Gayana Bot. 62(2): 88-97, 2005 ISSN 0016-5301

ARTÍCULOS REGULARES

NITROGEN FLUXES IN A NOTHOFAGUS OBLIQUA FOREST AND A PINUS RADIATA PLANTATION IN THE CENTRAL VALLEY OF SOUTHERN CHILE

FLUJOS DE NITROGENO EN UN BOSQUE DE NOTHOFAGUS OBLIQUA Y UNA PLANTACION DE PINUS RADIATA EN EL VALLE CENTRAL DEL SUR DE CHILE

Carlos Oyarzún¹, Roberto Godoy^2, Jeroen Staelens³, Claudia Aracena¹ & Juan Proschle¹

¹Instituto de Geociencias, ²Instituto de Botánica, Universidad Austral de Chile, Casilla 567, Valdivia, Chile.
³Laboratory of Forestry, Ghent University, B-9090 Gontrode, Belgium. coyarzun@uach.cl

ABSTRACT

Forest structure and tree species can have a significant impact on total atmospheric nitrogen (N) deposition; for example, deciduous forests have higher N requirements than coniferous forests. However, knowledge about the effect of the conversion of native Chilean vegetation cover by exotic plantations on N cycling is scarce. The aim of this study was to determine the effect of the replacement of a Nothofagus obliqua native forest by a Pinus radiata plantation in southern Chile on the chemistry of throughfall, stemflow, soil water infiltration and percolation. Pinus radiata stemflow was more acidic (pH 4.7) than precipitation (pH 5.2) and throughfall (pH 5.6), while in the Nothofagus forest stemflow and throughfall pH were 6.3 and 6.1, respectively. In the Nothofagus forest, the soil water infiltration and percolation at 150 cm depth pH were 6.2 and 6.3, while in the Pinus plantation pH were 5.9 and 6.0, respectively. Throughfall and stemflow were enriched in DIN (NH₄⁺-N + NO₃^--N) and DON in both forests. DIN fluxes were higher in throughfall in the Nothofagus forest (7.5 kg ha^-1 yr^-1) than in the Pinus plantation (6.4 kg ha^-1 yr^-1). DIN infiltration fluxes were much higher in the Pinus plantation (NH₄⁺-N = 3.1 kg ha^-1 yr^-1, NO₃^--N = 5.4 kg ha^-1 yr^-1) than in the Nothofagus forest (NH4⁺-N = 0.4 kg ha^-1 yr^-1, NO3^--N = 1.5 kg ha^-1 yr^-1), suggesting a lower N immobilization and/or plant uptake in the Pinus plantation.

Keywords: Ammonium, infiltration, nitrate, soil percolation, throughfall.

RESUMEN

Es conocido que las especies arbóreas y la estructura de los bosques tienen un significativo impacto sobre la depositación de N, y que los requerimientos de nitrógeno de los bosques deciduos son mayores que los bosques de coníferas. Sin embargo, el conocimiento acerca de los efectos de la sustitución de vegetación nativa chilena por plantaciones exóticas sobre el ciclo del N es escaso. El objetivo de este estudio fue determinar los efectos del reemplazo de un bosque nativo de Nothofagus obliqua por una plantación de Pinus radiata sobre la química de la precipitación directa, escurrimiento fustal, infiltración y percolación del agua en el suelo. El agua del escurrimiento fustal en Pinus radiata fue más ácida (pH 4,7) que la precipitación directa (pH 5,6) y la precipitación (pH 5,2), mientras que en el bosque nativo de Nothofagus el pH del escurrimiento fustal y la precipitación directa fueron 6,3 y 6,1, respectivamente. En el bosque de Nothofagus, el pH del agua de infiltración y percolación a 150 cm de profundidad fueron de 6,2 y 6,3, respectivamente; mientras que en la plantación de pinos fueron de 5,4 y 6,0, respectivamente. La precipitación directa y el escurrimiento fustal fueron enriquecidos en nitrógeno inorgánico (DIN) y orgánico (DON) en ambos ecosistemas forestales. Los mayores flujos de DIN ocurrieron en la precipitación directa, tanto en el bosque de Nothofagus (7,5 kg ha^-1 año^-1) como en la plantación de Pinus (6,4 kg ha^-1 año^-1). En el agua de infiltración del suelo, los flujos de DIN fueron mayores en la plantación de Pinus (NH₄⁺-N = 3,1 kg ha^-1 año^-1, NO₃^--N = 5,4 kg ha^-1 año^-1) que en el bosque de Nothofagus (NH₄⁺-N = 0,4 kg ha^-1 año^-1, NO₃^--N= 1,5 kg ha^-1 año^-1), sugiriendo una inmovilización en el suelo o consumo por la vegetación.

Palabras Claves: Amonio, infiltración, nitrato, percolación, precipitación directa.

INTRODUCTION

Precipitation passing through forest canopies as throughfall or stemflow is enriched or depleted in certain ions depending on ion reactivity and on the nature of the canopy (Houle et al. 1999). Furthermore, when water infiltrates the forest floor and mineral soil, its chemistry is further modified depending on soil characteristics, litterfall quality and tree species. Throughfall and stemflow have been reported to have significant impacts on forest biogeochemical cycles (Parker 1983, Likens & Bormann 1995). Throughfall has been estimated at 60-86% of total precipitation for broadleaved temperate forests and 55-82% for conifer plantations in south-central Chile, and stemflow has been shown to account for 1 - 8% in the broadleaved forests and 1 - 13% in the coniferous stands (Huber & Iroumé 2001). A few studies of throughfall and stemflow chemistry were undertaken in forests of southern Chile, of which the majority is focusing at native temperate forests such as Fitzroya cupressoides (Molina) I.M.Johnst. located in the Coastal Range (Oyarzún et al. 1998) and Nothofagus pumilio (Poepp. & Endl.) Krasser and N. betuloides (Mirb.) Oerst. located in the Andes (Godoy et al. 1999, Oyarzún et al. 2004). Comparative studies with exotic plantations are scarce (Uyttendaele & Iroumé 2002), and consequently the effect of the substitution of Nothofagus forests by exotic plantations on soil water chemistry is unknown.

Research in the northern hemisphere (Cole & Rapp 1981) has reported that the uptake of nitrogen by temperate deciduous forests can be about 63% higher than by coniferous forests. Furthermore, forest structure, canopy density and tree species were reported to have a significant impact on total atmospheric nitrogen deposition onto forest stands (De Schrijver et al. 2004). Therefore, it is probable that the substitution of native vegetation cover by exotic plantations has an impact on the chemistry of throughfall, stemflow and soil water. Native forests in the Valdivian Eco-region (36º - 48º S) have been affected by human disturbances including degradation due to non-sustainable logging practices, and destruction due to human-set fires as well as conversion to agriculture and fast-growing plantations (Lara et al. 1996). It is estimated that between 1974 and 1992 around 11,000 ha y^-1 of native forest was converted into exotic plantations of Pinus radiata D.Don and Eucalyptus spp., especially in the coastal range of south-central Chile. The Nothofagus obliqua (Mirb.) Oerst. forests of the central valley have suffered a particularly drastic reduction in land cover and original species composition (San Martín et al. 1991, Veblen et al. 1996b). Ecosystem remnants of this forest type are poorly represented in the Chilean system of national parks and reserves (SNASPE) (Armesto et al. 1998). These small areas located at the Central Valley in southern Chile have a great significance in biodiversity conservation (Veblen et al. 1996a), and based on research in other regions, likely also in the maintenance of groundwater water quality (e.g. De Schrijver et al. 2000).

This paper analyzes the effect of the replacement of a Nothofagus obliqua native forest by a Pinus radiata plantation on the concentrations and fluxes of nitrogen in the canopy and soil water. We hypothesized that the nitrogen fluxes in throughfall, soil water infiltration, and percolation water would be altered due to the different composition and structure of the vegetation.

MATERIALS AND METHODS

STUDY AREAS

The Nothofagus obliqua forest and the Pinus radiata plantation are located in the Central Valley (40º 07' S, 72º 51' W, 160 m above sea level) at a distance of 0.5 km of each other. The climate is classified as temperate with less than four dry months and the annual precipitation for the period June 2003-May 2004 was 1187 mm. Maximum precipitation occurred during the period April-September with 75% of the rainfall. Both soils originate from volcanic ash. Soils of the Nothofagus forest are denominated "trumaos", while in the plantation red clay soils occur, originating from older volcanic ash. The geological substrate is a metamorphic complex, mainly consisting of mica schist with quartz lenses.

The dominant tree species in the Nothofagus forest is N. obliqua. The stand has a subcanopy dominated by Aextoxicon punctatum Ruiz et Pav., and an understorey of the bamboo Chusquea quila Kunth and the small tree Rhaphithamnus spinosus (A.L.Juss) Molina. The aspect of the stand is southerly and the mean slope is 35-40%. The growing season lasts from September till April. The Pinus radiata site is a fast-growing monoculture with an understorey of Aristotelia chilensis (Molina) Stuntz, Chusquea quila and Rubus contrictus P.F.Müll. et Lefèvre. The aspect of the stand is northerly and the mean slope is 35-40%. Tree density and basal area are much higher in the pine plantation than in the Nothofagus forest (Table I).

DATA COLLECTION

Bulk precipitation, throughfall, and stemflow water were collected monthly over a period of 18 months (June 2003 - November 2004). Bulk precipitation was measured with three funnel collectors, each with a surface area of 200 cm², attached to a plastic 2-l bottle. The bottles were set inside an opaque tube in order to avoid light penetration that could promote algae growth. The bulk precipitation was measured in a grassland about 50 m outside the Nothofagus forest. In addition to the rain collectors, a HOBO standard rain gauge was installed for measuring the amount of rainfall.

The throughfall collectors, their maintenance, and sample collection procedures were identical to those used for bulk precipitation, according to Kleemola & Soderman (1993). In both the Nothofagus forest and the pine plantation, 15 throughfall collectors were systematically located on a 0.1 ha plot. The stemflow collectors consisted of plastic collars attached to 25 L containers, for 4 trees that were representative of the tree diameter distribution.

Soil water infiltration was collected with 6 small zero-tension lysimeters (900 cm²). These collectors were installed at 10 cm depth and were covered by an undisturbed soil layer. Percolation water was collected at 80 and 150 cm depth using a Pressure Vacuum Soil Water Sampler. Sixty centibars of soil suction was applied 24 hr before collection. Two replicates for each depth were located in the same plots as the throughfall, stemflow and infiltration collectors. Volume of water percolation at 150 cm depth was estimated using the water balance method, according to the references of Oyarzún & Huber (1999).

CHEMICAL ANALYSES

The samples were frozen, stored and analyzed within 2 weeks after collection. For each plot, the samples were pooled to one monthly sample of bulk precipitation, throughfall, stemflow, soil water infiltration and percolation before chemical analysis. After filtration of the water samples through a borosilicate glass filter (Whatmann) of 0.45 µm, pH and conductivity were determined using specific electrodes. NO₃-N was determined by a colorimetric method based on the reduction of cadmium (Clesceri et al. 1998). NH₄-N was measured by the phenate method (Clesceri et al. 1998). Organic-N was calculated by subtracting NH₄-N concentration of total Kjeldahl nitrogen (sum of organic nitrogen and NH₄-N) measured by the Kjeldahl method (Clesceri et al. 1998).

DATA ANALYSES

Element fluxes were calculated for the period June 2003-May 2004, by multiplying the measured amount of water in the different compartments of the forest and plantation with the element concentration (volume-weighted averages). Bulk deposition was subtracted from the sum of throughfall and stemflow to obtain net canopy exchange (NCE). Negative NCE values indicate uptake within the canopy (Houle et al. 1999). One-way ANOVA and LSD tests were applied to find differences in nutrient concentrations in precipitation, throughfall, stemflow and soil solution between the experimental sites.

RESULTS

PRECIPITATION CHEMISTRY

Volume-weighted bulk precipitation pH averaged 5.2 and average electrical conductivity was 11.6 µS cm-1 (Table II). Concentrations of NO₃--N, NH₄⁺-N and DON in the precipitation were 26 µg L^-1, 180 µg L^-1 and 121 µg L^-1, respectively.

THROUGHFALL AND STEMFLOW CHEMISTRY

Average throughfall pH was 6.1 and 5.6 for the Nothofagus forest and 5.6 for the Pinus plantation, respectively (Table II). Electric conductivity increased from 11.6 µS cm^-1 in bulk precipitation to 37.9 and 24.5 µS cm^-1 in the Nothofagus and pine forest, respectively (Table II). In both plots the concentration of nitrate in throughfall was significantly higher than in bulk precipitation (p < 0.05) but not between the stands (p > 0.05). Ammonium concentration in the throughfall of the Nothofagus forest (1213 µg L^-1) was significantly (p < 0.05) higher than in bulk precipitation, but not significantly different from throughfall in the Pinus plantation (861 µg L^-1, p = 0.29) (Table II). There was a trend for DIN and DON concentrations to be higher in throughfall than in the bulk precipitation in both forest stands, particularly for nitrate, but the difference was not significant (Fig. 1). The enrichment concentration ratios of throughfall to precipitation were as follows: 6.7 and 4.8 for NH₄+-N, 7.7 and 7.4 for NO₃^--N, and 1.8 and 5.7 for DON in the Nothofagus and the Pinus stand, respectively (Table II).

FIGURE 1. Monthly nitrogen concentrations (µg L-1) in precipitation (P) and throughfall (TF) for a Pinus radiata plantation and a Nothofagus forest from June 2003 till November 2004. FIGURA 1. Concentraciones mensuales de nitrógeno (µg L-1) en la precipitación (P) y precipitación directa (TF) en una plantación de Pinus radiata y un bosque de Nothofagus, desde junio 2003 hasta noviembre 2004.

Stemflow pH was higher (6.3) in the Nothofagus stand and lower (4.7) in the Pinus plantation than in precipitation (Table II). In the two stands, electrical conductivity of stemflow was greater than in precipitation. Nothofagus stemflow concentrations of NO₃^--N, NH₄⁺-N and DON were significantly higher than in precipitation (p < 0.05), but this was not the case in the pine plantation. The enrichment concentration ratios of stemflow to precipitation were as follows: 5.6 and 2.2 for NH₄⁺-N, 13.0 and 12.2 for NO₃^--N, and 4.8 and 2.9 for DON in the Nothofagus and the Pinus stand, respectively (Table II).

SOIL SOLUTION CHEMISTRY

The pH of the soil water infiltration under Pinus (5.4) was similar to that of precipitation (5.2), whereas pH under Nothofagus was slightly higher (6.2). At 80 cm depth the soil solution pH had further risen to 5.9 under the Pinus plantation and had remained similar in Nothofagus forest (6.2). The difference between soil solution pH at 80 cm and at 150 cm depth was negligible in both plots (Table II). NO3--N concentrations in soil water infiltration were significantly higher in the Pinus plantation (814 µg L^-1, p < 0.01) and the Nothofagus forest (591 µg L^-1, p < 0.05) than in precipitation (26 µg L^-1). Ammonium concentrations were similar under Nothofagus (180 µg L^-1) and in precipitation (180 µg L^-1), but higher under Pinus (401 µg L^-1). In both plots, DON concentrations in soil water infiltration were not significantly different compared with precipitation (Table II). There was a general trend for NO₃^--N, NH₄⁺-N and DON concentrations to be lower at 80 and 150 cm depth than in bulk precipitation (Fig. 2). Nitrate, ammonium and DON concentrations at 80 and 150 cm depth were lower than those in soil water infiltration.

FIGURE 2. Monthly nitrogen concentrations (µg L-1) in precipitation (P) and soil water infiltration (INF) for a Pinus radiata plantation and a Nothofagus forest from June 2003 till November 2004. FIGURA 2. Concentraciones mensuales de nitrógeno (µg L-1) en la precipitación (P) e infiltración de agua en el suelo (INF) en una plantación de Pinus radiata y un bosque de Nothofagus, desde junio 2003 hasta noviembre 2004.

WATER AND NUTRIENT FLUXES

Net precipitation (throughfall plus stemflow) represented 73.1% of the gross precipitation in the Nothofagus stand and 86.9% in the Pinus stand (Table III). Therefore, canopy interception amounted 26.9 and 13.1% in the two stands, respectively. The soil percolation at 150 cm depth was estimated to be 20.3 and 23.5% of the gross precipitation in the Nothofagus forest and the Pinus plantation.

Bulk precipitation deposition of DIN and DON were 2.5 and 2.1 kg ha^-1 yr^-1, respectively (Table III). In the two stands, NH₄⁺ and NO₃^- throughfall fluxes were higher than bulk deposition. Stemflow fluxes of DIN and DON were very low in both stands due to the small quantities of water flowing down the trunks, especially in the Nothofagus forest (Table III). The highest fluxes of inorganic N in the soil water infiltration were found in the P. radiata plantation (NH₄⁺-N = 3.1 kg ha^-1 yr^-1, NO₃^--N = 5.4 kg ha^-1 yr^-1), while the total DIN flux in the soil water infiltration in the Nothofagus stand was only 1.9 kg ha^-1 yr^-1. The NH₄⁺-N flux was reduced in both stands after passing through the humus layer, with the decrease being more pronounced in the Nothofagus stand. NO₃- fluxes in the soil water infiltration increased in both stands compared to net precipitation, with the increase in NO₃^--N amount being most clearly in the Pinus stand. Nitrogen fluxes at 150 cm depth were very low in both stands (Table III).

DISCUSSION

A few previous studies on the interaction of precipitation with forest canopies have been carried out in southern Chile (Godoy et al. 1999, 2001, 2003; Oyarzún et al. 1998, 2002, 2004; Uyttendaele & Iroumé 2002). The pH of bulk precipitation in the present study (Table II) was slightly lower (pH = 5.2) than those found at sites located in the Andean Range (pH = 5.7 - 6.1) (Godoy et al. 2001) and a site near Valdivia (pH = 6.1) (Uyttendaele & Iroumé 2002), probably because of moderate anthropogenic inputs. The study site is influenced by light industries and probably by a small town located approximately 2 km to the north. In regions subject to strong anthropogenic influence, rain generally has pH values lower than 5 (Likens & Bormann 1995). Electrical conductivity was similar (11.6 µS cm^-1) to the reports for forested sites located in the Andean Range (10.9 - 11.4 µS cm^-1) and slightly lower than found in agriculture-cattle sites at the Central Valley (13.7 - 22.9 µS cm^-1) (Oyarzún et al. 2002). Most coniferous canopies show a tendency toward net acidification of bulk precipitation inputs (Cronan & Reiners 1983, Edmonds et al. 1995). In our study, only the pH of stemflow in the Pinus radiata stand was slightly lower than the precipitation pH. Other studies in southern Chile (Uyttendaele & Iroumé 2002) and southeastern Australia (Crockford et al. 1996) have found similar results, suggesting that stemflow pH is strongly influenced by organic acids released by stem tissues.

In general, bulk precipitation deposition of N in southern Chile is considered to be among the lowest of the world. Bulk deposition of dissolved inorganic nitrogen (DIN= NH₄⁺-N + NO₃^--N) ranged between less than 1 kg N ha-1 yr-1 in the Costal Range to about 5 kg N ha^-1 yr^-1 for the Andean Range (Godoy et al. 2003). In the agricultural region of Osorno, Central Valley (40 S), the annual bulk deposition was 6.9 kg N ha^-1 yr^-1 indicating the influence of livestock farming (Oyarzún et al. 2002). In our study site, the nitrogen bulk deposition was 2.5 kg N ha^-1 yr^-1 (Table II) suggesting a small anthropogenic influence. However, the present study does not consider the input from dry deposition, which may be important in the Central Valley of southern Chile (Godoy et al. this issue, Staelens et al. this issue). Data from forested sites in the USA and Europe (Lovett 1992) showed that net canopy exchange of N (throughfall plus stemflow minus bulk deposition) was negative for NH₄⁺ and NO₃^- at all sites, indicating that canopies were clearly sinks for inorganic N. In our study both DIN and DON concentrations increased in stemflow and throughfall relative to precipitation, indicating a net N enrichment when passing through the forest canopies. This net enrichment is the result of two processes: the washing off of the unquantified N input by dry deposition, on the one hand, and the N uptake from wet, dry particulate and gaseous deposition by leaves, twigs, stem surfaces, and lichens, on the other hand (Staelens et al. this issue). Oyarzún et al. (2004) have shown the importance of the epiphytic lichen Sticta living in the canopies and branches of old-growth trees in the Puyehue National Park at the Andean Range.

The inorganic N concentrations in soil water infiltration at 10 cm depth (Table II) were 10-15 times higher than corresponding measurements in Nothofagus pumilio and N. betuloides forests located in Andean Range (Godoy et al. 2001). The throughfall and stemflow input of about 7 kg DIN ha^-1 yr^-1 decreased to almost 2 kg ha^-1 yr^-1 in the soil water infiltration in the Nothofagus stand and increased to more than 8 kg ha^-1 yr^-1 in the Pinus stand. Compared to net precipitation, the ammonium flux decreased and the nitrate flux increased in both stands by passing trough the humus and upper soil layer. This can be attributed to mineralization, nitrification and plant uptake occurring in the topsoil. The clear difference in nitrate soil water infiltration might be related to species-specific nitrogen uptake, because nitrate is often assumed to be a more important mineral N source for broadleaved trees than for conifers (De Schrijver et al. 2004). However, forest trees are able to use different organic and inorganic N compounds, depending on N availability in soil and the composition of the below-ground mycorrhizal community (Wallenda et al. 2000).

Inorganic and organic nitrogen concentrations were much lower in the soil percolation water than at the forest floor in both stands. Similarly, the nitrogen fluxes at 150 cm depth were negligible (Table III) in comparison with N fluxes at the forest floor. The low soil water fluxes at 80 and 150 depth reflect strong N retention due to abiotic and biotic immobilization (Johnson et al. 2000, Boeckx et al. 2004). Furthermore, inorganic nitrogen could be exported from the plantations via subsurface flow to streams, given the steep slope of both forest sites. However, this should be confirmed by further research.

ACKNOWLEDGMENTS

This study was supported by Fondecyt Project N 1030344. This paper is a contribution to the Millenium Project Forecos P01-057-F (Mideplan). J. Staelens was funded as a research assistant of the Research Foundation - Flanders (FWO-Vlaanderen, Belgium).

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Recibido 07/02/05
Aceptado 13/06/05