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INTRODUCTION
The establishment of fast-growing plantations with short-rotation woody crops in marginal and degraded lands is globally prevalent and has been the focus of research at every national and international level (Silva et al., 2019). However, such plantations are not only emerging as the primary source for meeting the ever-increasing need for wood, bioenergy, and raw materials for a variety of forest-based industries (Arce, 2019), but also hold great potential to restore the productivity and biodiversity of the degraded ecosystems (Silva et al., 2019). Apart from bolstering the ecological functions of degraded lands, they also help to preserve dwindling natural forests (Farooq et al., 2021), reduce atmospheric CO2, and improve the socioeconomic status of local communities.
At a global level, bamboo is regarded as the most versatile, sustainable, and replenishable plant resource, with enormous economic, social, and environmental benefits and covering an area of around 37 million ha (Goh et al., 2020). Bamboo has multifaceted uses like food, fuel, fiber, and construction material, playing an integral role in the sustainable development of equatorial countries (Akinlabi et al., 2017). For a few decades, bamboo planting has become more popular around the world, with Asia accounting for 30% of all bamboo resources planted (Lobovikov et al., 2007). In India alone, around 136 bamboo species in 29 genera are prevalent, both in cultivated plantations as well as forest ecosystems, with maximum species diversity in the forest subgroups: 2B, 3B, and 3C (Sharma & Nirmala, 2015).
Different bamboo species have distinctive growth rates and biomass allocation patterns, and they require various environmental conditions to proliferate. The assessment and evaluation of the growth performance of bamboo species are integral to calculating their adaptive capacity, production potential, and nutrient transport during harvest (Alemayehu et al., 2015). Comparative studies on the growth behavior of the various bamboo species are integral for the selection for commercial plantations, agroforestry/social forestry programs, and plantations for ecosystem conservation activities, among other things.
Bamboo plantations play many ecologically beneficial and environmentally sustainable roles, i.e., in controlling soil erosion depletion, reclaiming degraded lands, water resource preservation, and climate change mitigation (Tu et al., 2013). Bamboo’s fast growth with thick foliage and dense fibrous interconnected root systems, which results in a thick litter layer on the soil, helps to conserve soil moisture, improve soil fertility, and act as a barrier to soil nutrient loss. The impact of bamboo on soil is species-specific and has a varied effect on soil physicochemical properties, with most species tending to improve soil aeration, bulk density, and soil microbial biomass due to the large root surface area and the secretion of root exudes in the rhizosphere (Liu et al., 2021). Therefore, the goal of this study is to understand the growth patterns, nutrient uptake, and effects on soil qualities of several bamboo species. We hypothesized that the aboveground growth parameters, nutrient allocation, and soil properties of bamboo are species-specific.
MATERIALS AND METHODS
Study area
This study was conducted in a 14-year-old bamboo plantation at the Agroforestry Research Center, G. B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand (29°03′73′′ N latitude, 79°45′25′′ E longitude, and 236 m altitude) during the 2021-2022 period. This area is well known as the Tarai region, a narrow strip located in the foothills of the Shivalik range of the Himalaya. The climate of the experimentation site is humid semi-tropical, with chilled winters and hot dry summers, falling in the Cwa category of the Koppen-Geiger system. During the research period, the mean maximum temperature reached 45 °C in May, and the average lowest temperature was 0.5 °C in January. Around 80-90% of the rainfall is received from the south-west monsoon in July-September, with a mean annual rainfall of 1405 mm. Alluvial materials with medium to moderately coarse textures make up soils of the Terai region, which are mostly influenced by tall vegetation and moderate to well-drained conditions. The soils are referred to as mollisols because they have mollic horizons and weakly developed mollic epipedons.
Experimental design
The plantation was established in 2005 with six bamboo species: Dendrocalamus hamiltonii, Bambusa nutans, Dendrocalamus asper, Bambusa bambos, Bambusa balcooa, and Dendrocalamus strictus. The bamboo stand was established with a 5 m × 5 m clump spacing in a randomized block statistical design with four replications. For each bamboo species, four plants were planted in each plot (block plantation), occupying an area of 10 m2. In total, there were 24 plots, covering a total area of 240 m2.
METHODOLOGY
Biomass estimation
A complete enumeration was carried out, counting the number of culms per clump with the help of the cross-stick method. Then, the circumference of the bamboo clump (i.e., clump girth) was determined at breast height (DBH at 1.37 m) using measuring tape. Three culms of different ages (1, 2, or 3 years old or above) from each replication (12 for each species) were selected based on their physiological appearance, i.e., the color of the culm, the presence of culm sheath. The length, diameter, number of nodes, and internodal length of selected culms were measured, which were destructively felled from ground level during December. To determine the fresh weight, branches were parted, and leaves were removed from the felled culms of each bamboo species. To calculate the culm, branch, and leaf biomass of various bamboo species, representative samples were taken from each bamboo species and then oven dried. The oven dry weight of the representative sample was then multiplied by the total number of culms. The total aboveground biomass was calculated by adding the leaf, branch, and culm biomass for each bamboo species.
Nutrient uptake
To estimate the nutrient uptake of the six bamboo species under study, the samples of culm, branches, and leaves from specimens of different ages (1, 2, or 3 years old or above) from each replication were collected and kept under sun, left to dry for 2-3 days. The samples were further dried for two days in a hot air oven at 65±5ο C and then ground to fine powder. After grinding, the samples passed through 0.5 mm sieve and were chemically analyzed for nitrogen (Jackson, 1967, i.e., modified KEL PLUS auto N-analyzer), phosphorus (vanado-molybdate method), and potassium (flame emission spectrophotometer). The following formulas were used to determine the nutrient (NPK) uptake by the culm, branch, and leaves of six bamboo species (Singh & Rai, 2012):
Soil physicochemical properties and microbial activities
The soil samples were collected randomly from between the rows of bamboo and from the plot devoid of bamboo (control plot) at three levels of soil depth, i.e., 0-40, 40-80, and 80-120 cm. A composite sample was then prepared for each soil depth, combining soils from three sampling sites and producing one sample per level from each plot. Following collection, the samples were air dried, ground, and sieved. The bulk density was estimated by using the core sampler technique (Blake & Hartge, 1986), and a steel measuring cylinder was used to gauge the particle density. Soil porosity was calculated via the method described by Danielson & Southerland (1986). The soil pH was measured with a 1:2.5 soil: water suspension ratio (Jackson, 1967), and the electrical conductivity of the soil was measured using a digital microprocessor-based electrical conductivity meter (Bower & Wilcox, 1965). The quick titration method was used to determine the amount of organic carbon in the soil (Walkley & Black, 1934). The alkaline potassium permanganate method was used to determine the available soil nitrogen (Subbaiah & Asija, 1956), the available phosphorus was estimated via Bray no. 1 extraction followed by the reduced molybdate blue color estimation method (Watanabe & Olsen, 1965), and the available potassium was determined by means of neutral normal ammonium acetate solution extraction followed by flame photometry (Jackson, 1958). Regarding the soil microbial population, the soil samples were collected from each plot at a depth of up to 20 cm and air dried, ground, and sieved. The serial dilution plate technique to soil samples from each plot (Chhonkar et al., 2007). As for fungi, Martin's Rose-Bengal medium was used, Thornton's medium for bacteria, and Ken-Knight and Munaier’s medium for actinomycetes.
Statistical analysis
A thorough statistical study was carried out, and the range of deviation was calculated using the standard error of the mean. A randomized complete block design with four replications was used to compare the treatment effect (bamboo species) on aboveground growth parameters, nutrient uptake, and soil physicochemical properties at different soil depths for the same bamboo species and at the same depth for different bamboo species and soil microbial population. A one-way analysis of variance (ANOVA) was performed using R (version 4.2.1) for Windows. To identify significant differences in the treatment means, a post hoc test, i.e., Tukey’s honest significant difference (HSD), was used at a 0.05 level of significance.
RESULTS
Growth parameters and biomass
The post hoc analysis via Tukey’s HSD test (Figures 2 and 3) revealed that all the growth parameters of six bamboo species were significantly different (p<0.05), i.e., the clump girth (CG), culm length (CuL), culm diameter (CuD), number of culms per clump (NCu/Cl), number of nodes (NN), and internodal length (IL).
Figure 2 Variations in growth parameters: a) clump girth (m), b) number of culms, c) culm length (m), d) culm diameter (cm), e) number of nodes, and f) internode length (cm) for six bamboo species. Means followed by different letters are significantly different (p<0.05). The error bar signifies the mean standard deviation
Figure 3 Variations in above ground biomass a) leaf biomass, b) branch biomass, c) culm biomass, and d) total aboveground biomass for six bamboo species. Means followed by different letters are significantlydifferent (p<0.05). The error bar signifies the mean standard deviation
The highest CG (8.60 m), CuL (15.68 m), NN (50), and IL (35.37 cm) were observed in B. nutans. However, the CuL (15.39 m) and NN (48) of D. hamiltonii and the NN of B. balcooa (49) exhibited insignificant differences with the CuL and NN of B. nutans. Among all bamboo species, the highest CuD (5.70 cm) was recorded in D. hamiltonii, followed by B. nutans (4.70 cm). D. strictus exhibited the highest NCu/Cl (65), followed by B. balcooa (51), and the lowest value was recorded in B. bambos (35). Likewise, for the six bamboo species, Tukey’s HSD test revealed significant variations in different components of the biomass (Figures 4a-d), i.e, leaf (LB), branch (BB), culm (CuB), and total aboveground biomass (TAGB) (p<0.05). D. hamiltonii reported the highest LB (29.54 Mg.ha-1), CuB (296.12 Mg.ha-1), and TAGB (362.56 Mg.ha-1), followed by B. nutans (LB: 27.18 Mg.ha-1, CuB: 198.48 Mg.ha-1, TAGB: 277.50 Mg.ha-1). B. nutans showed the highest BB (51.84 Mg.ha-1), followed by D. hamiltonii (36.90 Mg.ha-1). However, D. asper revealed minimal BB (7.54 Mg.ha-1), CuB (38.08 Mg.ha-1), and TAGB (51.14 Mg.ha-1), as well as no significant differences with the TAGB of B. bambos(63.14 Mg.ha-1).
Note: NUL: nitrogen uptake by leaves; PUL: phosphorus uptake by leaves; KUL; potassium uptake by leaves; NUB: nitrogen uptake by branches; PUB: potassium uptake by branches; KUB; potassium uptake by branches; NUC; nitrogen uptake by culms; PUC: phosphorus uptake by culms; KUC: potassium uptake by culms; NUAGB: nitrogen uptake by aboveground biomass; PUAGB: phosphorus uptake by aboveground biomass; KUAGB: potassium uptake by aboveground biomass.
Figure 4 Variations in total nutrient uptake by a) leaves, b) branches, c) culms, and d) aboveground biomass for six bamboo species. Means followed by different letters are significantly different (p<0.05). The error bar signifies the mean standard deviation
Nutrient uptake
In this research, the nutrient uptake, i.e., nitrogen (N), phosphorus (P), and potassium (K) by various growth components (Figure 3) of the six bamboo species exhibited significant differences (p<0.05). Regarding the nutrient uptake by leaves, Tukey’s HSD test revealed that the highest N (0.25 Mg.ha-1), P (0.07 Mg.ha-1), and K (0.36 Mg.ha-1) uptake was recorded by D. hamiltonii, which remained statistically on par with the N (0.24 Mg.ha-1) and P (0.06 Mg.ha-1) uptake by leaves of B. nutans. Significant variations in the nutrient uptake by branches were also observed in bamboo species, with the highest N (0.42 Mg.ha-1), P (0.07 Mg.ha-1), and K (0.46 Mg.ha-1) uptake being recorded by B. nutans, followed by D. hamiltonii ( N: 0.28 Mg.ha-1, P: 0.06 Mg.ha-1, K: 0.34 Mg.ha-1), whereas the lowest N (0.06 Mg.ha-1), P (0.01 Mg.ha-1), and K (0.06 Mg.ha-1) uptake was recorded by D. asper. As for the nutrient uptake by culms, D. hamiltonii showed the highest N (1.78 Mg.ha-1), P (0.35 Mg.ha-1), and K (2.34 Mg.ha-1) uptake values, followed by the N (1.34 Mg.ha-1), P (0.25 Mg.ha-1), and K (1.57 Mg.ha-1) culm uptake of B. nuatns. The lowest N, P, and K uptake values reported by culms correspond to D. asper (0.22, 0.04, and 0.28 Mg.ha-1) which was insignificantly different from the N (0.23 Mg.ha-1), P (0.05 Mg.ha-1), and K (0. Mg.ha-1) culm uptake of B. bambos. Among the bamboo species, the overall total aboveground nutrient uptake also varied significantly (p<0.05). The highest total aboveground N, P, and K uptake (2.31, 0.47, and 3.04 Mg.ha-1) was observed in D. hamiltonii, followed by B. nutans (2.00, 0.38, and 2.36 Mg.ha-1).
Soil physicochemical properties
In the bamboo plantations, there was a significant reduction regarding soil properties (Table 1), i.e., soil porosity (POR), soil electrical conductivity (EC), soil organic carbon (SOC), available soil nitrogen (ASN), available soil phosphorus (ASP), and available soil potassium (ASK), as well as an increase in soil parameters like bulk density (BD), particle density (PD), and pH, as the soil depth increased (p<0.05).
Table 1 Soil physicochemical properties of six bamboo species in 14-year-old plantation and a control plot in the Terai region of the Central Himalayas
These values correspond to the mean ± standard deviation (n = 4). Different lower-case letters indicate significant differences in the soil characteristics of bamboo species and the control plot in the same soil layer. Different capital letters indicate significant differences in the soil characteristics of different soil layers for the same treatment (ANOVA and LSD test, p<0.05). BD: bulk density; PD: particle density; EC: electrical conductivity; POR: soil porosity; ASN: available soil nitrogen; ASP: available soil phosphorous; and ASK: available soil potassium
In the soil layer (0-40 cm), the highest BD (1.36 g.cm3) was observed in the control plots, followed by D. asper (1.23 g.cm3), whereas the lowest value was observed in D. hamiltonii (1.07 g.cm3). Similarly, the open plots exhibited the highest PD (2.84 g.cm3), but no significant difference was observed in the different bamboo species. The soil pH of the control plots was found to be slightly alkaline while also exhibiting the highest value (7.75), whereas the lowest soil pH was recorded in D. asper (6.66), on par with B. bambos (6.83) and D. hamiltonii (6.81). The highest soil POR was recorded in D. hamiltonii (59.00%), close to those of B. nutans (58.20 %), B. bambos (58.10 %), and B. balcooa (56.30 %). The highest soil EC was observed in D. strictus (0.54 ds m-1), followed by B. balcooa (0.49 ds m-1). The highest SOC (1.54 %) was recorded in D. hamiltonii, also followed by B. balcooa (1.40 %). The open plots had the lowest soil POR (52.20 %), EC (0.34 ds m-1), and SOC (0.74 %). Among the bamboo species, the highest ASN (228.29 kg.ha-1) and ASK (216.72 kg.ha-1) were recorded in D. hamiltonii, on par with the ASN (223.28 kg.ha-1) and ASK (211.68 kg.ha-1) of B. balcooa. In contrast, the open plots had the lowest ASN (169.96 kg.ha-1) and ASK (172.54 kg.ha-1). Regarding ASP, there was no significant variation between the bamboo species and the open plot.
Tukey’s HSD test also revealed that, in the 40-80 cm soil layer, the open plots had the highest soil BD (1.45 g.cm3), and D. hamiltonii showed the lowest value (1.21 g.cm3). As for the soil POR, D. hamiltonii exhibited the highest value (56.40 %), and the lowest was recorded in the open plots (50.20 %) and D. asper (52.10 %), which exhibited no significant difference. The highest soil EC (0.36 ds m-1) was observed in D. strictus, on par with B. balcooa (0.35 ds m-1). Among the bamboo species, D. hamiltonii exhibited the highest SOC (0.72 %), followed by B. bambos (0.65 %), the highest ASN (173.73 kg.ha-1), followed by D. strictus (164.32 kg.ha-1), and the highest ASP (14.78 kg.ha-1) in the 40-80 cm soil layer, while the open plots had the lowest SOC (0.40 %), ASN (144.25 kg.ha-1), and ASP (10.49 kg.ha-1). However, there was no significant difference in the soil PD, pH, and ASK between the bamboo species and the open plots.
In the 80-120 cm soil layer, the variations in soil BD, PD, pH, EC, ASN, ASP, and ASK were not statistically significant, except for SOC, whose highest value was recorded in D. strictus (0.21 %), followed by B. bambos (0.18%), whereas the lowest was reported by D. asper (0.12 %) (Tukey’s HSD test).
Soil microbial population
The soil microbial population exhibited significant variations between the bamboo species and the open plot (p<0.05). The highest fungi count was observed in B. nutans (101.20 ×104 cfu.g-1), which did not differ significantly from that of D. hamiltonii (100.47 ×104 cfu.g-1), followed by D. strictus (97.14 ×104 cfu.g-1) and B. bambos (95.61 ×104 cfu.g-1). The open plot had the lowest fungi count (54.40 ×104 cfu.g-1). Among the different bamboo species, the highest actinomycetes count was reported by D. hamiltonii (54.16 ×105cfu.g-1), followed by B. nutans (51.33 ×105cfu.g-1), and the lowest value was recorded in the open plot (30.45 ×105cfu.g-1). Similarly, the bacterial count was significantly higher in D. hamiltonii (68.30 ×106cfu.g-1), followed by B. nutans (65.11×106cfu.g-1), and it was minimal in open plots (36.22 ×106cfu.g-1).
DISCUSSION
Growth parameters and biomass
In this research, the variations in the growth parameters of the six different bamboo species, which grew under similar environmental conditions, may be indicative of their unique genetic makeup (Kaushal et al., 2020b), as well as due to internal factors such as the plant's origin, species type, and resilient system. All bamboo species in the plantations are sympodial and are prone to growing centrifugally, i.e., growing from the center outwards. Due to the varying adaptive potentials of these bamboo species with regard to their surroundings and environment, numerous external factors may have an impact on the growth rate (Hassan et al., 2022).
The variation in the clump girth, i.e., the measurement of the enclosing boundary of the cluster of bamboo poles, depends on the mortality and recruitment of new culms in the bamboo clumps (Franklin et al., 2010; Iyer, 2019). Chen et al. (2022) argued that internodal length is affected by the growth of fiber and parenchyma cells in the bamboo culm. Similarly, Chen (2021) suggested that this aspect is dependent on the function of intercalary meristem present at the base of the internode, which increases the internodal length by elongation of the meristematic cells. This could be the case of B. nutans in this research. Similar findings were reported by Singh et al. (2018) and Gaikwad et al. (2021), who recorded the highest clump girth and internodal length of B.nutans in bamboo plantations. Likewise, Kaushal et al. (2020b) reported the highest clump girth for B. nutans (15.40 m) and the lowest one for D. asper (5.10 m) in 12-year-old bamboo plantations, which is consistent with our findings.
The culm length growth of the bamboo species, which shows a substantial association with the aboveground biomass according to the adaptive potential regarding the given environmental conditions (Noguchi & Yoshida, 2005), corroborates the highest culm length reported by B. nutans, which was followed by D. hamiltonii. Similarly, Singh et al. (2018) recorded the significant growth (8.70 m) of B. nutans in the central plains of India, which was lower than that reported in this study. Likewise, the study by Kanwal (2011) attributed the highest culm height to B. nutans over eight other bamboo species in the central Himalayan zone of India.
Large-diameter bamboo culms are increasingly in demand as sustainable raw materials for the wood industries (Zhang et al., 2013), which depends on the growth of ground and vascular tissue in the intermediate layer of bamboo culms (Xie et al., 2014; Wang et al., 2022). This might be the reason for the higher culm diameter growth of D. hamiltonii in our study. Similar findings were published by Pattanaik and Hall (2014) and Zhan et al. (2016), who observed a greater diameter growth in D. hamiltonni in the east Khasi hills of Meghalaya, India (7.16 cm) and in the Yunnan province, China (10-18.5 cm). These values are higher than ours. Likewise, Kaushal et al. (2020b) reported a great culm diameter growth for D. hamiltonii over other bamboo species in the central Himalayan region.
In the bamboo species, the aggregation of culms per clump was dependent on the activity of the adventitious buds present on the rhizome, which ultimately develop into new culm recruits (Zhao-Hua et al., 2005). Likewise, Li et al. (2021) found that the number of culms per clump is positively correlated with the vitality and growth of the belowground rhizome, which varies from species to species and is determined by capacity of a bamboo species to adapt to a given environment, which might be the reason for the higher culm density per clump in D. strictus. Similarly, Kittur et al. (2016) recorded 75 culms per clump of D. strictus in a seven-year-old bamboo plantation in the humid climate of India, which is comparable to our findings.
The variations in the aboveground growth parameters, i.e., the leaves, branches, and culm of the bamboo species, is dependent on species’ characteristics and their ability to compete for limited resources. Similarly, different bamboo species have distinct phylogenetic, morphological, and physiological features, as well as distinct biomass allocation strategies to optimize resource uptake (Mensah et al., 2016). Oli & Kandel (2005) and Puangchit et al. (2019) recorded the branch biomass (12.24 t. ha-1; 10 t. ha-1) of B. nutans in the eastern Terai region of Nepal and in the reserve forest of Myanmar, with values lower than those of our findings. Similarly, Kaushal et al. (2022) observed the highest branch biomass in B. nutans in the foothills of western Himalaya. Likewise, in comparison with our study, Shanmughavel & Francis (2003) recorded higher values for the culm (431.66 Mg. ha-1) and the total aboveground biomass (623.05 t. ha-1) of D. hamiltonii in wastelands of Coimbatore, India.
Nutrient uptake
Various factors such as species, age, developmental phase, soil fertility, climatic factors, and other variables have a general impact on the uptake, accumulation, and distribution of nutrients throughout the various plant parts (Ovington, 1965). According to Kim et al. (2018), the variations in the nutrient uptake by different biomass components of bamboo species can be linked to their response to soil nutrition and their propensity to accumulate various amounts of dry matter in different parts. Similarly, resource allocation and their utilization by various structural and functional organs for habitat adaptation might be another feasible explanation for the observed variations in nutrient uptake (Guo et al., 2021).
Many researchers (Singh & Kochhar, 2005; Singh & Rai, 2012; Kumar et al., 2006) have documented the nutrient uptake by different biomass components of various bamboo species. Like any other plant species, the primary organ for organic matter production is the leaf, while branches and stems serve as storage organs, particularly in older bamboos (Wu et al., 2009). The differences in nutrient uptake by the leaves of the bamboo species could be ascribed to their specific photosynthesis strategy, which is influenced by genetic variability (Luo et al., 2021). Our findings are consistent with those of Gaikwad et al. (2021), who recorded the highest N uptake by leaves and the highest P and K uptake by branches in a two-year-old B. nutans plantation in semi-arid tracts of Madhya Pradesh, India. Likewise, Kumari & Bharadwaj (2017) recorded the N and P uptake of D. hamitonii leaves (0.19 Mg. ha-1-; 0.02 Mg. ha-1) in the subtropical climate of Himachal Pradesh, values lower than those of our findings.
The maximal nutrient uptake by culms is attributed to bamboo species with a high biomass production and nutrient export potential via culm harvesting (Singh & Rai, 2012), which corroborates the high nutrient uptake by the culms of D. hamiltonii in our plantation. Similar findings were published by Kumari & Bharadwaj (2019) who reported the total N (0.21 t. ha-1), P (0.03 t. ha-1), and K (0.19 t. ha-1) uptake by D. hamiltonii in ten-year-old bamboo plantations, with values lower than those of our findings. Likewise, Gaikwad et al. (2021) observed lower values of total N (0.45 t. ha-1), P (0.04 t. ha-1), and K (0.37 t. ha-1) uptake by B. nutans in comparison with our study.
Soil physicochemical properties
There are significant differences regarding the impact of different bamboo species on soil physicochemical properties and the microbial population. The soil BD, which has a substantial impact on soil fertility, water flow, and gas penetration, is impacted by a variety of factors, including the aboveground biomass, compaction, and the plant species' rooting system, among others (Nawaz et al., 2013). The reduced soil BD in the 0-40 cm soil layer of our bamboo plantations is most likely due to an increased root dispersion, which can loosen the soil (Zhou & Shangguan, 2007). The rooting systems of different bamboo species may be the reason for the differences in soil BD (Cao et al., 2011). In the 0-40 and 40-80 cm soil layers, D. hamiltonii reported the lowest soil BD, which could be attributed to higher production of fine roots and aboveground biomass, litterfall rate, and decomposition (Seobi et al., 2005; Udawatta et al., 2009). Kaushal et al. (2020a) reported higher fine root production and litterfall as well as a lower soil BD under D. hamiltonii in comparison to other bamboo species. The slight decrease in soil particle density when compared to the control plot might be due to maximal root spread in the upper soil layer, which generates channels in the soil and might have accelerated the downward movement of finer soil particles (silt and clay), thus reducing soil particle density as suggested by El-amin et al. (2001). Similarly, Selassie & Ayanna (2013) and Pandey (2019) observed a decline in soil particle density with the addition of soil organic matter in bamboo plantations.
Soil porosity is the proportion of pore space in a given volume of soil, which is determined by the soil BD, organic matter, and particle shape and size, among others (Robinson et al., 2022). The smaller pore space in the 40-80 and 80-120 cm soil layers when compared to the 0-40 cm level of the bamboo plantations could be attributed to reductions in root distribution, biomass, total root length, and the root area per unit of soil volume (Kodešová et al., 2006; Hao et al., 2019). Arunachalam & Arunachalam (2002) and Kumar et al. (2021) estimated the soil pore space under different bamboo species. Similarly to our study, Patra et al. (2022) observed maximum values for soil porosity under D. hamiltonii in the northwestern Himalayan region. The slightly more acidic nature of the 0-40 cm soil layer in comparison with the 40-80 and 80-120 cm levels may be explained by the presence of litterfall, which causes more humus to accumulate. The decomposition of this humus may have resulted in organic acids, causing a decrease in the soil pH (Kumar et al., 2022; Qian et al. 2021)
The electrical conductivity of the soil is determined by its particle size, dispersion, porosity, water retention, and temperature (Bai et al., 2013). The enhanced electrical conductivity of the 0-40 cm soil layer under the bamboo plantations might be due to a better water retention, as a result of the larger pore space (Bai et al., 2013; Fu et al., 2021). Likewise, the decomposition of litterfall, which leads to the leaching of bases under the bamboo species, could be a plausible explanation for the increased soil electrical conductivity (Kim et al., 2018; Gaikwad et al., 2021; Kumar et al., 2022). The higher soil organic carbon in the 0-40 cm soil layer under the bamboo plantations could be due to the species' rapid growth rate, resulting in the continuous addition of litterfall to the soil (Lei et al., 2019; Kaushal et al., 2020b; Benbi et al., 2015). Similarly, the dense mat-like root systems of rhizomatous bamboo species, along with their small and finer roots in the upper soil layer, may contribute to more stable soil aggregates, less soil erosion, and an increase in the organic carbon content of the soil (Qiao et al., 2016; Dong & Kou, 2022). Likewise, variations in above- and belowground biomass production, as well as the disintegration rate, could be valid explanation for the high soil organic carbon content under D. hamiltonii.
The addition of soil organic matter and nutrient return through litterfall, as well as the extensive fibrous interconnected root structure of bamboo plantations, have the potential to reduce the surface water flow velocity and increase the retention of more nutrients in the soil, which would otherwise be lost due to erosion (Shiau et al., 2017). This could be the reason for the greater availability of soil N, P, and K under the bamboo plantations in comparison to the control plot. Singh and Singh (2006) posited that bamboo species that allocate more biomass to the foliage are likely to have more nutrient-rich litterfall and enhanced soil fertility, which may be the reason for the higher soil nutrient availability under D. hamiltonii. Similarly to our study, Kanwal et al. (2011) reported the ASP (24.84 kg.ha-1) and ASK (191 kg.ha-1) values of the 0-40 cm soil layer in a six-year-old D. hamiltonii plantation in the Central Himalayas.
Soil microbial population
The greater microbial populations found under the bamboo plantations in comparison with the control plot could be attributed to litterfall, which is a carbon and nitrogen source for the soil. This would boost soil enzymatic activities and soil organic carbon, consequently increasing microorganism diversity in the soil (Tian et al., 2007; Chang et al., 2019). Researchers such as Xue et al. (2017) and Zhang et al. (2020) have suggested a positive relationship between the ASN, the SOC, and the diversity of soil microorganisms. Likewise, bamboo roots exude various metabolic products, which attract diverse soil microflora into the rhizosphere (Trivedi et al., 2020; Zheng & Lin, 2020), which might be a plausible explanation for the higher microbial population under the bamboo plantations. Similarly to our study, Kaushal et al. (2020b) revealed that D. hamiltonii had the highest actinomycetes count (8.4 ×105cfu. g-1) compared to other species in a ten-year-old bamboo plantation. On contrary, Arunachalam & Arunachalam (2002) reported the lowest bacterial (7.080 ×105cfu. g-1) and fungal (22.53 ×103 cfu. g-1) soil count under D. hamiltonii among other bamboo species in the humid tropics of India.
CONCLUSIONS
For homogeneous climate conditions, this study revealed that the aboveground growth characteristics of several bamboo species can vary, indicating their adaptive and production potential, which is an important indicator for species selection in plantations. B. nutans and D. hamiltonii, due to their larger and thicker culm walls, can act as excellent sustainable building materials, as they can support great loads. These species also accumulated the highest amount of total aboveground biomass, making them excellent raw materials for bio-energy, pulp, paper, and paperboard production, as well as a more effective terrestrial carbon sink for climate change mitigation. The assessment of nutrient uptake by bamboo species is critical for optimizing soil nutrient management in order to enchance the plantation output potential. D. hamiltonii performed best in improving the soil physicochemical properties and enhancing the soil microbial population. Thus, it is suitable for the restoration of fallow lands in the Terai region of the Central Himalayas.
