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a Dep. of Biological Sciences, Univ. of Denver, Denver, CO 80208 USA
b Dep. of Agronomy and Soil Sci., Univ. of Hawaii, Manoa Beaumont Research Center, 461 West Lanikaula St., Hilo, HI 96720 USA
giardina{at}hawaii.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Abbreviations: Pi, inorganic phosphorus • Po, organic phosphorus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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Dry forests represent 
42% of all tropical forest vegetation (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.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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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 high-intensity 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 remained 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 10000 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 two-sample 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 (
and
, respectively). Control forest P data were analyzed with the sample as the experimental unit (
). Bulk density data for the control plot (
) and the treatment plots (
) 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.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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There were no significant differences (
) 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.
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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 fraction was nearly significant (
) and could explain a portion of the discrepancy.
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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 pre-burn 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-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 plant-available 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 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;
). The sizes of these changes were not significantly different from one another (
; paired t-test), suggesting that the increase in plant-available P was largely supplied by the decrease in non-plant-available Po and occluded P.
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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 plant-available 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, 1995b). Heating soil for 10 min at 70°C kills non-spore-forming 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, NH+4 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 soil-heating 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 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.
Management Implications
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 post-burn 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-and-burn management on soil nutrient availability may need to be reevaluated.
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ACKNOWLEDGMENTS |
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Received for publication February 27, 1998.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionREFERENCES |
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