Article (refereed) -postprint Aggenbach, Camiel J.S.; Kooijman, Annemieke M.; Fujita, Yuki; van der Hagen, Harrie; van Til, Mark; Cooper, David; Jones, Laurence. 2017. Does atmospheric nitrogen deposition lead to greater nitrogen and carbon accumulation in coastal sand dunes? Contact CEH NORA team at noraceh@ceh.ac.uk The NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Abstract 23 24Atmospheric nitrogen (N) deposition is thought to accelerate ecological succession, causing a 25 loss of diversity in species-rich dune grasslands and hampering restoration goals. We tested 26 whether elevated atmospheric N deposition results in faster accumulation of soil C and soil N, 27 using three high-resolution chronosequences of up to 162 years in coastal sand dunes with 28 contrasting N deposition and soil base status (high N deposition calcareous and acidic dunes 29 in Luchterduinen, the Netherlands (LD) and low N deposition calcareous dunes in 30Newborough, UK (NB)). We also used the process model CENTURY to evaluate the relative 31 contribution of N deposition, climate, and soil pH. In contrast to our hypothesis we found that 32 accumulation of soil C and N was greatest at the low N deposition site NB. Model simulations 33 indicated a negative interaction between high N deposition and symbiotic N2 fixation. From 34 this we conclude that high N deposition suppresses and replaces N2 fixation as a key N 35 source. High N deposition led to lower soil C:N only in the early stages of succession (<20 36 years). The data also revealed accelerated acidification at high N deposition, which is a major 37 concern for restoration of dune grasslands. More data are needed from acidic dunes from low 38

We present an overview of high nitrogen deposition effects on coastal dune grasslands in NW-Europe (H2130), especially concerning grass encroachment in calcareous and acidic Grey Dunes. The problem is larger than previously assumed, because critical loads are still too high, and extra N-input from the sea may amount to 10 kg ha −1 yr −1 . Grass encroachment clearly leads to loss of characteristic plant species, from approximately 16 in open dune grassland to 2 in tall-grass vegetation. Dune zones differ in grass encroachment, due to the chemical status of the soil. In calcareous and iron-rich dunes (Renodunal district), grass encroachment showed a clear gradient over the dune area. Grass encroachment is low in calcareous foredunes, due to low P-availability, and large grazers were not needed to counteract grass encroachment after 2001. In partly decalcified middle dunes, P-availability and grass encroachment are high due to dissolution of calcium phosphates, and grazing only partially helped to control this. In acidic, iron-rich hinterdunes, grass encroachment gradually increased between 1990 and 2014, possibly because P-availability increased with time due to increased soil organic matter content. In acidic, iron-poor dunes (Wadden district), grass encroachment is a large problem, because chemical P-fixation with Ca or Fe does not occur. Large grazers may however reduce tall-grass cover. High cumulative Ndeposition could theoretically lead to increased N-storage and N-mineralization in the soil. Mineralization indeed increased with N-deposition, but in 15 N experiments, most ammonium was converted to nitrate, and storage in soil organic matter was low. Soil N-storage is probably reduced by high nitrate leaching, which will favour dune restoration when N-deposition levels decrease.

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2020. Resilience in coastal dune grasslands: pH and soil organic matter effects on P nutrition, plant strategies, and soil communities. Ecosphere 11(5):Abstract. Soil organic matter (SOM) and pH are key ecosystem drivers, influencing resilience to environmental change. We tested the separate effects of pH and SOM on nutrient availability, plant strategies, and soil community composition in calcareous and acidic Grey dunes (H2130) with low, intermediate, and/or high SOM, which differ in sensitivity to high atmospheric N deposition. Soil organic matter was mainly important for biomass parameters of plants, microbes, and soil animals, and for microarthropod diversity and network complexity. However, differences in pH led to fundamental differences in P availability and plant strategies, which overruled the normal soil community patterns, and influenced resilience to N deposition. In calcareous dunes with low grass-encroachment, P availability was low despite high amounts of inorganic P, due to low solubility of calcium phosphates and strong P sorption to Fe oxides at high pH. Calcareous dunes were dominated by low-competitive arbuscular mycorrhizal (AM) plants, which profit from mycorrhiza especially at low P. In acidic dunes with high grass-encroachment, P availability increased as calcium phosphates dissolved and P sorption weakened with the shift from Fe oxides to Fe-OM complexes. Weakly sorbed and colloidal P increased, and at least part of the sorbed P was organic. Acidic dunes were dominated by nonmycorrhizal (NM) plants, which increase P uptake through exudation of carboxylates and phosphatase enzymes, which release weakly sorbed P, and disintegrate labile organic P. The shifts in P availability and plant strategies also changed the soil community. Contrary to expectations, the bacterial pathway was more important in acidic than in calcareous dunes, possibly due to exudation of carboxylates and phosphatases by NM plants, which serve as bacterial food resource. Also, the fungal AM pathway was enhanced in calcareous dunes, and fungal feeders more abundant, due to the presence of AM fungi. The changes in soil communities in turn reduced expected differences in N cycling between calcareous and acidic dunes. Our results show that SOM and pH are important, but separate ecosystem drivers in Grey dunes. Differences in resilience to N deposition are mainly due to pH effects on P availability and plant strategies, which in turn overruled soil community patterns.