Changes in Soil Phosphorus and Nitrogen During Slash-and-Burn Clearing
of a Dry Tropical Forest
C. P. Giardina,* R. L. Sanford, Jr., and I. C. Døckersmith
ABSTRACT
tation (Murphy and Lugo, 1986), yet relative to moist
forests, the biogeochemistry of dry forests has received
little attention (Jaramillo and Sanford, 1995). Dry forests have been extensively modified by humans and fire,
and continue to be exposed to pressures for land use
change (Murphy and Lugo, 1986; Janzen, 1988; Maass,
1995). The nutrient-rich ash hypothesis has been extrapolated to dry forests (Maass, 1995), but no experimental
investigations have examined soil fertility changes following slash-and-burn disturbance of this globally important forest type. Here we test the hypotheses proposed by Nye and Greenland (1960) at a dry forest site
located on the Pacific coast of Mexico. We examine soil
and aboveground pools of P and N, two nutrients widely
limiting to plant production in the tropics, to test
whether ash is the primary source of P to post-burn
increases in P availability, and whether soil P and N are
relatively immune to the direct effects of heating during
slash-and-burn conversion of dry forest to agriculture.
Slash-and-burn clearing of forest typically results in an increase in
soil nutrient availability. Throughout the tropics, ash from consumed
vegetation has been accepted as the primary nutrient source for this
increase. In contrast, soil heating has been viewed as a secondarily
important mechanism of nutrient release. Through the use of multiple
burn plots and intensive pre-burn and post-burn sampling of mineral
soil, this study quantified changes in total P and N, P fractions, and
KCl-extractable N in soil during the slash-and-burn conversion of a
Mexican dry forest to agriculture. Slash burning resulted in large
transformations of non-plant-available P and N in soil into mineral
forms readily available to plants. Anion-exchange resin, NaHCO3extractable P, and KCl-extractable N in soil increased by 37 kg P ha⫺1
and 82 kg N ha⫺1. Organic and occluded P (sequentially extracted
with NaOH, sonication ⫹ NaOH, and NaOH fusion) and organic N
(total N minus KCl-extractable N) decreased after burning by
25 kg P ha⫺1 and 150 kg N ha⫺1. Immediately after burning, ash
from consumed aboveground biomass contained 11 kg P ha⫺1 and
27 kg N ha⫺1, of which 55 and 74%, respectively, were quickly transported off the site by wind. At this dry forest site, soil heating had a
much larger influence on soil P and N availability than inputs of ash.
MATERIALS AND METHODS
Study Site, Treatments, and Soil Sampling
I
n the tropics, humans have long used slash-and-burn
to clear forested land for agriculture (Nye and
Greenland, 1960; Ewel et al., 1981). Rates of nutrient
loss from slash fires are among the highest of any fires
known (Kauffman et al., 1995), and sustaining site fertility depends on a detailed understanding of the nutrient
fluxes and losses that accompany such fires (Raison,
1979; Raison et al., 1985). Most studies of slash-andburn document increased soil nutrient availability after
burning (Nye and Greenland, 1964; Seubert et al., 1977;
Tiessen et al., 1992; De Rouw, 1994). Nye and Greenland (1960) proposed that this increase is caused by the
transfer of nutrients contained in slash biomass to soil
following conversion of the biomass to nutrient-rich ash.
Nye and Greenland (1960) also proposed that soils are
relatively immune to the direct effects of burning, but
that increases in soil pH, due to incorporation of ash
during rainfall and planting, can have a meliorative effect on soil nutrient availability. Post-burn increases in
soil fertility have been attributed to nutrient-rich ash in
nearly all tropical forest types where slash-and-burn has
been examined (Sanchez et al., 1991; Van Reuler and
Janssen, 1993; De Rouw, 1994; Maass, 1995); however,
few field studies have rigorously tested these ideas
(Ewel et al., 1981).
Dry forests represent ≈42% of all tropical forest vege-
To investigate the effects of slash burning on soil and
aboveground P and N, we selected a dry forest site on a
north–south ridge located 10 km north of the Chamela Biological Station National Autonomous University of Mexico, near
the town of San Mateo, Jalisco, on the Pacific coast of central
Mexico (19⬚31⬘ N, 105⬚06⬘ W). The region of the study is
characterized by steep hilly topography, a pronounced dry
season from November to May, 750 mm of average annual
rainfall (Bullock, 1986), and forests that are among the most
biologically diverse in Mexico (Toledo and Ordonez, 1993).
The soils, isohyperthermic Typic Ustorthents, are shallow
(⬍1 m), sandy loam in texture, and derived from rhyolite.
Slopes for the site range from 0 to 30%. During the growing
season, P is regionally limiting to forest production (Jaramillo
and Sanford, 1995).
Three 1-ha blocks of intact dry forest (⬎100 yr old) were
cleared following local methods. Forest vegetation was cut by
machete and chainsaw in March 1993, allowed to dry in place
for about 2 mo, then broadcast burned from downslope to
upslope. Forest floor and slash were not removed or manipulated before burning. As is common practice in the region,
burning took place at the end of the dry season to maximize
the intensity of the burn, a management objective believed
by local farmers to be important for good crop yield. Before
clearing, each block was divided into three 33 by 100 m plots,
which were randomly assigned low-, medium-, and high-intensity burn treatments. Here we present results from the highintensity burn plots, the treatment most closely approximating
local practices. It was not possible to include an unburned
control treatment in each of the randomized blocks. Therefore, a 100 by 100 m plot of intact forest, immediately adjacent
to treated plots, served as the control. This control plot re-
C.P. Giardina, R.L. Sanford, Jr., and I.C. Døckersmith, Dep. of Biological Sciences, Univ. of Denver, Denver, CO 80208; C.P. Giardina,
Dep. of Agronomy and Soil Sci., Univ. of Hawaii, Manoa Beaumont
Research Center, 461 West Lanikaula St., Hilo, HI 96720. Received
27 Feb. 1998. *Corresponding author ([email protected]).
Published in Soil Sci. Soc. Am. J. 64:399–405 (2000).
Abbreviations: Pi, inorganic phosphorus; Po, organic phosphorus.
399
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SOIL SCI. SOC. AM. J., VOL. 64, JANUARY–FEBRUARY 2000
mained undisturbed through the study period and was sampled
for soils simultaneously with the high-intensity burn plots.
Before burning, soil sample points were marked with metal
stakes. Before soil sampling in April and May 1993 (21 d
before and 1 d after burning), mineral soil surfaces were carefully cleaned of forest floor or ash. Pre- and post-burn soils
for soil P, N, and C analyses were sampled at spatially paired
locations (50 cm apart) from 0- to 2-cm and 2- to 5-cm depths.
Within the unburned control forest plot and each of the three
burned plots, soil sample points were located at four stratified
positions along each of two 100-m transects per plot. The
four stratified sample points were 25 to 30 m apart from one
another and represented four different topographic positions.
To collect soils, 20-cm deep pits were dug with a trowel, and
soils were sampled from the side wall of the pits. Soils were
sieved to 2 mm and moisture content was determined on a
subset of soils following oven drying at 104⬚C for 24 h. All
soils contained ⬍4% moisture at the time of sampling. Samples
were stored at room temperature for up to 1 yr before analysis.
Ash, Soil Temperature, and Bulk Density
Ash was sampled at 27 stratified points 1 d (May 1993)
and 28 d (June 1993) after burning. The second ash sampling
occurred 10 d before the first rain of the season. Sample points
represented the top, middle, and bottom of each of the nine
plots. At each sample point for both sampling dates, ash from
three randomly located, 50- by 50-cm quadrats was collected
with a vacuum cleaner, composited, oven-dried, and weighed.
Nutrient data for the May ash samples are presented by Steele
(1999). For the June sampling, eight randomly selected ash
samples were analyzed for total N by dry combustion on a
Leco 1000 CHN analyzer (Leco, St. Joseph, MI). Following
digestion by NaOH fusion (Smith and Bain, 1982), ash samples
were analyzed for total P on a Lachat Instruments AE Flow
Injection Autoanalyzer (Lachat Instruments, Milwaukee, WI)
according to Lachat Instruments (1992) molybdate–ascorbic
acid QuikChem Method 10-115-01-1-B.
Soil temperatures were measured at 12 topographically
stratified points in the high-intensity burn plots using temperature-sensitive paints on mica sheets with a range from 60 to
812⬚C (one sheet per sampling point, four sheets per plot,
three plots). The mica sheets were placed vertically into the
ground before burning (Fenner and Bentley, 1960). Bulk densities for ⬍2-mm size fraction were determined for 0- to 5-cm
depth soils near these 12 points using a core method (Blake
and Hartge, 1986). Bulk density samples were collected immediately following burning in the three high-intensity burn plots
and in the adjacent control forest plot. Pre-burn bulk density
was assumed to be that of the unburned control forest.
Soil Phosphorus Analyses
Soil samples from the burned treatment plots and unburned
control forest plot were composited within each plot by topographic position. A modified Hedley soil P–fractionation
method (Hedley et al., 1982) was used to separate total soil P
into organic (Po) and inorganic (Pi) fractions. The fractionation
scheme involves (i) extraction of solution Pi with an Ionics
anion-exchange resin (Type 103-QZL-386, Ionics, Boston,
MA); (ii) extraction of readily solubilized Pi and readily mineralized Po with 0.5 M NaHCO3, adjusted to a pH of 8.5; (iii)
extraction of Pi and Po chemisorbed to Fe and Al surfaces in
soil, partially stabilized as soil organic matter, or immobilized
within microorganisms with 0.2 M NaOH; (iv) extraction of
Pi bound to Ca minerals with 1 M HCl; (v) extraction of
residual Pi and Po held by Fe, Al, and Ca minerals within soil
aggregates with 0.2 M NaOH following sonication; and (vi)
extraction of total P remaining in the final pellet by NaOH
fusion (Smith and Bain, 1982). The NaOH fusion method
removes the most stabilized or occluded Pi and Po in soil, but
does not permit separation into Pi and Po.
All soils were fractionated at the same time to ensure that
results were comparable for the two sampling dates. For each
composited sample, 1 g of air-dried soil was placed into a
50-mL centrifuge tube, along with 30 mL of DI water and one
10- by 50-mm anion-exchange resin strip. Resin strips had
been washed five times with 1 M HCl, then loaded with HCO3⫺
during five washes with 0.5 M NaHCO3. The tubes were
capped with rubber stoppers and shaken on a reciprocating
shaker for 16 h. After the 16-h shake, tubes were uncapped
and the exchange resin strip removed with tweezers, rinsed
with DI water to remove any soil that was attached to the
strip, and extracted for 1 h with 1 M HCl on a reciprocating
shaker to remove P from the resin strip. The tubes were then
centrifuged at 10 000 rpm for 15 min, and the DI water decanted. This process was repeated for each extract solution.
Two empty centrifuge tubes were run through the fractionation process as blanks.
Total P (Pi ⫹ Po) in the NaHCO3, HCl, NaOH, and NaOH ⫹
sonication extracts was determined after acidified (H2SO4)
ammonium persulfate digestion (45 min) in an autoclave. For
these fractions, Pi was measured directly on acidified, undigested samples; Po was then calculated by difference (total
P – Pi). The DI water in the first extraction step contained
only background levels of Pi, and no Po was detected in the
HCl fraction. All extracts were appropriately neutralized and
diluted, then analyzed on a Lachat Instruments AE Flow Injection Autoanalyzer according to Lachat Instruments (1992)
molybdate–ascorbic acid QuikChem Method 10-115-01-1-B.
Soil Nitrogen and Carbon Analyses
Soils sampled from two of the three burned plots were
analyzed for total C and N on a Leco 1000 CHN analyzer
following grinding on a ball mill. Soils from the 0- to 2-cm
depth were not composited; soils from the 2- to 5-cm depth
were composited within plots by topographic position.
Statistical Analyses
Pre-burn to post-burn comparisons were made using twosample paired t-tests (Wilkinson, 1991). Pre- to post-burn
changes in soil P fractions and total P, N, and C were analyzed
with the plot as the experimental unit (n ⫽ 3 and n ⫽ 2,
respectively). Control forest P data were analyzed with the
sample as the experimental unit (n ⫽ 4). Bulk density data
for the control plot (n ⫽ 15) and the treatment plots (n ⫽ 18)
were compared using a two-group t-test with pooled variance
estimates and the sample as the experimental unit. All P data
were log transformed to meet variance or normality assumptions, and a 0.05 significance level was used for Type I errors.
RESULTS AND DISCUSSION
Ash, Soil Bulk Density, and Soil Temperature
Ash contained 11.2 kg P ha⫺1 and 27.2 kg N ha⫺1
immediately after burning (Steele, 1999). Ash sampled
28 d later contained 5.3 kg P ha⫺1 and 7.2 kg N ha⫺1,
suggesting that 55% of the P and 74% of the N in ash
had been lost from the site before the first rains of the
growing season. The larger losses of N than P may be
attributed to volatilization losses of ammonia (NH3),
the primary form of mineral N in ash. Large, wind-
GIARDINA ET AL.: SLASH AND BURN CLEARING EFFECTS ON SOIL P AND N
related losses of ash have been observed in dry (Kauffman et al., 1993) and humid forests (Ewel et al., 1981).
Quantities of P and N in ash reported here are comparable with previous studies (Seubert et al., 1977; Ewel et
al., 1981; Kauffman et al., 1993).
There were no significant differences (P ⫽ 0.291) in
soil bulk density between the control forest plot (0.75 g
cm⫺3) and post-burn treatment plots (0.79 g cm⫺3). The
apparent lack of change in bulk density was likely due
to the sandy loam texture and low C content of these
forest soils (generally ⬍4% by weight). Also, high temperatures were limited to the top 1 cm of soil. Maximum
soil temperatures averaged above 500⬚C in the surface
0.5 cm, but 200⬚C at 2 cm, and 100⬚C at 3 cm (Fig. 1).
Changes in bulk density were possible in the surface 1
cm of soil, but the 5-cm long cores used to sample for
bulk density did not permit detection of these potential
changes. A bulk density of 0.79 g cm⫺3 was used to
convert pre- and post-burn soil nutrient concentrations
to an area basis.
Soil Phosphorus
The supply of P to plants is controlled by complex
biological and geochemical processes (Lindsay 1979;
Cross and Schlesinger, 1995). The Hedley P–fractionation method chemically separates soil P into plantavailable and non-plant-available forms (Hedley et al.,
1982; Cross and Schlesinger, 1995), but the extent to
which these fractions index P supply to perennial plants
is not well understood (Gahoonia and Nielsen, 1992).
The P removed by anion-exchange resin and NaHCO3
is viewed as being plant-available (Hedley et al., 1982;
Cross and Schlesinger, 1995). Portions of the Po and Pi
removed during the NaOH–extraction step are of organic matter and microbial origin (Hedley et al., 1982)
and are likely to be plant-available over longer periods
of time. The Po or Pi removed during the HCl, NaOH ⫹
sonication, and NaOH fusion–extraction steps are generally considered to be non-plant-available except over
long time periods (Cross and Schlesinger, 1995). Notably, some evidence indicates that plants can access most
P fractions in soil (Gahoonia and Nielsen, 1992). Here,
we define the anion resin and NaHCO3-extractable P
fractions as plant-available and all other Po and Pi fractions as non-plant-available.
Plant-available P increased by 24.8 kg ha⫺1 in 0- to
2-cm depth soils, while non-plant-available Po and occluded P decreased by 25.3 kg ha⫺1 (Table 1). In 2- to
5-cm depth soils, plant-available P increased significantly by 12.9 kg ha⫺1, but no other changes at this
depth were significant (Table 1). Total amounts of P in
0- to 2-cm depth soils increased significantly after burning by 6.4 kg P ha⫺1 (Table 1), indicating that a portion
of the P contained in the aboveground biomass was
transferred to soil during burning. After accounting for
the 6.4 kg P ha⫺1 increase in total P, a deficit of
≈9 kg P ha⫺1 in the soil P budget (Table 1) suggests
that non-significant declines in 2- to 5-cm depth soil P
fractions may have been real (i.e., Type II statistical
error). The 4.9 kg P ha⫺1 decrease in the NaOH Po
401
fraction was nearly significant (P ⫽ 0.06) and could
explain a portion of the discrepancy.
The significant increase in total P in 0- to 2-cm depth
soils has two interpretations. First, aboveground ash was
included with soils during sampling. This interpretation
is unlikely because soils were carefully cleaned of ash
by blowing, until no visible ash remained. We estimate
that at least 90% of the ash on the soil surface was
removed before sampling. The P content of ash immediately after burning was 11.2 kg P ha⫺1 (Steele, 1999),
indicating that at most, 1 kg P ha⫺1 may have been
included with soils during sampling. We suggest that
the increase in total P was due to the condensation of
volatilized aboveground Po and Pi onto soil surfaces in
the top 2 cm of soil because temperatures 1 cm beneath
the soil surface did not surpass 300⬚C. The formation
of hydrophobic layers in soil has been ascribed to the
downward flux of volatilized organic compounds during
burning (DeBano et al., 1970). This flux can be large
when large quantities of fuel are consumed near the soil
surface, as is the case with slash burning. Inorganic P
is volatilized at temperatures above 774⬚C, while Po is
volatilized at lower temperatures (Raison et al., 1985).
Burning at our site resulted in soil surface temperatures
of more than 812⬚C, and flaming combustion of wood
is known to generate temperatures above 1100⬚C (Raison et al., 1985).
The soil temperatures observed during burning at this
dry forest site were sufficiently high to reduce quantities
of total Po and increase quantities of plant-available Pi
in soil (Giovannini et al., 1990). The effects of burning
on occluded P fractions have not been previously examined; however, the occluded fraction likely contained
stabilized Po that would be thermally mineralized during
combustion of stabilized soil organic matter. Heating
may have also reduced aggregate stability in 0- to 2-cm
depth soils (Giovannini and Lucchesi, 1983; Giovannini
et al., 1988), such that during the fractionation procedure, a portion of the P that had been occluded in preburn soils was released earlier in the fractionation of
post-burn soils. The HCl-extractable Pi fraction increased significantly following burning (Table 1), indicating that burning may have affected aggregate stability. Alternatively, the post-burn increase in HCl Pi may
be due to the higher pH of post-burn soils (7.0 vs. 8.3;
Døckersmith et al., 1999). Increased soil pH would increase the affinity of Ca2⫹ for P and the potential for
precipitation of Ca phosphate minerals during the fractionation procedure. These precipitation products
would be removed during extraction with HCl. Because
post-burn increases in the HCl Pi fraction were small
(3.6 kg P ha⫺1; Table 1), the effects of heating on aggregate stability were likely small.
The interpretation that soil heating was responsible
for the transformation of soil P is supported by several
observations. First, P fractions in soil sampled from the
adjacent, unburned control forest did not change between sampling events, ruling out seasonally related
explanations (Table 1). Second, pre-burn soil pH for
our site (7.0; Døckersmith et al., 1999) was near optimal
for soil P availability (Lindsay, 1979), ruling out a pH-
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SOIL SCI. SOC. AM. J., VOL. 64, JANUARY–FEBRUARY 2000
Table 1. Pools and changes in mineral soil P fractions resulting from slash burning in dry tropical forest. n ⫽ 4 samples for the unburned
control forest and n ⫽ 3 plots for the burned treatment.
0–2 cm soil depth
Unburned forest
P fraction
Resin Pi
Bicarbonate Pi
NaOH Pi
HCl Pi
Residual Pi
Bicarbonate Po
NaOH Po
Residual Po
Occluded P
Total P
Sampling event
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
mean
2.9
3.5
0.6
0.9
1.4
0.5
2.9
3.2
0.3
4.2
3.4
⫺0.7
2.5
2.6
0.1
3.7
3.4
⫺0.3
17.8
18.4
0.5
7.7
8.7
1.1
47.4
49.7
2.3
90.0
94.3
4.3
2–5 cm soil depth
Burned plots
mean
2.7
19.3
16.5**
1.5
9.2
7.7**
3.7
6.5
2.7
2.9
6.5
3.6**
2.5
3.0
0.5
2.9
3.5
0.6
20.7
10.6
⫺10.1**
11.4
6.4
⫺5.0*
50.1
39.9
⫺10.2**
98.3
104.7
6.4*
Unburned forest
SE
kg P
0.0
1.0
0.9
0.1
0.5
0.5
0.5
0.5
0.7
0.5
0.7
0.3
0.3
0.3
0.4
0.1
0.3
0.4
0.8
0.4
0.6
0.7
0.9
0.4
1.6
1.5
0.5
3.7
5.0
1.3
Burned plots
mean
mean
SE
3.0
2.8
⫺0.3
1.0
0.7
⫺0.3
3.7
3.7
⫺0.1
4.5
3.7
⫺0.8
3.6
3.6
0.1
4.2
4.6
0.4
20.2
19.9
⫺0.4
11.8
10.8
⫺1.0
71.2
66.8
⫺4.4
123.3
116.5
⫺6.8
2.5
10.5
8.0*
1.2
4.4
3.2**
4.6
5.2
0.6
3.9
5.7
1.8
3.8
4.0
0.2
3.9
5.5
1.7**
27.5
22.6
⫺4.9
18.6
15.4
⫺3.3
69.6
67.6
⫺1.9
135.6
140.9
5.3
0.1
2.2
2.3
0.1
0.7
0.6
0.4
0.6
0.3
0.7
0.5
0.7
0.5
0.4
0.2
0.2
0.1
0.1
1.6
0.3
1.5
2.3
0.7
1.7
3.7
2.6
1.2
7.3
5.9
1.9
ha⫺1
*, **, *** Change significant at the 0.05, 0.01, and 0.001 probability levels, respectively.
based explanation for increased P availability. In fact,
the post-burn pH increase of 1.3 units (Døckersmith et
al., 1999) likely reduced the size of the increase in plantavailable P because Ca2⫹ affinity for P increases in this
pH range (Lindsay, 1979). Notably, pH-related effects
on P availability at our site contrast with those encountered at humid sites, where pre-burn soil pH is often
acidic and suboptimal for P availability, and where any
post-burn increase in soil pH would increase P availability (Sanchez, 1976). Finally, plant-available P in 0- to
5-cm depth soils increased significantly after burning by
Fig. 1. Soil depth vs. temperature in the top 6 cm of soil during slashand-burn conversion of a tropical dry forest to agriculture. Standard
error bars represent variation in depth to which soils were heated
and recorded by six temperature-sensitive paints (n ⫽ 12).
38 kg P ha⫺1 (Fig. 2; P ⬍ 0.01), while non-plant-available
Po and occluded P in 0- to 5-cm depth soils declined
significantly after burning by 35 kg P ha⫺1 (Fig. 2; P ⫽
0.01). The sizes of these changes were not significantly
different from one another (P ⫽ 0.62; paired t-test),
Fig. 2. Comparison of pre-burn and post-burn pools of plant-available P (resin P, NaHCO3 P) and non-plant-available organic
(NaOH P, NaOH ⫹ sonication P) plus occluded P in 0- to 5-cm
depth soils. Standard error bars were based on n ⫽ 3 plots for
burned treatment and n ⫽ 4 samples for the unburned control
forest.
403
GIARDINA ET AL.: SLASH AND BURN CLEARING EFFECTS ON SOIL P AND N
suggesting that the increase in plant-available P was
largely supplied by the decrease in non-plant-available
Po and occluded P.
Table 2. Pools and changes in mineral soil total C and N resulting
from slash burning in dry tropical forest. n ⫽ 2 plots for the
burned treatment.
Soil Nitrogen
During the burn, total N in 0- to 2-cm depth soils
decreased by 68 kg ha⫺1 (Table 2). At this site, KClextractable N in the top 10 cm of soil increased significantly, from 56 to 138 kg ha⫺1 (Døckersmith et al., 1999).
The loss of total N and the increase in mineral N suggest
that about 150 kg ha⫺1 of non-plant-available N were
transformed by heat, of which 82 kg ha⫺1 supplied the
increase in mineral N and 68 kg N ha⫺1 were lost from
the soil. The effect of slash burning on the long-term
soil N supply are typically negative because of very
large aboveground losses (Nye and Greenland, 1960;
Kauffman et al., 1993, 1995). At our site, the negative
effects of burning on N supply may have been partially
offset in the short term by the thermal release of N
from soil organic matter.
Heating Effects
At our dry forest site, the quantity of non-plant-available Po and occluded P that was thermally transformed
into plant-available forms (35 kg P ha⫺1; Fig. 2) was
larger than the total quantity of P contained in pre-burn
slash biomass and the forest floor (27 kg P ha⫺1; Steele,
1999). All but 3.1 kg of the 27 kg of total aboveground
biomass P were released by burning (Steele, 1999), of
which 11.2 kg were recovered in ash and 6.4 kg were
transferred belowground. Therefore, approximately
7.6 kg of biomass P ha⫺1 were lost from the site. Of the
11.2 kg P ha⫺1 contained in ash, 55% was lost from
the site in the 28 rainless days following burning. By
comparison, very little P in soil appears to have been
lost during the burn (Table 1) or in the 28 d following
the burn (Giardina, 1999). The quantity of non-plantavailable soil N that was transformed by heat into plantavailable forms was much smaller than the 943 kg N
ha⫺1 contained in total aboveground biomass (Steele,
1999). However, of the 794 kg N ha⫺1 released by fire,
⬎95% was lost from the site during the burn (Steele,
1999). Of the 27.2 kg N ha⫺1 contained in ash 1 d after
burning (Steele, 1999), only 7.2 kg N ha⫺1 remained on
the soil surface by the second ash sampling in June. The
quantity of soil N that was thermally transformed from
non-plant-available forms into plant-available forms
was much larger than the quantity of N contained in
either the May or June ash samples.
Our findings are supported by several laboratory
studies demonstrating that heat alone can profoundly
affect the availability of P and N in soil (Sertsu and
Sanchez, 1978; Kang and Sajjapongse, 1980; Andriesse
and Koopmans, 1984; DeBano and Klopatek, 1988; Sibanda and Young, 1989; Giovannini et al., 1990; Serrasolsas and Khanna, 1995a, 1995b). Increases in plantavailable P and N, and reductions in Po following soil
heating can be attributed, in part, to the heat-induced
death of soil microbial populations and the release of
microbial nutrients (Serrasolsas and Khanna, 1995a,
Measure
Total C
Total N
Sampling event
Pre-burn
Post-burn
Change
Pre-burn
Post-burn
Change
0–2 cm soil depth
2–5 cm soil depth
Burned plots
Burned plots
mean
7476
6008
⫺1468
757
689
⫺68*
SE
mean
SE
kg ha⫺1
106
7090
289
7118
183
28
33
706
35
750
1.4
44
238
492
255
47
72
25
* Change significant at the 0.05 probability level.
1995b). Heating soil for 10 min at 70⬚C kills non-sporeforming fungi, protozoa, and some bacteria, while temperatures above 127⬚C would nearly sterilize soil (Raison, 1979). We did not measure changes in microbial P;
however, soil temperatures measured during our experimental field burns (Fig. 1) were high enough to kill most
microorganisms in the top 3 cm of soil. Døckersmith et
al. (1999) found that net N mineralization rates in 0- to
10-cm depth soils were substantially depressed after
slash burning at our site, indicating that microorganisms
were impacted by the burn.
Between 170 and 300⬚C, soil Po is thermally mineralized with little loss of organic matter (Giovannini et al.,
1990). At temperatures above 300⬚C, organic matter
begins to oxidize with little remaining at temperatures
above 500⬚C (Raison, 1979). Between 300 and 500⬚C,
Po is therefore mineralized during the combustion of
organic matter (Sertsu and Sanchez, 1978; Kang and
Sajjapongse, 1980; Andriesse and Koopmans, 1984; Giovannini et al., 1990). The observed losses of soil Po, the
increases in labile soil Pi, but no net loss of total P from
soil (Table 1) are consistent with the 700⬚C volatilization
temperature for Pi (Raison et al., 1985).
In soils heated above 100⬚C, NH4⫹ levels generally
increase dramatically. These increases are due to the
release of N during the lysis of microbial biomass (Dunn
et al., 1979; DeBano and Klopatek, 1988; Serrasolsas
and Khanna, 1995a, 1995b), the thermal decomposition
of organic matter (Russell et al., 1974; Sertsu and Sanchez, 1978; Raison, 1979; Sibanda and Young, 1989), and
the desiccation of soil minerals (Raison, 1979). Nitrogen
can be lost from soil at temperatures below 100⬚C
through volatilization of NH3, nitric acid, and volatile
organic N compounds. At temperatures above 300⬚C,
soil N is lost as oxidized N gases and N2 during the
combustion of organic N (Raison, 1979). Soil temperatures during our experimental burns were high enough
to cause the observed decrease in total soil N and the
large increase in mineral N.
Our findings agree with results from previous soilheating experiments, but they contradict the classic view
of nutrient cycling during shifting cultivation that has
been generalized to all of the tropics (Nye and Greenland, 1960; Sanchez et al., 1991; Van Reuler and Janssen,
1993; De Rouw, 1994; Maass, 1995). First, the quantities
of P contained in ash cannot explain the large increase in
404
SOIL SCI. SOC. AM. J., VOL. 64, JANUARY–FEBRUARY 2000
soil P availability following burning. Second, the thermal
transformation of non-plant-available soil P and N was
of major, rather than minor, importance to changes in
soil P and N availability. Because quantities of thermally
transformed P and N in soil were much larger than
quantities of total P and N measured in ash, and elevated
levels of plant-available Pi in soil persisted into a second
growing season (Giardina, 1999), we conclude that
heated soil, not burned vegetation, was the primary
source of plant-available P and N supplying post-burn
increases in soil fertility.
and-burn management on soil nutrient availability may
need to be reevaluated.
ACKNOWLEDGMENTS
Financial support was provided by the National Science
Foundation (Grant BSR 91-18854). We thank Mr. Ramiro
Peña for the use of his land and assistance with the conversion.
We thank V. Jaramillo, J. Kauffman, D. Binkley, X. Zou, M.
Bashkin, and F. Garcı́a-Oliva for helpful comments on earlier
versions of this manuscript and S. Huffman and T. Boardman
for valuable technical assistance.
Management Implications
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