Biomulch Treatment Effects on Weed Control and Soil Properties in Cassava Plantation

Legume Cover Crop (LCC) are plants grown as live mulch so that it is beneficial to the agro-ecosystem and can be included in cropping systems to regulate soil fertility and suppress weed growth. This study aims to determine the benefits of using Arachis pintoi as biomulch applied at different times on weed composition and soil fertility. The experiment was carried out at the Teaching farm of IPB, Jonggol, West Java, Indonesia; the soil type is Ultisol. This study used two factors and was designed in a factorial randomized block design. The first factor was four accessions of cassava “Ketan Malang”, “Genjah Bayam”, “IR Jonggol”, and “Manggu”; the second factor is the time of the biomulch planting: 4, 8, and 12 weeks before planting the cassava, at cassava planting, and without biomulch + manual weeding as the control. Weed vegetation was analyzed before land preparation and before cassava harvest. The physical and chemical properties of the soil were analyzed before and after the biomulch treatment. Our study demonstrated that Melastoma malabathricum, Tetracera indica, Oxalis barrelieri, Mimosa invisa, Ottochloa nodosa, Ipomoea sp and Cyperus sp. are the dominant weeds in the cassava plantation. Biomulch planted four and eight weeks before cassava was able to reduce weed dry weight. All biomulch treatments were able to improve soil density and total pore space. Soil with biomulch planted eight weeks before cassava planting had the highest C-organic (7.59%) and total nitrogen (0.41%).


Introduction
Cassava (Manihot esculenta Crantz) is a native American species that has become an important food crop in Africa and Asia, including Indonesia. Cassava is a high calorie food source with the most affordable price. Cassava carbohydrate contents are 20% higher than those in corn and 40% higher than those in rice (Tonukari, 2004). Cassava has high tolerance to drought and highly acidic environment. In Indonesia, cassava is an important raw material for the food industry which comes from processing starch and its derivatives (Howeler, 2012). The increasing population and the development of cassava-based industries will further increase the demand for cassava in the future (Nintania et al., 2021). According to Widyatmoko et al. (2018), several processed cassava products such as tapioca flour, modified cassava flour (mocaf), and cassava rice, are developing into industrial products for the domestic as well as international markets.
Cassava is widely grown on Alfisol, Ultisol, Entisol, and Inceptisol soil types, and it can grow on soils with low nutrient and organic matter or marginal land, where the soil's physical, chemical, and mineral content is sub-optimal. Therefore, cassava can be one of the crops that can utilize marginal lands (Saidi, 2020). Ultisol develops under forest vegetation containing argillic horizons, and like Alfisol, have a low alkaline cation content so it tends to be acidic (Yaalon, 1996). Ultisols are Indonesia's second largest soil area after Inceptisols, with a total area of 24.3% or equivalent to 45.8 million ha (Nursyamsi, 2006). The fertility of ultisol is affected by erosion as the organic matter in the top layer is prone to erosion, thereby reducing its organic matter and nutrients (Prasetyo and Suriadikarta, 2014). The low fertility of ultisols is reflected in their low pH, low soil C-organic, CEC and Our current study aimed to determine the optimal time to plant biomulch A. pintoi to suppress weed growth and to improve soil fertility in the cassava plantation.

Experimental Site
The experiment was conducted in the experimental field of the Faculty of Agriculture, IPB University, at Singasari village, Jonggol District, Bogor Regency, West Java. The land in the study area is dry Ultisol with a slope of 15 o to 26.79%. The experiment was performed from November 2021 to September 2022. The study used stem cuttings of the ornamental bean A. pintoi and four cassava accessions, "Ketan Malang", "Genjah" "Bayam", "IR Jonggol", and "Manggu". The planting medium for A. pintoi cuttings consisted of soil and rice husks with a volume ratio of 2:1. Other materials used were cow manure, Urea, SP-36, and KCl fertilizers, herbicide (a.i. glyphosate), insecticide (a.i. carbofuran), 10x15 cm polybags, 0.5 m x 0.5 m 2 square frames for weed sampling, and a digital scale. The herbicide was applied before land preparation, insecticides were applied only when necessary.

Experimental Design
The experimental design used was a factorial randomized block design. The first factor was the cassava varieties: "Ketan Malang", "Genjah" "Bayam", "IR Jonggol", and "Mangu". The second factor was the planting time of the Arachis pintoi, i.e., B1 = planted at the time of cassava planting, B2 = four weeks, B3 = eight weeks, and B4 = 12 weeks before planting cassava, and without A. pintoi /B0, as a control. There were 20 treatments repeated four times to obtain 80 experimental units. Each experimental unit consisted of 25 cassava plants and 80 of the 4-weekold A. pintoi cuttings. Prior to planting in the field, a stem cutting of A. pintoi per polybag was prepared; the A. pintoi were planted in the field with a 20 cm distance between plants when they had 3-4 shoots. Cassava cuttings 25 cm in length were planted with a spacing of 100 x 100 cm, one cutting per planting hole. C-Organic, total N, available P, K 2 O, and CEC, were determined prior to planting.

Vegetation Analysis
Weed vegetation was analyzed before land preparation, and six months after planting cassava. Weed vegetation was analyzed using the quadratic method measuring 0.5 m x 0.5 m with three random throws in the experimental plots. Weed vegetation analysis data includes relative density (RDs), relative frequency (RF), relative dominance (RD), and sum dominance ratio (SDR) ((Marianan and Warso, 2016). The weed dominance ratio is calculated using the formula described in Listyowati et al. (2022): Relative density (RDs) = density of one species x 100% density of all species Relative frequency (RF) = frequency of one species x 100% frequency of all species Relative dominance (RD) = dominance of one species x 100% dominance of all species Sum dominance ratio (SDR) = RD + RF + RD 3

Soil Analysis
The soil sample for chemical and physical property analysis were collected twice, before the land preparation, and the after the cassava was harvested. The soil chemical properties tested include C-Organic (Walkey and Black method), total N (Kjedahl method), available P (Bray method), K 2 O (Morgan method), and CEC (basic percolation Method) as described in Eviati and Sulaeman (2009). Soil sampling (1000 g each) was collected diagonally at five sample points with a depth of 0-30 cm from the soil surface. The soil samples were homogenized and placed in containers for later analysis in the soil chemistry laboratory. Soil samples for the soil physical property analysis were collected using the sample ring with a diameter of 7.35 cm and a height of 4 cm. The ring has a cylindrical shape and was inserted to a certain soil depth then dismantled carefully so the volume of the soil remains constant. The sample points were determined randomly according to the ground cover treatments. Soil physical property analysis was conducted on soil bulk density, soil-specific gravity, and the total pore space at the soil physics laboratory at the Soil Research Institute, Bogor.

Result and Discussion
The growth of A. pintoi that were planted 12 weeks before cassava (B4) was even and covered almost the entire surface of the beds (85-90%). Arachis pintoi planted at eight weeks before cassava (B3) had a coverage of 60-75%, whereas those planted four weeks before cassava (B2) had the coverage of 30-45%. Arachis pintoi planted at the time of cassava planting (B1) grew very slowly and did not have much coverage ( Figure 1).
Arachis pintoi planted at 12 weeks (B4) had a greater fresh weight (232.6 g.m -2 ) and dry weight (65.1 g.m -2 ) compared to those planted at the other times (Table  1). Arachis pintoi planted at cassava planting (B1) had the lowest weight, likely because they grew slower due to competition with the weeds and the cassavas.
Arachnis pintoi planted at 8 or 12 weeks before planting cassava was able to suppress weed growth (Table 2). At 6 MAP, the weed dry weight with A. pintoi planted at 8 weeks before planting cassava was 15.55 g per 0.25 m 2 , whereas with A. pintoi planted 12 weeks before cassava was 14.65 g per 0.25m 2 ( Table 2).

Summed Dominance Ratio
Vegetation analysis before land preparation and after cassava harvest are described in Table 3. Before planting cassava, 11 weed species were identified, nine of which were broadleaf weeds, one species of grass, and one species of sedge. Broadleaf weeds appeared to be more dominant at the beginning (before planting cassava) with an SDR value of 51.18% compared to the grass weeds (11.41%) and the sedge weeds (37.41%) ( Table 3).
There was a shift in the weed dominance at six months after cassava planting, especially the broadleaf weeds, and this was influenced by biomulch treatments (Figure 2). Our results agreed with Silmi and Chozin (2014)    Honestly Different test at α=0.05. MAP: months after planting cassava. B0: without biomulch, B1: biomulch was planted at the time of cassava planting, B2: biomulch was planted four weeks before cassava planting, B3: biomass was planted eight weeks before cassava planting, B4: biomass was planted twelve weeks before cassava planting.

Summed Dominance Ratio
Vegetation analysis before land preparation and after cassava harvest are described in Table 3. Before planting cassava, 11 weed species were identified, nine of which were broadleaf weeds, one species of grass, and one species of sedge. Broadleaf weeds appeared to be more dominant at the beginning (before planting cassava) with an SDR value of 51.18% compared to the grass weeds (11.41%) and the sedge weeds (37.41%) ( Table 3).  (Figure 3). It is suspected that these weed species are adaptable and tolerant to the environmental stress, making it difficult to be suppressed by ground cover plants. According to a  Listyowati et al. (2022), the dominance of broadleaf weeds is due to by the growth of weed seeds after tillage; soil tillage brings the weed seeds to the soil surface and exposed them to light required for germination. The dominant weed species has vine and creeping growth habits, such as Tetracera indica, Ottochloa nodosa, Dioscorea hispida, and Ipomoea obscura. According to Sudira et al. (2017) root system greatly affects plant growth, those with shallow and creeping roots get more nutrients and water, whereas those with taproot can grow deep into the soil to access water.
The important characters for selecting cover crop are fast growth and tolerance to shades and drought, and these characters are very important for a long-term weed control (Schappert et al., 2019). Using A. pintoi as biomulch can be an option in environmentally friendly weed management because it can reduce the use of chemicals from herbicides. According to Monteiro and Santos (2022), using herbicides in weed control often causes weed resistance and ecosystem imbalance. Environmental pollution, weed resistance, and increased production costs are the effects of an excessive herbicide use (Pradhan et al., 2022). Long-term weed control for sustainable agriculture must be safe and efficient; it is important to select ground cover that can increase soil fertility, pore space (Pavlovic et al., 2022), and soil moisture (Yuniarti et al., 2018). According to Kaluba et al., (2021), intercropping of cassava and legumes can help suppress weed growth, use less fertilizer, and reduce labor input.
Cassava productivity can increase if weeded at least twice during cassava growing season (Kintché et al., 2017). Weed control by application of glyphosate herbicide is equivalent to five times manual weeding but is more expensive than the manual weeding conducted twice (Chikoye et al., 2002). Therefore, the use of biomulch can potentially reduce the use of herbicides and labor.

Soil Chemical Quality
The soil analysis before tillage showed that it had an acidic pH (4.12), low total nitrogen (0.14%), and low cation exchange capacity (28.53 cmol.kg -1 ), and the soil has a dominant clay texture (51.79%), 25.34% sand, and 22.87% silt. The results of the soil chemical analysis are presented in Table 2. The soil chemical analysis six months after planting cassava showed an increase in C-organic, N-total, available P, and CEC (Table 3). According to McKenzie-Gopsill et al. (2022), cover crops can produce large amounts of biomass so they can effectively suppress weeds. Biomulch planted 12 weeks before planting cassava had the highest biomass weight (Table 1), showing that a longer time is required for the biomass to cover the soil surface. In addition, the cassava canopy also reduces weed growth under the cassava plants.
According to Mzabri et al., (2022) 70% of shade can inhibit weed growth, although this depends on the weed species and their tolerance to environmental stress condition.
The dominant weed species from the broadleaf group include Melastoma malabathricum, Mimosa invisa, Oxalis barrelieri, Dioscorea hispida, Tertacera indica, Ipomoea obscura, and from the grass group Ottochloa nodosa (Figure 3). It is suspected that these weed species are adaptable and tolerant to the environmental stress, making it difficult to be suppressed by ground cover plants. According to a study by Listyowati et al. (2022), the dominance of broadleaf weeds is due to by the growth of weed seeds after tillage; soil tillage brings the weed seeds to the soil surface and exposed them to light required for germination. The dominant weed species has vine and creeping growth habits, such as Tetracera indica, Ottochloa nodosa, Dioscorea hispida, and Ipomoea obscura. According to Sudira et al. (2017) root system greatly affects plant growth, those with shallow and creeping roots get more nutrients and water, whereas those with taproot can grow deep into the soil to access water.
The important characters for selecting cover crop are fast growth and tolerance to shades and drought, and these characters are very important for a long-term weed control (Schappert et al., 2019). Using A. pintoi as biomulch can be an option in environmentally friendly weed management because it can reduce the use of chemicals from herbicides. According to Monteiro and Santos (2022), using herbicides in weed control often causes weed resistance and ecosystem imbalance. Environmental pollution, weed resistance, and increased production costs are the effects of an excessive herbicide use (Pradhan et al., 2022). Long-term weed control for sustainable agriculture There was a decrease of C-organic content in the control and in the biomulch B1 treatment (planted at the same time as cassava planting). The initial C-organic value was 3.69% (moderate), in the treatment without biomulch it was 1.97% (low), and in the B1 treatment it was 1.79% (low). With treatment B2 the C-organic value was 5.12% (very high), B3 was 7.54% (very high), and treatment B4 was 2.60% (moderate). These results show that the C-organic content, which is abundant in the topsoil, can be easily leached by soil erosion. On the other hand, with the biomulch treatment B2 and B3 the amount of specific C-organic increased (Table 5). According to Nariratih et al. (2013), the amount of carbon in the soil is influenced by several factors including evapotranspiration, activity of soil microorganisms in breaking down organic matter, soil erosion, and carbon removal at harvest. High residual biomass from the ground cover increases soil organic matter, and the biomass production is affected by rainfall. Lower rainfall reduces growth, hence lower the biomass production (Blanco-Canqui et al., 2015).
The soil total-N value before tillage is 0.14% which is in the low N category (Table 4), and this value is similar to the value before the cassava harvest. The total-N in the control (without biomulch) was 0.18%, and with biomulch B1 was 0.16% (also in the low category, Table 5). With the biomulch treatments B2, B3, and B4, there was an increase of 0.22-0.41% in the total N, which is in the moderate category. This shows that the biomulch treatment four, eight, and twelve weeks before planting cassava can increase nitrogen in the soil. Increased nitrogen levels in the   Parvin et al., (2022) the increase in total N-was influenced by the activity of rhizobium, and the use of legume cover crops under optimal growth conditions was able to fix up to 50 kg N/ha-1. According to De Notaris et al., (2020), using legume plants as ground cover increases soil carbon and nitrogen. Increasing the organic matter content in the soil may also increase the N content in the soil (Asbur et al., 2018;Plaza-Bonilla et al., 2015). Using A. pintoi cover crops can increase the availability of soil nitrogen and maintain soil moisture (Chozin et al., 2015). The use of cover crops can minimize N loss due to climate change (Kaye and Quemada, 2017), and reduce NO 3 leaching regardless of the type of soils (Lapierre et al., 2022) .
The available soil P before the study was classified as very low (8.6 ppm, Table 4) but it increased after the biomulch treatment, except for the B4 treatment. This shows that A. pintoi biomulch can increase the available phosphorus in the soil. According to Lizcano Toledo et al. (2022), the use of cover crops for years can increase the soil available P. However, the increase in phosphorus does not guarantee that the plants can use the phosphorus because of the high levels of Fe, which may bind P and make P unavailable to the plants. The solubility of Fe and Al in acidic soils is relatively high and form strong bonds of Al-P and Fe-P, making them unavailable to the plants (Daras et al., 2012). According to Syahputra et al., (2015), ultisols' high and low P content depends on the P content in the parental materials. The unavailability of P for plants is also affected by the presence of P bonds in Al and Fe minerals. This is in line with a study by Mardamootoo et al. (2021), which stated that phosphorus is present in most soils, but it is not necessarily available for plants.
The CEC value before tillage was 28.53 cmol. kg -1 , which is classified as high. The high CEC values may be due to the soil texture at the study site which is dominated by clay fraction (Table 4). At the time of harvest, the results of soil analysis showed an increase in CEC compared to without cover crop treatment, whereas soil CEC values increased in all biomulch treatments (Table 4). The biomulch B2 and B3 treatments showed the highest CEC values, i.e., 48.7 cmol.kg -1 and 28.8 cmol.kg -1 (Table 5). This shows that cover crops can increase the soil CEC value. According to Husni et al. (2016) and Sari et al. (2022), soils that predominantly has the clay fraction tend to have a high capacity for ion exchange and bind water, so they have highly stable soil aggregates. In addition, these soils contain Ca, Mg, K, and Na, which also affects the CEC values (Husni et al. 2016, Sari et al., 2022. Soils with high CEC values can hold more nutrients in the form of cations (Taisa et al., 2019).

Soil Physical Quality
Our study showed that the biomulch treatment reduced soil bulk density (Table 6). At the beginning of planting, treatment B1 to B4 had a bulk density value of 0.77 g.cm -3 to 0.81 g.cm -3 , lower than without biomulch treatment (Table 6). At the time of harvest, the soil with biomulch treatment had a lower soil bulk density than those without the biomulch treatment (Table 6). The lowest bulk density was in the B2 treatment of 0.65 g.cm -3 or decreased by about 35% by weight. This shows that A. pintoi biomulch increased the soil organic matter and reduced soil bulk density. Study by Muddarisna et al. (2009) reported that the use of manures and green manure A. pintoi reduced the soil weight, increased soil porosity, and increased soil permeability and soil aggregates. Soil with low bulk density makes it easier for plant roots to penetrate the deeper soil layers (Maysarah et al., 2021), whereas high bulk density reduces soil pores hence reduces aeration (Meli et al., 2018).
Biomulch treatment B1, B2, and B3 reduced the specific gravity of the soil by 2.12 g.cm -3 , 2.14 g.cm - Table 6. Effect of biomulch treatments on the soil bulk density, specific gravity, and total pore space at the beginning of planting (0 MAP) and at cassava harvest (6 months after planting). 119 3 , and 2.15 g.cm -3 , respectively, whereas the control had 2.23 g.cm -3 (Table 6). These values show that the use of biomulch increases the soil organic matter, which presumably reduce the soil-specific gravity and bulk density as according to Safitri et al. (2018), the higher the organic matter applied to the soil, the lower the soil specific gravity. Increasing organic matter content improves soil physical properties, resulting in a more stable soil structure, and increased soil N and K (Yuliani and Rahayu, 2016). With the optimal crop and soil management, Ultisols can be utilized as agricultural land for food crops (Syahputra et al., 2015).
The total pore space at the study site was significantly increased by the A. pintoi biomulch treatment (Table  6), and treatment B2 had a total pore space increase of 20.7% (Table 6). This indicates that the decrease in bulk density increases the total pore space. Our results agree with Khair et al. (2017) in that soil bulk density is inversely proportional to the total pore space and is affected by the soil organic matter. The increase in the soil total pore space is related to the formation of soil aggregates by soil organic matter, hence increasing the soil volume and pores (Ardiansyah et al., 2015). The soil pores formed between soil particles affects the growth of plant roots; the high soil porosity allows the plant roots to penetrate deeper soil and access more water and nutrients (Taisa et al., 2019).

Conclusion
Arachnis pintoi planted four and eight weeks before planting cassava controlled the weed growth, demonstrated by the lowest weed dry weight of 15.55 g per 0.25 m -2 and 14.65 g per 0.25 m -2 , respectively at six months after cassava planting. All biomulch treatments increased the soil density and pore spaces. The soil with biomulch planted eight weeks before cassava had the highest C-organic (7.59%) and total N (0.41%) compared to the control/without A. pintoi biomulch. The dominant weeds in the cassava study site are Melastoma malabathricum, Tetracera indica, Oxalis barrelieri, Mimosa invisa, Ipomoea sp, Ottochloa nodosa and Cyperus sp.