The role of Leguminosae plants for soil fertility in oil palm plantations

The greatest and most lucrative plantation product in Indonesia is palm oil. The availability of nutrients in the soil affects how oil palm plants grow and develop. To overcome the low availability of nutrients in the soil to meet the needs of plants, fertilization operations are required. The concept of a green economy necessitates that the palm oil industry limits its usage of chemical fertilizers as well as chemical pest and disease management. Oil palm plants require a variety of fertilizers that are rich in macronutrients (N, P, and K), secondary nutrients (Ca and Mg), and micronutrients (B, Zn, and Cu). The potential use of biodiversity as a soil fertility agent, such as Leguminosae plants, must be researched. The purpose of this study is to define the function of Leguminosae plants in oil palm farms’ soil fertility. Several scientific papers that were pertinent to the research issue were studied for literature as part of the data collection process. Oil palm plantations in Indonesia have implemented the planting of legume cover crops (LCC). Planting LCC suppresses weed growth, protects the soil from direct sunlight and raindrops, reduces surface runoff, maintains soil moisture, and increases soil fertility. Neustanthus phaseoloides var. javanicus, Calopogonium caeruleum, C. mucunoides, Mucuna bracteata, M. pruriens var. utilis, and Centrosema pubescens are a few Leguminosae plants that can be used as cover crops. In addition to these species, planting Parkia timoriana, P. speciosa, and Archidendron pauciflorum trees can help improve soil fertility. Oil palm plantations with legumes have a higher clay texture, an average pH of 6.0, and high levels of N-total, P, K, and C-organic, whereas non-legume oil palm plantations have a sandy texture and high dust, an average pH of 5.3, and very low levels of N-total, P, K, and C-organic. Legumes, in addition to being a cover crop, can also be used as animal feed and to increase nutrient levels in plants. Planting LCC in oil palm plantations can affect the nutrient content of forages that grow under oil palm trees because the root nodules of LCC contain rhizobium bacteria that function to fix nitrogen from the air and are converted by the nitrogenase enzyme into ammonium and nitrate as a source of N for the LCC. The concept of integrated plantations can be applied to oil palm plantations by using LCC, which has hampered the growth of oil palm plants as animal feed, and P. timoriana, P. speciosa, and A. pauciflorum trees as economically valuable food plants.


Introduction
Palm oil is the largest and most profitable plantation commodity in Indonesia.Based on data from Ditjenbun [1], most oil palm plantations in Indonesia are cultivated by large private companies (PBS) 1243 (2023) 012016 IOP Publishing doi:10.1088/1755-1315/1243/1/012016 2 of 7,942,335 ha (54.94%) and large state enterprises (PBN) of 617,501 ha (4.27%), while smallholder plantations 5,896,755 ha (40.79%).Indonesia's palm oil production in 2019 was 47,120,247 tons with an export value of 14.7 billion US$ [1].Important factors affecting oil palm growth are soil fertility, water availability and climate, age, and planting patterns [2].The availability of nutrients in the soil affects how oil palm crops grow and develop.
Fertilization activities are necessary in overcoming the low availability of nutrients in the soil to meet the needs of plants.Fertilization is a major factor in calculating the production costs of oil palm plantations, this is because more than 50% of the costs are used for these activities [3].However, fertilization activities (mainly applied in the long term) can cause environmental problems and damage ecosystems, such as groundwater pollution, greenhouse gas emissions, changes in the physicochemical properties of the soil, and implications for the soil food chain [4].The concept of a green economy requires the palm oil industry to limit the use of chemical fertilizers as well as the control of chemical pests and diseases.
Oil palm plants need a variety of fertilizers that are rich in macronutrients (N, P, and K), secondary nutrients (Ca and Mg), and micronutrients (B, Zn, and Cu) [5].The administration of micronutrients is carried out through the injection of infusion on actively growing roots.Macronutrients in oil palm plants are given through chemical fertilizers (inorganic) by sowing on a disk [6].The use of legume plants in oil palm plantations can be carried out as a sustainable land use practice.Due to its ability to bind nitrogen, many species of legumes are cultivated to increase soil fertility, such as Neustanthus phaseoloides var.javanicus, Calopogonium caeruleum, C. mucunoides, and Mucuna bracteata.In addition, legumes are commonly used to increase net primary productivity, including grain, wood, and forage yields, in anthropogenic ecosystems [7].The presence of legumes is also reported to increase soil carbon sequestration [8].
The opposition of nitrogen-rich legume plants is more easily degraded by soil microorganisms, and this effect can achieve high trophic levels through bottom-up control [4,9].Soil microbial communities are a key component of the nitrogen cycle and greatly affect soil fertility.In many ecosystems, the presence of soil microbes can affect plant biomass, regulate plant carbon content (C), nitrogen (N) and phosphorus (P), and affect the replacement of plant species over time [10,11].Many studies conclude that the presence of soil microbes affects plant growth through plant microbial interactions [11,12] and potentially encourage ecological succession through the accumulation of soil pathogens [13], changes in soil nutrition [14], and the effects of symbiotic microbes [12].Applying inorganic nitrogen fertilizers to soil microbial communities will increase soil microbial biomass in the short term but in the long run can disrupt soil microbes [15].Legume species can also increase the complexity of food webs in the soil [16].The purpose of this study is to determine the role of Leguminosae plants in the soil fertility of oil palm plantations.

Method
The method used in this study is a literature study.Some scientific works related to research issues are studied for literature as part of the data collection process.The data collected is sourced from various scientific publications such as books, proceedings, theses, accredited national journals, and reputable international journals.This research reviews eight books, such as The Oil Palm, Exploring Inclusive Palm Oil Production, and Oil Palm and Biodiversity: A Situation Analysis by the IUCN Oil Palm Task Force.In addition to books, this study examined two proceedings, four theses, two accredited national journals (Warta Perkaretan and Berita Biologi), 39 publications from reputable international journals (Ecology, Conservation Letter, Nature, and so on), and six publications from other journals.

Legume plants for soil fertility
Legume planting can increase soil fertility and plant species diversity in oil palm plantations.Crop productivity will increase along with the richness of plant species [17].Several studies have shown the positive effect of plant diversity on crop productivity and soil C absorption [18,19].Based on these two studies, there is one group of plants that provides the strongest effect, namely Leguminosae.Legume has been identified as a major driver of primary productivity, C absorption, N accumulation and availability, mineralization, and increasing crop productivity [20,21,22,23].A mixture of ground cover crops (Centrosema, Calopogonium, and Neustanthus) when grown in between plantations is estimated to contribute 3-14 Mg of green material equivalent to 20-111 kg of N, 2-9 kg of P, and 12-56 kg of K ha/year [24].
Legumes are symbiotic with Rhizobium bacteria that fix N in the atmosphere and allocate it to plants instead of carbohydrates.Rhizobium cannot fix nitrogen without legume plants.On the contrary, without legume plants, Rhizobium can also not fix nitrogen.Nitrogen is fixed in the nodules and occurs only if there is a symbiotic relationship between bacteria and legume plants.The symbiosis between Rhizobium and the roots of legume plants will produce nitrogen-fixing organs, namely root nodules [25].Based on this, the presence of legumes can increase N input into the ecosystem, which will have an impact on increasing crop productivity and increasing absorption in the soil [26,27].
Some species of legume cover crop (LCC), including N. phaseoloides var.javanicus, C. caeruleum, C. mucunoides, M. bracteata, M. pruriens var.utilis, and Centrosema pubescens (Figures 1-6).Some of these species have been used as crop covers in oil palm plantations.The right ground cover plant as an LCC must have rapid growth and density, be able to symbiotic mutualism with nitrogen fixation bacteria, and the biomass produced is easily decomposed so that the cover crop does not become a competitor to the main crop [28].According to [29], LCC plants can provide organic matter inputs of 2-3 tons/ha at the age of 3 months and 3-6 tons/ha until the age of 6 months.
LCC planting can also play a role in soil and water conservation, namely reducing soil density as a place to store carbon, affecting hydrology and protecting the soil from erosion caused by water and wind, as well as increasing the rate of water infiltration [30,31,32,33].M. bracteata is one of the LCC species capable of producing high biomass and contains more N than other cover crops [34].[35] in his research found that an increased dose of organic mulch M. bracteata resulted in an increase in volume, dry weight and root occupy in yielding oil palm crops.In addition to LCC planting, to maintain and improve soil fertility in oil palm plantations can be combined with tree species, including kedawung (Parkia timoriana), petai (P.speciosa), and jengkol (Archidendron pauciflorum).P. timoriana is a group of legume plants that can maintain soil fertility as well as a strategic and important medicinal plant for public health development [36].Based on several research results, it can be seen that legume plants can increase soil fertility by significantly increasing the levels of total C, total N, and organic C. As a result, all biochemical variables related to the microbial activity (general parameters) and enzyme activity involved in the C, N, P, and S cycles (specific parameters) are significantly activated to different degrees in the soil.Ground cover crops, however, have a variable effect on total soil N, organic C, and biochemical parameters, mainly due to variations in the quantity of biomass produced.Therefore, it seems clear that repeated planting of ground cover and incorporation of in situ will lead to the long-term sustainability of soil fertility by increasing soil organic matter levels, microbial activity, and enzymes [37].

Comparison of oil palm plantations with LCC and without LCC
Oil palm plantations planted with LCC showed significant differences compared to oil palm plantations without LCC.These differences include the physical and chemical properties of the soil in oil palm plantations.Based on research by [38], the physical properties of soil in oil palm plantations with LCC show a higher percentage of clay texture than in oil palm plantations without LCC, which is 96.74%.Oil palm plantations without LCC have a higher percentage of sand and dust texture than oil palm plantations with LCC, namely 2.68% and 1.65% (Table 1).One of the roles of LCC is a contributor to soil organic matter so that soil fertility increases.The results of the research showed that oil palm plantations with LCC have a higher total N-level than oil palm plantations without LCC, which is 0.79% [38].This is because LCC can fix nitrogen.The lack of N-total value in oil palm plantations without LCC can be caused by the influence of evaporation, water washing, and absorption by plants.The following is a comparison of soil chemical properties in oil palm plantations with LCC and without LCC (Table 2).Another study conducted by [7] also showed that the presence of legumes tends to increase the resistance of some soil properties to the logging of undergrowth in mixed plantation forests.The main reason for the increased resistance of the soil to the removal of undergrowth may be the ability of legumes to bind N, which not only increases the nitrogen levels of the soil but also increases the complexity of the soil food web [16].Plant diversity can increase the absorption of C and N soils by allowing for complementarity in the use of resources.For example, plant diversity can increase N retention depending on soil N transformation, including N mineralization, and nitrification [39].
LCC planting in oil palm plantations also affects the morphology of oil palm crops.Based on research conducted by [40] in South Kalimantan, shows that oil palm plantations planted with LCC (M.bracteata) have a higher number of fronds, leaflets, leaf area, and leaf area index compared to oil palm plantations that are not planted with LCC but overgrown with reeds (Imperata cylindrica).This is because M. bracteata is a plant that can fix high nitrogen.Nitrogen is one of the important nutrients that affect plant growth, development, and productivity.Nitrogen is usually taken up by plants in the form of nitrates and ammonia present in the soil [41].A comparison of oil palm crop gaps in plantations with LCC and without LCC is presented in Figures 7-10.Source: [40] Some research results show that M. bracteata can produce high biomass and contains more N than other ground cover plants.In addition, an increased dose of organic mulch of M. bracteata resulted in an increase in root volume, dry weight of roots, and root area in yielding oil palm crops.[28] added that M. bracteata in the shade of oil palm can produce 8.7 tons of litter (equivalent to 236 kg of NPKMg, the concentration of N 75-83%), while in open areas as much as 19.6 tons (equivalent to 513 kg of NPKMg, the concentration of N 75-83%).M. bracteata gives a significant increase in the content of C, P total, and K, as well as the cation exchange capacity in the soil.This is certainly better when compared to land that is overgrown with weeds and not planted with LCC.

The concept of integrated oil palm plantations
Palm oil is an economically important crop for the food and cosmetic industries and as a feedstock for biofuels to offset greenhouse gas (GHG) emissions [42,43,44].Oil palm plantations are estimated to have an area of about 17 million ha of land and most of them are in tropical areas [45].Oil palm cultivation on degraded land is considered one of the best strategies for biofuel production and emission savings [2,46].The promotion of oil palm cultivation has been carried out not only to improve climate regulation services but also to provide alternative energy sources and contribute to economic development and rural livelihoods [47,48].However, the expansion of oil palm plantations is largely disputed due to their negative impacts on biodiversity, GHG emissions, and reducing ecosystem services in the long term [49,50].Therefore, sustainable oil palm cultivation through an integrated plantation system has the potential to improve additional socioeconomic and ecosystem services and help reduce its negative impact on the ecosystem.The concept of integrated oil palm plantation is a combination of oil palm, legume crops, and animal husbandry.
The length of time for oil palm (3-4 years) to start bearing fruit is a major problem for smallholders who invest money and energy to earn income from these plantations.Therefore, intercropping in oil palm especially during the tillering phase can be beneficial for farmers in reducing these costs during the growth period.Intercropping of oil palms combined with various food crops [51,52] and with cocoa, black pepper, and other spices [53] have been studied in different parts of the world.The complexity of intercropping can cause more stress on ecosystems if crop choices, planting techniques, and management practices are inappropriate [54].Intercropping is mostly done as subsistence and a means to increase income, due to the delayed income from oil palm which is a major setback for oil palm cultivation [55].
Oil palm is planted at a distance of 9 x 9 m both monoculture and agroforestry.Plant spacing will affect the effectiveness of nutrient absorption by plants [56].Palm oil reportedly consumes a lot of nutrients that can be supplemented by organic mulch and recycled byproducts from empty bunches [57].To make the system more sustainable, legume crops can be planted with oil palm, especially in suboptimal conditions.This will reduce C and N losses from palm oil and increase nutrient stocks.LCC planting can not only be used for soil fertility but can also be used as animal feed.LCC whose growth has interfered with oil palm crops can be harvested as animal feed.
Intercropping oil palm with crops of economic value has recently received attention [58].Some researchers conducted studies on oil palm yields in diversified plantations and reported that considerable improvement in economic and environmental systems seems feasible through mixed palm oil systems, especially in sub-optimal climates where oil palm is experiencing some seasonal water pressures [59,60,61].Planting kedawung, petai, and jengkol crops as intermittent crops in oil palm plantations can provide benefits as an additional source of food and economy for the community.Petai and jengkol plants are types of plants that are often used in agroforestry as fruit-producing plants.

Conclusion
Planting Leguminosae on oil palm plantations can increase soil fertility, crop fertility, and oil palm productivity.Integrated plantation development can be carried out in oil palm plantations by utilizing LCC plants whose growth has interfered with oil palm crops as animal feed.The oil palm planting system combined with kedawung, petai, and jengkol trees can provide benefits as an additional source of food and economy for the community.

Figure 7 .
Figure 7.The number of fronds on oil palm plantations with M. bracteata and I.cylindricaSource:[40]

Figure 8 .
Figure 8.The number of leaflets on oil palm plantations with M. bracteata and I. cylindricaSource:[40]

Figure 10 .
Figure 10.Leaf area on oil palm plantations with M. bracteata and I. cylindrica.

Table 1 .
Comparison of soil physical properties in oil palm plantations with LCC and without LCC

Table 2 .
Comparison of soil chemical properties in oil palm plantations with LCC and without LCC.