Assessing Soil Biological Properties of Natural and Planted Forests in the Malaysian Tropical Lowland Dipterocarp Forest

Problem statement: A study was conducted to evaluate and compare the soil biological properties of a natural forest and an 18-year-old s tand of Shorea leprosula in Chikus Forest Reserve, Perak, Malaysia. Approach: Soils were sampled at depths of 0-15 cm (topsoil) a nd 15-30 cm (subsoil) in six subplots (20×20 m) of natural forest (C1) an d of a planted S. leprosula (C2) plot. Fresh composite soil samples were kept in UV-sterilized p olyethylene bags prior to analysis in the laboratory. The microbial population count was dete rmined using a spread-plate count technique. The microbial enzymatic activity was elucidated using a Fluorescein Diacetate (FDA) hydrolysis assay; microbial biomass was extracted using a rapid chlor oform fumigation extraction method. The Microbial Biomass C (MBC) was determined by wet dic hromate oxidation; Kjeldahl digestion and a distillation method were used for evaluation of Mic robial Biomass N (MBN). Results: Results indicate that only the microbial biomass N and the populatio n c unt in the soil at the 0-15 cm depth were found to be higher in C1 compared to C2. The higher micro bial population count in the soil at the 0-15 cm depth of C1 compared to C2 was enhanced by the larg mount of organic matter that serves as a suitable medium for soil microbial growth. The high er MBN in the C1 soil was also influenced by the high content of organic material available that encourages activities of decomposing bacteria to take place. Similarities in the soil biological pro erties of the plots with regard to enzymatic activ ity and microbial biomass Care believed to be influence d by the same topographic gradient. The higher MBC/MBN ratios found in soils of C2 compared to C1 were due to the low availability of N compared to C, might result from N utilization by s oil microbes for organic material decomposition. Conclusion: There are similarities in microbial enzymatic activ ity and biomass C, but not in microbial population counts and biomass N, between a natural forest and an 18-year-old stand of S. leprosula in Chikus Forest Reserve, Perak, Malaysia.


INTRODUCTION
Forest rehabilitation is one of the vital strategies used to restore a degraded forest to its initial state (Singh et al., 2011;Arifin et al., 2008;Hamzah et al., 2009;Zaidey et al., 2010: Heryati et al., 2011a2011b). Rehabilitation also helps to reduce the demand for woody and non-woody products from natural forest. Therefore, it is crucial to carry out soil fertility evaluations of rehabilitated forests to evaluate the success of rehabilitation efforts.
It is undeniable that soil biological activity plays a key role in the catalyzing cycles of nutrients in the soil. Bollard and Stotzky (1990) found that the sensitive nature of microbial activity to environmental changes made it a good indicator of soil fertility. Islam and Weil (2000) also justified the importance of including soil biological parameters in determining soil fertility; these parameters can show how land management affects soil microbial biomass. In addition, Van Dobben and McConnell (1975) found that 90% of the energy cycle in the soil was induced by microbial decomposition activity, which provided a preview of soil organic matter turnover in a particular area.
The Chikus Forest Reserve, Perak, Malaysia is considered to be a lowland dipterocarp forest and it was subjected to a forest rehabilitation program by the Forestry Department of Peninsular Malaysia (FDPM) and the Japan International Cooperation Agency (JICA) via the Multi-Storied Forest Management System (MSFMS) (Arifin et al., 2008). Many soil fertility evaluations were carried out, focusing on the physicochemical and mineralogical properties of the soil in these forests (Arifin et al., 2008;Abdu et al., 2007;Saga et al., 2010); however, no information on the soil biological properties was collected. Hence, this study was carried out to evaluate and compare the soil biological properties of natural and rehabilitated forests.

Study site and soil sampling:
The study was carried out in a natural forest (N 04.10076° E101.19411°, ± 28 m) and an 18-year-old stand of Shorea leprosula (N 04.09197° E 101.19499°, ± 28 m) in January, 2011. The area has an average annual precipitation of 3223 mm and a mean temperature of 27.7°C. S. leprosula was planted in 1992 through collaborative work of the Multi-Storied Forest Management System involving the Forestry Department Peninsular Malaysia, the Perak State Forestry Department and the Japan International Cooperation Agency (JICA). The planting distance was 10 m x 3 m. Natural forest and planted S. leprosula plots were marked as C1 and C2, respectively. Six subplots were established at each plot and a composite sample was randomly collected from depths of 0-15 cm (topsoil) and 15-30 cm (subsoil). The fresh composite soil samples were kept in UV-sterilized polyethylene bags under 0-4°C in ice-filled polystyrene boxes prior to analysis.

Soil analyses:
The spread-plate technique described by Sleytr et al. (2007) was used for microbial population counts. The microbial enzymatic activity was evaluated using the Fluorescein Diacetate (FDA) hydrolysis assay (Sanchez-Monedero et al., 2008). Microbial biomass was extracted via a rapid ethanol-free chloroform fumigation extraction technique (Witt et al., 2000).
Microbial Biomass C (MBC) was analysed by wet dichromate oxidation (Vasquez-Murrieta et al., 2007;Vance et al., 1987) and microbial biomass N (MBN) was elucidated through Kjeldahl digestion and a distillation technique (Simmone et al., 1997;Brookes et al., 1985). The loss-on-ignition method used in Ahmadpour et al. (2010) was used for soil organic matter and organic carbon determination. Kjeldahl digestion was used for total N evaluation and soil acidity was determined using a glass electrode and a soil: water suspension ratio of1:2.5 (Akbar et al., 2010).

Data analyses:
Mean values were compared using the Student's t-test. Linear regression analysis using SPSS ver. 16.0 was performed to determine any correlations or relationships derived from comparisons between microbial biomass C and organic matter and between microbial biomass N and total N at the same soil depths.

Microbial biomass C to microbial biomass N (MBC/MBN) ratio:
The ratio of MBC/MBN was found to be higher in C2 compared to C1 (Fig. 5)

Organic matter, organic C, total N and soil acidity:
The soil organic matter and organic C were significantly higher (P≤0.05) in C1 compared to C2 at the same soil depth (Table 1). In addition, total N was also found to be significantly different (P≤0.05), except at the 15-30 cm soil depth. No significant difference was found in soil acidity at the 0-15 cm soil depth, but the soil of C2was significantly more acidic (P≤0.05) at the15-30 cm soil depth than the soil of C1.  No linear relationships were found between microbial biomass C and organic matter or between microbial biomass N and total N in the soils of C1 and C2 at either soil depth.

DISCUSSION
Higher microbial population counts in soil of natural forest compared to the S. leprosula plot could be due to the high content of organic matter and humic substances in the soil at the 0-15 cm depth. The high content of organic material in the soil would trigger soil microbial activities and, thus, could subsequently result in a larger population in the C1 soil.
The similarities between the plots in soil microbial enzymatic activity and MBC could be due to the fact these plots are located at the same elevation; both plots were located about 28 m above sea level, which is considered to be lowland forest area. Furthermore, both areas are classified as tropical rainforest and receive heavy rainfall; hence, these factors promote the same range of microbial diversity and activity (Barbhuiya et al., 2004). Arunachalam and Pandey (2003) found that soil microbial biomass could be a more reliable indicator of ecosystem recovery than other ecological approaches because it is the most labile and sensitive portion of organic matter and determines soil fertility. Arifin et al. (2008) and Carter (2002) reported that the distribution of soil organic biodiversity is also influenced by the type and carrying capacity of soil, the weather and how the land at a particular site was managed. The soil MBN was found to be significantly different between the plots for both soil depths. This is could be due to low availability of total N in the S. leprosula plot. In the field, the natural forest area was waterlogged, whereas the S. leprosula plot was welldrained. In the natural forest, adequate supply of water or suitable humidity leads to enhanced growth of nitrogen-fixing bacteria and allows increases in the decomposition of biomass N to take place. In addition, Behera and Sahani (2003) found that optimum microbial activity occurs when there is an adequate supply of growth substrates. Hence, drought or inadequate supply of water itself will inhibit or slow down nutrient cycling and the decomposition of organisms by affecting the microbial activity in the soil. However, the lower level of MBN compared to MBC in both plots could be due to the loss of MBN from the ecosystem that is caused by low N mobilization (Maithani et al., 1998).
Differences in the MBC/MBN ratio between the natural forest and S. leprosula plots indicate that qualitative changes take place in microbial biomass; these changes are believed to be enhanced by the cycling and turnover processes of micro flora in soils (Behera and Sahani, 2003). The MBC/MBN ratios found at the 0-15 cm and 15-30 cm depths in the natural forest were within the optimal range of 5-8 (Joergensen et al., 1995). However, the higher MBC/MBN ratio in the S. leprosula plot compared to the natural forest could be due to a higher ratio in the C2 soil of fungi to bacteria or actinomycetes (Behera and Sahani, 2003;Campbell et al., 1994). Furthermore, fungal domination of microbial biomass also contributes to the higher MBC/MBN ratio we observed in soil of the S. leprosula plot; the higher MBC/MBN ratio in the lower soil layer of both the natural forest and S. leprosula plots indicates a low level of accumulation of organic matter and fine roots (Bremer and Van Kessel, 1992;Maithani et al., 1996). The higher MBC/MBN ratio in soil of the S. leprosula plot could be caused by the higher organic matter and organic C content compared to the availability of total N in the soil at natural forest. The low N in the soil of the S. leprosula plot also could be due to the available N being utilized by microbes during the decomposition of soil organic matter or humus. This result indicates that reforestation of a particular site requires more time to populate adequate detrital medium for microbial growth (Arunachalam and Pandey, 2003).
The dynamics of soil biological properties provide us with data regarding the current conditions and early effects of land management, monitoring and sustainability status in a particular area. Furthermore, the abundance of microbial communities, which catalyze numerous enzymatic cycles of nutrients in the soil, will subsequently increase and maintain the soil fertility and productivity (Behera and Sahani, 2003).

CONCLUSION
Analysis of the biological activities of the soils in a natural forest and the S. leprosula plantation in Chikus Forest Reserve, Perak showed no significant differences, with the exceptions of microbial biomass N and the microbial population counts at the 0-15 cm soil depth. Hence, the data prove that rehabilitation of degraded forestland helps restore soil biological activities to their original state or to the extent seen in natural forests.

ACKNOWLEDGEMENT
We wish to thank the Perak South District, Department of Forestry, Perak who allowed us to carry out the research project. This study was financially supported by the Fundamental Research Grant Scheme (FRGS) from the Ministry of Higher Education of Malaysia (MOHE) through the Universiti Putra Malaysia (UPM), Malaysia. We would also like to express our gratitude to all of the Department of Forestry staff in Perak who helped us with the fieldwork.