Changes in Soil Phosphorus and Nitrogen During Slash-and ... - ENVIS

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.

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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

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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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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.

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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

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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 Pen˜a 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. Garcı´a-Oliva for helpful comments on earlier versions of this manuscript and S. Huffman and T. Boardman for valuable technical assistance.

Management Implications

REFERENCES

Physiognomic differences between dry, moist, and humid forests challenge tendencies to generalize about tropical forests. However, these differences may not be adequately appreciated in the context of slash-and-burn agriculture. Tropical dry forests support considerably less biomass than do humid forests with generally smaller quantities of nutrients in that biomass (Murphy and Lugo, 1986; Kauffman et al., 1993, 1995). Consequently, burning dry forest slash would return smaller quantities of nutrients to soil than would burning humid forest slash. Ash generated during the burning of humid forest slash at a Brazilian site contained more than 80 kg P ha⫺1 (Kauffman et al., 1995). Dry forest slash is generally finer and, contingent on management practices, drier at the time of burning than humid forest slash. As a result, consumption rates can exceed 80% in dry forest with high oxidation, volatilization, and convective losses for N and P (Kauffman et al., 1993; Steele, 1999), further reducing the already small return of aboveground nutrients to soil. Finally, at the time of burning, soils will likely be drier at dry forest sites than humid forest sites (Seubert et al., 1977; Ewel et al., 1981; Kauffman et al., 1993). Because elevated soil moisture can buffer the flux of heat into soil, and slashed sites with long dry seasons will have drier soils than sites with short dry seasons, dry forest soils may be heated to higher temperatures than moist forest soils. Alternatively, soil heating may be an important, but overlooked, mechanism of nutrient release in humid forests. Thermal transformations of soil nutrients have rarely been examined in humid forests. The relative importance of soil heating as a mechanism of nutrient release in humid forests may be substantial if large quantities of forest slash are consumed during burning (Kauffman et al., 1995), if quantities of nutrients contained in ash are small (Seubert et al., 1977), or if postburn losses of ash are large (Ewel et al., 1981). Forests with long dry seasons (e.g., dry deciduous, monsoonal, or semi-deciduous moist forests) represent well over half of all tropical forest cover and support the highest densities of people in the tropics (Murphy and Lugo, 1986). Our results indicate that nutrient transformation due to soil heating may be an important, but underestimated, mechanism of soil P and N release in these agro-ecosystems. Because few field studies in the tropics have adequately assessed the effects of slash burning on soil nutrients, the immediate impact of slash-

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