Atmospheric nitrogen deposition is related to plant biodiversity loss at multiple spatial scales

Accounting for the uncertainty in nitrogen deposition estimates in support of policy

Deposition of reactive nitrogen (Nr) onto sensitive habitats in exceedance of Critical Load (CL) thresholds can drive biodiversity loss and affect ecosystem function. Nr deposition is a highly complex process that is difficult to measure and model, leading to large uncertainties. We assess the implications for policy development and target setting of the large range in estimates provided by different modelling approaches. We considered three UK models (UKIAM, EMEP4UK, CBED), used to inform national policy and responses to the UN-ECE Air Convention. We used a scaling method to project the range in current estimates to future scenarios, and a risk-based approach to provide a probabilistic assessment of exceedances. We considered two future scenarios, a 2040 baseline and a 2040 high ambition technological measures scenario, in relation to a 2018 baseline. The 2018 baseline CL exceedances are highly dependent on the model used - Average Accumulated Exceedance of 1.3-9.1 kg.N.ha-1.yr-1 across all habitats. The relative reduction in exceedances for future scenarios also depends on the model, with a range of 30-66 % achieved by 2040 for the high ambition scenario, posing a challenge for target setting. Despite this, it's clear that a much greater level of ambition is required to protect the majority of habitat areas. Our risk-based approach shows that implementing only technological measures is likely to leave most areas in exceedance in 2040. This uncertainty in the assessment of Nr deposition and the benefits of abatement measures poses a challenge for policy development that is not unique to the UK.

Responses of dissolved organic nitrogen to varied nitrogen deposition: A 16-year nitrogen-addition and cessation experiment in a subtropical forest

The leaching of dissolved organic nitrogen (DON) signals N loss in forest ecosystems, connecting terrestrial and aquatic ecosystems. The response of DON to varied inorganic N (Nin) deposition remains unclear. A 16-year continuous monitoring of DON in throughfall, soil water, and stream water was conducted under a field N manipulation (10-year Nin addition and subsequent cessation of Nin addition) in a subtropical forest in China. Under reference conditions, the average organic N (Nor) deposition was 23.0 ± 4.9 kg N ha-1 yr-1, with approximately one-third leaching at a soil depth of 30 cm, a value that was higher than those reported for temperate forests. Over a 16-year period, DON concentrations tended to increase in throughfall, soil solutions and stream water. The long-term increase in DON concentrations in soil solutions was primarily driven by increasing Nor deposition and indirectly influenced by decreasing acid deposition. Due to a doubling of monthly Nin input, either as sodium nitrate or as ammonium nitrate, the soil DON leaching doubled, causing the soil to transition from a DON sink to a DON source. After Nin addition ceased, the DON leaching returned to natural levels, and the soil reverted to being a DON sink. This study reveals the importance of Nor deposition and decreasing acid deposition as drivers of long-term trends in DON concentrations in soil solutions, in addition to elucidating the response patterns of DON to increasing and decreasing Nin deposition.

The impact of ecosystem nitrogen enrichment on pollen allergy: a cross-sectional paired comparison study

BACKGROUND

The prevalence of allergy to aeroallergens is rising, driven by both environmental and lifestyle changes. However, the role of ubiquitous nitrogen enrichment in exacerbating pollen allergy remains unclear. This study aimed to investigate the impact of nitrogen on pollen allergenicity by connecting the resulting ecological changes with allergic outcomes.

METHODS

We conducted a cross-sectional paired comparison study, examining differences between nitrogen-enriched (fertilised) and non-enriched common semi-natural grasslands in Belgium. Pollen from paired grasslands (n=50, enriched [n=25] vs non-enriched [n=25]) based on their common geography, were sampled following a standardised protocol. We analysed grassland pollen abundance, quantified pollen species composition via DNA sequencing, and assessed pollen allergenicity using basophil activation testing and specific IgE measurements in a cross-sectional sample of adults who were allergic to grass pollen (n=20). Basophil activation test outcome measures included area under the dose-response curve, maximal reactivity (CD63max), and effective concentration eliciting 50% basophil activation.

FINDINGS

Nitrogen-enriched grasslands produced significantly more pollen, with a 6·2-fold increase compared with their unfertilised counterparts (3·6 mg/m2vs 0·6 mg/m2). When normalised to protein content, pollen from these enriched grasslands showed increased allergenic potential, with 5·1 times higher basophil activation test sensitivity and a 1·3-fold increase in specific IgE titres compared with their unfertilised counterparts (geometric mean fertilised 3·63 kUA/L vs unfertilised 2·81 kUA/L).

INTERPRETATION

Nitrogen enrichment substantially increased pollen abundance and allergenicity, indicating a heightened allergy burden in nitrogen-rich environments. These findings underscore the need for policies addressing nitrogen pollution to mitigate its public health impacts.

FUNDING

Belgian Science Policy Office.