Large losses of inorganic nitrogen from tropical rainforests suggest a lack of nitrogen limitation.

Inorganic nitrogen losses from many unpolluted mature tropical forests are over an order of magnitude higher than losses from analogous temperate forests. This pattern could either reflect a lack of N limitation or accelerated plant-soil N cycling under tropical temperatures and moisture. We used a simple analytical framework of the N cycle and compared our predictions with data of N in stream waters of temperate and tropical rainforests. While the pattern could be explained by differences in N limitation, it could not be explained based on up-regulation of the internal N cycle without invoking the unlikely assumption that tropical plants are two to four times less efficient at taking up N than temperate plants. Our results contrast with the idea that a tropical climate promotes and sustains an up-regulated and leaky - but N-limited - internal N cycle. Instead, they are consistent with the notion that many tropical rainforests exist in a state of N saturation.

Global distribution and climate sensitivity of the tropical montane forest nitrogen cycle.

Tropical forests are pivotal to global climate and biogeochemical cycles, yet the geographic distribution of nutrient limitation to plants and microbes across the biome is unresolved. One long-standing generalization is that tropical montane forests are nitrogen (N)-limited whereas lowland forests tend to be N-rich. However, empirical tests of this hypothesis have yielded equivocal results. Here we evaluate the topographic signature of the ecosystem-level tropical N cycle by examining climatic and geophysical controls of surface soil N content and stable isotopes (δ15N) from elevational gradients distributed across tropical mountains globally. We document steep increases in soil N concentration and declining δ15N with increasing elevation, consistent with decreased microbial N processing and lower gaseous N losses. Temperature explained much of the change in N, with an apparent temperature sensitivity (Q10) of ~1.9. Although montane forests make up 11% of forested tropical land area, we estimate they account for >17% of the global tropical forest soil N pool. Our findings support the existence of widespread microbial N limitation across tropical montane forest ecosystems and high sensitivity to climate warming.

Maintenance of terrestrial nutrient loss signatures during in-stream transport.

Small streams account for the majority of channel length in river basins worldwide and are the primary conveyors of terrestrial nutrients to rivers and ultimately the oceans. The controls of stream nutrient fluxes, however, are debated. Classical models emphasize that nutrient transport in streams integrates nutrient cycling in the terrestrial watershed while others argue that in-stream processes control nutrient flux. Recent studies have shown that in-stream cycling can be important in determining downstream nutrient fluxes, but results have not been reconciled with mass-balance calculations at the small-watershed scale. Here we use a simple analytical framework to assess nutrient cycling in streams and show that, under most conditions, longitudinally static nutrient concentrations reflect in-stream biotic balance between uptake and regeneration and groundwater inputs. Using measures of nutrient concentrations in small streams across four biomes, we provide evidence for generality of biogeochemical steady state (inputs = outputs) in stream ecosystems: overall, longitudinal profiles were flat for nitrogen and phosphorus and were similar in concentration to soil and ground waters. Deviation from flat longitudinal profiles was associated with seasonal or successional biomass growth and small groundwater inputs relative to in-stream sink strength. We conclude that streams tend strongly toward nutrient balance, allowing use of their chemistry as an integrated measure of terrestrial nutrient losses.

Symbiotic N fixation is sufficient to support net aboveground biomass accumulation in a humid tropical forest.

Symbiotic nitrogen (N) fixation has been shown to support carbon storage in young regenerating tropical forests, but N-fixing trees can also be strong competitors with non-fixing trees, making it unclear which mechanism drives long term patterns in biomass accretion. Many tropical forests have excess N, but factors such as rising atmospheric CO2 or selective cutting practices might induce additional N demand. Here we combine decades of stem inventory data, in-situ measures of symbiotic N fixation, and simulations of N demand to evaluate demographic and biogeochemical controls on biomass dynamics in legume-rich lowland forests of Trinidad. We document sustained net biomass accumulation and high rates of N fixation in these forests, regardless of the timing of selective timber harvests, including an old growth stand. The biomass accumulation was explained by growth of non-fixing trees, not N-fixing trees, but the total amount of symbiotic N fixation was sufficient to account for most of net above ground N demands, suggesting that N-fixers could contribute to the long-term C sink in these forests via fertilizing non-fixers.

Long-term decline in grassland productivity driven by increasing dryness.

Increasing aridity and drought severity forecast for many land areas could reduce the land carbon (C) sink. However, with limited long-term direct measures, it is difficult to distinguish direct drying effects from counter effects of CO2 enrichment and nitrogen (N) deposition. Here, we document a >50% decline in production of a native C3 grassland over four decades and assign the forcing and timing to increasing aridity and specifically to declining late-summer rainfall. Analysis of C and N stable isotopes in biomass suggests that enhanced water use efficiency via CO2 enrichment may have slightly ameliorated the productivity decline but that changes in N had no effects. Identical declines in a long-term snow-addition experiment definitively identified increasing late-summer dryness as the cause. Our results demonstrate lasting consequences of recent climate change on grassland production and underscore the importance of understanding past climate-ecosystem coupling to predicting future responses to changing climate.

Convergence of soil nitrogen isotopes across global climate gradients.

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the (15)N:(14)N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in (15)N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ(15)N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ(15)N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss.