Article pubs.acs.org/est
Tracing the Sources of Atmospheric Phosphorus Deposition to a Tropical Rain Forest in Panama Using Stable Oxygen Isotopes A. Gross,*,† B. L. Turner,‡ T. Goren,†,§ A. Berry,† and A. Angert† †
The Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Republic of Panama
‡
ABSTRACT: Atmospheric dust deposition can be a significant source of phosphorus (P) in some tropical forests, so information on the origins and solubility of atmospheric P is needed to understand and predict patterns of forest productivity under future climate scenarios. We characterized atmospheric dust P across a seasonal cycle in a tropical lowland rain forest on Barro Colorado Nature Monument (BCNM), Republic of Panama. We traced P sources by combining remote sensing imagery with the first measurements of stable oxygen isotopes in soluble inorganic phosphate (δ18OP) in dust. In addition, we measured soluble inorganic and organic P concentrations in fine (1 μm) aerosol fractions and used this data to estimate the contribution of P inputs from dust deposition to the forest P budget. Aerosol dry mass was greater in the dry season (December to April, 5.6−15.7 μg m−3) than the wet season (May to November, 3.1−7.1 μg m−3). In contrast, soluble P concentrations in the aerosols were lower in the dry season (980−1880 μg P g−1) than the wet season (1170−3380 μg P g−1). The δ18OP of dry-season aerosols resembled that of nearby forest soils (∼19.5‰), suggesting a local origin. In the wet season, when the Trans-Atlantic Saharan dust belt moves north close to Panama, the δ18OP of aerosols was considerably lower (∼15.5‰), suggesting a significant contribution of longdistance dust P transport. Using satellite retrieved aerosol optical depth (AOD) and the P concentrations in aerosols we sampled in periods when Saharan dust was evident we estimate that the monthly P input from long distance dust transport during the period with highest Saharan dust deposition is 88 ± 31 g P ha−1 month−1, equivalent to between 10 and 29% of the P in monthly litter fall in nearby forests. These findings have important implications for our understanding of modern nutrient budgets and the productivity of tropical forests in the region under future climate scenarios.
1. INTRODUCTION
Here we present results from aerosol measurements across a seasonal cycle in lowland tropical forest on Barro Colorado Nature Monument, Republic of Panama (Figure 1). The area supports semideciduous moist tropical forest,12 with a strong four month dry season between December and April.13 Forest productivity in the area responds to experimental addition of P as well as other nutrients,14 but there is little information on atmospheric aerosol deposition. To establish the contribution of atmospheric P inputs from long distance dust transport to the forest P budget, we first aimed to identify the origins of the aerosol P, as this has a marked effect on soluble P concentrations.6,15 To do this, we took advantage of the natural variation of stable oxygen isotope ratios in phosphate (expressed as δ18OP). These ratios can be used to partition atmospheric P sources, because the isotopic signature of the source is preserved in the atmosphere.8,15 We hypothesized that the aerosol mass concentrations and their P sources would vary between the dry and wet seasons, as shown previously for lowland tropical forest in the Yucatan Peninsula.11 We expected that local P sources would dominate
The productivity of terrestrial ecosystems often depends on aeolian inputs of nutrients such as phosphorus (P).1−3 This is especially true for many tropical forests, which grow on highly weathered soils in which P availability represents a major constraint on primary productivity.4,5 However, atmospheric P inputs to tropical forests remain poorly characterized for two reasons. First, long-term measurements of atmospheric aerosols in tropical forests are scarce and rarely involve P analysis. Second, P concentrations in aerosols depend on their origins, with marked variation in soluble P between regions,6,7 yet an adequate technique for identifying the terrestrial sources of atmospheric aerosol P has been developed only recently.8 In tropical forests that experience a strong dry season, the origins of atmospheric P may shift seasonally between local and external sources. During the dry season, deposition can be dominated by local sources such as biomass burning, primary biogenic particles, and particles emitted from local soils.6,9 During the wet season, when emissions from local sources are reduced (e.g., soils are wet, wind speeds lower, reduced biomass burning), long-range transport of aerosols might become the dominant P source. This is particularly true in tropical forests of the Americas, which are located downwind of the Saharan desert,1,3,10,11 the most important source of dust on earth. © 2015 American Chemical Society
Received: Revised: Accepted: Published: 1147
October 8, 2015 December 14, 2015 December 28, 2015 December 28, 2015 DOI: 10.1021/acs.est.5b04936 Environ. Sci. Technol. 2016, 50, 1147−1156
Article
Environmental Science & Technology
Figure 1. Location map of the western Caribbean sea and Central America, showing the sampling station at BCNM, Panama (black dot). Spatial extent of MODIS AOD products used to estimate dust transport is indicated with the dashed box.
μm. The particles were divided to fine and coarse fractions: the fine fraction was defined as particles 1.11 μm. The air sampler was positioned ∼5 m above the ground to decrease the contribution of particles from close proximity to the sampling station. Before sampling, the filters were preheated at 40 °C and then weighed. The filters were replaced every 10−30 sampling days, based on the particle load on the filters. After sampling, the filters were dried again at 40 °C to minimize biological activity, reweighed and temporarily stored in sealed plastic bags at 4 °C until analysis. The combination of drying and low temperature minimize potential changes in the isotopic composition of the aerosol P through biological activity during storage.8,19 2.3. Aerosol P Determination. We measured soluble inorganic P and soluble organic P in the aerosol mass fractions of all the 19 filter groups (a total of 114 filters). Subsamples of 6.7% of filter area were cut from each filter for P determination. The measurements were performed on duplicates from the same filter. For the determination of soluble inorganic P, filters were shaken for 16 h on an orbital shaker with 45 mL of double deionized water (DDW). Phosphorus concentrations in the solution were determined colorimetrically20 on a subsample of the extracting solutions in duplicates. For soluble organic P determination, the filters were shaken in sealed quartz test tubes under UV irradiation (18 UV−C lamps, 30W each) for 16 h with 40 mL of DDW and 5 mL of phosphate-free H2O2 following the method of Zamora et al.21 This procedure hydrolyses soluble organic P compounds and therefore approximates the total soluble P in the sample. The total soluble P was determined colorimetrically as described above. Soluble organic P was calculated as the difference between total soluble P and soluble inorganic P. Blank values from unused filters were subtracted from measured values. For filters with low aerosol dry mass, P concentrations were close to blank values, so we report values only for filters that contained at least 10 mg of dust. The concentrations of total, inorganic and organic soluble P were measured for both the fine (1.11 μm) fractions. 2.4. δ18OP Determinations. To obtain sufficient particulate matter for the determination of the aerosol P δ18OP values,22
in the dry season, when dust is mobilized from the local dry soils by strong trade winds.13 In the wet season, when the Trans-Atlantic Saharan dust belt moves north just over Panama,16 we expected that external sources such as longrange dust transport would dominate. To address this, we collected a total of 114 filters of dry aerosol particles, with six size fractions, and measured aerosol mass concentrations and their soluble P content and δ18OP values. We focused on soluble inorganic and organic P in the aerosols, since these are likely to be relatively bioavailable following deposition. Based on these data, we estimated the relative contribution of local and long-range dust to the forest P budget.
2. MATERIALS AND METHODS 2.1. Study Site. The study was performed in semideciduous rain forest, on Barro Colorado Nature Monument (BCNM), Republic of Panama (9.063°N; 79.50°W, Figure 1). The tropical monsoon (Köppen system) climate has a mean annual temperature of 27 °C and mean annual rainfall of 2600 mm.13 Approximately 90% of annual rainfall occurs in the wet season between May and December.8 The rainfall, wind direction and wind speed at BCNM differ markedly between wet and dry seasons.17 In the wet season Panama is affected by easterly winds, whereas northerly winds prevail in the dry season. Nearby soils are characterized by extremely low readily available phosphate concentrations ( 8 μm) and an additional backup filter that collected particles smaller than 0.5 1148
DOI: 10.1021/acs.est.5b04936 Environ. Sci. Technol. 2016, 50, 1147−1156
a
15/9/12 15/10/12 1/11/12 10/11/12 1/12/12 15/12/12 1/1/13 15/1/13 1/3/13 20/3/13 10/4/13 20/4/13 10/5/13 20/5/13 5/6/13 20/6/13 1/7/13 15/7/13 1/8/13
340 270 270 420 250 250 70 70 32 36 50 95 280 280 270 270 280 280 310
1149
29.5 11.5 26.5 19.7 16.7 50.6 99.9 69.2 249.1 327.6 166.3 37.8 61.7 21.6 37.3 41.6 15.1 36.8 20.8
91.4 67.3 103.9 86.3 105.6 156.6 270.5 104.5 212.7 299.2 206.0 82.6 182.1 109.8 126.1 120.0 165.0 178 4 158.1
fine fraction massa (mg) 120.9 78.8 130.2 106.0 122.3 207.2 370 4 173.7 461.8 626.8 372.3 120.4 243.8 131.4 163.4 161.6 180.1 215.2 178.9
TSPa (mg) 1.7 0.4 0.8 1.0 0.4 1.4 2.7 6.3 7.6 7.3 4.7 1.4 1.6 0.6 1.0 1.0 0.4 1.1 0.7
coarse fraction conc in airb (μg m−3)
Analytical standard error of
