Abstract:We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreated); and WS3 (24-year-old, treated with (NH 4 ) 2 SO 4 since 1989 at the rate of 35 kg N·ha -1 ·year -1 ). Net nitrification was 141, 114, and 115 kg N·ha -1 ·year -1 , for WS3, WS4, and WS7, respectively, essentially 100% of net N mineralization for all watersheds. Temporal (seasonal) patterns of nitrification were significantly related to soil moisture and ambient temperature in untreated watersheds only. Spatial patterns of soil water NO 3 -of WS4 suggest that microenvironmental variability limits rates of N processing in some areas of this N-saturated watershed, in part by ericaceous species in the herbaceous layer. Spatial patterns of soil water NO 3 -in treated WS3 suggest that later stages of N saturation may result in higher concentrations with less spatial variability. Spatial variability in soil N variables was lower in treated WS3 versus untreated watersheds. Nitrogen additions have altered the response of N-processing microbes to environmental factors, becoming less sensitive to seasonal changes in soil moisture and temperature. Biotic processes responsible for regulating N dynamics may be compromised in N-saturated forest ecosystems. , soit essentiellement 100% de la minéralisation nette pour tous les bassins versants. Les patrons temporels (saisonniers) de nitrification étaient significativement reliés à l'humidité du sol et à la température ambiante seulement dans les bassins versants non traités. Les patrons spatiaux de NO 3 -de la solution de sol du WS4 indiquent que la variabilité micro-environnementale limite le taux de recyclage de N dans certaines aires de ce bassin versant saturé en N, en partie à cause des espèces d'éricacées pré-sentes dans la strate herbacée. Les patrons spatiaux de NO 3 -de la solution du sol dans le WS3 traité indiquent que des stades plus avancés de saturation de N peuvent se traduire par de plus fortes concentrations avec moins de variabilité spatiale. La variabilité spatiale des variables de N du sol était plus faible dans le WS3 comparativement aux bassins versants non traités. Les ajouts de N ont altéré la réponse des microorganismes associés au cycle de N vis-à-vis les facteurs environnementaux en les rendant moins sensibles aux changements saisonniers d'humidité et de température du sol. Les processus biotiques responsables de la régulation de la dynamique de N peuvent être compromis dans les éco-systèmes forestiers saturés en N. Résumé[Traduit par la Rédaction] Gilliam et al. 1785

Among the current environmental concerns for forests of the eastern United States is nitrogen (N) saturation, a result of excessive inputs of N associated with acidic deposition. We studied nutrient responses on N-treated and untreated watersheds of the Fernow Experimental Forest, West Virginia, to test for evidence of N saturation on the treated watershed. The watersheds were WS7 (23-year-old even-aged control), WS4 (mature mixed-aged control), and WS3 (23-year-old even-aged treatment). WS3 has received aerial applications of (NH4)2SO4 from 1989 to the present (a total of 4 years for the study period) at 3 × ambient inputs of N and S (54 and 61 kg•ha−1•year−1, respectively). Base-flow stream samples were collected weekly from each watershed and analyzed for NO3− and Ca2+. Mineral soil was incubated in situ, placed in bags, and buried about every 30 days during the growing season in each of seven sample plots within each watershed. Moist samples of soil from the bags were analyzed for extractable NH4+ and NO3−. In addition, forest floor material and leaves of an herbaceous species (Violarotundifolia Michx.) from each plot were analyzed for N and other nutrients. Violarotundifolia was present on all 21 plots and used as an additional indicator of N availability and soil fertility. Foliage tissue was sampled from overstory tree species (Liriodendrontulipifera L., Prunusserotina Ehrh., Betulalenta L., and Acerrubrum L.) from WS3 and WS7 and analyzed for nutrient content. Results from the 1993 growing season showed few, if any, differences among watersheds for (1) N content and C/N ratio of the mineral soil and forest floor and (2) relative proportion of NH4+ and NO3− produced in the buried bags. Nitrification rates were equally high in soils of all watersheds; N concentrations were significantly higher in foliage tissue of overstory tree species and of V. rotundifolia in the treatment versus control watersheds; plant tissue Ca was significantly lower for the treatment watershed than for the control watersheds. Our results support the conclusions of earlier studies that high amounts of ambient N deposition have brought about N saturation on untreated watersheds at the Fernow Experimental Forest. This is suggested by minimal differences among watersheds in N mineralization and nitrification and soil and forest floor N. However, aggravated N saturation on our treated watershed can be seen in differences in plant tissue nutrients among watersheds and streamflow data, indicating increased losses of NO3− with accompanying losses of Ca2+ in response to further N additions to a N-saturated system.

Abstract:We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreated); and WS3 (24-year-old, treated with (NH 4 ) 2 SO 4 since 1989 at the rate of 35 kg N·ha -1 ·year -1 ). Net nitrification was 141, 114, and 115 kg N·ha -1 ·year -1 , for WS3, WS4, and WS7, respectively, essentially 100% of net N mineralization for all watersheds. Temporal (seasonal) patterns of nitrification were significantly related to soil moisture and ambient temperature in untreated watersheds only. Spatial patterns of soil water NO 3 -of WS4 suggest that microenvironmental variability limits rates of N processing in some areas of this N-saturated watershed, in part by ericaceous species in the herbaceous layer. Spatial patterns of soil water NO 3 -in treated WS3 suggest that later stages of N saturation may result in higher concentrations with less spatial variability. Spatial variability in soil N variables was lower in treated WS3 versus untreated watersheds. Nitrogen additions have altered the response of N-processing microbes to environmental factors, becoming less sensitive to seasonal changes in soil moisture and temperature. Biotic processes responsible for regulating N dynamics may be compromised in N-saturated forest ecosystems.Résumé : Nous avons étudié les patrons temporels et spatiaux de la dynamique de l'azote (N) du sol, de 1993 à 1995, dans trois bassins versants de la forêt expérimentale de Fernow : WS7 (âgé de 24 ans et non traité); WS4 (mature et non traité); WS3 (âgé de 24 ans et traité avec (NH 4 ) 2 SO 4 depuis 1989 au taux de 35 kg N·ha -1 ·an -1 ). La nitrification nette était respectivement de 141, 114 et 115 kg N·ha -1 ·an -1 pour WS3, WS4 et WS7, soit essentiellement 100% de la minéralisation nette pour tous les bassins versants. Les patrons temporels (saisonniers) de nitrification étaient significativement reliés à l'humidité du sol et à la température ambiante seulement dans les bassins versants non traités. Les patrons spatiaux de NO 3 -de la solution de sol du WS4 indiquent que la variabilité micro-environnementale limite le taux de recyclage de N dans certaines aires de ce bassin versant saturé en N, en partie à cause des espèces d'éricacées pré-sentes dans la strate herbacée. Les patrons spatiaux de NO 3 -de la solution du sol dans le WS3 traité indiquent que des stades plus avancés de saturation de N peuvent se traduire par de plus fortes concentrations avec moins de variabilité spatiale. La variabilité spatiale des variables de N du sol était plus faible dans le WS3 comparativement aux bassins versants non traités. Les ajouts de N ont altéré la réponse des microorganismes associés au cycle de N vis-à-vis les facteurs environnementaux en les rendant moins sensibles aux changements saisonniers d'humidité et de température du sol. Les processus biotiques responsables de la régulation de la dynamique de N peuvent être compromis dans les éco-systèmes forestiers saturés en N...

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