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Abstract: Global warming in the Arctic may alter decomposition rates in Arctic soils and therefore nutrient availability. In addition, changes in the length of the growing season may increase plant productivity and the rate of labile C input below ground. We carried out an experiment in which inorganic nutrients (NH4NO3 and NaPO4) and organic substrates (glucose and glycine) were added to soils sampled from across the mountain birch forest-tundra heath ecotone in northern Sweden (organic and mineral soils from the forest, and organic soil only from the heath). Carbon dioxide production was then monitored continuously over the following 19 days. Neither inorganic N nor P additions substantially affected soil respiration rates when added separately. However, combined N and P additions stimulated microbial activity, with the response being greatest in the birch forest mineral soil (57% increase in CO2 production compared with 26% in the heath soil and 8% in the birch forest organic soil). Therefore, mineralisation rates in these soils may be stimulated if the overall nutrient availability to microbes increases in response to global change, but N deposition alone is unlikely to enhance decomposition. Adding either, or both, glucose and glycine increased microbial respiration. Isotopic separation indicated that the mineralisation of native soil organic matter (SOM) was stimulated by glucose addition in the heath soil and the forest mineral soil, but not in the forest organic soil. These positive ‘priming’ effects were lost following N addition in forest mineral soil, and following both N and P additions in the heath soil. In order to meet enhanced microbial nutrient demand, increased inputs of labile C from plants could stimulate the mineralisation of SOM, with the soil C stocks in the tundra-heath potentially most vulnerable. [Copyright &y& Elsevier]

Abstract: Radiocarbon analysis of soil CO2 can provide information on the age, source and turnover rate of soil organic C. We developed a new method for passively trapping respired CO2 on molecular sieve, allowing it to be returned to the laboratory and recovered for C isotope analysis. We tested the method on a soil at a grassland site, and using a synthetic soil created to provide a contrasting isotopic signature. As with other passive sampling techniques, a small amount of fractionation of the 13C isotope occurs during sampling, which we have quantified, otherwise the results show that the molecular sieve traps a sufficiently large and representative sample of CO2 for C isotope analysis. Since 14C results are routinely corrected for mass-dependent fractionation, our results show that passive sampling of soil respiration using molecular sieve offers a reliable method to collect soil-respired CO2 for 14C analysis. [Copyright &y& Elsevier]

Respiratory denitrification is controlled by a number of environmental and soil factors including pH, but some of the interactions between pH and denitrification in soil are still uncertain. The objective of the present study was to assess the relationship between natural soil pH and denitrifying enzyme activity (DEA), to investigate the impact of two different techniques used for determining of potential denitrification, namely short-term DEA and the long-term denitrification potential (DP), on resulting pH optimum, especially with respect to the length of measurement, and to elucidate the effect of soil pH on the nature of denitrification products (N2O, N2). For this purpose, five mineral soils similar in a texture but differing in pH, were investigated. In addition to examining the soils in their natural pH values, pHs were adjusted by amendment of sulphuric acid or sodium hydroxide solutions prior to DEA or DP determination. The study showed that there was no simple relationship between DEA and soil pH, as very high DEA was found both in acid and alkaline soil. The study also indicated that the populations of soil denitrifiers were adapted to prevailing natural soil pH in the sense that they exhibited the highest DEA at or near natural soil pH (in experiments where the pH was adjusted to values from about 4 to 11 before DEA determination). Suppression of nitrate reductase activity by introducing nitrite instead of nitrate as an electron acceptor had no effect on this interaction between the DEA and soil pH. If the soil incubation was prolonged under optimum conditions for denitrification (surplus of nitrate or nitrite and available carbon, very low partial pressure of O2) and DP was estimated, the optimum pH for evolution of denitrification products was shifted towards neutrality. It is proposed that this is either due to the development of a community of denitrifiers which can grow better at the neutral pH or due to accommodation of the existing populations to new conditions. The shift in pH optimum occurred after about 12 h, but it took about 1–2 days to fully appear. In agreement with previous studies, it was confirmed that at pH above 7, N2 is much more important denitrification product than N2O. The study also suggests that the expressions like optimum pH for denitrification should be avoided, as different denitrification characteristics obtained using different methodology can be quite differently related to the soil reaction. Such general expression should be specified by introducing the particular denitrification parameters to which it is being applied. [Copyright &y& Elsevier]

Abstract: In arctic ecosystems, winter soil respiration can contribute substantially to annual CO2 release, yet the source of this C is not clear. We analysed the 14C content of C released from plant-free plots in mountain birch forest and tundra-heath. Winter-respired CO2 was found to be a similar age (tundra) or older (forest) than C released during the previous autumn. Overall, our study demonstrates that the decomposition of older C can continue during the winter, in these two contrasting arctic ecosystems. [Copyright &y& Elsevier]

Abstract: Quantifying the potential for eroding agricultural soils to act as sinks or sources of atmospheric carbon relies on accounting for the pools and fluxes of soil organic carbon (SOC) and nutrients, e.g. nitrogen (N), affected by erosion. Herein, we report the outcomes of an experiment where a C4 maize (Zea mays) crop (δ 13C = −12.1‰) was cultivated and incorporated for 2 years to introduce a ‘pulse’ of 13C-enriched SOC to a C3 arable soil (δ 13C = −27.4‰). Soils were sampled at eroding (top slope and upper slope) and depositional (lower slope and slope foot) positions of an accelerated erosion pathway that were confirmed using 137Cs measurements. The sand particle-sized fraction (63–2000 μm) was predominant and increased in the depositional slope positions due to selective loss of fine particles and preferential deposition of the coarsest fraction of transported sediment. There was a significant isometric relationship between the percentage SOC and total N: top slope > upper slope > lower slope, with similar values in the slope foot to the top slope. The δ 15N values of the soils were enriched (7.3‰) at the slope foot, compared with the other slope positions (average 6.3‰), suggesting increased denitrification rates. The δ 13C values of the soil microbial biomass C extracted from surface soils (0–5 cm) at each slope position showed that the proportion of maize C being incorporated into the soil microbial biomass declined in the downslope direction from 54% (top slope) to 43% (upper slope) to 18% (lower slope) in inverse proportion to the size of the soil microbial biomass, and increased to 41% at the slope foot. This suggests dynamic replacement of the SOC with crop C in the eroding slope positions and dilution of the transported C by C3-SOC in the depositional slope positions. This paper is evidence that erosional distribution of soil carbon leads to differential microbial utilisation of SOC between eroding and depositional sites. [Copyright &y& Elsevier]

Abstract: We investigated the effects of increased soil temperature, water and nutrient availability on soil bacterial communities at Wynn Knolls on Signy Island (60 °S) in the northern maritime Antarctic and at Mars Oasis (71 °S) in the southern maritime Antarctic. After 10–12 months, analyses of the concentrations of ester linked fatty acids (ELFAs) in soil indicated that bacterial communities responded positively to single applications of substrates at both locations, with 20% and 49% increases in total Gram positive and Gram negative bacterial markers, respectively, in response to the application of tryptic soy broth (TSB; a complex substrate containing organic carbon and nitrogen, plus other nutrient elements) at Wynn Knolls, and 120% and 44% increases in Gram positive bacterial markers at Mars Oasis in response to the application of TSB and the amino acid glycine (a relatively simple source of organic carbon and nitrogen), respectively. Responses to the warming treatment were not detected at Wynn Knolls, where open top chambers (OTCs) increased mean monthly soil temperatures by up to 0.7 °C, but at Mars Oasis, where OTCs increased monthly soil temperatures by up to 2.4 °C, warming led to 41% and 46% reductions in the concentrations of Gram positive bacterial markers in soil to which glycine and TSB had been applied, respectively. Warming also led to 55% and 51% reductions in the ratio of Gram positive to Gram negative markers in soils at Mars Oasis to which glycine and TSB had been applied. These data suggest that warming may constrain the responses of bacterial communities to carbon and nitrogen inputs arising from dead plant matter entering maritime Antarctic soils in future decades. [Copyright &y& Elsevier]

Abstract: Most nitrogen (N) enters many Arctic and Antarctic soil ecosystems as protein. Soils in these polar environments frequently contain large stocks of proteinaceous organic matter, which has decomposed slowly due to low temperatures. In addition to proteins, considerable quantities of d-amino acids and their peptides enter soil from bacteria and lengthy residence times can lead to racemisation of l-amino acids in stored proteins. It has been predicted that climate warming in polar environments will lead to increased rates of soil organic N turnover (i.e. amino acids and peptides of both enantiomers). However, our understanding of organic N breakdown in these soils is very limited. To address this, we tested the influence of chain length and enantiomeric composition on the rate of breakdown of amino acids and peptides in three contrasting tundra soils formed under the grass, moss or lichen-dominated primary producer communities of Signy Island in the South Orkney Islands. Both d- and l-enantiomers of the amino acid monomer were rapidly mineralized to CO2 at rates in line with those found for l-amino acids in many other terrestrial ecosystems. In all three soils, l-peptides were decomposed faster than their amino acid monomer, suggesting a different route of microbial assimilation and catabolism. d-peptides followed the same mineralization pattern as l-peptides in the two contrasting soils under grass and lichens, but underwent relatively slow decomposition in the soil underneath moss, which was similar to the soil under the grass. We conclude that the decomposition of peptides of l-amino acids may be widely conserved amongst soil microorganisms, whereas the decomposition of peptides of d-amino acids may be altered by subtle differences between soils. We further conclude that intense competition exists in soil microbial communities for the capture of both peptides and amino acids produced from protein breakdown. [Copyright &y& Elsevier]

The relationships between the fluxes of nitrous oxide (N2O) and carbon dioxide (CO2), and their concentrations in the soil air, three different measures of potential denitrification, soil moisture, soil temperature and precipitation were investigated in soils from beneath ryegrass (Lolium multiflorum Lam.), red clover (Trifolium pratense L.) and mixture of ryegrass–red clover stands on a gleic cambisol. Investigations were carried out in order to test the hypothesis that the measure(s) of potential denitrification are good predictor(s) of N2O fluxes and thus may be used in empirical models of N2O emission. Potential denitrification characteristics used in this study involved (i) short-term denitrifying enzyme activity (DEA), (ii) long-term denitrification potential (DP), both determined in soils amended with nitrate and glucose, and (iii) denitrification rate (DR) measured using intact soil cores. Flux measurements were made using cylindrical chambers (internal diameter 31 cm, volume 0.015 m3). The fluxes of N2O and CO2 and many other characteristics showed large spatial and temporal variability. Emissions of N2O from the grass plots were closely related to N2O concentrations in the soil atmosphere at 22.5 cm depth. Most soil properties did not correlate with N2O fluxes. It was concluded that DP was not a good predictor for N2O flux. DEA did not show significant relationship with N2O flux, but it is suggested that if determined in representative, large soil samples, DEA could be a predictor of N2O fluxes; this assumption needs, however, verification. The only potential denitrification characteristic which was significantly related to N2O emission both in grass and clover treatments was DR, which was determined in soil cores. [Copyright &y& Elsevier]