Greening boosts soil formation and soil organic matter accumulation in Maritime Antarctica

Global warming in the Antarctic Peninsula, Maritime Antarctica, within the past 45 years has accelerated rapid glacier retreatment, forming temporal gradients of soil development that concurs with the colonization of the ice-free soils by phototrophs. In the past decade the paradigm emerged that above- and belowground processes are interconnected, e.g. recently gained carbon fuels microbial activity and thus drives soil organic matter built-up and decomposition as well as mineral weathering. Studies of carbon allocation for Antarctic ecosystems, occurring in harsh conditions are lacking. Little is also known about the contribution of bacteria and fungi to decomposition of different soil carbon pools with different turnover rates in these soils, which is of utmost importance for the prediction of the future feedback of the Antarctic carbon balance to climate change. We followed soil horizon formation, soil organic carbon accumulation and carbon exchange with the atmosphere along a gradient of phototrophs of different trophic complexity level at King George Island by combining soil chemical analyses, field CO2 flux measurements, C-13 in situ labeling and molecular methods (PLFA and metabolomics). Our study revealed that colonization of the ice-free soils by vascular plant (Deschampsia antarctica) was leading to the formation of well-developed soil, with high contents of organic carbon and with a relatively high rates of photosynthesis and CO2 soil efflux. The soils sampled under D. antarctica showed the impact of this higher plant on the soil organic matter, containing significantly higher amounts of carbohydrates and amines, presumably as a result of root exudation. As determined by the C-13 labeling experiment more than 15% of the carbon recently assimilated by D. antarctica was transferred belowground, with a major flow into soil fungi. This suggests that not bacteria, but rather fungi preferentially and faster utilize the recently assimilated low molecular compounds allocated to the soil. Probably, successful performance of vascular plants in Maritime Antarctica may significantly foster biological weathering via enhanced microbial activity

Hot moments in the Antarctic due to climate warming?

Climate warming is severely affecting maritime Antarctica, causing accelerated glacier retreat and thus leading to an ongoing exposure of once ice- covered land. This initiates a succession of plant and soil development. Nevertheless, the temporal dynamics and controlling factors of these processes, like C and N status of soils and the effect of root exudation are widely unknown under these harsh climatic conditions. Topsoil samples from three different sites of a chronological soil sequence in the forefront of a retreating glacier of the Fildes Peninsula, King George Island, were collected and incubated at 2 °C for three weeks. To mimic the influence of C and N containing root exudates (primers) on the mineralization of soil C, we added 13C labeled glucose or alanine and compared CO2 evolution in comparison to samples without C and N addition. Soil microbes covered up to 90% of their C demand for anabolic functions with the added C-sources in the case of late soil successions while it was only 50% for the young soils. These findings were independent of the form of primer. Both primers increased the mineralization of soil carbon in the young soils as compared to the control. For the later stages of soil development, we found negative priming which was strongest for the latest stage. These results give evidence for a clear shift in the microbial community of the three investigated sites. While sites with initial soil formation seem to be dominated by k-strategists with low turnover rates that rather use complex C-sources, a significant number of r-strategists in the soils of the older sites uses simple C-substrates very efficiently. As this leads to a relative decrease in SOM mineralization for the late stages of soil development, it is questionable if higher plants can improve their nutrition by stimulating free living soil microbes with root exudates or if they rather have to rely on mycorrhiza

Biogenic weathering bridges the nutrient gap in pristine ecosystems - a global comparison

In many pristine ecosystems there seems to be negative nutrient budget existent, meaning that export exceeds the input received by aeolian deposition and physico-chemical weathering. Such ecosystems should degrade rather quickly, but are often found surprisingly stable on the long run. Our hypothesis was that this nutrient gap is an artefact caused by not considering the contribution of photoassimilatory-mediated biogenic weathering to the overall nutrient input, which might constitute an additional, energetically directed and demand driven pathway. Here, we firstly evaluated the evolution of mutualistic biogenic weathering along an Antarctic chronosequence and secondly compared the biogenic weathering rates under mycorrhized ecosystems over a global gradient of contrasting states of soil development. We found the ability to perform biogenic weathering increasing along its evolutionary development in photoautotroph-symbiont interaction and furthermore a close relation between fungal biogenic weathering and available potassium across all 16 forested sites in the study, regardless of the dominant mycorrhiza type (AM or EM), climate, and plant-species composition. Our results point towards a general alleviation of nutrient limitation at ecosystem scale via directional, energy driven and on-demand biogenic weathering

Variation in Soil Respiration across Soil and Vegetation Types in an Alpine Valley.

BACKGROUND AND AIMS: Soils of mountain regions and their associated plant communities are highly diverse over short spatial scales due to the heterogeneity of geological substrates and highly dynamic geomorphic processes. The consequences of this heterogeneity for biogeochemical transfers, however, remain poorly documented. The objective of this study was to quantify the variability of soil-surface carbon dioxide efflux, known as soil respiration (Rs), across soil and vegetation types in an Alpine valley. To this aim, we measured Rs rates during the peak and late growing season (July-October) in 48 plots located in pastoral areas of a small valley of the Swiss Alps. FINDINGS: Four herbaceous vegetation types were identified, three corresponding to different stages of primary succession (Petasition paradoxi in pioneer conditions, Seslerion in more advanced stages and Poion alpinae replacing the climactic forests), as well as one (Rumicion alpinae) corresponding to eutrophic grasslands in intensively grazed areas. Soils were developed on calcareous alluvial and colluvial fan deposits and were classified into six types including three Fluvisols grades and three Cambisols grades. Plant and soil types had a high level of co-occurrence. The strongest predictor of Rs was soil temperature, yet we detected additional explanatory power of sampling month, showing that temporal variation was not entirely reducible to variations in temperature. Vegetation and soil types were also major determinants of Rs. During the warmest month (August), Rs rates varied by over a factor three between soil and vegetation types, ranging from 2.5 μmol m-2 s-1 in pioneer environments (Petasition on Very Young Fluvisols) to 8.5 μmol m-2 s-1 in differentiated soils supporting nitrophilous species (Rumicion on Calcaric Cambisols). CONCLUSIONS: Overall, this study provides quantitative estimates of spatial and temporal variability in Rs in the mountain environment, and demonstrates that estimations of soil carbon efflux at the watershed scale in complex geomorphic terrain have to account for soil and vegetation heterogeneity

Pasture degradation modifies the water and carbon cycles of the Tibetan highlands

© Author(s) 2014. The Tibetan Plateau has a significant role with regard to atmospheric circulation and the monsoon in particular. Changes between a closed plant cover and open bare soil are one of the striking effects of land use degradation observed with unsustainable range management or climate change, but experiments investigating changes of surface properties and processes together with atmospheric feedbacks are rare and have not been undertaken in the world's two largest alpine ecosystems, the alpine steppe and the Kobresia pygmaea pastures of the Tibetan Plateau. We connected measurements of micro-lysimeter, chamber, 13C labelling, and eddy covariance and combined the observations with land surface and atmospheric models, adapted to the highland conditions. This allowed us to analyse how three degradation stages affect the water and carbon cycle of pastures on the landscape scale within the core region of the Kobresia pygmaea ecosystem. The study revealed that increasing degradation of the Kobresia turf affects carbon allocation and strongly reduces the carbon uptake, compromising the function of Kobresia pastures as a carbon sink. Pasture degradation leads to a shift from transpiration to evaporation while a change in the sum of evapotranspiration over a longer period cannot be confirmed. The results show an earlier onset of convection and cloud generation, likely triggered by a shift in evapotranspiration timing when dominated by evaporation. Consequently, precipitation starts earlier and clouds decrease the incoming solar radiation. In summary, the changes in surface properties by pasture degradation found on the highland have a significant influence on larger scales

Spring in the boreal environment: observations on pre- and post-melt energy and CO₂ fluxes in two central Siberian ecosystems

A range of observations points towards earlier onset of spring in northern high latitudes. However, despite the profound effects this may have on vegetation-atmosphere exchange of carbon (NEE), vegetation-atmosphere physical coupling, or the location of the tundra-taiga interface, the number of studies that investigate winter-spring transition fluxes in contrasting northern vegetation types is limited. Here, we examine spring ecosystem-atmosphere energy and carbon exchange in a Siberian pine forest and mire. Divergent surface albedo before and during snow-melt resulted in daytime net radiation (R-n) above the forest exceeding R. above the mire by up to 10 MJ m(-2). Until stomata could open, absorbed radiation by the green pine canopy caused substantial daytime sensible heat fluxes (H > 10 MJ m(-2)). H above the mire was very low, even negative (<-2 MJ M-2), during that same period. Physiological activity in both ecosystems responded rapidly to warming temperatures and snow-melt, which is essential for survival in Siberia with its very short summers. On days with above-zero temperatures, before melt. was complete, low rates of forest photosynthesis (1-2 mu mol m(-2) s(-1)) were discernible. Forest and mire NEE became negative the same day, or shortly after, photosynthesis commenced. The mire lagged by about two weeks behind the forest and regained its full carbon uptake capacity at a slower rate. Our data provide empirical evidence for the importance the timing of spring and the relative proportion of forest vs. mire has for late winter/spring boundary-layer growth, and production and surface-atmosphere mixing of trace gases. Models that seek to investigate effects of increasingly earlier spring in high latitudes must correctly account for contrasting physical and biogeochemical ecosystem-atmosphere exchange in heterogeneous landscapes. [References: 79

Organic carbon and total nitrogen variability in permafrost-affected soils in a forest tundra ecotone

Soils of the high latitudes are expected to respond sensitively to climate change, but still little is known about carbon and nitrogen variability in them. We investigated the 0.44-km(2) Little Grawijka Creek catchment of the forest tundra ecotone (northern Krasnoyarsk Krai, Russian Federation) in order (i) to relate the active-layer thickness to controlling environmental factors, (ii) to quantify soil organic carbon (SOC) and total nitrogen (NT) stocks, and (iii) to assess their variability with respect to different landscape units. The catchment was mapped on a 50 x 50 m grid for topography, dominant tree and ground vegetation, organic-layer and moss-layer thickness, and active-layer thickness. At each grid point, bulk density, and SOC and NT concentrations were determined for depth increments. At three selected plots, 2-m deep soil cores were taken and analysed for SOC, NT and C-14. A shallow active layer was found in intact raised bogs at plateaux situations and in mineral soils of north-northeast (NNE) aspect. Good drainage and greater solar insolation on the south-southwest (SSW) slopes are reflected in deeper active layers or lack of permafrost. Organic carbon stocks to a soil depth of 90 cm varied between 5 and 95 kg m(-2). The greatest stocks were found in the intact raised bogs and on the NNE slopes. Canonical correspondence analysis indicates the dominant role of active-layer thickness for SOC and NT storage. The 2-m soil cores suggest that permafrost soils store about the same amount of SOC from 90 to 200 cm as in the upper 90 cm. Most of this deep SOC pool was formed in the mid-Holocene (organic soils) and the late Pleistocene (mineral soils). Our results showed that even within a small catchment of the forest tundra, active-layer thickness and, hence, SOC and NT storage vary greatly within the landscape mosaic. This has to be taken into account when using upscaling methods such as remote sensing for assessing SOC and NT storage and cycling at a regional to continental level. [References: 44

Organic matter dynamics along a salinity gradient in Siberian steppe soils

Salt-affected soils will become more frequent in the next decades as arid and semiarid ecosystems are predicted to expand as a result of climate change. Nevertheless, little is known about organic matter (OM) dynamics in these soils, though OM is crucial for soil fertility and represents an important carbon sink. We aimed at investigating OM dynamics along a salinity and sodicity gradient in the soils of the southwestern Siberian Kulunda steppe (Kastanozem, non-sodic Solonchak, Sodic Solonchak) by assessing the organic carbon (OC) stocks, the quantity and quality of particulate and mineral-associated OM in terms of non-cellulosic neutral sugar contents and carbon isotopes (δ13C, 14C activity), and the microbial community composition based on phospholipid fatty acid (PLFA) patterns. Aboveground biomass was measured as a proxy for plant growth and soil OC inputs. Our hypotheses were that (i) soil OC stocks decrease along the salinity gradient, (ii) the proportion and stability of particulate OM is larger in salt-affected Solonchaks compared to non-salt-affected Kastanozems, (iii) sodicity reduces the proportion and stability of mineral-associated OM, and (iv) the fungi : bacteria ratio is negatively correlated with salinity. Against our first hypothesis, OC stocks increased along the salinity gradient with the most pronounced differences between topsoils. In contrast to our second hypothesis, the proportion of particulate OM was unaffected by salinity, thereby accounting for only  &lt;  10 % in all three soil types, while mineral-associated OM contributed  &gt;  90 %. Isotopic data (δ13C, 14C activity) and neutral sugars in the OM fractions indicated a comparable degree of OM transformation along the salinity gradient and that particulate OM was not more persistent under saline conditions. Our third hypothesis was also rejected, as Sodic Solonchaks contained more than twice as much mineral-bound OC than the Kastanozems, which we ascribe to the flocculation of OM and mineral components under higher ionic strength conditions. Contrary to the fourth hypothesis, the fungi : bacteria ratio in the topsoils remained fairly constant along the salinity gradient. A possible explanation for why our hypotheses were not affirmed is that soil moisture covaried with salinity along the transect, i.e., the Solonchaks were generally wetter than the Kastanozems. This might cause comparable water stress conditions for plants and microorganisms, either due to a low osmotic or a low matric potential and resulting in (i) similar plant growth and hence soil OC inputs along the transect, (ii) a comparable persistence of particulate OM, and (iii) unaffected fungi : bacteria ratios. We conclude that salt-affected soils contribute significantly to the OC storage in the semiarid soils of the Kulunda steppe, while most of the OC is associated with minerals and is therefore effectively sequestered in the long term