The Role of Biomulch Arachis pintoi In Increasing Soil Infi ltration Rate on Sloping Land of Oil Palm Plantation

The slope of land in oil palm plantation areas is the one of the primary causes of low soil water content due to low rates of soil infi ltration. Biomulch is one of the conservation methods that can be used to cover and shield the soil from weeds, prevent soil erosion, and increase the rate of soil infi ltration. Arachis pintoi is a perennial, stoloniferous legume crop that has potentials to be used as biomulch. The objective of the research was to study the role of Arachis pintoi in increasing the rate of soil infi ltration on a sloping land of oil palm plantation. The research was conducted on the slope land (22.8%) of the Bukit Kemuning Farmer Group, Mersam, Batanghari, Jambi, Indonesia (01036’21”, 102057’11”) from September 2017 to March 2018. The environmental design used in this study was a one-factor randomized block design (RBD) with fi ve ground cover treatments, i.e. natural vegetation, Arachis pintoi, Centrosema pubescens, Pueraria javanica and Calopogonium mucunoides. The results showed that the average growth rate of A. pintoi was 2.47 cm per week, which was lower than the growth of other treatments. The root length of A. pintoi was 50.36 cm at 20 weeks after planting. A. pintoi can be used as biomulch; sloping land planted with A. pintoi had an infi ltration rate of 49.30 cm per hour at 20 week after planting, i.e. an increase of 32.47% compared to the infi ltration rate with the natural vegetation.


Introduction
Oil palm (Elaeis guineensis Jacq) plantation continues to expand in Indonesia. The productive land area covered by oil palm plantations in Indonesia in 2015 was 11,300,370 ha (BPS, 2016). Oil palm in Indonesia is generally cultivated on marginal lands such as dry and hilly land (Farni et al., 2012), which may aff ect the availability of water in the soil caused by low infi ltration and high erosion on the soil surface. Therefore, to prevent land degradation, conservation system such as the use of cover crops or biomulch should be implemented (Ministry of Agriculture, 2015). Biomulch includes living plants that are planted as ground cover (Silmi and Chozin, 2014). The use of biomulch has many advantages for marginal land, including improving soil fertility, inhibiting weed growth, reducing erosion rates and increasing soil infi ltration (Refl ianty et al., 2009;Kumar et al., 2010). In addition, the benefi ts of biomulch can increase soil porosity and soil absorption (Arsyad, 2006). Utaya and Sugeng (2008) stated that diff erences in infi ltration rates in various land uses indicate that vegetation have a large role in determining soil infi ltration. It is hypothesized that the infi ltration capacity of soils under cover crop vegetation could be higher than that of bare soil.
Arachis pintoi is a herbaceous, perennial legume species that can be used as biomulch because it can cover the soil surface for erosion control (Silmi and Chozin, 2014;Carvalho et. al., 2009). A. pintoi growth does not interfere with the growth of the main crop and can potentially increase soil moisture content (Yuniarti, 2016;Yuniarti et al., 2018). A study by Mudarisna and Pujiono (2015) showed that the biomass of A. pintoi increased soil porosity and permeability, and improved soil aggregate. There has been limited information about the benefi ts of A. pintoi on soil physical improvement of the oil palm plantation.
C. pubescens, P. javanica and C. mucunoides are leguminous species that have been widely grown as cover crops. C. pubescens is resistant to drought and shade, so it is widely used to suppress the growth of weeds such Imperata cylindrica (Sutedi, 2015). P. javanica is a broadleaf leguminous creeper that has roots in each node and can cover the soil surface quickly; it is a very popular cover crop for plantations worldwide (Arsyad, 2012). C. mucunoides is a vigorous, annual trailing legume; it can reach several meters in length and form dense foliage with shallow root system. Leaf and stem of C. mucunoides have fi ne hairs. C. mucunoides is used as a pioneer in rehabilitating degraded land due to erosion but is not resistant to shade (Purwanto, 2007). The purpose of this study was to determine the potential uses of A. pintoi to increase soil infi ltration on sloping oil palm lands, and to compare advantages of growing A. pintoi as biomulch on oil palm plantations was compared to those of the natural vegetation, conventional biomulch Centrosema pubescens, Pueraria javanica and Calopogonium mucunoides.

Materials and Methods
The study was conducted at the Bukit Kemuning Farmer Group's land, Mersam District, Batanghari Regency, in the Jambi Province (01 0 36'21", 102 0 57'11") with slope topography of 20.8%. This research was carried out from September 2017 to March 2018. The materials used were stem cuttings of Arachis pintoi, seeds of Centrosema pubescens, Calopogonium mucunoides and Pueraria javanica. Two-year-old oil palm "Sriwijaya" variety was planted with a spacing of 9 m x 9 m. The equipment used included analytic scales, ovens, double ring infi ltrometer, clinometer and ombrometer. The environmental design used in this study was a one-factor randomized complete block design (RCBD) with fi ve treatments. The treatments consisted of natural vegetation cover crops C. pubescens, C. mucunoides and P. javanica, and A. pintoi on 9m x 3m plots, and replicated four times. The total plots of 20 were made in the middle of oil palm plantation. Biomulch was planted one day after the rain; the biomulch seeds and cuttings of A. pintoi were planted at a depth of 5 cm. Cutting of A. pintoi consists of one internode with a pair of mature leaves; it was expected that these cuttings rooted at about the same time with germination of the other biomulches. Biomulch crops were planted with a spacing of 40 cm x 40 cm. The total number of seeds used in each biomulch treatment was 675 seeds.
Measurements were made on tendrils and root length, soil infi ltration rate and soil infi ltration capacity from 4 to 20 weeks after planting (WAP) of the four cover crops. Infi ltration rate is calculated by the formula used by Budiarto et al. (2004): where F : infi ltration rate (cm per hour) ΔH : High decrease of water in a certain time interval (cm) t : Time needed by water at ΔH to enter the ground (hour) Infi ltration capacity is calculated by the formula (Horton, 1939): where F : infi ltration capacity at time t (cm/hour) f c : rate of infi ltration at a constant time (cm/hour) f 0 : initial infi ltration rate (cm/hour) e : 2.718 t : Time needed by water at fc to enter the ground (hour) K : constant for certain types of soil and soil cover Horton infi ltration parameters are used in the calculation of Horton's infi ltration, to obtain the value of k, a decrease in Horton's infi ltration formula (Beven, 2004;Dagadu, 2012):

Tendril Length of A. pintoi
Tendril is a stem that grows on the buds of plants and propagates on the soil surface. Based on our results, there was a signifi cant diff erence in the eff ect of the biomulch on the tendril length starting 4 WAP. The data showed longer growth of tendrils in C. pubescens compared to A. pintoi, P. javanica and C. mucunoides at 8 to 20 WAP (Table 1). Table 1 show that the average growth rate of tendrils of C. pubescens of 8.62 cm per week was higher than the average vine length of A. pintoi, P. javanica and C. mucunoides with a length of 2.47, 6.10 and 5.73 cm per week, respectively. The diff erence in the length of tendrils was possibly caused by the type of biomulch and the biomulch responses to the environment where they are grown. C. pubescens leaves and girth are relatively small, resulting in a dominant tendril growth while P. javanica and C. mucunoidez have large girth with broad leaf and long tendrils, resulting in slightly slower growth. In addition, the growing environment is also very infl uential on the growth of various types of biomulch. Indiana and Setiadi (2011) reported that the percentage of survival of C. mucunoides was 27.5% in straw husk media, which was higher than those of C. pubescens and P. javanica, i.e 20% and 14%, respectively. In other media such the leaf compost media, the percentage of C. mucunoides survival was 14% lower than C. pubescens and P. javanica, i.e. 18.33% and 20%, respectively.
The results showed that the growth rate of tendrils A. pintoi was lower than other biomulch species (Table  1). The slow growth of tendrils in A. pintoi was possibly caused by the excessive number of branches and the number of leaves which tend to reduce the growth of the tendrils. The average vine length of A. pintoi in this study was similar to the results of Dianita and Abdulah (2011) where the average growth rate of tendrils in A. pintoi was 1.60 cm per week. Yuniarti (2016) demonstrated that the average number of tendrils and branch numbers of A. pintoi was more than C. pubescens, P. javanica and C. mucunoides.

Root Length
Roots of biomulch have important roles in the conservation of plantation land; they improve soil structure and increase soil pores through root penetration to the soil. In addition roots hold the soil in position and prevent it from being washed away. In our study the growth of P. javanica roots signifi cantly faster (4.52 cm per week) compared to those of the other biomulches from 4 to 20 WAP (< 3 cm per week; Table 2).
The longest root of P. javanica was 15.90 cm at 20 week, which was longer than the root length of A. pintoi, C. pubescens and C. mucunoides, i.e. 50.36, 59.93 and 60.06 cm at 20 week, respectively. (Table 2). This rapid root growth in P. javanica was possibly related to the ability of P. javanica to absorb nitrogen better than other biomulches. As shown by Darmawati et al. (2015) that P. javanica biomulch is very responsive to nitrogen applications, and the nitrogen content in the tissues of P. javanica was higher than C. pubescens and C. mucunoides.  The results also showed that the root growth of A. pintoi was slower relative to other biomulch at 20 WAP. This may be due to in the diff erences in the planting materials used in the study. The A. pintoi were grown from stem cuttings and had fi brous roots, so that the number of roots is more dominant than the root length growth. The use of stem cuttings as planting material in A. pintoi is due to the fact it produces very few seeds, even though it produces abundant blooms; only 4 to 8% fl owers developed seeds (Adjolohoun et al., 2013). Planting material from stem cuttings will predominantly produce numerous shorter roots rather than long roots. A study by Sumiahadi et al. (2016) showed that the average number and length of roots in A. pintoi were 42.4 and 17.1 cm at 12 WAP.

Soil Infi ltration
The results showed that the biomulch treatments aff ected the actual infi ltration rate at 20 WAP ( Figure  1). Infi ltration is the process of absorbing water into the soil vertically through the soil surface and thoroughly passing through the soil pores. Infi ltration rate is the speed of water entering the soil for a certain time, while infi ltration capacity is the minimum rate of movement of water into the soil in saturated conditions (Hanks and Ashcroft, 1986).
The actual infi ltration rate curve in Figure 1 describes the rate of infi ltration of each biomulch species. Infi ltration rate is calculated by the formula used by Budiarto et al. (2004). The infi ltration rate of soil covered with P. javanica was 58.75 cm per hour, which is faster than the infi ltration rate in the treatment of natural vegetation, A. pintoi, C. pubescens and C. mucunoides of 33.29, 49.30, 51.52 and 44.87 cm per hour, respectively. The high rate of soil infi ltration in P. javanica may be related to the faster growth of root P. javanica as compared with other biomulch, thus may increase the number of soil pores (Table  2). Soils covered with P. javanica could increase ultisol pores by 27.47% compared to the natural vegetation treatment, and the number of soil pores in P. javanica roots was higher than C. pubescens which was 15.39% and C. mucunoides by 17.81% (Refl iaty et al, 2009). Figure 1 also shows soil infi ltration rate of A. pintoi which is higher than natural vegetation treatment. The ability of A. pintoi to increase soil infi ltration rate may be caused by the long roots of A. pintoi. Sumiahadi et al. (2016) reported that A. pintoi has an average number of roots of 42.4 with an average root length of 17.10 cm at 12 WAP; it also formed numerous roots in each node on each of the tendrils. The high infi ltration rates of the diff erent biomulch are evidence that biomulch can increase the rate of soil infi ltration compared to the natural vegetation. Gao-Lin et al. (2016) investigated the eff ects of diff erent artifi cial grasslands on soil physical properties and soil infi ltration capacity, and reported that mixtures of Legume (Astragalus adsurgens and Artemisia desertorum) and Poaceae (Bromus inermis) to create grasslands provided an eff ective ecological restoration approach to increase soil infi ltration properties by 72.38% due to their greater root biomasses. In addition to soil infi ltration rate, the soil infi ltration capacity needs to be determined. Soil infi ltration capacity is a soil hydrological parameter that can be used as an indicator of soil degradation and the drought potential of the soil (David et al., 2015). Table 3 shows that the soil infi ltration capacity of each biomulch treatment consistently increased from 4 to 20 WAP. P. javanica had a signifi cantly higher soil infi ltration capacity compared to other biomulch at 20 WAP. The increase in soil infi ltration capacity by P. javanica may be caused by a faster root length in this crop compared to the other biomulch (Table  2). Roots can improve soil structure and increase soil pores which in turn increased the soil fi eld capacity. In Refl iaty et al. (2009) study P. javanica increased the fi eld capacity of ultisol by 48.26% compared to that of the natural vegetation treatment, and that the fi eld capacity of P. javanica was higher than C. pubescens (28.68%) and C. mucunoides (30.73%).  The results of this study showed that the soil infi ltration capacity of soil with A. pintoi, C. pubescens and C. mucunoides were not signifi cantly diff erent from that of the natural vegetation treatment (Table 3). Although it was not signifi cantly diff erent, biomulch treatment signifi cantly increased soil infi ltration capacity from 4 to 20 WAP. The role of biomass is very important to protect the soil surface from direct collision with raindrops and to improve soil structure. Infi ltration capacity can be maintained if soil porosity is not disturbed during rain (Arsyad, 2012). Closed soil pores reduce soil infi ltration capacity, whereas soil with stable aggregates will maintain high infi ltration capacity. The value of infi ltration capacity of 12.5 to 25 cm per hour is considered rapid according to Lee (1980).
The results of this study demonstrated that A. pintoi can potentially be used as biomulch in oil palm plantations. A. pintoi can increase the rate of soil infi ltration so it can reduce the run off and soil erosion rates, as demonstrated in Table 3. Erosion is the main cause of land degradation which can reduce soil fertility, especially on sloping land. The ability of A. pintoi in suppressing the rate of erosion and increasing the rate of soil infi ltration will optimize nutrient absorption in sloping lands, hence preventing land degradation and creating a more eco-friendly oil palm plantation system.

Conclusion
The growth rate of tendrils of A. pintoi (2.47 cm per week) was slower than the growth of other biomulch treatments. The root length of A. pintoi (50.36 cm at 20 weeks after planting) was shorter than P. javanica but it was not signifi cantly diff erent from C. pubescens and C. mucunoides. Soil infi ltration rate with A. pintoi as biomulch was 49.30 cm per hour at 20 weeks after planting, or an increase in soil infi ltration rate of by 32.47% compared to that of the natural vegetation.
The soil infi ltration capacity with A. pintoi (14.95 cm per hour at 20 WAP) was lower than P. javanica, but it was not signifi cantly diff erent from C. pubescens and C. mucunoides. The result of this study demonstrated that A. pintoi can increase the rate of soil infi ltration in sloping land so it can be used as an bio mulch for oil palm plantation.