TY - JOUR
AU - Oelbermann, Maren
AB - In forest ecosystems, litterfall collected in trapping devices is exposed to periods of wetting and drying, which may initiate the first stages of decomposition. This could lead to an underestimation of organic matter and nutrient input due to leaching or an overestimation due to immobilization. The objectives of this study were to quantify changes in mass and nutrient stocks of sugar maple (Acer saccharum Marsh.), basswood (Tilia americana L.), and beech (Fagus grandifolia Ehrh.) leaves under in situ conditions and to quantify changes in leaf mass and nutrient stocks and leachate concentration when exposed to different quantities of moisture (high = 100 mm, medium = 60 mm, and low = 30 mm) under ex situ conditions. Results from this study showed that sugar maple and basswood had a significantly greater (P < 0.05) mass loss than beech in the in situ and ex situ study. Nutrient stocks either decreased significantly (P < 0.05) or remained the same, depending on species in the in situ study. Similar results were observed in the ex situ study, in which carbon, nitrogen, phosphorus, and potassium stocks decreased significantly (P < 0.05) with increasing exposure to moisture, but calcium and magnesium stocks showed less pronounced changes. Mean concentrations of dissolved organic carbon, dissolved organic nitrogen, and ammonium were significantly different (P < 0.05) between species and moisture treatments, whereas nitrite showed no such differences. Results from this study suggested that the collection of leaf litter should take place frequently during the peak leaf abscission period and during periods of high precipitation. This would provide a more accurate quantification of the quantity of nutrients entering the forest ecosystem in the within-system pathway between live vegetation and the forest floor detritus pool. In addition, more frequent litterfall collection may also minimize litter decomposition and nitrification. deciduous forest zone, dissolved inorganic nitrogen, dissolved organic carbon, dissolved organic nitrogen, leaching, littertraps Major sources of nutrient input in forest ecosystems are derived from throughfall, stemflow, and litterfall (Pedersen and Bille-Hansen 1999). Although senesced leaf litter is nutrient-poor because of nutrient retranslocation and leaching (Hagen-Thorn et al. 2006), litterfall contributes the greatest input of nutrients (Ukonmaanaho and Starr 2001) to within-system pathways between live vegetation and the forest floor detritus pool. For example, Salazar et al. (2011) found that in deciduous tree species, up to 50% of the nitrogen (N) and 1% of phosphorus (P) can be retranslocated from leaves and that the process of retranslocation of P may begin as early as June in the north temperate zone (Staaf and Stjernquist 1986). Nutrient input from litterfall in forest ecosystems is evaluated through biweekly or monthly collection of leaves and small branches (<2 mm diameter) by open traps installed underneath the forest canopy (Corrigan and Oelbermann 2010). However, between sampling periods, litterfall may be exposed to precipitation, leading to the wetting and drying of leaves, which thus may initiate the first stages of decomposition (Ukonmaanaho and Starr 2001, Corrigan and Oelbermann 2010). During the early stages of leaf litter decomposition, the majority of mass and nutrients may be lost due to leaching from rainfall, dew, mist, and/or fog (Prescott 2005, Tietema and Wessel 1994). After leaching, there may be an absolute increase in mass due to immobilization and subsequent mass loss due to mineralization (Berg and Eckbohm 1983, Upadhyay and Singh 1989). Gessner and Konstanz (1989) found that up to 33% of leaf mass may be lost due to leaching in addition to the loss of nutrients such as P and potassium (K). For example, Taylor and Parkinson (1988) determined that aspen (Populus tremuloides Michx.) leaves lost 14% of their mass during the early phase of decomposition as a result of leaching. Salamanca et al. (2003) found that precipitation leached labile compounds from leaf litter and noted that this occurs because freshly abscised leaves have a higher concentration of inorganic nutrients in their intercellular spaces. Although nutrients lost from leaching and decomposition of senescent leaves are returned to the forest soil, in litterfall studies this may not provide an accurate quantification, leading to an underestimation of the actual quantity of carbon (C) and nutrients cycling within forest ecosystem pathways due to mineralization (Ukonmaanaho and Starr 2001, Corrigan and Oelbermann 2010) or an overestimation due to immobilization. The objectives of this study were to quantify changes in leaf mass (%) and nutrient stocks (g m−2) of sugar maple (Acer saccharum Marsh.), American basswood (Tilia americana L.), and American beech (Fagus grandifolia Ehrh.) leaves under in situ conditions. We also determined changes in mass and nutrient stocks of sugar maple, basswood, and beech leaves when exposed to different quantities of moisture under ex situ conditions. In addition, we quantified changes in leachate concentration (mg l−1) as a result of exposing leaves to different quantities of moisture (ex situ). It was hypothesized that leaves exposed to higher moisture levels would have a greater mass and nutrient loss, which would correspond to greater leachate concentrations, and that this would differ among species. Results from this study contribute to the current gap in the literature on the potential role of precipitation in leaching nutrients during the initial phase of decomposition in litterfall studies. This study also contributes to the future design of litterfall and decomposition studies and will help in the calibration of empirical decomposition models. Materials and Methods Study Site The in situ study was conducted at the Laurel Creek Nature Centre (LCNC) (43°27′ N, 80°22′ W). Materials for the ex situ study were also collected at the LCNC. The LCNC is located in the peninsular region of southern Ontario, Canada, which has a climate modified by the Great Lakes. The climate is temperate with hot and humid summers and cold winters and a mean annual frost-free period of 134 days, a mean annual precipitation of 820 mm, and a mean annual temperature of 7.2° C (Environment Canada 2006). The LCNC is located 317 m above sea level. The predominant vegetation includes sugar maple (Acer saccharum Marsh.), bitternut hickory (Carya cordiformis [Wangenh.] K. Koch), American basswood (Tilia americana L.), American beech (Fagus grandifolia Ehrh.), and eastern white cedar (Thuja occidentalis L.) interspersed with trembling aspen (Populus tremuloides Michx.), white elm (Ulmus americana L.), red osier dogwood (Cornus stolonifera Michx.), and Canada yew (Taxus canadensis Marsh.) (Oelbermann and Gordon 2000). The soil is a gray-brown Luvisol, which developed on a well-drained fine sand, loam, and silt loam forming part of the Waterloo Moraine. The parent material of the soil is lacustrine fine and very fine sand. The A-horizon consists of a fine sandy loam with a pH of 6.9 (Tupman et al. 2004). This study site was chosen for the in situ and ex situ experiments because the LCNC represented typical forest vegetation in this region of southern Ontario and is similar to forest ecosystems extending to the eastern United States and north to the Great Lakes-St. Lawrence forest region (Watkins 2006). The specific tree species used in the in situ and ex situ experiments included sugar maple, American basswood, and American beech. These species were chosen because they are the most representative within this forest region and have also been the featured species in numerous litterfall studies. Experimental Design and Analysis In Situ Study A total of nine study plots (28 m × 30 m) were randomly selected in the LCNC. Within each study plot, a 10 × 10 m net (0.01 × 0.01 m mesh size, Vantage Utility netting, VN1250; Vantage Ltd., Mississauga, ON, Canada) was suspended above the ground for leaf collection. Freshly fallen, undamaged, and naturally abscised leaves from sugar maple, basswood, and beech were collected over a 2-day period in mid-October 2007 (peak of autumnal litterfall) for the in situ and ex situ experiments. Leaves were collected daily to ensure that no exposure to precipitation occurred. The collected leaves for the ex situ experiment were stored in a cooler and transported immediately to the University of Waterloo, Waterloo, ON, Canada. A portion (500 g fresh weight) of the collected litter was dried at 65° C for 48 hours, weighed, and analyzed for nutrient concentration (C, N, P, K, calcium [Ca], and magnesium [Mg]) for baseline (control) comparisons. For the in situ experiment, three litter decomposition bags per tree species (n = 3) were randomly placed under the forest canopy. The litterbags were constructed of netting with a 0.01 × 0.01 m mesh size. Each litterbag had a 0.45 m2 area and was suspended 0.4 m above the forest floor. A total of 200 g (fresh weight) of leaves for each sugar maple, basswood, and beech was placed in the litterbags. Each litterbag was covered with netting to minimize leaf loss or addition during the study period. Leaves were left in situ during the last 14 days of October, which is representative of the maximum litterfall period in this region of southern Ontario. During this time, the total amount of precipitation was 23.2 mm, with a mean daily maximum temperature of 16.1° C and a mean daily minimum temperature of 6.9° C. After 14 days, leaves were collected, weighed for fresh weight, dried at 65° C for 48 hours, and weighed for dry weight. Dried leaves for the baseline (control) comparisons and those from the 14-day experiment were ground in a Kinematica Polymix plant grinder (Px-MFC 90D; Kinematica, Lucerne, Switzerland) followed by grinding in a Retsch ball mill (model MM200; Retsch, Haan, Germany). The ground samples were analyzed for C and total N using a Costech 4010 Elemental Analyzer (Costech, Cernusco, Italy); P was analyzed using a Technicon Autoanalyzer II (Technicon Industrial Systems, Tarrytown, NY), and K, Ca, and Mg were analyzed using a Varian AA-20 atomic absorption spectrophotometer (Varian, Santa Clara, CA). The percent change in dry weight was determined as the percent difference between the baseline dry weight (defined as the dry weight of freshly abscised leaf litter) and the dry weight of leaf litter after the 14-day litterbag experiment. Ex Situ Study In the laboratory, Vantage utility netting was secured on top of a plastic tank (0.32 × 0.27 × 0.12 m). For each species and treatment, there was a replicate of three plastic tanks (n = 3). A total of 20 g of fresh leaves per treatment and per species was placed on top of the utility netting. The treatment consisted of leaves exposed to high (HI = 100 mm), medium (MED = 60 mm), low (LOW = 30 mm), and no (CTRL = 0 mm) levels of moisture over a 14-day experimental period. Water was applied using a small watering can. The 14-day experimental period represented the minimum time that leaf litter typically accumulates in litter traps before its collection. The amount of moisture applied to the LOW treatment corresponded to that in the in situ study. The ambient temperature (21° C) and relative humidity (65%) were kept constant throughout the experiment. On days 1, 10, and 14, leachate was collected 2 hours after application of HI, MED, and LOW treatments. This time was sufficient for the water to percolate through the leaves and collect in the plastic tanks. Visual observations showed that the leaf surface, between treatment applications, showed little water retention, which represented conditions similar to those in the field. The collected leachate was filtered immediately using a 1.5-μm pore size Whatman glass microfiber filter (934-AH) and was stored in the dark at 4° C until further analysis. All leachate was analyzed immediately after the last day of its collection (day 14 of the experimental period). On day 14, leaves from each treatment were prepared and analyzed for nutrients as described for the in situ study. The percent change in dry weight was determined as the percent difference between the baseline dry weight (defined as the dry weight of freshly abscised leaf litter) and the dry weight of leaf litter after the 14-day litter trap experiment. Leachate and water were analyzed for dissolved organic C (DOC) using a Dohrmann Total Carbon Analyzer (DC190). Total dissolved N (TDN) and dissolved inorganic N (DIN), consisting of ammonium (NH4+) and nitrate (NO3−), were analyzed using a Technicon Autoanalyzer II. Dissolved organic N (DON) was calculated by subtracting NH4+-N and NO3−-N from TDN (Campbell et al. 2000). All experimental values for the ex situ experiment were corrected by subtracting the concentration of the experimental leachate from that of the water used in the treatment applications. The quantity of water applied, representing HI, MED, and LOW treatments, was determined from actual precipitation data collected at this site over a 24-year period, representing the peak litterfall period in this region of southern Ontario. Precipitation data for the month of October, from 1981 to 2005 (Environment Canada 2006) was divided into three categories (HI, MED, and LOW). The mean value within each category was used in the following equation to quantify the daily volume of water application for each treatment:   where DAQW is the daily application quantity of water (ml) assuming an equal amount of precipitation per day, DT is the dimension of the tank (m2), and TQ is the treatment quantity (mm) of moisture (HI, MED, and LOW). Ambient temperature (21° C) and relative humidity (65%) were kept constant throughout the experiment. Water used to simulate the different quantities of moisture was collected from natural rainfall events by placing a large circular-shaped container (1.5 m diameter and 0.5 m height) in an open area to avoid nutrient enrichment from throughfall and stemflow. The rainfall collector was thoroughly cleaned before and after each collection. The collected rainwater was frozen immediately, thawed in the refrigerator, and kept at 4° C before and during its use in the individual treatment applications. Statistical Analysis All data were examined for homogeneity of variance using Levene's test and found to have a normal distribution. Our dependent variables (weight, nutrient stocks, and leachate concentrations) were assessed by the Shapiro-Wilk test and found to have a normal distribution. To quantify differences within and between species nutrient concentrations and between treatments for both in situ and ex situ experiments, data were analyzed using the general linear model (analysis of variance [ANOVA]) in SPSS (2009; SPSS Science, Inc.). Significantly different main effects were further tested using Tukey's multiple comparison test (Steel et al. 1997). Significant simple effects were tested using the estimated marginal means function in SPSS. Differences between control (before leaching) and after leaching (treatment) in the in situ study were determined using a t test (Steel et al. 1997). Repeated measures of ANOVA in SPSS were used to compare each of the measured parameters of the leachate (DOC, DON, NH4+, and NO3−) between treatments over the study period. Sampling time (days 1, 10, and 14) was the repeated factor (within subjects), and leachate (DOC, DON, NH4+, and NO3−) was the main factor (between subjects) (Steel et al. 1997). Before the statistical analysis, data for the leachate was corrected with respect to nutrient values obtained from the rainwater. For all statistical analyses, the threshold probability level for determining significant differences was P < 0.05. Results In Situ and Ex Situ Leaf Mass Change Changes in leaf mass (actual percent weight) were significantly different between species in the in situ study (Table 1). The greatest loss in mass was observed in sugar maple followed by basswood and beech. Interaction effects of the actual percent weight change with respect to species-by-moisture treatment was significant in the ex situ experiment [F(6, 24) = 25.237, P = 0.001]. Simple effects showed that weight loss was significantly greater in the HI treatment within species. Main effects showed that a significantly greater mass loss occurred in the HI and MED treatments for sugar maple followed by basswood and beach (Table 1). When HI, MED, and LOW treatments were averaged, the greatest mass loss occurred in sugar maple (8.9%) followed by basswood (8.1%) and beech (1.9%). Table 1. Actual percent weight change for sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaves after 14 days in an in situ experiment (n = 3) in southern Ontario, Canada (n = 3) and changes after exposure to different moisture levels for 14 days, using the same leaf species in an ex situ experiment (n = 3). Values followed by the same upper case letters, comparing differences between treatments HI, MED, and LOW within leaf species are not significantly different at P < 0.05 for the ex situ experiment. Values followed by the same lower case letters, comparing differences between leaf species for in situ and ex situ experiments, are not significantly different at P < 0.05. View Large Table 1. Actual percent weight change for sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaves after 14 days in an in situ experiment (n = 3) in southern Ontario, Canada (n = 3) and changes after exposure to different moisture levels for 14 days, using the same leaf species in an ex situ experiment (n = 3). Values followed by the same upper case letters, comparing differences between treatments HI, MED, and LOW within leaf species are not significantly different at P < 0.05 for the ex situ experiment. Values followed by the same lower case letters, comparing differences between leaf species for in situ and ex situ experiments, are not significantly different at P < 0.05. View Large In Situ and Ex Situ Leaf Carbon and Nutrient Stocks Interaction effects of nutrient stocks (g m−2) with respect to species-by-moisture treatment were not significant in the in situ study. Main effects showed significant differences in leaf C, N, Ca, and Mg stocks between species before leaching (control) and showed significant decreases in leaf C, N, K, Ca, and Mg stocks after leaching (treatment) (Table 2). Within species, sugar maple had significantly lower C, N, K, and Ca stocks after leaching (treatment) compared with the control (Table 2). Basswood had significantly lower C, K, Ca, and Mg stocks and beech had significantly lower N, Ca, and Mg stocks after leaching than the control. Table 2. Changes in sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaf carbon and nutrient stocks (g m−2) before leaching (control) and after leaching (treatment) over 14 days in an in situ experiment in southern Ontario, Canada. Standard errors are given in parentheses (n = 3). SEs are given in parentheses (n = 3). Values followed by the same upper case letters, comparing differences between leaf species within the control or treatment for each nutrient, are not significantly different at P < 0.05. Values followed by the same lower case letters, comparing differences between the control (before leaching) and the treatment (after leaching) within leaf species, are not significantly different at P < 0.05. View Large Table 2. Changes in sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaf carbon and nutrient stocks (g m−2) before leaching (control) and after leaching (treatment) over 14 days in an in situ experiment in southern Ontario, Canada. Standard errors are given in parentheses (n = 3). SEs are given in parentheses (n = 3). Values followed by the same upper case letters, comparing differences between leaf species within the control or treatment for each nutrient, are not significantly different at P < 0.05. Values followed by the same lower case letters, comparing differences between the control (before leaching) and the treatment (after leaching) within leaf species, are not significantly different at P < 0.05. View Large Interaction effects of nutrient stocks (g m−2) with respect to species-by-moisture treatment were significant only for C [F(6, 24) = 75.023, P = 0.001] and N [F(6, 24) = 84.312, P = 0.001] in the ex situ experiment. Simple effects showed that C and N stocks were significantly lower in the HI treatment than in the control between species. Main effects showed significant differences in leaf C and nutrient stocks (N, Ca, and Mg) before leaching (CTRL) and in LOW, MED, and HI treatments between species (Table 3). When the influence of moisture level on leaves was compared, results showed that the greatest loss of C and nutrient stocks occurred in the HI treatment followed by the MED and LOW treatments (Table 3). For example, C, N, and Mg stocks for sugar maple followed a sequence of CTRL > LOW > MED > HI, whereas Ca stocks followed a sequence of CTRL ≈ LOW ≈ MED > HI. For basswood, C and N stocks followed a sequence of CTRL > LOW > MED > HI, whereas the sequence for Ca was CRTL ≈ LOW ≈ MED > HI and that for Mg was CTRL > LOW ≈ MED > HI. Beech followed a sequence of CTRL > LOW > MED > LOW for C and N stocks and CTRL ≈ LOW > MED > HI for Mg stocks. Comparisons between species for P and K nutrient stocks and within sugar maple and beech were not possible because concentrations were below the detection limit of the analytical equipment. Table 3. Changes in sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaf C and nutrient (N, P, K, Ca, and Mg) stocks when exposed to different moisture levels compared with a control in an ex situ experiment over 14 days. SEs are given in parentheses (n = 3). Values followed by the same upper case letters, comparing differences between leaf species, within CRTL, HI, MED, and LOW treatments, are not significantly different at P < 0.05. Values followed by the same lower case letter comparing differences between CRTL, HI, MED, and LOW, within each leaf species for each are not significantly different at P < 0.05. View Large Table 3. Changes in sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaf C and nutrient (N, P, K, Ca, and Mg) stocks when exposed to different moisture levels compared with a control in an ex situ experiment over 14 days. SEs are given in parentheses (n = 3). Values followed by the same upper case letters, comparing differences between leaf species, within CRTL, HI, MED, and LOW treatments, are not significantly different at P < 0.05. Values followed by the same lower case letter comparing differences between CRTL, HI, MED, and LOW, within each leaf species for each are not significantly different at P < 0.05. View Large Ex Situ Leaf Leachate Concentration Interaction effects of leachate concentrations (mg l−1) with respect to species-by-moisture treatment were significant only for DOC [F(4, 27) = 3.071, P = 0.033]. Simple effects showed that the DOC concentration was significantly greater in the HI and MED treatments for sugar maple and basswood. Main effects showed significantly greater DON concentrations in the MED treatment for sugar maple and beech (Table 4). In a comparison of differences within species, basswood had a significantly greater DON concentration in the HI treatment followed by the MED and LOW treatments. When NH4+ was compared between species, beech and basswood had a significantly greater concentration in the HI treatment, whereas sugar maple had a significantly lower NH4+ concentration in the HI treatment. No significant differences were observed for NO3− concentration between and within species. Table 4. Leachate concentrations of DOC, DON, NH4+, and NO3− expressed as a mean value of three different sampling points (days 1, 10, and 14) over the entire 14-day ex situ experimental period for sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaves exposed to different levels of moisture. SEs are given in parentheses (n = 3). Values followed by the same upper case letters, comparing differences between leaf species, within HI, MED, and LOW treatments, are not significantly different at P < 0.05. Values followed by the same lower case letter comparing differences between HI, MED, and LOW treatments, within each leaf species for each leachate (DOC, DON, NH4+, and NO3−) are not significantly different at P < 0.05. View Large Table 4. Leachate concentrations of DOC, DON, NH4+, and NO3− expressed as a mean value of three different sampling points (days 1, 10, and 14) over the entire 14-day ex situ experimental period for sugar maple (A. saccharum), basswood (T. americana), and beech (F. grandifolia) leaves exposed to different levels of moisture. SEs are given in parentheses (n = 3). Values followed by the same upper case letters, comparing differences between leaf species, within HI, MED, and LOW treatments, are not significantly different at P < 0.05. Values followed by the same lower case letter comparing differences between HI, MED, and LOW treatments, within each leaf species for each leachate (DOC, DON, NH4+, and NO3−) are not significantly different at P < 0.05. View Large Changes in leachate concentrations with time revealed that DOC concentrations were significantly greater on day 1 for sugar maple in all treatments than on days 10 and 14, but basswood and beech showed this tendency only in the HI and MED treatments (Figure 1). However, DON concentrations were significantly greater on day 14 in sugar maple (HI and MED treatments), but the LOW treatment was significantly greater on days 10 and 14 compared (Figure 2). DON concentrations were significantly different only in the MED treatment for basswood, whereas that for beech was significantly greater on day 14 (HI) and on days 10 and 14 in the LOW treatment. Sugar maple and beech leaves presented a similar pattern for NH4+ concentration, with a significantly greater concentration on day 10, whereas basswood showed a significantly greater concentration on day 14 (Figure 3). However, NO3− concentrations in sugar maple and basswood were significantly greater on day 1 compared with those on days 10 and 14 in all treatments, whereas that for beech was significantly lower on day 1 (Figure 4). Figure 1. View largeDownload slide Concentration of DOC (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Figure 1. View largeDownload slide Concentration of DOC (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Figure 2. View largeDownload slide Concentration of DON (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Figure 2. View largeDownload slide Concentration of DON (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Figure 3. View largeDownload slide Concentration of NH4+ (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Figure 3. View largeDownload slide Concentration of NH4+ (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Figure 4. View largeDownload slide Concentration of NO3− (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Figure 4. View largeDownload slide Concentration of NO3− (mg l−1) in leachate from sugar maple (A. saccharum) (a), basswood (T. americana) (b), and beech (F. grandifolia) (c) leaves when exposed to HI, MED, and LOW levels of moisture over 14 days. Letters above each bar denote significant differences (P < 0.05), comparing differences between sampling dates and within each moisture level (n = 3). Discussion In Situ and Ex Situ Leaf Mass Change It was expected that exposure to moisture in the in situ experiment would result in a decline in leaf mass and that higher levels of moisture (HI) (ex situ experiment) would lead to the greatest mass loss regardless of species. The findings from the in situ experiment were similar to those of Parsons et al. (1990), Huang and Schoenau (1997), and Lensing and Wise (2006). Consistent with the ex situ experiment, Knutson (1997) found increased leaf mass loss in an Acer-Tilia-Quercus-dominated deciduous forest when exposed to high levels of moisture. Wieder et al. (2009) and Ventura et al. (2010) suggested that a higher mass loss with increasing exposure to moisture may be related to enhanced microbial activity on the leaf surface, initial litter solubility, and/or the loss of particulate organic matter. The dissimilar amount of mass loss between species (in situ and ex situ studies) suggested that species may respond differently to moisture (Pereira et al. 1998). This may be due to variations in leaf chemical concentrations and physical quality (Wieder et al. 2009), such as leaf toughness and cuticle thickness (Gallardo and Merino 1993), and the presence of cuticle and epicuticular waxes that act as a barrier to water-soluble compounds (Gessner and Konstanz 1989). Melillo et al. (1982) and Joergensen and Meyer (1990) suggested that variation in leaf chemical and physical characteristics may affect overall mass loss during the initial stages of decomposition. For example, the lower mass loss of beech leaves in our study may be due to their waxy and thick cuticle, in addition to a higher lignin/N ratio, rendering them more resistant to decomposition (Pereira et al. 1998, Madritch and Cardinale 2007, Page and Mitchell 2008). Jacob et al. (2009) also observed a lower concentration of N, Ca, and Mg and a lower density of microorganisms on the surface of beech leaves. They suggested that these results may have led to a slower rate of decomposition and nutrient mineralization (Jacob et al. 2009). In Situ and Ex Situ Leaf Carbon and Nutrient Stocks In situ and ex situ leaf nutrient concentrations before moisture exposure (control) showed substantial between-species variation in litter chemistry, which was also observed by Osono and Takeda (2004) and Wieder et al. (2009). As a result of exposure to moisture in the in situ and ex situ studies, nutrient stocks decreased significantly but varied among species. This occurred because moisture is considered to be the major factor in controlling litter decomposition (Ibrahim et al. 2010), but differences in leaf chemical and physical characteristics may affect the amount of nutrient loss between species (Page and Mitchell 2008). Variation in nutrient stocks between the in situ and ex situ studies were probably due to differences in environmental conditions between the field and laboratory. Exposure to moisture, including that of the LOW treatment, was greater in the ex situ study compared with that in the field study. Other factors such as temperature, which was kept constant at 21° C in the ex situ environment compared with a mean temperature of 16.1° C in the in situ study, was probably more favorable for microbial activity, leading to a greater loss in mass and nutrients (Qui et al. 2005). Under ex situ conditions, significant losses in C and N could already be observed at LOW levels of moisture in all species. This result suggested that a small increase in moisture availability provided a favorable environment for microbial activity when a sufficient amount of N was available to mineralize leaf C (Berg and Eckbohm 1983). Manzoni et al. (2010) also suggested that with time, a reduction in decomposer C-use efficiency may occur, leading to the leaching of nutrients when exposed to high levels of moisture. Losses of P and K, especially in sugar maple and beech, were triggered with exposure to minimal amounts of moisture. This may be due to initially low concentrations of P in leaves (Moore et al. 2010), because most of the P is retranslocated before leaf abscission (Duchesne et al. 2001). However, Rutigliano et al. (1998) and Ventura et al. (2010) found that P that is not retranslocated may be rapidly consumed by microbes. Ukonmaanaho and Starr (2001) observed that K leached readily from litterfall, and Rutigliano et al. (1998) found that beech leaves are especially prone to K leaching. This leaching occurs because K is highly mobile and not structurally bonded (Mahmood et al. 2009). For example, in a laboratory study, Moore (1996) observed that the proportion of leaf K lost increased when leaves were washed in deionized water. The relatively small changes (sugar maple and basswood) and lack of change (beech) in Ca stocks suggested that this nutrient is not readily leached (Osono and Takeda 2004). A possible reason is that Ca is part of the structural plant tissue and is relatively immobile during the first phase of decomposition (Jacob et al. 2009). Ventura et al. (2010) found that Ca was released more gradually and in smaller quantities than N, P, and K, and the majority of this nutrient was lost during the second phase of decomposition (Rees et al. 2006), which, according to Osono and Takeda (2004), occurs after 5 months. Similar to the results of our study, Osono and Takeda (2004) observed that the Mg stock in Acer rufinerve Siebold & Zucc. and Fagus crenata Blume leaves decreased during the initial phase of decomposition in a temperate Japanese forest, possibly because Mg occurs in plant cells in solution rather than as part of the structural plant tissue, allowing it to be readily leached during the initial phase of decomposition (Osono and Takeda 2004). In Situ Leachate Concentration Water-soluble compounds are leached from leaves within hours to days after exposure to water (Wallace et al. 2008); this may contribute up to 30% of the total C loss from leaf litter (Magill and Aber 2000). Increasing exposure to moisture resulted in greater DOC and DON concentrations in the leachate in our study, which was also reported by Michalizik et al. (2001), Schmidt et al. (2010), Artigas et al. (2011), and Kammer and Hagedorn (2011). The different species exhibited contrasting patterns of DOC release, and the lower DOC concentration from beech leaf leachate was similar to that reported by Hagedorn and Machwitz (2007). Wieder et al. (2009) suggested that variation in DOC concentrations between species may be due to differences in leaf chemical and physical qualities, in addition to differences in the initial carbon chemistry (Silveira et al. 2011) or due to variation in the degree of water repellency of the leaf cuticle (Czech and Kappen 1997, Wallace et al. 2008). Wallace et al. (2008) found that leaves with a high water repellency produced lower concentrations of leachate. This finding suggested that DOC may be released through complex interactions between microbial production and consumption and exposure to moisture, which could result in various amounts of DOC leached from different species (Park et al. 2002). Temporal trends of changes in DOC concentration were similar to that reported by Hansson et al. (2010) and Magill and Aber (2000). For example, Magill and Aber (2000) found a high DOC concentration from sugar maple and red maple (Acer rubrum L.) within the 1st week of leaching followed by a rapid decrease. Similarly, Hansson et al. (2010) observed an initially high DOC concentration from Norway spruce (Picea abies L. [Karst.]) needles followed by a quick decrease within the first 20 days of their 125-day study. However, Magill and Aber (2000) noted that temporal changes in DOC concentration varied between species and, similar to our study, found the least change in beech leaves. They also noted that the overall amount of DOC from beech leaves was lower compared with that from maple leaves (Magill and Aber 2000). Kammer and Hagedorn. (2011) suggested that differences in DOC concentrations between litter types may be due to different microbial communities present on the leaves, which could be related to differences in initial litter chemistry (Artigas et al. 2011) and changes in substrate quality as decomposition proceeds (Hansson et al. 2010). Hansson et al. (2010) found that the degree of decomposition of the substrate is important in controlling DOC production, and lower quantities of DOC were released from more decomposed materials. Such differences may be related to the different phases of litter decomposition, for which losses of nutrients are more rapid in the first phase of decomposition compared with that in later phases (Kalbitz et al. 2007). Results from this study suggested that leaves are an important source of DOC and that they are readily metabolized by microbes while collecting in litter traps, even when exposed to low amounts of moisture. Only a few studies have evaluated DON and DIN from leached leaves. Aerts and de Caluwe (1997) found that up to 20% of the initial N was removed from Carex sp. when immersed in distilled water for 96 hours. Thus, water may be the major controlling agent of leaf DON leaching in temperate forest ecosystems (Schmidt et al. 2010). However, the concentration of DON varied among species, and its concentrations were lower compared with that of DIN (Magill and Aber 2000), which was similar to the result in the current study. Temporal trends in DON concentrations were similar to that reported by Wallace et al. (2008) who observed that the majority of DON was leached within the first 6 days of exposure to moisture. Similar to the current study, Hansson et al. (2010) did not detect a clear pattern of increasing or decreasing DIN concentrations during the first 14 days of leaching. However, after 6 weeks of leaching Hansson et al. (2010) observed a decrease in NH4+ and an increase in NO3−. On the basis of our results, this finding suggested that collecting leaf litter from litter traps on a 2-week rotation probably does not initiate the process of nitrification. Conclusions The present research showed that exposing leaves collecting in littertraps to low levels of moisture can induce initial stages of decomposition. Differences in environmental conditions, including greater exposure to moisture and temperature, may lead to enhanced leaf microbial activity and an increased loss of mass and nutrients under laboratory conditions (ex situ study). Nonstructurally bonded nutrients had the greatest loss with increasing moisture levels, whereas structurally bonded nutrients such as Ca remained more stable with increasing exposure to moisture. Our results also showed that the amount of mass and nutrient loss varied among species, which may be due to differences in initial leaf chemistry in addition to variation in physical characteristics including leaf thickness and waxiness. Such variation between species in chemical and physical characteristics also influenced the concentration of leachate (DOC, DON, and DIN) when exposed to different levels of moisture. Temporal trends in DOC concentration showed a decrease in this leachate over the 14-day experimental period for all species and corresponded to a pattern of an increasing DON concentration with time. However, DIN did not show a clear pattern, suggesting that nitrification may not have taken place within the 14-day experimental period. The loss in leaf mass and nutrient stocks at a low level of moisture suggested that the collection of deciduous litterfall should take place more frequently during the peak of leaf abscission. To accurately quantify nutrient inputs via litterfall in the within-system pathway between live vegetation and the forest floor detritus pool, more frequent litter collection should also be considered during periods of high precipitation. Future researchers should consider the interaction of various litter components and conduct leaching studies using a mixture of leaves from a variety of species. Because the leaching of DOC occurs almost immediately after the first wetting of leaf tissue, it is imperative to collect sufficient volumes of leachate to account for temporal changes, especially in studies that take place over a period longer than 14 days. Long-term studies will also provide further insight into the temporal pattern of DIN and may provide an opportunity to identify at what period nitrification processes commence. Acknowledgments: We thank the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation, the Ontario Research Fund, and the University of Waterloo for providing financial assistance and research infrastructure to carry out this work. 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TI - Mass and Nutrient Loss of Leaf Litter Collecting in Littertraps: An In Situ and Ex Situ Study
JF - Forest Science
DO - 10.5849/forsci.12-016
DA - 2013-08-01
UR - https://www.deepdyve.com/lp/springer-journals/mass-and-nutrient-loss-of-leaf-litter-collecting-in-littertraps-an-in-s0X0qFeQDa
SP - 484
EP - 493
VL - 59
IS - 4
DP - DeepDyve
ER -