Growth evaluation of several types of energy crops from tropical shrubs species

Introduction

The Indonesian government has issued a national energy policy to encourage the development of alternative energy. This policy targets the replacement of diesel and gasoline with biodiesel and bio-ethanol by 5% in Indonesia by 2025. The Government commissioned the Ministry of Forestry to play an active role in the development of biomass production. The use of raw material biomass to replace fossil fuels for heat and electricity production is prioritized, including permits for the utilization of plantations in the form of unproductive areas and permits for natural forest utilization^1,2.

Different types of forest plants and lignocellulose weeds have the potential to be developed as biomass feedstock for electricity production. Woody shrub species, such as Vernonia amygdalina Delile, Piper aduncum L., Gliricidia sepium (Jacq.) Kunth ex Walp., Calliandra calothyrsus Meissner., Bridelia tomentosa Blume, Vitex pinnata L., Vernonia arborea Buch.-Ham. and Bauhinia purpurea var. corneri de Wit., are suitable for use as sustainable raw materials for electrical energy³. This type of bush plant is a pioneer species that can easily be grown in secondary forests and open lands that have formerly been cultivated and logged. Energy crops are defined as cultivated plants that are developed and grown specifically for fuel and are rapidly growing, resistant to pests and droughts and can quickly be harvested, so they could have competitive prices if used as fuel⁴.

Wood is a renewable resource as it is sustainable and its supply will always be available. Meanwhile, cellulotic bio-fuel is produced from non-food (feed) stocks that play a critical role in reducing dependence on oil imports⁵. Woody biomass is produced, among others, by establishing plantations of energy crops. Coppice is a tree process regenerated with new shoots from the stumps that have been harvested and the Short Rotation Coppice (SRC) from hardwoods is more promising in generating biomass for bio-energy because it consists of fast-growing tree species and high-yield planted varieties (5000-15000 stems per ha), harvested in a two to six year rotation cycle.

This study focused on the growth and nutrient status of leaves of several types of energy crops from tropical shrub species with NPK fertilizer application, as a first step in the preparation of ready-to-plant if they are cultivated in the future as raw materials for biomass for renewable electricity.

Methods

Experimental design

Complete randomized block design (CRBD) was used as the experimental design. There were 8 plant species and their growth responses were evaluated based on 5 treatments using NPK fertilizer (commercial by YARA International ASA, Oslo, Norway; 16% N, 16% P₂O₅, 16% K₂O₅, 1.5% MgO, and 5% CaO). Five groups in five plots (each plant species): T0: with no fertilizer as control group (0 g/plant); T1: supplemented with NPK 40 g/plant; T2: supplemented with NPK 80 g/plant; T3: supplemented with NPK 120 g/plant; T4: supplemented with NPK 160 g/plant. The fertilizer treatments were conducted 2 weeks after sowing. There were 10 sample plants (as replication) for each treatment, a total of 50 plants for each species (divided into 5 experimental plots). Therefore, in total, there were 40 experimental plots, each plant was separated 1 m in length within each plot while it was separated 3 m in length between each plot (Figure 1).

Figure 1. Seedling planting diagrams for each treatment.

Results

The first part was the result of planting several types of tropical shrubs, including height growth, stem diameter, number of branches and leaves, while second part included the results of the analysis of nutrient content that was accumulated on the leaves of several types of tropical shrubs, namely nitrogen, phosphorus, potassium, calcium and magnesium.

Based on Table 1, the best stem height was measured from 67.00 cm to 320.50 cm in V. pinnata and V.amygdalina, respectively, where both results were observed with T4 treatment. The best stem diameter ranged from 0.68 cm to 1.60 cm in B. purpurea and P. aduncum, respectively. The B. purpurea result was observed in T4 treatment, whereas the P. aduncum result was in T1 treatment. The highest leaf number ranged from 34.10 in B. purpurea to 271.50 in V.amygdalina. The first result from B. purpurea was observed in T1 treatment while the second from V. amygdalina was in T4 treatment. The highest branch number varied from 2.90 in B. purpurea to 12.00 in V. amygdalina. Similar to the leaves, B. purpurea result was observed in T1 treatment while V. amygdalina was different, because it was obtained from only T2 treatment. Other results on several plant species also show highest numbers however they are not significant within each species.

The results show that fertilization treatment affects the growth of superior energy tropical shrub plants. In plants V. amygdalina, P. aduncum, V. pinnata, C. callothyrsus, S. fasciculata and G. sepium, the best growth was found in T4 treatment, whereas in M. malabathricum and B. purpurea, the best growth was obtained by T1 treatment. This indicates that the response of plants varies according to the fertilization treatment. Stem height and stem diameter of the various plants after 5 months of planting are presented in Figure 2.

Figure 2.

(A) Height and (B) diameter comparison of tropical shrub stems 5 months after planting measured from all fertilizer applications.

According to Figure 2, the highest stem height was observed in V. amygdalina while the lowest was in V. pinnata, S. fasciculata and B. purpurea. Moreover, the large stem diameter was measured in P. aduncum, while the lowest was in V. pinnata and B. purpurea.

The accumulation of nutrients in the leaves of the tropical shrub plants varied widely. The highest nitrogenous nutrients accumulated in the leaves of M. malabatricum, C. calothyrsus and V. pinnata, while the highest phosphorus and potassium nutrients accumulated in the leaves of V. amygdalina, P. aduncum and G. sepium. Calcium nutrients accumulated the most in the leaves of P. aduncum, G. sepium and S. fasciculata. On the other hand, the highest accumulation of magnesium nutrients occurred in the leaves of B. purpurea, V. amiqdalina and G. sepium (Figure 3). Full data of the effect of fertilizer applications on tropical shrubs growth (height, diameter, and leaf and branch number) and leaves’ nutrient concentration (N, P, K, Ca and Mg) are available⁸.

Figure 3.

Nutrient accumulation in leaves of tropical shrubs 5 months after planting measured from all fertilizer applications (A. Nitrogen, B. Phosphorus, C. Potassium, D. Calcium, and E. Magnesium).

Discussion

In this study, the growth and accumulation of nutrients in the leaves of tropical shrub plants varied greatly. The tropical shrubs that grew the best were V. amygdalina, G. sepium and C. calothyrsus, which accumulated mostly phosphorus and potassium nutrients in the leaves, while nitrogen, calcium and magnesium were least accumulated. These plants responded to NPK fertilizer up to 160 g per plant, resulting in best growth and high production of biomass, showing their suitable as raw material for biomass to create electricity. A previous study has shown that V. amygdalina provides 2.25 MWh, G. sepium 2.08 MWh and C. calothyrsus 2.01 MWh per ton of dry biomass³. As reported by Susanto and Amirta, fast-growing pioneer species such as M. gigantea absorbs the most potassium reaching 35% in every ton of plant biomass^6,9. For V. amygdalina, growth parameters are positively correlated to rainfall, relative humidity and cloud cover¹⁰. On the other hand, G. sepium can be harvested at a residual height of 70 cm, with better agronomic characteristics and chemical composition occurring in the fall¹¹. C. calothyrsus also has good growth in planting plots in previous research¹², and it was previously reported that mycorrhizae such as Glomus sp. and Acaulospora sp. have significant influence on its height. In Colombia, planted fallows using C. calothyrsus have an additional benefit of producing large quantities of wood for household use¹³. Based on the growth and nutrient analysis in the present study, we believe that these tree plants species can be developed widely to support a sustainable supply of biomass feedstock for the green electricity program in Indonesia.

The growth of five plants species, namely M. malabatricum, P. aduncum, V. pinnata, B. purpurea, S. fasciculata was lower (less than half) than the three plants species discussed earlier (Table 1). In the present study, several plant species that grew slower actually accumulated more nutrients of nitrogen and phosphorus in the leaves, such as S. fasciculata, P. aduncum, V. pinnata, B. purpurea and M. malabatricum, P. aduncum and S. fasciculata also accumulated a large amount of potassium and calcium nutrients in their leaves. Moreover B. purpurea accumulated magnesium mostly in its leaves (Figure 3). For M. malabathricum, the availability of phosphorus and aluminum in the rhizosphere increases its growth, it can also adapt to low soil pH¹⁴ and absorb heavy metals in contaminated soils¹⁵. On the other hand, the mean foliar aluminum concentration in wild plants of M. malabathricum had positive correlation with foliar calcium, total nitrogen, calcium and magnesium concentrations within this species¹⁶ while Symplocos sp. mean foliar aluminum concentrations were detected at 4107 (±1474 mg kg^-1) and 4290 (±4025 mg kg^-1) for seedlings and saplings, respectively¹⁷. P. aduncum can be propagated with seeds and shoot cuttings¹⁸ and can accumulate large amounts of potassium, as previously reported¹⁹; at 23 months, it had accumulated 222 kg N, 50 kg P, 686 kg K, 255 kg Ca, 75 kg Mg, and 24 kg S ha^{− 1}. More than half of the P, K, Ca and Mg nutrients were found in the stem (wood). Its leaf litter is significant and becomes an easily decomposable source of potassium, but G. sepium’s leaf litter contains much nitrogen¹⁸ while B. purpurea, which is a light-demanding tree, only grew 25% in a shady house with full sunlight²⁰. Therefore, each tropical shrubs species in this study varies in accumulating plant nutrients N, P, K, Ca and Mg in their leaves.

Based on this study, it is necessary to pay attention to the fast developing plant type, especially for tropical shrubs that have the potential as raw materials for biomass energy. Many beneficial nutrients also accumulated mostly in the leaves. This accumulation reflects the nutrient requirements of those plants, which will be cultivated as energy raw materials; therefore at the initial stage, the demand for plant fertilizer can be predicted. The carrying capacity of soil nutrients requires serious attention for the sustainment of plant cultivation. These wood shrubs species (V. amygdalina, G. sepium and C. calothyrsus) were also able to re-grow naturally by generation more than single shoots on their coppice trees. The scheme of Short Rotation Coppices (SRC) was an effort to achieve forest energy plantation using fast growing trees and wood shrubs species for aiming for a sustainable cycle³.

Conclusion

Of the eight types of tropical shrubs in this study, three species, namely V. amygdalina, G. sepium and C. calothyrsus, had the best growth and could potentially be developed as Energy Crops. The most accumulated nutrients in the leaves of these three species of plants are phosphorus and potassium.

Data availability

Open Science Framework: Growth Evaluation of Several Types of Energy Crops from Tropical Shrubs Species, https://doi.org/10.17605/OSF.IO/3G8FH⁸.

Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).