Impacts of atmospheric nitrogen deposition: responses of multiple plant and soil parameters across contrasting ecosystems in long‐term field experiments

Abstract

Atmospheric nitrogen (N) deposition is a global and increasing threat to biodiversity and ecosystem function. Much of our current understanding of N deposition impacts comes from field manipulation studies, although interpretation may need caution where simulations of N deposition (in terms of dose, application rate and N form) have limited realism. Here, we review responses to simulated N deposition from the UKREATE network, a group of nine experimental sites across the UK in a diversity of heathland, grassland, bog and dune ecosystems which include studies with a high level of realism and where many are also the longest running globally on their ecosystem type. Clear responses were seen across the sites with the greatest sensitivity shown in cover and species richness of bryophytes and lichens. Productivity was also increased at sites where N was the limiting nutrient, while flowering also showed high sensitivity, with increases and declines seen in dominant shrub and forb species, respectively. Critically, these parameters were responsive to some of the lowest additional loadings of N (7.7–10 kg ha−1 yr−1) showing potential for impacts by deposition rates seen in even remote and ‘unpolluted’ regions of Europe. Other parameters were less sensitive, but nevertheless showed response to higher doses. These included increases in soil %N and ‘plant available’ KCl extractable N, N cycling rates and acid–base status. Furthermore, an analysis of accumulated dose that quantified response against the total N input over time suggested that N impacts can ‘build up’ within an ecosystem such that even relatively low N deposition rates can result in ecological responses if continued for long enough. Given the responses have important implications for ecosystem structure, function, and recovery from N loading, the clear evidence for impacts at relatively low N deposition rates across a wide range of habitats is of considerable concern.

Long-term nitrogen deposition depletes grassland seed banks

“…Cover and flowering of the other species was not affected significantly by 35N or 140N treatment (GLM; P40.1 in all the cases). Although exotic species in the seed bank can increase after N deposition 11 , we did not record any weeds or invasive species in the soil, which is consistent with the aboveground vegetation responses 5 .…”

“…Moreover, there was a 41% and 29% reduction in species richness in the high and low nitrogenaddition treatments (Fig. 1b), an effect not recorded in the aboveground vegetation 5 . Although seed bank composition was changed significantly by N deposition, the correlation between N treatment and seed bank composition was weak ( Supplementary Fig.…”

“…Despite widespread evidence of impacts of N pollution on floristic diversity and plant community structure 4,5 , studies on soil seed banks are scarce [10][11][12][13] and most are limited to short-term effects. For example, a 3-year study of N addition in a desert community found no change in the density or species richness of the seed bank 11 , while another 1-year study found that N addition increased seedling emergence 12 .…”

“…Anthropogenic activities have caused an increase in atmospheric N deposition, which in turn has altered the global N cycle radically, threatening the biodiversity of terrestrial and marine ecosystems 4 . In terrestrial ecosystems, several effects of N deposition on aboveground plant communities have been identified, including changes in plant cover, abundance, species composition and richness [4][5][6] , a decline in resistance to pathogens and pests 4,5 , an increase in susceptibility to drought and frost 4,5 and a decrease in the stability of biomass production (by the effects of N eutrophication) 6 .…”

“…The Wardlow site is both one of the earliest and longest running N deposition experiments in the world, simulating atmospheric N deposition on grassland ecosystems 5 . Seed bank analysis was further supported by an evaluation of the vegetative cover and flowering.…”