Lucy J. Sheppard

The global nitrogen cycle in the twenty- first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic Nr are on land (240 Tg N yr−1) within soils and vegetation where reduced Nr contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer Nr contribute to nitrate (NO3−) in drainage waters from agricultural land and emissions of trace Nr compounds to the atmosphere. Emissions, mainly of ammonia (NH3) from land together with combustion related emissions of nitrogen oxides (NOx), 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 Nr. Some of the marine Nr 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 Nr 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 Nr 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 Nr from which it is produced.

The relationship between nitrogen deposition, species composition and foliar nitrogen concentrations in woodland flora in the vicinity of livestock farms

Measurements of atmospheric ammonia concentration along a gradient of decreasing concentration, species composition and tissue nitrogen content of a range of plant species were made in woodland in the vicinity of four intensive animal units in Scotland. Ammonia concentrations were large at woodland edges close to the livestock buildings (annual means 20–60 μg m−3) and exceed critical levels for NH3 (8 μg m−3 annual mean). Surveys of species composition of ground flora along an 0.5 km transect from livestock buildings show marked changes within 300 m downwind of the buildings. Species such as Deschampsia flexuosa, Holcus lanatus, Rubus idaeus and Urtica dioica were abundant close to livestock units and their percentage cover decreased rapidly with distance from source, while the more N-sensitive species such as Oxalis acetosella, Galium odoratum, mosses and ferns which are found upwind and outside the influence of the NH3 source, were scarce at all sites receiving >25 kg ha−1 N year−1. Visible injury to pine and spruce needles was observed immediately downwind of the buildings. Foliar nitrogen concentration of a number of species was large close to the buildings and declined with distance. Total nitrogen deposition at the woodland boundaries is estimated to range from 40 to 80 kg N ha−1 year−1 at the 4 sites and exceeds critical loads for acidic coniferous forest, i.e. 15–20 kg N ha−1 year−1 to protect ground flora, and is also often in excess of that (11–50 kg N ha−1 year−1) proposed to protect tree health. Foliar nitrogen content of mosses, (LN, % dry weight) is related to nitrogen deposition (FN, kg N ha−1 year−1) according to LN = 3.81(1-e−0.04FN).

Bioindicators of enhanced nitrogen deposition

Increased deposition of atmospheric N largely from intensive agriculture is affecting biodiversity and the composition of natural and semi-natural vegetation in Europe. The value of species based bioindicators such as the Ellenberg N index and measurements of total tissue N and free amino acids in key plant species, is described with reference to a mixed woodland downwind of a livestock farm in the Scottish Borders, operated for over 20 years with a measured spatial gradient of ammonia concentration (29-1.5 microg m(-3)). All the indicators examined showed a relationship with N deposition and provided some indication of vegetation change. Total tissue N and arginine concentrations were most closely linked with ammonia concentrations and N deposition, with r(2) values of >0.97 and >0.78 respectively.

Evidence for changing the critical level for ammonia.

The current critical level for ammonia (CLE(NH3)) in Europe is set at 8mug NH(3) m(-3) as an annual average concentration. Recent evidence has shown specific effects of ammonia (NH(3)) on plant community composition (a true ecological effect) at much smaller concentrations. The methods used in setting a CLE(NH3) are reviewed, and the available evidence collated, in proposing a new CLE(NH3) for different types of vegetation. For lichens and bryophytes, we propose a new CLE(NH3) of 1 microg NH(3) m(-3) as a long-term (several year) average concentration; for higher plants, there is less evidence, but we propose a CLE(NH3) of 3+/-1 microg NH(3) m(-3) for herbaceous species. There is insufficient evidence to provide a separate CLE(NH3) for forest trees, but the value of 3+/-1 microg NH(3) m(-3) is likely to exceed the empirical critical load for N deposition for most forest ecosystems.

The effect of nitrogen deposition and seasonal variability on methane oxidation and nitrous oxide emission rates in an upland spruce plantation and moorland

Abstract Rates of CH 4 oxidation (-) and N 2 O emission were measured at upland moorland and coniferous forest sites in Southern Scotland to investigate seasonal variations in flux and the influence of atmospheric N inputs on the land atmosphere exchange of these two gases. CH 4 oxidation rates ranged from −0.4 to −16.7 ng CH 4 m −2 s −1 and showed a strong seasonal response with a summer maxima. Total annual uptake of CH 4 was estimated at 1.8, 0.7 and 1.3 kg ha −1 yr −1 at the moorland, high altitude forest and low altitude forest sites, respectively. Highly significant correlations were observed between CH 4 oxidation rates and soil temperature, with activation energies between 60 and 140 kJ mol −1 . CH 4 oxidation rates were on average 46 and 61% smaller at the high altitude forest site than at the the low altitude forest and moorland sites, respectively, coincident with a large concentration of soil NH 4 + N caused by large rates of N deposition at the hill summit. Addition of N as NO 3 to soil cores resulted in a 86% reduction in oxidation rates and addition of N as NH 4 Cl in a 70% reduction. However, even for NaCl treated cores, a 75% reduction in oxidation rates was observed. N 2 O fluxes ranged from −0.8 to 15.5 ng Nm −2 s −1 . Fluxes were significantly higher in 1994 than in 1995, corresponding to less rainfall during the week prior to flux measurements in 1995. In 1994 significant correlations were observed between N 2 O emission rates, soil temperature, soil available NH 4 + N concentrations and CH 4 oxidation rates. N 2 O fluxes were greatest at the hill summit. Total annual emissions were estimated at 0.3 kg N ha − yr −1 at the high altitude forest and moorland site compared to 0.1 kg N ha −1 yr −1 at the low altitude forest sites. It was calculated that 1% of the N deposited in the forest at the hill summit was emitted as N 2 O.

Nitrogen Deposition Effects on Mediterranean-type Ecosystems: An Ecological Assessment

We review the ecological consequences of N deposition on the five Mediterranean regions of the world. Seasonality of precipitation and fires regulate the N cycle in these water-limited ecosystems, where dry N deposition dominates. Nitrogen accumulation in soils and on plant surfaces results in peaks of availability with the first winter rains. Decoupling between N flushes and plant demand promotes losses via leaching and gas emissions. Differences in P availability may control the response to N inputs and susceptibility to exotic plant invasion. Invasive grasses accumulate as fuel during the dry season, altering fire regimes. California and the Mediterranean Basin are the most threatened by N deposition; however, there is limited evidence for N deposition impacts outside of California. Consequently, more research is needed to determine critical loads for each region and vegetation type based on the most sensitive elements, such as changes in lichen species composition and N cycling.

Soil environmental variables affecting the flux of methane from a range of forest, moorland and agricultural Soils

Measurements of the net methane exchange over a range of forest, moorland, and agricultural soils in Scotland were made during the period April to June 1994 and 1995. Fluxes of CH4 ranged from oxidation −12.3 to an emission of 6.8 ng m−2 s−1. The balance between CH4 oxidation and emission depended on the physical conditions of the soil, primarily soil moisture. The largest oxidation rates were found in the mineral forest soils, and CH4 emission was observed in several peat soils. The smallest oxidation rate was observed in an agricultural soil. The relationship between CH4 flux and soil moisture observed in peats (FluxCH4 = 0.023 × %H2O (dry weight) − 7.44, p > 0.05) was such that CH4 oxidation was observed at soil moistures less than 325%( ± 80%). CH4 emission was found at soil moistures exceeding this value. A large range of CH4 oxidation rates were observed over a small soil moisture range in the mineral soils. CH4 oxidation in mineral soils was negatively correlated with soil bulk density (FluxCH4 = −37.35 × bulk density (g cm−3) + 48.83, p > 0.05). Increased nitrogen loading of the soil due to N fixation, atmospheric deposition of N, and fertilisation, were consistently associated with decreases in the soil sink for CH4, typically in the range 50 to 80%, on a range of soil types and land uses.

Climatic modifiers of the response to nitrogen deposition in peat-forming Sphagnum mosses: a meta-analysis

Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data. We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m(-2)  yr(-1) for each 1°C increase. Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum-dominated vegetation.

Soil nitrous oxide and nitric oxide emissions as indicators of elevated atmospheric N deposition rates in seminatural ecosystems

Elevated N deposition caused by ammonia emissions from poultry and pig farms, and supplemented N concentrations in acid mist in field and chamber experiments increased soil available NH4+ and NO3− concentrations and emissions of N2O and NO. In a ‘pristine’ soil, not previously exposed to high N deposition rates, an initial threshold of 40 kg N ha−1 year −1 was required to increase N2O emissions. For all data described here on average 0.76% (range 0.2 to 15%) of the elevated N deposited was emitted as N2O. For soils exposed to long-term and large N deposition rates N2O losses>3% of the N deposition rate were calculated. This suggests that N2O losses of more than 3% of the N input can be indicative of soil ecosystems where the N input exceeds its demand. For NO a more limited data set showed losses ranging from 1.3 to 20% of the elevated N input. It was calculated that NH3 emissions from all intensive pig and poultry farms in Great Britain accounted for 18 t N2O---N year−1 and that poultry farms accounted for less than 3 t NO---N year−1.

The Role of Nitrogen Deposition in Widespread Plant Community Change Across Semi-natural Habitats

Experimental studies have shown that deposition of reactive nitrogen is an important driver of plant community change, however, most of these experiments are of short duration with unrealistic treatments, and conducted in regions with elevated ambient deposition. Studies of spatial gradients of pollution can complement experimental data and indicate whether the potential impacts demonstrated by experiments are actually occurring in the ‘real world’. However, targeted surveys exist for only a very few habitats and are not readily comparable. In a coordinated campaign, we determined the species richness and plant community composition of five widespread, semi-natural habitats across Great Britain in sites stratified along gradients of climate and pollution, and related these ecological parameters to major drivers of biodiversity, including climate, pollution deposition, and local edaphic factors. In every habitat, we found reduced species richness and changed species composition associated with higher nitrogen deposition, with remarkable consistency in relative species loss across ecosystem types. Whereas the diversity of mosses, lichens, forbs, and graminoids declines with N deposition in different habitats, the cover of graminoids generally increases. Considered alongside previous experimental studies and survey work, our results provide a compelling argument that nitrogen deposition is a widespread and pervasive threat to terrestrial ecosystems.