Throughfall Dissolved Organic Matter as a Terrestrial Disinfection

May 29, 2019 - (31) Consequently, the throughfall DOM originating from canopies via ... by the dissolved organic carbon (DOC) level] from different ca...
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Article Cite This: ACS Earth Space Chem. 2019, 3, 1603−1613

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Throughfall Dissolved Organic Matter as a Terrestrial Disinfection Byproduct Precursor Huan Chen,† Kuo-Pei Tsai,† Qiong Su,‡ Alex T. Chow,† and Jun-Jian Wang*,∥,⊥ †

Biogeochemistry & Environmental Quality Research Group, Clemson University, Clemson, South Carolina 29442, United States Water Management & Hydrological Science, Texas A&M University, College Station, Texas 77843, United States ∥ Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control, School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China ⊥ State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control, School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China

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S Supporting Information *

ABSTRACT: More than half of the drinking water supply in the United States originates from forest watersheds, where terrestrial dissolved organic matter (DOM) is known as an important disinfection byproduct (DBP) precursor. Throughfall-derived DOM, a significant portion of terrestrial DOM, has seldom been evaluated for its formation potential of DBPs. Here, we collected throughfall and leaf extracts of an evergreen (loblolly pine, Pinus taeda L.) and a deciduous tree species (turkey oak, Quercus cerris L.) in South Carolina to explore their seasonal DOM quantity, optical properties, and DBP formation potential. Elevated dissolved organic carbon (DOC) from rainwater (1.2 ± 0.4 mg/L) to pine (26.0 ± 19.7 mg/L) and oak throughfall (38.8 ± 37.8 mg/L) indicated canopy can be a significant DOM source. DOM aromaticity (indicated by specific ultraviolet absorbance at 254 nm) was higher in oak than pine throughfall and higher in throughfall than leaf extracts. The throughfall DOM characteristics were seasonally more stable for the evergreen pine than for the deciduous oak. The specific DBP formation potential of pine and oak throughfall both varied greatly across seasons, with values of 52.7 ± 17.3 and 58.6 ± 15.1 μg/g-DOC for trihalomethanes, 0.82 ± 0.35 and 0.64 ± 0.11 μg/g-DOC for haloacetonitriles, 0.59 ± 0.60 and 0.22 ± 0.05 μg/g-DOC for haloketones, and 4.51 ± 2.25 and 4.20 ± 2.76 μg/g-DOC for chloral hydrate, respectively. We estimated the contribution of canopies on runoff DOC yield, and results suggested that the highest contribution would occur in the fall season. Results suggest that throughfall DOM is an important and overlooked terrestrial DBP precursor, and its seasonal variation is forest-type-dependent. KEYWORDS: Throughfall DOM, optical characteristics, disinfection byproducts, seasonal variations, pine, oak stream DOM.23 Therefore, understanding of the role of treeDOM, a subset of DOM in forested watersheds, as a possible DBP precursor, will help reduce DBPs by implementing effective and targeted management strategies.24 In forested catchments, when first intercepted by trees, precipitation or rainwater makes its way to the forest floor by two hydrological flow paths: throughfall (rainwater that drips after passing through the canopy) and stemflow (rainwater funnelled to the stem by the canopy).20,21 Most DOM reaches the forest floor as throughfall (40−200 kg C/ha/yr) instead of stemflow (0.2−50 kg C/ha/yr).20,25 The characteristics of DOM in rainwater are altered when rainwater drains from the canopy as throughfall.20,21,23,26 Throughfall DOM, mainly composed of an autochthonous fraction from trees and an

1. INTRODUCTION Dissolved organic matter (DOM) in source water can react with chemical disinfectants to form carcinogenic disinfection byproducts (DBPs), such as trihalomethanes (THMs) and haloacetonitriles (HANs).1−4 The reactivity of DOM in forming DBPs largely depends on its chemical composition,5,6 which is related to DOM sources.7−9 Terrestrial DOM from the forest floor is widely considered as an important DBP precursor,10−12 particularly during and after storm events.13−15 Forested watersheds can usually provide water with higher quality, which can be used as drinking water after less expensive treatments.16−18 Therefore, they contribute more than half of the drinking water supply in the contiguous United States.16,19 Within the forested watershed, tree-derived DOM (tree-DOM) has recently been unveiled as one of the critical terrestrial DOM sources.20−22 Trees are estimated to yield comparable amounts of DOM per unit area of landscapes as the streams draining those landscapes, but the chemical nature of tree-DOM is less understood than that of soil DOM or © 2019 American Chemical Society

Received: Revised: Accepted: Published: 1603

April 7, 2019 May 24, 2019 May 29, 2019 May 29, 2019 DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613

Article

ACS Earth and Space Chemistry allochthonous fraction from atmospheric deposition,21 can be enriched with aromatic, humic-like, and protein-like compounds.20,21 Aromatic compounds, such as lignin phenols and aromatic amino acids, are considered to be highly reactive THM precursors.27,28 After reaching the surface of the forest floor/soil, rainwater will travel downhill as surface runoff or will percolate into the groundwater,29 depending on land use and soil cover.30 In southeastern US such as South Carolina, the groundwater is often shallow, which favors the surface runoff over soil infiltration during rainfall events.31 Consequently, the throughfall DOM originating from canopies via the hydrological flow path of throughfall is likely a significant DBP precursor in forested watersheds in this region. However, it is still poorly understood how much throughfall DOM could be exported from the watersheds in South Carolina, and how reactive the throughfall DOM could be to form different types of DBPs. Leaves in the canopy are usually considered as an important source of throughfall DOM and often have large seasonal variations in their biochemistry. For example, Salminen et al.32 found that hydrolyzable tannins, a dominant phenolic content in oak (Quercus robur L.) leaves, decreased by 54% from late May to September. Klamerus-Iwan and Witek33 reported that the water storage capacity and wettability of oak (Quercus robur L.) canopy and the aromatic hydrocarbon in leaves increased from May to September. Therefore, a deep understanding of the seasonal changes in throughfall DOM characteristics and chlorine reactivity would help optimize the treatment processes in drinking water facilities to ensure water safety.34,35 Leaf extraction was sometimes used as an easy approach to simulate the throughfall chemistry.36,37 However, it is unknown whether leaf-extract DOM could be used to understand the characteristics and chlorine reactivity of throughfall DOM across different seasons. If the leaf-extract DOM could be a good proxy for throughfall DOM, leaves of various tree species could be rapidly tested to examine the characteristics of their throughfall DOM without requirement of real rainfall events. Here, loblolly pine (Pinus taeda L.) and turkey oak (Quercus cerris L.), one representative evergreen and one representative deciduous tree species in the southeastern US, were studied. We collected their throughfall samples during the four seasons and prepared leaf-extract samples. This study aimed to (1) determine the seasonal variations in DOM concentration [indicated by the dissolved organic carbon (DOC) level] from different canopies; (2) characterize the optical properties and chlorine reactivity of throughfall DOM and evaluate whether the seasonal variations are forest-type-specific; and (3) examine whether the leaf-derived DOM we extracted could be a good proxy for throughfall DOM. Moreover, the DOC yields from the throughfall and runoff from pine and oak canopies in South Carolina were estimated to explore the potential impact of throughfall DOM as a DBP precursor in forested watersheds.

loblolly pine trees and three healthy turkey oak trees in the woodland (33°21′41″N, 79°13′25″W) near the Belle W. Baruch Institute of Coastal Ecology and Forest Science were selected. All six trees were mature (>10 years old), with a height greater than 4 m and a diameter at breast height that exceeded 10 cm. None of the trees hosted epiphyte cover, which is likely to alter the amount and chemistry of throughfall.20,22 Each tree was approximately 30−50 m away from the closest neighboring tree. To collect the throughfall, three precombusted aluminum trays with a diameter of 25 cm were placed beneath the canopy hours before a rainfall event. After a rainfall event (each event >20 mm and lasting more than 4 h), the throughfall in trays was immediately transferred to the laboratory. All throughfall samples were filtered through 0.45-μm poly(ether sulfone) membranes and then stored at 4 °C in clean amber bottles without headspace before analyses. The throughfall samples were collected in December 2014 (winter), April 2015 (spring), August 2015 (summer), and October 2015 (fall), and there was no rainfall for at least 2 weeks prior to the sample collection. In parallel with the throughfall collection, three rainwater samples were also collected during the same rainfall events over an open lawn that was approximately hundreds of meters away from the woodland. To explore the seasonal variation of leaf-derived DOM (leaf-extract DOM) without the interference of rainfall, fresh pine needles and oak leaves were collected from the same trees during the same months approximately 1 week before the rainfall events. Six locations on each tree were randomly selected for leaf collection. The fresh leaf samples were immediately extracted using a fixed leaf-to-water extraction ratio to obtain leaf-extract DOM. Specifically, fresh leaves were extracted using a 5 g to 200 mL of leaf-to-water ratio in a 500 mL Erlenmeyer flask on a shaker at 200 rpm for 2 h. After passing through a 0.45 μm poly(ether sulfone) membrane, the leaf extract was stored at 4 °C in a clean bottle without headspace before analyses. The water content of leaves was measured by oven drying the fresh leaf samples at 50 °C for 48 h. 2.2. Chemical Analyses. All filtered rainfall, throughfall, and leaf extract samples were analyzed by a TOC/TN analyzer (Shimadzu, Japan) to determine the concentrations of DOC and total dissolved nitrogen (TDN).38 The samples were also analyzed by UV−visible spectrometry (UV-1800, Shimadzu, Japan) and 3D spectrofluorometry (RF5301, Shimadzu, Japan).39 The specific UV absorbance at 254 nm (SUVA254; in L/mg-C/m) is an indicator of DOM aromaticity and was calculated by dividing the UV absorbance at 254 nm by the DOC concentration.40 The E2/E3 ratio was calculated by dividing the absorbance at 254 nm by that at 365 nm.41 The raw fluorescence excitation−emission matrices (EEMs) were corrected for instrument-dependent effects, inner-filter effects, and Raman effects and then standardized to Raman units.42 The fluorescence index (FI) is an index used to differentiate terrestrial and microbial origins of DOM and was determined as the ratio of fluorescence intensity at emission wavelengths (Ems) of 470 and 520 nm at excitation wavelength (Ex) of 370 nm.43 The freshness index (β/α) was determined as the ratio of the fluorescence signal at Em of 380 nm to the maximum signal between Ems of 420 and 435 nm at Ex of 310 nm.44 The humification index (HIX) was calculated by dividing the peak area under Em range of 435−480 nm by the peak area under Em range of 300−345 nm at Ex of 254 nm.45 Through fluorescence regional integration based on Simpson’s rule,

2. MATERIALS AND METHODS 2.1. Sample Collection. The experiment was conducted at Hobcaw Barony in North Winyah Bay, South Carolina, US. According to the US climate data (www.usclimatedata.com), the local mean annual precipitation is 1429 mm, and the mean annual temperature is 18 °C. In this area, loblolly pine (Pinus taeda L.) and turkey oak (Quercus cerris L.) are the common evergreen and deciduous species, respectively. Three healthy 1604

DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613

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Table 1. Variations (Mean ± Standard Deviation) of Nutrients, Optical Properties, Formation Potentials of Disinfectant Byproducts (DBP-FPs), and Specific Formation Potentials of Disinfectant Byproducts (SDBP-FPs) Across Sample Typesa throughfall variables

units

rainwater

Nutrients DOC mg/L 1.24 ± 0.43d TDN mg/L 0.35 ± 0.09d DOC/TDN / 4.01 ± 2.37c Optical Properties SUVA254 L/mg-C/m HIX / E2/E3 / FI / β/α / PI,n % PII,n % PIII,n % PIV,n % PV,n % Disinfection Byproduct Formation Potentials (DBP-FPs) THMs μg/L HANs μg/L HKTs μg/L CHD μg/L Specific Disinfection Byproduct Formation Potentials (SDBP-FPs) THMs μg/mg-DOC HANs μg/mg-DOC HKTs μg/mg-DOC CHD μg/mg-DOC -

leaf extracts

pine

oak

pine

oak

26.0 ± 19.7c 0.65 ± 0.34c 38.5 ± 14.6b

38.8 ± 37.8bc 0.55 ± 0.18c 71.3 ± 63.1b

162 ± 230b 2.00 ± 2.14b 59.9 ± 27.8b

323 ± 311a 3.57 ± 3.37a 151 ± 134a

1.63 2.25 8.43 1.38 0.55 13.6 19.8 25.1 21.7 19.8

± ± ± ± ± ± ± ± ± ±

0.34b 0.48b 1.31b 0.07b 0.05a 4.02b 1.20c 3.04a 1.87b 2.37a

3.11 3.38 6.36 1.43 0.47 13.4 17.7 26.7 18.7 23.6

± ± ± ± ± ± ± ± ± ±

1.63a 1.88a 1.58c 0.18ab 0.06b 6.04b 3.83c 4.83a 3.48b 7.20a

0.50 0.24 20.1 1.50 0.46 36.1 24.7 6.60 28.0 4.53

± ± ± ± ± ± ± ± ± ±

0.18c 0.09d 7.67a 0.09a 0.15b 4.05a 1.16a 1.17c 3.08a 1.43b

2.88 0.44 9.72 1.68 0.32 35.9 21.7 11.9 22.9 7.56

± ± ± ± ± ± ± ± ± ±

2.13ab 0.17c 2.79b 0.51a 0.24b 6.47a 2.66b 5.44b 6.63b 4.06b

1584 21.7 14.9 139

± ± ± ±

1821b 16.0b 15.9ab 168b

2165 24.5 8.84 113

± ± ± ±

2071b 23.0b 11.6b 68.2b

2348 58.7 38.4 288

± ± ± ±

3133b 76.6b 51.4a 434b

15736 96.5 30.5 802

± ± ± ±

15915a 62.6a 40.3a 990a

52.7 0.82 0.59 4.51

± ± ± ±

17.3a 0.35a 0.60a 2.25a

58.6 0.64 0.22 4.20

± ± ± ±

15.1a 0.11ab 0.05b 2.76a

17.5 0.47 0.30 1.52

± ± ± ±

7.83b 0.38bc 0.24b 0.45c

57.7 0.39 0.12 2.43

± ± ± ±

33.0a 0.14c 0.09c 1.21b

a

Different letters after the numbers indicate significant differences between groups. /: not applicable; -: not included.

EEMs were operationally divided into five regions, including (I) tyrosine-like, (II) tryptophan-like, (III) fulvic acid-like, (IV) soluble microbial byproduct-like, and (V) humic acidlike.46,47 We also calculated the percent fluorescence response in each region (Pi,n).46 A chlorination test was used to evaluate the DBP formation potential (DBP-FP) and chlorine reactivity of DOM.48 The filtered samples were diluted, buffered to pH 8.0, and reacted with freshly prepared NaOCl/H3BO3 solution at 25 °C and pH of 8 in the dark for 24 h as described in Wang et al.49 The reaction was then stopped by adding small amount of 10% Na2SO3 solution. The resulting solution was saturated by Na2SO4 and the DBPs were then extracted with MTBE. DBPs in the MTBE solution were analyzed by an Agilent 7890 gas chromatography−electron capture detector using the EPA method 551.1. The analytes included four types of THMs (trichloromethane, dichlorobromomethane, dibromochloromethane, and tribromomethane), four types of HANs (trichloroacetonitrile, dichloroacetonitrile, bromochloroacetonitrile, and dibromoacetonitrile), three types of haloketones (HKTs; 1,1-dichloro-2-propanone, 1,1,1-trichloro-2-propanone, and 1,2,3-trichloropropanone), and chloral hydrate (CHD). The minimum reporting levels of the present study were about 0.1−0.3 μg/L. We also calculated the specific DBPFP (SDBP-FP) in the unit of μg-DBP/mg-DOC as the ratio of the DBP concentration to the DOC concentration. The SDBPFP is an indicator of the DOM reactivity in forming DBPs. 2.3. DOC Yields from Throughfall and Runoff. We made a rough estimation of the DOC yields in throughfall and runoff from the pine and oak canopies at 34 watersheds in South Carolina based on the assumption that the measured

data here could be extrapolated (eqs 1−3; Figure S1). The surface runoff volume was estimated by eqs 4 and 5.50 The daily rainfall between December 1, 2014 and November 31, 2015 for a total of 168 stations in South Carolina was retrieved from Climate Data Online managed by the National Oceanic and Atmospheric Administration (https://www.ncdc.noaa. gov/cdo-web/) (Figure S2). We employed the ordinary kriging method with a 30 m spatial resolution for spatial rainfall interpolation, and the performance was evaluated by the mean absolute error as well as the root-mean-square error (Table S1). Land cover data were obtained from the 2014 cropland data layer at 30 m resolution (https://www.nass.usda. gov/Research_and_Science/Cropland/SARS1a.php), and the regions of evergreen and deciduous forests were considered as the distribution of pine and oak trees because of their dominance (https://data.fs.usda.gov/geodata/rastergateway/ forest_type/) (Figure S3). The hydrologic soil group data were obtained from the STATSGO data set managed by the National Cooperative Soil Survey (https://www.nrcs.usda. gov/wps/portal/nrcs/detail/soils/survey/geo/?cid= nrcs142p2_053629) (Figure S4). Ythroughfall =

Yrunoff =

∑i ∑k (Vthroughfall, i , k DOCthroughfall ) nA

∑i ∑k (Q runoff, i , k DOCrunoff ) (2)

nA

Vthroughfall, i , k = 10A(α0Pi , k + β0) 1605

(1)

(3)

DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613

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ACS Earth and Space Chemistry Vrunoff, i , k = 10A

(Pi , k − Ia , k)2 Pi , k − Ia , k + Sk

ji 1000 zy Sk = 25.4jjj − 10zzz j CNk z k {

canopies with varied density. The throughfall DOC could be lower when the throughfall samples had less canopy interaction. Although studies on the spatial variability of throughfall DOC concentration for a single tree are rare, it is generally assumed that the characters of throughfall DOM do not vary greatly for a single species without epiphytes.20,21 The DOC concentrations in leaf extracts were significantly and consistently higher than those in the corresponding throughfall samples, probably because of the relatively high leaf-to-water ratio and/or the intensive physical strength exerted during extraction. Similarly, the TDN concentration (in mg-N/L) increased in the order of rainwater (0.35 ± 0.09) < oak throughfall (0.55 ± 0.18) ≈ pine throughfall (0.65 ± 0.34) < pine-leaf extracts (2.00 ± 2.14) < oak-leaf extracts (3.57 ± 3.37). The large differences in DOC and TDN concentrations between throughfall and rainfall support that significant amounts of carbon and nitrogen could be leached from tree canopies.20,21,54−57 The DOC/TDN ratio was ranked as rainwater (4.01 ± 2.37) < pine throughfall (38.5 ± 14.6) ≈ pine-leaf extracts (59.9 ± 27.8) ≈ oak throughfall (71.3 ± 63.1) < oak-leaf extracts (151 ± 134). The DOC/TDN ratios in both throughfall and leaf extracts were higher than those in previous studies (i.e., 9−44 in temperate forests58 and 16−21 in rainforests59) and significantly exceeded the stoichiometry of the microbial requirement for biomass building.20 The concentrations of DOC and TDN and the DOC/TDN ratio showed wide seasonal variations, and such seasonal variations also depended on plant species (Figure 1). The maximum to minimum DOC ratio across the four sampling periods (DOCmax/min) was 4.27 (maximum in fall/minimum in winter) in pine throughfall, 6.87 (spring/summer) in oak throughfall, 18.6 (fall/spring) in pine-leaf extracts, and 9.18 (fall/summer) in oak-leaf extracts (Figure 1). The TDNmax/min was 2.63 (fall/summer) in pine throughfall, 1.91 (fall/summer) in oak throughfall, 10.4 (fall/spring) in pine-leaf extracts, and 15.6 (fall/spring) in oak-leaf extracts. The highest DOC and TDN concentrations were found in fall for pine throughfall, pine-leaf extracts, and oak-leaf extracts. This result may be a consequence of the leaf aging in canopy. The wettability of oak leaf surface has been found higher in September than May,33 which may allow more organics leaching from the leaves. In contrast, oak throughfall had the highest DOC concentration in spring, which is in agreement with findings in other oakdominated areas.60,61 This result implies that sources other than leaves may have contributed to the DOC concentrations in the spring oak throughfall. We observed that oak trees produced a large amount of pollen during spring, which could have strongly contributed to the highest DOC concentration measured in oak throughfall in spring.20,62 The (DOC/ TDN)max/min was 2.21 (spring/winter) in pine throughfall, 5.71 (spring/fall) in oak throughfall, 3.22 (fall/winter) in pineleaf extracts, and 8.84 (summer/winter) in oak-leaf extracts. The DOCmax/min, TDNmax/min, and (DOC/TDN)max/min values were higher in leaf-extract DOM than in throughfall DOM, and higher values were measured in the oak compared with the pine sources (Figure 1). This suggests that leaf-extract DOM was a highly season-sensitive contributor to the throughfall DOM pool, and there might be lower seasonal variations in DOM chemistry associated with pine compared with oak. 3.2. DOM Optical Properties. SUVA254 (L/mg-C/m) was significantly lower in pine-leaf extracts (0.50 ± 0.18) than in oak-leaf extracts (2.88 ± 2.13) and in pine throughfall (1.63 ± 0.34) than in oak throughfall (3.11 ± 1.63) (p < 0.05; Table

(4)

(5)

where Ythroughfall is the DOC yield from the pine or oak canopy that reaches the forest floor via throughfall, kg-C/ha; Yrunoff is the runoff DOC yield from the pine or oak canopy, kg-C/ha; i indicates the ith rain event; k is the kth grid cell; A is the area of a grid cell (30 × 30 m2 = 0.09 ha), ha; n is the number of grid cells occupied by pine or oak forests in each watershed in South Carolina; DOCthroughfall is the DOC concentration in throughfall, kg/m3; DOCrunoff is the DOC concentration in runoff and is assumed to be equal to DOCthroughfall, kg/m3; Vthroughfall,i,k is the volume of throughfall during the ith rain event in the kth grid cell; m3; α0 and β0 are the empirical constants to estimate throughfall from gross rainfall as determined by regression analysis [α0 = 0.88 and β0=-0.8 (mm) for pine51 and α0 = 0.87 and β0 = 0.35 (mm) for oak20]; Pi,k is the gross rainfall depth of the ith rainfall event in the kth grid cell, mm; Vrunoff,i,k is the runoff volume from the ith rainfall event in the kth grid cell, m3; Iα,k is the initial abstraction including surface storage, interception, and infiltration (approximated as 0.2Sk), mm; Sk is the soil water retention parameter in the kth grid cell; and CNk is the curve number in the kth grid cell, according to the USDA Soil Conservation Service.52 2.4. Data Analyses. RStudio Desktop v1.0.44 (Boston, MA, US) was used for statistical analyses. A paired t-test (using the t.test function in the stats package) or Wilson test (while the difference was not normally distributed; using the wilcox.test function in the stats package) was conducted to test the difference between groups of DOM sources and seasons. The Shapiro-Wilk normality test (using the shapiro.test function in the stats package) was conducted to test the normality. After obtaining the p values, the orderPvalue function in the agricolae package was used to group the average values. For correlation analysis, correlation coefficients and p values were calculated by the cor and cor.test functions, respectively, in the stats package. Factor analysis of optical properties and SDBP-FPs was performed using the fa function (nfactors = 2, rotate = “varimax”, SMC = FALSE, f m = “minres”) in the psych package. To evaluate the greatness of seasonal variations, the ratio of maximum and minimum values among the four seasons was calculated for each parameter.

3. RESULTS AND DISCUSSION 3.1. DOC and TDN Concentrations. The DOC concentration (mean ± standard deviation; in mg-C/L) was lowest in rainwater (1.2 ± 0.4), followed by pine (26.0 ± 19.7) and oak throughfall (38.8 ± 37.8), and then pine-leaf (162 ± 230) and oak-leaf extracts (323 ± 311; Table 1). The DOC concentrations in throughfall measured here were at the high end of the wide concentration range (5.7−54.0 mg-C/L) of forest throughfall reported in previous studies (Table S2), indicating that pine and oak canopies are important sources of throughfall DOM. We also observed the high spatial variability of the throughfall DOC concentrations in our study,53 suggesting that higher number of repeated samples would be favorable to better quantify the DOM fluxes. The reason for the high spatial variability in our throughfall DOC could be that the water samples were collected from the areas beneath 1606

DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613

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Figure 1. Concentrations of (a) dissolved organic carbon (DOC) and (b) total dissolved nitrogen (TDN), and (c) the DOC/TDN ratio in rainwater and in throughfall and leaf extracts of pine and oak. TF(pi), pine throughfall; TF(oa), oak throughfall; LE(pi), pine-leaf extracts; LE(oa), oak-leaf extracts. The ratio of maximum and minimum values among the four seasons is presented at the top of each sample type.

1). As SUVA254 is widely considered to be a good indicator of DOM aromaticity,40,63,64 the results suggest that the pinederived DOM had lower aromaticity than the oak-derived DOM in both throughfall and leaf extracts. Moreover, the (SUVA254)max/min value was 1.41 (winter/spring) in pine throughfall, 2.98 (spring/winter) in oak throughfall, 2.14 (spring/fall) in pine-leaf extracts, and 10.0 (spring/fall) in oakleaf extracts (Figure 2). This indicates that the seasonal variations in DOM aromaticity were much greater in oakderived DOM than in pine-derived DOM and were greater in leaf-derived DOM than in throughfall DOM. Specifically, SUVA254 decreased from spring to summer and then to fall for both oak throughfall and oak-leaf extracts, which could be attributed to the decreasing content of aromatic components (e.g., hydrolyzable tannins, phenolics, and flavonoid glycosides) from new to senescing oak leaves.32 The E2/E3 ratio was in the order of oak throughfall (6.36 ± 1.58) < pine throughfall (8.43 ± 1.31) ≈ oak-leaf extracts (9.72 ± 2.79) < pine-leaf extracts (20.1 ± 7.67) (Table 1 and Figure 2), suggesting that throughfall DOM from oak trees contains more highmolecular-weight (HMW) compounds than pine trees, and throughfall DOM contains more HMW compounds than leafextract DOM.65,66 The E2/E3 ratio was lowest in winter for both oak and pine throughfall, which implies more HMW DOM in throughfall in the winter. It is possible that the lowmolecular-weight DOM in “old” leaves was preferentially washed out during the fall, leaving more HMW DOM in the canopy in the winter.

Figure 2. Characteristics of dissolved organic matter in rainwater and in throughfall and leaf extracts of pine and oak: (a) SUVA254; (b) HIX; (c) E2/E3; (d) FI; (e) β/α; (f) PI,n; (g) PII,n; (h) PIII,n; (i) PIV,n; and (j) PV,n. TF(pi), pine throughfall; TF(oa), oak throughfall; LE(pi), pine-leaf extracts; LE(oa), oak-leaf extracts. The ratio of maximum and minimum values among the four seasons is presented at the top of each sample type.

The HIX value was lower in pine throughfall (2.25 ± 0.48) than in oak throughfall (3.38 ± 1.88) (p < 0.05; Table 1). Ohno45 reported an HIX value of 1.33 ± 0.03 for non- or lesshumified DOM from plant residues, 5.18 ± 0.22 for soil watersoluble DOM, and 15.9 ± 5.9 for soil fulvic acid. The HIX values here reflect the fact that the throughfall DOM was nonor less-humified. Compared to the throughfall DOM, the leafextract DOM had a significantly lower HIX value of 0.24 ± 0.09 in pine-leaf extracts and 0.44 ± 0.17 in oak-leaf extracts (p < 0.05), indicating fresher and less humified compounds in leaf extracts. The HIXmax/min value was 1.40 (winter/spring) in pine throughfall, 2.92 (summer/spring) in oak throughfall, 2.56 (summer/fall) in pine-leaf extracts, and 2.78 (winter/summer) in oak-leaf extracts (Figure 2). This implies that throughfall DOM in evergreen pine needles had more stable HIX across seasons compared to deciduous oak leaves. The fluorescence index is an indicator for differentiating the microbial and terrestrial origins of DOM.43,67 This index was similar (p > 1607

DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613

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ACS Earth and Space Chemistry 0.05) between pine throughfall (1.38 ± 0.07) and oak throughfall (1.43 ± 0.18) and showed very small variations across seasons (Figure 2). It is also similar to the FI values reported in previous studies (1.14−1.6; Van Stan et al.68 and references cited therein). This suggests that the DOM leached from canopy was not greatly processed by microorganisms and thus had relatively low microbial contribution.20,69 The β/α ratio is the freshness index that indicates the relative abundance of recently derived DOM.70 The order of oak-leaf extracts (0.32 ± 0.24) ≈ pine-leaf extracts (0.46 ± 0.15) ≈ oak throughfall (0.47 ± 0.06) < pine throughfall (0.55 ± 0.05) for the β/α ratio suggested the largest proportion of fresh DOM in pine throughfall.71 The lowest β/α ratio always occurred in fall for all four types of DOM (Figure 2), suggesting a lower proportion of fresh DOM in fall. The throughfall and leaf extract samples had relatively higher PI,n and PII,n compared to common soil leachate or water samples that we tested in a previous study using the same instrument setting.47 The tyrosine- and tryptophan-like (PI,n and PII,n) components have been hypothesized to be more biolabile than fulvic- and humic-acid-like (regions III and V) materials.72,73 Although the fluorescent signals in regions I and II can be also contributed by polyphenols, our result is consistent with the previous finding that throughfall DOM contains higher biolabile proportion (>30%) relative to the previously published data of litter and soil leachate (10%− 30%).22 Pine and oak throughfall samples had similar Pi,n values (p > 0.05) among all five fluorescent regions, suggesting the insignificant influence of tree species on regional fluorescence responses of throughfall DOM. Compared to throughfall, the leaf-extract DOM had higher PI,n (tyrosinelike) and PII,n (tryptophan-like) and lower PIII,n (fulvic acidlike) and PV,n (humic acid-like) for both pine and oak (p < 0.05). It implies that the leaf-extract DOM may be more biolabile than the throughfall DOM. This is likely due to the contribution of branch and bark DOM in throughfall, as well as the relatively intensive extraction of fresh and biolabile intracellular DOM from leaf materials. Overall, oak throughfall not only showed distinct optical characteristics (greater aromaticity and higher molecular size) compared to pine throughfall but also displayed different seasonal patterns and greater seasonal variations for almost all parameters. Note that the SUVA and HIX values reached 3fold differences across seasons for oak throughfall (Figure 2). This evidence demonstrates that throughfall DOM from the deciduous oak trees would have greater seasonal variation in chemistry compared to that from evergreen pine trees. Therefore, the temporal variation of throughfall DOM could be considerable, and frequent sampling is desirable to comprehensively understand throughfall chemistry, especially for deciduous trees such as oaks. Moreover, there were large differences in optical properties between throughfall DOM and leaf-extract DOM, and the seasonal patterns of throughfall and leaf-extract DOM were highly inconsistent. Compared to the leaf-extract DOM, throughfall DOM had more HMW (indicated by E2/E3) and humic-like components (indicated by HIX and PV,n), fewer protein-like components (indicated by PI,n and PII,n), and weaker seasonal variations for most optical characteristics. Clearly, the leaf extraction method used here failed to capture the DOM characteristics in the real throughfall samples, probably because sources other than leaves (e.g., twigs, barks, and pollen) may have contributed DOM to the throughfall.

3.3. DBP Formation. After passing through the canopies as throughfall, the rainwater gained considerable amounts of additional DBP-FPs for all studied DBP groups (p < 0.05; Table S3). Specifically, THM-FP (μg/L) was 52.0 ± 49.8 in rainwater but increased to 1584 ± 1821 (∼30.5-fold increase) in pine throughfall and 2165 ± 2071 (∼41.6-fold increase) in oak throughfall (Table 1). These high THM-FP levels of throughfall DOM are within the same order of magnitude as the previously reported THM-FP of organic-rich peat soil leachates.74 These high FPs of the throughfall demonstrate that the poststorm throughfall DOM transported to the source water could be another important yet overlooked terrestrial THM precursor in drinking water. Consistent with THMs, the DBP-FPs for HANs and CHD followed the order of pine throughfall ≈ oak throughfall ≈ pine-leaf extracts < oak-leaf extracts (Figure 3). In contrast, HKT-FPs (μg/L) followed the

Figure 3. Formation potentials (a−d) and specific formation potentials (e−h) of disinfection byproducts. TF(pi), pine throughfall; TF(oa), oak throughfall; LE(pi), pine-leaf extracts; LE(oa), oak-leaf extracts. The ratio of maximum and minimum values among the four seasons is presented at the top of each sample type.

order of oak throughfall (8.84 ± 11.6) < oak-leaf extracts (30.5 ± 40.3) ≈ pine-leaf extracts (38.4 ± 51.4), while HKT-FP of pine throughfall (14.9 ± 15.9) did not differ significantly from those of the other three DOM sources. All four types of DOM showed extensive and inconsistent seasonal variations in DBPFP. Specifically, the DBP-FPmax/min value was 5.13−23.9 for THMs, 4.53−21.1 for HANs, 3.82−26.9 for HKTs, and 2.72− 25.7 for CHD (Figure 3), indicating that the seasonal DBP-FP of throughfall can differ by more than one order of magnitude. Interestingly, the highest FPs for all studied DBP groups occurred in fall for pine throughfall (except HKTs in spring), pine-leaf extracts, and oak-leaf extracts, but occurred in spring 1608

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Figure 4. Factor analysis (FA) of optical properties (including SUVA254, HIX, E2/E3, FI, β/α, PI,n, PII,n, PIII,n, PIV,n, and PV,n) and specific formation potentials of disinfection byproducts (including STHM-FPs, SHAN-FPs, SHKT-FPs, and SCHD-FPs): (a) loadings and (b) scores.

were spring for pine and summer for oak, in both throughfall and leaf extracts. 3.4. Linking Chlorine Reactivity to Optical Properties of DOM. Correlation analysis (Table S4 and Figure S5) and factor analysis of all optical properties and SDBP-FPs indicated that the first two factors (F1 and F2) explained 37.1% and 20.4%, respectively, of the total variance (Figure 4). The variables E2/E3, PI,n, and PII,n had high negative F1 loadings (≤-0.50), while high positive F1 loadings (≥0.50) were observed for HIX, PIII,n, PV,n, STHM-FPs, and SCHD-FPs (Table S5). F2 loadings less than or equal to −0.50 were found for FI and PIV,n, while the variables β/α and PIII,n had F2 loadings ≥0.50. Accordingly, the high F1 loading could be interpreted as more fulvic/humic acid-like compounds with high reactivity in forming THMs or CHD and less tyrosine/ tryptophan-like and LMW components, while the positive F2 loading reflected more fulvic acid-like compounds with high freshness and less microbial-originated compounds. It supports the fulvic/humic-acid-like DOM components as the reactive THM and CHD precursors. Samples from the four DOM sources were separated by their scores in factor analysis (Figure 4). Specifically, throughfall samples had higher F1 and F2 scores than leaf-extracts, suggesting that throughfall DOM had more HMW fulvic/humic acid-like compounds with high freshness and much higher chlorine reactivity to form all four types of DBPs (Figure 4). It further supports that the leafextract DOM is not a good proxy for throughfall DOM from the considerations of optical properties and chlorine reactivity. Moreover, the oak samples showed wider distribution compared to pine samples on the score plot (Figure 4), regardless of leaf extract or throughfall samples. It suggests that oak throughfall and leaf extracts had greater seasonal variations in the DOM chemistry. The higher F1 score of samples from oak than pine source supports that the oak-derived DOM had more HMW fulvic/humic acid-like compounds than pinederived DOM. 3.5. Environmental Implications. Here, we show that after passing through the canopies, the DOC concentration and all DBP-FP values increased considerably (p < 0.05) in the throughfall of pine and oak trees. Specifically, the pine and oak throughfall had ∼30.5 and ∼41.6 times, respectively, the amount of THM-FPs compared with rainwater. The relatively

for oak throughfall (Figure 3) due to high DOC level in oak throughfall in spring. The SDBP-FP (μg-DBP/mg-DOC) was similar (p > 0.05) between pine and oak throughfall in forming THMs (52.7 ± 17.3 versus 58.6 ± 15.1), HANs (0.82 ± 0.35 versus 0.64 ± 0.11), and CHD (4.51 ± 2.25 versus 4.20 ± 2.76), but was lower (p < 0.05) in oak throughfall (0.22 ± 0.06) than pine throughfall (0.59 ± 0.60) in forming HKTs (Table 1). Specifically, the STHM-FP (μg-THMs/mg-DOC) from throughfall were higher than the range (15.9−36.0) for forest floor leachates reported by Chow et al.,75 at the relatively high end of the range (19.2−64.2) for forest floor leachates reported by Beggs and Summers,10 and similar to the range (40.0−74.9) for forest floor leachates reported by Jian et al.76 Although most fresh plant leachates have been hypothesized as relatively weak precursors for THMs,8,77 the present study showed that throughfall DOM can be reactive precursors, and thus, its role in THM formation should not be ignored. Note that the SDBP-FP (μg-DBP/mg-DOC) of throughfall was consistently higher than that of leaf extracts in forming all studied DBP groups (except THMs for oak in summer; Figure 3). The relatively low chlorine reactivity of leaf-extract DOM was consistent with previous findings of low chlorine reactivity of plant leachates.77 However, the higher reactivity of throughfall DOM relative to leaf-extract DOM or plant leachates indicates that other sources besides leaf-extract DOM, such as twig, bark, or pollen, might be more reactive sources of DBP precursors. Although the DOM from these other sources was not examined, stemflow passing over branches and barks was recently found to contain more aromatic DOM,21,78 which supports the finding that twig and bark DOM could be more aromatic and thus probably more reactive in forming THMs.64 The SDBP-FPsmax/min value was 1.70−4.05 for THMs, 1.45−4.55 for HANs, 1.32−6.70 for HKTs, and 1.53−4.17 for CHD (Figure 3). The highest SDBP-FPs for all studied DBP groups occurred in spring for pine throughfall (except STHM-FP and SCHD-FP in fall), summer for oak throughfall (except SHAN-FP in fall), spring for pine-leaf extracts (except SCHD-FP in fall), and summer for oak-leaf extracts (except SHAN-FP in winter). The seasons for relatively high DOM reactivity in terms of DBP formation 1609

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Figure 5. Seasonal and annual dissolved organic carbon (DOC) yields (kg-C/ha/mo) in throughfall and runoff from pine and oak canopies across 34 watersheds in South Carolina: (a) pine throughfall; (b) oak throughfall; (c) runoff DOC yield from pine canopy; and (d) runoff DOC yield from oak canopy.

high THM-FPs compared to forest floor or soil-derived leachates suggest an important role of throughfall DOM as a previously unrecognized terrestrial THM precursor. Based on the DOC levels, landscape, and climate information, a very rough estimate of the annual throughfall DOC yield in South Carolina was 347 ± 128 kg-C/ha/yr from pine and 304 ± 81 kg-C/ha/yr from oak (Figure 5 and Table S6). The annual runoff DOC yield was estimated to be 71.7 ± 28.0 kg-C/ha/yr from pine and 39.7 ± 11.6 kg-C/ha/yr from oak in South Carolina. These values were at the relatively high end of the summarized ranges, i.e., 41−340 kg-C/ha/yr for throughfall DOC yield in temperate forests in a recent review by Van Stan and Stubbins23 and 6.45−84 kg-C/ha/yr for runoff DOC yield (Table S2). The seasonal throughfall DOC yield (kg-C/ha/ mo) could amount to 88.9 ± 36.2 in the fall for pine and 39.5 ± 9.72 in the fall and 39.0 ± 16.0 in the spring for oak. The highest monthly runoff DOC yield was estimated in the fall for both pine (21.9 ± 8.89 kg-C/ha/mo) and oak (8.75 ± 2.15 kgC/ha/mo). Compared to the fall, the spring season was estimated to have similar throughfall DOC yield but lower runoff DOC yield from oak canopies because the lower rainfall intensity in spring caused more infiltrated rainfall and less generated runoff. Therefore, throughfall DOM is likely to play an important role as the DBP precursor of water supplies in the fall season. Considering its high DOC yield per unit area of landscape and high reactivity in forming THMs, this study suggests that throughfall DOM could be a significant terrestrial DBP precursor.

4. CONCLUSIONS In summary, this study revealed that throughfall DOM could be a significant terrestrial DBP precursor. Based on the seasonal variations in concentration levels, spectroscopic characteristics, and chlorine reactivity of pine and oak throughfall DOM, forest type was confirmed as a critical factor that alters the seasonal patterns of DOM quantity and quality of throughfall. The estimation of the contribution of canopies on runoff DOC yield in South Carolina suggests that the greatest contributions would occur in the fall (pine, 21.9 ± 8.89 kg-C/ha/mo; oak, 8.75 ± 2.15 kg-C/ha/mo) among all seasons. Furthermore, the throughfall DOM derived from pine and oak trees contained much more aromatic and fulvic/ humic-like components than their leaf extracts. It implies that the leaf extraction method we used is not a satisfactory approach to study throughfall DOM. The results in the present study would be applied to improve the modeling of DOC export from forest watersheds to downstream receiving water and to support the future forest management and water supplies in the context of environmental and landscape change.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.9b00088. 1610

DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613

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(10) Beggs, K. M. H.; Summers, R. S. Character and chlorine reactivity of dissolved organic matter from a mountain pine beetle impacted watershed. Environ. Sci. Technol. 2011, 45 (13), 5717−5724. (11) Mitra, S.; Bianchi, T. S.; Guo, L. D.; Santschi, P. H. Terrestrially derived dissolved organic matter in the Chesapeake Bay and the Middle Atlantic Blight. Geochim. Cosmochim. Acta 2000, 64 (20), 3547−3557. (12) Mikkelson, K. M.; Dickenson, E. R. V.; Maxwell, R. M.; McCray, J. E.; Sharp, J. O. Water-quality impacts from climateinduced forest die-off. Nat. Clim. Change 2013, 3 (3), 218−222. (13) Majidzadeh, H.; Uzun, H.; Ruecker, A.; Miller, D.; Vernon, J.; Zhang, H. Y.; Bao, S. W.; Tsui, M. T. K.; Karanfil, T.; Chow, A. T. Extreme flooding mobilized dissolved organic matter from coastal forested wetlands. Biogeochemistry 2017, 136 (3), 293−309. (14) Inamdar, S.; Singh, S.; Dutta, S.; Levia, D.; Mitchell, M.; Scott, D.; Bais, H.; McHale, P. Fluorescence characteristics and sources of dissolved organic matter for stream water during storm events in a forested mid-Atlantic watershed. J. Geophys. Res. 2011, 116, G03043. (15) Yang, L.; Hur, J.; Lee, S.; Chang, S. W.; Shin, H. S. Dynamics of dissolved organic matter during four storm events in two forest streams: source, export, and implications for harmful disinfection byproduct formation. Environ. Sci. Pollut. Res. 2015, 22 (12), 9173− 9183. (16) Murphy, S. F.; Writer, J. H.; McCleskey, R. B.; Martin, D. A. The role of precipitation type, intensity, and spatial distribution in source water quality after wildfire. Environ. Res. Lett. 2015, 10 (8), No. 084007. (17) Cunha, D. G. F.; Sabogal-Paz, L. P.; Dodds, W. K. Land use influence on raw surface water quality and treatment costs for drinking supply in Sao Paulo State (Brazil). Ecol Eng. 2016, 94, 516− 524. (18) Elias, E.; Dougherty, M.; Srivastava, P.; Laband, D. The impact of forest to urban land conversion on streamflow, total nitrogen, total phosphorus, and total organic carbon inputs to the converse reservoir, Southern Alabama, USA. Urban Ecosyst. 2013, 16 (1), 79−107. (19) Brown, T. C.; Hobbins, M. T.; Ramirez, J. A. Spatial distribution of water supply in the coterminous United States. J. Am. Water Resour. Assoc. 2008, 44 (6), 1474−1487. (20) Van Stan, J. T.; Wagner, S.; Guillemette, F.; Whitetree, A.; Lewis, J.; Silva, L.; Stubbins, A. Temporal dynamics in the concentration, flux, and optical properties of tree-derived dissolved organic matter in an epiphyte-laden oak-cedar forest. J. Geophys. Res.: Biogeosci. 2017, 122 (11), 2982−2997. (21) Stubbins, A.; Silva, L. M.; Dittmar, T.; Van Stan, J. T. Molecular and optical properties of tree-derived dissolved organic matter in throughfall and stemflow from live oaks and eastern red cedar. Front Earth Sci. 2017, 5, 1−13. (22) Howard, D. H.; Van Stan, J. T.; Whitetree, A.; Zhu, L. X.; Stubbins, A. Interstorm variability in the biolability of tree-derived dissolved organic matter (tree-DOM) in throughfall and stemflow. Forests 2018, 9 (5), 236. (23) Van Stan, J. T.; Stubbins, A. Tree-DOM: Dissolved organic matter in throughfall and stemflow. Limnol. Oceanogr. Lett. 2018, 3 (3), 199−214. (24) Kraus, T. E. C.; Anderson, C. A.; Morgenstern, K.; Downing, B. D.; Pellerin, B. A.; Bergamaschi, B. A. Determining sources of dissolved organic carbon and disinfection byproduct precursors to the Mckenzie River, Oregon. J. Environ. Qual. 2010, 39 (6), 2100−2112. (25) Wanek, W.; Hofmann, J.; Feller, I. C. Canopy interactions of rainfall in an off-shore mangrove ecosystem dominated by Rhizophora mangle (Belize). J. Hydrol. 2007, 345 (1−2), 70−79. (26) Bischoff, S.; Schwarz, M. T.; Siemens, J.; Thieme, L.; Wilcke, W.; Michalzik, B. Properties of dissolved and total organic matter in throughfall, stemflow and forest floor leachate of central European forests. Biogeosciences 2015, 12 (9), 2695−2706. (27) Hua, G. H.; Kim, J.; Reckhow, D. A. Disinfection byproduct formation from lignin precursors. Water Res. 2014, 63, 285−295. (28) Wang, J. J.; Ng, T. W.; Zhang, Q.; Yang, X. B.; Dahlgren, R. A.; Chow, A. T.; Wong, P. K. Technical Note: Reactivity of C1 and C2

Tables showing performance of ordinary kriging methods, previously reported concentrations and yields of DOC in throughfall and runoff; mean and standard deviation of nutrients, optical properties, and (S)DBPFPs; correlations between optical properties and SDBPFPs; loadings in factor analysis; calculated monthly and annually DOC yield from throughfall and runoff in pine and oak forests. Figures showing basin distribution, seasonal rainfall, pine and oak distribution, hydrologic soil group in South Carolina, and correlation among optical properties and SDBP-FP (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Huan Chen: 0000-0001-9998-1205 Kuo-Pei Tsai: 0000-0003-2620-3861 Qiong Su: 0000-0003-2420-5898 Alex T. Chow: 0000-0001-7441-8934 Jun-Jian Wang: 0000-0002-3040-0924 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the National Science Foundation (1529927), the Natural Sciences Foundation of China (41807360), Southern University of Science and Technology (G01296001), and Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (No. 2017B030301012).

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DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on June 12, 2019. A partially corrected version was published on June 12 and the fully corrected version was reposted on June 13, 2019

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DOI: 10.1021/acsearthspacechem.9b00088 ACS Earth Space Chem. 2019, 3, 1603−1613