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vol.62 número2ALTERACION DEL CICLO DEL NITROGENO POR LAS ACTIVIDADES AGRICOLAS, SUS CONSECUENCIAS AMBIENTALES Y SOBRE LA SALUD HUMANAMONITOREO ECOHIDROLOGICO EN ECOSISTEMAS NATURALES Y MANEJADOS EN EL SUR DE CHILE índice de autoresíndice de materiabúsqueda de artículos
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Gayana. Botánica

versión impresa ISSN 0016-5301versión On-line ISSN 0717-6643

Gayana Bot. v.62 n.2 Concepción  2005

http://dx.doi.org/10.4067/S0717-66432005000200006 

 

Gayana Bot. 62(2): 110-119, 2005 ISSN 0016-5301

ARTÍCULOS REGULARES

 

Dry deposition of nitrogen to passive samplers in grassland and forest canopies in the Central depression of southern Chile

DEPOSITACION SECA DE NITROGENO EN PRADERA Y DOSEL DE BOSQUES EN LA DEPRESION INTERMEDIA DEL SUR DE CHILE

 

Roberto Godoy1, Jörgen Haneke2, Jeroen Staelens3, Carlos Oyarzún4, Leandro Paulino1 & Mónica Barrientos1

1Universidad Austral de Chile, Instituto de Botánica, Casilla 567, Valdivia, Chile.
2Technische Universität Braunschweig, Institut für Mikrobiologie, Spielmannstraße 7, D38106 Braunschweig, Germany.
3Ghent University, Laboratory of Forestry, Geraardsbergsesteenweg 267, B-9090 Gontrode, Belgium.
4Universidad Austral de Chile, Instituto de Geociencias, Casilla 567, Valdivia, Chile. rgodoy@uach.cl


ABSTRACT

The growth of intensive livestock farming in the central depression of southern Chile (40° S) during the last century has resulted in elevated emissions of inorganic nitrogen (N) compounds into the atmosphere, particularly of NH3. Consequently, atmospheric N has been deposited to lakes, vegetation, and soil. Much attention has been paid to wet N deposition in southern Chile, while almost nothing is known about dry N deposition in this region. Our aim is to use an exploratory approach with SAM (Standard Air Monitoring) filters to investigate the potential contribution of the dry N deposition to the total N deposition in the central depression of southern Chile. Furthermore, the relationship between micro-climatological patterns and dry deposition was studied, and dry deposition to passive samplers in grasslands was compared to the deposition to samplers in two forest canopies. Wet and dry deposition of N were measured during one year and six months, respectively, in a grassland area, a Nothofagus obliqua (Mirb.) Oerst. forest and a Pinus radiata D. Don. plantation located in the central depression of southern Chile. After twelve months, the wet atmospheric N deposition was 3.4 kg N ha-1, including organic and inorganic N fluxes, while the amount of dry deposited NH4-N on passive samplers in the grassland was 12.2 kg N ha-1 during six months. No clear seasonal pattern in wet deposition was found over the year and no trend in dry deposition appeared after six months of observation. The results reflect the large variations between the deposition of NH4-N to passive samplers in grassland and samplers within the canopy of Nothofagus forest and Pinus plantation. After inclusion of an estimate of dry deposited N, the atmospheric N load is greater than reported values for evergreen and deciduous forests in southern Chile.

Keywords: Atmospheric deposition, nitrogen, Nothofagus obliqua forest, Pinus radiata plantation, surrogate surface.

RESUMEN

Las crecientes actividades de agropecuaria intensiva en la depresión intermedia del sur de Chile (40° S) durante los últimos siglos han resultado en emisiones elevadas de compuestos de nitrógeno inorgánico (N) en la atmósfera, particularmente de NH3. Consecuentemente, el N atmosférico ha sido depositado en lagos, en la vegetación y en el suelo. Mucha atención ha sido dada a la depositación húmeda de N en el sur de Chile, mientras que casi nada se sabe sobre la depositación seca de N en esta región. Nuestro objetivo fue usar una aproximación exploratoria a través de filtros SAM (Monitoreo Estándar del Aire) para investigar la potencial contribución de la depositación seca de N sobre la depositación total de N en la depresión intermedia del sur de Chile. Además, se estudió la relación entre los patrones micro-climatológicos y la depositiación seca de nitrógeno, y la comparación de la depositación seca en filtros pasivos en una pradera y el dosel de dos bosques. La depositación seca y húmeda de N fueron medidas durante un año y seis meses, respectivamente, en un área de pradera, en un bosque de Nothofagus obliqua (Mirb.) Oerst. y en una plantación de Pinus radiata D.Don., ubicados en la depresión intermedia del sur de Chile. Después de doce meses, la depositación húmeda de N fue de 3,39 kg ha-1, incluyendo los flujos de N orgánico e inorgánico, mientras que la cantidad de depositación seca de NH4-N en colectores pasivos en la pradera fue de 12,18 kg ha-1 durante seis meses. No se encontró un claro patrón estacional en la depositación húmeda de N durante un año y tampoco alguna tendencia en la depositación seca después de seis meses de observación. Los resultados reflejan la gran variación entre la depositación de NH4-N en colectores pasivos en la pradera y bajo el dosel del bosque de Nothofagus y la plantación de Pinus. Al incluir una estimación de la cantidad de depositación seca de N, el aporte atmosférico de N es más alto que valores reportados para bosques siempreverdes y deciduos en el sur de Chile.

Palabras claves: Bosque de Nothofagus obliqua, depositación N, plantación de Pinus radiata.


INTRODUCTION

The chemistry of precipitation in southern Chile reflects one of the closest known approximations to pre-industrial atmospheric conditions (Weathers & Likens 1997, Godoy et al. 2001). Wet deposition of inorganic nitrogen is reported to be < 1 kg ha-1 yr -1 in coniferous forests of Fitzroya cupressoides (Molina) I.M.Johnst. in the Cordillera de la Costa in southern Chile (Hedin et al. 1995, Oyarzún et al. 1998). Godoy et al. (1999) and Oyarzún et al. (2004) found an inorganic nitrogen deposition < 5 kg ha-1 yr -1 in Nothofagus forests of the Cordillera de Los Andes.

Most biogeochemical research in southern Chile has been carried out in mountain areas. Much less is known about nutrient fluxes in the forest ecosystems in the central depression, where there is a potential impact of nitrogen deposition (Oyarzún et al. 2002, Staelens et al. submitted). However, in order to quantify the actual atmosheric N input onto forest ecosystems, the total deposition has to be separated into wet and dry components. Similarly to other regions with extensive cattle breeding (e.g. De Schrijver et al. 2004), dry deposition may be an important part of the total atmospheric input to forest ecosystems in the central depression of Chile (Oyarzún et al. 2002). Unfortunately, no estimates of dry N deposition in the forests of southern Chile are available at present.

Atmospheric ammonia (NH3) is a pollutant of increasing interest, being along with sulphur dioxide and nitrogen oxide (NOx) one of the three main primary pollutants leading to acidifying deposition. The NOx and NH3 emissions may result from natural processes, food production, and energy production (Galloway et al. 2004). During the last decades, the global introduction of reactive N into the biosphere by food and energy production increased considerably (Galloway et al. 2004). Agriculture contributes about two-thirds of global NH3 emissions to the atmosphere (Davison & Mosier 2004). In addition, the short atmospheric life time of NH3 means that the concentrations vary substantially in space and time (Fangmeier et al. 1994).

Anthropogenic land cover change in the central depression of southern Chile (40° - 42° S) is the most evident processes of deforestation and agricultural expansion (Neira et al. 2002). A large fraction of the Nothofagus forests in the same region was cleared for agriculture during the last century. As a result, patches of second-growth forest cover vast areas of the regional landscape, leaving only scattered remnants of intensive agriculture activity (Veblen et al. 1996). Anthropogenic activities such as transport, industry and agriculture have been increasing in the central depression of this region and can substantially alter the atmospheric N load and subsequently the N deposition (Godoy et al. 2003). Oyarzún et al. (2002) found in an altitudinal transect of this area that wet inorganic-N deposition has a markedly seasonal variation in this agricultural region, with the heighest values occurring in spring and summer. The annual rates of wet NH4-N depositions range from 2.8 kg N ha-1 yr -1 in the mountain forests to 6.4 kg N ha-1 yr -1 in the agricultural region.

Cattle pastures located in the central depression of Chile are responsible for about 70 % of the national milk production and 45 % of the national meat production (Salazar et al. 2003). NH3 can be emitted from pastures, cowshed, dung and manure or mineral applications, and subsequently can be deposited to forest ecosystems, especially during the summer period near to the emission source. The total ammonia emission of this region was estimated to be 5,623 Mg NH3-N yr-1 of which about 3,630 Mg NH3-N yr-1 came from cattle (Salazar, personal communication).

Natural and semi-natural ecosystems are considered as important sinks for pollutants like NH3 because of their low nitrogen status compared to managed crop ecosystems (Sutton et al. 1992). Forest structure, canopy density and tree species composition were reported to have a significant impact on the dry deposition of gas and particulate pollutants. Dry deposition to coniferous forests is generally higher than to deciduous forests due to their evergreen character, the often higher leaf area index and the higher collecting efficiency of needles compared to leaves (De Schrijver et al. 2000).

Here we explore the potential contribution of dry N deposition to the total N deposition in the central depression of southern Chile, a region with intensive agriculture-livestock activity. Dry deposition was estimated by passive sampling using Standard Air Monitoring filters. Furthermore, the relationship between micro-climatological patterns and dry deposition was studied, and dry deposition to passive samplers in grasslands was compared to the deposition to samplers in the canopy of a Nothofagus obliqua forest and a Pinus radiata plantation.

MATERIAL AND METHODS

STUDY AREA

The study area is located in Paillaco, province of Valdivia, in the central depression of southern Chile (40°07' S, 72°51' W, 160 m a.s.l.). The mean annual precipitation in the region is about 1,500 mm, while the mean monthly temperature varies from 8.8 to 15.1 °C with an annual mean of 10.8 °C. The soils of the area are classified as andisols, typic dystrandepts (Tosso 1985). The prevailing wind direction is north-west during winter and west during summer. Experimental sites were established on grassland, in a Nothofagus obliqua (Mirb.) Oerst. forest and in a Pinus radiata D.Don. Plantation, located near each other. The grassland had a species composition consisting of Lolium perenne L. and Trifolium repens L., and is ploughed and newly sown every two years. The second-growth forest of N. obliqua has a mean tree height of 35 m, a stand density of 757 trees ha-1, and a canopy cover of 70 % during the growing season (October-March). The average tree age is 120 years. The P. radiata plantation is 30 years old, had a mean tree height of 24 m, and a stand density of about 1,743 trees ha-1.

MICROCLIMATE AND WET DEPOSITION

The present study was carried out from May 2003 to April 2004. Air temperature was measured with a data recorder in a microclimate station. The amount of precipitation was determined from hourly rain-gauge data recorded with a Heater Recording HOBO (Ben Meadows Company). Precipitation for chemical analyses was sampled monthly using two plastic rain gauges with a surface area of 200 cm2 attached to a 2 L collection bottle, installed in an open area in grassland (no trees within 20 m of the sampling point). The rain gauges were covered with plastic nets in order to prevent insects and leaves entering the collection bottles, and designed with a plastic ring in order to exclude bird droppings (Kleemola & Soderman 1993). The two monthly samples were pooled to one sample for chemical analyses within 48 h after collection. Samples were filtered through a borosilicate glass filter (Whatman) of 0.45 µm. 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). The detection level was 1 µg L-1 for nitrate and ammonium. Organic-N was calculated by subtracting NH4-N concentration of Kjeldahl nitrogen (sum of organic nitrogen and NH4-N) measured by the Kjeldahl method (Clesceri et al. 1998). Monthly element fluxes were calculated by multiplying the measured amount of water in the bulk precipitation with the element concentration in the rain gauges (volume-weighted averages).

DRY DEPOSITION

Dry deposition of NH3 gas and NH4+ aerosol was measured by passive sampling with the Standard Air Monitoring (SAM) filter method according to De Temmerman (1996), supported on methological recomendation of Kleemola & Sodermann (1993). Filter papers (Watmann N° 1, 12.5 cm diameter) were treated with 3.5 mL citric acid and dried for 20 min at 130 °C. Ammonia gas reacts with citric acid to form ammonium hydroxide. The NH3 binding capacity of this amount of acid is in excess of the greatest amount for the collecting period. Although equipments for measuring some gaseous pollutant concentrations are based on continuously recording detectors, impregnated filter for total ammonium is considered a valid monitoring method (Kleemola & Soderman 1993).

Dry deposition was sampled over six months (October 2003 - March 2004) at representative places in the grassland, the N. obliqua forest and the P. radiata plantation. At each location, six papers were exposed for two weeks inside a standard meteorological box to protect from rain. The described method is similar to passive samplers solution to collect NH3 as in Vandré & Kaupenjohann (1998) which provide only a relative response to NH3 concentration in the air. In the forest and the plantation, the boxes were installed approximately 3 m under the canopy at a height of 30 and 20 m, respectively. In the grassland the box was set at a height of 1.5 m above soil level. After a fortnight's exposure, the filter papers were extracted by shaking (300 rpm) 2*1/12 of the 6 filter papers for 30 min in 50 mL deionized water (De Temmerman 1996). The ammonium concentration in the extract was measured by spectrophotometry (UV mini 1240 Schimadzu, 660 nm). The amount of dry deposition of NH3 gas and NH4+ aerosol (µg NHy-N dm-2 two weeks-1) on the filter papers was calculated using the two-sided surface area of the filter papers. The dry deposition amount to the filter papers was expressed as kg NHy-N ha-1 month -1 by simple extrapolation.

DATA ANALYSIS

Wilcoxon matched pairs tests were used to compare the monthly nitrogen input by the bulk precipitation deposition and dry deposition. This non-parametric equivalent of the paired t-test was used because of the small sample sizes (n = 6 for dry N input, and n = 12 for wet N input). Differences among means were determined at the p < 0.05 level. The influence of micro-climate factors on N deposition was examined by non-parametric Spearman rank analysis.

RESULTS

WET DEPOSITION

During the study period total precipitation was 1,223 mm yr-1. The precipitation had a clear seasonal distribution with much higher monthly amounts during the winter (June-September) than during the summer (December-January) (Fig. 1). The maximal monthly rainfall was 263 mm in June. The mean annual temperature in the studied year was 11°C. The maximal air temperatures > 20°C from October to March, with a maximum of 30 °C in March 2004. The monthly mean temperature for the same period was 10.3 °C to 16.3 °C. (Fig. 1).


FIGURE 1. Monthly bulk precipitation (mm) and mean air temperature (°C) from may 2003 to april 2004 at the central depression of southern Chile (40° S).

FIGURA 1. Precipitación mensual total (mm) y temperatura media del aire (°C) de mayo 2003 a abril 2004 en la depresión intermedia del sur de Chile (40° S).

The monthly wet deposition of ammonium-N was much higher than that of nitrate-N (Table I). The annual wet depositions in the study area were 2.38 and 0.16 kg ha-1 yr-1 for NH4-N and NO3-N, respectively. Hence, the ratio of NH4:NO3 was about 15. The monthly wet deposition of ammonium showed large seasonal variations, with a maximal contribution from November 2003 to April 2004 (Table I). At the period of study, the wet deposition of ammonium was the only variable that fitted significantly to climate variables, in this case minimum and mean temperature (Table II).


The wet deposition of organic-N was 0.85 kg ha-1 yr-1 , which is the third part of the inorganic N fluxes by wet deposition (2.54 kg ha-1 yr-1 ) and 1/4 of the total wet N deposition (Table I). Organic nitrogen showed great monthly variations without a clear seasonal pattern (Table II).


DRY DEPOSITION

The estimated dry deposition onto the passive samplers in the grassland was 12.18 kg NH4N ha-1 over the six-month period. The dry deposition amounts measured within the canopy of the forest vegetations were 16.71 and 7.88 kg NH4-N ha-1 for the Nothofagus forest and the Pinus plantation, respectively. The amount of dry deposited N onto the samplers was significantly higher (p < 0.05) in the Nothofagus forest than in the Pinus stand (Table III).


During the six measured months (October-March), the dry deposition onto the SAM filters in the grassland of 12.18 kg NH4-N ha-1 was clearly higher than the total wet deposition of 1.96 kg NH4-N ha-1 (Table I). The mean value for dry N deposition was significantly higher (p < 0.05) than that of wet N and wet NH4-N for the same period (Fig. 2).


FIGURE 2. Mean values of dry N, wet ammonium-N and wet N depositions from October 2003 to March 2004 in open air condition at the study area. Bars mean ± 1 standard deviation; asterisks mean significant differences between dry N deposition and wet (NH4-N and total N) depositions, p < 0.05.

FIGURA 2. Valores promedios de la depositación seca de N, húmeda de N-amonio y húmeda de N, de octubre de 2003 a Marzo de 2004 en condiciones de aire abierto en el área de estudio. Barras significan ± 1 desviación estándar; asteriscos significan diferencias significativas entre la depositación seca y húmeda (N-NH4 y N total), p < 0.05.

DISCUSSION

WET DEPOSITION

The observed precipitation pattern is typical of dry years in the central depression of southern Chile (Di Castry & Hajek 1976). Precipitation is very low compared to the Andean and Coastal mountain area in the same region. (Godoy et al. 1999, Oyarzún et al. 1998, 2004). The importance of NH4-N deposition compared with NO3-N can be attributed to the fact that agriculture is the only important activity in this area. Lower NH4/NO3 ratios of about 5 were found in a remote mountain area in the same region (Oyarzún et al. 2004).

The measured wet organic N deposition (0.85 kg ha-1 yr-1 ) was much lower than the 8.2 kg ha-1 yr-1 measured in a mountain area of the same region (Oyarzún et al. 2004). Organic N fluxes are particularly important in mountain areas, where forests are very efficient in trapping nutrients, especially nitrogen and cations of clouds and fogs (Weathers 1999). The fluxes of dissolved organic nitrogen (DON) represent an important source of N, but are often an unmeasured component of the nutrient budget. The atmospheric organic deposition of 41 measurements in the northern hemisphere suggests that organic N averages about one-third of the total atmospheric N deposition (Neff et al. 2002), which is higher than in the present study. The flux of inorganic N via wet deposition was 2.54 kg ha-1 yr-1 , which is an important contibution of the total N due to the agricultural activity at central depression. The total contribution of the wet nitrogen deposition in the present study was 3.39 kg ha1 yr1 , which can be considered as a low input given the extent of the agricultural livestock in Chile. Finally, it should be noted that wet deposition was measured using continuously open funnels, and not by wet-only samplers that are only uncovered during rain events. This implies that the bulk wet deposition results include some contribution of dry deposition onto the funnels. However, it is known that the difference between bulk and wet-only deposition is much smaller for nitrogen compounds than e.g. for calcium, magnesium, and potassium (Staelens et al. 2005).

DRY DEPOSITION

The dry deposition of N measured during spring and summer is often an indication of ammonia emissions. The highest value was observed in October (spring), probably as a result of the start of higher activities in the agricultural area. Dry and wet deposition both depend on atmospheric air concentrations. Since wet deposition showed maximum values during the summer months, a similar seasonal pattern might be expected for dry N deposition. However, the short measurement period did not allow to derive seasonal patterns, and no significant relationships were found between the dry deposition and mean temperature or precipitation at the monthly level (Table II).

Several studies in more polluted areas of the northern hemisphere have reported dominance of the dry inorganic-N deposition over the wet deposition (Andersen & Hovmand 1999, Fangmeier et al. 1994, De Schrijver et al. 2004). The N-losses from agricultural operations to the atmosphere occur during slurry application, animal housing, slurry storage, grassing, fertilizer application and from crops. The main factor at the studied area is the slurry application. Ammonia is usually deposited within a few kilometres to perhaps hundred of kilometres downwind of the source, so local and regional budgets may be more relevant than global budgets (Galloway et al. 2004).

The use of SAM filters is valuable as an exploratory method and indicates the seasonal pattern of dry deposition (Steubing et al. 2002). However, the measured dry deposition values express the amount of N deposited onto acidified paper filters rather than the dry deposition onto vegetation at the sample locations. Therefore, the measured dry deposition amounts should not be interpreted as exact deposition amounts onto the grassland or forest canopies. Table II. Non-parametric correlations (Spearman Rank Order analysis) between N deposition and micro-climatic variables.

Nevertheless, the results strongly suggest that dry deposition may significantly contribute to the atmospheric N input of ecosystems in the central depression of southern Chile. This is in agreement with studies in North America and Europe, where dry deposition was estimated using more advanced methodologies. In a study in North America were the total N deposition to forests was measured using micrometeorological approaches, dry deposition represented the major component of N input (Lovet 1994) and was approximately twice as high as the wet deposition for low-elevation forests in the USA. For central European countries, a similar ratio between dry and wet deposited nitrogen has been observed (Fangmeier et al. 1994). In the present study, a much higher dry deposition contribution has been measured. We found that dry deposition of NH4-N onto surrogate surfaces was six times higher than the total bulk deposition of N during spring/summer time. This contrasting values are probably related to the affinity of SAM acidified filters to ammonium.

The amount of dry deposition at a given time and place depends on meteorology, atmospheric chemistry, and canopy structure of the vegetation (Andersen & Hovmand 1999). Additionally, several factors influence the dry deposition of pollutants onto forest canopies like turbulent transport, wind speed, tree height and canopy closure (Erisman & Draaijers 2003). Since the studied Nothofagus forest is clearly less dense and has a lower leaf area index than the Pinus plantation, a lower dry N deposition onto the samplers in the Nothofagus canopy was expected, which was not the case. This finding indicates higher NH3 and NH4+ concentrations in the Nothofagus canopy, which may be caused by an increased atmospheric turbulence in the more open and irregular Nothofagus canopy compared to the Pinus stand plantation. Furthermore, the low amount of dry N deposition onto samplers in the Pinus canopy may be related to a higher absortion of NH3 in Pinus needles.

Temperature and wind velocity gradients strongly influenced NH3 deposition onto pasive acidified filters in heather and forest ecosystems in the northern hemisphere, mainly where high nitrogen depositions were detected (Duyzer et al. 1992, Hansen 1999). However, no significant influence of precipitation and temperature on dry deposition could be detected through this exploratory survey, probably because of the relatively short study period of 6 months. Therefore, more research is necessary to study the relationship between dry N depostion and climatological variables in the central depression.

ACKNOWLEDGMENTS

This work was supported by Fondecyt projects N°1020989 and N° 1030344, and a Bilateral Chile-Flanders Project of the Ministry of the Flemish Government. This is a contribution to the Scientific Nucleous Millenium P01-057-F. R. Godoy gratefully acknowledges Mecesup AUS 0111. L Paulino thanks Conicyt for a PhD scholarship. J. Staelens was funded as a research assistant of the Research Foundation - Flanders (FWO-Vlaanderen, Belgium). The authors would like to thank the reviewers and editor for the valuable comments on an earlier version of the manuscript.

 

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Received 17/02/05
Accepted 10/07/05

 

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