SummaryGlobal warming may have profound effects on terrestrial ecosystems. However, a comprehensive evaluation of the effects of warming on ecosystem nitrogen (N) pools and dynamics is not available.Here, we compiled data of 528 observations from 51 papers and carried out a meta-analysis of experimental warming effects on 13 variables related to terrestrial N pools and dynamics.We found that, on average, net N mineralization and net nitrification rate were increased by 52.2 and 32.2%, respectively, under experimental warming treatment. N pools were also increased by warming, although the magnitude of this increase was less than that of N fluxes. Soil microbial N and N immobilization were not changed by warming, probably because microbes are limited by carbon sources. Grassland and shrubland/heathland were less responsive to warming than forest, probably because the reduction of soil moisture by warming offset the temperature effect in these areas. Soil heating cable and all-day treatment appeared to be the most effective method on N cycling among all treatment methods.Results of this meta-analysis are useful for better understanding the response of N cycling to global warming and the underlying mechanism of warming effects on plants and ecosystem functions.

Higher aridity and more extreme rainfall events in drylands are predicted due to climate change. Yet, it is unclear how changing precipitation regimes may affect nitrogen (N) cycling, especially in areas with extremely high aridity. Here we investigate soil N isotopic values (d 15 N) along a 3,200 km aridity gradient and reveal a hump-shaped relationship between soil d 15 N and aridity index (AI) with a threshold at AI ¼ 0.32. Variations of foliar d 15 N, the abundance of nitrification and denitrification genes, and metabolic quotient along the gradient provide further evidence for the existence of this threshold. Data support the hypothesis that the increase of gaseous N loss is higher than the increase of net plant N accumulation with increasing AI below AI ¼ 0.32, while the opposite is favoured above this threshold. Our results highlight the importance of N-cycling microbes in extremely dry areas and suggest different controlling factors of N-cycling on either side of the threshold.

No abstract

Loss of nitrogen (N) from land limits the uptake and storage of atmospheric CO2 by the biosphere, influencing Earth's climate system and myriads of the global ecological functions and services on which humans rely. Nitrogen can be lost in both dissolved and gaseous phases; however, the partitioning of these vectors remains controversial. Particularly uncertain is whether the bacterial conversion of plant available N to gaseous forms (denitrification) plays a major role in structuring global N supplies in the nonagrarian centers of Earth. Here, we use the isotope composition of N ( 15 N/ 14 N) to constrain the transfer of this nutrient from the land to the water and atmosphere. We report that the integrated 15 N/ 14 N of the natural terrestrial biosphere is elevated with respect to that of atmospheric N inputs. This cannot be explained by preferential loss of 14 N to waterways; rather, it reflects a history of low 15 N/ 14 N gaseous N emissions to the atmosphere owing to denitrifying bacteria in the soil. Parameterizing a simple model with global N isotope data, we estimate that soil denitrification (including N2) accounts for Ϸ1/3 of the total N lost from the unmanaged terrestrial biosphere. Applying this fraction to estimates of N inputs, N2O and NOx fluxes, we calculate that Ϸ28 Tg of N are lost annually via N2 efflux from the natural soil. These results place isotopic constraints on the widely held belief that denitrifying bacteria account for a significant fraction of the missing N in the global N cycle.denitrification ͉ isotope ͉ nitrogen ͉ ecosystem ͉ microbe

Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N 2 ) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO 3 − ) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6-30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO 3 − leaching, pointing to widespread dominance of denitrification in removing NO 3 − from forest ecosystems across a range of conditions. Further, we report that the loss of NO 3 − to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.denitrification | nitrate isotopes | nitrogen cycling | forested watersheds

Soil carbon (C) and nitrogen (N) stoichiometry is a main driver of ecosystem functioning. Global N enrichment has greatly changed soil C : N ratios, but how altered resource stoichiometry influences the complexity of direct and indirect interactions among plants, soils, and microbial communities has rarely been explored. Here, we investigated the responses of the plant-soil-microbe system to multi-level N additions and the role of dissolved organic carbon (DOC) and inorganic N stoichiometry in regulating microbial biomass in semiarid grassland in northern China. We documented a significant positive correlation between DOC and inorganic N across the N addition gradient, which contradicts the negative nonlinear correlation between nitrate accrual and DOC availability commonly observed in natural ecosystems. Using hierarchical structural equation modeling, we found that soil acidification resulting from N addition, rather than changes in the plant community, was most closely related to shifts in soil microbial community composition and decline of microbial respiration. These findings indicate a down-regulating effect of high N availability on plant-microbe interactions. That is, with the limiting factor for microbial biomass shifting from resource stoichiometry to soil acidity, N enrichment weakens the bottom-up control of soil microorganisms by plant-derived C sources. These results highlight the importance of integratively studying the plant-soil-microbe system in improving our understanding of ecosystem functioning under conditions of global N enrichment.

Anthropogenic nitrogen (N) deposition has accelerated terrestrial N cycling at regional and global scales, causing nutrient imbalance in many natural and seminatural ecosystems. How added N affects ecosystems where N is already abundant, and how plants acclimate to chronic N deposition in such circumstances, remains poorly understood. Here, we conducted an experiment employing a decade of N additions to examine ecosystem responses and plant acclimation to added N in an N-rich tropical forest. We found that N additions accelerated soil acidification and reduced biologically available cations (especially Ca and Mg) in soils, but plants maintained foliar nutrient supply at least in part by increasing transpiration while decreasing soil water leaching below the rooting zone. We suggest a hypothesis that cation-deficient plants can adjust to elevated N deposition by increasing transpiration and thereby maintaining nutrient balance. This result suggests that long-term elevated N deposition can alter hydrological cycling in N-rich forest ecosystems.

Agriculturally fertilized soil is a dominant source of NOx pollution in California.