Does atmospheric nitrogen deposition lead to greater nitrogen and carbon accumulation in coastal sand dunes?

Atmospheric nitrogen (N) deposition is thought to accelerate ecological succession, causing a loss of diversity in species-rich dune grasslands and hampering restoration goals. We tested whether elevated atmospheric N deposition results in faster accumulation of soil C and soil N, using three high-resolution chronosequences of up to 162 years in coastal sand dunes with contrasting N deposition and soil base status (high N deposition calcareous and acidic dunes in Luchterduinen, the Netherlands (LD) and low N deposition calcareous dunes in Newborough, UK (NB)). We also used the process model CENTURY to evaluate the relative contribution of N deposition, climate, and soil pH. In contrast to our hypothesis we found that accumulation of soil C and N was greatest at the low N deposition site NB. Model simulations indicated a negative interaction between high N deposition and symbiotic N2 fixation. From this we conclude that high N deposition suppresses and replaces N2 fixation as a key N source. High N deposition led to lower soil C:N only in the early stages of succession (<20 years). The data also revealed accelerated acidification at high N deposition, which is a major concern for restoration of dune grasslands. More data are needed from acidic dunes from low N deposition areas to assess pH effects on soil C and N pools. Therefore, while N accumulation in soils may not be an issue, both acidification and plant community change due to elevated availability of mineral N remain major conservation problems. Restoration in degraded dune grasslands should focus on maintaining habitat suitability, rather than N removal from soil pools

Restoration of Central European fens – the larger context

Fens are groundwater-fed wetlands that once covered substantial surfaces in Central Europe. They deliver important services to society including carbon fixation, water buffering, biodiversity and nutrient retention. Nowadays most of these wetlands have been lost or highly decreased in size or quality. This has led to enormous losses in water buffering capacity and biodiversity and huge releases of carbon and nutrients. Despite all this negative effects of drainage even today freshwater wetlands still face the highest loss rate of all European habitat types. On the other hand, many countries have started restoration programs to get at least some of the functions of the lost wetlands back. The present contribution will address factors that affect the sustainability of wetland restoration in relation to spatial scale and landscape connectivity. We will translate these findings into practical knowledge, aimed at evaluating restoration scenarios focusing on the optimisation of different services in restored fen systems. We will evaluate to what degree there are synergies possible between restoration activities aimed at increasing ecosystem resilience and those that seek to enhance other goals.peerReviewe

Data from: Restoration of endangered fen communities: the ambiguity of iron-phosphorus binding and phosphorus limitation

1.Low phosphorus (P) availability limits plant biomass production in fens, which is a prerequisite for the persistence of many endangered plant species. We hypothesized that P limitation is linked to soil iron (Fe) content and soil Fe:P ratios as iron compounds provide binding sites for dissolved P, presumably reducing P availability to plants. 2.We sampled 30 fens in a trans-European field survey to determine how soil Fe pools relate to pools of P and Fe-bound P, and we measured vegetation P uptake and N:P ratio to assess where P limitation occurs. Next, we determined P uptake by Carex rostrata in experimental fen mesocosms to investigate interactive effects of soil Fe- and P pools (and -NDASH-fractions) and water levels (drained or rewetted). 3.The field survey revealed that soil P pools correlate positively with soil Fe pools, regardless of fen degradation level, location, or sampling depth. Moreover, soil Fe- and P pools correlated positively with P uptake by the vegetation and negatively with vegetation N:P ratios. Generally, N:P ratios dropped below 10 g g−1 whenever thresholds of 15 mmol Fe L−1 soil and 3.3 mmol P L−1 soil were exceeded. Endangered fen species mainly thrived in Fe- (and thus P-) poor fens. 4.The mesocosm experiment further showed that interactions between water levels and P pools determined plant P uptake: although fen rewetting led to an overall increase in P uptake, plants that had grown on drained Fe-rich soils with large acid-extractable P pools (>1.6 mmol Pacid L−1) could still sequester large quantities of P. Soil Fe:P ratio had no effect on P uptake. 5.Synthesis and applications. Our findings have important implications for the management and restoration of endangered fen communities. We demonstrated the existence of an iron-phosphorus (Fe-P) binding ambiguity in fens: large Fe pools “trap” mobile P, thereby enhancing overall P availability to plants rather than diminishing it. For P limitation we suggest an empirical threshold of < 3.3 mmol P L−1 soil, which is mainly found in Fe-poor fens. Restoring fens by rewetting increases the relative availability of P and may not always result in favourable conditions for endangered fen communities. Rewetting of drained fens is most likely to be successful if soil P and Fe pools are well below 3.3 mmol L−1 and 15 mmol L−1 respectively