Recovery of aboveground biomass and plant species richness with the decline in N deposition depends on pH and SOM effects on P and N availability

The response to declining atmospheric N deposition between 1992 and 2021 was studied in ungrazed coastal dune grasslands representative for NW Europe, in a synthesis of 11 separate studies. With the decline from 25 to 15 kg N ha −1 year −1 , average aboveground biomass decreased from 450 to 150 g m −2 , while plant species richness increased from 7 to 14, albeit only in tallgrass vegetation. Apart from N deposition, soil and landscape factors such as pH and SOM were important drivers. Low biomass levels could be reached at high pH and/or low SOM, but not at low pH and high SOM. The vegetation was N-limited at low SOM, and higher aboveground biomass with high SOM could be explained by higher N mineralization. Decrease in biomass with pH was associated with shifts in P-availability and plant strategies for nutrient uptake. At high pH, insoluble calcium phosphate was the major source of P, which favours arbuscular mycorrhizal (AM) plants. Plant species richness increased, but aboveground biomass decreased, probably due to high costs to the fungal partner. However, even at high pH, the vegetation was not P-limited, which may explain its sensitivity to high N deposition. At low pH, organic and sorbed P predominated, which favour nonmycorrhizal plants. Aboveground biomass increased at low pH with higher P availability and higher plant N use efficiency. This study provides strong evidence that lower N deposition indeed helps recovery of vegetation, but also stresses the importance of pH and SOM as key controls for nature management. Our study presents empirical evidence that the decline in nitrogen (N) deposition over the past decades led to a significant decrease in aboveground vascular plant biomass in ungrazed Dutch coastal dune grasslands ( p = 0.006), and in tallgrass also to higher species richness ( p = 0.018). However, as visualized above, the actual responses to N deposition depended on pH and soil organic matter content (SOM), through their influence on availability of N and P, as well as plant strategies for nutrient uptake. Soil pH and SOM are also key controls for nature management, and can be improved by stimulation of aeolian activity. • Aboveground biomass decreased with the decline in N deposition in coastal dune grasslands. • Plant species richness increased with lower N deposition, but only in tallgrass vegetation. • Effects of high N deposition were enlarged by high SOM, but mitigated by high pH. • High SOM increased N mineralization, while high pH decreased P availability and N use efficiency. • N deposition always increased biomass as plants were sometimes N-limited, but never P-limited.