Explaining the large-scale diversity of soil organisms that drive biogeochemical processes-and their responses to environmental change-is critical. However, identifying consistent drivers of belowground diversity and abundance for some soil organisms at large spatial scales remains problematic. Here we investigate a major guild, the ectomycorrhizal fungi, across European forests at a spatial scale and resolution that is-to our knowledge-unprecedented, to explore key biotic and abiotic predictors of ectomycorrhizal diversity and to identify dominant responses and thresholds for change across complex environmental gradients. We show the effect of 38 host, environment, climate and geographical variables on ectomycorrhizal diversity, and define thresholds of community change for key variables. We quantify host specificity and reveal plasticity in functional traits involved in soil foraging across gradients. We conclude that environmental and host factors explain most of the variation in ectomycorrhizal diversity, that the environmental thresholds used as major ecosystem assessment tools need adjustment and that the importance of belowground specificity and plasticity has previously been underappreciated.

Phosphorus availability may shape plantmicroorganism-soil interactions in forest ecosystems. Our aim was to quantify the interactions between soil P availability and P nutrition strategies of European beech (Fagus sylvatica) forests. We assumed that plants and microorganisms of P-rich forests carry over mineral-bound P into the biogeochemical P cycle (acquiring strategy). In contrast, P-poor ecosystems establish tight P cycles to sustain their P demand (recycling strategy). We tested if this conceptual model on supply-controlled P nutrition strategies was consistent with data from five European beech forest ecosystems with different parent materials (geosequence), covering a wide range of total soil P stocks (160-900 g P m -2 ; \1 m depth). We analyzed numerous soil chemical and biological properties. Especially P-rich beech ecosystems accumulated P in topsoil horizons in moderately labile forms. Forest floor turnover rates decreased with decreasing total P stocks (from 1/5 to 1/40 per year) while ratios between organic carbon and organic phosphorus (C:P org ) increased from 110 to 984 (A horizons). High proportions of fine-root biomass in forest floors seemed to favor tight P recycling. Phosphorus in fine-root biomass increased relative to microbial P with decreasing P stocks. Concomitantly, phosphodiesterase activity decreased, which might explain increasing proportions of diester-P remaining in the soil organic matter. With decreasing P supply indicator values for P acquisition decreased and those for recycling increased, implying adjustment of plantmicroorganism-soil feedbacks to soil P availability. Intense recycling improves the P use efficiency of beech forests.

Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.

Fine root distribution and turnover were investigated in ca. 40-year-old pure Norway spruce (Picea abies Karst.) stands in Germany, growing on four sites that differed in soil acidity (Ebergötzen < Barbis < Fichtelgebirge = Harz). The density of fine root biomass and necromass in different soil horizons differed among the sites. At one of the most acidic sites (Harz), fine root density in the humus layer was more than twice that at the least acidic site (Ebergötzen). At the two most acidic sites, Fichtelgebirge and Harz, the ratio of biomass to necromass was significantly lower than at Ebergötzen and Barbis, particularly in the subsoil layer. In each stand, clear vertical gradients in fine root length density and root tip density were observed. Most of the roots and the root tips were in the humus layer and in the first mineral soil horizon (0-10 cm). There was a significantly different decrease in specific root length (cm gDM (-1)) and specific root tip density (root tips gDM (-1)) in the more acidified stands Fichtelgebirge and Harz compared with Ebergötzen and Barbis. Fine root production estimated by ingrowth cores and a net method was approximately twice as high in the more acidic stands Fichtelgebirge and Harz compared with Ebergötzen and Barbis. Rates of living fine root biomass turnover were higher at the Fichtelgebirge and Harz sites than at the Ebergötzen site. Rates of necromass turnover were similar at all sites. The results suggest that the accumulation of necromass was not due to a slower disappearance at the more acid sites, but to earlier root death. Roots contributed 46% to root + needle litter and 32% to root + total aboveground litter at the Harz site in 1997.

Local biodiversity trends over time are likely to be decoupled from global trends, as local processes may compensate or counteract global change. We analyze 161 long-term biological time series (15-91 years) collected across Europe, using a comprehensive dataset comprising 6,200 marine, freshwater and terrestrial taxa. We test whether (i) local long-term biodiversity trends are consistent among biogeoregions, realms and taxonomic groups, and (ii) changes in biodiversity correlate with regional climate and local conditions. Our results reveal that local trends of abundance, richness and diversity differ among biogeoregions, realms and taxonomic groups, demonstrating that biodiversity changes at local scale are often complex and cannot be easily generalized. However, we find increases in richness and abundance with increasing temperature and naturalness as well as a clear spatial pattern in changes in community composition (i.e. temporal taxonomic turnover) in most biogeoregions of Northern and Eastern Europe.

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Dissolved organic carbon (DOC) concentrations have risen in upland waters across large areas of Europe and North America. Two proposed drivers of these increases are (1) deposition of atmospheric pollutant nitrogen (N) with consequent effects on plant and decomposer carbon dynamics, and (2) soil recovery from acidification associated with decreasing sulphur deposition. Examination of 12 European and North American field N addition experiments showed inconsistent (positive, neutral, and negative) responses of DOC to N addition. However, responses were linked to the form of N added and to resulting changes in soil acidity. Sodium nitrate additions consistently increased DOC, whereas ammonium salts additions usually decreased DOC. Leachate chemistry was used to calculate an index of ''ANC forcing'' of the effect of fertilization on the acid-base balance, which showed that DOC increased in response to all de-acidifying N additions, and decreased in response to all but three acidifying N additions. Exceptions occurred at two sites where N additions caused tree mortality, and one experiment located on an older, unglaciated soil with high anion adsorption capacity. We conclude that collectively these experiments do not provide clear support for the role of N deposition as the sole driver of rising DOC, but are largely consistent with an acidity-change mechanism. It is however possible that the unintended effect of acidity change on DOC mobility masks genuine effects of experimental N enrichment on DOC production and degradation. We suggest that there is a need, more generally, for interpretation of N manipulation experiments to take account of the effects that experimentally-induced changes in acidity, rather than elevated N per se, may have on ecosystem biogeochemistry.

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