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Climate change drives a northward shift of biomes in high-latitude regions. This might have consequences on the decomposition of plant litter entering the soil, including its lignin component, which is one of the most abundant components of vascular plants. In order to elucidate the combined effect of climate and soil characteristics on the decomposition pattern of lignin, we investigated lignin contents and its degree of oxidative decomposition within soil profiles along a climosequence in western Siberia. Soil samples were collected from organic topsoil to mineral subsoil at six sites along a 1500-km latitudinal transect, stretching from tundra, through taiga and forest steppe to typical steppe. The stage of lignin degradation, as mirrored by decreasing organic carbon-normalized lignin contents and increasing oxidative alteration of the remnant lignin (acid-to-aldehyde ratios of vanillyl- and syringyl-units [(Ac/Al)V and (Ac/Al)S]) within soil horizons, increased from tundra to forest steppe and then decreased to the steppe. Principal component analysis, involving also climatic conditions such as mean annual temperature and aridity index, showed that the different states of lignin degradation between horizons related well to the activity of phenoloxidases and peroxidases, enzymes involved in lignin depolymerization that are produced primarily by fungi and less importantly by bacteria. The low microbial lignin decomposition in the tundra was likely due to low temperature and high soil moisture, which do not favour the fungi. Increasing temperature and decreasing soil moisture, facilitating a higher abundance of fungi, led to increased fungal lignin decomposition towards the forest-steppe biome, while drought and high pH might be responsible for the reduced lignin decomposition in the steppe. We infer that a shift of biomes to the north, driven by climate change, might promote lignin decomposition in the northern parts, whereas in the south a further retardation mig

Semi-arid regions of Central Asia suffer from wind erosion due to expanding steppe conversion and unsustainable farming practices. Empirical data from field observations are needed to support the implementation of adapted management. In this study, a mobile wind tunnel was used for the first time in Kazakhstan to assess the soil's erodibility under real conditions. Field experiments were conducted on loamy sands with different initial conditions that are typical for the most erosive time of the year: a bare surface with a cloddy structure after recent steppe conversion, a weak crust on a plot with barley (Hordeum vulgare L.), and a plot with loose material in the rows of maize plants (Zea mays L.). Subsequently, different levels of mechanical stress (low, moderate, high) were considered to analyze the effect of disruptive forces soils experience during field cultivation (light cultivator, disc harrow, tractor tires) on possible soil losses. The results of wind tunnel experiments showed already great differences under initial conditions. The cloddy structure of the recent steppe conservation had the lowest susceptibility against wind erosion due to a good aggregation and a large roughness, resulting in soil loss of 12 g m−2. The plot grown with barley was less affected by wind erosion due to the weak crust, smaller distances between plants, and leaves close to the ground (soil loss of 34 g m−2). Maize was also the most problematic crop in the study area because wind can blow below the plant canopy without considerable resistance during the early growth stages. Additionally, existing deposits in the maize rows from previous erosion events led to the highest soil loss of 1609 g m−2. Mechanical stress by seedbed preparation generally increased the erodible fraction, resulting in higher soil losses (light cultivator: 198 ± 129 g m−2, disc harrow: 388 ± 258 g m−2). The most severe disruption of soil structure occurred on tractor tire tracks, causing a loss of 2767 ± 1810

Permafrost-affected soils in the northern circumpolar region store more than 1,000 Pg soil organic carbon (OC), and are strongly vulnerable to climatic warming. However, the extent to which changing soil environmental conditions with permafrost thaw affects different compounds of soil organic matter (OM) is poorly understood. Here, we assessed the fate of lignin and non-cellulosic carbohydrates in density fractionated soils (light fraction, LF vs. heavy fraction, HF) from three permafrost regions with decreasing continentality, expanding from east to west of northern Siberia (Cherskiy, Logata, Tazovskiy, respectively). In soils at the Tazovskiy site with thicker active layers, the LF showed smaller OC-normalized contents of lignin-derived phenols and plant-derived sugars and a decrease of these compounds with soil depth, while a constant or even increasing trend was observed in soils with shallower active layers (Cherskiy and Logata). Also in the HF, soils at the Tazovskiy site had smaller contents of OC-normalized lignin-derived phenols and plant-derived sugars along with more pronounced indicators of oxidative lignin decomposition and production of microbial-derived sugars. Active layer deepening, thus, likely favors the decomposition of lignin and plant-derived sugars, that is, lignocelluloses, by increasing water drainage and aeration. Our study suggests that climate-induced degradation of permafrost soils may promote carbon losses from lignin and associated polysaccharides by abolishing context-specific preservation mechanisms. However, relations of OC-based lignin-derived phenols and sugars in the HF with mineralogical properties suggest that future OM transformation and carbon losses will be modulated in addition by reactive soil minerals.

In nitrogen (N) -limited agricultural systems, a high microbial immobilization of applied fertilizer-N can limit its availability to plants. However, there is scarce information on the effect of the form of fertilizer used on the plant–microorganism competition in clay-rich soils under a severe semi-arid climate. In a field study, we investigated the wheat–microorganism competition after the direct application of NH415NO3 closely to seeds in arable fields in North Kazakhstan, documenting the effect of the use of liquid versus granular fertilizer and mini-tillage versus no-tillage. Our results barely showed any fertilizer-N translocation in the soil. Plants outcompete microorganisms for fertilizer-N during the vegetation period. Microbial-to-plant 15N ratios revealed a predominant fertilizer-15N uptake by plants. The strong competition for N was mainly related to the placement of the fertilizer close to the seeds. Moreover, the long time interval between fertilization and sampling enhanced the competition for N, meaning that previously microbially immobilized N became available to plants through the death of microorganisms and their subsequent mineralization. The fertilizer distribution between microorganisms and plants did depend on the form of fertilizer used, owing to the good solubility of granular fertilizer. The smaller fertilizer-N uptake under the no-tilling condition was probably due to the more intense soil compaction, which caused a reduction in plant growth. The application of fertilizer close to the seeds and the small fertilizer translocation during the vegetation period ultimately resulted in a high level of plant N being derived from the fertilizer.

Climate change in Arctic ecosystems fosters permafrost thaw and makes massive amounts of ancient soil organic carbon (OC) available to microbial breakdown. However, fractions of the organic matter (OM) may be protected from rapid decomposition by their association with minerals. Little is known about the effects of mineral-organic associations (MOA) on the microbial accessibility of OM in permafrost soils and it is not clear which factors control its temperature sensitivity. In order to investigate if and how permafrost soil OC turnover is affected by mineral controls, the heavy fraction (HF) representing mostly MOA was obtained by density fractionation from 27 permafrost soil profiles of the Siberian Arctic. In parallel laboratory incubations, the unfractionated soils (bulk) and their HF were comparatively incubated for 175 days at 5 and 15 degrees C. The HF was equivalent to 70 +/- 9% of the bulk CO2 respiration as compared to a share of 63 +/- 1% of bulk OC that was stored in the HF. Significant reduction of OC mineralization was found in all treatments with increasing OC content of the HF (HF-OC), clay-size minerals and Fe or Al oxyhydroxides. Temperature sensitivity (Q10) decreased with increasing soil depth from 2.4 to 1.4 in the bulk soil and from 2.9 to 1.5 in the HF. A concurrent increase in the metal-to-HF-OC ratios with soil depth suggests a stronger bonding of OM to minerals in the subsoil. There, the younger C-14 signature in CO2 than that of the OC indicates a preferential decomposition of the more recent OM and the existence of a MOA fraction with limited access of OM to decomposers. These results indicate strong mineral controls on the decomposability of OM after permafrost thaw and on its temperature sensitivity. Thus, we here provide evidence that OM temperature sensitivity can be attenuated by MOA in permafrost soils.

Permafrost soils preserve huge amounts of organic carbon (OC) prone to decomposition under changing climatic conditions. However, knowledge on the composition of soil organic matter (OM) and its transformation and vulnerability to decomposition in these soils is scarce. We determined neutral sugars and lignin-derived phenols, released by trifluoroacetic acid (TFA) and CuO oxidation, respectively, within plants and soil density fractions from the active layer and the upper permafrost layer at three different tundra types (shrubby grass, shrubby tussock, shrubby lichen) in the Northeast Siberian Arctic. The heavy fraction (HF; > 1.6 g mL(-1)) was characterized by a larger enrichment of microbial sugars (hexoses vs. pentoses) and more pronotmced lignin degradation (acids vs. aldehydes) as compared to the light fraction (LF; < 1.6 g mL(-1)), showing the transformation from plant residue-dominated particulate OM to a largely microbial imprint in mineral-associated OM. In contrast to temperate and tropical soils, total neutral sugar contents and galactose plus mannose to arabinose plus xylose ratios (GM/AX) decreased in the HE with soil depth, which may indicate a process of effective recycling of microbial biomass rather than utilizing old plant materials. At the same dine, lignin-derived phenols increased and the degree of oxidative decomposition of lignin decreased with soil depth, suggesting a selective preservation of lignin presumably due to anaerobiosis. As large parts of the plant-derived pentoses are incorporated in lignocelluloses and thereby protected against rapid decomposition, this might also explain the relative enrichment of pentoses with soil depth. Hence, our results show a relatively large contribution of plant derived OM, particularly in the buried topsoil and subsoil, which is stabilized by the current soil environmental conditions but may become available to decomposers if permafrost degradation promotes soil drainage and improves the soil oxyg

Background: Maritime Antarctica is severely affected by climate change and accelerating glacier retreat forming temporal gradients of soil development. Successional patterns of soil development and plant succession in the region are largely unknown, as are the feedback mechanisms between both processes. Here we identify three temporal gradients representing horizontal and vertical glacier retreat, as well as formation of raised beaches due to isostatic uplift, and describe soil formation and plant succession along them. Our hypotheses are (i) plants in Antarctica are able to modulate the two base parameters in soil development, organic C content and pH, along the temporal gradients, leading to an increase in organic carbon and soil acidity at relatively short time scales, (ii) the soil development induces succession along these gradients, and (iii) with increasing soil development, bryophytes and Deschampsia antarctica develop mycorrhiza in maritime Antarctica in order to foster interaction with soil. Results: All temporal gradients showed soil development leading to differentiation of soil horizons, carbon accumulation and increasing pH with age. Photoautptroph succession occurred rapidly after glacier retreat, but occurrences of mosses and lichens interacting with soils by rhizoids or rhizines were only observed in the later stages. The community of ground dwelling mosses and lichens is the climax community of soil succession, as the Antarctic hairgrass D. antarctica was restricted to ornithic soils. Neither D. antarctica nor mosses at the best developed soils showed any sign of mycorrhization. Conclusion: Temporal gradients formed by glacier retreat can be identified in maritime Antarctic, where soil development and plant succession of a remarkable pace can be observed, although pseudo-succession occurs by fertilization gradients caused by bird feces. Thus, the majority of ice-free surface in Antarctica is colonized by plant communities which interact with soil b

The input of recently photosynthesized C has significant implications on soil organic C sequestration, and in paddy soils, both plants and soil microbes contribute to the overall C input. In the present study, we investigated the fate and priming effect of organic C from different sources by conducting a 300-day incubation study with four different 13C-labelled substrates: rice shoots (shoot-C), rice roots (root-C), rice rhizodeposits (rhizo-C), and microbe-assimilated C (micro-C). The efflux of both 13CO2 and 13CH4 indicated that the mineralization of C in shoot-C-, root-C-, rhizo-C-, and micro-C-treated soils rapidly increased at the beginning of the incubation and decreased gradually afterwards. The highest cumulative C mineralization was observed in root-C-treated soil (45.4 %), followed by shoot-C- (31.9 %), rhizo-C- (7.90 %), and micro-C-treated (7.70 %) soils, which corresponded with mean residence times of 39.5, 50.3, 66.2, and 195 days, respectively. Shoot and root addition increased C emission from native soil organic carbon (SOC), up to 11.4 and 2.3 times higher than that of the control soil by day 20, and decreased thereafter. Throughout the incubation period, the priming effect of shoot-C on CO2 and CH4 emission was strongly positive; however, root-C did not exhibit a significant positive priming effect. Although the total C contents of rhizo-C- (1.89 %) and micro-C-treated soils (1.90 %) were higher than those of untreated soil (1.81 %), no significant differences in cumulative C emissions were observed. Given that about 0.3 and 0.1 % of the cumulative C emission were derived from labelled rhizo-C and micro-C, we concluded that the soil organic C-derived emissions were lower in rhizo-C- and micro-C-treated soils than in untreated soil. This indicates that rhizodeposits and microbe-assimilated C could be used to reduce the mineralization of native SOC and to effectively improve soil C sequestration. The contrasting behaviour of the different photosynthe