New techniques have identified a wide range of organisms with the capacity to carry out biological nitrogen fixation (BNF)—greatly expanding our appreciation of the diversity and ubiquity of N fixers—but our understanding of the rates and controls of BNF at ecosystem and global scales has not advanced at the same pace. Nevertheless, determining rates and controls of BNF is crucial to placing anthropogenic changes to the N cycle in context, and to understanding, predicting and managing many aspects of global environmental change. Here, we estimate terrestrial BNF for a pre-industrial world by combining information on N fluxes with 15 N relative abundance data for terrestrial ecosystems. Our estimate is that pre-industrial N fixation was 58 (range of 40–100) Tg N fixed yr −1 ; adding conservative assumptions for geological N reduces our best estimate to 44 Tg N yr −1 . This approach yields substantially lower estimates than most recent calculations; it suggests that the magnitude of human alternation of the N cycle is substantially larger than has been assumed.

The intermittent upwelling hypothesis (IUH) predicts that the strength of ecological subsidies, organismal growth responses, and species interactions will vary unimodally along a gradient of upwelling from persistent downwelling to persistent upwelling, with maximal levels at an intermediate or “intermittent” state of upwelling. To test this model, we employed the comparative‐experimental method to investigate these processes at 16–44 wave‐exposed rocky intertidal sites in Oregon, California, and New Zealand, varying in average upwelling and/or downwelling during spring–summer. As predicted by the IUH, ecological subsidies (phytoplankton abundance, prey recruitment rates), prey responses (barnacle colonization, mussel growth), and species interactions (competition rate, predation rate and effects) were unimodally related to upwelling. On average, unimodal relationships with upwelling magnitude explained ∼50% of the variance in the various processes, and unimodal and monotonic positive relationships against an index of intermittency explained ∼37% of the variance. Regressions among the ecological subsidies and species interactions were used to infer potential ecological linkages that underpinned these patterns. Abundance of phytoplankton was associated with increases in rates of barnacle colonization, intensity of competition and predation, and predation effects, and rates of barnacle recruitment were associated with increases in mussel growth, barnacle colonization, and species interactions. Positive effects on interactions were also seen for rates of colonization, competition, predation, and predation effects. Several responses were saturating or exponential, suggestive of threshold effects. These results suggest that the IUH has geographic generality and are also consistent with earlier arguments that bottom‐up effects and propagule subsidies are strongly linked to the dynamics of higher trophic levels, or top‐down effects, as well as to nontrophic interactions. The ∼50% of the variance not explained by upwelling is likely due to more regional‐to‐local influences on the processes examined, and future efforts should focus on incorporating such effects into the IUH.

The structure of ecological communities reflects a tension among forces that alter populations. Marine ecologists previously emphasized control by locally operating forces (predation, competition, and disturbance), but newer studies suggest that inputs from large-scale oceanographically modulated subsidies (nutrients, particulates, and propagules) can strongly influence community structure and dynamics. On New Zealand rocky shores, the magnitude of such subsidies differs profoundly between contrasting oceanographic regimes. Community structure, and particularly the pace of community dynamics, differ dramatically between intermittent upwelling regimes compared with relatively persistent downwelling regimes. We suggest that subsidy rates are a key determinant of the intensity of species interactions, and thus of structure in marine systems, and perhaps also nonmarine communities. Many ecological processes determine the structure and dynamics of communities and ecosystems. Theoretical and experimental advances have led to a growing awareness that different processes operate at different spatial and temporal scales (1, 2). Capturing the full richness of ecosystem dynamics thus requires studies ranging across a wide range of scales and allowing effective evaluation of the relative impact of the relevant factors. Doing so can be challenging. The logistics are daunting, especially in using the power of experimentation over large spatial scales. One solution, the ''comparative-experimental'' approach (3), involves replicated local-scale experimentation at multiple sites spanning larger scales, coupled with local-scale repeated sampling of factors that vary at characteristically larger scales.In its early development, marine community ecology focused on the dynamic consequences of primarily local-scale processes such as species interactions and physical disturbance (4). More recently, marine and nonmarine ecologists alike have documented the influence on communities of larger-scale phenomena including subsidies of materials and propagules transferred between adjacent ecosystems (5-8). Although evidence for the importance of subsidies is growing, questions remain about their impact, generality, magnitude, and interdependence and the physical and biotic mechanisms that underlie them.Here we use the comparative-experimental approach to address the role of large-scale oceanographic phenomena in structuring communities on New Zealand rocky shores. Based on earlier results (6, 9), we predicted that intertidal community structure and dynamics would reflect the coastal oceanographic regime. We hypothesized that the influences of oceanographically modulated subsidies (propagules, as a cause of increases in population density of benthic species, and the concentration of phytoplankton and detritus, as food for filter feeders) would be high with upwelling and low with downwelling. Study SystemPrevious research on the west and east coasts of the South Island of New Zealand (pairs of sites 100-500 m apart on each coast) revealed strik...

Symbiotic nitrogen (N) fixers are critical components of many terrestrial ecosystems. There is evidence that some N fixers fix N at the same rate regardless of environmental conditions (a strategy we call obligate), while others adjust N fixation to meet their needs (a strategy we call facultative). Although these strategies are likely to have qualitatively different impacts on their environment, the relative effectiveness and ecosystem-level impacts of each strategy have not been explored. Using a simple mathematical model, we determine the best facultative strategy and show that it excludes any obligate strategy (fixer or nonfixer) in our basic model. To provide an explanation for the existence of nonfixers and obligate fixers, we show that both costs of being facultative and time lags inherent in the process of N fixation can select against facultative N fixers and also produce the seemingly paradoxical patterns of sustained N limitation and N richness. Finally, we speculate on why the costs and lags may differ between temperate and tropical regions and thus whether they can explain patterns in both biomes simultaneously.

Biological nitrogen fixation (BNF) is the major nitrogen (N) input in many terrestrial ecosystems, yet we know little about the mechanisms and feedbacks that control this process in natural ecosystems. We here examine BNF in four taxonomically and ecologically different groups over the course of forest ecosystem development. At nine sites along the Franz Josef soil chronosequence (South Westland, New Zealand) that range in age from 7 to 120000 yr old, we quantified BNF from the symbiotic plant Coriaria arborea, cyanolichens (primarily Pseudocyphellaria spp.), bryophytes (many species), and heterotrophic bacteria in leaf litter. We specifically examined whether these groups could act as "nitrostats" at the ecosystem level, turning BNF on when N is scarce (early in primary succession) and off when N is plentiful (later in succession and retrogression). Coriaria was abundant and actively fixing (approximately 11 kg N x ha(-1) x yr(-1)) in the youngest and most N-poor site (7 yr old), consistent with nitrostat dynamics. Coriaria maintained high BNF rates independent of soil N availability, however, until it was excluded from the community after a single generation. We infer that Coriaria is an obligate N fixer and that the nitrostat feedback is mechanistically governed by species replacement at the community level, rather than down-regulation of BNF at the physiological scale. Biological nitrogen fixation inputs from lichens (means of 0-2 kg N x ha(-1) x yr(-1)), bryophytes (0.7-10 kg N x ha(-1) x yr(-1)), and litter (1-2 kg N x ha(-1) x yr(-1)) were driven primarily by changes in density, which peaked at intermediate-aged sites (and increased with soil N availability) for both lichens and bryophytes, and grew monotonically with soil age (but did not change with soil N) for litter. This non-nitrostatic link between soil N availability and lichen/bryophyte BNF likely stems from increased tree biomass in more fertile sites, which increases epiphytic moisture conditions and habitable surface area. This apparent positive feedback could produce N-rich conditions.

The rarity of symbiotic nitrogen-fixing trees in higher-latitude compared to lower-latitude forests is paradoxical because higher-latitude soils are relatively N poor. Using national-scale forest inventories from the United States and Mexico, we show that the latitudinal abundance distribution of N-fixing trees (more than 10 times less abundant poleward of 35 degrees N) coincides with a latitudinal transition in symbiotic N-fixation type: rhizobial N-fixing trees (which are typically facultative, regulating fixation to meet nutritional demand) dominate equatorward of 35 degrees N, whereas actinorhizal N-fixing trees (typically obligate, maintaining fixation regardless of soil nutrition) dominate to the north. We then use theoretical and statistical models to show that a latitudinal shift in N-fixation strategy (facultative vs. obligate) near 35 degrees N can explain the observed change in N-fixing tree abundance, even if N availability is lower at higher latitudes, because facultative fixation leads to much higher landscape-scale N-fixing tree abundance than obligate fixation.

Ecosystem carbon (C) balance is hypothesised to be sensitive to the mycorrhizal strategies that plants use to acquire nutrients. To test this idea, we coupled an optimality-based plant nitrogen (N) acquisition model with a microbe-focused soil organic matter (SOM) model. The model accurately predicted rhizosphere processes and C-N dynamics across a gradient of stands varying in their relative abundance of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) trees. When mycorrhizal dominance was switched - ECM trees dominating plots previously occupied by AM trees, and vice versa - legacy effects were apparent, with consequences for both C and N stocks in soil. Under elevated productivity, ECM trees enhanced decomposition more than AM trees via microbial priming of unprotected SOM. Collectively, our results show that ecosystem responses to global change may hinge on the balance between rhizosphere priming and SOM protection, and highlight the importance of dynamically linking plants and microbes in terrestrial biosphere models.

In the Jasper Ridge Global Change Experiment -an annual grassland with elevated carbon dioxide (CO 2 ), nitrate deposition, temperature, and precipitation -we used six indices of phosphorus (P) limitation to test the hypothesis that global changes that increase net primary production (NPP) increase P demand or limitation. All indices indicated that nitrate deposition, the only factor that stimulated NPP, increased P demand or limitation: (1) soil phosphatase activity increased by 14%; (2) P concentration in green and (3) senescent leaves of the dominant grass genus, Avena, dropped by 40% and 44%, respectively; (4) N : P ratios in green and (5) senescent Avena widened by 99% and 161%, respectively; and (6) total aboveground plant P decreased by 17% with elevated nitrate deposition. The other three factors, which did not stimulate NPP, did not increase P demand: based on two indices, enhanced precipitation decreased P demand (11% decrease in phosphatase activity, 19% increase in total aboveground P), and there was no evidence that elevated CO 2 or temperature altered P demand. In a metaanalysis to assess the generality of P constraints on growth increases from global change factors, we found that six of 11 N-limited ecosystems responded to N deposition with enhanced P limitation or demand, but did not detect significant effects of elevated CO 2 or warming.