Changes in soil chemical and microbial properties after a wildfire in a tropical rainforest in Sabah, Malaysia

Soil Biology & Biochemistry 35 (2003) 1071–1078 www.elsevier.com/locate/soilbio

Changes in soil chemical and microbial properties after a wildfire in a tropical rainforest in Sabah, Malaysia U. Ilstedt*, R. Giesler, A. Nordgren, A. Malmer Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83 Umea˚, Sweden Received 15 July 2002; received in revised form 28 January 2003; accepted 14 March 2003

Abstract Changes in soil caused by drought and wildfire in a Dipterocarp rainforest in Sabah, Malaysia were assessed by phosphorus fractionation, extractable nitrogen and nutrient limited respiration kinetics (after addition of glucose þ N or P). Fire increased the concentration of total phosphorus (P) in the litter layer (per ha and per dry soil) by raising the 0.2 M NaOH extractable-P. In the soil organic layer, membrane exchangeable P was reduced by fire while 1.0 M HCl extractable-P, and 0.5 M NaHCO3 extractable-P increased. Microbially available P increased after the fire and was most closely related to NaOH extractable-P that has been considered available to plants only over long timescales. Total nitrogen (N) increased in the litter layer (per ha and per dry soil) due to post-fire litter fall, while the NO2 3 increased up to 10-fold down to the 10 cm mineral soil. In contrast, the microbially available N decreased by 50%. Basal respiration and substrate-induced respiration increased in the litter layer and decreased in the organic horizon (per dry soil and per organic matter). P limited microbial growth resulted in a slow and non-exponential increase in respiration, presumably reflecting the P-fixing nature of the soils, while N limitation resulted in a fast exponential increase. However, higher respiration rates were eventually achieved under P limitation than under N limitation. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Acrisol; Ultisol; Soil quality; Resilience; Fertility; Hedley fractionation

1. Introduction It has only recently been realized that fire plays an important ecological role in shaping natural rainforest ecosystems (Whitmore and Burslem, 1998; Nepstad et al., 1999). However, increased population pressures and logging activities have increased the frequency and severity of wild fires dramatically (Nepstad et al., 1999; Siegert et al., 2001). Most research concerning wildfires in Southeast Asian humid forests have examined vegetation changes, land use or policy issues (Dennis, 1999). There have been few studies of changes in soil caused by wildfires in natural humid forest. Investigations of soil after fire in humid tropical forests have largely considered fires associated with clearing for agriculture (Kwari and Batey, 1991; Sommer et al., 2000), pasture (Luizao et al., 1992; Kauffman et al., 1998) or prescribed burning in forestry (Ellis et al., 1982; Romanya et al., 1994). The effects of wildfires in natural forests, secondary forests, and controlled fire for agricultural * Corresponding author. Tel.: þ 46-90-7865800; fax: þ 46-90-7867750. E-mail address: [email protected] (U. Ilstedt).

purposes can be fundamentally different (Malmer et al., 2004). In a recent study in Malaysia, streams draining areas in secondary vegetation were shown to have substantially higher concentrations of nutrients after wildfire than streams after fire in natural forest (Malmer, 2003). This was mainly explained by the increased openness, greater drought susceptibility and larger amounts of potential fuel on the ground in the secondary vegetation. In combination these factors result in more intense fires, more ashes and reduced nutrient retention due to reductions in cation exchange capacity and plant uptake. In 1998, extensive wildfires swept over Borneo (Siegert et al., 2001), including areas in Sabah, Malaysia, where long-term ecosystem studies on nutrient budgets in Dipterocarp and plantation forest were conducted (Malmer, 2003). Monitoring of stream flow water chemistry showed little temporal variation in solute concentrations suggesting that seasonal fluctuations are minor unless disturbed (Malmer, 2003). In contrast, the wildfire increased leaching losses of several nutrient elements, suggesting changes in soil chemical properties. In a part of this research area, covered by Dipterocarp forest, a soil sampling had been

0038-0717/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-0717(03)00152-4

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performed in December 1997 to test a bioassay method for P and N availability, previously only used in temperate and boreal forests ecosystems (Nordgren, 1992). After the fire caught the area, it was decided that a repeated sampling would provide a unique possibility to assess changes in the soil caused by drought and wildfire in a rainforest with no previously known fires. The objective of the study was to evaluate the effect of wildfire in a Dipterocarp rainforest by soil respiration kinetics, in comparison to total soil C, N and P, as well as different extractable forms of N and P.

2. Methods 2.1. Research area The Mendolong research area is situated at an altitude of about 700 m in the foothills of Mt Lumako, 35 km southeast of the coastal city Sipitang (115.58E, 5.08N) in Sabah, Malaysia (Northern Borneo). In young plantations the monthly minimum temperatures range between 20 and 22 8C and maximum temperatures between 27 and 31 8C (Malmer, 1993). The mean annual precipitation was 3350 mm with no pronounced dry season, and the mean yearly runoff was 1995 mm. The vegetation consisted of hill Dipterocarp forest, which was lightly, selectively logged in 1981. The extraction intensity at that time was only 10 trees ha21, and the structure of the remaining forest resembled natural forest. There had been no previous fires in this forest. The soils were Haplic Acrisols (FAO, 1988) or Isothermic Typic Hapludults (Soil Survey Staff, 1996) developed on sandstone and shale with textures ranging from sandy loam to clay loam in the top 50 cm (Clay 10 – 40%; Silt 18 – 27%; Sand 36 –72%). The porosity in the undisturbed soil ranged from 60 to 70%, and the average bulk density was 0.83 (0.09) g cm23 in the uppermost 5 cm of the soil (Malmer et al., 1998). The average carbon content (% C) was 14 (4.4), 4.0 (0.42) and 2.4 (0.09) in the organic (O) horizon, and the 0 – 5 and 5 –10 cm uppermost layers of mineral soil, respectively. The original thickness of the litter (L) and O horizons averaged 0.78 (0.24) and 2.25 (1.17) cm, respectively (figures in parentheses are standard deviations). No measurements of fire intensity or temperature were available, due to the accidental nature of the fire. However, the fire appears to have been of light intensity because there was not much fuel on the forest floor, the O horizon was not thinner at the post-fire sampling, and the ashes were not white. The fire did not consume the standing trees but most were killed and all trees shed their leaves shortly after the fire. This was observed to result in a litter layer that protected the newly formed ashes from erosion. In contrast, nearby plantations with high fuel loads on the ground were burned more harshly, sometimes down to the mineral soil, and were severely eroded.

2.2. Sampling The soil was sampled December 1997 and September 1998, before and after a wildfire in April 1998 (97/98 ENSO event). At each time, the soil samples were collected from six soil pits, distributed with 15 m between each pit along two parallel 70 m catenas, sloping 5 – 308 towards a small stream. The first catena was selected randomly within the approximately 10 ha large watershed and the second was located at a fixed, predetermined distance of 60 m from the first. The uppermost pits were 30 m from the water divide and the lowest pits were about 10 m from the stream. The randomization of catenas was performed in the same way before as well as after the fire, which located the first postfire catena approximately 30 m from the pre-fire catena. There were no unburned control in this study meaning that changes due to climatic or other seasonal effects that coincided with the samplings would interfere with the results. However, in this per-humid tropical area normal seasonal changes would be minor in comparison with the drought connected with the fire. Samples from the litter layer and the O horizon were collected from 20 £ 20-cm squares. Samples with known volumes were collected from the mineral soil (0 –5 and 5– 10 cm) using a cylindrical sampling device (diameter 7.2 cm and height 5 cm), frozen within 6 h of sampling for later processing, and transported by air to Sweden (inside a foam-box cooler). Freezing of tropical soils is likely to induce changes in soil properties, including lysis of microbial cells. Basal respiration, substrate induced respiration (SIR) and the most easily extractable N and P is the most likely to be affected. Freezing was, however, considered unavoidable because the alternative to store samples at prolonged time at temperatures above 0 8C would be even more detrimental (Verchot, 1999). 2.3. Microbial respiration Microbial measurements were performed in May and June 2000 (pre- and post-fire samples were analyzed simultaneously). Soils were adjusted to 2 15 kPa water potential on suction plates (Klute, 1986), to optimize conditions for microbial growth (Ilstedt et al., 2000). Samples of 20, 10 and 3 g of moisture-adjusted mineral, O horizon and litter layer soil were weighed into 250 ml plastic jars in a respirometer (Nordgren, 1988; Rospicond IV, Nordgren Innovations, Dja¨kneboda, Sweden). Soil respiration was measured every hour at 20 8C. Basal respiration rate (BR) was calculated as the average of 40 hourly measurements, after the respiration increase caused by soil handling had leveled off. Immediately after the basal respiration period a substrate containing glucose with P or N was added as a powder (0.4 g glucose plus 10 mg KH2PO4 or 65 mg (NH4)2SO4) on duplicate samples. The respiration rate 12 h after substrate addition (Nlim12 and Plim12) was used as a measure of the rate of

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respiration increase possible given the N and P in the soil available to the microbial community, respectively (Fig. 1). We assume Nlim12 and Plim12 to be limited by the rate of N and P supply (extraction rate) to the microbial community, respectively. After some time the respiration rate peaks (Fig. 1), presumably when all N (glucose and P added) or P (glucose and N added) available to the microbes has been depleted, thus providing an index of the amount of microbially available N and P (Nlim and Plim, respectively). 2.3.1. P, N and C analyses In March 2001, a sequential extraction of P was performed using a modified form (Giesler et al., 2003) of the Hedley fractionation procedure (Hedley et al., 1982) with an anion exchange membrane (Saggar et al., 1990), 0.5 M NaHCO3, 0.2 M NaOH and, 1.0 M HCl. Litter, humus and mineral soil samples of about 1, 10 and 20 g (dw), respectively, were weighed into 250 ml centrifuge bottles for the extraction. The anion exchange membrane and 0.5 M NaHCO3 solution is assumed to extract readily available inorganic and organic P fractions, the 0.2 M NaOH is assumed to extract Al and Fe surface-bound inorganic P and partially stabilized organic P in soil organic matter, and 1.0 M HCl removes inorganic P in Ca phosphates and inorganic P occluded within Al and Fe oxides (Cross and Schlesinger, 1995). Inorganic P (Pi) was determined in all extracts. Total P was determined in the 0.5 M NaHCO3 and 0.2 M NaOH extracts using acidified ammonium persulfate digestion. Organic P (Po) was calculated as the difference between total P and Pi Residual P was calculated as the difference between

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acid-digestible P (see below) and the sum of all extraction steps. Ammonium and nitrate were extracted (shaken 2 h) with 25 ml of 2.0 M KCl using 2 – 5 or 10 g of soil (dw) from the litter layer and the O, 0 –5 and 5 –10 cm soil layers, respectively. Sub-samples for total carbon, nitrogen and acid-digestible phosphorus were milled in a ball mill, dried in a vacuum drier (70 8C, 48 h) and digested in concentrated HNO3 and HClO4 (s:s ratio 10:1). All extracts and acid digests were analyzed for P content using a flow injection analyzer (Tecator 5012 and 5020, Foss Tecator, Ho¨gana¨s, Sweden) and total C and N were analyzed by an elemental analyzer (Perkin Elmer 2400 CHN, Norwalk, Connecticut, USA). 2.4. Statistics The data were analyzed by linear regression in a factorial model (1) with the distance along the transects ðXi1 Þ; fire ðXi2 Þ and transect-fire interaction ðXi1 Xi2 Þ as factors (Umetrics, 1999). Because different soil layers were sampled from the same pits they were not independent measures, and thus were analyzed as separate response variables ðyi Þ: yi ¼ b0 þ b1 Xi1 þ b2 Xi2 þ b3 Xi1 Xi2 þ ei

ð1Þ

The distance along transects was used in the model to account for variability due to fire intensity or hydrology. Significant differences refer to Student t-tests of b2 on the p , 0:05 level unless otherwise stated. Any response that was not normally distributed was transformed by logarithmic or power transformation, as appropriate. For Pearson correlations, the variables were expressed per LOI to

Fig. 1. Hypothetical example of respiration kinetics. The definitions of basal respiration, substrate induced respiration (SIR), nutrient limited respiration after 12 h (Nlim12/Plim12) and maximum nutrient limited respiration (Nlim/Plim) are indicated in the figure.

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account for the strong covariation of most variables with organic matter.

3. Results 3.1. Effects of fire on P and N The total P concentration in the litter layer was 33% greater after the fire, due to increases in the NaOHextractable P fractions and the residual P fraction (Fig. 2). In the O horizon membrane-P decreased whereas NaOH- and HCl-extractable Pi increased after the fire (Fig. 2). There was, however, no change in total P concentration in the O horizon. In the mineral soil (5 – 10 cm) only concentrations of membrane-P increased and NaOH-Pi decreased after the fire. No change was found in the total P concentrations in the mineral soil.

Total N concentrations increased by about 20% in the litter layer after fire, but decreased by about 16% in the 0– 5 cm mineral soil (Table 1). Nitrate concentrations increased between 7- and 12-fold after the fire, whereas NHþ 4 concentrations remained unchanged (Table 1). Thus, extractable inorganic N increased in all layers and the relative proportion of NO3 – N increased, especially in the O layer and mineral soil (Table 1). The C concentration was reduced by fire in the litter layer while C levels in the O layer and mineral soil were unaffected (Table 1). The total amount (kg ha21) of C, P and N in the litter layer was 56, 73 and 67%, greater respectively, after the fire (Table 2). This was a combined effect of the increased thickness and bulk density (40 and 36%), together with changes in elemental concentrations. The amount of nitrate (kg ha21) after the fire increased approximately 10-fold in all horizons. Despite an increase in the average value of the ammonium pool from 0.94 to 4.1 kg ha21 in the litter layer

Fig. 2. Concentrations (mg kg21) of phosphorus fractions in the different horizons for unburned and burned soil. Error bars indicate the standard errors. Significant differences for fractions before and after the fire at the 0.05 and 0.01 probability levels are indicated by * and **, respectively.

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Table 1 Averages and standard errors ðn ¼ 6Þ for concentrations of nutrients before and after the fire Horizon

L O Mineral (0– 5 cm) Mineral (5– 10 cm)

Total C (%)

21 NHþ 4 (mg kg )

21 NO2 3 (mg kg )

Total N (%)

Before

After

Before

After

Before

After

Before

After

49 (0.63) 14 (1.8) 4.0 (0.17) 2.4 (0.29)

45 (1.6)* 11 (1.0) 3.5 (0.44) 2.2 (0.61)

1.3 (0.048) 0.72 (0.10) 0.26 (0.0056) 0.16 (0.018)

1.7 (0.12)* 0.60 (0.052) 0.22 (0.023)* 0.12 (0.015)

2.5 (0.27) 1.3 (0.54) 0.82 (0.44) 0.41 (0.090)

17 (6.2)* 16 (1.7)*** 5.9 (0.67)*** 4.3 (0.88)***

210 (30) 40 (5.3) 11 (1.0) 6.7 (0.44)

290 (71) 43 (5.5) 16 (5.1) 9.4 (1.5)

*,***Change significant at the 0.05 and 0.001 probability levels, respectively.

these amounts were not significantly different due to a large increase in variation after the fire. 3.2. Effect of fire on respiration kinetics Fire reduced the microbially available N (Nlim) in the O, 0 –5 and 5 –10 cm layer, while microbially available P (Plim) increased in the O layer. Expressed per unit C concentration (%), Plim values for both the litter layer and the O layer were significantly higher after burning. In contrast, the microbial P-extraction rate (Plim12) was unaffected by fire in all layers (Table 3). Respiration rate always increased much more rapidly following substrate addition under N limitation than under P limitation (Fig. 3). However, the N limited respiration rate reached its maximum peak earlier, and at lower values, compared to P limited respiration. Thus, microbially available N was much lower than microbially available P (Table 3). Fire decreased basal respiration in the O and 0 – 5 cm mineral horizons when expressed in terms of C concentration. Expressing the respiration per C concentration took away most of the effect of layer, but not the effect of fire. Fire decreased the microbial biomass (SIR) in the O layer. 3.3. N and P relationships with depth in undisturbed forest soil Concentrations of N and P in unburned soil decreased sharply with depth, the highest concentrations being found in the litter layer (Table 1; Fig. 2(a)). In contrast, the amounts of N and P (kg ha21) were greatest in the 0 –5 cm

mineral soil. The membrane-P fraction had the sharpest decline with depth of all P fractions, decreasing from 29 mg kg21 in the O horizon to 2.3 mg kg21 in the upper mineral soil (0 –5 cm). Microbially available P in unburned soil decreased with depth, while microbially available N had a maximum in the O horizon (Table 3). 3.4. Correlations between microbial and chemical methods Microbially available P correlated most strongly with NaOH-extractable P (r ¼ 0:83; p , 0:001), while correlations with residual P were less (r ¼ 0:75; p , 0:001) and with readily available P (membrane P þ NaHCO3 Pi þ NaHCO3 Po) were not significant at all (r ¼ 0:33; p ¼ 0:331; Fig. 4). Microbially available N correlated to total N but not to the extractable nitrate or ammonium (Table 4).

4. Discussion 4.1. Effects of fire on N and P Fire resulted in a 10-fold increase in nitrate concentrations in the soil (representing 6.0 kg ha21 down to 10 cm). If not taken up by the vegetation or immobilized this N is likely to be leached or denitrified. During the establishment of nearby forest plantations, about 10 – 13 kg ha21 of N was leached, mainly after the burning of slash (Malmer, 1993). The 50% decrease in microbially available N after the fire might seem to conflict with the increase in KCl-extractable inorganic N. Previous studies

Table 2 Averages and standard errors ðn ¼ 6Þ for bulk density and total amounts of nutrients before and after the fire Horizon

L O Mineral (0– 5 cm) Mineral (5– 10 cm)

Bulk density (ton m23)

Depth (cm)

Before

After

Before

0.068 (0.012) 0.49 (0.088) 0.84 (0.048) 1.0 (0.047)

0.11 (0.027) 0.62 (0.082) 0.91 (0.048) 1.1 (0.069)

0.9 (0.098) 2.4 (0.48)

*Change significant at the 0.05 probability levels.

Total C (ton ha21)

Total P (kg ha21)

After

Before

After

Before

After

1.5 (0.29) 2.4 (0.48)

2.6 (0.62) 12 (2.2) 17 (1.0) 12 (1.0)

5.8 (2.2)* 11 (4.8) 16 (1.4) 11 (2.5)

1.9 22 38 29

6.9 (2.6)* 26 (9.4) 50 (8.8) 29 (2.5)

(0.56) (3.6) (3.0) (1.7)

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Table 3 Averages and standard errors ðn ¼ 6Þ for respiration measurements before and after the fire Horizon

SIR

Nlim12

Nlim

Plim12

Plim

Before

After

Before

After

Before

After

Before

After

Before

After

Before

After

7.6 (0.53) 3.5 (0.36) 1.2 (0.088) 0.69 (0.067)

11 (1.3) 1.8 (0.15)*** 0.75 (0.088)** 0.57 (0.053)

17.0 (1.8) 13 (1.8) 5.8 (1.1) 3.4 (0.7)

22.0 (2.4) 6.5 (1.1)* 7.0 (2.1) 5.9 (1.5)

21 (5) 38 (10) 20 (4.8) 15 (3.6)

37 23 21 18

43 (7.0) 160 (9.8) 94 (5.4) 63 (8.1)

43 (4.7) 80 (9.1)*** 51 (6.3)*** 36 (4.6)*

25 (2.4) 32 (5.7) 18 (4.7) 10 (3.7)

32 (4.1) 22 (6.1) 15 (5.4) 12 (4.1)

98 (26) 180 (15) 110 (12) 82 (13)

210 (30)* 330 (46)* 140 (28) 77 (14)

(2.7) (5.4) (3.4) (2.7)

*,**,***Change significant at the 0.05, 0.01 and 0.001 probability levels, respectively. Values are in mg CO2 g21 dry soil h21.

Fig. 3. Example of microbial respiration kinetics for a soil with induced N and P limitation. In the N limited curve (Nlim indicates the peak) glucose and P was added and in the P limited curve (Plim indicates the peak) glucose and N was added.

Fig. 4. Relationships between Plim and (a) readily available P (membrane P þ NaHCO3 Pi þ NaHCO3 Po), (b) surface-bound P (NaOH Pi þ NaOH Po), and (c) residual P. All values are expressed per unit soil C content (%). The value in parentheses was excluded from the Pearson correlation.

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L O Mineral (0–5 cm) Mineral (5–10 cm)

BR

U. Ilstedt et al. / Soil Biology & Biochemistry 35 (2003) 1071–1078 Table 4 Pearson correlations (upper figures) and p-values (lower figures) for Nlim, þ total-N, NO2 3 and NH4 expressed per unit carbon content Total-N NH4 –N NO3 –N Nlim

20.107 (0.471) 0.392 (0.006) 0.844 (0.000)

NHþ 4

0.331 (0.022) 20.18 (0.217)

NO2 3

0.191 (0.193)

have shown that micro-organisms efficiently utilize various organic forms of N, such as amino acids (Owen and Jones, 2001). Alternatively, methodological limitations associated with the method to derive Nlim are possible. The increase of total N and P in the litter layer was due to the shedding of leaves after the fire, which compensated for losses caused by the fire. Other studies of fire in humid tropical forests have concluded that total N is decreased in the soil (Neary et al., 1999). For nitrate and ammonium observations are mixed, and are probably dependent on fire intensity and initial factors such as pH and soil organic content and vegetation. The increase of NaHCO3-Po in the O horizon may be explained by transport of some of the organic material (including partly combusted ash) from the litter layer. The increase in microbially available P in the litter layer and O horizon suggest changes in substrate quality and available P pools. However, the fact that microbial P-extraction rate was unaffected suggests P adsorption was unchanged. Other studies have concluded that P availability is temporally improved, and then declines to normal or lower than pre-fire values if followed by cropping (Ellis et al., 1982; Romanya et al., 1994; Saharjo, 1999). However, these studies involved fires with more fuel on the ground and less litter fall from the vegetation remaining after the fire.

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available P and different soil P pools was that between microbially available P and NaOH-extractable P (Fig. 4). This P fraction is assumed to consist mainly of surfacebound inorganic P and organically bound P, and has been considered to be available to plants only on a long-term scale (Cross and Schlesinger, 1995). The slow and nonexponential increase in respiration seen under P limitation compared to the fast and exponential increase under N limitation (Fig. 3) indicates that uptake of P required a higher C cost than uptake of N. This is reasonable since utilization of surface-bound or organically bound P would involve either exudation of organic acids to release surface-bound P or production of phosphatases to hydrolyze organic P compounds. However, higher respiration rates were eventually attained under P limitation than under N limitation. It was therefore more available P than N in total (relative to the needs of the micro-organisms).

5. Conclusions This wildfire had mostly positive effects on soil fertility. The main reason for this was the low intensity of the fire, due to the low amount of fuel on the ground. The shedding of green leaves shortly after the fire contributed to the high amounts of available nutrients in the upper layers of the soil and protected the soil from erosion. However, nitrate concentrations increased 10-fold and could be vulnerable to leaching if not taken up by the vegetation. Respiration kinetics after substrate and nutrient addition showed potential as a method for assessing changes in soil nutrient status. Microbes can access P resources over a time-scale of hours to a few days in P-fixing soils that may be available to plants only over much longer time-scales.

4.2. Effect of fire on basal respiration and SIR In the O horizon there was no decrease in C quantity, however, the decrease in BR and SIR indicated a change in quality. However, some studies in boreal forests suggested that decreases in microbial biomass and respiration were an effect of fire and subsequent changed moisture, and not an effect of charcoal addition (Fritze et al., 1994; Pietikainen et al., 2000). In similar systems also substrate availability has been proposed as a regulating mechanism (Ba˚a˚th et al., 1995). Also in other humid tropical forests (after clearing and burning) decreases in microbial activity has been noted (Luizao et al., 1992; Funakawa et al., 1997). 4.3. Correlations between microbial and chemical methods The absence of correlation between the extractable N pool and microbially available N may indicate that the micro-organisms utilized sources of N that would not normally be considered readily available to plants. Similarly, the strongest relationship between microbially

Acknowledgements This study was funded in equal parts by the Swedish Agency for Research Cooperation with Developing Countries (SAREC) and Sabah Forest Industries Sdn Bhd (SFI). Reiner Giesler was supported by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS).

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