Effect of linear view approach of weed management in agro-ecosystem: A review

Weeds are considered a pest that is detrimental to human interest. Weeds compete with cultivated and desirable plants for space, soil nutrient, and sunlight and soil moisture thereby reducing crop productivity. Weeds also serve as a host for pests and pathogens that cause plant diseases. Weeds typically produce large numbers of seeds, assisting their spread, and rapidly invade disturbed sites. Seeds spread into natural and disturbed environments, via wind, waterways, people, vehicles, machinery, birds and other animals. To prevent the effect of weeds on crop productivity, farmers mostly use a linear approach for weed management. The linear view approach aims at eradicating weeds without concern about the environment. This paper reviews the effects of the linear view approach of weed management in the agro-ecosystem. A review using forty-one articles reveals that herbicides application, bush burning and soil tillage are the most often used linear view approach for controlling weeds in the farm ecosystem. The paper highlights the contribution of these weed control methods to climate change through loss of biodiversity, pollution of water bodies, soil degradation, deterioration of fruit quality and release of carbon dioxide to the atmosphere.


INTRODUCTION
A weed is any plant growing in the wrong place, that is, undesirable areas and conflicts with human interest. Moreover, good for nothing plants are also termed as weeds (Holzner, 1982). Ecologists view weeds as colonizers. According to Holzner (1982) and Hahn (2019), weed types can be categorized into various classes which include; agrestal, ruderals, grassland weeds, water weeds, forestry weeds, and environmental weeds. Agrestals are weeds of farmed or arable land which includes root crops, cereals, orchards, and even plantations. Ruderals are plants occurring on debris such as the earth heaps. Grassland weeds are also referred to *Corresponding author. E-mail: kalii.gideon@stud.umu.ac.ug. Author(s)

Lifecycle of weeds Definition Examples Sources
Annual Weeds that complete their lifecycle within one growing season Redroot pigweed(Amaranthus retroflexus) Knezevic et al. (1994); Pacifico et al. (2008) Biennial Weeds that complete their lifecycle in over two growing season Musk thistle(Carduus nutans) Smith and Kok (1984) Perennial Weeds that continue to regrow over several seasons to many seasons, that is, they live more than a single season.
Couch grass (Elytrigia repens) Glinwood et al. (2003) as pasture weeds and are mainly perennial. Water weeds hinder human activities especially in slowing down the flow of irrigation water in waterways. Forestry weeds are the forest plants that hinder the growth of young trees. Lastly, environmental weeds are the species that colonize natural vegetation and suppress the indigenous species to a certain extend. Weeds can be annual, perennial, or biennial depending on their lifecycle (Table  1).
In the arable lands, both the annual and perennial weeds have various characteristics attributed to them. Both annual and perennial crops experience weed as a common problem (Karim et al., 2004). However, annual crops are not only attacked by annual weeds but also by perennial weeds. Some of the worst weeds in annual crops are perennial plants whereas annual weed species have difficulties in thriving among perennial crops with dense canopies (Håkansson, 1982). Biennial weeds are also found in the arable lands since they take over two seasons to complete their cycle. Weeds are detrimental while in the field since they compete for water, nutrients, space, and sunlight with the crops; (Benayas et al., 2002). They have been acting as hosts to harmful pests and diseases which have led to a reduction in the expected crop yields. As a result of this, weed control has become a major area of concern in the agricultural sector which has led them to use any means available to eradicate them. Linear view approach of weed management aims at eradicating weeds without concern about the environment Most of these methods used affect the agro-ecosystem negatively and they include the use of herbicides, tillage, and bush burning (slushing and burning).
The continuous use of chemical weed control methods contaminates the soil since the chemical residues accumulate in the soil containing heavy metals such as lead (Pb), zinc (Zn), and copper (Cu) (Rose et al., 2016;Apori et al., 2018). This affects the soil physicochemical properties as well as the distribution of micro-organisms in the soil. Once it rains, the chemical residues are carried by the water runoffs or washed into water reservoirs hence contaminating the surface water, groundwater, and the underground water. Continuous tillage which is the main way of weed eradication leads to soil compaction which enhances hardpan development (Welch et al., 2016). Moreover, bush burning also releases gases that contribute to climate change into the atmosphere. The opposite of linear view approach of weed management is the wide view approach of weed management which ensures that the environment is taken care of during weed management so as to enhance biodiversity sustainability. Therefore, this article seeks to review the effect of herbicides application, tillage practices, and bush burning in the agroecosystem.

Herbicides application
Herbicides are agrochemicals that belong to the class of pesticides used to control unwanted plants. They are categorized based on the time of application, mode of application, and biological effect (Table 2). Under the time of application, they can be categorized into preplant, pre-emergent, and post-emergent herbicides (Willoughby et al., 2003). Pre-plant herbicides are applied before the crop is planted/transplanted into the main field such as glyphosate ( Figure 1) which kills weeds before planting (Duke and Powles, 2008;Bradberry et al., 2004).
On a different note, the pre-plant herbicide can be into the soil especially in rice-growing fields (Karim et al., 2004). The pre-emergent ones target the grassy weeds and the broad-leaf weeds and they are applied to the soil surface such as butachlor (Yu et al., 2003) while the postemergent are selective (target specific weed species) and non-selective (broad-spectrum) in nature (Kaeser and Stewart, 2010;Dear et al., 2006). An example of the post-emergent herbicide is propanil (Villarroel et al., 2003).
Under the mode of application, herbicides can either be foliar applied or soil applied. Foliar applied herbicides are directly sprayed on the leaves such as 2, 4dichlorophenoxyacetic acid (Garabrant and Philbert, 2002). On the other hand, soil-applied herbicides are sprayed on the soil surface targeting germinating weeds. Under the biological effect category, herbicides can be grouped according to the mode of action, and according to selectivity (selective or non-selective). The mode of action involves contact herbicides and systemic herbicides. The contact herbicides are applied to the leaves and kill plant tissues, e.g. paraquat (Smith et al., 1976). The systemic herbicides transit within the plant, e.g. bispyribac sodium (Toni et al., 2010). Different concentrations of herbicides affect the environment differently. According to Gaupp-Berghausen et al. (2015), glyphosate-based herbicides reduce the activity and the reproduction of earthworms and lead to the rising of soil nutrient concentrations. Yusof et al. (2014) found out that glyphosate herbicide reduced the hatching and the survival percentage of Java medaka (Oryzias javanicus) fish as its concentration increased.

Tillage
Tillage is an important weed control tool in organic farming. According to Kuipers (1970), weed control is the only sure effect of tillage on crop production. Tillage has various systems such as reduced/minimum tillage and zero tillage which substitute herbicides for tillage (Kladivko, 2001;Lee and Stewart, 1983;Rusu, 2014). In most cases, tillage for weed control depends on the prevailing weather conditions for it to be successful. Mixtures of weed species are affected by the presence or absence of tillage operations. Zero-tillage systems experience challenges from perennial weeds and weeds that are dispersed by wind (Derksen et al., 1996). In conventional tillage system, the soil storage capacity of plant biomass is reduced which results to reduced moisture and high temperature hence low microbial activity (Aziz et al., 2013). Moreover, weed seeds are buried in the soil and end up germinating due to the favorable conditions (Forcella and Lindstrom, 1988).

Bush burning
Bush burning is the act of setting weeds and grasses in uncultivated farmlands on fire. Farmers can also burn the crop residues left after harvesting in preparation for the next planting season. This mainly takes place during the dry periods. Bush burning has negative effects on humankind's health and also pollutes the environment (Table 3). It releases gases such as carbon monoxide, hydrogen sulfide, nitrogen oxides, sulphur oxides, and even ozone (Izah et al., 2017). Uncontrolled bush burning is harmful to the agro-ecosystem as it destroys gardens, livestock, crops, soil micro-organisms, and soil physicochemical properties. Some farmers within the tropics slush the unwanted plants during land preparation then set them on fire.

Effect of herbicides concentration on soil quality indicators and water quality
In the past years, the contamination of soils by inorganic toxins due to the application of herbicides and other chemicals in agricultural production has been on the rise . The continued use of herbicides in weed management has affected soil physiochemical and biological properties. The moisture content in soils that have herbicides applied in them reduces drastically within a short period. Moreover, herbicides remove essential soil minerals and make them unavailable for uptake by plants (Sebiomo et al., 2012). This results in the alteration of soil chemical properties which ends up affecting plant growth and the potential yield. In the case of round-up which is made up of glyphosate, it increases the soil concentrations of both nitrate and phosphate which possess a risk of leaching (Gaupp-Berghausen et al., 2015). Herbicide concentration takes advantage of the cation exchange capacity (CEC) of the soil and bind to the soil colloids. This concentration can be maintained at toxic levels. These toxic levels leave behind chemical residues which could dissolve into the available groundwater since the herbicides bind with the soil colloids and are less available to crops (Johnson and Colmer, 1955). Soil biodiversity brings out the variability among living organisms such as fungi, bacteria, and protozoa in the soil. The application of both pre-plant, pre-emergent, and post-emergent herbicides in crop production impacts soil biodiversity. Most studies point out that herbicide application impacts are only minor but according to Rose et al. (2016), herbicides consistently alter soil functions. Application of glyphosate and atrazine herbicides interferes with the earthworm ecology in soils. Moreover, sulfonylurea herbicides inhibit the soil nitrogen cycle (including biological nitrogen fixation) in low organic matter soils (Rose et al., 2016). Moreover, the concentration of the increasing herbicide harms the soil microorganism population (Pochron et al., 2019). According to Johnson and Colmer (1955), the microorganism population is reduced as a result of the herbicide concentration in the soil. In their study which involved different treatments, the population of bacteria counted in the soil drastically reduced after the herbicides application. In an experiment, Gaupp-Berghausen et al. (2015) established that glyphosate-based herbicide reduced the casting activity of vertically burrowing earthworms (Lumbricus terrestris) and also reduced the production of soil-dwelling earthworms.
Herbicide application affects both the surface and the groundwater. The application of herbicides compromises with the water quality since they are water-soluble and can leach into the groundwater and the surface water (Thurman et al., 1991). The use of herbicides has led to reduced water quality which is one of the existing environmental problems that the world is facing today (Misra et al., 1996). When it rains, the herbicides dissolve into the water and the runoff water washes them into water bodies like lakes, rivers, and groundwater aquifers. Water inhabiting animals like fish have had to tolerate weed killers from agronomic activities that have been washed into the water bodies (Mullison, 1970). Table 3. Key findings of bush burning in the environment.

Source Key findings
Birnin-Yaur et al. (2008) It affects different weed species differently. It leads to termination of grass weed species while increasing the prevalence of broadleaved weed species Bebawi and Campbell (2002) It reduces the weed seed bank in various soils. However, the deeper the weed seeds in the soil the less the impact of the fire temperatures which leaves them viable for germination Inyang and Esoh (2014) It is harmful to human beings and the environment as it involves production of air pollutants (Carbon monoxide, hydrogen sulphide, nitrogen oxides, sulphur oxides, ozone) Izah et al. (2017) Bush burning reduces the soil microbial population and diversity Ohwoghere- Asuma (2012) Bush burning reduces the value of carbon, organic matter, nitrogen and pH. Vågen et al. (2005) Bush burning reduces soil organic carbon on the upper soil profile. Bagoora (1988) Bush burning increases soil erosion risk in highland areas.

Herbicides concentration on fruit quality
Herbicide application in fruit production affects the yield and fruit quality. According to Mohammed and Ali (1986), the yield of tomato fruit reduced significantly when herbicides were used for weed control as compared to hand cultivation. Moreover, the level of ascorbic acid in the tomato fruits significantly increased as compared to the untreated ones. Some herbicides like treflan reduce the general fruit acidity. However, according to Maia et al. (2012), the use of herbicides such as diuron fluazifop-pbutyl and atrazine+ S-metolachlor does not affect the quality of some fruits like pineapple.

Effect of tillage on soil compaction, soil infiltration, and porosity
Soil compaction is the soil structure degradation (SSD) which is one of the most popular and harmful forms of land degradation in agriculture (McGarry, 2003). Compacted soils have low porosity, low air as well as low water permeability. Since tillage (conventional, zero and reduced) is the most used means of weed control in organic farming, it could contribute to soil compaction that affects soil water availability, nutrient availability and could contribute to soil erosion (Badalíková, 2010). According to Badalíková (2010), conventional tillage result in higher bulk density which connotes reduced water infiltration resulting in lower porosity. The lower range of tilling the soil favors reduced bulk density which increases water infiltration. Tillage also affects soil infiltration as some tillage methods interfere with surface connected pores and prevent the accumulation of soil organic matter. This also ends up reducing the porosity. Reduced tillage improves soil infiltration and also reduces surface runoff (Mupangwa et al., 2013)

Effect of bush burning in the environment
Bush burning is used as a means of weed control especially after harvesting in preparation for the next season. In most cases, slashing is done before the burning (Akobundu, 1979). Weeds, grasses and crop residues are set on fire and end up releasing various gases which pollute the atmosphere. Such gases include carbon monoxide, carbondioxide, hydrogen sulphide, nitrogen oxides, sulphur oxides and even ozone. These gases contribute to the greenhouse gas effect, acid rain, and ozone layer depletion which intensifies climate change. The acidic rain that is received alters the soil pH. On the other hand, bush burning causes a decrease in composition and abundance of biodiversity (Izah et al., 2017;Jamala et al., 2012;Birnin-Yauri et al., 2008). According to Schoch and Binkley (1986), bush burning reduces nutrient pool sizes through ash transport, leaching and erosion. At the same time, bush burning improves the availability of nutrients since fire converts the nutrients held in soil surfaces and plant debris to more available forms. According to Hough (1981), the concentration of some nutrients such as calcium, potassium and magnesium ions can be unaltered by bush burning while nitrogen and sulphur depreciate. The intensity of the fire in bush burning determines the kind of effects imposed on soil microbes. Moreover, there is decreased microbe abundance as a result of bush burning. According to Poth et al. (1995), the level of activity by microbes is higher than that of the microbial community preceding bush burning fire.

Contribution of linear view approach of weed management to greenhouse gases emissions
The linear view approach of weed management practices contributes to greenhouses gases either positively or negatively. The effects of herbicides on greenhouse gas emissions remain unclear (Jiang et al., 2015). According to Brookes et al. (2017), some herbicides such as glyphosate and butachlor reduce N 2 O and CO 2 but do not affect CH 4 . Herbicide application reduces the use of fuel by machines that are used in conventional tillage which reduces greenhouse gas emission (Gianessi, 2013). Zero tillage reduces greenhouse gas emissions due to the curtailing of the use of heavy machinery (Maraseni and Cockfield, 2011). According to Mangalassery et al. (2014), conventional tillage contributes to the change of state of CO 2 and CH 4 gases while it has no impact on N 2 O. Moreover, according to Lognoul et al. (2017), N 2 O and CO 2 emissions are higher in reduced tillage ascompared to conventional tillage. Conventional tillage distributes crop residues within the soil profile raising the organic C and N content. Straw returning enhances carbon sequestration. Burning biomass releases CO 2 , N 2 0, CH 4 , and other greenhouse gases into the atmosphere and burning of crop residues that store carbon releases CO 2 into the atmosphere which is a greenhouse gas (Guoliang et al., 2008).

Knowledge gap
Fruit quality is affected differently by various herbicides applications as they decrease fruit yield and rise in ascorbic acid while as per Maia et al. (2012) concluded that fruit yield and ascorbic acid remained constant after herbicide application. There is a need for further research to find out the actual effects on growth, yield, and quality in other fruits since the effects seem to differ in each fruit. Nutrients such as calcium, potassium and magnesium ions can be unaltered by bush burning while nitrogen and sulphur depreciate. However, to what extent will bush burning affects potassium, calcium, magnesium, and nitrogen dynamics in different soils.

Conclusion
The linear view approach of weed management remains a key area affecting the agroecosystem (Figure 2). Application of herbicides, tillage and bush burning affect the physicochemical and biological properties of soil resulting in low soil fertility. It also pollutes surface and underground water quality. Furthermore, it contributes to greenhouse gases emission. There is a need to carry out awareness among farmers and policymakers as it relates to proper ways of herbicide handling, proper tillage practices and controlled bush burning to avoid their detrimental effects on the environment. There is a need for emphasis on the adoption of WIDE VIEW APPROACH of weed management to enhance sustainable agriculture and meet the sustainable development goals.