Vertical Distribution of Soil Organic Carbon and Nitrogen in a Tropical Community Forest of Nepal.
Soils are considered as viable sinks of atmospheric carbon (C) and may significantly contribute to climate change mitigation [1-3]. Soil carbon sequestration is something that we cannot afford to ignore . Soils store a large share of organic carbon--twice more than vegetation and two-thirds more than the atmosphere . Soils serve as both source and sink of C[O.sub.2]. Therefore, they have a great potential to reduce emissions and enhance carbon sequestration through better soil management. In recent years, soil organic carbon (SOC) has received worldwide attention in the context of international policy agendas of C[O.sub.2] emission . Forests are the largest carbon stock in terrestrial ecosystems . Soil nitrogen (N) stocks and carbon-nitrogen ratio (C/N ratio) are considered as indicators of carbon sequestration potential in soils [6, 7]. Soil pH and C/N ratio are considered as major factors in the soil microbial structure and activities [8-10]. The C/N ratio may be treated as an approximate indicator of the quality of organic matter inflowing to forest soils and of its transformation to SOC. To increase SOC and nitrogen or to maintain these quantities close to native levels is one of the goals of conservation . Vertical patterns of SOC can contribute as an input or as an independent validation for biogeochemical models and thus provide valuable information for examining the responses of terrestrial ecosystems to global change . Previous studies (e.g., ) primarily focused on the topsoil carbon stock, and carbon dynamics in deeper soil layers and driving factors behind vertical distributions of SOC remain poorly understood . SOC in subsoil horizons has become the subject of intensive research only in recent years because it was recognised that subsoil carbon contributes greatly to the total carbon stocks within a soil profile . Song et al.  suggests that deep soil layers contribute considerable amounts and should not be omitted in soil carbon estimates. Nitrogen plays a crucial role through the interaction with carbon in the ecosystem productivity and carbon sequestration [17-19]. Many previous studies on the topsoil layers, i.e., 0-20 cm or slightly more [20, 21], are indication for need for studies on deeper soil layers. Thus, improved knowledge of distribution of SOC and nitrogen across different soil depths is essential to determine whether carbon in deep soil layers will react to global change and accelerate the increase in atmospheric carbon dioxide (C[O.sub.2]) concentration . Applying the concept of biogeochemistry provides a framework to identify and evaluate the sources and fates of chemical elements in a system approach. Predicting the future nutrient balance in forest ecosystems requires an estimation of forest carbon pools and fluxes . Estimating carbon pools under existing land uses and their distribution within the soil profile provides baseline data to carbon sequestration over time . Field data on C and N pools are also needed for the estimation of sources and sinks of greenhouse gases at the national level to be reported to the UNFCCC, to predict the carbon sequestration potential of nation's forest resources and to promote nutrient balance for enhanced productivity of the ecosystems. Modelling of biogeochemistry of major nutrients is useful in predicting the changes in the carbon pools in the forest lands over time in the context of changing management practices and climate. Several authors have indicated about the uncertainties associated with estimating SOC due to lack of extensive soil sampling data [15, 25, 26]. In this context, the objective of this research was to quantify SOC and nitrogen in different soil layers in a tropical community forest of Nepal up to 1 m soil depth.
2. Materials and Methods
2.1. Research Site. The research was conducted in Kankali community forest (27.65[degrees]N, 84.57[degrees]E, and 749.18 ha) in Chitwan district located in the tropical region of Nepal (Figure 1). It is a natural tropical forest on 220-580 m above mean sea level handed over to local community for management in 1995. The average annual maximum and minimum temperatures in the research site were 31 degree Celsius and 18 degree Celsius, respectively, and the average relative humidity and average annual rainfall of the area were 78% and 190 mm, respectively . The forest is dominated by Shorea robusta. Other associated plant species are Semecarpus anacardium, Lagerstroemia parviflora, Holarrhena pubescens, Buchanania latifolia, Terminalia alata, Dalbergia sissoo, and Acacia catechu [27, 28]. The forest contains sandstone and mudstone of the lower and middle Siwaliks with quartz, biotite, muscovite, feldspar, and opaque minerals .
2.2. Soil Sampling. Soil samples, each around half kilogram, were collected from 0 to 20 cm, 21 to 40 cm, 41 to 60 cm, 61 to 80 cm, and 81 to 100 cm depths of soil profile from 12 randomly distributed pits dug in both Dalbergia Acacia/ Riverine forest and Shorea robusta forest. The collected soil samples were packed in plastic bags, labelled, air-dried, and taken to the Regional Soil Test Laboratory, Makwanpur, for analysis. The overall field measurement methods were based on guidelines of ANSAB .
2.3. Soil Analysis. Soil bulk density was determined using the soil core sampler having a diameter of 5.7 cm . The SOC concentration was determined by dry combustion of oven-dry soil samples . The percentage of soil organic matter was converted to SOC by multiplying with 0.58, as soil organic matter is assumed to contain 58% of organic carbon. Total nitrogen (TN) was determined by the Kjeldahl digestion-distillation method . Soil pH of a composite sample (of all soil layers and soil pits) was determined using a pH probe with a glass-calomel electrode (Suntronics pH electrode, Suntronics India), keeping 1: 1 soil: water ratio :
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The significance difference of bulk density, SOC, nitrogen, and C/N ratio was tested using ANOVA at 5% level of significance. The correlation test was applied to find out relationship between the variables. SPSS software (IBM SPSS Statistics, IBM Corporation, Version 20) was used for statistical analysis of the data.
3. Results and Discussion
The average soil organic matter and nitrogen concentration in 1 m soil profile depth of tropical community forest was found to be 0.99% [+ or -] 0.45% and 0.10% [+ or -] 0.05%, respectively (Table 1). These values were close to the values reported by Paudel and Sah  (1.74% [+ or -] 0.31% SOC and 0.111% [+ or -] 0.01% soil nitrogen in the mixed S. robusta forest), Sharma and Chhetri  (0.09% soil nitrogen) and Ghimire et al.  (0.95% SOC in the S. robusta-dominated forest). This may be due to a similar forest type containing S. robusta as dominant forest species, physiographical range, climatic condition, and soil sampling. Our finding on soil organic matter was lower than the value reported by Sharma and Chhetri  (1.28%) in the same forest. It maybe due to the fact that they reported the soil organic matter concentration of only 0-30 cm soil profile depth.
In this research, the average C/N ratio of the soil was found to be 9.90 (Table 1). Calculations were made on the basis of Kirkby et al. . Data presented by Ostrowska and Porcbska  indicated that the mean C/N ratio for humus was 12.7, the mean C/N ratio in the arable soils examined by the authors was 13.6, and the mean C/N ratio for an international soil amounted to 11.6, close to the value reported in this research (9.90).
The average soil pH was found to be 5.3 [+ or -] 0.67. The pH range in this research was close to the values reported by Paudel and Sah  (5.26 [+ or -] 0.58 in the mixed S. robusta forest) and Singh and Singh  (4.5-5.5 in the S. robusta forest). This may be due to similar forest types containing S. robusta as dominant species. pH was lower than the values reported by Sigdel  (5.90-6.42), by Karki  (6.4-7.1), Sharma and Chhetri  (4.64), or by Singh and Singh  (6.7-6.8). This may be due to variation in topography, forest condition, and soil sampling.
The average bulk density of the soil in 1m soil profile depth was found to be 1.28 [+ or -] 0.21 gram per cubic centimetre (Table 1), close to the value 1.17 gram per cubic centimetre  and 1.18 gram per cubic centimetre found by Ghimire et al.  in two S. robusta-dominated tropical forests of Nepal.
The ANOVA test shows that there was significant difference of SOC (p = 0.001) and nitrogen (p = 0.001) between different soil layers at 5% level of significance. There was no significant difference in bulk density of soil between different soil layers (p = 0.193) at 5% level of significance (Table 1).
The stock of SOC and nitrogen in tropical community forest was found to be 122.36 t/ha and 12.74 t/ha, respectively, within 1 m soil profile depth (Table 2). Ghimire et al.  quantified 110 t/ha SOC in 1 m soil profile depth in a tropical forest dominated by Shorea robusta in Makwanpur district of Nepal. Our study district (Chitwan) is the neighboring district to Makwanpur district where Ghimire et al.'s  study was conducted and our finding on SOC stock within 1 m soil profile depth is close to theirs. Result of this research on SOC stock in 1 m soil profile depth is more than that reported by Gurung et al.  (96.53 t/ha) in S. robusta-dominated forests. This variation could be due to their different soil sampling techniques (up to 30 cm soil depth) and forest management regimes.
The SOC and nitrogen were found to decrease with increased soil depth; for example, in 0-20 cm soil depth, SOC stock was found to be 34.35 t/ha, but in 81100 cm soil depth, it was found to be 20.34 t/ha (Table 2). The higher organic carbon percentage in the top layer may be due to rapid decomposition of forest litter in a favorable environment. Soils with rich organic carbon levels generally indicate high fertility, and therefore, it is important to maintain its optimum level that requires a careful land use and management practices . A study by Tiwari et al.  also reported declining trend of total nitrogen with increasing soil depths. Gautam and Mandal  also reported a decreasing trend of SOC and nitrogen with increased depths of soil in a tropical moist forest in eastern Nepal. Similar findings on the decline of stocks of SOC with the increase in soil depth were reported by Pandey and Bhusal  in S. robusta-dominated forests of hills and Terai regions of Nepal and Ghimire et al.  in the S. robusta-dominated tropical community forest of Nepal. Song et al.  also reported that the concentrations of SOC and nitrogen decreased with depth, and the greatest concentration was in the 0-10 cm topsoil in selected forests of China.
Bulk density of the soil was negatively correlated with SOC (r = -0.425) and nitrogen (r = -0.308). However, there was a positive correlation between SOC and nitrogen (r = 0.429). Gautam and Mandal  also reported positive correlation between SOC and nitrogen in a tropical moist forest in eastern Nepal. Negative correlation between SOC and bulk density of soil was also reported by Ali et al.  and Ghimire et al. .
This article indicates the first reporting of SOC and nitrogen in Kankali community forest, Chitwan, up to 1 m soil profile depth. Results of this study show evidence of a tropical community forest having role in storage of SOC and nitrogen within 1 m soil profile depth in Nepal. The average bulk density of the soil in 1 m soil profile depth was 1.28 gram per cubic centimetre. The average carbon to nitrogen ratio (C/N ratio) of the soil was found to be 9.90. The SOC and nitrogen were found decreasing with increased soil depths, with a statistically significant difference in values among across different soil layers. Bulk density and C/N ratio of the soil was found increasing with increased soil depths. The forest soil was found slightly acidic. It is found that the soil of tropical Shorea robusta-dominated community forest had high potentiality of storage of organic carbon and nitrogen and is playing a role in global climate change mitigation. The topsoil contained higher amounts of SOC and nitrogen, so the forest management practices should consider retaining organic matter in the forest floor.
The data used to support the findings of this study are available from the corresponding author upon request.
This research was a part of author's regular academic activity.
Conflicts of Interest
The author declares that there are no conflicts of interest regarding the publication of this paper.
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Gandhiv Kafle [ID]
Faculty of Forestry, Agriculture and Forestry University, Hetauda, Nepal
Correspondence should be addressed to Gandhiv Kafle; firstname.lastname@example.org
Received 23 June 2019; Revised 23 October 2019; Accepted 4 November 2019; Published 12 December 2019
Academic Editor: Qing-Lai Dang
Caption: Figure 1: Kankali community forest (source: ).
Table 1: Bulk density, organic carbon, nitrogen, and C/N ratio in different soil layers. Soil layer (cm) Average bulk density P Average organic carbon (gm/[cm.sup.3]) value (%) 0-20 1.18 0.193 1.46 21-40 1.28 1.10 41-60 1.24 0.84 61-80 1.34 0.77 81-100 1.36 0.76 Mean (0-100 cm) 1.28 0.99 Soil layer (cm) P Average nitrogen P C/N value (%) value ratio 0-20 0.001 * 0.17 0.001 * 8.59 21-40 0.13 8.46 41-60 0.09 9.33 61-80 0.07 11.00 81-100 0.05 15.20 Mean (0-100 cm) 0.0 9.90 Soil layer (cm) p value 0-20 0.001 * 21-40 41-60 61-80 81-100 Mean (0-100 cm) * Significance difference at 5% level of significance. A p value is a measure of probability used for hypothesis testing. If we have a p value less than 0.05, the null hypothesis would be rejected in favor of the alternative hypothesis indicating dissimilarity of at least one mean from the rest. Table 2: Organic carbon and nitrogen stocks in different soil layers. Soil layer (cm) SOC stock (t/ha) Soil nitrogen stock (t/ha) 0-20 34.35 3.97 21-40 27.33 3.22 41-60 20.23 2.13 61-80 20.11 1.98 81-100 20.34 1.44 Total 122.36 12.74
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|Publication:||International Journal of Forestry Research|
|Date:||Dec 1, 2019|
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