Leaf litter age, chemical quality, and photodegradation control the fate of leachate dissolved organic matter in a dryland river

Journal of Arid Environments 89 (2013) 30e37

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Leaf litter age, chemical quality, and photodegradation control the fate of leachate dissolved organic matter in a dryland river J.B. Fellman a, *, K.C. Petrone b, P.F. Grierson a a b

Ecosystems Research Group, School of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia CSIRO Land and Water, Private Bag 5, Wembley, WA 6913, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 March 2012 Received in revised form 10 August 2012 Accepted 5 October 2012 Available online 4 November 2012

We used biodegradable dissolved organic carbon (BDOC) incubations, specific ultraviolet absorbance (SUVA254, indicator of aromatic carbon content) and laboratory experiments to determine the bioavailability and chemical composition of dissolved organic matter (DOM) leached from fresh leaves and litter aged on a seasonally dry floodplain for 2, 4, and 6 months. Our objective was to elucidate how litter age and solar radiation affect the bacterial utilization of DOM released from floodplain leaf litter when inundated. Leachate percent BDOC ranged from 22 to 47% for three different leaf species and significantly decreased (p < 0.05) with increasing litter age. However, total BDOC (mg C L
1) was unrelated to litter age. Bacterial utilization of DOM leachate collected from litter aged on the floodplain for four and six months significantly increased following 48 h of irradiation for all species but there was no difference for leachate from fresh and two month old litter. The photo-mediated increase in percent BDOC was concomitant with a decrease in aromatic carbon content, as SUVA254 values decreased on average 9  6% for light exposure experiments. Our findings demonstrate that sunlight moderates the degradation of plant litter in the terrestrial environment through the photo-mediated shift in DOM composition and its bioavailability in streams. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Bioavailability Dissolved organic matter Dryland river Floodplain Leaf litter Photodegradation Pilbara Specific ultraviolet absorbance

1. Introduction The input of allochthonous organic matter to streams is widely recognized as a key component of ecosystem energy budgets (Minshall, 1967; Wallace et al., 1997). In forested, headwater streams, as much as 99% of the annual energy input is allochthonous and leaf litter accounts for 29% of this energy input (Fisher and Likens, 1973). After submersion, leaves can be colonized by bacteria, fungi and invertebrates, thereby providing both a physical substrate and food source (Sanpera-Calbet et al., 2009). A large fraction of initial leaf mass can also be leached (up to 40%) as dissolved organic matter (DOM) within the first few days after submersion (Boulton, 1991; McDowell and Fisher, 1976), and its lability is an important control of stream metabolism. Consequently, there is a rich history of studies evaluating stream uptake of DOM leached from leaf litter (Bernhardt and McDowell, 2008; Kaplan and Bott, 1983; Lock and Hynes, 1976). The uptake of DOM by stream ecosystems depends on a number of factors including its chemical composition (e.g., elemental ratio,

* Corresponding author. E-mail address: [email protected] (J.B. Fellman). 0140-1963/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaridenv.2012.10.011

molecular weight) and inorganic nutrient status, both of which can be altered by photolytic processes (Moran et al., 2000). For instance, photochemical release of NH4 from DOM can alleviate N limitation and increase bacterial metabolism of humic DOM (Bushaw et al., 1996). Photolytic processes may also degrade recalcitrant DOM into a range of lower molecular weight compounds (e.g., acetate) and amino acids (e.g., glycine and alanine) that are more readily available for bacterial uptake (Buffam and McGlathery, 2003; Moran and Zepp, 1997). In contrast, irradiation of bioavailable, algal-derived DOM may form more biorefractory photoproducts, such as carbon gases (e.g., carbon monoxide) and radicals (e.g., hydrogen peroxide), which reduce bacterial growth (Obernosterer et al., 1999; Tranvik and Kokalj, 1998). These studies have all converged on the hypothesis that initial DOM quality influences the bacterial growth response following irradiation, where light transforms allochthonous DOM mainly into more labile compounds but algal-derived DOM into compounds of decreased bacterial substrate quality (Moran and Covert, 2003; Obernosterer et al., 1999; Tranvik and Bertilsson, 2001). However, little is known about how solar radiation affects bacterial utilization of allochthonous DOM of varying lability. Rivers draining dryland landscapes are prime examples of aquatic environments where DOM cycling could be strongly

J.B. Fellman et al. / Journal of Arid Environments 89 (2013) 30e37

influenced by photolytic processes because they typically have low riparian forest coverage and receive extensive solar radiation. Dryland rivers have variable and unpredictable flow regimes, especially in Australia where river discharge is nearly three times as variable as the world average (Davies et al., 1994). Many Australian rivers also have extensive periods when surface water is limited to a series of temporary pools or waterholes (Bunn et al., 2003). During dry periods, leaf litter can fall directly into pools. However, large amounts of detritus (e.g., of Eucalyptus or Melaleuca spp. that generally dominate these systems) also accumulate on the floodplain and dry stream channel (Briggs and Maher, 1983), where both biotic (Melillo et al., 1982) and photolytic processes (Austin and Vivanco, 2006; Pancotto et al., 2005) can degrade plant litter and modify its chemical composition. Therefore, DOM leached from aged and partially degraded plant litter during an ensuing flood could be largely biorefractory but more susceptible to photomediated increases in bacterial availability. To date, the interaction of litter age and photo-alteration on DOM lability has not been directly addressed. In this study, we use specific ultraviolet absorbance (SUVA254, indicator of aromatic carbon content), laboratory light exposure experiments and DOM bioavailability incubations to determine the chemical composition and bioavailability of DOM leached from fresh leaves (time ¼ 0 months) and litter aged on a seasonally dry floodplain for 2, 4, and 6 months. Our objective was to elucidate how litter age and light exposure affect the bacterial utilization of DOM released from floodplain leaf litter when inundated. Although the bioavailability of leachate DOM may decrease with increasing litter degradation (Baldwin, 1999), we assess whether solar radiation may enhance DOM bioavailability depending on its initial lability and chemical composition. 2. Methods 2.1. Site description Leaf litter was collected from the Marillana Creek catchment in the Pilbara region of northern Western Australia (22 460 S, 119 100 E). Marillana Creek is an intermittent stream characterized by highly variable surface water flow. The creek is typical of other dryland river systems in the Pilbara where surface water is largely constrained to pools along drainage lines and those with groundwater inputs to surface water. The Pilbara has a semi-arid to subtropical climate with mild winters averaging 11e26  C and hot summers averaging 24e40  C. Solar radiation is high and generally in excess of 30 Megajoules m
2 for much of the year (Australian Bureau of Meteorology, 2011). Rainfall in the region averages 350 mm yr
1 and varies greatly both within and among years, but the majority falls during the summer cyclone season from December to March. Droughts of more than three years duration are common and the inter-annual variability in precipitation (coefficient of variation) is more than 100%. Rainfall for the study period was <50 mm and there was no surface flow in Marillana Creek. The geology of the Pilbara is complex and includes some of the oldest exposed rock on the earth’s surface. As a result, floodplain soils and sediments have low organic matter and nutrient pools (<1% for total C, <0.1% for total N, and <0.05% for total P; McIntyre et al., 2009). Riparian environments of Marillana Creek are typical of the Pilbara region in that they often have dense stands of trees adjacent to the main channel, in stark contrast to the open grasslands and shrublands of the surrounding floodplains. These riparian woodlands are generally dominated by Eucalyptus camaldulensis subsp. refulgens and E. victrix. Other common trees are the legumes, Acacia coriacea and Acacia citrinoviridis, which often form a patchy scrub

31

mixed with Eucalyptus spp. especially on secondary channels or along broad reaches of rivers. Melaleuca spp. are also common in the region, particularly Melaleuca argentea, which can often be the dominant tree around waterholes or where there is permanent water 1e2 m within the soil surface. Leaf fall in Marillana Creek is greatest during the Austral winter months of June through September and Eucalyptus spp. typically account for >70% of total leaf fall (Fellman, unpubl.). 2.2. Field procedures We collected foliage during May of 2009 from then dominant trees, E. camaldulensis subsp. refulgens, A. coriacea, and M. argentea. While extractable DOM and nutrients may be greater from live foliage compared to naturally senescent litter (e.g., Cleveland et al., 2004), we used live foliage in this study: 1) to ensure all leaf material was similar in age at the start of field incubations, and 2) windfall of branches and live foliage is quite common in the Pilbara such that non-senescent leaves can contribute significantly to litter fall. Foliar samples were randomly collected from trees located along a 50 m transect extending parallel to the creek channel by clipping leaves from trees at a height of 1.5 m. Foliage was removed from large stems prior to placement in the litter bags. Approximately 50 g of leaf material for each species was returned to the laboratory for initial extractions, while the remaining leaves were placed in mesh litter bags for field incubations. Approximately 15 g of leaf litter for each species was placed into each litter bag (40 cm  70 cm, mesh size ¼ 1 mm), and litter was evenly spread throughout each litter bag. In total, 27 litter bags (three tree species  three time intervals and three replicates) were placed at different locations along the riparian transect and adjacent to foliage collection sites. After 2, 4, and 6 months, three replicate litter bags for each tree species were collected from different locations along the riparian transect, composited into one litter bag for each species, and returned to the laboratory for analysis. 2.3. Chemical analysis of leaf litter and litter leachate Leaf litter extractions of DOM were performed on fresh leaves (time ¼ 0 months) and litter aged on the dry floodplain for 2, 4, and 6 months following a similar procedure outlined in Cleveland et al. (2004). All litter was lightly brushed to remove any soil or particulates collected during field incubations. Four replicate extractions were performed for each tree species by cutting approximately 10 g of air-dried litter into small pieces (<2 cm2). Cut litter was placed into 1000 mL glass beakers and extracted in 500 mL of Milli-Q water for 2 h at room temperature (w20e25  C). All extractions were gently stirred on a shaker table. Litter leachates were immediately filtered through a pre-combusted, Whatman GF/F filter (nominal pore size 0.7 mm) and stored in acid-washed, HDPE bottles for initial chemical analyses or laboratory experiments (see below). Dissolved organic carbon (DOC) and total dissolved N (TDN) concentrations were measured by high temperature catalytic oxidation on a Shimadzu TOC/TN-V analyzer. All DOC and TDN data were reported as the mean of three to five replicate injections, for which the coefficient of variance was always <2%. Nitrateenitrogen (NO3eN) and ammoniumenitrogen (NH4eN) were measured using a colorimetric detection method on a Technicon auto-analyzer (Technicon, 1977). Dissolved organic N (DON) was calculated as the difference between TDN and dissolved inorganic N (DIN ¼ NH4e N þ NO3eN). Soluble reactive phosphorus (SRP) was measured using the ascorbic acid method with a 1 cm quartz cell (Murphy and Riley, 1962). We used specific ultraviolet absorbance (SUVA254) of DOM, which is an indicator of aromatic carbon content (Weishaar et al.,

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J.B. Fellman et al. / Journal of Arid Environments 89 (2013) 30e37

2003), to determine the chemical composition of DOM. SUVA254 was measured at 254 nm using a 1 cm, quartz flow cell and Milli-Q water as a blank. SUVA254 values were determined by dividing sample absorbance at 254 nm by DOC concentration multiplied by cell length (1 cm) with units of L mg C
1 m
1. A Shimadzu UV-1601 spectrophotometer was used for all absorbance measurements. SUVA254 was measured on all leaf extractions and during DOM light exposure experiments on water samples warmed to room temperature (see below). 2.4. Laboratory photodegradation experiment Laboratory irradiation experiments were completed on litter leachate exposed to light that resembled natural solar radiation of the study area. Each experiment occurred immediately following extraction of fresh leaves (time ¼ 0 months) and litter aged in the field for 2, 4, and 6 months. Litter leachate was initially filtered through a 0.2 mm nylon syringe filter to remove the majority of microbial biomass. After filtration, sample water was placed in 48 glass vials (three species  four time intervals  four replicates) filled to capacity (volume of 40 mL) and capped tightly. Twelve of the vials (one replicate for each species) were wrapped in foil and used as a dark control. All samples including the dark control were placed in front of a lamp. The UVB bandwidth (280e 320 nm) of the light source was 0.2 W m
2 (measured using a Solarmeter digital UV meter). Light experiments were performed at room temperature and a cooling fan was placed next to the samples to ensure sample temperature never exceeded 25  C. Samples were swirled periodically during experiments to ensure the sample was well mixed. Three replicate vials and one dark control were collected after 0, 8, 24, and 48 h. At each time interval, sample absorbance (SUVA254), NH4eN, and DOC were measured, and incubations quantifying DOC lability were conducted on samples irradiated for 0 and 48 h using the methods below. 2.5. Biodegradable DOC incubations Laboratory incubations examining biodegradable DOC (BDOC) were completed immediately following DOM extraction from fresh leaves (time ¼ 0 months) and litter aged on the floodplain for 2, 4, and 6 months. Laboratory BDOC incubations were conducted following a modified method from Servais et al. (1989). Three replicates were completed for each species and for both freshly extracted (time ¼ 0 h) and irradiated DOM (time ¼ 48 h). Extractions were initially filtered through a 0.2 mm nylon syringe filter to remove the majority of microbial biomass. After filtration, 23 mL of the filtrate was placed into pre-combusted (400  C for 4 h), glass

bottles and 2 mL of a microbial inocula was added. The microbial inoculum was prepared by leaching stream sediments collected from Marillana Creek with Milli-Q water for 24 h followed by filtration through a Whatman GF/D filter (2.7 mm). Samples were diluted if necessary to 20 mg C L
1 to prevent excessive microbial growth. Concentrations of DOC were measured at the start of the experiment and samples were incubated for 28 days in the dark at room temperature. After the incubation, the solution was refiltered through a 0.2 mm filter, DOC was re-measured, and BDOC was calculated as the difference in DOC before and after the incubation. 2.6. Data analysis We used linear regression to assess the relationship between litter age on the dry floodplain (0, 2, 4, and 6 months), BDOC, and SUVA254 for the three tree species (n ¼ 12 for each regression analysis). Paired t tests for BDOC, SUVA254, and NH4eN at time 0 and 48 h were also completed to assess changes in DOM biogeochemistry during light exposure experiments. For paired t tests, data for BDOC concentrations were log transformed to normalize the distribution. Sigmastat software was used for all statistical comparisons (Sigmastat, 2009). 3. Results 3.1. Extractable DOM and nutrient concentrations Water extractable DOC and DON varied widely among tree species and also with litter age but concentrations were always greatest for Eucalyptus leachate (Table 1). Extractable DOC ranged from 0.4% to 2.9% of dry mass, while TDN solubility ranged from 0.01% to 0.03% of dry mass. Concentrations of DOC and DON were greatest for litter aged on the floodplain for two months followed by a gradual decrease with increasing litter age. Despite this decrease in DOC with litter age, concentrations were still greater in six month old litter (mean of 10.8  6.7 mg C g
1) than for freshly extracted leaves (mean of 9.1  6.3 mg C g
1) for all tree species. The DOC:DON ranged from 19 to 108 and was always >60 for Melaleuca and Eucalyptus and <35 for Acacia. Mean extractable DIN (NH4eN þ NO3eN) and SRP concentrations were variable across tree species and litter age (Table 1). Concentrations of DIN accounted for 3e17% of TDN, but concentrations were always <6.5% of TDN for Melaleuca and Eucalyptus and between 14% and 17% of TDN for Acacia leachate. Acacia leachate had the highest mean concentrations of DIN but lowest SRP, while mean SRP for Eucalyptus was nearly twice that of Melaleuca and six-fold greater than that of Acacia. Similar to DOM,

Table 1 Mean (SE, n ¼ 4) water extractable concentrations of dissolved C, N, and P for fresh leaves (time ¼ 0 months) and litter aged on the dry floodplain for 2, 4, and 6 months. Tree species were Eucalyptus camaldulensis subsp. refulgens, Acacia coriacea, and Melaleuca argentea. Tree genera

Time months

DOC mg C g
1

DON mg N g
1

DOC:DON

NH4eN mg N g
1

SRP mg P g
1

Eucalyptus Eucalyptus Eucalyptus Eucalyptus

0 2 4 6

16.2 29.2 20.4 18.2

(1.2) (2.2) (0.5) (2.1)

240.2 290.4 213.9 171.3

(21.5) (27.9) (18.7) (21.7)

67 101 95 106

(5) (7) (6) (10)

NO3eN mg N g
1 3.0 6.1 3.2 2.1

(0.3) (0.4) (0.9) (0.2)

10.5 13.1 10.4 3.7

(2.5) (1.1) (2.0) (1.4)

39.4 50.8 44.4 34.6

(5.8) (4.8) (6.3) (3.9)

Melaleuca Melaleuca Melaleuca Melaleuca

0 2 4 6

7.1 12.1 11.9 9.4

(0.8) (0.4) (1.0) (0.4)

94.6 130.2 110.5 91.1

(4.1) (10.2) (5.6) (6.3)

75 93 108 104

(5) (9) (5) (3)

1.2 2.6 2.1 1.0

(0.1) (0.1) (0.4) (0.1)

5.4 6.3 4.9 2.6

(0.3) (0.2) (0.1) (0.4)

21.8 38.2 26.7 18.3

(1.4) (3.3) (2.8) (2.4)

Acacia Acacia Acacia Acacia

0 2 4 6

4.0 7.1 6.4 4.9

(0.6) (1.3) (0.4) (0.4)

204.1 252.0 206.9 146.7

(14.5) (23.2) (8.9) (10.2)

19 28 31 34

(1) (2) (1) (1)

20.1 24.4 14.2 18.3

(1.6) (3.7) (2.9) (2.5)

13.0 17.0 14.9 11.2

(0.5) (0.2) (0.6) (1.6)

9.8 11.1 6.0 3.1

(1.6) (0.3) (0.3) (1.0)

J.B. Fellman et al. / Journal of Arid Environments 89 (2013) 30e37

extractable DIN and SRP were greatest for two month old litter followed by a gradual decrease thereafter. 3.2. Specific UV absorbance of DOM The SUVA254 values ranged from 2.7 to 3.4 L mg C
1 m
1 for all tree species and values were greatest for Eucalyptus leachate (Fig. 1). This range in values corresponds to an aromatic carbon content of 21e26% using the linear model of Weishaar et al. (2003). There was a significant increase in SUVA254 with increasing litter age for all trees species (r2 ¼ 0.62, p ¼ 0.003, n ¼ 12, Fig. 1), as average aromaticity increased from 22% (freshly extracted) to 25% after six months of field incubation. This enrichment of SUVA254 with litter age was greatest for Acacia when aromatic carbon content increased from 21% to 25% for six month old litter. Light exposure (photodegradation experiments) significantly decreased aromatic carbon content of DOM leachate collected from four and six month old litter, as SUVA254 values decreased 7.5  3.4% for four month old litter and 17.3  4.7% for six month old litter (paired t test, p < 0.050, Fig. 2). However, there was no significant difference for DOM leachate from fresh leaves and two month old litter (paired t test, p > 0.050, Fig. 2). Concentrations of DOC decreased slightly during light exposure experiments with a mean change of 1.3  0.2 mg C g
1 for Eucalyptus, 0.4  0.2 mg C g
1 for Acacia, and 0.7  0.5 mg C g
1 for Melaleuca across light exposure experiments. Consequently, the change in SUVA254 from four and six month old litter was driven largely by a decrease in absorbance. 3.3. DOM bioavailability incubations The percent BDOC ranged from 22.2% to 46.7% across the four incubations and was greatest for Melaleuca leachate (Table 2). In contrast, BDOC concentrations were greatest for Eucalyptus leachate, as average concentrations were 6.1  1.7 mg C g
1 for Eucalyptus, 4.1  0.9 mg C g
1 for Melaleuca and 1.9  0.7 mg C g
1 for Acacia (Table 2). Percent BDOC decreased with increasing litter age for all trees species (r2 ¼ 0.52, p ¼ 0.037, n ¼ 12). However, concentrations were unrelated to litter age because the greater DOM concentration in leachate from aged leaves offset its lower percent BDOC (r2 ¼ 0.05, p ¼ 0.600, n ¼ 12). Percent BDOC was also

3.6

-1

-1

SUVA254 (L mg C m )

r2=0.62, p=0.003 3.4

3.2

3.0

2.8

2.6 0

2

4

6

Field incubation time (months) Fig. 1. Linear regression of litter age and mean (SE, n ¼ 3) specific ultraviolet absorbance (SUVA254) values for dissolved organic matter (DOM) leachate collected from fresh leaves (time ¼ 0 months) and litter aged on the dry floodplain for 2, 4, and 6 months. C represents Eucalyptus camaldulensis subsp. refulgens, B represents Acacia represents Melaleuca argentea. coriacea,

33

negatively correlated with SUVA254 (r2 ¼ 0.76, p < 0.001) indicating a strong relationship between decreasing aromatic carbon content and bioavailability. Bacterial utilization of DOM leachates collected from litter aged on the floodplain for four and six months increased following 48 h of irradiation for all tree species (paired t test, p < 0.050, Fig. 3a). On average, the photo-mediated increase in percent BDOC was 6.9  1.1% for four month old litter and 13.5  3.3% for six month old litter. In contrast, there was no difference in percent BDOC for leachate from fresh leaves and two month old litter (paired t test, p > 0.050, Fig. 3a). Concentrations of BDOC also increased after light exposure for leachate collected from litter aged four and six months on the floodplain (paired t test, p < 0.050, Fig. 3b). Similar to percent BDOC, there was no change in BDOC concentrations for fresh leaves and two month old litter (paired t test, p > 0.050, Fig. 3b). 3.4. Photoproduction of ammonium Leachate concentrations of NH4eN for the tree species significantly increased after 48 h of light exposure for fresh leaves (time ¼ 0 months) and litter aged 2 and 4 months on the floodplain (paired t test, p < 0.050, Fig. 4aec). However, there was no difference in leachate NH4eN following irradiation for six month old litter (paired t test, p > 0.050, Fig. 4d). The photoproduction of NH4eN averaged 0.04  0.02 mM h
1 for Eucalyptus, 0.06  0.01 mM h
1 for Acacia, and 0.03  0.02 mM h
1 for Melaleuca leachate across the light exposure experiments. Photoproduction rates of NH4eN were also positively correlated with initial DON concentrations (r2 ¼ 0.43; p ¼ 0.007). 4. Discussion This study highlights how litter age and solar radiation may interact to affect the bacterial utilization of DOM released from floodplain litter when inundated. To illustrate, using an Eucalyptus leaf litter fall of 166 g m
2 yr
1 (Briggs and Maher, 1983) and the leachate BDOC concentrations for Eucalyptus litter reported here (leaf litter fall in g m
2 yr
1*BDOC in mg C g
1), we estimate the potential BDOC flux to Marillana Creek during a flood to be: 0.90 g BDOC m
2 yr
1 and 1.79 g recalcitrant DOC m
2 yr
1 for fresh leaves, and 0.73 g BDOC m
2 yr
1 and 2.29 g recalcitrant DOC m
2 yr
1 for six month old litter. Including the influence of solar radiation on leachate DOM (leaf litter fall in g m
2 yr
1*BDOC in mg C g
1 following irradiation), we revised our potential estimate of bioavailable DOM from Eucalyptus litter to be: 0.91 g BDOC m
2 yr
1 and 1.78 g recalcitrant DOC m
2 yr
1 for fresh leaves, and 1.15 g BDOC m
2 yr
1 and 1.88 g recalcitrant DOC m
2 yr
1 for six month old litter. These varying estimates clearly demonstrate that total BDOC decreases with litter age but photodegradation processes can in part offset this temporal decrease in total BDOC by enhancing the availability of labile DOM to stream communities. Therefore, plant litter degradation in the terrestrial environment and photodegradation of leachate DOM in the stream work in opposite directions affecting the incorporation of carbon and nutrients into aquatic food webs. Some caution in interpretation of the results from the light exposure experiments is necessary because we used glass vials rather than quartz tubes. Quartz tubes are widely used in laboratory DOM photodegradation experiments because light penetration in the UV range is high. We recognize that glass vials selectively remove UV light and that using glass adds a level of uncertainty to our findings. For instance, it may be possible that different leaf species or aged leaves could be more susceptible to photodegradation at longer wavelengths (e.g., visible light). Given that

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J.B. Fellman et al. / Journal of Arid Environments 89 (2013) 30e37

3.4

a 0 months

b 2 months t test, p =0.053

Eucalyptus Melaleuca Acacia

t test, p=0.087

3.2

SUVA254 (L mg C-1 m-1)

3.0 2.8 2.6 2.4 3.4

d

c 4 months

6 months t test, p=0.026

t test, p =0.046

3.2 3.0 2.8 2.6 2.4 0

10

20

30

40

50

0

10

20

30

40

50

Hours Fig. 2. Changes in specific ultraviolet absorbance (SUVA254; mean  SE, n ¼ 3) for dissolved organic matter (DOM) leachate collected from fresh leaves (time ¼ 0 months) and litter aged on the dry floodplain for 2, 4, and 6 months and exposed to light for 0, 8, 24, and 48 h. Paired t tests were performed on SUVA254 values at time 0 and 48 h of light exposure experiments for all tree species (Eucalyptus camaldulensis subsp. refulgens, Acacia coriacea, and Melaleuca argentea) taken together.

photolysis occurs mainly at UV wavelengths in natural environments, the observed changes in DOM composition and its bioavailability may not be representative of natural photolytic processes. We found considerable bacterial utilization of leachate DOM for all tree species, consistent with previous studies that show microbial communities can readily utilize extractable DOM (Baldwin, 1999; Cleveland et al., 2004). In particular, percent BDOC for Eucalyptus leachate reported here (26e33%) is consistent with the range of Eucalyptus (20e40%) in southeastern Australia (Baldwin, 1999; O’Connell et al., 2000). These results were somewhat surprising because Eucalyptus leaves are conventionally Table 2 Mean (SE, n ¼ 3) concentration and percent of biodegradable dissolved organic carbon (BDOC) for the 28-day laboratory incubations for freshly extracted (0 h) and irradiated dissolved organic matter (DOM) (48 h). Leachates were collected from fresh leaves (time ¼ 0 months) and litter aged on the dry floodplain for 2, 4, and 6 months. Tree species were Eucalyptus camaldulensis subsp. refulgens, Acacia coriacea, and Melaleuca argentea. Tree genera

Time months

Freshly extracted DOM

Irradiated DOM

BDOC % C loss

BDOC mg C g
1

BDOC % C loss

BDOC mg C g
1

Eucalyptus Eucalyptus Eucalyptus Eucalyptus

0 2 4 6

33.3 28.7 28.3 24.1

(2.6) (4.1) (2.8) (2.4)

5.4 8.4 5.8 4.4

(1.4) (1.1) (0.7) (0.6)

34.3 26.0 34.7 37.9

(2.9) (3.1) (2.7) (1.5)

5.5 7.6 7.1 6.9

(0.6) (1.5) (0.4) (0.8)

Melaleuca Melaleuca Melaleuca Melaleuca

0 2 4 6

46.7 43.4 37.1 34.6

(2.4) (3.2) (4.6) (2.4)

3.3 5.3 4.4 3.4

(0.2) (0.6) (0.5) (0.3)

47.0 44.3 45.3 44.7

(5.3) (3.9) (4.6) (2.6)

3.3 5.4 5.4 4.2

(0.5) (0.6) (0.5) (0.4)

Acacia Acacia Acacia Acacia

0 2 4 6

40.3 38.3 29.0 22.2

(2.4) (2.9) (3.1) (2.6)

1.6 2.7 1.9 1.1

(0.3) (0.6) (0.2) (0.3)

41.7 40.8 35.1 38.9

(2.4) (2.3) (2.7) (2.6)

1.7 2.9 2.2 1.9

(0.3) (0.6) (0.2) (0.2)

considered recalcitrant due to high polyphenol and low N content (Pressland, 1982). However, polyphenolics are a diverse group of molecules and concentrations have previously been positively correlated with bacterial growth during laboratory incubations of coniferous plant litter leachate (McArthur and Richardson, 2002). Given Eucalyptus leachate contains a diverse group of polyphenolic compounds (Chapuis-Lardy et al., 2002), it is also possible that less inhibitory phenolic compounds (e.g., Gallic acid) may be responsible for supporting bacterial growth. Our finding of a significant negative relationship between SUVA254 and percent BDOC indicates that the decrease in percent BDOC with litter age was driven in part by an increase in aromatic carbon content. It is clear from these findings and others (e.g., Guillemette and del Giorgio, 2011; Volk et al., 1997) that bacterial utilization of DOM is strongly influenced by its chemical composition (e.g., aromaticity, Kalbitz et al., 2003). We therefore suggest leaf litter that accumulates on the floodplain or riparian environment may play very different roles in ecosystem carbon cycling than litter that falls directly into streams. In this regard, refractory organic compounds will be selectively preserved during litter biodegradation (Kalbitz et al., 2003) and may alter ecosystem carbon dynamics by reducing the microbial utilization of DOM leached from aged compared to freshly abscised leaves (Baldwin, 1999). Although percent BDOC decreased with increasing litter age, total BDOC (mg C g
1) was greatest for two month old litter because the greater DOC concentration in leachate from aged litter offset its lower percent BDOC. Previous research has shown that for recently abscised leaves, the leaf matrix remains relatively intact and leaching of soluble materials will be gradual but leaf degradation can cause rapid leaching of soluble materials due to a decrease in leaf structural integrity (Gessner, 1991). Consequently, extractable DOM (bioavailable versus recalcitrant) is linked to leaf litter decomposition on the dry floodplain. Overall, we suggest that

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Fig. 3. a) Mean (SE, n ¼ 3) percent of biodegradable dissolved organic carbon (BDOC) and b) concentrations of BDOC (mg C g
1) at 0 and 48 h for dissolved organic matter (DOM) leachate collected from fresh leaves (time ¼ 0 months) and litter aged on the dry floodplain for 2, 4, and 6 months and exposed to light for 48 h. Paired t tests were performed at time 0 and 48 h of light exposure experiments for all tree species (Eucalyptus camaldulensis subsp. refulgens, Acacia coriacea, and Melaleuca argentea) taken together.

timescales of bacterial DOM utilization (e.g., short versus long-term bacterial utilization; Guillemette and del Giorgio, 2011) are key to understanding the cycling of allochthonous DOM in rivere floodplain systems. During flooding, highly labile compounds, such as simple amino and organic acids, are likely to support bacterial metabolism (Volk et al., 1997). However, as rivers dry up and surface water is constrained to temporary pools, we hypothesize that increased water residence time will allow for more recalcitrant dissolved humic substances to play an increasingly important role in bacterial carbon demand as highly labile pools become exhausted. Our findings show that the utilization of leachate DOM by bacterial communities will reflect not only its origin and litter age, but also photolytic processes that alter the chemical composition and bioavailability of DOM. Leachate percent BDOC from four and six month old litter significantly increased following irradiation and provides evidence that photolysis of DOM can directly affect bacterial utilization through a shift in DOM lability (Daniel et al., 2006; Moran and Zepp, 1997). The photo-mediated increase in

35

BDOC may be attributed to the formation of oxygen-rich compounds (Kim et al., 2006), increased concentrations of amino acids and carbohydrates (Jorgensen et al., 1998), or the breakdown of high molecular weight, photoreactive compounds into lower molecular weight, more UV transparent compounds (Jorgensen et al., 1998). Alternatively, there was a negligible change in BDOC following irradiation for freshly extracted leaves, which had the greatest initial percent BDOC. These results support the hypothesis that initial DOM quality regulates the bacterial growth response following irradiation (Judd et al., 2007; Obernosterer et al., 1999; Tranvik and Bertilsson, 2001). Overall, bioavailable DOM leached from recently fallen leaves might become more refractory following exposure to solar radiation but recalcitrant DOM leached from aged litter may be rendered more bioavailable to stream bacteria following irradiation. We observed a significant decrease in SUVA254 values for DOM leachate collected from four and six month old litter and exposed to light, which is consistent with the photodegradation of highly conjugated and aromatic compounds (Helms et al., 2008; Judd et al., 2007). Previous research has shown that high molecular weight, aromatic chromophores are largely degraded during irradiation (due to bond cleavage and/or disaggregation) resulting in an increased portion of lower molecular weight compounds that contribute to light absorption (Helms et al., 2008). Similar to percent BDOC, the decrease in SUVA254 following irradiation was negligible for fresh litter suggesting the potential for photolytic shifts in DOM quality and lability in streamwater increases as litter degrades on the dry floodplain. These results also suggest that the photo-mediated increase in DOM lability was driven by a decrease in aromaticity, which supports previous studies that report a relationship between BDOC and aromaticity (Kalbitz et al., 2003). Our finding of measurable photoproduction of N from DOM shows that solar radiation may also influence the entry of nutrients into microbial food webs (Daniel et al., 2006; Wiegner and Seitzinger, 2001). Photoproduction rates of NH4 vary widely in surface water, ranging from 0 to as much as 5 mM h
1 in highly humic, bayou and river water in Louisiana (Wang et al., 2000). However, NH4 photoproduction rates are typically below 0.1 mM h
1, and our rates (0.03e0.06 mM h
1) fall within the range of reported values (reviewed by Buffam and McGlathery, 2003). Research has also shown that the photoproduction of NH4 on the southeastern continental shelf of the U.S.A. could increase available allochthonous N by an estimated 20% (Bushaw et al., 1996). Given that surface water concentrations of DIN in northwestern Australia are typically near or below detection and floodplain soils are highly weathered and contain extremely low N concentrations (McIntyre et al., 2009), the photoproduction of NH4 from refractory DON could be a key source of N to dryland rivers of the region. This may be especially important from litter aged on the floodplain because we found that extractable DIN from fresh litter was twice that of six month old litter. Based on our results on the variable effect of litter age and sunlight on DOM lability, we summarize how allochthonous DOM released from floodplain plant litter may enter the microbial food web in our study catchment. Most of the annual precipitation for the Pilbara region of northwestern Australia falls during the 3e5 month summer cyclone season, but drought of more than three years is common. Plant litter that falls on the floodplain during the dry season (when the majority of litter fall occurs) would slowly degrade, through both biotic and photochemical processes, at different rates largely determined by initial organic matter quality (Melillo et al., 1982). Thus, the supply of bioavailable DOM to stream microbial communities would be closely coupled to source material and length of degradation time before an ensuing flood. After a flood when leachate DOM is exposed to sunlight, photolytic processes may increase bacterial utilization of recalcitrant,

36

J.B. Fellman et al. / Journal of Arid Environments 89 (2013) 30e37

Fig. 4. Mean (SE, n ¼ 3) NH4eN concentration at 0 and 48 h for dissolved organic matter (DOM) leachate collected from fresh leaves (time ¼ 0 months) and litter aged on the dry floodplain for 2, 4, and 6 months and exposed to light for 48 h. Paired t tests were performed on NH4eN concentration at time 0 and 48 h of light exposure experiments for all tree species (Eucalyptus camaldulensis subsp. Refulgens, Acacia coriacea, and Melaleuca argentea) taken together.

allochthonous DOM through improved substrate quality coupled with slightly enhanced DIN availability from photoproduction of NH4. Our findings clearly show that the interaction of litter age and photolytic processes influence the availability of labile DOM to stream microbial communities following a flood. Hence, the net effect of litter age and solar radiation on leachate DOM uptake by bacterial communities is a result of counteracting processes that render DOM either more labile or recalcitrant. Furthermore, we suggest that light may play an especially important role in ecosystem carbon cycling for dryland environments that receive extensive solar radiation because of the potential influence on not just allochthonous DOM lability in streams, but also on plant growth (Pancotto et al., 2005) and subsequent litter decomposition in the terrestrial environment (Austin and Vivanco, 2006). 4.1. Ecosystem implications The large concentrations of extractable DOM and nutrients reported here have implications for biogeochemical processes in floodplain and riparian environments. However, it is important to recognize that these concentrations alone do not accurately predict how C, N, and P inputs from leaf litter affect stream productivity. For instance, natural variation in leaf nutrient content (Gosz et al., 1972) and nutrient resorption by plants can result in variable nutrient litter fall (Chapin, 1980; Gosz et al., 1972). Research has shown that up to 90% of total leaf N and P can be translocated out of leaves before natural abscission (Chapin, 1980), especially in nutrient poor ecosystems. This suggests that our concentrations of extractable DOM and nutrients are likely higher than those observed from naturally senesced leaves (Cleveland et al., 2004). In this context, elevated leaf nutrient content could also affect leachate BDOC and our estimate of bioavailable C flux to Marillana Creek given the strong control N exerts on DOM degradation. The relative proportion of leaf fall that enters the stream via both lateral and direct inputs as well as the seasonal timing of leaf

fall may also influence DOM and nutrient delivery to streams (Fisher and Likens, 1973). E. camaldulensis subsp. refulgens and A. coriacea are widely distributed along waterways across northwest Australia. However, M. argentea is constrained to areas where permanent water is 1e2 m within the soil surface, such as around permanent pools where it is often the dominant tree. Our finding that percent BDOC was greatest for Melaleuca leachate suggests that tree species may also influence stream ecosystem dynamics through the input of bioavailable DOM to permanent pools. Overall, the uptake of allochthonous DOM and nutrients by stream microbial communities will be determined by many factors including: initial litter nutrient content, litter age, photochemical processes, quality of leachate DOM, and stream nutrient status. Recognition of solar radiation as an important control on organic matter degradation is well documented in both terrestrial (Austin and Vivanco, 2006) and aquatic ecosystems (Moran and Covert, 2003). Although the importance of terrestrialeaquatic linkages to stream ecosystems has been well known for several decades (Likens and Bormann, 1974; Wallace et al., 1997), most organic matter photodegradation studies focus exclusively on either environment and do not adequately consider how sunlight modifies linkages between terrestrial and aquatic ecosystems (e.g., Pancotto et al., 2005). Our findings demonstrate that the photomediated shift in DOM composition and its bioavailability in streams may moderate the degradation of plant litter in the terrestrial environment. In this way, sunlight may influence terrestrialeaquatic linkages by enhancing the entry of recalcitrant, allochthonous DOM into microbial food webs. Acknowledgments The authors thank Anna Byrne, Kate Bowler, Sally Madden and Gerald Page for assistance in the field and laboratory, and two anonymous reviewers for helpful comments on an earlier version of

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