The global nitrogen cycle in the twenty-first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (N_r) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic N_r are on land (240 Tg N yr⁻¹) within soils and vegetation where reduced N_r contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer N_r contribute to nitrate (NO₃⁻) in drainage waters from agricultural land and emissions of trace N_r compounds to the atmosphere. Emissions, mainly of ammonia (NH₃) from land together with combustion related emissions of nitrogen oxides (NO_x), contribute 100 Tg N yr⁻¹ to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH₄NO₃) and ammonium sulfate (NH₄)₂SO₄. Leaching and riverine transport of NO₃ contribute 40–70 Tg N yr⁻¹ to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr⁻¹) to double the ocean processing of N_r. Some of the marine N_r is buried in sediments, the remainder being denitrified back to the atmosphere as N₂ or N₂O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of N_r in the atmosphere, with the exception of N₂O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 10²–10³ years), the lifetime is a few decades. In the ocean, the lifetime of N_r is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N₂O that will respond very slowly to control measures on the sources of N_r from which it is produced.     

Quantitative Analysis of Pedogenic Thresholds and Domains in Volcanic Soils

Ecosystems - Pedogenic thresholds describe where soil properties or processes change in an abrupt/nonlinear fashion in response to small changes in environmental forcing. Contrastingly, soil...     

Risky Business: Temporal and Spatial Variation in Preindustrial Dryland Agriculture

Traditional dryland agriculture in the Pacific island was often labor-intensive and risky, yet settlement and farming in dry areas played an important role in the development of Polynesian societies. We investigate how temporal and spatial climatic fluctuations shape variation in agricultural production across dryland landscapes. We use a model that couples plant growth, climate, and soil organic matter dynamics, together with data from Kohala, Hawai'i, to understand how temperature, rainfall, nitrogen availability, and cropping activity interact to determine yield dynamics through time and space. Due to these interactions, the statistical characterization of rainfall alone is a poor characterization of agricultural yield. Using a simple linear model of human population dynamics, we show that the observed yield variation can affect long-term population growth substantially. Our approach to analyzing spatial and temporal fluctuations in food supply, and to interpreting the population consequences of these fluctuations, provides a quantitative evaluation of agricultural risk and human carrying capacity in dry regions.     

Heritable variation in circulating glucocorticoids and endocrine flexibility in a free‐living songbird

Phenotypic flexibility is a central way that organisms cope with challenging and changing environments. As endocrine signals mediate many phenotypic traits, heritable variation in hormone levels, or their context‐dependent flexibility, could present an important target for selection. Several studies have estimated the heritability of circulating glucocorticoid levels under acute stress conditions, but little is known about the potential for either baseline hormone levels or rapid endocrine flexibility to evolve. Here, we assessed the potential for selection to operate on the elevation (circulating hormone levels) and flexibility of glucocorticoid reaction norms to acute restraint stress. Multivariate animal models revealed low but significant heritability in baseline (h² = 0.13–0.14) and stress‐induced glucocorticoids (h² = 0.18), and moderate heritability in glucocorticoid flexibility in response to acute stress (h² = 0.38) in free‐living juvenile tree swallows (Tachycineta bicolor; n = 408). Baseline glucocorticoids were not genetically correlated with either stress‐induced glucocorticoids or glucocorticoid flexibility. These findings indicate that baseline glucocorticoids and the acute stress response are distinct traits that can be independently shaped by selection. Microevolutionary changes that influence the expression or flexibility of these endocrine mediators of phenotype may be an important way that populations adapt to changing environments and novel threats.     

Organic matter and nitrogen accumulation and nitrogen fixation during early ecosystem development in Hawaii

We used a chronosequence comprised of 10 y, 52 y and 142 yold `a'a lava flows on Mauna Loa, Hawaii, to determine theaccumulation of organic matter and nitrogen and rates of nitrogenfixation through time. The mass of organic matter (live and deadbiomass and soil organic matter) on the 1984, 1942 and 1852 lavaflows was 0.6, 2.2 and 7.6 kg m^{– 2}, respectively, while total N was 4.8, 10.9 and 85.7 g m^{– 2}.We estimated the total rates of nitrogen fixation for thethree different aged ecosystems using an acetylene reduction assaycalibrated with ¹⁵N incubations. While mean rates of total N fixation remained largely constant across the three sites – between2.0 and 3.1 kg ha^{– 1} y^{– 1} – the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years ofecosystem development, the most important N fixer was a cyanoalga,while after 142 years, the predominant N fixers were heterotrophicbacteria associated with leaf litter, twigs and detritus. The totalamount of N accumulated after 52 years of ecosystem development wasequivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation couldonly account for between 27–59% of the total nitrogen accrued.We used fertilizer additions of all essential nutrients otherthan N to test whether the availability of lithophilic nutrientsregulated rates of N fixation in early ecosystem development. Ratesof nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter ofMetrosideros polymorpha also increased significantly when fertilized with lithophilic nutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems.     

Litter quality versus soil microbial community controls over decomposition: a quantitative analysis

The possible effects of soil microbial community structure on organic matter decomposition rates have been widely acknowledged, but are poorly understood. Understanding these relationships is complicated by the fact that microbial community structure and function are likely to both affect and be affected by organic matter quality and chemistry, thus it is difficult to draw mechanistic conclusions from field studies. We conducted a reciprocal soil inoculum × litter transplant laboratory incubation experiment using samples collected from a set of sites that have similar climate and plant species composition but vary significantly in bacterial community structure and litter quality. The results showed that litter quality explained the majority of variation in decomposition rates under controlled laboratory conditions: over the course of the 162-day incubation, litter quality explained nearly two-thirds (64 %) of variation in decomposition rates, and a smaller proportion (25 %) was explained by variation in the inoculum type. In addition, the relative importance of inoculum type on soil respiration increased over the course of the experiment, and was significantly higher in microcosms with lower litter quality relative to those with higher quality litter. We also used molecular phylogenetics to examine the relationships between bacterial community composition and soil respiration in samples through time. Pyrosequencing revealed that bacterial community composition explained 32 % of the variation in respiration rates. However, equal portions (i.e., 16 %) of the variation in bacterial community composition were explained by inoculum type and litter quality, reflecting the importance of both the meta-community and the environment in bacterial assembly. Taken together, these results indicate that the effects of changing microbial community composition on decomposition are likely to be smaller than the potential effects of climate change and/or litter quality changes in response to increasing atmospheric CO₂ concentrations or atmospheric nutrient deposition.     

Nutrient dynamics on a precipitation gradient in Hawai'i

We evaluated soil and foliar nutrients in five native forests in Hawai'i with annual rainfall ranging from 500 mm to 5500 mm. All of the sites were at the same elevation and of the same substrate age; all were native-dominated forests containing Metrosiderospolymorpha Gaud. Soil concentrations of extractable NO₃-N and PO₄-P, as well as major cations (Ca, Mg, and K), decreased with increasing annual precipitation, and δ¹⁵N values became more depleted in both soils and vegetation. For M.polymorpha leaves, leaf mass per area (LMA) and lignin concentrations increased significantly, while δ¹³C values became more depleted with increasing precipitation. Foliar phosphorus, and major cation (Ca, Mg, and K) concentrations for M.polymorpha all decreased significantly with increasing precipitation. For other native forest species, patterns of LMA, δ¹³C, and δ¹⁵N generally mirrored the pattern observed for M. polymorpha. Decreasing concentrations of available rock-derived nutrients in soil suggest that the effect of increased rainfall on leaching outweighs the effect of increasing precipitation on weathering. The pattern of decreased foliar nutrient concentrations per unit leaf area and of increased lignin indicates a shift from relatively high nutrient availability to relatively high carbon gain by producers as annual precipitation increases. For nitrogen cycling, the pattern of higher inorganic soil nitrogen concentrations in the drier sites, together with the progressively depleted δ¹⁵N signature in both soils and vegetation, suggests that nitrogen cycling is more open at the drier sites, with smaller losses relative to turnover as annual precipitation increases. Received: 24 March 1997 / Accepted: 19 September 1997     

Prevalence of Tree Regeneration by Sprouting and Seeding Along a Rainfall Gradient in Hawai'i

Drought stress in tropical dry forests is thought to result in greater asexual regeneration via vegetative sprouting ( e.g ., basal, root, and branch layering) than occurs in moister tropical forests. We tested this hypothesis by examining the prevalence of tree sprouting and seeding in tropical forests located along a rainfall gradient on the island of Hawai'i. Additionally, we examined the potential for novel disturbance, feral pig Sus scrofa rooting and trampling, to alter patterns in tree regeneration mode. We found greater sprouting (in terms of relative density and basal area) in dry forests than in mesic and wet forests, supporting the hypothesis. We also found that feral pig disturbance is negatively correlated with the relative density and basal area of seedlings in wet forests, but is positively correlated with the relative importance of sprouting, and the richness and diversity of sprouting species. Our results suggest rainfall regimes may be an important factor controlling broad-scale patterns in tree regeneration mode, and that exotic ungulates can significantly modify such patterns with potential consequences for the structure and dynamics of tree populations and communities.