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 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.

LINEAR VIEW APPROACH OF WEED MANAGEMENT

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 pre-plant, 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 post-emergent 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, 4-dichlorophenoxyacetic   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 linear view approach of weed management in agro-ecosystem

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 (Zhang et al., 2013). 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).

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-p-butyl 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 N₂O and CO₂ but do not affect CH₄. 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 CO₂ and CH₄ gases while it has no impact on N₂O. Moreover, according to Lognoul et al. (2017), N₂O and CO₂ 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 CO₂, N₂0, CH₄, and other greenhouse gases into the atmosphere and burning of crop residues that store carbon  releases   CO₂   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.

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.

The authors have not declared any conflict of interests.

The  authors   thank   African   Center   of   Excellence  in Agroecology and Livelihood System, Uganda Martyrs University for their financial support and encouragement.