Nitrogen saturation in UK moorlands: the critical role of bryophytes and lichens in determining retention of atmospheric N deposition.
Abstract
Anthropogenic nitrogen (N) deposition may have several impacts on upland moorland ecosystems, including changes in vegetation composition, eutrophication and surface water acidification through nitrate leaching, but few studies linking N deposition to key biogeochemical processes have been published. A stable isotope tracer (15N) was used to determine the fate of inorganic N inputs to four moorland catchments across gradients of N deposition and leaching, through 2-weekly additions to experimental plots on major soil types over 1 year. An apparent decline in total 15N recovery from soils and vegetation as the proportion of leached N deposition increased was not significant at the P=0.05 level, but a significant relationship was found for recovery in mosses and lichens. Vegetation retained 31-68% of 15N inputs, and 15N recovery increased significantly (P=0.01) with biomass for all compartments except woody shrubs. Mosses and lichens showed far greater 15N recovery per unit biomass than grasses or ericaceous shrubs. There was no significant variation in the proportion of 15N recovered in higher plants across the N deposition gradient (24-29%). In contrast, the proportion recovered in mosses and lichens declined from 44% to 2% as deposition increased, mirroring a decline in their biomass and showing a highly significant inverse relationship (P=0.01) with nitrate leaching. The proportion of 15N recovered in litter plus surface soils (33-39%) was remarkably constant across the deposition gradient for a variety of soil types. However, significantly declining recovery per unit biomass in litter (P<0.05) suggested progressive N saturation of this sink and increasing importance of retention in underlying surface soils as deposition increased. Synthesis and applications. Past studies have demonstrated a decline in mosses and lichens in response to increasing N deposition, but we show here for the first time that reduced N retention might result together with increased nitrate leaching into surface waters. The conservation of bryophyte and lichen flora on moorlands is therefore critical to prevent excessive nitrate leaching and associated surface water acidification and eutrophication. Ensuring management practices such as grazing or burning are at an intensity that does not further degrade the bryophyte and lichen communities may help minimize the impact of N deposition on freshwaters, but the only effective means to reduce the risk of N leaching is a reduction in N emissions.