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Effects of burning on nutrient balance in an area of gorse (Ulex europaeus L.) scrub
ELSEVIER
The Science
of the Total
Environment
204 (1997)
271-281
Effects of burning on nutrient balance in an area of gorse ( Ulex euvopaeus L.) scrub B. Soto”, R. Basanta, F. Diaz-Fierros Departamento
de Edafoloxia,
Facultad
de Farmacia,
Received
Universidad
18 April
de Santiago,
1997; accepted
15 706 Santiago
de Compostela,
Spain
19 June 1997
Abstract
Wildfires affect nutrient balanceasa result of combustionof biomass,increasedsurfaceand subsurfacerunoff and increasedsoil erosion.In the present study, nutrient inputs and outputs to burnt and unburnt Ulex scrubplots were monitored over a 2-year period. During burning, between 50 and 75% of the nutrients contained in above-ground plant tissueswere directly lost due to volatilization and upward movement of particulates to the atmosphere.Only small amounts(lessthan 3% for all elements)were depositedat the soil surfaceas ash.During the first rains after burning, N, P and K losseswere largely due to sedimenttransport in surface runoff, while Ca and Mg losseswere roughly equally distributed between sedimentlossesand soluble-formlosses(in surfacerunoff and subsurfaceflow) and Na losseswere largely in solubleform. Post-burningnutrient inputs to the soil in throughfall were lower than in the control plots for N and K; in the caseof the remainingelements(P, Ca, Mg and Na), inputs to the burnt plots and control plots differed little. In general, burning led to clear net lossesof nutrients; annual losseswere approximately 2.5-3.5 g m-’ in the caseof N and approximately 6.5-9.0 g m-* in the caseof K. In the unburnt plots, by contrast, outputs were approximately equal to inputs. 0 1997Elsevier ScienceB.V. Keywords:
Ecosystemdegradation;Burning; Plant nutrients; Air pollution; Water pollution
1. Introduction
During wildfires large amounts of the nutrients contained in the vegetation are released to the
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atmosphere in gaseous or particulate form and the remainder are deposited at the soil surface as ash (Woodmansee and Wallach, 1981; Feller, 1988). Furthermore, nutrients are lost after the fire as a result of leaching and particulate transport; such processes may affect not only vegetation ash, but also nutrients present in the soil (as a result of increased erosion) (Helvey et reserved.
272
B. Soto et al. /The Science of the Total Environment 204 (1997) 271-281
al., 1985; Cornish and Binns, 1987; Lavabre et al., 1993; Cerda et al., 1995). Under some circumstances, wildfires may thus provoke massive nutrient inputs to watercourses (Schindler et al., 1980; Davis, 1989). In addition, nutrient input to the soil in the months and years following a fire is often reduced, as a result of the destruction of vegetation cover and consequent effects on atmospheric deposition. In view of these various effects, wildfires may thus have a very considerable impact on soil nutrient balance and biogeochemical cycles. Fire is a frequent event in many scrub ecosystems, including the kermes scruboak (Quercus cocciferu) garrigue, tree heather (Etica arborea) maquis and broom (Calycotome spinosa) maquis of Mediterranean regions (Trabaud, 1994), the chaparral of California (Keeley, 1982) and the gorse scrub of northwest Spain (Basanta et al., 1988). The high frequency of fire in ecosystems of this type (fire recurs every lo-20 years) leads to generalized degradation and a gradual decline in the system’s capacity to recover. Studies of the different processes by which nutrients may enter and leave such ecosystems and of the effects of fire on such processes, are of fundamental importance for understanding the effect of fire in environments of this type and will facilitate assessment of the risk of contamination due to nutrient losses to the atmosphere or watercourses. In the work reported here we monitored nutrient losses from Ulex scrub plots during burning and subsequent effects on nutrient input and output. 2. Materials and methods
2.1. Study area The study was performed within an area of UIex scrub of approx. 2000 m2 in area, with mean slope 30% and altitude approximately 350 m, on the west slope of Monte Pedroso in Santiago de Compostela (northwest Spain). The bedrock is granite. The soil is a lithic haplumbret on the soil taxonomy classification (USDA, 1975) mean organic C content is 67 g kg-i, mean N content 6.8 g kg-‘, mean cation exchange capacity 28.4 meq
per 100 g; texture is sandy loam. The vegetation is 8-year-old gorse shrubland, with a biomass of approximately 4294 g rnp2, approx. 90% of this as gorse (UZ~Xeuropaeus); other common species are bell-heather (Utica cinerea), dorset heath (Etica umbelhta), St. DabeoCs heath (Daboecia cantabrica), heather (CuZluna uulguris) and rockrose (HaZimium alyssoides); mean height of the vegetation is approximately 1.2 m (Basanta et al., 1988). 2.2. Plots description Four 20 X 4 m plots were marked out within the study area and delimited with metal sheeting. Each plot had a collector for runoff at its lower end; this collector fed to a sedimentation tank, allowing determination of sediment losses. Each sedimentation tank in turn fed via a l/9 divider to a runoff collection tank. Throughfall was measured with the aid of two 0.44-m’ (4 x 0.11 m) channels in each plot, both channels feeding to a collection tank outside the plot. Subsurface flow samples were collected with the aid of a 3 x 0.15m drainpipe buried at the downslope end of each plot (see Fig. 1); note that these samples were used for evaluating the chemical composition of subsurface flow, but that subsurface flow volumes were estimated with the aid of the WEPP model (see below). Precipitation was measured with two gauges located close to the plots. 2.3. Experimental
bums
In September 1988, two of the four plots were subjected to controlled burns. Plot BP1 was burnt in the morning, with wind velocity 2.55 m s-l, air temperature 18.3”C and 60% relative humidity. Plot BP2 was burnt in the early evening, with wind velocity 1.56 m s-l, air temperature 21.o”C and 48% relative humidity. During burning, a total of 21 circular trays (139 cm2 each) were distributed at random within each plot for collection of ashes. The other two plots (CPl and CP2) were maintained as controls; in September 1989, however, one of these plots (CP2) was affected by a wildfire and is thus referred to as the wildfire plot (WF) from that time on.
B. Soto et al. /The Science of the Total Environment 204 (1997) 271-281
213
eis
Drainpipe
Fig. 1. Schematic surface.
drawing
of the system
for collection
of subsurface
Additional plots were used for estimation of phytomass and related data (see Table 1) and determination of soil properties (see Table 2). 2.4. Estimation of runofl throughfall and subsueace JEow Rainfall intensity in several rainfall events (one in the first year, three in the second year) was so high that it was not possible to accurately determine runoff and throughfall volumes. In these cases, runoff was predicted with the aid of the WEPP model (Flanagan and Nearing, 1995) and throughfall with the aid of Calder’s model (Calder, 1986). Both models have been validated for these plots (Soto and Diaz-Fierros, 1997; Soto and Diaz-Fierros, 1997). Subsurface flow, Iwhich was not measured, was predicted with the aid of the WEPP model; note that subsurface flow includes subsurface lateral flow and percolation out of the root zone. 2.5. Chemical analysisof water, sedimentsand soils Runoff and throughfall water sampleswere collected after each rainfall event. Subsurface flow samples were collected 28 times during the study period. All water sampleswere maintained at 4°C
flow. The
top of the plastic
sheet was 25 cm below
the soil
and filtered (pore size 0.45 pm) before analysis. Sodium and K contents were determined by flame emission spectroscopy, Ca and Mg contents by atomic absorption spectroscopy, P content by the molybdate blue method and N content by titration following Kjeldahl distillation. Sediments lost from each plot were collected six times in the course of the study (4, 7, 10, 15, 18 and 21 months after burning). All samples were oven-dried at 60°C before analysis. Nitrogen was determined as per Guitian and Carballas (1976). Ca, Mg, Na, K and P were determined after digestion of the sample with a mixture of perchloric, hydrochloric and hydrofluoric acids in a sand bath at 200°C. Calcium and Mg were determined by atomic absorption spectroscopy, Na and K by flame emission spectroscopy and P by the molybdate blue method. The nutrients amount in runoff water (volume of water by concentration) represents the losses in soluble-form and the nutrients amount in sediments (mass of soil removed by concentration) represents the lossesin particulate-form. Soil analyses were as follows. Nitrogen was determined as in sediments. Plant-available cations (Ca, Mg, Na and K) were extracted with ammonium acetate (pH 7) and determined as in sediments. Available phosphorus was determined
274
B. Soto et al. / The Science of the Total Environment
Table 1 Total phytomass in all cases)
and nutrient
mass of major
components
-
of the vegetation,
Total phytomass
204 (1997)
before
271-281
and inmediately
after burning
(units
are g m-*
N
P
Ca
Mg
Na
K
Pre-fire
Ulex europaeus Vlex europaeus Ericaceae Other species Litter Total
Shoots Stems
1398.4 2448.7 309.1 137.4 1060.1 5353.7
16.46 12.72 0.72 1.23 16.88 48.01
0.70 0.48 0.05 0.05 0.42 1.70
3.24 2.48 0.20 0.24 6.28 12.44
2.23 1.44 0.11 0.16 2.36 6.30
1.28 1.24 0.06 0.10 0.32 3.00
6.49 7.08 0.42 0.55 1.10 15.64
Post-fire BP1
Ulex europaeus Other species Litter Ash Total
Stems
2373.9 43.5 467.3 8.4 2893.1
12.33 0.39 7.44 0.10 20.26
0.47 0.02 0.19 0.05 0.73
2.40 0.07 2.77 0.19 5.43
1.40 0.05 1.04 0.11 2.60
1.20 0.03 0.14 0.21 1.58
6.87 0.17 0.48 0.21 7.73
Post-fire BP2
Ulex europaew Other species Litter Ash Total
Stems
1888.1 107.8 109.5 8.7 2114.1
9.81 0.97 1.74 0.10 12.62
0.37 0.04 0.04 0.05 0.50
1.91 0.19 0.65 0.19 2.94
1.11 0.13 0.25 0.11 1.60
0.95 0.08 0.03 0.22 1.28
5.46 0.43 0.11 0.21 6.21
Percentage BP1 BP2
reduction
in total
after fire 46.0 60.5
by the Bray-2 method (Bray and Kurtz, 1945). Soil pH was determined in water and 0.1 N KC1 (ratio soil:solution, 1:2.5). 3. Results
3.1. Nutrient losses to the atmosphere Nutrient losses to the atmosphere (estimated as the difference between the nutrient content of the vegetation and litter before burning and that of the vegetation, litter and ash remaining after burning) are listed for each element (N, P, Ca, Mg, Na and K) and each plot in Table 1. Losses were in the range 47-59% from plot BP1 and Sl-76% from plot BP2; in both cases, percentage loss of K was considerably lower than that of other elements. The highest absolute losses were of N (27.8 g m-’ from BPl, 35.4 g me2 from BP2) and of Ca and K (7-10 g m-‘>.
57.8 73.7
57.1 70.6
56.4 76.4
58.7 74.6
47.3 57.3
50.6 60.3
3.2. Ash deposition
Total ash deposition to the soil surface was 8.4 g me2 in plot BP1 and 8.7 g me2 in plot BP2 (84 and 87 kg ha-r). These values are lower than those reported in previous studies of similar characteristics (loo-10 000 kg ha-‘) (Raison, 1979; Carreira and Niell, 1995), which probably reflects the low intensities of our experimental burns (the soil temperature at S-cm depth during the fire was less than 50°C in BP plots) (Diaz-Fierros et al., 1990). In accordance with this, phytomass (including litter) remaining after the fire was approx. 54% (BPl) and 39% (BP2). The ash was highly alkaline (pH 10.5-11, as determined in a 1:lOOO aqueous solution), implying deposition of 2.66 (BPI) or 2.75 (BP2) meq OH- m-‘. Six months after burning, soil pH in the BP plots had increased with respect to preburn levels by approximately 0.73 units in the O-2.5 cm layer, 0.45 units in the 2.5-5 cm layer
B. Soto et al. /The Science of the Total Environment 204 (1997) 271-281
275
Table 2 Physicochemical properties of the soil of the BP plots, before and 6 months after burning. Carbon and N contents percentages w/w. Other element contents (pa = plant-available) are expressed as mg per 100 g. Each value determination of six samples Pre-fire O-2.5 PH H@ pH KC1 C N Na pa Kpa Ca pa Mg pa Ppa
4.44 3.48 6.74 0.68 5.01 6.81 4.56 1.18 2.75
as of
Post-fire cm
f + + k f f. + * +
are expressed is the mean
0.14 0.19 0.55 0.10 0.75 1.26 1.16 0.37 1.29
2.5-5 4.40 3.51 7.91 0.58 3.57 3.73 1.58 0.13 1.91
cm + + + + f & + * f
0.07 0.06 1.18 0.06 1.30 0.60 0.58 0.10 0.59
5-10
cm
4.41 3.68 7.42 0.54 3.30 2.76 0.62
and 0.23 units in the 5-10 cm layer (Table 2). In the wildfire plot WF, soil pH increased more markedly: 5 months after burning, pH had increased with respect to pre-burn levels by approximately 1.22 units in the O-2.5 cm layer, 0.87 units in the 2.5-5 cm layer and 0.63 units in the 5-10 cm layer. This greater increase in pH in the WF plot is attributable to two factors. First, phytomass remaining after the fire (though not quantified precisely) was clearly much lower than in the BP plots, so that ash deposition was undoubtedly more significant. Second, partial com’bustion of soil organic matter occurred in the WF plot (carbon content in the O-2.5 cm layer dropped by approx. 27%). Mean element contents in ash were 11.6 mg g-l for N, 5.4 mg gg’ for P, 22.2 mg g-l for Ca, 12.6 mg 8-l for Mg, 24.9 mg g-l for Na and 24.7 mg g-l for K. In view of the relatively small amounts of ash deposited (see Table l), nutrient input to the soil surface by this route can be considered of minor importance (between approximately 0.05 g m-2 for P and 0.22 g rnp2 for Na). Nevertheless, soil levels of available cations except Na and P were higher after burning than before (Table 2). The most marked increases were in Ca and Mg levels, all of which remained above pre-burn levels for at least 6 months (Table 2), indeed 18 months in the case of K and Ca (results not shown). Note that the observed increases in cation levels are too high to be explained by ash
+ & f * * + f
0.07 0.07 0.26 0.04 0.88 0.53 0.40
* 0.35
O-2.5
cm
2.5-5
em
5.17 3.88 1.22 0.52 4.53 10.15 14.20 5.45 2.58
k 0.18 + 0.14 + 2.61 f 0.15 + 0.85 &- 1.26 f 4.43 + 1.39 + 1.33
4.85 +_ 0.25 3.89 + 0.16 5.60 + 2.16 0.50 f 0.07 2.51+ 0.68 5.90 + 1.28 3.15 f 1.35 0.97 + 0.39 1.66 + 1.11
5-10 4.64 3.92 7.38 0.54 1.74 4.66 1.48 0.57 1.17
cm k f i + + + + + +
0.19 0.07 1.42 0.05 0.36 0.67 0.67 0.27 0.45
deposition and are probably due to the increased mineralization of soil organic matter resulting from the increased temperatures in the surface layer (due to increased insolation and reduced reflectance as a result of blackening) and the increased soil water content in the months after burning (Raison, 1979). 3.3. Nutrient uptake by the vegetation The concentration of nutrients in the different species was obtained from bibliographical data concerning this type of scrub (Egunjobi, 1971b; Forgeard and Frenot, 1987) the concentration of nutrients in the main species was similar in both studies. Estimates of the amount of nutrients incorporated into the vegetation over the 2 years after burning are listed in Table 3. The total increase in phytomass over the first year was approximately 273 g rnp2, approx. 90% of which was due to Ulex europaeus (Pereiras and Casal, 1994). The increase in phytomass over the second year was much more marked (1608 g me2, approx. 85% due to Ulex europaeus, 5% to Ericaceae and 10% to other taxa). 3.4. Nutrient input in throughfall Nutrient concentrations in throughfall varied considerably among plots and with time of year,
276
B. Soto et al. /The
Table 3 Estimated net annual nutrient uptakes the first and second years after burning Increase biomass
in
(g m-‘>
Science of the Total Environment
to above-ground
N
tissues
204 (1997) 271-281
of lJlex europaeus
and other
species
in the burnt
plots,
P
Ca
W
Na
K
First year vlex Other species Total
245.5 27.3 272.8
2.89 0.25 3.14
0.12 0.01 0.13
0.57 0.05 0.62
0.39 0.03 0.42
0.22 0.02 0.24
1.14 0.11 1.25
Second year ulex Ericaceae Other species Total
1366.6 80.4 160.8 1607.8
16.09 0.76 1.44 18.29
0.69 0.05 0.06 0.80
3.17 0.26 0.28 3.71
2.18 0.16 0.19 2.53
1.25 0.08 0.12 1.45
6.35 0.37 0.64 7.36
ranging from 0.5 to 10 mg 1-i for N, 0.01-0.26 mg 1-l for P, 1.3-10.0 mg 1-l for Ca, 0.3-5.2 mg 1-l for Mg, 1.2-14.0 mg 1-l for Na and 0.2-5.2 mg 1-r for IS. In the control plot, concentrations of most nutrients were highest during the first major rainfall episodes following the summer drought, reflecting lavage of accumulated nutrients (in leaf exudates and/or due to dry deposition from the atmosphere) from above-ground plant structures (Egunjobi, 1971a; Christensen, 1973) in the burnt plot, concentrations of most nutrients were highest during the first rainfall episodes after burning, principally reflecting leaching of ash nutrients from above-ground plant structures. Estimated mean nutrient inputs to the soil surface via throughfall are listed in Table 4. In the burnt plots, N and K inputs over the first year were lower than in the control plot. Calcium input was slightly higher in the burnt plots. Inputs of the other elements differed little between the burnt and control plots. As indicated above, nutrient concentrations in throughfall varied considerably with time. During the first two rainfall episodes of the first year (together accounting for approximately 22.5% of the year’s total throughfall in both the control and burnt plots), throughfall inputs of N to the burnt plots accounted for approximately 50.7% (BP11 and 43.8% (BP2) of the year’s total, while throughfall inputs of Ca to the burnt plots accounted for approximately 64.3% and 41.7% of the year’s total; note, however, that throughfall
in
nutrient inputs to the control plots were also somewhat high in relation to throughfall volume (approximately 30.9% and 27.9% of the annual totals for N and Ca, respectively). Likewise, mean N and Ca inputs to the burnt plots in throughfall collected during October of year 2 accounted for 14.4% and 12.3% of that year’s inputs, respectively (25.7% and 10.9%, respectively in the control plot), despite the fact that throughfall volume during this period accounted for 3.1-3.4% of the year’s total in the BP plots and only 2.6% of the year’s total in the control plots. For all elements measured, total throughfall nutrient input in the control plots was higher during the second year of study than during the first year. The increase in throughfall input was particularly marked in the case of Na. These results are probably attributable to the higher rainfall during the second year (1499 vs. 995 mm); the study area is only approx. 30 km from the coast and Na concentration in rainwater is typically high. Total throughfall nutrient inputs in the burnt plots differed little between the 2 years of study, except in the case of N, input of which was considerably higher in the second year (Table 4).
Nutrient concentrations in subsurface flow samples were low, as expected given the high nutrient (particularly P) retention capacities of this soil (Soto and Diaz-Fierros, 1993). We did
B. Soto et al. / i%e Science of the Total Environment
204 (1997)
277
271-281
Table 4 Estimated inputs and outputs of nutrients (g me2 ) to/from the control (CPl) and burnt plots (BPl, BP2 and WF) during the first and second years of study. The outputs considered are soluble-form losses in surface runoff, soluble-form losses in subsurface flow and particulate-form losses in surface runoff. The amount column shows throughfall, runoff and subsurface flow depths in mm and particulate-form losses in surface runoff in g m-* Amount First year (1988/1989) CPl outputs:
N
P
Ca
Mg
Na
K
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
621.9 51.8 190.8 50.0
1.74 0.28 0.39 0.36
0.021 0.005 0.003 0.025
2.23 0.15 0.54 0.20
0.68 0.04 0.20 0.12
4.85 0.31 1.28 0.33
2.26 0.10 0.09 1.54
BP1
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
892.0 82.4 339.7 225.7
0.39 0.41 0.69 1.79
0.014 0.016 0.005 0.166
3.22 0.26 0.96 0.83
0.79 0.29 0.36 0.61
5.02 1.12 2.27 1.73
1.02 0.37 0.16 6.92
BP2
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
913.8 64.1 339.7 219.8
0.34 0.47 0.69 1.85
0.019 0.010 0.005 0.156
3.11 0.25 0.96 0.87
0.72 0.20 0.36 0.55
3.65 0.92 2.27 1.58
0.51 0.42 0.16 6.24
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
1038.9 215.7 612.7 153.8
1.88 0.32 1.25 1.42
0.048 0.003 0.010 0.112
2.62 0.39 1.72 0.52
1.13 0.20 0.65 0.40
8.44 1.30 4.10 0.13
2.65 0.22 0.28 4.22
BP1
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
1325.4 238.4 646.6 247.7
0.84 0.29 1.32 2.52
0.015 0.004 0.010 0.177
2.75 0.39 1.82 0.99
0.74 0.21 0.68 0.67
5.79 1.16 4.32 2.19
0.56 0.18 0.30 7.11
BP2
Inputs: outputs:
Througbfall Runoff, soluble Subsurface, soluble Runoff, particulate
1315.1 219.8 646.6 318.8
0.50 0.29 1.32 2.25
0.024 0.003 0.010 0.180
3.07 0.31 1.82 1.12
0.79 0.18 0.68 0.70
6.27 1.10 4.32 2.33
0.65 0.18 0.30 9.48
WF
outputs:
Runoff, soluble Subsurface, soluble Runoff: particulate
262.9 742.0 1314.0
0.27 1.51 12.92
0.010 0.012 0.988
0.44 2.09 6.24
0.21 0.78 3.58
1.43 4.97 11.16
0.39 0.34 39.12
Second CPI
year (1989/1990)
not determine nutrient concentrations in samples corresponding to all subsurface runoff episodes; however, the variation among the 28 samples taken over the study period was low and there was no evidence of any within-year trend. Mean concentrations were 2.04 + 0.54 mg 1-l for N, 0.016 -t 0.01 mg 1-i for P, 2.81 i 1.67 mg 1-i for Ca, 1.05 _+ 0.15 mg 1-l for Mg, 6.70 + 1.09 mg 1-l for Na and 0.46 + 0.30 mg 1-i for K. Subsurface flow volumes were estimated with the aid of the WEPP model (Soto and Diaz-Fierros, 1997). The estimates for each year and the corresponding estimates of nutrient losses, are
listed in Table 3. Note that 77% of nitrogen in subsurface flow was in nitrate form (Soto, 1993), which is of particular importance (despite the fact that subsurface losses of N were smaller than particulate-form surface runoff losses) given that this is the principal form taken up by plants.
3.6. Soluble-form
nutrient losses in su@ace runoff
Surface runoff volumes and corresponding estimates of soluble-form nutrient losses, are listed in Table 4. In the first year, soluble-form losses of all nutrients in surface runoff were higher from
278
B. Soto et al. /The
Science of the Total Environment
the burnt plots than from the control plots. During the second year, soluble-form losses from the burnt plots were in all cases similar to losses from the control plot. Interestingly, soluble-form losses from plot CP2/WF were only slightly higher during the year following the wildfire that affected this plot than during the previous year, despite the high year-2 runoff volume in this plot by comparison with the other plots; indeed, P and K were the only elements for which soluble-form runoff losses in year 2 were markedly higher for plot WF than for the other plots. Nutrient concentrations in runoff from CP plots remained more or less constant over the first year of study. For example, the first runoff event from CP plots accounted for 32.7% of total runoff volume for the first year and similar percentages of total nutrient losses (35.3% for P, 34.5% for Ca, 32.6% for Mg, 33.2% for Na and 32.5% for K); only percentage N loss (14.9%) was markedly different from percentage total runoff. However, losses during this runoff episode from the burnt plots were proportionally higher: runoff during this episode accounted for 41.2% (BPl) and 25.3% (BP2) of the first-year total volumes, but percentages of the year’s total soluble-form losses were 65.0 and 55.6%, respectively (BP1 and BP2) for Ca, 87.9 and 84.6%, respectively for Mg, 80.9 and 77.9%, respectively for Na and 83.9 and 82.2%, respectively for K. By contrast, soluble-form losses during the second years first runoff event represented, for all nutrients and all plots except WF, a similar percentage of the years total (approx. 10%) as was represented by that events runoff volume. In plot WF, soluble-form losses during the second year’s first runoff event were high with respect to runoff volume, except for N (i.e. the pattern observed in the BP plots in the first year after burning was repeated); specifically, runoff during this first event represented 9.6% of the year’s total, while N loss represented only 5.0% of the year’s total loss and other nutrient losses ranged from 21.8% (Ca) to 29.7% (K) of the year’s total. 3.7. Particulate-form
nutrient losses in sur$ace runoff
In general, nutrient
concentrations
in sediment
204 (1997) Z-281
differed little between the burnt plots and the control plot, though total losses from the burnt plots were higher because of the greater erosion losses (Table 4). Nutrient concentrations in sediment from individual plots differed little over the study period. Losses of P and K from the control plot were chiefly in particulate form, while losses of Na, Mg and Ca were chiefly in soluble form. In the case of N, soluble-form losses were the same or only slightly higher than particulate-form losses. Considering the BP plots, losses of P, K and N were chiefly in particulate form, while losses of Na were chiefly in soluble form and losses of Mg and Ca were about equally distributed between the soluble and particulate forms. Considering the WF plot, particulate-form losses were in all cases greater than soluble-form losses, which can be attributed to the intense erosion from this plot. 4. Discussion
The amount of nutrients lost to the atmosphere during wildfires varies depending on the fire’s intensity and the characteristics of the vegetation (species, stage in the season etc.) (Carreira and Niell, 1995). This explains why reports in the literature have varied so widely. For example, Evans and Allen (1971) observed that 57% of the N content of Calluna vulgaris was lost to the atmosphere during laboratory burning at 700°C whereas losses of other cations ranged between 10 and 20%. Likewise on the basis of laboratory trials, Arianotsou and Margaris (1981) found that 96% of the N content of above-ground biomass was lost to the atmosphere, while losses for the remaining elements ranged between 36% (Ca) and 59% (Mg), except for P, losses of which were close to zero. Gillon and Rapp (1989) found that controlled burning of the Quercus coccifera understorey of a Pinus halepensis wood led to a 77% loss of biomass; N loss was likewise 77%, while K loss was 54%, P loss 35% and Ca loss negligible. In the present study, estimated N losses to the atmosphere were 57% (plot BPl) and 70% (plot BP2). These values are similar to those reported by Evans and Allen (1971) and Gillon and Rapp
B. Soto et al. /The
Science of the Total Environment
204 (1997) 271-281
279
I
(a>
First
year
after
fire
1
Ca
Na
1
0
-6 -7 I
(b)
Second N
P
Ca
year
after
fire Na
4
2
0
-2 E Is -4
-6
-8
-10
Fig. 2. Nutrient balance in the control plot CPl and the. burnt plots BP1 and BP2, in the first (Fig. 2a> and second (Fig. 2b) years after burning. Nutrient balance (mg mm’) IS estimated as Throughfall input - Soluble-form runoff output - Soluble-form subsurface output ~ Particulate-form runoff output (see Table 4).
280
B. Soto et al. /The
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(1989), despite the fact that our burns were only of moderate intensity; this is probably attributable to the fact that most nitrogen in gorse scrub is located in the litter layer and in Alex shoots, which are almost totally destroyed even by fires of moderate intensity. Losses to the atmosphere of the other elements were similar those observed for N, except for Na and K, losses of which were slightly lower. For all elements considered, the percentage loss of elements to the atmosphere exceeds the percentage loss of biomass. Our estimates of throughfall inputs of P, N and Na in the control plot are similar to those reported in a previous study of Egunjobi (1971a,b) of a 7.5-year-old Ulex scrub with biomass of approx. 5910 g m-* and annual throughfall of approx. 565 mm. Egunjobi’s estimate of mean throughfall input of K (5.6 g m-‘) was somewhat higher than ours (2.26-2.65 g m-‘), while his estimate for Ca (1.4 g rn-’ ) was somewhat lower than ours (2.23-2.62 g m-‘). Our results as regards nutrient balance (i.e. input in throughfall minus outputs in surface runoff and percolation) are summarized in Fig. 2a (year 1) and Fig. 2b (year 2). In the control plot in year 1, there was a net gain of all elements except P, for which input was about the same as output. In the control plot in year 2, there was a net loss of N and K (attributable to particulate-form losses in runoff) and a net gain of Na; in the case of the remaining elements, input was about the same as output. In both years of study, the burnt plots showed a net loss of all elements except Ca. The net losses of N and K in year 2 were considerably greater than those from the control plot, which is attributable to the higher particulate-form losses in surface runoff from the BP plots (see Table 4). In the WF plot, nutrient inputs with throughfall were not determined; however, net losses from this plot were undoubtedly much higher than from the other burnt plots, in view of the very high particulate-form losses in surface runoff (see Table 4). Comparison of nutrient losses (a) to the atmosphere during burning and (b) over the subsequent two years in runoff and subsurface flow
204 (1997) 271-281
indicates that N and P losses from the BP plots were largely immediate losses to the atmosphere. Sodium losses were largely long-term losses in runoff and subsurface flow. The remaining elements showed similar losses by the two routes; this is probably attributable to the high erosion losses during year 2. 5. Conclusions
In the present study, burning of Ulex matorral led to major losses of phytomass nutrients to the atmosphere (whether in gaseous or particulate form). Nitrogen losses were particularly large, as expected given the high concentrations of this element in plant tissues. Nutrient export in surface runoff and subsurface flow, both in soluble and particulate form, was also increased after burning. In general, the relative importances of soluble and particulate losses can be expected to depend on the hydrological regime at the site in question. The post-burning increases in export of N, P and K observed in the present study were largely due to increases in particulate-form export. Note, however, that the single most important nutrient-export events were those occurring as a result of the first rain after burning, due to leaching of soluble nutrients in ashes. Nutrient inputs in throughfall over the 2 years following burning were likewise reduced, due to the reduced capacity for entrapment of airborne particles. In our study area, Na input is largely in rainwater and as a result total input was not so severely affected by burning. As a result of the increased export and reduced import, the burnt plots showed clear net losses of all nutrients (except Ca) both in absolute terms and with respect to the control plot (Fig. 2). Note that Fig. 2 does not include losses to the atmosphere during burning; if these losses are taken into account, the total loss of nutrients is clearly considerable. However, it should be stressed that net N loss may not be as severe as is suggested by our results, since we did not take into account the capacity of UZex europaeus (a leguminous) to fix this element from the atmosphere.
B. Soto et al. /The
Science of the Total Environment
Acknowledgements
This research was supported by the Commission of the European Communities DGXII [contract EV4V-0103(E)]. References Arianotsou M, Margaris NS. Fire-induced nutrient losses in a phryganic (East Mediterranean) ecosystem. Int J Biometeorol 1981;25:341-347. Basanta M, Vizcaino E, Casal M. Structure off shrubland communities in Galicia (NW Spain). In: During HJ, Werger MJ, Willems JH, editors. Diversity and pattern in plant communities. The Hague: SPB Academic Publishing, 1988:25-36. Bray RH, Kurtz LT. Determination of total, organic and available forms of phosphorus in soil. Soil Sci 1945;59:39-45. Calder IR. The influence of land use on water yield in upland areas of the U.K.. J Hydrol 1986;88:201-211. Carreira JA, Niell FX. Mobilization of nutrients by fire in a semiarid gorse-shrubland ecosystem of Southern Spain. Arid Soil Res Rehabil 1995;9:73-89. Cerda A, Imeson AC, Calvo A. Fire and aspect differences on the erodibility and hydrology of soils at La Costera Valencia, southeast Spain. Catena 1995;24:289-304. Christensen NL. Fire and the nitrogen cycle in California chaparral. Science 1973;181:66-68. Cornish PM, Binns D. Streamwater quality following logging and wildfire in a dry sclerophyll forest in southeastern Australia. For Ecol Manage 1987;22:1-28. Davis EA. Prescribed fire in Arizona chaparral: effects on stream water quality. For Ecol Manage 1989;26:189-206. Diaz-Fierros F, Benito E, Vega JA, Castelao A, Soto B, Perez R, Taboada T. Solute loss and soil erosion in burnt soil from Galicia (NW Spain). In: Goldammer JG, Jenkins M, editors. Fire and ecosystem dynamics. The Hague: SPB Academic Publishing, 1990:103-116. Egunjobi JK. Ecosystem processes in a stand of Ulex europaeus L. I. Dry matter production, litter fall and efficiency of solar energy utilization. J Ecol 1971a;59:31-38. Egunjobi JK. Ecosystem processes in a stand of Ulex europaeus L. II. The cycling of chemical elements in the ecosystem. J Ecol 1971b;59:669-678. Evans CC, Allen SE. Nutrient losses in smoke produced during heather burning. Oikos 1971;22:149-154. Feller MC. Relationships between fuel properties and slashburning-induced nutrient losses. For Sci 1988;34:998-1015. Flanagan DC, Nearing MA, editors. USDA-water erosion prediction project. Technical documentation. NSERL Report No. 10. West Lafayette, Indiana: USDA-ARS NSERL, 1995:285.
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Forgeard F, Frenot Y. Suivi de quelques caracteristiques physico-chimiques dun sol de lande A Ulex europaeus aprCs un incendie de printemps. Rev Ecol Biol Sol 1987;24:715-728. Gillon D, Rapp M. Nutrient losses during a winter-low-intensity prescribed fire in a Mediterranean forest. Plant Soil 1989;120:69-77. Guitian F, Carballas T. Tecnicas de analisis de suelos. In: Pica Sacro, editor. Santiago de Compostela, Spain, 1976:288. Helvey JD, Tiedemann AR, Anderson TD. Plant nutrient losses by soil erosion and mass movement after wildfire. J Soil Water Conserv 1985;40:168-173. Keeley JE. Distribution of lightning-and man-caused wildfires in California. In Proceedings of the Symposium on Dynamics and Management of Mediterranean-type Ecosystems. USDA For Serv Gen Tech Rep PSW-58, 1982:431-437. Lavabre J, Sempere D, Cernesson F. Changes in the hydrological response of a small Mediterranean basin a year after a wildfire. J Hydrol 1993;142:273-299. Pereiras J, Casal M. Vegetation cover evolution in experimental plots after controlled burning. Proceedings Second International Conference Forest Fire Research. Vol. II, D. 13. Coimbra, 1994:923-935 Raison RJ. Modification of the soil environment by vegetation fires, with particular reference to nitrogen transformations: a review. Plant Soil 1979;51:73-108. Schindler DW, Newburg RW, Beaty KG, Prokopowich J, Ruszczynski T, Dalton JA. Effects of a windstorm and forest fire on chemical losses from forested watersheds and on the quality of receiving streams. Can J Fish Aquat Sci 1980;37:328-334. Soto B. Efectos de 10s incendios forestales en la fertilidad y erosionabilidad de 10s suelos de Galicia. Ph.D. Dissertation. University of Santiago de Compostela, Spain, 1993:340. Soto B, Diaz-Fierros F. Interactions between plant ash leachates and soil. Int J Wildland Fire 1993;3:207-216. Soto B, Diaz-Fierros F. Soil water balance as affected by throughfall in gorse (Ulex europeus L.) shrubland after burning. J Hydrol, 1997 (in press). Soto B, Diaz-Fierros F. Runoff and soil erosion from areas of burnt scrub: Comparison of experimental results with those predicted by the WEPP model. Catena, 1997 (sent to publication). Trabaud L. Postfire plant community dynamics in the Mediteranean basin. In: Moreno JM, Oechel WC, editors. The role of fire in Mediterranean-type ecosystems. New York: Springer-Verlag, 1994:1-15. USDA. Soil taxonomy. A basic system of soil classification for making and interpreting soil surveys. Agric. Handbook 436. Washington DC: US Government Printing Office, 1975:754. Woodmansee RG, Wallach LS. Effects of fire regimes on biogeochemical cycles. Ecol Bull 1981;33:649-669.
The Science
of the Total
Environment
204 (1997)
271-281
Effects of burning on nutrient balance in an area of gorse ( Ulex euvopaeus L.) scrub B. Soto”, R. Basanta, F. Diaz-Fierros Departamento
de Edafoloxia,
Facultad
de Farmacia,
Received
Universidad
18 April
de Santiago,
1997; accepted
15 706 Santiago
de Compostela,
Spain
19 June 1997
Abstract
Wildfires affect nutrient balanceasa result of combustionof biomass,increasedsurfaceand subsurfacerunoff and increasedsoil erosion.In the present study, nutrient inputs and outputs to burnt and unburnt Ulex scrubplots were monitored over a 2-year period. During burning, between 50 and 75% of the nutrients contained in above-ground plant tissueswere directly lost due to volatilization and upward movement of particulates to the atmosphere.Only small amounts(lessthan 3% for all elements)were depositedat the soil surfaceas ash.During the first rains after burning, N, P and K losseswere largely due to sedimenttransport in surface runoff, while Ca and Mg losseswere roughly equally distributed between sedimentlossesand soluble-formlosses(in surfacerunoff and subsurfaceflow) and Na losseswere largely in solubleform. Post-burningnutrient inputs to the soil in throughfall were lower than in the control plots for N and K; in the caseof the remainingelements(P, Ca, Mg and Na), inputs to the burnt plots and control plots differed little. In general, burning led to clear net lossesof nutrients; annual losseswere approximately 2.5-3.5 g m-’ in the caseof N and approximately 6.5-9.0 g m-* in the caseof K. In the unburnt plots, by contrast, outputs were approximately equal to inputs. 0 1997Elsevier ScienceB.V. Keywords:
Ecosystemdegradation;Burning; Plant nutrients; Air pollution; Water pollution
1. Introduction
During wildfires large amounts of the nutrients contained in the vegetation are released to the
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*Corresponding author. +34 81 594912.
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atmosphere in gaseous or particulate form and the remainder are deposited at the soil surface as ash (Woodmansee and Wallach, 1981; Feller, 1988). Furthermore, nutrients are lost after the fire as a result of leaching and particulate transport; such processes may affect not only vegetation ash, but also nutrients present in the soil (as a result of increased erosion) (Helvey et reserved.
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al., 1985; Cornish and Binns, 1987; Lavabre et al., 1993; Cerda et al., 1995). Under some circumstances, wildfires may thus provoke massive nutrient inputs to watercourses (Schindler et al., 1980; Davis, 1989). In addition, nutrient input to the soil in the months and years following a fire is often reduced, as a result of the destruction of vegetation cover and consequent effects on atmospheric deposition. In view of these various effects, wildfires may thus have a very considerable impact on soil nutrient balance and biogeochemical cycles. Fire is a frequent event in many scrub ecosystems, including the kermes scruboak (Quercus cocciferu) garrigue, tree heather (Etica arborea) maquis and broom (Calycotome spinosa) maquis of Mediterranean regions (Trabaud, 1994), the chaparral of California (Keeley, 1982) and the gorse scrub of northwest Spain (Basanta et al., 1988). The high frequency of fire in ecosystems of this type (fire recurs every lo-20 years) leads to generalized degradation and a gradual decline in the system’s capacity to recover. Studies of the different processes by which nutrients may enter and leave such ecosystems and of the effects of fire on such processes, are of fundamental importance for understanding the effect of fire in environments of this type and will facilitate assessment of the risk of contamination due to nutrient losses to the atmosphere or watercourses. In the work reported here we monitored nutrient losses from Ulex scrub plots during burning and subsequent effects on nutrient input and output. 2. Materials and methods
2.1. Study area The study was performed within an area of UIex scrub of approx. 2000 m2 in area, with mean slope 30% and altitude approximately 350 m, on the west slope of Monte Pedroso in Santiago de Compostela (northwest Spain). The bedrock is granite. The soil is a lithic haplumbret on the soil taxonomy classification (USDA, 1975) mean organic C content is 67 g kg-i, mean N content 6.8 g kg-‘, mean cation exchange capacity 28.4 meq
per 100 g; texture is sandy loam. The vegetation is 8-year-old gorse shrubland, with a biomass of approximately 4294 g rnp2, approx. 90% of this as gorse (UZ~Xeuropaeus); other common species are bell-heather (Utica cinerea), dorset heath (Etica umbelhta), St. DabeoCs heath (Daboecia cantabrica), heather (CuZluna uulguris) and rockrose (HaZimium alyssoides); mean height of the vegetation is approximately 1.2 m (Basanta et al., 1988). 2.2. Plots description Four 20 X 4 m plots were marked out within the study area and delimited with metal sheeting. Each plot had a collector for runoff at its lower end; this collector fed to a sedimentation tank, allowing determination of sediment losses. Each sedimentation tank in turn fed via a l/9 divider to a runoff collection tank. Throughfall was measured with the aid of two 0.44-m’ (4 x 0.11 m) channels in each plot, both channels feeding to a collection tank outside the plot. Subsurface flow samples were collected with the aid of a 3 x 0.15m drainpipe buried at the downslope end of each plot (see Fig. 1); note that these samples were used for evaluating the chemical composition of subsurface flow, but that subsurface flow volumes were estimated with the aid of the WEPP model (see below). Precipitation was measured with two gauges located close to the plots. 2.3. Experimental
bums
In September 1988, two of the four plots were subjected to controlled burns. Plot BP1 was burnt in the morning, with wind velocity 2.55 m s-l, air temperature 18.3”C and 60% relative humidity. Plot BP2 was burnt in the early evening, with wind velocity 1.56 m s-l, air temperature 21.o”C and 48% relative humidity. During burning, a total of 21 circular trays (139 cm2 each) were distributed at random within each plot for collection of ashes. The other two plots (CPl and CP2) were maintained as controls; in September 1989, however, one of these plots (CP2) was affected by a wildfire and is thus referred to as the wildfire plot (WF) from that time on.
B. Soto et al. /The Science of the Total Environment 204 (1997) 271-281
213
eis
Drainpipe
Fig. 1. Schematic surface.
drawing
of the system
for collection
of subsurface
Additional plots were used for estimation of phytomass and related data (see Table 1) and determination of soil properties (see Table 2). 2.4. Estimation of runofl throughfall and subsueace JEow Rainfall intensity in several rainfall events (one in the first year, three in the second year) was so high that it was not possible to accurately determine runoff and throughfall volumes. In these cases, runoff was predicted with the aid of the WEPP model (Flanagan and Nearing, 1995) and throughfall with the aid of Calder’s model (Calder, 1986). Both models have been validated for these plots (Soto and Diaz-Fierros, 1997; Soto and Diaz-Fierros, 1997). Subsurface flow, Iwhich was not measured, was predicted with the aid of the WEPP model; note that subsurface flow includes subsurface lateral flow and percolation out of the root zone. 2.5. Chemical analysisof water, sedimentsand soils Runoff and throughfall water sampleswere collected after each rainfall event. Subsurface flow samples were collected 28 times during the study period. All water sampleswere maintained at 4°C
flow. The
top of the plastic
sheet was 25 cm below
the soil
and filtered (pore size 0.45 pm) before analysis. Sodium and K contents were determined by flame emission spectroscopy, Ca and Mg contents by atomic absorption spectroscopy, P content by the molybdate blue method and N content by titration following Kjeldahl distillation. Sediments lost from each plot were collected six times in the course of the study (4, 7, 10, 15, 18 and 21 months after burning). All samples were oven-dried at 60°C before analysis. Nitrogen was determined as per Guitian and Carballas (1976). Ca, Mg, Na, K and P were determined after digestion of the sample with a mixture of perchloric, hydrochloric and hydrofluoric acids in a sand bath at 200°C. Calcium and Mg were determined by atomic absorption spectroscopy, Na and K by flame emission spectroscopy and P by the molybdate blue method. The nutrients amount in runoff water (volume of water by concentration) represents the losses in soluble-form and the nutrients amount in sediments (mass of soil removed by concentration) represents the lossesin particulate-form. Soil analyses were as follows. Nitrogen was determined as in sediments. Plant-available cations (Ca, Mg, Na and K) were extracted with ammonium acetate (pH 7) and determined as in sediments. Available phosphorus was determined
274
B. Soto et al. / The Science of the Total Environment
Table 1 Total phytomass in all cases)
and nutrient
mass of major
components
-
of the vegetation,
Total phytomass
204 (1997)
before
271-281
and inmediately
after burning
(units
are g m-*
N
P
Ca
Mg
Na
K
Pre-fire
Ulex europaeus Vlex europaeus Ericaceae Other species Litter Total
Shoots Stems
1398.4 2448.7 309.1 137.4 1060.1 5353.7
16.46 12.72 0.72 1.23 16.88 48.01
0.70 0.48 0.05 0.05 0.42 1.70
3.24 2.48 0.20 0.24 6.28 12.44
2.23 1.44 0.11 0.16 2.36 6.30
1.28 1.24 0.06 0.10 0.32 3.00
6.49 7.08 0.42 0.55 1.10 15.64
Post-fire BP1
Ulex europaeus Other species Litter Ash Total
Stems
2373.9 43.5 467.3 8.4 2893.1
12.33 0.39 7.44 0.10 20.26
0.47 0.02 0.19 0.05 0.73
2.40 0.07 2.77 0.19 5.43
1.40 0.05 1.04 0.11 2.60
1.20 0.03 0.14 0.21 1.58
6.87 0.17 0.48 0.21 7.73
Post-fire BP2
Ulex europaew Other species Litter Ash Total
Stems
1888.1 107.8 109.5 8.7 2114.1
9.81 0.97 1.74 0.10 12.62
0.37 0.04 0.04 0.05 0.50
1.91 0.19 0.65 0.19 2.94
1.11 0.13 0.25 0.11 1.60
0.95 0.08 0.03 0.22 1.28
5.46 0.43 0.11 0.21 6.21
Percentage BP1 BP2
reduction
in total
after fire 46.0 60.5
by the Bray-2 method (Bray and Kurtz, 1945). Soil pH was determined in water and 0.1 N KC1 (ratio soil:solution, 1:2.5). 3. Results
3.1. Nutrient losses to the atmosphere Nutrient losses to the atmosphere (estimated as the difference between the nutrient content of the vegetation and litter before burning and that of the vegetation, litter and ash remaining after burning) are listed for each element (N, P, Ca, Mg, Na and K) and each plot in Table 1. Losses were in the range 47-59% from plot BP1 and Sl-76% from plot BP2; in both cases, percentage loss of K was considerably lower than that of other elements. The highest absolute losses were of N (27.8 g m-’ from BPl, 35.4 g me2 from BP2) and of Ca and K (7-10 g m-‘>.
57.8 73.7
57.1 70.6
56.4 76.4
58.7 74.6
47.3 57.3
50.6 60.3
3.2. Ash deposition
Total ash deposition to the soil surface was 8.4 g me2 in plot BP1 and 8.7 g me2 in plot BP2 (84 and 87 kg ha-r). These values are lower than those reported in previous studies of similar characteristics (loo-10 000 kg ha-‘) (Raison, 1979; Carreira and Niell, 1995), which probably reflects the low intensities of our experimental burns (the soil temperature at S-cm depth during the fire was less than 50°C in BP plots) (Diaz-Fierros et al., 1990). In accordance with this, phytomass (including litter) remaining after the fire was approx. 54% (BPl) and 39% (BP2). The ash was highly alkaline (pH 10.5-11, as determined in a 1:lOOO aqueous solution), implying deposition of 2.66 (BPI) or 2.75 (BP2) meq OH- m-‘. Six months after burning, soil pH in the BP plots had increased with respect to preburn levels by approximately 0.73 units in the O-2.5 cm layer, 0.45 units in the 2.5-5 cm layer
B. Soto et al. /The Science of the Total Environment 204 (1997) 271-281
275
Table 2 Physicochemical properties of the soil of the BP plots, before and 6 months after burning. Carbon and N contents percentages w/w. Other element contents (pa = plant-available) are expressed as mg per 100 g. Each value determination of six samples Pre-fire O-2.5 PH H@ pH KC1 C N Na pa Kpa Ca pa Mg pa Ppa
4.44 3.48 6.74 0.68 5.01 6.81 4.56 1.18 2.75
as of
Post-fire cm
f + + k f f. + * +
are expressed is the mean
0.14 0.19 0.55 0.10 0.75 1.26 1.16 0.37 1.29
2.5-5 4.40 3.51 7.91 0.58 3.57 3.73 1.58 0.13 1.91
cm + + + + f & + * f
0.07 0.06 1.18 0.06 1.30 0.60 0.58 0.10 0.59
5-10
cm
4.41 3.68 7.42 0.54 3.30 2.76 0.62
and 0.23 units in the 5-10 cm layer (Table 2). In the wildfire plot WF, soil pH increased more markedly: 5 months after burning, pH had increased with respect to pre-burn levels by approximately 1.22 units in the O-2.5 cm layer, 0.87 units in the 2.5-5 cm layer and 0.63 units in the 5-10 cm layer. This greater increase in pH in the WF plot is attributable to two factors. First, phytomass remaining after the fire (though not quantified precisely) was clearly much lower than in the BP plots, so that ash deposition was undoubtedly more significant. Second, partial com’bustion of soil organic matter occurred in the WF plot (carbon content in the O-2.5 cm layer dropped by approx. 27%). Mean element contents in ash were 11.6 mg g-l for N, 5.4 mg gg’ for P, 22.2 mg g-l for Ca, 12.6 mg 8-l for Mg, 24.9 mg g-l for Na and 24.7 mg g-l for K. In view of the relatively small amounts of ash deposited (see Table l), nutrient input to the soil surface by this route can be considered of minor importance (between approximately 0.05 g m-2 for P and 0.22 g rnp2 for Na). Nevertheless, soil levels of available cations except Na and P were higher after burning than before (Table 2). The most marked increases were in Ca and Mg levels, all of which remained above pre-burn levels for at least 6 months (Table 2), indeed 18 months in the case of K and Ca (results not shown). Note that the observed increases in cation levels are too high to be explained by ash
+ & f * * + f
0.07 0.07 0.26 0.04 0.88 0.53 0.40
* 0.35
O-2.5
cm
2.5-5
em
5.17 3.88 1.22 0.52 4.53 10.15 14.20 5.45 2.58
k 0.18 + 0.14 + 2.61 f 0.15 + 0.85 &- 1.26 f 4.43 + 1.39 + 1.33
4.85 +_ 0.25 3.89 + 0.16 5.60 + 2.16 0.50 f 0.07 2.51+ 0.68 5.90 + 1.28 3.15 f 1.35 0.97 + 0.39 1.66 + 1.11
5-10 4.64 3.92 7.38 0.54 1.74 4.66 1.48 0.57 1.17
cm k f i + + + + + +
0.19 0.07 1.42 0.05 0.36 0.67 0.67 0.27 0.45
deposition and are probably due to the increased mineralization of soil organic matter resulting from the increased temperatures in the surface layer (due to increased insolation and reduced reflectance as a result of blackening) and the increased soil water content in the months after burning (Raison, 1979). 3.3. Nutrient uptake by the vegetation The concentration of nutrients in the different species was obtained from bibliographical data concerning this type of scrub (Egunjobi, 1971b; Forgeard and Frenot, 1987) the concentration of nutrients in the main species was similar in both studies. Estimates of the amount of nutrients incorporated into the vegetation over the 2 years after burning are listed in Table 3. The total increase in phytomass over the first year was approximately 273 g rnp2, approx. 90% of which was due to Ulex europaeus (Pereiras and Casal, 1994). The increase in phytomass over the second year was much more marked (1608 g me2, approx. 85% due to Ulex europaeus, 5% to Ericaceae and 10% to other taxa). 3.4. Nutrient input in throughfall Nutrient concentrations in throughfall varied considerably among plots and with time of year,
276
B. Soto et al. /The
Table 3 Estimated net annual nutrient uptakes the first and second years after burning Increase biomass
in
(g m-‘>
Science of the Total Environment
to above-ground
N
tissues
204 (1997) 271-281
of lJlex europaeus
and other
species
in the burnt
plots,
P
Ca
W
Na
K
First year vlex Other species Total
245.5 27.3 272.8
2.89 0.25 3.14
0.12 0.01 0.13
0.57 0.05 0.62
0.39 0.03 0.42
0.22 0.02 0.24
1.14 0.11 1.25
Second year ulex Ericaceae Other species Total
1366.6 80.4 160.8 1607.8
16.09 0.76 1.44 18.29
0.69 0.05 0.06 0.80
3.17 0.26 0.28 3.71
2.18 0.16 0.19 2.53
1.25 0.08 0.12 1.45
6.35 0.37 0.64 7.36
ranging from 0.5 to 10 mg 1-i for N, 0.01-0.26 mg 1-l for P, 1.3-10.0 mg 1-l for Ca, 0.3-5.2 mg 1-l for Mg, 1.2-14.0 mg 1-l for Na and 0.2-5.2 mg 1-r for IS. In the control plot, concentrations of most nutrients were highest during the first major rainfall episodes following the summer drought, reflecting lavage of accumulated nutrients (in leaf exudates and/or due to dry deposition from the atmosphere) from above-ground plant structures (Egunjobi, 1971a; Christensen, 1973) in the burnt plot, concentrations of most nutrients were highest during the first rainfall episodes after burning, principally reflecting leaching of ash nutrients from above-ground plant structures. Estimated mean nutrient inputs to the soil surface via throughfall are listed in Table 4. In the burnt plots, N and K inputs over the first year were lower than in the control plot. Calcium input was slightly higher in the burnt plots. Inputs of the other elements differed little between the burnt and control plots. As indicated above, nutrient concentrations in throughfall varied considerably with time. During the first two rainfall episodes of the first year (together accounting for approximately 22.5% of the year’s total throughfall in both the control and burnt plots), throughfall inputs of N to the burnt plots accounted for approximately 50.7% (BP11 and 43.8% (BP2) of the year’s total, while throughfall inputs of Ca to the burnt plots accounted for approximately 64.3% and 41.7% of the year’s total; note, however, that throughfall
in
nutrient inputs to the control plots were also somewhat high in relation to throughfall volume (approximately 30.9% and 27.9% of the annual totals for N and Ca, respectively). Likewise, mean N and Ca inputs to the burnt plots in throughfall collected during October of year 2 accounted for 14.4% and 12.3% of that year’s inputs, respectively (25.7% and 10.9%, respectively in the control plot), despite the fact that throughfall volume during this period accounted for 3.1-3.4% of the year’s total in the BP plots and only 2.6% of the year’s total in the control plots. For all elements measured, total throughfall nutrient input in the control plots was higher during the second year of study than during the first year. The increase in throughfall input was particularly marked in the case of Na. These results are probably attributable to the higher rainfall during the second year (1499 vs. 995 mm); the study area is only approx. 30 km from the coast and Na concentration in rainwater is typically high. Total throughfall nutrient inputs in the burnt plots differed little between the 2 years of study, except in the case of N, input of which was considerably higher in the second year (Table 4).
Nutrient concentrations in subsurface flow samples were low, as expected given the high nutrient (particularly P) retention capacities of this soil (Soto and Diaz-Fierros, 1993). We did
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Table 4 Estimated inputs and outputs of nutrients (g me2 ) to/from the control (CPl) and burnt plots (BPl, BP2 and WF) during the first and second years of study. The outputs considered are soluble-form losses in surface runoff, soluble-form losses in subsurface flow and particulate-form losses in surface runoff. The amount column shows throughfall, runoff and subsurface flow depths in mm and particulate-form losses in surface runoff in g m-* Amount First year (1988/1989) CPl outputs:
N
P
Ca
Mg
Na
K
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
621.9 51.8 190.8 50.0
1.74 0.28 0.39 0.36
0.021 0.005 0.003 0.025
2.23 0.15 0.54 0.20
0.68 0.04 0.20 0.12
4.85 0.31 1.28 0.33
2.26 0.10 0.09 1.54
BP1
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
892.0 82.4 339.7 225.7
0.39 0.41 0.69 1.79
0.014 0.016 0.005 0.166
3.22 0.26 0.96 0.83
0.79 0.29 0.36 0.61
5.02 1.12 2.27 1.73
1.02 0.37 0.16 6.92
BP2
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
913.8 64.1 339.7 219.8
0.34 0.47 0.69 1.85
0.019 0.010 0.005 0.156
3.11 0.25 0.96 0.87
0.72 0.20 0.36 0.55
3.65 0.92 2.27 1.58
0.51 0.42 0.16 6.24
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
1038.9 215.7 612.7 153.8
1.88 0.32 1.25 1.42
0.048 0.003 0.010 0.112
2.62 0.39 1.72 0.52
1.13 0.20 0.65 0.40
8.44 1.30 4.10 0.13
2.65 0.22 0.28 4.22
BP1
Inputs: outputs:
Throughfall Runoff, soluble Subsurface, soluble Runoff, particulate
1325.4 238.4 646.6 247.7
0.84 0.29 1.32 2.52
0.015 0.004 0.010 0.177
2.75 0.39 1.82 0.99
0.74 0.21 0.68 0.67
5.79 1.16 4.32 2.19
0.56 0.18 0.30 7.11
BP2
Inputs: outputs:
Througbfall Runoff, soluble Subsurface, soluble Runoff, particulate
1315.1 219.8 646.6 318.8
0.50 0.29 1.32 2.25
0.024 0.003 0.010 0.180
3.07 0.31 1.82 1.12
0.79 0.18 0.68 0.70
6.27 1.10 4.32 2.33
0.65 0.18 0.30 9.48
WF
outputs:
Runoff, soluble Subsurface, soluble Runoff: particulate
262.9 742.0 1314.0
0.27 1.51 12.92
0.010 0.012 0.988
0.44 2.09 6.24
0.21 0.78 3.58
1.43 4.97 11.16
0.39 0.34 39.12
Second CPI
year (1989/1990)
not determine nutrient concentrations in samples corresponding to all subsurface runoff episodes; however, the variation among the 28 samples taken over the study period was low and there was no evidence of any within-year trend. Mean concentrations were 2.04 + 0.54 mg 1-l for N, 0.016 -t 0.01 mg 1-i for P, 2.81 i 1.67 mg 1-i for Ca, 1.05 _+ 0.15 mg 1-l for Mg, 6.70 + 1.09 mg 1-l for Na and 0.46 + 0.30 mg 1-i for K. Subsurface flow volumes were estimated with the aid of the WEPP model (Soto and Diaz-Fierros, 1997). The estimates for each year and the corresponding estimates of nutrient losses, are
listed in Table 3. Note that 77% of nitrogen in subsurface flow was in nitrate form (Soto, 1993), which is of particular importance (despite the fact that subsurface losses of N were smaller than particulate-form surface runoff losses) given that this is the principal form taken up by plants.
3.6. Soluble-form
nutrient losses in su@ace runoff
Surface runoff volumes and corresponding estimates of soluble-form nutrient losses, are listed in Table 4. In the first year, soluble-form losses of all nutrients in surface runoff were higher from
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the burnt plots than from the control plots. During the second year, soluble-form losses from the burnt plots were in all cases similar to losses from the control plot. Interestingly, soluble-form losses from plot CP2/WF were only slightly higher during the year following the wildfire that affected this plot than during the previous year, despite the high year-2 runoff volume in this plot by comparison with the other plots; indeed, P and K were the only elements for which soluble-form runoff losses in year 2 were markedly higher for plot WF than for the other plots. Nutrient concentrations in runoff from CP plots remained more or less constant over the first year of study. For example, the first runoff event from CP plots accounted for 32.7% of total runoff volume for the first year and similar percentages of total nutrient losses (35.3% for P, 34.5% for Ca, 32.6% for Mg, 33.2% for Na and 32.5% for K); only percentage N loss (14.9%) was markedly different from percentage total runoff. However, losses during this runoff episode from the burnt plots were proportionally higher: runoff during this episode accounted for 41.2% (BPl) and 25.3% (BP2) of the first-year total volumes, but percentages of the year’s total soluble-form losses were 65.0 and 55.6%, respectively (BP1 and BP2) for Ca, 87.9 and 84.6%, respectively for Mg, 80.9 and 77.9%, respectively for Na and 83.9 and 82.2%, respectively for K. By contrast, soluble-form losses during the second years first runoff event represented, for all nutrients and all plots except WF, a similar percentage of the years total (approx. 10%) as was represented by that events runoff volume. In plot WF, soluble-form losses during the second year’s first runoff event were high with respect to runoff volume, except for N (i.e. the pattern observed in the BP plots in the first year after burning was repeated); specifically, runoff during this first event represented 9.6% of the year’s total, while N loss represented only 5.0% of the year’s total loss and other nutrient losses ranged from 21.8% (Ca) to 29.7% (K) of the year’s total. 3.7. Particulate-form
nutrient losses in sur$ace runoff
In general, nutrient
concentrations
in sediment
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differed little between the burnt plots and the control plot, though total losses from the burnt plots were higher because of the greater erosion losses (Table 4). Nutrient concentrations in sediment from individual plots differed little over the study period. Losses of P and K from the control plot were chiefly in particulate form, while losses of Na, Mg and Ca were chiefly in soluble form. In the case of N, soluble-form losses were the same or only slightly higher than particulate-form losses. Considering the BP plots, losses of P, K and N were chiefly in particulate form, while losses of Na were chiefly in soluble form and losses of Mg and Ca were about equally distributed between the soluble and particulate forms. Considering the WF plot, particulate-form losses were in all cases greater than soluble-form losses, which can be attributed to the intense erosion from this plot. 4. Discussion
The amount of nutrients lost to the atmosphere during wildfires varies depending on the fire’s intensity and the characteristics of the vegetation (species, stage in the season etc.) (Carreira and Niell, 1995). This explains why reports in the literature have varied so widely. For example, Evans and Allen (1971) observed that 57% of the N content of Calluna vulgaris was lost to the atmosphere during laboratory burning at 700°C whereas losses of other cations ranged between 10 and 20%. Likewise on the basis of laboratory trials, Arianotsou and Margaris (1981) found that 96% of the N content of above-ground biomass was lost to the atmosphere, while losses for the remaining elements ranged between 36% (Ca) and 59% (Mg), except for P, losses of which were close to zero. Gillon and Rapp (1989) found that controlled burning of the Quercus coccifera understorey of a Pinus halepensis wood led to a 77% loss of biomass; N loss was likewise 77%, while K loss was 54%, P loss 35% and Ca loss negligible. In the present study, estimated N losses to the atmosphere were 57% (plot BPl) and 70% (plot BP2). These values are similar to those reported by Evans and Allen (1971) and Gillon and Rapp
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279
I
(a>
First
year
after
fire
1
Ca
Na
1
0
-6 -7 I
(b)
Second N
P
Ca
year
after
fire Na
4
2
0
-2 E Is -4
-6
-8
-10
Fig. 2. Nutrient balance in the control plot CPl and the. burnt plots BP1 and BP2, in the first (Fig. 2a> and second (Fig. 2b) years after burning. Nutrient balance (mg mm’) IS estimated as Throughfall input - Soluble-form runoff output - Soluble-form subsurface output ~ Particulate-form runoff output (see Table 4).
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(1989), despite the fact that our burns were only of moderate intensity; this is probably attributable to the fact that most nitrogen in gorse scrub is located in the litter layer and in Alex shoots, which are almost totally destroyed even by fires of moderate intensity. Losses to the atmosphere of the other elements were similar those observed for N, except for Na and K, losses of which were slightly lower. For all elements considered, the percentage loss of elements to the atmosphere exceeds the percentage loss of biomass. Our estimates of throughfall inputs of P, N and Na in the control plot are similar to those reported in a previous study of Egunjobi (1971a,b) of a 7.5-year-old Ulex scrub with biomass of approx. 5910 g m-* and annual throughfall of approx. 565 mm. Egunjobi’s estimate of mean throughfall input of K (5.6 g m-‘) was somewhat higher than ours (2.26-2.65 g m-‘), while his estimate for Ca (1.4 g rn-’ ) was somewhat lower than ours (2.23-2.62 g m-‘). Our results as regards nutrient balance (i.e. input in throughfall minus outputs in surface runoff and percolation) are summarized in Fig. 2a (year 1) and Fig. 2b (year 2). In the control plot in year 1, there was a net gain of all elements except P, for which input was about the same as output. In the control plot in year 2, there was a net loss of N and K (attributable to particulate-form losses in runoff) and a net gain of Na; in the case of the remaining elements, input was about the same as output. In both years of study, the burnt plots showed a net loss of all elements except Ca. The net losses of N and K in year 2 were considerably greater than those from the control plot, which is attributable to the higher particulate-form losses in surface runoff from the BP plots (see Table 4). In the WF plot, nutrient inputs with throughfall were not determined; however, net losses from this plot were undoubtedly much higher than from the other burnt plots, in view of the very high particulate-form losses in surface runoff (see Table 4). Comparison of nutrient losses (a) to the atmosphere during burning and (b) over the subsequent two years in runoff and subsurface flow
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indicates that N and P losses from the BP plots were largely immediate losses to the atmosphere. Sodium losses were largely long-term losses in runoff and subsurface flow. The remaining elements showed similar losses by the two routes; this is probably attributable to the high erosion losses during year 2. 5. Conclusions
In the present study, burning of Ulex matorral led to major losses of phytomass nutrients to the atmosphere (whether in gaseous or particulate form). Nitrogen losses were particularly large, as expected given the high concentrations of this element in plant tissues. Nutrient export in surface runoff and subsurface flow, both in soluble and particulate form, was also increased after burning. In general, the relative importances of soluble and particulate losses can be expected to depend on the hydrological regime at the site in question. The post-burning increases in export of N, P and K observed in the present study were largely due to increases in particulate-form export. Note, however, that the single most important nutrient-export events were those occurring as a result of the first rain after burning, due to leaching of soluble nutrients in ashes. Nutrient inputs in throughfall over the 2 years following burning were likewise reduced, due to the reduced capacity for entrapment of airborne particles. In our study area, Na input is largely in rainwater and as a result total input was not so severely affected by burning. As a result of the increased export and reduced import, the burnt plots showed clear net losses of all nutrients (except Ca) both in absolute terms and with respect to the control plot (Fig. 2). Note that Fig. 2 does not include losses to the atmosphere during burning; if these losses are taken into account, the total loss of nutrients is clearly considerable. However, it should be stressed that net N loss may not be as severe as is suggested by our results, since we did not take into account the capacity of UZex europaeus (a leguminous) to fix this element from the atmosphere.
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Acknowledgements
This research was supported by the Commission of the European Communities DGXII [contract EV4V-0103(E)]. References Arianotsou M, Margaris NS. Fire-induced nutrient losses in a phryganic (East Mediterranean) ecosystem. Int J Biometeorol 1981;25:341-347. Basanta M, Vizcaino E, Casal M. Structure off shrubland communities in Galicia (NW Spain). In: During HJ, Werger MJ, Willems JH, editors. Diversity and pattern in plant communities. The Hague: SPB Academic Publishing, 1988:25-36. Bray RH, Kurtz LT. Determination of total, organic and available forms of phosphorus in soil. Soil Sci 1945;59:39-45. Calder IR. The influence of land use on water yield in upland areas of the U.K.. J Hydrol 1986;88:201-211. Carreira JA, Niell FX. Mobilization of nutrients by fire in a semiarid gorse-shrubland ecosystem of Southern Spain. Arid Soil Res Rehabil 1995;9:73-89. Cerda A, Imeson AC, Calvo A. Fire and aspect differences on the erodibility and hydrology of soils at La Costera Valencia, southeast Spain. Catena 1995;24:289-304. Christensen NL. Fire and the nitrogen cycle in California chaparral. Science 1973;181:66-68. Cornish PM, Binns D. Streamwater quality following logging and wildfire in a dry sclerophyll forest in southeastern Australia. For Ecol Manage 1987;22:1-28. Davis EA. Prescribed fire in Arizona chaparral: effects on stream water quality. For Ecol Manage 1989;26:189-206. Diaz-Fierros F, Benito E, Vega JA, Castelao A, Soto B, Perez R, Taboada T. Solute loss and soil erosion in burnt soil from Galicia (NW Spain). In: Goldammer JG, Jenkins M, editors. Fire and ecosystem dynamics. The Hague: SPB Academic Publishing, 1990:103-116. Egunjobi JK. Ecosystem processes in a stand of Ulex europaeus L. I. Dry matter production, litter fall and efficiency of solar energy utilization. J Ecol 1971a;59:31-38. Egunjobi JK. Ecosystem processes in a stand of Ulex europaeus L. II. The cycling of chemical elements in the ecosystem. J Ecol 1971b;59:669-678. Evans CC, Allen SE. Nutrient losses in smoke produced during heather burning. Oikos 1971;22:149-154. Feller MC. Relationships between fuel properties and slashburning-induced nutrient losses. For Sci 1988;34:998-1015. Flanagan DC, Nearing MA, editors. USDA-water erosion prediction project. Technical documentation. NSERL Report No. 10. West Lafayette, Indiana: USDA-ARS NSERL, 1995:285.
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Forgeard F, Frenot Y. Suivi de quelques caracteristiques physico-chimiques dun sol de lande A Ulex europaeus aprCs un incendie de printemps. Rev Ecol Biol Sol 1987;24:715-728. Gillon D, Rapp M. Nutrient losses during a winter-low-intensity prescribed fire in a Mediterranean forest. Plant Soil 1989;120:69-77. Guitian F, Carballas T. Tecnicas de analisis de suelos. In: Pica Sacro, editor. Santiago de Compostela, Spain, 1976:288. Helvey JD, Tiedemann AR, Anderson TD. Plant nutrient losses by soil erosion and mass movement after wildfire. J Soil Water Conserv 1985;40:168-173. Keeley JE. Distribution of lightning-and man-caused wildfires in California. In Proceedings of the Symposium on Dynamics and Management of Mediterranean-type Ecosystems. USDA For Serv Gen Tech Rep PSW-58, 1982:431-437. Lavabre J, Sempere D, Cernesson F. Changes in the hydrological response of a small Mediterranean basin a year after a wildfire. J Hydrol 1993;142:273-299. Pereiras J, Casal M. Vegetation cover evolution in experimental plots after controlled burning. Proceedings Second International Conference Forest Fire Research. Vol. II, D. 13. Coimbra, 1994:923-935 Raison RJ. Modification of the soil environment by vegetation fires, with particular reference to nitrogen transformations: a review. Plant Soil 1979;51:73-108. Schindler DW, Newburg RW, Beaty KG, Prokopowich J, Ruszczynski T, Dalton JA. Effects of a windstorm and forest fire on chemical losses from forested watersheds and on the quality of receiving streams. Can J Fish Aquat Sci 1980;37:328-334. Soto B. Efectos de 10s incendios forestales en la fertilidad y erosionabilidad de 10s suelos de Galicia. Ph.D. Dissertation. University of Santiago de Compostela, Spain, 1993:340. Soto B, Diaz-Fierros F. Interactions between plant ash leachates and soil. Int J Wildland Fire 1993;3:207-216. Soto B, Diaz-Fierros F. Soil water balance as affected by throughfall in gorse (Ulex europeus L.) shrubland after burning. J Hydrol, 1997 (in press). Soto B, Diaz-Fierros F. Runoff and soil erosion from areas of burnt scrub: Comparison of experimental results with those predicted by the WEPP model. Catena, 1997 (sent to publication). Trabaud L. Postfire plant community dynamics in the Mediteranean basin. In: Moreno JM, Oechel WC, editors. The role of fire in Mediterranean-type ecosystems. New York: Springer-Verlag, 1994:1-15. USDA. Soil taxonomy. A basic system of soil classification for making and interpreting soil surveys. Agric. Handbook 436. Washington DC: US Government Printing Office, 1975:754. Woodmansee RG, Wallach LS. Effects of fire regimes on biogeochemical cycles. Ecol Bull 1981;33:649-669.