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TECHNICAL REPORTS

Landscape and Watershed Processes

Inorganic Nitrogen and Phosphorus in Sierran Forest O Horizon Leachate

^a Hydrologic Sciences Program, College of Agriculture, Biotechnology, and Natural Resources, Univ. of Nevada, Reno, NV 89512
^b Dep. of Natural Resources and Environmental Science, College of Agriculture, Biotechnology, and Natural Resources, Univ. of Nevada, Reno, NV 89512
^c Dep. of Natural Resources and Environmental Science, College of Agriculture, Biotechnology, and Natural Resources, Univ. of Nevada, Reno, NV 89512

^* Corresponding author (wilymalr{at}cabnr.unr.edu)

Received for publication December 19, 2005.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
High in situ concentrations of inorganic N and P have been reported in overland/litter interflow from Sierran forests, indicating that these nutrients are derived from the forest floor O horizons. To test this hypothesis, forest floor monoliths consisting of the combined O_e and O_i horizons were collected near the South Shore of Lake Tahoe, Nevada, for leaching experiments. Three monoliths were left intact, and three were hand-separated according to horizon for a total of three treatments (combined O_e+O_i, O_e only, and O_i only) by three replications. Samples were randomized and placed into lined leaching bins. Initial leaching consisted of misting to simulate typical early fall precipitation. This was followed by daily snow applications and a final misting to simulate spring precipitation. Leachate was collected, analyzed for NH₄⁺–N, NO₃^––N, and PO₄^3––P, and a nutrient balance was computed. There was a net retention of NH₄⁺–N, but a net release of both NO₃^––N and PO₄^3––P, and a net release of inorganic N and P overall. Total contributions (mg) of N and P were highest from the O_e and O_e+O_i combined treatments, but when expressed as per unit mass, significantly (p < 0.05) higher amounts of NO₃^––N and PO₄^3––P were derived from the O_i materials. The nutrients in forest floor leachate are a potential source of biologically available N and P to adjacent surface waters. Transport of these nutrients from the terrestrial to the aquatic system in the Lake Tahoe basin may therefore play a part in the already deteriorating clarity of the lake.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
DUE to the exclusion of fire and forest harvest in the eastern Sierra Nevada, forest stands have become dense and unhealthy, and are now characterized by reduced growth, increased disease, and increased insect infestation (Ansley and Battles, 1998; Stephens and Moghaddas, 2005). Although coniferous forests typically exhibit low concentrations of N and P in the soil and plant biomass compared with most other forested ecosystems (Stohlgren, 1988), Sierran forests have become dominated by young trees and have accumulated a very thick O horizon over time (Neary et al., 1999). In healthy coniferous ecosystems, rapidly decomposing litter can sustain a short-term cycling of nutrients from the canopies to the soils and back to the canopies. However, an accumulating forest floor similar to that present in the eastern Sierras tends to sequester nutrients leading to the buildup of large nutrient pools that tie up N and P, and at the same time may contribute to long-term nutrient discharge to surface waters (Piatek and Allen, 2001).

An environment where the forest litter builds up on the surface of the mineral soil faster than it degrades is thus undesirable for forest management because nutrient cycles develop progressively slower rates of turnover (Covington and Sackett, 1984), and the forest floor is no longer able to maintain a short-term nutrient cycle. In addition, the excess nutrients now present in the litter may either leach into the soil or discharge from the area by way of surface runoff and/or litter interflow. Recent research has identified the presence of high concentrations of biologically available nitrogen (NH₄⁺–N, NO₃^––N) and phosphorus (PO₄^3––P) in coniferous forest overland flow (Miller et al., 2005). This suggests that these nutrients are derived from the overlying forest floor O horizons, and that there has been little biological uptake, leaching, or direct contact with the mineral soil where strong retention of NH₄⁺–N and PO₄^3––P would be expected. Although commonly considered to be an unimportant water flow pathway in forest ecosystems because vegetation and litter layers typically promote maximum infiltration and soil moisture and nutrient storage, it now appears that overland flow may be an important source of dissolved nutrients discharged to nearby streams and lakes. The potential magnitude, however, remains largely unknown.

The hypothesis for our study was that forest floor O horizon materials are an important source of the biologically available N and P contained in litter interflow. The test objective was to quantify, under laboratory conditions, the discharge of these nutrients by measuring their concentrations in leachate derived from intact monoliths of O horizon materials collected in situ.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Intact O horizon monoliths were collected from a study site located approximately 2 km southeast of South Shore Lake Tahoe, in Stateline, NV (57.5° N, 119.92° W). The slope gradient ranges from 10 to 40%. Dominant tree species are Jeffrey pine (Pinus jeffreyi [Grev. and Balf.]) and California white fir (Abies concolor var. lowiana [Gord. and Glend.] Lindl.), with substantially lesser amounts of sugar pine (Pinus lambertianna [Dougl.]). The understory vegetation consists of Sierra chinquapin (Castanopsis sempervirens [Dudl.]) and currant (Ribes spp.), with some tobacco brush (Ceanothus velutinis [Dougl. ex Hook.]), greenleaf manzanita (Arctostaphylos patula [Greene]), and antelope bitterbrush (Purshia tridentata [Pursh]). The soils developed from granitic parent material, belong to the Cagwin-Rock Outcrop complex, and are classified as mixed Typic Cryopsamments.

Six intact monoliths composed primarily of Jeffrey pine and white fir needle litter and consisting of the combined O_a (highly decomposed unidentifiable organic material), O_e (partially decomposed litter), and O_i (fresh needle fall litter) horizons were collected for greenhouse leaching experiments (Fig. 1). Adjacent field samples were taken for determination of initial nutrient (N and P) status. Air dry monoliths were collected in late summer/early fall of 2004 using 0.63 cm thick 0.61 m by 2.44 m (1.49 m²) sheets of plywood covered with a 1.22 by 4.88 m vinyl polyethylene tarp to reduce friction. A 30 cm by 3 m swath of O horizon material was first excavated to the mineral soil surface. Thickness of the O horizon ranged from about 10 to 15 cm, of which approximately two thirds consisted of the O_e material. The leading edge along the length dimension of the plastic-covered plywood was placed at the now exposed O-A horizon interface, and manually inserted along the interface until the width dimension of the plywood sampler became flush with the sidewall of the original excavation. The O horizon was then vertically severed around the perimeter of the plywood sampler, the intact monolith removed, covered in plastic, and transported to the greenhouse for study. Very little, if any, mineral soil was collected using this technique.

View larger version (169K): [in this window] [in a new window]   Fig. 1. Intact forest floor monolith.

To simulate an early fall rainfall precipitation event common to the eastern Sierras, misting was applied to initially wet the O horizon material in each treatment. Municipal water was supplied via a suspended 3 m, 2.54 cm diam. PVC pipe, running the length of the 2.5 m leaching bin (Fig. 2). Seven misting nozzles (orifice size = 0.030 in.) were inserted along the supply line at distances of 1, 75, 90, 150, 210, 225, and 299 cm from the main water source. Before beginning the actual leaching experiment, several arrays of 300 mL beakers were distributed across the total misting area and the average minimum misting application rate was measured at approximately 0.05 cm min^–1. Leaching bins were then placed on separate bench areas, and the leaching experiments were initiated in February 2005. Before starting the experiment, six small beakers were randomly embedded in the litter materials throughout each bin. The intent was twofold: (i) to collect samples of source water from each leaching cycle for nutrient analysis; and (ii) to measure the average volume of water applied during each cycle.

View larger version (138K): [in this window] [in a new window]   Fig. 2. Leaching bin apparatus.

Following the seven snowmelt cycles, misting was used again to simulate the effects of a spring rainfall on snow event. The final misting was performed in the same manner as previously described; however, only 1 cm of water was applied to each bin over the course of 2 h. The final misting discharge period was about 5 h long, making the cumulative treatment period including the mist and snow applications approximately 10 d. The average total water application throughout the experiment for each bin was 19.6 cm, roughly 23% of the average annual precipitation for the Lake Tahoe basin.

Leachate discharged from misting and snowmelt applications was collected continuously throughout each treatment cycle, sequential and total discharge volume was measured, and subsamples were taken for nutrient analysis. During the initial and final misting applications, samples were collected on volume intervals, one for every 1000 mL that discharged from the outlet. During the snowmelt period, samples were instead collected on time intervals, waiting approximately 2 to 3 h between each sampling event. Source misting water and snow samples from each treatment cycle were also collected, and all samples were analyzed for NH₄⁺–N, NO₃^––N, and PO₄^3––P at the Oklahoma State University Soil, Water, and Forage Analytical Laboratory (Stillwater, OK). Ammonium N was analyzed on a Lachat flow-injection analyzer using QuickChem method 12-107-06-2-A (LACHAT, 1994), NO₂+NO₃^––N was analyzed on a Lachat flow-injection analyzer using QuickChem method 12-107-04-1-B (LACHAT, 1994), and PO₄^3––P was analyzed using an inductively coupled plasma (ICP) spectrometer (APHA, 1995). Average nutrient input concentrations from the municipal source and Sierra snow are presented in Table 1.

Following gravity drainage from the final misting application, the total content of each bin was weighed wet and subsamples were collected for moisture content determination. For the combined horizon treatments, the organic material was separated as previously described into O_e and O_i components and weighed, with separate subsamples collected for each. This was done because the moisture content of O_i litter would be less than that of the more highly decomposed O_e portion. Although rarely present, when the bins were being emptied any mineral soil deposits were manually isolated from the organic material. This small mineral fraction could have provided an additional source of NO₃^––N, but would generally be regarded as a potential sink for NH₄⁺–N and PO₄^3––P. Sticks, cones, and bark material were also separated from the rest of the bin contents. Needles and decomposed components have been shown to contain much higher concentrations of N and P than wood, bark, or cones (Smethurst and Nambiar, 1990). For this reason, and because of the different moisture storage capacities compared with that of the O_e and O_i materials, separate moisture contents were again determined for the wood, bark, and cones. The wet weight of the sticks, bark and cone materials was subtracted from the total wet weight of the bin contents before dry mass determination for the remaining organic materials.

An individual input/output water balance was developed for each of the nine leaching bins using the following relationship:

[1]

Due to the time of year (winter), ambient greenhouse temperatures, and the comparatively short duration of each wetting cycle, evaporation was assumed to be minimal compared with the total inflow and, therefore, disregarded as a major component of the water balance. The mass water content of each organic layer immediately following complete gravity drainage from the final misting treatment was taken as the maximum litter moisture storage after drainage for the overall experiment.

Combining the inflow/outflow water balance with measured concentrations of NH₄⁺–N, NO₃^––N, and PO₄^3––P in the misting water, each snow application, and discharged leachate, a nutrient mass balance was determined for each treatment from a similar relationship:

[2]

Where C_i is input concentration, C_o is output concentration, and r is nutrient discharge or retention. Any difference between the nutrient content entering the bins in the input water and that discharged during leaching/drainage was assumed to be either retained or released from the O horizon materials. The nutrient discharge or retention was then converted to nutrient mass per unit mass of organic litter using the total dry weight content of each bin. A simple analysis of variance (ANOVA) was performed for both the overall discharge mass and the mass per dry weight of bin material for each inorganic nutrient form. Finally, for overall nutrient loading throughout the experiment, the mass per unit mass of each nutrient retained or discharged was converted to kg ha^–1, based on the actual dry mass of bin material per unit area (kg m^–2) of each leaching bin.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Water Balance
The actual volume of water input proved difficult to quantify. The six drop can (beaker) placements in each bin were ineffective at accurately characterizing water input. This was due, in part, to changes in air flow and water drop distribution as a result of bin placement on each misting bench. In addition, snowmelt often circumvented collection due to preferential flow around the edges of the open mouth beakers. As a result, the initial sum of the moisture storage and the measured outflow was often larger than the estimated inflow volume based on projected misting rates and the moisture equivalent of snow applications. Because the total outflow plus the litter moisture storage cannot exceed the total inflow (Eq. [1]), the actual volume of water applied during the initial and final misting treatments had to be greater than the 0.05 cm min^–1 application rate would predict (i.e., 20832 cm³ bin^–1 and 13888 cm³ bin^–1, for first and last misting, respectively). Once the litter moisture storage was satisfied, the inflow for each subsequent leaching cycle could be assumed equal to the outflow. Assuming inflow to equal outflow for the final misting cycle, new input volumes during the initial and final misting treatments were estimated to be 21535 cm³ and 14357 cm³, respectively.

The water balance for the overall experiment is presented in Table 2. Based on these data and the constraints of Eq. [1], the average estimated total inflow across all treatments was 272558 cm³, or 19.6 cm of applied water, ± 1.4 cm. During the initial misting, the time was noted at which the first outflow of discharge water was observed. The O_i treatments generally demonstrated the shortest time to discharge, but the relationship was significant (t-test, p < 0.05) only when compared with the combined O_i+O_e treatments, in which the monoliths were more intact. Since the O_e layers in coniferous forests are largely water repellent, thus restricting infiltration and uniform wetting (Brock and DeBano, 1982; Naslas et al., 1994; Cleveland et al., 2004), the O_e only and the O_i+O_e combined treatments were expected to exhibit longer times to initial outflow. The relatively short time to outflow observed in two of the O_e only treatments may have been the result of disturbance caused in the natural framework of the litter mat when the layers were hand-separated, or to the presence of natural preferential flow paths that became exposed to positive pressure heads following removal of the overlying O_i materials. In any case, it was apparent that the moisture storage capacity of the O_i materials was easily satisfied during the initial misting cycle.

O Horizon Total Nitrogen and Phosphorus Concentrations
Initial total N and P content (mg kg^–1) of O horizon materials from the field samples are presented in Table 3 along with the water-extractable nitrogen and phosphorus loads determined from the leaching experiments. The NO₃^––N leached from the O horizon materials (mg kg^–1 dry mass) for the overall experiment was less than 1% of the total N concentration (mg kg^–1) measured in the litter before leaching. The PO₄^3––P leached was about 15% of the total P contained in the O_i horizon material, and less than 5% from the O_e.

View larger version (48K): [in this window] [in a new window]   Fig. 3. Nitrate-N leachate concentrations from each misting and snowmelt cycle for the (A) Oi, (B) Oe, and (C) the Oi + Oe combined treatments. Note the scale differences.

View larger version (50K): [in this window] [in a new window]   Fig. 4. Ammonium-N leachate concentrations from each misting and snowmelt cycle for the (A) Oi, (B) Oe, and (C) the Oi + Oe combined treatments. Note the scale differences.

View larger version (13K): [in this window] [in a new window]   Fig. 5. Nitrate-N vs. NH4+–N concentrations measured in the discharge leachate.

An exception to this pattern was noted during the last snowmelt cycle (cycle 7), which exhibited low concentrations of both NO₃^––N and NH₄⁺–N (<0.1 mg L^–1) in the discharge leachate. These data suggested that the slow leaching characteristic of snowmelt had discharged most of the soluble inorganic nitrogen by the time the last cycle was initiated. However, during the final misting concentrations of NO₃^––N and NH₄⁺–N in the leachate again increased. Since the misting application rate was greater than that during snowmelt, it may be that new areas of inherently water repellent pockets began to wet, or more likely that immobilized water containing higher nutrient concentrations during periods of slow snowmelt became mobilized at the higher application rate of the final misting process.

Phosphorus
Phosphate-P was discharged from all treatments during each misting and snowmelt period throughout the course of the experiment (Fig. 6). The highest discharge concentrations, however, were observed during the initial misting cycle for all treatment types. Phosphate-P concentrations in the leachate from each replicate of the O_i only treatment during the initial misting cycle reached the highest levels recorded throughout the experiment (5.6, 4.8, and 4.1 mg L^–1, respectively) (Fig. 6A). From the time of the second snowmelt cycle, concentrations of PO₄^3––P in the leachate remained fairly constant at <1.0 mg L^–1 throughout each individual sampling period.

View larger version (38K): [in this window] [in a new window]   Fig. 6. Phosphate-P leachate concentrations from each misting and snowmelt cycle for the (A) Oi, (B) Oe, and (C) the Oi + Oe combined treatments. Note the scale differences.

Comparison with Lake Tahoe Basin Water Quality Standards
Water quality data collected on various tributaries of the Tahoe basin by the Lake Tahoe Interagency Monitoring Program (LTIMP) between 1989 and 1996 have shown NH₄⁺-N concentrations typically less than 0.01 mg L^–1, but ranging up to 0.075 mg L^–1, and discharge-weighted concentrations of PO₄^3– (reported as phosphate) and total phosphorus (TP) ranging from <0.01 to 0.02 and 0.02 to 0.22 mg L^–1, respectively (Reuter and Miller, 2000). In terms of ground water quality, Loeb and Goldman (1979) reported the well chemistry from four wells in the Ward Valley watershed to contain average concentrations of less than 0.015 mg L^–1 NH₄⁺, 0.16 mg L^–1 NO₃^–, and 0.07 mg L^–1 soluble reactive P. Accordingly, the Lahontan Regional Water Quality Control Board (LRWCB) has established storm water effluent limitations for surface water runoff that enters directly into Lake Tahoe (or into lake tributaries) for total N concentrations of 0.5 mg L^–1 and total P of 0.1 mg L^–1; and surface water guidelines in the Lake Tahoe basin watershed for maximum allowable total N concentrations of 0.21 mg L^–1, and soluble reactive P concentrations of 0.009 mg L^–1 (CA-EPA, 1994).

Based on the above, the observed peak concentrations of NH₄⁺–N and NO₃^––N found in leachate from the overall course of this study (0.8 and 0.75 mg N L^–1, respectively) are clearly of a sufficient magnitude for concern. Furthermore, since the waters of Lake Tahoe are now considered to be trending toward a P-limiting condition (Goldman, 1988), the observed leachate concentrations of inorganic P are of even greater concern. Initial discharge concentrations of PO₄^3––P were as high as 5.6 mg P L^–1, 50 times greater than LRWCB surface runoff limitations and almost three orders of magnitude greater than surface water guidelines for the Lake Tahoe basin.

Nutrient Mass Balance and Discharge Loading
Nitrate Nitrogen
Variable amounts (mg) of NO₃^––N were discharged from all treatments during each leaching cycle, with sporadic peaks recorded throughout the entire experiment, indicating a continual discharge via O horizon leaching. This was anticipated because the soluble inorganic NO₃^– anion would not normally be expected to adsorb to organic colloidal materials. ANOVA showed that the combined O_i+O_e treatment contributed more N as NO₃^– throughout the overall experiment than did either of the individual litter layer treatments. This difference was found to be only moderately significant (p < 0.10). Although not significant, the O_e only treatment also discharged more NO₃^––N (mg) than did the O_i only, largely due to the comparative total mass of respective litter materials. In the area of monolith collection, O_i horizon materials averaged about 1.8 kg m^–2, with about 5.4 kg m^–2 of underlying O_e material.

When converted to mass N per unit mass of dry litter, the differences between treatments were not significant according to the ANOVA. However, the O_i only treatment tended to release more N as NO₃^– per dry unit mass of material than did either the O_e or the combined O_i+O_e horizon treatments.

Ammonium Nitrogen
Nitrogen in the form of NH₄⁺ was generally retained and/or transformed through nitrification in all litter treatments over the course of the experiment. Only two of the leaching bins (#4 and #9) exhibited a cumulative release of NH₄⁺–N, both of which contained the combined O_i+O_e treatments. This effect in itself was not surprising in that NH₄⁺ in particular is readily immobilized during the first 6 mo to 1 yr of litter decomposition (Rustad and Cronan, 1988; Monleon and Cromack, 1996). Alternatively, a predominant mechanism for abiotic nitrogen immobilization is thought to involve the reaction of NH₄⁺ with plant materials containing large concentrations of lignin (Schimel and Firestone, 1989). Since coniferous forest litter contains a substantial amount of lignin, exposure to these organic constituents at and within needle surfaces during early stages of decay may have led to abiotic chemical immobilization (commonly termed ligand exchange), and hence, the observed retention of NH₄⁺–N overall (Schimel and Firestone, 1989). Furthermore, in the absence of extremely acidic conditions (pH < 4.5), soluble NH₄⁺ cations could be expected to adsorb to the charged surfaces of colloidal organic or mineral components associated with the forest floor matrix.

Based on ANOVA, the amount of NH₄⁺–N retention in the O_i treatments was significantly higher for both total mass (p < 0.05), and mass per unit dry mass (p < 0.005) of litter material. Although net biotic N accumulation has frequently been reported during the early stages of the decay process (Klemmedson et al., 1985; Yavitt and Fahey, 1986; Stohlgren, 1988), the O_i horizon consists largely of intact only slightly decayed needle litter. Hence, the specific mechanism of NH₄⁺ retention in the needle litter remains unclear.

Less NH₄⁺–N retention and even some discharge was observed in leachate from the intact monoliths containing both the O_i+O_e horizons. These monolith treatments were undisturbed throughout the experiment and microbial mineralization (i.e., ammonification) may have remained more active compared with the separate, albeit slightly disturbed, litter layers. Elevated NH₄⁺ concentrations have been reported in forest leachate after the microbial population has consumed a substantial part of the extractable organic N (Cortina et al., 1995), but in the Sierras, this process appears to be most prominent during the winter due to enhanced moisture content and the incubating effect of the overlying snow pack (Stark, 1973).

Phosphate Phosphorus
The greatest amount (mg) of PO₄^3––P was discharged from all treatment types during the initial misting cycle and declined thereafter. Measured differences throughout the course of the experiment between the treatments were found to be nonsignificant (p > 0.1) based on ANOVA. However, the combined horizon treatment tended to contribute greater amounts of PO₄^3––P in the discharge leachate than did the O_i only treatment. When expressed on a per unit mass basis (mg kg^–1), ANOVA showed a significantly greater (p < 0.05) amount of PO₄^3––P discharged from the fresh litter (O_i only) than from either of the other treatment types. Although not found to be significant, the O_e only treatment also contributed larger amounts of PO₄^3––P than did the combined horizon treatment.

These findings are consistent with the literature in that P has been reported to be rapidly lost by leaching during the initial stages (the first 3 to 6 mo) of litter decomposition (Edmonds, 1979; Rustad and Cronan, 1988; Cortina et al., 1995), with continued P release over time but at a much slower rate (Monleon and Cromack, 1996). Indeed, the reported P loss was so rapid during the early stages of litter decay for two separate studies on litter nutrient release, that the proportion of P remaining in the litter was about 50% after 1 yr, and 30% after 6 mo of incubation, respectively (Rustad and Cronan, 1988; Monleon and Cromack, 1996). Compared with the newly fallen litter, the PO₄^3––P discharged per unit mass from the O_e only treatments remained lower throughout the experiment, most likely because the older organic material had already lost a substantial portion of its phosphorus from leaching during the previous season.

In summary, there was a net discharge of biologically available N and P from the combined O_i+O_e materials during leaching. Specifically, when considering the combined horizon treatments because they best represent in situ field conditions, net contributions of N were approximately 4 mg N kg^–1 (0.45 kg N ha^–1) and for P approximately 17 mg P kg^–1 (2 kg P ha^–1) (Table 4). By way of comparison to the field, we can multiply the N and P yields per unit O horizon mass from this study by the mass of O horizon material measured in the field near where the monoliths were collected (93200 kg ha^–1). This produces a value for N yield of 0.37 kg N ha^–1 and for P yield of 1.58 kg P ha^–1, numbers very close to those estimated above from the leaching bin area. The N leaching values calculated either way are similar to the annual leaching values using resin lysimeters as reported by Murphy et al. (2006) for the field site location (an unburned control location) from which the monoliths were collected (0.37 ± 0.07 kg N ha^–1 yr^–1). However, the P values contained in O horizon leachate reported here are much greater than those reported by Murphy et al. (2006) for soil leaching flux (0.48 ± 0.16 kg P ha^–1 yr^–1). In both cases it should be noted that the values measured by Murphy et al. (2006) represented 8 mo of soil leaching fluxes over the winter season whereas the values in this study were for a much shorter period. Thus, N and P in runoff (O horizon leachate) appear to be of greater magnitude than from in situ soil leaching, suggesting that O horizon runoff may be potentially discharged to adjacent surface waters by way of overland/litter interflow.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

The effect of almost a century of fire suppression has been a general decline of forest and watershed health, including the excessive build up of forest litter containing much larger pools of soluble N and P. These nutrients, when discharged by way of forest floor leachate through litter interflow represent a potential source of biologically available N and P to adjacent surface waters. Transport of these nutrients from the terrestrial to the aquatic system in the Lake Tahoe basin may therefore play a role in the deterioration of lake clarity.

	ACKNOWLEDGMENTS

 
We gratefully acknowledge the participation and support of the Nevada Agricultural Experiment Station, McIntyre-Stennis, and the US Forest Service Lake Tahoe Basin Management Unit and Joint Fire Sciences Program. NAES publication #52055642.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A contribution of the Dep. of Natural Resources and Environmental Science. Research supported in part by Nevada Agricultural Experiment Station, publ. # 52055642.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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