Plant seasonal cycles alter carbon (C) and nitrogen (N) availability for soil microbes, which may affect microbial community composition and thus feed back on microbial decomposition of soil organic material and plant N availability. The temporal dynamics of these plant–soil interactions are, however, unclear.Here, we experimentally manipulated the C and N availability in a beech forest through N fertilization or tree girdling and conducted a detailed analysis of the seasonal pattern of microbial community composition and decomposition processes over 2 yr.We found a strong relationship between microbial community composition and enzyme activities over the seasonal course. Phenoloxidase and peroxidase activities were highest during late summer, whereas cellulase and protease peaked in late autumn. Girdling, and thus loss of mycorrhiza, resulted in an increase in soil organic matter-degrading enzymes and a decrease in cellulase and protease activity.Temporal changes in enzyme activities suggest a switch of the main substrate for decomposition between summer (soil organic matter) and autumn (plant litter). Our results indicate that ectomycorrhizal fungi are possibly involved in autumn cellulase and protease activity. Our study shows that, through belowground C allocation, trees significantly alter soil microbial communities, which may affect seasonal patterns of decomposition processes.

h a u fl e r a , B . K i t z l e r a , A . S c h i n d l b a c h e r a , U . S k i b a b , M . A . S u t t o n b & S . Z e c h m e i s t e r -B o l t e n s t e r n a SummaryIn order to estimate potential greenhouse gas flux rates from soils under different land use and climate, and to particularly assess the influence of soil temperature and soil moisture, we measured fluxes of nitrous oxide (N 2 O), nitric oxide (NO), carbon dioxide (CO 2 ) and methane (CH 4 ) from intact soil cores obtained from 13 European sites under controlled laboratory conditions. The soils covered the different climates of Europe and included four different land-use types: croplands, forests, grasslands and wetlands. In a twofactorial experimental design, the soil cores were incubated under four temperatures (5-20 • C) and water contents (20-80% water-filled pore space). We found a non-linear increase of N 2 O, NO and CO 2 emissions with increasing temperature. Nitrous oxide emissions were positively correlated with soil moisture, while NO emission and CH 4 oxidation rates were negatively correlated with soil moisture. Maximum CO 2 emissions occurred at intermediate soil moisture. Different land-use types strongly affected greenhouse gas fluxes. Nitrous oxide and CO 2 emissions were highest in grassland soils, while NO emissions were highest in forest soils. In grasslands, high soil microbial activity stimulated by high carbon (C) and nitrogen (N) contents, dense root systems and high C input from aboveground decaying biomass was the most likely cause for high N 2 O and CO 2 emissions. High NO emissions from forest soils were mainly attributed to low pH and high soil porosity. Northern soils showed the greatest capacity to take up CH 4 under warmer and dryer soil conditions. Nitric oxide emissions were positively correlated with N input.

Soil emissions are largely responsible for the increase of the potent greenhouse gas nitrous oxide (N 2 O) in the atmosphere and are generally attributed to the activity of nitrifying and denitrifying bacteria. However, the contribution of the recently discovered ammonia-oxidizing archaea (AOA) to N 2 O production from soil is unclear as is the mechanism by which they produce it. Here we investigate the potential of Nitrososphaera viennensis, the first pure culture of AOA from soil, to produce N 2 O and compare its activity with that of a marine AOA and an ammonia-oxidizing bacterium (AOB) from soil. N. viennensis produced N 2 O at a maximum yield of 0.09% N 2 O per molecule of nitrite under oxic growth conditions. N 2 O production rates of 4.6 ± 0.6 amol N 2 O celland nitrification rates of 2.6 ± 0.5 fmol NO 2 À cell À 1 h À 1 were in the same range as those of the AOB Nitrosospira multiformis and the marine AOA Nitrosopumilus maritimus grown under comparable conditions. In contrast to AOB, however, N 2 O production of the two archaeal strains did not increase when the oxygen concentration was reduced, suggesting that they are not capable of denitrification. In 15 N-labeling experiments we provide evidence that both ammonium and nitrite contribute equally via hybrid N 2 O formation to the N 2 O produced by N. viennensis under all conditions tested. Our results suggest that archaea may contribute to N 2 O production in terrestrial ecosystems, however, they are not capable of nitrifier-denitrification and thus do not produce increasing amounts of the greenhouse gas when oxygen becomes limiting.

Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO2 (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.

Abstract. Soil emissions of NO and N 2 O were measured continuously at high frequency for more than one year at 15 European forest sites as part of the EU-funded project NOFRETETE. The locations represent different forest types (coniferous/deciduous) and different nitrogen loads. Geographically they range from Finland in the north to Italy in the south and from Hungary in the east to Scotland in the west.The highest NO emissions were observed from coniferous forests, whereas the lowest NO emissions were observed from deciduous forests. The NO emissions from coniferous forests were highly correlated with N-deposition. The site with the highest average annual emission (82 µg NO-N m −2 h −1 ) was a spruce forest in South-Germany (Höglwald) receiving an annual N-deposition of 2.9 g m −2 . NO emissions close to the detection limit were observed from a pine forest in Finland where the N-deposition was 0.2 g N m −2 a −1 . No significant correlation between N 2 O emission and N-deposition was found. The highest average annual N 2 O emission (20 µg N 2 O-N m −2 h −1 ) was found in an oak forest in the Mátra mountains (Hungary) receiving an annual N-deposition of 1.6 g m −2 . N 2 O emission was significantly negatively correlated with the C/N ratio.The difference in N-oxide emissions from soils of coniferous and deciduous forests may partly be explained by differences in N-deposition rates and partly by differences in characteristics of the litter layer and soil. NO was mainlyCorrespondence to: K. Pilegaard (kim.pilegaard@risoe.dk) derived from nitrification whereas N 2 O was mainly derived from denitrification. In general, soil moisture is lower at coniferous sites (at least during spring time) and the litter layer of coniferous forests is thick and well aerated favouring nitrification and thus release of NO. Conversely, the higher rates of denitrification in deciduous forests due to a compact and moist litter layer lead to N 2 O production and NO consumption in the soil.The two factors soil moisture and soil temperature are often explaining most of the temporal variation within a site. When comparing annual emissions on a regional scale, however, factors such as nitrogen deposition and forest and soil type become much more important.