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SOIL PROPERTIES DYNAMICS INDUCED BY PASSAGE OF FIRE DURING
AGRICULTURAL BURNING
Edem I. Dennis, Alphonsus D. Usoroh, Researchers University o f Uyo, Nigeria
Rosemary A. Essien, Researcher Akwa Ibom State University, Nigeria
E-mail: [email protected]
ABSTRACT
Characteristics of an ecosystem are altered both as sudden modifications induced by the passage of the fire and the delayed changes derived from the simultaneous modifications of various soil physical and chemical parameters. Effects of fire on soil properties was performed in experimental plots, whose fuel amount was altered in order to obtain different heating intensities with the aim of determining changes in the soil physico-chemical parameters at varying heating temperatures. The research was conducted in a continuous cropped arable experimental plots located at the University of Uyo Teaching and Research Farm (UUTRF), Use-Offot, Uyo, Nigeria for four growing seasons, between March, 2010 to October, 2011. Core and bulk samples from the burned and adjacent unburned plots (control) were collected for physico-chemical analysis using standard procedures. These induced temperatures were highly variable on the soil surface. Temperature differences significantly (P < 0.05) affected sand, total nitrogen, organic carbon and pH contents of the soils positively (r = 0.518, 0.478, 0.582, 0.595 respectively), whereas a reduction in the soil temperature increased the concentrations of clay, 1mm, 0.05mm and 0.25 mm stable soil aggregates in the soil (r = -0.619, -0.578, -0.780, -0.526 respectively) after burning. Exchange acidity increased to 5.12 cmolkg-1 at 400C from 0.80 cmolkg-1 at initial temperature of 250C at the surface soil. Though aggregates formation was significantly higher (P = 0.05) after burning than the control soil locations, this soil will easily be distressed with the least application of force. The pH decreased to 5.4 at higher temperatures following burning before ashes mineralized. However, both organic matter and ECEC increased at increasing soil temperature. Potassium content remained surprisingly constant as the soil temperature increased. Despite the merits of quick release of occluded nutrients, heating temperatures of slash-and-burn method of land clearing altered soil quality attributes.
KEY WORDS
Slash-and-burn; Traditional farming; Soil quality; Modification; Temperature.
Slash and burn method of land clearing is an integral part of the traditional farming system (bush fallow rotation) widely used as a means of land clearing to pave way to tillage in southern Nigeria. Depending on management practices being used, human activities like bush burning, fossil fuel uses and deforestation have alter the atmosphere’s composition and earth balance. The invention of fire ignition and its control by man started the anthropogenic modification of biosphere (Neff et al., 2005). Fire has long been recognized as a disturbance that maintains grasslands and savannas and prevents invasion of woody species (Archer et al., 1988; Blair, 1997; Ruddiman, 2003). Therefore, prescribed fire is often employed as a land management tool to suppress the encroachment of woody plants into grass-dominated ecosystems. In humid tropics, fire frequencies and interactions between fire and other disturbance factor (such as tillage equipment and tillage methods) determined to a large extent the balance between trees and grasses, stand structure and dynamics, and shrub cover abundance (Edem et al., 2012; Neary et al., 1999; Rice & Owensby, 2000; Ruddiman, 2003). Above and below ground productivity often increase following fire as a result of microclimatic modification due to removal of litter and standing crop and changes in nutrient
availability and distributions (Creighton & Sutherland; National Wildfire Coordinating Group, 2001; Peterson & Reich, 2001).
According to Edem et al (2012), most land that is left unused in a cropping year is often set on fire by farmers. This is common with the livestock farmers so that their animals could browse on young plants that grow after burning. Before the plants come up to cover the ground surface, the soil is exposed to rainfall. Subsequently, soil aggregates are dispersed: pores are clogged with particles which further result in higher rates of surface runoff (Mallik et al., 1984). The level of alteration may even be enormous if quantity of trash is large and the residence time of burning is long, or a thin dry litter is completely incinerated (Ruddiman, 2005). More severe burns may alter soil fundamental characteristics such as texture, mineralogy and cation-exchange capacity (Johnson & Matchett, 2001).
So far, most research assesses change in organic carbon due to bush burning and few efforts were made to assess the effect on other soil properties. Moreover, no studies are known to that assesses the spatial variability of soil properties at different heating temperature in humid tropics. Hence, tropical conditions are often under represented. These researches aimed at developing regional-specific approaches and improve estimates on soil quality factor modifications at varying temperatures.
Therefore, the objectives of this study were to assess; (i) changes in soil physical conditions at varying fire temperature and (ii) the fire temperature within which soil quality attributes are depleted.
MATERIALS AND METHODS
Study area. The research was conducted in a continuous cropped arable experimental plots located at the University of Uyo Teaching and Research Farm (UUTRF), Use-Offot, Uyo, Nigeria. Uyo is located between latitudes 40 30’ and 50 3N and longitudes 70 31’ and 80 20’ E and altitude 65 m from the sea level. The area is divided into two distinct seasons, the wet and dry seasons. The wet or rainy season begins from April and lasts till October. It is characterized by heavy rainfall of about 2500-4000 mm per annum. The rainfall intensity is very high and there is evidence of high leaching and erosion associated with slope and rainfall factors in the area (Edem et al., 2012). In the area measuring 720 m2 on a slope of 7 %, were prepared 10 sub plots; each 24 x 3 m2, separated from each other by fireproof tracts (20 cm). In preparing the plots, dry biomass treatments of 50, 100, and 150 kg/m2 were applied on the cleared plots in order to produce three levels of fire intensities, and progressively fire was set into 9 out of the 10 plots.
Pre-and-post burnt soil samplings. Profile pits (50 cm depth) were dug at the centre of each plot. Bulk soil, core and aggregate samples were collected at two depths of 15 cm interval before and after passage of fire before mineralization of the CaCO3 in the ash content. The core samples were obtained for saturated hydraulic conductivity and bulk density determinations. The soil samples were secured in a core, and one end of the core was covered with a piece of cheese cloth fastened with a rubber band and properly labeled while the bulk samples collected were secured in properly labeled polythene bags before taken to the University of Uyo Soil Science laboratory for physical, chemical and structural parameters determinations using standard methods and procedures (Danielson & Sutherland, 1986).
Experimental measurements and statistical analyses. The severity of burning in each site was measured qualitatively from the degree of litter consumption of the applied biomass. Immediately after burning, soil temperatures were read from the installed temperature sensors at the surface and subsurface of the respective plots for the four growing seasons the experiment lasted. To ensure representative sampling, bulk soil samples, which were analyzed for soil physico-chemical properties, were composite of five random samples taken at 0-15 and 15-30 cm depths within replicated plots.
Particle-size distribution was determined in the soil samples using hydrometer method. Bulk density was estimated by dividing the oven-dry mass of the soil by the volume of the soil. In addition, core samples were also used to determine saturated hydraulic conductivity
(Ks) in the laboratory using a constant head permeameter. Undisturbed soil samples were taken for the determination of water-Stable aggregates.
(WSA) and mean weight diameter (MWD) using a modified Kemper and Rosenau wet sieving method. Soil organic carbon (SOC) was determined by loss-on-ignition and the standard Van Bemmelen factor (1.724) was used for conversion of SOC into organic matter content. Total nitrogen was determined by dry combustion using Leco CHN Analyzer (Laboratory Equipment Corporation, St Joseph, MO, USA; Bremner and Mulvaney 1982).
pH was determined with the use of glass electrode pH meter to read the suspension of 10g soil sample with 20 ml 0.01 N CaCl2. Available phosphorus was determined using bicarbonate extraction, with acid reductant. Meanwhile, the exchangeable cations (calcium, Ca; magnesium, Mg; potassium, K; and sodium, Na) in the soil were determined by first extracting the soil sediment with 1M NH4 OAc (ammonium acetate) solution. The amounts of exchangeable Na and K in the extract were determined by flame photometry while Ca and Mg were determined by atomic absorption spectrometry.
Effective cation exchange capacity (ECEC) was obtained by addition of the values of exchangeable bases and exchangeable acidity. Base saturation was expressed as the fraction of the negative binding sites occupied by exchangeable cations .It was calculated by summing together the levels of Ca, Mg, K ,and Na found in the soil, then expressing this sum as a percentage of the ECEC value as follows (BS represents base saturation, %):
BS = 100 (Ca + Mg + K + Na)
ECEC
The experiment consisted of two treatments (burned and un-burned plots) arranged in a RCBD with three replicates. Data were statistically analyzed for variance (ANOVA), and significant means were compared using Duncan multiple range test. Paired t-test was used to compare means of the unburnt and burnt plots. For all tests, a threshold of P =0.05 was used to define statistical significance. All statistical analyses were performed using SigmaStat (3.5 Edition) and validated using SPSS 17.0. Pearson correlation coefficients were used to assess the degree of relationships among variables.
RESULTS AND DISCUSSIONS
Regardless of varying fire temperatures, some physical and chemical characteristics of soil Before and after experimental fire, clearly and strongly differed between burnt and unburnt Soils in this study area as shown in Table 1.
Particle size distribution and soil texture. The results show that total sand fraction with mean value of 838.50 gkg-1 in the burnt plot was significantly (P= .05) greater than the unburnt plot with the mean value of 772.60 gkg-1. The silt fraction was higher in the unburnt plot with the mean value of 78.86 gkg-1 than the burnt plot with the mean of 47.58 gkg-1. Although Hubbert et al., (2006) reported increase in silt fraction after burning, but this result in line with the report Kettering et al., (2000), that burning has effect on soil particle distribution. Clay fraction was greater in the unburnt plot with the mean of 148.53 gkg-1 than the burnt plot with the mean of 114.02 gkg-1 but was not significant. The result showed that the burnt and unburnt plots were loamy sand texture. Therefore, the textural class was not affected by burning even though there were significant changes in the distribution of particle sizes. This result conformed to the earlier report of Edem et al., (2012) that soil texture is a fundamental attribute of the soil and cannot easily alter by management practices. Intense heating temperature (>4000C) may permanently alter soil texture by aggregating clay particles into stable sand-sized particle making the soil texture more coarse and erodible (Chandler et al., 1983)
Bulk density (BD) and Total porosity (P). Bulk density responded to burning with increase in the mean value of 1.67 g/cm3 compared to 1.59 g/cm3 before burning but was not statistically significant. This result confirmed the earlier report of Klemmedson et al., (1952) that bulk density increased after slash and burn. They ascribed this change to the disruption
of soil aggregation and loss of organic matter. Also, there was 10 % decrease in Total porosity after burning. This observation is in consonance with Mallik et al., (1984) and Neary et al., (1999) who reported reduction in larger pores and total porosity following burning and ascribed it to the ash deposits in the larger pores. The reduction in total porosity can also be ascribed to increase in bulk density. Although, reduction in total porosity has been reported by Mallik et al., (1984), but Oguntunde et al., (2008) and Ajaji et al.,( 2009), reported reduction in bulk density due to burning of soils. It therefore appears that the reduction in total pore volumes was perhaps due to ash deposits in larger pores.
Table 1 - Mean and standard deviation of some soils’ physical and chemical properties before and
after experimental fire
Soil parameters Pre-burnt plot Burnt plot :
Sand, gkg 772.60 ± 59.01:: 838.50 ±41.85'’ :
Silt, gkg'1 78.86 ± 33.60a 47.58 ± 14.40" :
Cjay, gkg;1 i 148.53 ± 52.24a 114.02 ± 37.03a :
Texture Loamy sand Loamy sand !
Ks, cm/hr 3.30 ± 3.82:: 7.32 ±9.25a :
BD. g/crrf 1.59 ± 0.13a 1.67 ± 11 96'’ :
P, cm3cm'3 39.88 ± 4.98a 35.98 ± 13.58a i
©v, cm3/cm3 2.55 ±0.40a 7.93 ± 14.52'’ :
pH.. : 5.9 ± 0.15a 5.4 ± 0.19" :
EC, dsm 0 04 ±0.31a 0 02 ± 0 09:: :
TN,gkg; , : 0.36 ± 0.13:: 0.67 ± 0.12'’ :
AVP, mg kg 27.77 ± 4 12' 26.56± 2.75'’ :
Ca, cmoikg'1 3.12 ± 0.93:: 4.98 ± 2.39'’ :
Mg. cmoikg'1 1.86 ± 0.46b 3.92 ± 2.22a :
K, cmoikg'1 0.05 ± 0.03a 0.05 ± 0.01a :
Na, cmoikg'1 0.05 ± 0.01a 0 04 ± b.io" :
* Means followed by different letters along the rows are significantly different (P = .05)
Ks = saturated hydraulic conductivity; BD = bulk density; Qv= volumetric moisture content; P = total porosity; EC = electrical conductivity; TN = total N; AV. P =available phosphorus.
Volumetric moisture content (QV) and Saturated hydraulic conductivity (K,). A significant (P = .05) increase in saturated hydraulic conductivity in the burnt plot was observed with the mean of 7.23 cm/hr compared to the unburnt plot having a mean of 3.30 cm/hr . This observation is contrary to the report of Pyne & Goldammer (1997). They found that Ks of soil decreased approximately 50% in the burnt plots relative to adjacent unburned plots. But Ruddiman (2005), paid attention to the textural characteristics, organic matter content, and structure which appeared to have been responsible for high Ks values. Volumetric moisture content increased after burning with the mean of 7.93 cm3/cm3 compared to 2.55 cm3/cm3 in the un-burnt plot. This is in consonance with Mallik et al., (1984) who reported an increase in water retained after burning. The increased in volumetric moisture content in this study however contradict with Edem et al., (2012) who reported reduction in moisture content from 0.13 to 0.03 m-3m-3 at a depth of 0-0.5m in a steep chaparral watershed, southern California, following burning.
Soil pH and Electrical conductivity (EC). The pH of the soil significantly decreased after burning with the mean value at 5.4 compared to 5.9 in the unburnt plot (P =.05). Electrical conductivity of the soil significantly decreased after burning with the mean of 0.02 dSm-1 compared to 0.04dSm-1 in unburnt plot (P =.05). But according Austin & Baisinger, (1955) as reported by Hernandez et al., (1997), EC values of burnt plots were higher than that of the unburned plots. The reduction of pH and EC in this research after burning could be ascribed to lack of mineralization of CaCO3 in the ash content due to immediate soil sampling after burning.
Total nitrogen, Organic carbon and Available phosphorus. Total nitrogen responded to burning with a significant increase in the mean value of 0.67gkg-1 after burning and 0.36gkg-1 in the unburned plot. This observation agreed with the earlier work of Neary et al., (1999) who reported increase in availability of total nitrogen after burning. Surprisingly, organic
carbon significantly (P=.05) increased after burning with mean of 15.97 gkg"1 compared to 9.29 gkg-1 in the unburnt plot. But Pyne & Goldammer (1997) reported that loss of organic carbon in soil occurs as a result of fire depleting the litter on the surface. Although, they did not assess heat intensity at varying temperatures and depth. Available phosphorus decreased after burning with the mean of 26.56 mgkg"1 compared to 27.77 mgkg"1 in the unburnt plot but was not significant. This is against the report of Neff et al., (2005) and Schevner et al., (2004) who reported that the ash deposits after burning, helps to fertilize the soil by immediate release of available P and other mineral nutrients-Mg and Ca. However, in this study, the ash was not allowed to mineralize, as samples were collected immediately after burning in order to assess sudden modifications induced to soil properties at varying heating temperature.
Table 2 - Variation induced by experimental fires on some soils’ physical properties and erodibility
Heating temp. °C Sand, gkg"1 Silt, gkg"1 Clay, gkg'1 Ks cm/hr BD .-3 gem P cm3cm"3 MC •3 _ cm cm 3 PSS t/ha/yr K factor tha/MJmm
Surface soil layer
24(control) 802.44f 76.48a 121.08b 5.78e 1.50b 43.00b 2.59b 7.79d 0.41c
35 821.OOe 47.20d 131.80b 1.80g 1.75a 34.00c 3.14b 9.88b 0.53ab
40 821.OOe 47.20d 131.80b 8.40d 1.65a 38.00c 3.47b 11.02a 0.58a
48 841.00c 47.20d 111.80c 19.80b 1.50b 57.00a 3.15b 10.07a 0.57a
49 831. OOd 57.20c 151.80a 11.70c 1.53b 42.00b 3.07b 9.69b 0.58a
50 851.00b 50.70c 111.80c 20.70a 1.45c 45.00b 2.77b 8.55c 0.51b
58 861.00a 67.20b 71.80d 3.60f 1.76a 32.00c 2.95b 8.17c 0.55ab
60 821.OOe 47.20d 111.80c 5.40e 1.40c 37.51c 7.37a 8.55c 0.53ab
CV (%) 5.16 30.26 25.71 12.27 20.70 37.75 18.31 12.07 10.47
Sub-surface soil layer
24(control) 741.88c 85.53a 172.97b 0.80c 1.67b 36.00b 2.46b 7.96c 0.43c
25 761.00a 27.20c 171.80b 2.40b 1.64b 36.00b 2.95b 7.60c 0.45c
30 721.0b 40.53c 138.46c 3.60a 1.60b 39.00a 3.03b 10.07b 0.50b
33 761.00a 47.20c 191.80a 1.80b 1.75a 38.00a 2.97b 10.45b 0.42c
35 721.0b 47.20c 171.80b 1.80b 1.65b 39.00a 3.14b 11.35a 0.53b
36 761.00a 67.20b 171.80b 3.00a 1.50c 36.00b 2.79b 10.00b 0.45c
37 781.00a 37.20c 181.80ab 3.30a 1.62b 34.00c 8.49a 9.56b 0.55a
CV (%) 7.63 42.60 35.17 11.57 8.17 12.48 15.68 14.05 10.71
Means followed by different letter along the column within the soil layer are significantly (P =.05) different. BD = Bulk density; P = total porosity; PSS = potential soil loss; MC = moisture content.
Ks = Saturated hydraulic conductivity.
Exchangeable bases (Ca, Mg, K & Na) and Exchange acidity. Calcium (Ca) and magnesium (Mg) significantly (P= 0.05) increased after burning with the mean of 4.98 and 3.92 cmol/kg respectively compared to 3.12 and 1.86 cmol/kg respectively in the unburnt plot. P content remains 0.05 cmol/kg. Sodium (Na) significantly (P = 0.05) decreased after burning with the mean of 0.04 cmolkg-1 compared to 0.05 cmol/kg before burning. The result of Ca and Mg were similar to Opera-Nadi et al., (2010) who reported that burned surface soils tend to have higher concentrations of non combustible elements such as Ca, K, Mg and Na compared with unburned soil but the result of K is on the contrary. The significant increase (P =.05) in Ca and Mg in the burnt plots is important because they cause flocculation of soil particles there by encourages aggregation of particles. Decrease in Na is significant because high content of Na+ can destroy soil structure through dispersion of the particles which in turn heads to high erosion but in this case reduction in Na content after burning signified less susceptibility of this soil erosion. Exchange acidity significantly (P = 0.05) decreased after burning with the mean of 1.17 cmol/kg compared to 3.42 cmol/kg in the unburnt plot.
Effective cation exchange capacity (ECEC) and percentage base saturation (BS). The ECEC of the soil increased after burning with the mean of 10.37 cmolkg-1 compared to 8.40 cmolkg-1 in the unburnt plot. This increase however was not significant (P = .05). This could be ascribed to the vegetation burning despite the fact that ash in the burnt biomass was not
added or incorporated into the soil before sampling. The percentage base saturation significantly increased with the mean of 86.68% after burning and 61.67% before burning.
Table 3 - Variations induced by experimental fire on soil chemical properties
Temp. °C pH EC dSm" TN ОС C:N : AV.P EA Ca Mg K Na ECEC BS % !
«ку : mgkg'1 cmolkg"
Surface soil :
24 (control) і 5.9a 0.03a 0.40c 10.67c 24.07a і 27.42a 2.68b 2.96d 1.90d 0.05bc 0.54a 8.13c 63.27c і
35 : 5.3c 0.02b 0.60b 14.05d 23.6ab : 28.97a 0.80d 6.72b 5.04b 0.06b 0.04b 12.66b 92.76b :
40 : 5.9a 0.03a 0.80a 18.80a 23.5ab : 25.64b 5.12a 6.72b 5.28b 0.08a 0.05b 17.25a 70.32c :
48 : 5.5b 0.02b 0.80a 19.20a 24.00a : 25.97b 1.12c 2.88d 2.40c 0.06b 0.06b 6.52 82.85be :
49 : 5.6b 0.03a 0.70ab 17.05b 24.35a : 29.8a 0.88d 5.76c 4.80b 0.06b 0.04b 11.54b 92.13b :
50 : 5.5b 0.03a 0.80a 19.05a 23.8ab : 23.31c 0.56d 2.88d 1.92d 0.06b 0.04b 5.46d 87.37b :
58 : 5.6b 0.02b 0.80a 17.80b 22.25b : 25.64b 0.80d 3.36d 2.40c 0.04c 0.04b 6.64d 87.99b :
60 : 5.6b 0.01c 0.70ab 17.20b 24.57a : 25.64b 0.80d 9.12a 7.68a 0.05bc 0.05b 17.7a 95.48a :
Cv (%) : 2.6 75 36.11 36.38 14.7 : 14.87 54.09 29.8 24.73 5.17 9.43 45.14 16.66 :
Subsurface soil :
24 (control) : 5.9a 0.05a 0.31d 8.10c 23.5a : 28.14a 3.42a 3.14cd 1,86cd 0.05ns 0.54a 9.01b 62.47c і
25 : 5.5b 0.02b 0.60ab 13.2ab 22.0c : 25.97c 1.6b 2.40d 1.14d 0.06 0.04b 5.24c 71.12bc :
30 : 5.2c 0.02b 0.70a 15.26a 23.13b : 25.86bc 0.96c 4.32b 3.52b 0.05 0.04b 8.89b 86.16a :
33 : 5.3bc 0.02b 0.50bc 12.20b 24.4a : 26.31b 1.12b 3.80c 2.88c 0.07 0.05b 7.92bc 86.71a :
35 : 5.2c 0.02b 0.40cd 9.20c 23.Obc : 26.97b 0.80c 4.32b 2.4b 0.05 0.05b 7.62bc 90.12a :
36 : 5.3bc 0.01b 0.50b 11 .Obc 22.0c : 25.97c 0.96c 8.64a 7.2a 0.06 0.03b 16.89a 94.32a :
37 : 5.3bc 0.01b 0.60ab 14.4a 24.0a : 27.64a 1.12b 4.56b 3.60b 0.06 0.06b 9.40b 84.25a :
CV (%) : 3.48 40 17.91 17.74 3.87 : 10.35 87.17 47.99 56.63 1.69 25 39.11 9.25 :
Means followed by different letter along the column within the soil layer are significantly (P =.05) different.
ns = not significantly different; CV= coefficient of variation; EC = electrical conductivity, TN = total nitrogen, AV.P
available phosphorus, EA = exchange acidity, BS = base saturation, ECEC = effective cation exchange capacity
Paired Samples test for physical and chemical properties of pre and post-burn soils. The results of this study (Table 3) indicate a clear distinction of pair differences between soil properties of burnt and unburnt soils. Sand content was 8.52% higher in post-burnt plot than pre-burnt plot (37.28 g/kg). For silt, it was 65.75% (31.27 g/kg) higher in pre-burnt plot then post burnt plot while clay was 30.26% (4.51 g/kg) higher in pre-burnt plot than post burnt plot and saturated hydraulic conductivity had a percent mean difference of 121% (3.93 cm/hr) in post-burnt plot than pre-burnt plot. The major determining factor s for saturated hydraulic conductivity is the degree of disturbance to the surface of the soil by fire, which is usually organic debris that protects the underlying mineral soil (Valzano et al., (1997). But for bulk density, a percent change was only 5.03% (0.02 g/cm3) higher in post-burnt soils than preburnt soil and total porosity had a percent mean difference of 10.83% (3.90 cm3/cm3) higher in pre-burnt soil than post-burnt soil while that of moisture content was 210 % (5.38 cm3/cm3) in post-burnt soil than pre-burnt soil. According to National Wildfire Coordinating Group (2001), fire can either reduce or increase soil moisture content. It all depends on the distribution of pore sizes higher after the imposed treatment.
Soil pH was 9.25% (0.51) higher in pre-burnt soil than post-burnt soil but electrical conductivity had 100% change from pre-burnt plots (0.02 dSm-1) while total nitrogen had a percent mean difference of 86% (0.31 g/kg) higher in post-burnt soil that pre-burnt soil. For available phosphorus, it was 4.55% (1.20 cmolkg-1) higher in pre-burnt soil than post-burnt soil and calcium was 59% (1.86 cmolkg-1) higher in post-burnt soil than pre-burnt soil. Magnesium was 110% (2.05 cmolkg-1) higher in post-burnt soil than in pre-burnt soil. Potassium content did not change after passage of fire (0.001 cmolkg-1). But for sodium, percent change was only 25% (0.007 cmolkg-1) higher in pre-burnt plot than post-burnt plot. Paired difference for exchange acidity was 192% (2.24 cmolkg-1 ) higher in pre-burnt plot than post-burnt plot. While effective cation exchange capacity was 14.69% (1.97 cmolkg-1) higher in post-burnt plot than pre-burnt plot. But for organic carbon, percent change was 69% (6.50 g/kg) higher in post-burnt plot than pre-burnt plot and base saturation had a percent mean difference of 40.55% (25.00%) higher in post-burnt soil than pre-burnt soil. Fire significantly increased the concentration of non combustible elements (such as Ca++, Mg++, K++), hence increased the fertility status of the soil.
Thermal effect on soil physical properties. As shown in Table 2, fire increased the soil temperature from 240C (control) to 600C in both surface and sub-surface soil layer. Sand content in the soil surface layer increased to 861 gkg-1 at temperature of 580C from 821.00 gkg-1 when the initial temperature rise was 350C. Whereas in the sub-surface layer, sand content increased to 781.00 gkg-1 at 370C from 761 gkg-1 when the initial temperature rise was 250C. At the initial temperature rise of 350C, the silt content was 47.20 gkg-1 and increased to 67.20 gkg-1 at 580C in the surface soil. In the sub-surface soil, silt content equally increased to 67.20 gkg-1 at 360C from 27.20 gkg-1 when the initial temperature rise was 25oC. The increased temperature caused marked variations of the soil physical parameters. Particle-size-distribution showed a continuous increase of sand fraction with the increasing temperature, corresponding to a significant decrease of clay fraction at temperature above 49oC However, the silt content in surface layer was irregularly distributed but significantly reduced at sub-surface soil as the temperature increased. Thus the soil would continue to be classified as loamy sand up to 60oC.
Ks increased in the surface layer to 20.70 cmhr-1 at 500C from 1.80 cmhr-1 when the initial temperature rise was 350C. Where as in the sub-surface, saturated hydraulic conductivity increased to 3.60 cm/hr at temperature of 300C from 2.40 cm/hr when the initial temperature rise was 250C. At the initial temperature rise of 350C, bulk density was 1.75 gcm-3 but increased to 1.76 gcm-3 at 580C in the soil surface whereas, in the sub-surface soil, bulk density increased to 1.75 gcm-3 at 330C from 1.64 gcm3 when the initial temperature rise was 250C. At the initial temperature rise of 350C, total porosity was 34.00 cm3 cm-3 but increased to 57.00 cm3 cm-3 at 300C and 350C from 36.00 cm3 cm-3 when the initial temperature was 250C. The average density values reported in literature (Hillel, 1980) for organic and mineral soils (average) are 1.3 g/cm3 and 2.65 g/cm3 respectively. Therefore, increases in bulk density after fire is considered attributable to an increased contribution, weighted according to their volume fraction of minerals characterized by higher density.
At temperature of 600C, moisture content increase to 7.37 cm3 cm-3 from 3. 14 cm3 cm-3 when the initial temperature rise was 350C in the surface soil. In the sub-surface soil,
OO n o o
moisture content increased to 8.49 cm3 cm-3 at 37°C from 2.95 cm3 cm-3 when the initial temperature rise was 250C. Heat transfer in the soil occurs mainly by thermal conduction, and the conductivity increases with the moisture content (Edem et al., 2012). Thus, heating dry soil should cause a greater rise in surface temperature, but less heat penetration compared with moist soil. If we consider the percentage decrease of the moisture content, we note that at lower temperature, from 48oC up to 58oC, the decrease ranges between 31.5 and 29. 5 %, whereas at higher temperature, 60oC, it reaches the value of 73.7 %. In the surface soils, highest content of sand, silt, clay and saturated hydraulic conductivity change was noticed at 580C and 490C whereas the least change in bulk density, total porosity and moisture content was observed at 600C, 580C and 500C respectively. In the sub-surface soil, highest content of sand, clay and saturated hydraulic conductivity changes was noticed at 300C, 250C, and 330C whereas the least change in silt, bulk density, total porosity, and moisture content was observed at 250C, 360C and 370C. Overall, the most varied physical property at the soil surface was total porosity (CV = 37.74%) and the least varied was sand (CV = 5.16%). In the sub-surface layer, the must varied physical property was silt (CV = 42. 17%) while the least varied was sand (Cv = 7.63%).
Thermal effects on soil chemical properties. The thermal effect on soil chemical properties of both surface and sub-surface soil are presented in Table 3. Following burning, different temperatures were measured at surface and sub-surface soil layers. In the surface layer, the temperatures were 350C, 400C, 490C, 500C, 580C and 600C while the temperatures for sub-surface soil were 250C, 300C, 330C and 370C.
Soil pH (Table 3) decreased with increasing temperature up to 60oC. this was probably due to the lowering he buffer action associated with denaturing of the colloids and the combustion of organic matter. The successive increase between 35oC and 40oC is probably attributable to the loss of OH groups resulting from the denaturing of clay mineral (Giovannini et al., 1990). At 240C electrical conductivity was 0.03 dSm-1, however, electrical conductivity was irregularly distributed as temperature increased in the surface soil. But in the sub-
surface layer, electrical conductivity decreased to 0.01 dSm-1 at heating temperature of 360C, and 370C from 0.0 2 dSm-1 when the initial temperature rise was 250C.
Relative highest value of total nitrogen (0.80gkg1) was noticed at 350C, 400C, 480C, 500C and 580C in the surface soil whereas in the sub-soil, high value of total nitrogen (0.70 gkg-1) was noticed only at 300C. Was it a matter of compensation between the decrease caused by volatilization in the sub-surface layer or is the soil N not affected by increase heating? At this time we are unable to account for the balance between the outputs and inputs of N.
At the initial temperature of 350C, the content of available phosphorus was 30.97 mgkg-1 but decreased to 29.80 mgkg-1 at 490C. At the sub-surface soil, available phosphorus increased to 27.64 mgkg-1 at 300C from 25.97 mgkg-1 when the initial temperature rise was 250C. Increased in available P with temperature increase at subsurface layer, confirming the report of Giovannini et al., (1990) that the available phosphorus is the outcome of the mineralization process of organic phosphorus.
At the surface soil, highest calcium content (9.12 cmolkg-1) was observed at 350C and 600C. whereas at the sub-surface soil, calcium increase to 8.64 cmolkg-1 at 360C from 2.40 cmolkg-1 at initial temperature of 250C . At the sub-face soil calcium increases to 8. 64 cmolkg-1 at 360C from 2.40 cmolkg-1 at initial temperature of 250C. At the soil surface, highest magnesium content (7.68 cmolkg-1) was observed at 350C and 600C whereas at the subsurface soil Mg increased to 7.20 cmolkg-1 at 360C from 1.14 cmolkg-1 at initial temperature of 250C. Potassium increased to 0.08 cmolkg-1 at 350C and 0.05 cmolkg-1 at 400C at the surface soil, whereas at the sub-surface soil, K increased to 0.07 cmolkg-1 at 330C from 0.06 cmolkg-1 at initial temperature rise of 250C. Sodium decreased in the surface layer to 0.06 cmolkg-1 at 480C from 0.04 cmolkg-1 when the initial temperature rise was 35oC whereas at the subsurface soil, Na increased to 0.06 cmolkg-1 at 370C from 0.04 cmolkg-1 from the initial temperature rise of 250C.
Exchange acidity increased to 5.12 cmolkg-1 at 400C from 0.80 cmolkg-1 at initial temperature of 250C at the surface soil but at the sub-surface, exchange acidity decreased to 1.12 cmolkg-1 at 330C and 360C, from 1.60 cmolkg-1 when the initial temperature rise was 250C. Effective cation exchange capacity increased to 17.71 cmolkg-1 at 600C from 17.70 cmolkg-1 when the initial temperature rise was 350C at the surface soil. At the sub-surface soil, effective cation exchange capacity increased to 16.89 cmolkg-1 at 360C from 5.54 cmolkg-1 when the initial temperature rise was 250C.
At the surface soil, organic carbon increased to 19.20 gkg-1 at 480C from 18.90 gkg-1 at the initial temperature of 350C whereas, at the sub-surface soil, organic carbon increased to 15.26 gkg-1 at 300C from 13.20 gkg-1 at initial temperature of 250C. Whereas, base saturation increased to 95.48% at 350C from 95.40% when the initial temperature rise was 350C whereas at the sub-surface, base saturation increase to 94.32% at 360C from 71.12% at the initial temperature of 250C. C:N ratio increased to 24.57 at 600C from 23.63 when the initial temperature rise was 350C at the surface layer. At the sub-surface soil layer, C:N ratio increased to 24.40 at 330C from 22.00 when initial temperature rise was 250C. Despite pronounced variability in soil chemical properties at different heat intensity, the most varied chemical property of the soil at the surface was electrical conductivity (CV = 75.00%) while the least varied was pH (CV =2.63%). In the sub-surface soil, the most varied chemical property was exchange acidity (CV = 87.17%) while the least varied was potassium (CV = 1.69%).
Correlation of heating temperatures, and depth with soil properties. As summarized in Table 4, the correlation of heating temperatures and depths with soil properties in the preburnt and burnt plots of arable field revealed that, clay, 1mm, 0.5 mm stable aggregate and organic carbon relates positively and highly significant (P=0.05) with depth in the burnt plots (r = 0.648**, 0.718**, 0.712**, 0.840* respectively). This implies that these parameters increase with corresponding increase in depth. But total nitrogen stock, sand, saturated hydraulic conductivity, total nitrogen, soil carbon stock, pH and electrical conductivity correlated negatively and highly significant with soil depth (r = -0.617**, -0.656**, -0.478*, -0.753**, -0.697**, -0.835**, -0.544* respectively). Therefore, increase in soil depth decreased
the concentration of these soil parameters (acidity increases) under burnt condition. While significant, the high coefficient of determination indicates that most of the variability noticed in the burnt plots could be explained by the measured parameters.
Table 4 - Significantly Related Soil Properties with Depth and Temperature in Burnt and Pre-burnt
soils
I Treatments Depth i Temperature i
TN (r = -0.617**) : Sand (r = 0.518*) :
WSA0.5 (r = 0.820**) : Clay (r =-0.619**) :
Clay (r = 0.648**) i WSA 1 (r = -0.578*) :
Ks(r = -0.478*) i WSA0.5(r=-0.780**) i
WSA1mm(r= 0.718**) : WSA0.25 (r = -0.526*) :
: Post-Burnt WSA0.5mm (r = 0.712**) i TN ( r = 0.478*) i
TNS(r = -0.753**) : oc(r = 0.582*) :
OC (r = 0.840**) : pH (r = 0. 595 **) :
: : scs (r =-0.697**) :
: : pH (r= - 0.835**) :
! ! EC (r =-0.544*) :
: ! Clay (r= 0.481*) :
; ; Ks(r=-0.673**) :
: Pre burnt BD (r— 0.636 **)
: : P (r— -0.643 **) :
! i WSA1 (r— 0.773**) :
** Correlation is significant at the 0.01 level. *Correlation is significant at the 0.05 level.
Temperature differences affect sand, total nitrogen, organic carbon and pH contents of the soils positively (r = 0.518*, 0.478*, 0.582*, 0.595** respectively), whereas a reduction in the soil temperature increased the concentrations of clay, 1mm, 0.05mm and 0.25 mm stable soil aggregates in the soil (r = -0.619**, -0.578*, -0.780, -0.526* respectively) after burning. Thus, based on the correlation results, soil management in burnt plot based on soil aggregates of 1 mm, 0.05 mm, 0.25 m and Total N, organic C, and pH fertility would lead to better management decisions.
Under pre-burnt condition, depth correlates positively and significantly with clay, bulk density, 1 mm and 0.5mm stable soil aggregates to water (r = 0.481*, 0.636**, 0.773* and 0.820** respectively). This means that as the soil depth increase, clay, bulk density, 1 mm and 0.5mm water stable aggregate also increases. As expected, sand, saturated hydraulic conductivity and total porosity decreased with an increase in depth (r = -0.542*, 0.673**, and -0.643** respectively) in the un-burnt plots. This shows that increase in soil depth decrease sand fraction, Ks and total porosity. The negative relationships would seem to relate poor pore tortuosity down the profile.
CONCLUSION
Burning results in changes in soil temperature, soil moisture and nutrient availability. Fire significantly affects soil properties due to rapidly combusted organic matter on the soil surface. The organic matter acts as the primary reservoir for several nutrients, stable aggregates and infiltration. Also, this may reduce the resistance of the soil to erosion due to tensile cracks and excess pore-water associated with burning during the first down pour. However, this research has shown that there is immediate increase in plant nutrients due to the release of occluded minerals after burning, but sure consequences of repeated vegetation burning might be detrimental to soil health.
RECOMMENDATIONS FOR FUTURE RESEARCH
The results of this study indicate the need for a review of the method of land clearing for sustainable agricultural production. Therefore, sequential soil samplings should be carried out after slash-and-burn land clearing say, monthly for four growing seasons, to assess further changes in the soil quality attributes.
ACKNOWLEDGMENTS
We thank Miss. Ndifreke Etim, an undergraduate student working on Agricultural burning project based at University of Uyo. We thank Mr. Idongesit Ambrose, a staff of Akwa Ibom State Ministry of Environment and BGI-resources LTD. Laboratory staff Port Harcourt, for access to facilities to complete soil sample analyses. We acknowledge support from the Department of Soil Science, University of Uyo for providing the experimental site for this study and the anonymous reviewers for their useful contributions.
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