Declines in soil-water nitrate in nitrogen-saturated watersheds

Abstract: Two forested watersheds (WS3 and WS9) in the central Appalachians were artificially acidified with ammo-nium sulfate fertilizer. WS9 was treated for 8 years, whereas WS3 has been treated for approximately 15 years. Soil leachate was collected from a depth of 46 cm (B horizon) in WS9 and below the A. B, and C horizons in WS3. Ni-trate concentrations from WS3 increased for approximately 10-1 2 years (depending upon horizon) and then did not in-crease from 2000 through 2003 despite continued fertilizer treatments. Nitrate concentrations in WS9 soil water increased for the first 3 years of fertilization, and then declined for another 2 years. After that time, the concentrations remained relatively constant at approximately 15 pequiv.-L-'; this period of low nitrate concentrations included 2.3 years of fertili~ation followed by 8.2 years of no fertili~ation. Stream-water nitrate concentrations from both water-sheds indicate they were in stage 2 of nitrogen saturation; however, the soil-water nitrate behavior observed within the setting of continued elevated nitrogen inputs is at odds with responses predicted in current nitrogen saturation theory. We believe that the cessation of nitrate increases in at least the B and C horizons were due primarily to abiotic reten-tion, with recalcitrant forms of dissolved organic carbon providing the carbon needed to induce retention. These results show that nitrogen cycling in forest soil ecosystems is more complex than current nitrogen saturation theory suggests. RCsumC: Deux bassins versants bois6s (WS3 et WS9) dans les Appalaches centrales ont 6tC artificiellement acidifies par une fertilisation au sulfate d'ammonium. Le bassin WS9 a kt6 trait6 pendant huit ans tandis que le bassin WS3 a 6t6 trait6 pendant approximativemerit 15 ans. Le lessivat du sol a kt6 collect6 B une profondeur de 46 crn (horizon B