Leon J.L. van den Berg

Ecosystem responses to reduced and oxidised nitrogen inputs in European terrestrial habitats

While it is well established that ecosystems display strong responses to elevated nitrogen deposition, the importance of the ratio between the dominant forms of deposited nitrogen (NH(x) and NO(y)) in determining ecosystem response is poorly understood. As large changes in the ratio of oxidised and reduced nitrogen inputs are occurring, this oversight requires attention. One reason for this knowledge gap is that plants experience a different NH(x):NO(y) ratio in soil to that seen in atmospheric deposits because atmospheric inputs are modified by soil transformations, mediated by soil pH. Consequently species of neutral and alkaline habitats are less likely to encounter high NH(4)(+) concentrations than species from acid soils. We suggest that the response of vascular plant species to changing ratios of NH(x):NO(y) deposits will be driven primarily by a combination of soil pH and nitrification rates. Testing this hypothesis requires a combination of experimental and survey work in a range of systems.

Sphagnum mosses - masters of efficient n-uptake while avoiding intoxication

Peat forming Sphagnum mosses are able to prevent the dominance of vascular plants under ombrotrophic conditions by efficiently scavenging atmospherically deposited nitrogen (N). N-uptake kinetics of these mosses are therefore expected to play a key role in differential N availability, plant competition, and carbon sequestration in Sphagnum peatlands. The interacting effects of rain N concentration and exposure time on moss N-uptake rates are, however, poorly understood. We investigated the effects of N-concentration (1, 5, 10, 50, 100, 500 µM), N-form (15N - ammonium or nitrate) and exposure time (0.5, 2, 72 h) on uptake kinetics for Sphagnum magellanicum from a pristine bog in Patagonia (Argentina) and from a Dutch bog exposed to decades of N-pollution. Uptake rates for ammonium were higher than for nitrate, and N-binding at adsorption sites was negligible. During the first 0.5 h, N-uptake followed saturation kinetics revealing a high affinity (Km 3.5–6.5 µM). Ammonium was taken up 8 times faster than nitrate, whereas over 72 hours this was only 2 times. Uptake rates decreased drastically with increasing exposure times, which implies that many short-term N-uptake experiments in literature may well have overestimated long-term uptake rates and ecosystem retention. Sphagnum from the polluted site (i.e. long-term N exposure) showed lower uptake rates than mosses from the pristine site, indicating an adaptive response. Sphagnum therefore appears to be highly efficient in using short N pulses (e.g. rainfall in pristine areas). This strategy has important ecological and evolutionary implications: at high N input rates, the risk of N-toxicity seems to be reduced by lower uptake rates of Sphagnum, at the expense of its long-term filter capacity and related competitive advantage over vascular plants. As shown by our conceptual model, interacting effects of N-deposition and climate change (changes in rainfall) will seriously alter the functioning of Sphagnum peatlands.

Territory selection by the Dartford warbler (Sylvia undata) in Dorset, England: the role of vegetation type, habitat fragmentation and population size

The Dartford warbler (Sylvia undata) is of conservation concern throughout western Europe. In the UK conservation is centred on management of the warblers lowland heath habitat. This study analysed the warblers habitat requirements in Dorset, its UK population centre. Maps of breeding territories from 1974, 1984 and 1994 were linked to maps of heathland vegetation and adjacent land use from 1978, 1987 and 1996. Logistic regression showed territories were positively associated with the area of dry/humid heath, mature Ulex europaeus, bare soil, and more rarely with wet heath, young U. europaeus, U. gallii and Pinus scrub. There were negative responses to fragmentation (isolation and area of a heath), and the proximity of woodland, urban areas and intensive agriculture. Sub-optimal habitat was used in years of high population size. Habitat management for this species should consider not only heathland vegetation but also landscape factors such as fragmentation and the proximity of avoided land use types.

Dissolved organic carbon (doc) concentrations in uk soils and the influence of soil, vegetation type and seasonality

Given the lack of studies which measured dissolved organic carbon (DOC) over long periods, especially in non-forest habitat, the aim of this study was to expand the existing datasets with data of mainly non-forest sites that were representative of the major soil and habitat types in the UK. A further aim was to predict DOC concentrations from a number of biotic and abiotic explanatory variables such as rainfall, temperature, vegetation type and soil type in a multivariate way. Pore water was sampled using Rhizon or Prenart samplers at two to three week intervals for 1 year. DOC, pH, organic carbon, carbon/nitrogen (C:N) ratios of soils and slope were measured and data on vegetation, soil type, temperature and precipitation were obtained. The majority of the variation in DOC concentrations between the UK sites could be explained by simple empirical models that included annual precipitation, and soil C:N ratio with precipitation being negatively related to DOC concentrations and C:N ratio being positively related to DOC concentrations. Our study adds significantly to the data reporting DOC concentrations in soils, especially in grasslands, heathlands and moorlands. Broad climatic and site factors have been identified as key factors influencing DOC concentrations.

Heathland restoration in The Netherlands: Effects of turf cutting depth on germination of Arnica montana

Abstract Germination experiments were conducted in a heathland after turf cutting and in a climate chamber to investigate the effects of turf cutting depth, aluminium toxicity and aluminium detoxification by humic acids and base cations on the germination and establishment of Arnica montana. Turfs were cut at three different depths, creating a gradient from organic to mineral soils. Germination and establishment of A. montana were negatively correlated with turf cutting depth. The removal of organic matter resulted in a major decrease in organic fraction of the soil and its nutrients. It also resulted in a considerable decrease in moisture content and humic acids. Additional liming after turf cutting increased germination and establishment in all plots and at all depths. Germination experiments under controlled conditions in a climate chamber revealed a significantly higher germination at low aluminium/calcium (Al:Ca) ratios. High Al:Ca ratios resulted in poor germination, suggesting Al toxicity. Addition of humic acids increased germination, even at high Al:Ca ratios, suggesting immobilization of Al by humic acids. It is concluded that turf cutting can have a marked effect on the success of heathland restoration. It results in the intended removal of the eutrophic layer but also in the unintentional removal of much of the buffering mechanisms and/or Al immobilizing compounds. Additional buffering and/or less deep turf cutting may be necessary to allow germination and establishment of rare herbaceous species such as A. montana. Abbreviations: DOC = Dissolved organic carbon; GLM = General Linear Model; ICP = Inductive coupled plasma emission spectrophotometer.

Hydrogen fluoride damage to vegetation from peri-urban brick kilns in Asia: A growing but unrecognised problem?

The rapid urbanisation of many cities in south and south-east Asia has increased the demand for bricks, which are typically supplied from brick kilns in peri-urban areas. We report visible foliar damage to mango, apricot and plum trees in the vicinity of traditional Bulls Trench brick kilns in Peshawar, Pakistan. Visible injury symptoms, hydrogen fluoride concentrations in air, and foliar fluoride concentrations were all greater in the vicinity of brick kilns than at more distant sites, indicating that fluoride emissions from brick kilns were the main cause of damage. Interviews with local farmers established the significant impact of this damage on their livelihoods. Since poorly regulated brick kilns are often found close to important peri-urban agricultural areas, we suggest that this may be a growing but unrecognised environmental problem in regions of Asia where emission control in brick kilns has not been improved.

Evidence for differential effects of reduced and oxidised nitrogen deposition on vegetation independent of nitrogen load

Nitrogen (N) deposition impacts natural and semi-natural ecosystems globally. The responses of vegetation to N deposition may, however, differ strongly between habitats and may be mediated by the form of N. Although much attention has been focused on the impact of total N deposition, the effects of reduced and oxidised N, independent of the total N deposition, have received less attention. In this paper, we present new analyses of national monitoring data in the UK to provide an extensive evaluation of whether there are differences in the effects of reduced and oxidised N deposition across eight habitat types (acid, calcareous and mesotrophic grasslands, upland and lowland heaths, bogs and mires, base-rich mires, woodlands). We analysed data from 6860 plots in the British Countryside Survey 2007 for effects of total N deposition and N form on species richness, Ellenberg N values and grass:forb ratio. Our results provide clear evidence that N deposition affects species richness in all habitats except base-rich mires, after factoring out correlated explanatory variables (climate and sulphur deposition). In addition, the form of N in deposition appears important for the biodiversity of grasslands and woodlands but not mires and heaths. Ellenberg N increased more in relation to NHx deposition than NOy deposition in all but one habitat type. Relationships between species richness and N form were habitat-specific: acid and mesotrophic grasslands appear more sensitive to NHx deposition while calcareous grasslands and woodlands appeared more responsive to NOy deposition. These relationships are likely driven by the preferences of the component plant species for oxidised or reduced forms of N, rather than by soil acidification.

Effects of ozone on crops in north-west Pakistan

Although ozone is well-documented to reduce crop yields in the densely populated Indo-Gangetic Plain, there is little knowledge of its effects in other parts of south Asia. We surveyed crops close to the city of Peshawar, in north-west Pakistan, for visible injury, linking this to passive measurements of ozone concentrations. Foliar injury was found on potato, onion and cotton when mean monthly ozone concentrations exceeded 45 ppb. The symptoms on onion were reproduced in ozone fumigation experiments, which also showed that daytime ozone concentrations of 60 ppb significantly reduce the growth of a major Pakistani onion variety. Aphid infestation on spinach was also reduced at these elevated ozone concentrations. The ozone concentrations measured in April-May in Peshawar, and used in the fumigation experiment, are comparable to those that have been modelled to occur over many parts of south Asia, where ozone may be a significant threat to sensitive crops.

Effects and Empirical Critical Loads of Nitrogen for Europe

Empirical critical loads of nitrogen (N) were first presented in a background document for a workshop in 1992 in Sweden. Since their first presentation, the critical loads of N have been updated at regular intervals and for a large number of habitats. This chapter presents a brief history of the empirical critical loads and explains the process of determination of empirical critical loads for nitrogen and their reliability. For European habitats (defined as EUNIS and Natura 2000 habitat classes), current empirical critical loads for nitrogen are presented. For each of these habitats, the main effects of enhanced nitrogen inputs are discussed that have formed the basis for the determination of the empirical critical loads. Factors other than nitrogen, that may affect ecosystem processes or ecosystem functioning, are discussed as these may modify the nitrogen critical load under specific conditions.

Effects of airborne ammonium and nitrate pollution strongly differ in peat bogs, but symbiotic nitrogen fixation remains unaffected

Pristine bogs, peatlands in which vegetation is exclusively fed by rainwater (ombrotrophic), typically have a low atmospheric deposition of reactive nitrogen (N) (<0.5kgha-1y-1). An important additional N source is N2 fixation by symbiotic microorganisms (diazotrophs) in peat and mosses. Although the effects of increased total airborne N by anthropogenic emissions on bog vegetation are well documented, the important question remains how different N forms (ammonium, NH4+, versus nitrate, NO3-) affect N cycling, as their relative contribution to the total load strongly varies among regions globally. Here, we studied the effects of 11years of experimentally increased deposition (32 versus 8kgNha-1y-1) of either NH4+ or NO3- on N accumulation in three moss and one lichen species (Sphagnum capillifolium, S. papillosum, Pleurozium schreberi and Cladonia portentosa), N2 fixation rates of their symbionts, and potential N losses to peat soil and atmosphere, in a bog in Scotland. Increased input of both N forms led to 15-90% increase in N content for all moss species, without affecting their cover. The keystone species S. capillifolium showed 4 times higher N allocation into free amino acids, indicating N stress, but only in response to increased NH4+. In contrast, NO3- addition resulted in enhanced peat N mineralization linked to microbial NO3- reduction, increasing soil pH, N concentrations and N losses via denitrification. Unexpectedly, increased deposition from 8 to 32kgha-1y-1 in both N forms did not affect N2 fixation rates for any of the moss species and corresponded to an additional input of 5kgNha-1y-1 with a 100% S. capillifolium cover. Since both N forms clearly show differential effects on living Sphagnum and biogeochemical processes in the underlying peat, N form should be included in the assessment of the effects of N pollution on peatlands.