwater to marine transect, further resolution regarding OM input changes along this landscape was obtained through principal component analysis (PCA), successfully grouping the study sites according to the OM source strengths. The data show the potential for this n-alkane based multi-proxy approach as a means of assessing OM inputs to complex ecosystems.

Age determination, uranium, carbon and oxygen isotopes ratios of a stalagmite of the Villars cave

The signature of Dansgaard-Oeschger events - millennial-scale abrupt climate oscillations during the last glacial period - is well established in ice cores and marine records (Labeyrie, 2000, doi:10.1126/science.290.5498.1905; Blunier and Brook, 2001, doi:10.1126/science.291.5501.109: Bond et al., 2001, doi:10.1126/science.1065680). But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of 14C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using 234U/230Th ratios. We find rapid climate oscillations coincident with the established Dansgaard-Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel (Bar-Mathews et al., 2000, doi:10.1016/S0009-2541(99)00232-6), and deep-sea records (Bond et al., 1993, doi:10.1038/365143a0; Shackleton and Hall, 2001, doi:10.1029/2000PA000513), indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 +/-0.6 and 67.4 0.9 kyr ago.

Increase in soil organic carbon by agricultural intensification in northern China

Acknowledgements. This research was supported by National Natural Science Foundation of China (no. 31370527 and 31261140367) and the National Science and Technology Support Program of China (no. 2012BAD14B01-2). The authors gratefully thank the Huantai Agricultural Station for providing of the Soil Fertility Survey data. We also thank Zheng Liang from China Agricultural University for the soil sampling and analysis in 2011. Thanks are extended to Jessica Bellarby for helpful discussion and suggestions.

Radiocarbon dating, carbon storage and soil properties of samples from Tulemalu Lake, central Canadian Arctic

We investigated total storage and landscape partitioning of soil organic carbon (SOC) in continuous permafrost terrain, central Canadian Arctic. The study is based on soil chemical analyses of pedons sampled to 1 m depth at 35 individual sites along three transects. Radiocarbon dating of cryoturbated soil pockets, basal peat and fossil wood shows that cryoturbation processes have been occurring since the Middle Holocene and that peat deposits started to accumulate in a forest-tundra environment where spruce was present (~6000 cal yrs BP). Detailed partitioning of SOC into surface organic horizons, cryoturbated soil pockets and non-cryoturbated mineral soil horizons is calculated (with storage in active layer and permafrost calculated separately) and explored using principal component analysis. The detailed partitioning and mean storage of SOC in the landscape are estimated from transect vegetation inventories and a land cover classification based on a Landsat satellite image. Mean SOC storage in the 0-100 cm depth interval is 33.8 kg C/m**2, of which 11.8 kg C/m**2 is in permafrost. Fifty-six per cent of the total SOC mass is stored in peatlands (mainly bogs), but cryoturbated soil pockets in Turbic Cryosols also contribute significantly (17%). Elemental C/N ratios indicate that this cryoturbated soil organic matter (SOM) decomposes more slowly than SOM in surface O-horizons.

Soil organic carbon storage and soil properties for 50 soil profiles in the Lena River Delta including land form description and map

To project the future development of the soil organic carbon (SOC) storage in permafrost environments, the spatial and vertical distribution of key soil properties and their landscape controls needs to be understood. This article reports findings from the Arctic Lena River Delta where we sampled 50 soil pedons. These were classified according to the U.S.D.A. Soil Taxonomy and fall mostly into the Gelisol soil order used for permafrost-affected soils. Soil profiles have been sampled for the active layer (mean depth 58±10 cm) and the upper permafrost to one meter depth. We analyze SOC stocks and key soil properties, i.e. C%, N%, C/N, bulk density, visible ice and water content. These are compared for different landscape groupings of pedons according to geomorphology, soil and land cover and for different vertical depth increments. High vertical resolution plots are used to understand soil development. These show that SOC storage can be highly variable with depth. We recommend the treatment of permafrost-affected soils according to subdivisions into: the surface organic layer, mineral subsoil in the active layer, organic enriched cryoturbated or buried horizons and the mineral subsoil in the permafrost. The major geomorphological units of a subregion of the Lena River Delta were mapped with a land form classification using a data-fusion approach of optical satellite imagery and digital elevation data to upscale SOC storage. Landscape mean SOC storage is estimated to 19.2±2.0 kg C/m**2. Our results show that the geomorphological setting explains more soil variability than soil taxonomy classes or vegetation cover. The soils from the oldest, Pleistocene aged, unit of the delta store the highest amount of SOC per m**2 followed by the Holocene river terrace. The Pleistocene terrace affected by thermal-degradation, the recent floodplain and bare alluvial sediments store considerably less SOC in descending order.

Eating from the same plate? Revisiting the role of labile carbon inputs in the soil food web

An increasing number of empirical studies are challenging the central fundamentals on which the classical soil food web model is built. This model assumes that bacteria consume labile substrates twice as fast as fungi, and that mycorrhizal fungi do not decompose organic matter. Here, we build on emerging evidence that points to significant consumption of labile C by fungi, and to the ability of ectomycorrhizal fungi to decompose organic matter, to show that labile C constitutes a major and presently underrated source of C for the soil food web. We use a simple model describing the dynamics of a recalcitrant and a labile C pool and their consumption by fungi and bacteria to show that fungal and bacterial populations can coexist in a stable state with large inputs into the labile C pool and a high fungal use of labile C. We propose a new conceptual model for the bottom trophic level of the soil food web, with organic C consisting of a continuous pool rather than two or three distinct pools, and saprotrophic fungi using substantial amounts of labile C. Incorporation of these concepts will increase our understanding of soil food web dynamics and functioning under changing conditions.

Factors and mechanisms controlling soil organic carbon turnover in subsoils of agricultural sites

Two-third of the terrestrial C is stored in soils, and more than 50% of soil organic C (SOC) is stored in subsoils from 30 – 100 cm. Hence, subsoil is important as a source or sink for CO2 in the global carbon cycle. Especially the stable organic carbon (OC) is stored in subsoil, as several studies have shown that subsoil OC is of a higher average age than topsoil OC. However, there is still a lack of knowledge regarding the mechanisms of C sequestration and C turnover in subsoil. Three main factors are discussed, which possibly reduce carbon turnover rates in subsoil: Resource limitation, changes in the microbial community, and changes in gas conditions. The experiments conducted in this study, which aimed to elucidate the importance of the mentioned factors, focused on two neighbouring arable sites, with depth profiles differing in SOC stocks: One Colluvic Cambisol (Cam) with high SOC contents (8-12 g kg-1) throughout the profile and one Haplic Luvisol (Luv) with low SOC contents (3-4 g kg-1) below 30 cm depth. The first experiment was designed to gain more knowledge regarding the microbial community and its influence on carbon sequestration in subsoil. Soil samples were taken at four different depths on the two sites. Microbial biomass C (MBC) was determined to identify depth gradients in relation to the natural C availability. Bacterial and fungal residues as well as ergosterol were determined to quantify changes in the in the microbial community composition. Multi-substrate-induced-respiration (MSIR) was used to identify shifts in functional diversity of the microbial community. The MSIR revealed that substrate use in subsoil differed significantly from that in topsoil and also differed highly between the two subsoils, indicating a strong influence of resource limitations on microbial substrate use. Amino sugar analysis and the ratio of ergosterol to microbial biomass C showed that fungal dominance decreased with depth. The results clearly demonstrated that microbial parameters changed with depth according to substrate availability. The second experiment was an incubation experiment using subsoil gas conditions with and without the addition of C4 plant residues. Soil samples were taken from topsoil and subsoil of the two sites. SOC losses during the incubation, were not influenced by the subsoil gas conditions. Plant-derived C losses were generally stronger in the Cam (7.5 mg g-1), especially at subsoil gas conditions, than in the Luv (7.0 mg g-1). Subsoil gas conditions had no general effects on microbial measures with and without plant residue addition. However, the contribution of plant-derived MBC to total MBC was significantly reduced at subsoil gas conditions. This lead to the conclusion that subsoil gas conditions alter the metabolism of microorganisms but not the degradation of added plant residues is general. The third experiment was a field experiment carried out for two years. Mesh bags containing original soil material and maize root residues (C4 plant) were buried at three different depths at the two sites. The recovery of the soilbags took place 12, 18, and 24 months after burial. We determined the effects of these treatments on SOC, density fractions, and MBC. The mean residence time for maize-derived C was similar at all depths and both sites (403 d). MBC increased to a similar extent (2.5 fold) from the initial value to maximum value. This increase relied largely on the added maize root residues. However, there were clear differences visible in terms of the substrate use efficiency, which decreased with depth and was lower in the Luv than in the Cam. Hence freshly added plant material is highly accessible to microorganisms in subsoil and therefore equally degraded at both sites and depths, but its metabolic use was determined by the legacy of soil properties. These findings provide strong evidence that resource availability from autochthonous SOM as well as from added plant residues have a strong influence on the microbial community and its use of different substrates. However, under all of the applied conditions there was no evidence that complex substrates, i.e. plant residues, were less degraded in subsoil than in topsoil.

Soil organic carbon stocks in estuarine and marine mangrove ecosystems are driven by nutrient colimitation of P and N

Mangroves play an important role in carbon sequestration, but soil organic carbon (SOC) stocks differ between marine and estuarine mangroves, suggesting differing processes and drivers of SOC accumulation. Here, we compared undegraded and degraded marine and estuarine mangroves in a regional approach across the Indonesian archipelago for their SOC stocks and evaluated possible drivers imposed by nutrient limitations along the land-to-sea gradients. SOC stocks in natural marine mangroves (271–572 Mg ha-1 m-1 were much higher than under estuarine mangroves (100–315 Mg ha-1 m-1 with a further decrease caused by degradation to 80–132 Mg ha-1 m-1. Soils differed in C/N ratio (marine: 29–64; estuarine: 9–28), δ15N (marine: 0.6 to 0.7‰; estuarine: 2.5 to 7.2‰), and plant-available P (marine: 2.3–6.3 mg kg-1; estuarine: 0.16–1.8 mg kg-1). We found N and P supply of sea-oriented mangroves primarily met by dominating symbiotic N2 fixation from air and P import from sea, while mangroves on the landward gradient increasingly covered their demand in N and P from allochthonous sources and SOM recycling. Pioneer plants favored by degradation further increased nutrient recycling from soil resulting in smaller SOC stocks in the topsoil. These processes explained the differences in SOC stocks along the land-to-sea gradient in each mangrove type as well as the SOC stock differences observed between estuarine and marine mangrove ecosystems. This first large-scale evaluation of drivers of SOC stocks under mangroves thus suggests a continuum in mangrove functioning across scales and ecotypes and additionally provides viable proxies for carbon stock estimations in PES or REDD schemes.