N, P and K budgets along nutrient availability-productivity gradients in wetlands

Abstract

Nutrient enrichment in Western Europe is an important cause of wetland deterioration and the concomitant loss of biodiversity. We quantified nitrogen, phosphorus, and potassium budgets along biomass gradients in wet meadows and fens (44 field sites) to evaluate the importance of various nutrient flows (atmospheric deposition, flooding, groundwater flow, leaching, soil turnover rates) for availability of the growth-limiting nutrient(s). From the nutrient budgets, we assessed N, P, and K availabilities for plants and compared them with N, P, and K in aboveground biomass. Also, potential long-term effects of annual hay harvesting on nutrient limitation were assessed. Comparing N, P, and K availabilities with N, P, and K amounts in the vegetation revealed that (1) the assessed availabilities could explain amounts and variation of nutrients in the vegetation along the biomass gradients, and (2) N was likely the major limiting nutrient along the gradients and P and K could (co)limit growth in some of the sites. Increasing N availabilities along the biomass gradients were caused by increasing N turnover rates in the soil. The contribution of atmospheric N deposition (43 kg N.ha-1·yr-1 at all sites) to N availability varied from ~63–76% in low-productivity meadows and fens to 24–42% in highly productive meadows and fens. P and K availabilities along the biomass gradients were primarily influenced by soil processes, as indicated by soil extractable nutrient pools. Flooding could explain 20–30% of K in aboveground higher plants but was less important for P or N availabilities. Nutrient input and output by groundwater flow were more or less negligible for nutrient availability. At low-productivity sites, N output by hay harvesting just accounted for N input from atmospheric deposition, whereas there was net output of P and K. At highly productive sites, there was net output of all three nutrients. Compared to total N, P, and K pools in the top soil, net K output (1–20% of soil K pool) was at many sites much larger than that of P (generally 0.5–3%) or N (0–3%). Hay harvesting particularly seems to create K limitation. Our results indicate that conservation or restoration of low productivity wetlands in Western Europe requires (1) stable site conditions controlling low N, P, and K turnover rates in the soil, and (2) in case of N limitation, annual removal of biomass by harvesting hay, or another management measure to counterbalance the N input from atmospheric deposition.