Leaching of nutrient cations from the forest floor: effects of nitrogen saturation in two long-term manipulations

Nitrogen saturation results in greater mobility of nitrate, which in turn is often correlated with concentrations of nutrient cations in soil solution and streamwater. At the Harvard Forest, U.S.A., under long-term NH4NO3 inputs, a Pinus resinosa Ait. forest has exhibited signs of N saturation more rapidly than a mixed-Quercus forest. We test the hypothesis that increased nitrate leaching causes increased concentrations of nutrient cations in soil solution. Over 2 years (years 6 and 7 of treatment) we measured SO4 2–, NO3 –, Cl–, Ca2+, K+, Mg2+, Na+, H+, and NH4 + in throughfall solution and in forest-floor (Oa) leachate. Concentrations of NO3 – in forest-floor leachate increased with rates of N amendment and correlated positively with cation concentrations, with stronger overall correlations in the pine forest: r2 values were 0.51 (pine forest) and 0.39 (oak forest) for Ca2+, 0.45 (pine) and 0.16 (oak) for K+, and 0.62 (pine) and 0.50 (oak) for Mg2+. In summer and fall, the oak forest showed some negative relationships between nutrient cation leaching and rate of N amendment. These contrasts showed retention of cations and N to occur together in an Nlimited system, whereas increased nitrate mobility occurred with increased cation losses in an N-saturated system. Résumé : La saturation en azote se traduit par une plus grande mobilité des nitrates, laquelle est alors corrélée aux concentrations des cations nutritifs dans la solution de sol et dans le cours d’eau. À la forêt de Harvard aux États-Unis, une forêt de Pinus resinosa Ait. soumise à des apports à long terme de NH4NO3 a présenté des signes de saturation en N plus rapidement qu’une forêt mélangée de Quercus. Nous avons vérifié l’hypothèse voulant qu’un accroissement du lessivage des nitrates causerait une augmentation des concentrations des cations nutritifs dans la solution de sol. Sur deux années (ans 6 et 7 du traitement), nous avons mesuré SO4 2–, NO3 –, Cl–, Ca2+, K+, Mg2+, Na+, H+ et NH4 + dans la solution de pluviolessivage et dans le lessivat de la couverture morte (Oa). Les concentrations de NO3 – dans le lessivat de la couverture morte ont augmenté avec les taux d’ajout de N et étaient positivement corrélées avec les concentrations des cations, les plus fortes corrélations étant observées dans la forêt de pin : les valeurs de r2 étaient de 0,51 (forêt de pin) et 0,39 (forêt de chêne) pour Ca2+, de 0,45 (pin) et 0,16 (chêne) pour K+, et de 0,62 (pin) et 0,50 (chêne) pour Mg2+. Durant l’été et l’automne, la forêt de chêne a montré quelques relations négatives entre le lessivage de cations nutritifs et le taux d’ajout de N. Ces contrastes montrent que la rétention des cations et de N peut être observée simultanément dans un système pauvre en N tandis qu’une mobilité accrue des nitrates est survenue avec une augmentation des pertes de cations dans un système saturé en N. [Traduit par la rédaction] Currie et al. 620 The dominant anions associated with cation leaching from forest soils may either be naturally occurring organic anions, bicarbonate, or the strong acid anions, sulfate and nitrate (Cronan et al. 1978). Increased production or input of any of these mobile anions could potentially accelerate cation losses from soils. Assessments of the role of SO4 2– in cation leaching are more straightforward than NO3 – because SO4 2– leaching is generally thought to be controlled more by conservative transport or abiotic soil adsorption, while inputs of nitrogen (N) are generally strongly retained by biotic processes (Johnson et al. 1982; Johnson 1992; Mitchell et al. 1996). However, high levels of nitrate leaching have been observed in sites with N-fixing tree species (Johnson et al. 1988), high N inputs in atmospheric deposition (Gundersen et al. 1998), disturbance (Vitousek et al. 1979; Hornung and Reynolds 1995), or N inputs in experimental manipulations (Kahl et al. 1993). Concern over nitrate leaching in northeastern North America has been heightened by findings such as increasing nitrification across gradients in increasing N deposition (McNulty et al. 1990; Gundersen et al. 1998). The theory of N saturation in forest ecosystems predicts that nitrate leaching may increase seasonally or over the longer term even under conditions when elevated N inputs are retained (Ågren and Bosatta 1988; Aber et al. 1989). Long-term N amendments at the Harvard Forest in central Can. J. For. Res. 29: 609–620 (1999) © 1999 NRC Canada 609 Received September 25, 1998. Accepted February 20, 1999. W.S. Currie.1 Appalachian Laboratory, University of Maryland Center for Environmental Science, 301 Braddock Road, Frostburg, MD 21532, U.S.A. J.D. Aber. Complex Systems Research Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, U.S.A. C.T. Driscoll. Department of Civil and Environmental Engineering, 220 Hinds Hall, Syracuse University, Syracuse, NY 13244, U.S.A. 1Author to whom all correspondence should be addressed. e-mail: [email protected] I:\cjfr\cjfr29\cjfr-05\X99-033.vp Thursday, April 22, 1999 3:53:56 PM Color profile: Disabled Composite Default screen