Effects of enhanced nitrogen deposition and phosphorus limitation on nitrogen budgets of semi‐natural grasslands

Abstract

Increased reactive atmospheric N deposition has been implicated in floristic changes in species‐rich acidic and calcareous grasslands, but the fate of this pollutant N in these ecosystems is unknown. This paper reports the first analysis of N budgets and N fluxes for two grasslands in the White Peak area of Derbyshire, one of the most heavily N‐polluted locations in the UK. N fluxes were monitored in lysimeter cores (retaining the original turfs) taken from field plots of unimproved acidic and calcareous grasslands that had received (in addition to ambient N deposition) simulated enhanced N deposition treatments of 3.5 and 14 g N m−2 yr−1 for 6 years. The influence of reducing phosphorus limitation was assessed by factorial additions of P. Seasonal leached losses of nitrate, ammonia and organic N were monitored in detail along with estimates of N removal through simulated grazing and gaseous losses through denitrification and volatilization. The rates of N fluxes by these pathways were used to create N budgets for the grasslands. Both grasslands were found to be accumulating much of the simulated additional N deposition: up to 89% accumulated in the calcareous grassland and up to 38% accumulated in the acidic grassland. The major fluxes of N loss from these grasslands were by simulated grazing and leaching of soluble organic N (constituting 90% of leached N under ambient conditions). Leached inorganic N (mainly nitrate) contributed significantly to the output flux of N under the highest N treatment only. Loss of N through ammonia volatilization accounted for less than 6% of the N added as simulated deposition, while denitrification contributed significantly to output fluxes only in the acidic grassland during winter. The implications of the results for ecosystem N balances and the likely consequences of N accumulation on these grasslands are discussed.

Long-term nitrogen deposition depletes grassland seed banks

“…On the basis of previous studies 20,32 , an acidic grassland was chosen to investigate the effects of long-term atmospheric nitrogen deposition on seed banks. The grassland is located on the south-east facing slope (5-10°) of a conical hill at Wardlow Hay Cop, in the South Pennines, in Cressbrook Dale National Nature Reserve within the Peak District National Park, UK (53°15'44N, 1°44'02W), and 370 m.a.s.l (ref.…”

“…20). During the past century, Wardlow Hay Cop has received one of the highest rates of N pollutant deposition in the United Kingdom 20,32 . Therefore, although the highest application treatment (140N) exceeds the current deposition rates reported for the UK, this treatment was chosen because the South Pennines region received cumulative N deposition in the 20th century amounting to 2,200-3,200 kg N ha À 1 , equating to between 22-33 kg ha À 1 per year 33 .…”

“…Therefore, although the highest application treatment (140N) exceeds the current deposition rates reported for the UK, this treatment was chosen because the South Pennines region received cumulative N deposition in the 20th century amounting to 2,200-3,200 kg N ha À 1 , equating to between 22-33 kg ha À 1 per year 33 . Our treatment application rates thus ranged from rates similar to historical deposition rates (35 kg N ha À 1 per year above ambient values in 1995) to the 140N treatment that equates to application rates used in agricultural practices 32 . Since the main form of N deposition in upland areas is wet deposition, the treatments have been applied monthly in 2 l of distilled water, per plot per application, as a mist by using a backpack sprayer 20 .…”

“…Plots (3 Â 3 m) have been receiving N treatments of 35 (35N) or 140 (140N) kg N ha À 1 per year of NH 4 NO 3 in factorial combination with 35 kg P ha À 1 per year of NaH 2 PO 4 , or no added P, since September 1995; controls receive distilled water only 20,32 . Since January 2005, half of each plot has continued to receive one of the treatments while the other half ('recovery plots') has not.…”

“…Since January 2005, half of each plot has continued to receive one of the treatments while the other half ('recovery plots') has not. The treatments were applied in a fully randomized block design replicated three times (3N Â 2P Â 3 blocks) 32 . Each replicate block consisted of six 3 Â 3 m plots, split in half (3 Â 1.5 m) to establish the recovery treatments.…”