No abstract

Belowground organisms play critical roles in maintaining multiple ecosystem processes, including plant productivity, decomposition, and nutrient cycling. Despite their importance, however, we have a limited understanding of how and why belowground biodiversity (bacteria, fungi, protists, and invertebrates) may change as soils develop over centuries to millennia (pedogenesis). Moreover, it is unclear whether belowground biodiversity changes during pedogenesis are similar to the patterns observed for aboveground plant diversity. Here we evaluated the roles of resource availability, nutrient stoichiometry, and soil abiotic factors in driving belowground biodiversity across 16 soil chronosequences (from centuries to millennia) spanning a wide range of globally distributed ecosystem types. Changes in belowground biodiversity during pedogenesis followed two main patterns. In lower-productivity ecosystems (i.e., drier and colder), increases in belowground biodiversity tracked increases in plant cover. In more productive ecosystems (i.e., wetter and warmer), increased acidification during pedogenesis was associated with declines in belowground biodiversity. Changes in the diversity of bacteria, fungi, protists, and invertebrates with pedogenesis were strongly and positively correlated worldwide, highlighting that belowground biodiversity shares similar ecological drivers as soils and ecosystems develop. In general, temporal changes in aboveground plant diversity and belowground biodiversity were not correlated, challenging the common perception that belowground biodiversity should follow similar patterns to those of plant diversity during ecosystem development. Taken together, our findings provide evidence that ecological patterns in belowground biodiversity are predictable across major globally distributed ecosystem types and suggest that shifts in plant cover and soil acidification during ecosystem development are associated with changes in belowground biodiversity over centuries to millennia.

Identifying the global drivers of soil priming is essential to understanding C cycling in terrestrial ecosystems. We conducted a survey of soils across 86 globally-distributed locations, spanning a wide range of climates, biotic communities, and soil conditions, and evaluated the apparent soil priming effect using 13 C-glucose labeling. Here we show that the magnitude of the positive apparent priming effect (increase in CO 2 release through accelerated microbial biomass turnover) was negatively associated with SOC content and microbial respiration rates. Our statistical modeling suggests that apparent priming effects tend to be negative in more mesic sites associated with higher SOC contents. In contrast, a single-input of labile C causes positive apparent priming effects in more arid locations with low SOC contents. Our results provide solid evidence that SOC content plays a critical role in regulating apparent priming effects, with important implications for the improvement of C cycling models under global change scenarios.

Summary1. Conceptual models of ecosystem development commonly predict a phase of initial colonization characterized by the nucleation, growth and coalescence of discrete patches of pioneer plants. Spatiotemporal dynamics during subsequent development may follow one of three different models: the classical model, in which initially discrete patches of competitive dominant (secondary) colonists coalesce to form a homogeneous cover; the patch dynamics model, in which renewal mechanisms such as disturbance create a shifting mosaic of patches at different stages; and the geoecological model, in which the vegetation gradually differentiates along edaphic gradients related to the underlying physical template. 2. These models of spatiotemporal dynamics were tested using vegetation and soil data from an 850-year chronosequence, comprised of seven lava flows on Mt Hekla, Iceland. The scale and intensity of spatial pattern were quantified on each flow using spatial analyses (mean-variance ratios, quadrat variance techniques and indices of autocorrelation). Changes in spatial pattern with increasing terrain age were compared with predicted trajectories, in order to identify which of the models of spatiotemporal dynamics was most consistent with the observations. 3. The early stages of ecosystem development were characterized by colonization of the pioneer species, especially Racomitrium mosses, in discrete patches ('Pioneer colonization stage', < 20 years), which then grew laterally and coalesced to form a continuous, homogeneous carpet ('Pioneer expansion stage', 20-100 years). Later in the sequence, higher plants established in discrete patches within this pioneer matrix ('Higher plant colonization stage', 100 -600 years). Over time, heterogeneity re-emerged at a larger spatial scale as the vegetation differentiated according to topographic variations in the underlying substrate ('Differentiation stage', > 600 years). 4. Synthesis . The spatiotemporal dynamics observed in the early stages of this succession were consistent with a model of pioneer nucleation in micro-scale safe sites, followed by growth, coalescence and eventual fragmentation of pioneer patches. The spatial patterns which emerged later in development support the geoecological model, with spatial differentiation of vegetation related to meso-scale substrate topography. The findings provide insight on how vegetation patterns emerge at different stages of ecosystem development in response to differing scales of heterogeneity in the underlying physical environment.

Article (refereed) -postprintCutler, Nick A.; Chaput, Dominique L.; van der Gast, Christopher J. 2014. Long-term changes in soil microbial communities during primary succession.Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. little is known about long-term (decades-centuries) structural changes in these communities. 22The development of aboveground-belowground linkages during century-scale succession is 23 also poorly understood. Our study addressed this knowledge gap by investigating SMC and 24 plant communities undergoing primary succession on an 850-year chronosequence of lava 25 flows in Iceland. We hypothesised that communities of microfungi and bacteria would 26 respond to progressive changes in vegetation and that SMC diversity would increase with 27 terrain age. Soil samples were collected from three lava flows at different stages of primary 28 succession (165, 621 and 852 years after lava flow emplacement). Plant community 29 composition was surveyed as the samples were collected. The composition of the SMCs 30 present in the soil was determined using amplicon pyrosequencing. The physical and 31 chemical properties of the soil were also analysed. The results of the study indicated 32 changes in plant and fungal communities with increasing terrain age. Distinct plant and 33 fungal assemblages were identified on the three sites and both communities became richer 34 and more diverse with increasing terrain age. There was also evidence to suggest the 35 development of mycorrhizal associations on older sites. In contrast, the composition and 36 structure of the bacterial communities did not change systematically with terrain age. 37Similarly, there were few changes in soil properties: SOM concentrations and pH, both of 38 which have been demonstrated to be important to SMCs, were constant across the 39 chronosequence. These results suggest that plant community composition is significant for 40 fungal communities, but less relevant for bacterial communities. This finding has implications 41 for studies of primary succession and the biogeochemical impact of vegetation change in 42 high-latitude ecosystems. 43 44