Throughfall Deposition Research Articles

Deposition and soil leaching in stands of Norway spruce and European Beech: Results from the Höglwald research in comparison with other European case studies

Stands of Norway spruce (Picea abies K.) and European beech (Fagus sylvatica L.) were investigated at the Hoglwald research area, Southern Germany from 1985–1988 and from 1994–1997 in order to determine the effects of tree species on deposition and soil solution fluxes. The results were compared to 15 European case studies representing different deposition levels and site conditions. At the Hoglwald site, which is characterised by a high nitrogen and a moderate sulphur load, throughfall deposition of nitrogen and sulphur compounds was about two-fold higher in spruce stands compared to beech stands. The differences in elemental input were clearly reflected in soil solution chemistry with a higher leaching of nitrate and sulphate in the spruce stands. The turnover of sulphur and nitrogen compounds induced a strong soil internal production of protons especially in the spruce stands. These results are in accordance with the other European case studies. Throughfall deposition and soil leaching of nitrogen and sulphur compounds was generally higher for spruce stands compared to beech stands. The species-related differences were mainly caused by dry deposition and were relatively small in remote areas. The consequences for the forest ecosystem itself and for the hydrosphere are discussed.

DEPOSITION AND LEACHING OF SULPHATE AND BASE CATIONS IN A MIXED BOREAL FOREST IN EASTERN FINLAND

Monthly fluxes of sulphate (SO4 2-) and base cations(Ca2+, Mg2+, K+) were studied from 1993 to 1996 as precipitation passed through forest vegetation and surfacesoil layers in an area receiving low and declining levels of atmospheric sulphate pollution. The canopy was dominated by mature Norway spruce (Picea abies Karsten) and the soilwas a podzol developed on glacial till material. The mean annual bulk deposition of SO4 2- collected in the open was 136 molc ha-1 and that of Ca2+, Mg2+ and K+ was 44, 11 and 25 molc ha-1, respectively. The annual total throughfall deposition of SO4 2- was 318 molc ha-1 and that of Ca2+, Mg2+ and K+ was 151, 64 and 181 molcha-1, respectively. Sulphate was the dominant anion accompanying the base cations leached from the canopy. More than half (58%) of the annual total throughfall deposition ofSO4 2- was retained by the O-horizon and only 15% leached from below the B-horizon. The annual leaching of Ca2+, Mg2+ and K+ from below the B-horizon was14, 25 and 9% of the annual total throughfall deposition, respectively. The transport of base cations through the soil was predominantely countered by SO4 2- anions.

Dry deposition and foliar leaching of mercury and selected trace elements in deciduous forest throughfall

The estimated annual throughfall deposition flux of Hg in a northern mixed-hardwood forest in the Lake Huron Watershed was 10.5±1.0 μg m −2 compared to an annual precipitation Hg flux of 8.7±0.5 μg m −2 (June 1996–June 1997). The source of this additional Hg in throughfall is often attributed to wash-off of dry deposition, but foliar leaching of Hg may also be important. To determine the influence of both dry deposition and foliar leaching of Hg and other elements in throughfall, we measured a suite of trace elements (Hg, Al, Mg, V, Mn, Cu, Zn, As, Rb, Sr, Cd, Ba, La, Ce, and Pb) in throughfall, precipitation, and ambient air samples from a northern mixed-hardwood forest. Based on a multiple linear regression model, dry deposition had the most important influence on Hg, Al, La, Ce, V, As, Cu, Zn, Cd, and Pb fluxes while foliar leaching strongly influenced Mg, Mn, Rb, Sr, and Ba fluxes in net throughfall. The Hg dry deposition flux was estimated using gaseous and aerosol Hg measurements and modeled deposition velocities. The calculated dry deposition flux (∼12–14 μg m −2) of Hg to the canopy indicated that atmospheric deposition of Hg could easily account for all of the Hg deposited in net throughfall (1.9±0.1 μg m −2). Although there is a large uncertainty associated with these techniques, the modeling estimates indicate that atmospheric Hg may account for all of the Hg deposited in litterfall (11.4±2.8 μg m −2).

Two options to explain the ammonia gap in The Netherlands

Abstract This paper addresses two hypothesis that try to explain the difference observed between the estimated NH 3 emission levels in The Netherlands and those indicated by atmospheric measurements, the so called ‘ammonia gap’: the role of SO 2 emissions regulating ambient NH 3 concentrations through co-deposition, and long-term NH 3 emissions after slurry injection. It was found that throughfall measurements of NH 4 + could not be used as indicator for changes in NH 3 emissions. The throughfall deposition of NH 4 + is in close equilibrium to SO 4 2− and NO 3 − and is thus regulated by the equilibrium of ambient NH 3 and NH 4 + in wet deposition and canopy water layers. When SO 2 emissions decrease, the amount of available SO 4 2− decreases, which imposes a limit on the deposition of (NH 4 ) 2 SO 4 . Long-term emissions of NH 3 after application of manure were monitored using a new technique, which continuously measures the concentration of NH 3 in a cross-section of the emission plume downwind of the source. The emissions could be registered for 3 weeks after application of manure. The results indicate that the long-term emissions only contribute 1–2% to the total emission level. Both the effect of SO 2 on the NH 3 deposition levels and the long-term emission fluxes are not enough to explain the observed ammonia gap. It seems that several counteracting effects, some of them emerging from the new emission reduction regulations, contribute to the ammonia gap. An integrated approach to abate ammonia emissions is, therefore, needed. The implementation and regulation of production ceilings for reactive nitrogen might be a good option.

Elevational Patterns of Acid Deposition into a Forest and Nitrogen Saturation on Mt. Oyama, Japan

Virgin fir trees have been dying on Mt. Oyama, which is located in the southwestern part of Kanto Plain, although the frequency of death seems to be reducing recently. We report elevational patterns of acid deposition in precipitation and throughfall under fir and cedar canopies and nitrogen saturation in the forest ecosystem on Mt. Oyama. The deposition fluxes of major inorganic ions in precipitation were nearly constant regardless of elevation except for hydrogen and ammonium ions, whereas the deposition fluxes of all major inorganic ions in throughfall among cedar increased. The 5-year average of annual nitrate deposition in precipitation from 1994 to 1998 showed 19.3–23.5 kg ha-1 yr-1 (annual inorganic total N deposition: 9.6–10.7 kgN ha-1 yr-1) at four sites ranging in elevation from 500 to 1252 m, whereas the deposition in both cedar and fir throughfall was over 6 times greater than that in precipitation. The average soil surface nitrate concentration in 1998 was 140 μg g-1 (the range: 21.1-429 μg g-1, n=80) and the 7-year average of nitrate concentration in stream water from 1992 to 1998 was 4.81 mg L-1 (the range: 2.38–20.6 mg L-1, n=317). Our results indicate that nitrogen saturation is occurring in the forest ecosystem because of high N deposition, probably via acid fog, on Mt. Oyama.

Throughfall Chemistry in a Sitka Spruce Plantation in Response to Six Different Simulated Polluted Mist Treatments

A mixed provenance Sitka spruce plantation, planted in 1986 on a drained deep peat, has been exposed to 6 different simulated mist treatments in 4 replicated blocks since 1996. Treatments provided N and/or S at a concentration of 1.6 mol m−3, supplying ca. 50 kg S and/or N ha−1 yr−1 as N (NH4NO3), S (Na2SO4), NS Acid (NH4NO3 + H2SO4 at pH 2.5), 2NS Acid (double dose by application at twice frequency), a control treatment supplied with additional rainwater only and a 'no treatment' set of plots. Throughfall, preserved with thymol in the field, was collected using gutters with a surface area of 1 m2 in all the replicate plots, and was analysed for all major ions. Prior to treatment in 1999, S deposition in throughfall exceeded that in rain because of dry deposition of SO2 and SO42− to the canopy; NH4+ and NO3− ions were both retained in the canopy. During treatment, only 20–40% of the applied N in the high-N treatments was retained in the canopy. Acidity in the applied mist was partly neutralised by the canopy, but not primarily through exchange of base cations, leading to the conclusion that weak organic acids, in solution or in situ in the canopy, contributed to the buffering of the H+ ion deposition in the acid treatments.

Dynamics of mercury and methylmercury in forest floor and runoff of a forested watershed in Central Europe

Forested watersheds are an important part of the terrestrialmercury and methylmercury cycle, and a link between theatmospheric and aquatic environment. This study was conducted todetermine the contribution of the forest floor to the pools andfluxes of total Hg (Hgtotal) and methylmercury (MeHg) in aforested catchment, and to identify factors influencing themobility of both compounds. Throughfall deposition, litterfall,runoff and fluxes with forest floor percolate of Hgtotal and MeHgwere sampled during one year in a coniferous catchment inGermany. Total deposition of Hgtotal was 552 mg ha−1 a−1 withlitterfall contributing one third. Nearly 60% of the total inputof Hgtotal reached the mineral soil with the forest floorpercolate, but less than half of this fraction was found in therunoff of the catchment. Total deposition of MeHg was 2.6 mg ha−1a−1, with litterfall as the dominating pathway. Only 19% of theMeHg deposition was discharged from the forest floor, but theflux of MeHg with runoff was nearly twice as high. Only fewcorrelations with other solution parameters were found. Fluxes ofboth compounds with forest floor percolates depended mainly onwater fluxes, which was not true for the runoff. The forest floorof the upland soil is an effective sink for MeHg, but not forHgtotal. Differences in the mobility of both compounds in theforest floor disappeared at the catchment scale, probably becauseother processes (i.e. Hgtotal immobilization and MeHg formation)dominated.

Atmospheric Deposition to Oak Forests along an Urban−Rural Gradient

To determine the patterns of atmospheric deposition and throughfall in the vicinity of a large city, bulk deposition, oak forest throughfall, and particulate dust deposition were measured at sites along a transect within and to the north of New York City. Concentrations and fluxes of NO3-, NH4+, Ca2+, Mg2+, SO42-, and Cl- in throughfall all declined significantly with distance from the city, while hydrogen ion concentration and flux increased with distance from the city. Most of the change in concentrations and fluxes occurred within 45 km of the city. Throughfall deposition of inorganic N was twice as high in the urban sites as compared to the suburban and rural sites. Bulk deposition patterns were similar to those of throughfall, but changes along the transect were much less pronounced. The water-extractable component of dust deposition to Petri plates also was substantially higher in the urban sites for Ca2+, Mg2+, SO42-, NO3-, and Cl-. The dust particles had little alkalinity, suggesting that alkaline...

Throughfall and fog deposition of nitrogen and sulfur at an N-limited and N-saturated site in the San Bernardino Mountains, southern California

Inorganic nitrogen (N) and sulfur (S) deposition in bulk throughfall and fog were determined at two sites located at opposite ends (42 km apart) of a pollution gradient in the San Bernardino Mountains. Plot-level averages for total annual N and S deposition in throughfall in 1996 were 18.8 and 2.9 kg·ha-1, respectively, at Camp Paivika (CP) and 2.9 and 0.4 kg·ha-1, respectively, at Barton Flats (BF). Deposition of N in throughfall in the four transects at CP ranged from 12.1 to 31.7 kg·ha-1·year-1. Spatial variability was high because of heterogenous canopy cover and varying exposure to air pollution. Annual estimated stand-level deposition of N and S in fog were 10.9 and 1.9 kg·ha-1, respectively, at CP, and 0.6 and 0.2 kg·ha-1, respectively, at BF. We estimated that N deposition in fog contributed 35% of the total annual N deposition at CP and 13% at BF. Analogous values for S were 39% at CP and 26% at BF. Ammonium deposition in throughfall and fog at CP were 72 and 122% as high as NO3- deposition because of elevated NH3 emissions from dairy farms in the Chino-Norco area 34 km southwest of CP.

Height and radial growth trends of Corsican pine in western France

Height and radial growth trends were analysed in Corsican pine (Pinus nigra Arnold ssp. laricio var. Corsicana) plantations in western France. Difference in height growth was tested by comparing the site index of stands established before and after 1950 and the height growth development curves of 13 pairs of young and old stands growing side by side on the same soil type. The site index of the young stands was 20-30% greater than the site index of the old stands. From the period 1921-1991, radial growth increased 45, 31, and 50% in earlywood, latewood (LW) and total ring (TR) area, respectively. The amount of increase depended on cambial age. The LW/TR ratio decreased by 8%. The regional climatic data revealed a significant increase in mean annual temperature of 1.1°C, mean annual minimum temperature (1.5°C), mean summer temperature (2.2°C), and minimum summer temperature (2.3°C) for the period 1950-1997. Because of the negative correlation between summer temperature and ring widths, increased temperature cannot explain the observed increases in growth. Effects of nitrogen inputs, which averaged 6.3 and 11 kg·ha-1·year-1 for bulk and throughfall depositions, respectively; land use history; improvement in silvicultural practices (wider initial spacing, higher thinning); and CO2 fertilization are discussed as possible causes of the observed growth trends.

Throughfall and fog deposition of nitrogen and sulfur at an N-limited and N-saturated site in the San Bernardino Mountains, southern California

Inorganic nitrogen (N) and sulfur (S) deposition in bulk throughfall and fog were determined at two sites located at opposite ends (42 km apart) of a pollution gradient in the San Bernardino Mountains. Plot-level averages for total annual N and S deposition in throughfall in 1996 were 18.8 and 2.9 kg·ha-1, respectively, at Camp Paivika (CP) and 2.9 and 0.4 kg·ha-1, respectively, at Barton Flats (BF). Deposition of N in throughfall in the four transects at CP ranged from 12.1 to 31.7 kg·ha-1·year-1. Spatial variability was high because of heterogenous canopy cover and varying exposure to air pollution. Annual estimated stand-level deposition of N and S in fog were 10.9 and 1.9 kg·ha-1, respectively, at CP, and 0.6 and 0.2 kg·ha-1, respectively, at BF. We estimated that N deposition in fog contributed 35% of the total annual N deposition at CP and 13% at BF. Analogous values for S were 39% at CP and 26% at BF. Ammonium deposition in throughfall and fog at CP were 72 and 122% as high as NO3- deposition because of elevated NH3 emissions from dairy farms in the Chino-Norco area 34 km southwest of CP.

Annual nitrogen budget of a temperate coastal barrier salt-marsh system along a productivity gradient at low and high marsh elevation

Abstract An annual nitrogen budget was established for a temperate back barrier salt-marsh system along a productivity gradient at low and high marsh elevation. We measured plant biomass and nitrogen content in three plant compartments to deduce plant N-allocation patterns. Measurements were done along a successional sequence in a salt-marsh system. In addition, N-mineralization, wet and dry atmospheric N-deposition and sediment N-deposition were measured. Plant-species dominance changed along the successional sequence. In early stages, Elymus farctus and Spergularia media formed a large part of total plant biomass. Festuca rubra and Puccinellia maritima were dominant at intermediate stages, whereas Elymus pycnanthus and Limonium vulgare were dominant at late stages of succession. Shoot biomass was highest in June, whereas litter biomass was highest in September and December. Root biomass formed by far the largest fraction of total plant biomass, especially at a low-marsh elevation. Wet deposition of nitrate and ammonium was 1.7 g N m −2 yr −1 , whereas throughfall deposition (dry and wet deposition) amounted to 2.1–3.6 g N m −2 yr −1 , and was positively related to the height of an artificial plant canopy. Sediment organic nitrogen deposition rate was 0.3–5.4 g N m −2 yr −1 , and negatively related to marsh elevation. Nitrogen mineralization rate increased from 2.5–2.8 g N m −2 yr −1 in young marshes towards 8.0–12.7 g N m −2 yr −1 at older marshes, depending on marsh elevation. At a low-marsh elevation, plant N-availability depended equally on tidal N, atmospheric N and mineralized N, especially in young marshes, whereas the decomposition pathway became more important in older marshes. Tidal N contributed most to ecoystem N-accumulation rate at early successional stages, whereas atmospheric N was more important at later stages. Tidal influence was low at high-marsh elevation sites. Here, atmospheric deposition was the dominant exogenous nitrogen source both in young and old marshes.

Throughfall chemistry in a loblolly pine plantation under elevated atmospheric CO2 concentrations

Accelerated tree growth under elevatedatmospheric CO2 concentrations may influencenutrient cycling in forests by (i) increasingthe total leaf area, (ii) increasing the supplyof soluble carbohydrate in leaf tissue, and (iii) increasing nutrient-use efficiency. Here wereport the results of intensive sampling andlaboratory analyses of NH 4 + , NO 3 − , PO 4 3− , H+, K+, Na+,Ca2+, Mg2+, Cl−, SO 4 2− , and dissolved organic carbon (DOC) in throughfallprecipitation during the first 2.5+ years of the DukeUniversity Free-Air CO2 Enrichment (FACE)experiment. After two growing seasons, a largeincrease (i.e., 48%) in throughfall deposition of DOCand significant trends in throughfall volume and inthe deposition of NH 4 + , NO 3 − , H+, and K+ can be attributed to the elevatedCO2 treatment. The substantial increase indeposition of DOC is most likely associated withincreased availability of soluble C in plant foliage,whereas accelerated canopy growth may account forsignificant trends toward decreasing throughfallvolume, decreasing deposition of NH 4 + ,NO 3 − , and H+, and increasing deposition of K+ under elevated CO2. Despiteconsiderable year-to-year variability, there wereseasonal trends in net deposition of NO 3 − ,H+, cations, and DOC associated with plant growthand leaf senescence. The altered chemical fluxes inthroughfall suggest that soil solution chemistry mayalso be substantially altered with continued increasesin atmospheric CO2 concentrations in the future.

Interactions of atmospheric deposition with a mixed hardwood and a coniferous forest canopy at the Lake Clair Watershed (Duchesnay, Quebec)

From 1989 to 1996, ion deposition in precipitation, throughfall, and stemflow were measured under a deciduous and a coniferous stand, located in the Lake Clair Watershed, during the growing and the dormant seasons. During the growing season, throughfall deposition under both stands was significantly depleted in H+ and NH4+ compared with wet deposition, and a significant uptake of NO3- was observed under the coniferous canopy. Deposition of Ca2+, Mg2+, K+, Na+, Cl-, and SO42- was significantly higher in the throughfall than in the wet precipitation. During the growing season, the coniferous stand was more efficient in retaining nitrogen (NH4+ and NO3-), while H+ was more intensively retained in the deciduous stand. Significant interactions between precipitation and forest canopies were also observed during the dormant season: throughfall depositions of Ca2+, Mg2+, K+, Na+, and Cl- were significantly higher than wet precipitation under both canopies, while throughfall SO42- was significantly enriched only under the coniferous stand. Using a Na+ ratio method, foliar leaching was found mostly responsible for the throughfall enrichment on a full-year basis in both stands, with values averaging 61, 73, and 96% of the total throughfall fluxes for Ca2+, Mg2+, and K+, respectively. Under both stands, net canopy exchange (NCE) of base cations, expressed on a monthly basis, were correlated to water volume and to H+ and SO42- deposition. Multiple regression models including wet SO42- deposition and an estimate of dry S deposition, explained up to 88% (Ca2+ in the coniferous stand) of the variance in base cation NCE under both stands.

Determination of sample size for estimating ion throughfall deposition under a mixed hardwood forest at the Lake Clair Watershed (Duchesnay, Quebec)

Variability of throughfall volume and ion deposition were measured under a mixed hardwood forest using 36 or 72 collectors from 1988 to 1994 during the growing period (May to October) for 163 events. The ions showing the highest variability in results were (in decreasing order) NH4+, H+, Mg2+, K+, Ca2+, Cl-, Na+, NO3-, and SO42-, based on the median coefficient of variation of deposition. The variability observed was related to the reactivity of the ion in the canopy, with the more reactive ions showing higher spatial variability. The number of collectors required to obtain estimates of throughfall ion depositions within a 10% error and a confidence interval of 95% varied from 21 (SO42-) to 165 (NH4+) on a weekly basis based on events of median variability. The error associated with deposition measurements was decreased considerably from weekly, to monthly, to seasonal deposition (3 months). The number of collectors necessary to measure throughfall deposition depends on the data quality objectives of the study (i.e., error and confidence interval chosen as acceptable), the variability specific to a given ion, and the time period needed to study a given process with adequate temporal resolution. A figure is proposed that may be used as a guideline to determine the number of collectors needed in mixed hardwood forests based on the latter information.

Throughfall deposition of nitrogen and sulfur in a Jeffrey pine forest in the San Gabriel Mountains, southern California

Deposition of N and S in bulk precipitation and throughfall, and dry deposition to surrogate surfaces were determined in 1994 in a high-elevation mixed conifer forest dominated by Jeffrey pine ( Pinus jeffreyi Grev. & Balf.) in the San Gabriel Mountains northeast of Los Angeles, California. Three sites 2 km apart, with a range of N deposition inputs, were chosen for study. N deposition in throughfall and dry deposition fluxes of N varied among the three sites, presumably because of differing pollutant transport with varying air flow patterns as a function of ridge and canyon orientation and the prevailing northeasterly winds. Net throughfall, an estimate of dry deposition, was the dominant input source of NH 4 + and NO 3 −, except at Vista where wet deposition of NH 4 + was greater than dry deposition of NH 4 +. Wet deposition of SO 4 2− was much greater than dry deposition of SO 4 2− at all three sites. Higher SO 4 2− deposition in bulk precipitation and throughfall in this study compared to montane sites further from the coast was presumably because of marine aerosols transported inland during storm events and possibly by the prevailing winds. The location of the study sites in relation to the ocean (65 km inland) and to the urban smog source, resulted in throughfall inputs which were higher in S (7.8–12.5 kg ha −1 year −1), but lower in N (6.0–11.5 kg ha −1 year −1) compared to montane sites in the lower San Gabriel Mountains (23 kg N ha −1 year −1) or in the western end of the San Bernardino Mountains further inland (19 kg N ha −1 year −1; 3 kg S ha −1 year −1). S concentrations in pine foliage (0.69–0.83 g kg −1) were also greater in the San Gabriel Mountain sites than in four sites located further inland in the San Bernardino Mountains (0.48–0.56 g kg −1). Results of this and other studies suggest that the threshold for throughfall N deposition levels which lead to symptoms of N saturation in southern California montane ecosystems is ca. 16–19 kg ha −1 year −1.

Atmospheric Inputs of Mercury and Organic Carbon into a Forested Upland/Bog Watershed

Inputs of mercury (Hg) and dissolved organic carbon (DOC) in throughfall and stemflow waters were measured for an upland/bog watershed in northern Minnesota, and were compared to the deposition in a nearby opening to determine the influence of tree canopies on Hg and DOC deposition. Twice as much Hg and seven times as much DOC was deposited in the forested watershed compared to the opening. Mass balance studies that are based on wet-only deposition in openings severely underestimate atmospheric deposition of Hg in forests. Conifer canopies are more efficient filters of airborne, particulates than are deciduous canopies as indicated by much higher Hg concentrations and total deposition in throughfall and stemflow waters under conifers. Significant positive relationships existed between Hg and DOC in both throughfall (36-57% of the variation) and stemflow waters (55-88% of the variation). Hg complexation by DOC appears to be related to the contact time between precipitation and carbon sources.

Nitrogen and sulfur deposition and forest nutrient status in the Valley of Mexico

Mexico City experiences some of the most severe air pollution in the world. Ozone injury has been documented in sensitive tree species in urban and forested areas in the Valley of Mexico. However, little is known of the levels of other atmospheric pollutants and their ecological effects on forests in the Valley of Mexico. In this study bulk throughfall deposition of inorganic nitrogen (N) and sulfur (S) was measured for one year at a forested site upwind (east) and downwind (southwest) of Mexico City. Edaphic and plant (Pinus hartwegii Lindl.) indicators of N and S nutrient status were also measured. Streamwater NO3 - and SO4 2- concentrations were also determined as an indicator of watershed-level N and S loss. Annual bulk throughfall deposition of inorganic N and S at the high-pollution forested site 23 km southwest of Mexico City (Desierto de los Leones National Park; DL) was 18.5 and 20.4 kg ha-1. Values for N and S deposition at Zoquiapan (ZOQ), a relatively low-pollution site 53 km east of Mexico City, were 5.5 and 8.8 kg ha-1 yr-1. Foliar concentrations of N, foliar N:P and C:N ratios, extractable soil NO3 -, and streamwater NO3 - concentrations indicate that the forest at DL is N enriched, possibly as a result of chronic N deposition. Sulfur concentrations in current-year foliage were also slightly greater at DL than at ZOQ, but S concentrations in one-year-old foliage were not statistically different between the two sites. Streamwater concentrations of NO3 - ranged from 0.8 to 44.6 μEq L-1 at DL compared to 0.0 to 11.3 μEq L-1 at ZOQ. In summary, these findings support the hypothesis that elevated N deposition at DL has increased the level of available N, increased the N status of P. hartwegii, and resulted in export of excess N as NO3 - in streamwater.

Determination of sample size for estimating ion throughfall deposition under a mixed hardwood forest at the Lake Clair Watershed (Duchesnay, Quebec)

Determination of sample size for estimating ion throughfall deposition under a mixed hardwood forest at the Lake Clair Watershed (Duchesnay, Quebec)

Internal nutrient fluxes in a Puerto Rican rain forest

Throughfall and soil solution chemistry were studied for 1 y in a tropical montane forest in the Luquillo Mountains of Puerto Rico. Passage of precipitation through the forest canopy resulted in an increase in the concentration and flux of all solutes except H+ and NO3−. Throughfall chemistry showed no strong seasonal patterns, but concentrations of many solutes declined during weeks of high rainfall. Enrichment in throughfall relative to precipitation was similar to values reported recently for several other tropical sites, with the exception of NH4+, which was particularly high at this site. Based on net throughfall deposition of Cl−, dry deposition of marine aerosols appears to be a relatively minor component (c. 15%) of total deposition. In soil solution, no seasonal patterns in concentration were evident and variability was highest for elements with high biological activity (especially N). Concentrations and fluxes of K+ and dissolved organic carbon (DOC) showed the largest declines as throughfall passed through the soil profile; concentrations of most other elements increased or were relatively constant. Declining DOC flux through the soil profile appears to be due to sorption processes similar to those observed in many temperate forests. Concentrations and fluxes of HCO3− and SiO2 increased substantially in soil solution, but never approached those observed in the stream. This suggests that additional weathering must occur as groundwater moves from this ridgetop site to the stream.