Top 4 Residual Effects of Herbicides

Residues can prevent the growth of sensitive weeds residual effects of herbicides on soil and soil bioagents are of great concern. These residues can also restrict crop growth in the following year. Herbicides pose different effects on biotic components of the environment where they persist for a long time.

1. It is accumulated in the soil from excessive use of herbicides
2. Death and retarded growth of soil-inhabiting microorganisms.
3. Injury to succeeding sensitive crops [1,2].
4. Unjust use of herbicide and then residual effects of herbicides persists in the following crop.

Herbicide residues have multiple effects on soil health, micro and macro flora and fauna and the physio-chemical properties of soil. This makes it compulsory to manage herbicide residues. The effects of Residues of herbicides like that of Triazine, vary in the damage they cause according to the seasonal conditions. Its effects will be much less when the season starts and is uniform on rainy compared to dry.

Root disease will exacerbate the effect of triazine residues as the young seedlings cannot grow away from these which are concentrated in the cultivation layer. Sulfonylurea and imidazolinone residues are less affected by soil moisture as they are more soluble. The first effect of the sulfonylureas is to prune roots. Be careful in duplex soils that have sand over alkaline clay. The sulfonylureas can leach down to the clay where they will be more persistent due to the high pH.

Residual Effects of Herbicides to Contaminate Soil

Soil life and soil health are of great concern for farmers and scientists. Soil contains microorganisms like bacteria, yeast, fungi, algae, and macro-organisms like protozoa, earthworms, nematodes, mites, spiders, etc. The function of this complex biota is to carry out decomposition, storage, and cycling of nutrients. Although herbicides are meant for plant protection, the residual effects of herbicides on these micro and macro-organisms cannot be ignored.

An imbalance in the diversity of these beneficial organisms has a drastic effect on the agro-ecosystem. This concern was well documented at the United Nations Conference on the Environment and Development (UNCED) in 1992. Residual effects of herbicides may also indirectly destroy soil biota by disturbing their habitat and food supply.

Residual Effects on Soil Microflora

Total microbial population changes directly after herbicide application. However, studies have shown that in actual field conditions and at the recommended dose, no serious or prolonged effect on soil biota is observed [3]. Except for initial suppression, butachlor stimulated microbial biota of rice rhizosphere [4]. Pendimethalin in cotton crops as a pre-emergence herbicide, steadily increased the total population of actinomycetes, bacteria and fungi however, its residues affected soil microorganisms after crop harvest [5]. Repeated application of pendimethalin on the same soil also led to rapid degradation of it in successive years [6]. However, traces of pendimethalin found in soil were not able to translocate into plant parts [7].

Generally in the field conditions, herbicides initially depress the population of microbes, and later, their population increase to the normal level. This initial suppression could be due to the residual effects of herbicides on susceptible strains and a subsequent increase could be due to an increase in the population of relatively resistant strains with time. The subsequent increase in number could also be related to the increased nutrient status of the soil after weeds were killed by the treatment. Thus a shift in soil microbial composition may occur after the treatment of residual effects of herbicides in soil [8].

Residual Effects on Soil Biochemical Processes

Due to some limitations, data cannot distinguish between active and inactive microbes in the soil. Assessment of enzymes (dehydrogenase) CO2 (evolution) offers an integrative index of the biological status of the soil. Dehydrogenase activity is the measure of the metabolic activity of microbes in the soil. Respiratory activity and the extent of CO2 evolution are greatly reduced as a result of a buildup of residual effects of herbicides. The process of ammonification and nitrification showed a temporary suppression after herbicide application. Later on, the increased availability of ammonium (NH⁴⁺) and nitrate (NO₃) in the rhizosphere has been reported [9].

Symbiotic nitrogen fixation in legumes is of immense importance. The residual effects of herbicides may also alter the process of nodulation by limiting the growth and survival of Rhizobia. Treating soybean with rhizobial culture and fluchloralin at recommended rate promoted nitrogenase enzyme and nodulation activity [10].

Ammonification of organic compounds in the soil is carried out by a wide range of microbes. It has also been observed that residues of butachlor, fluchloralin, and oxyfluorfen had no significant effect on urease activity in sandy loam soils [11].

The process of nitrification is controlled by a wide range of microorganisms thus there is no significant effect of residues. However, nitrification, as is carried out by a small group of organisms (Nitrosomonas and Nitrobacter) and thus any residue limiting these bacteria can limit the process of nitrification to great extent [3].

Residual Effects of Herbicides on Soil Fauna

The soil fauna may be beneficial in improving soil structure and fertility or injurious e.g. parasitic nematode. Beneficial soil fauna increases aeration and drainage and contributes to humus formation. The residual effects of herbicides on soil fauna depend upon soil type, stage of the crop, and type of species found. Residues of Butachlor and pretilachlor were reported as the most toxic ones. Acute toxicity of butachlor to Drawida willsi (earthworm) was reported by Panda et al. (2004). However, the application of fluchloralin and alachlor increased soil nematode population to great extent in alluvial soil [12].

References:

1. Sondhia S. 2013. Harvest time residues of pendimethalin in tomato, cauliflower and radish under field conditions. Toxicological and Environmental Chemistry 95(2): 254-259.
2. Brandenberger LP. 2007. Injury potential from carryover of watermelon herbicide residues. Weed Technology 21: 473-476.
3. Barman, K.K. and J.G. Varshney. 2008. Impact of herbicides on soil environment. Indian Journal of Weed Science 40 (1&2):10-17.
4. Shetty SVR. 1977. Effect of some new rice herbicides on total soil microflora. Current Research Monthly Newsletter, University of Agricultural Sciences, Bangalore 6: 37-40.
5. Balasubramanian K and Sankaran S. 2001. Effect of pendimethalin on soil microorganisms. Indian Agriculturist 45: 93-98.
6. Pahwa SK and Bajaj K 1997. Persistence of trifluralin and pendimethalin in soils incubated at different temperature, Indian Journal of Weed Science 29(3&4): 187-182.
7. Arora S and Gopal M. 2004. Residues of pendimethalin after weed control in cabbage crop (Brassica oleracea.Capitata). Bulletin of Environmental Contamination
   and Toxicology 73: 106-110.
8. Lupwayi NZ, Harker KN, Clayton GW, Turkington TK, Rice WA and O’-Donovan JT. 2004. Soil microbial biomass and diversity after herbicide application. Canadian Journal of Plant Science 84: 677-685.
9. Debnath A, Das AC and Mukherjee D. 2002a. Rhizosphere effect of herbicides on nitrogen fixing bacteria in relation to availability of nitrogen in rice soil. Journal of the Indian Society of Soil Science 50: 463-466.
10. Murali Gopal, Alka Gupta, Magu SP, Gopal M and Gupta A. 2002. Effect of fluchloralin on N fixation and growth of rhizobia and fluorescent pseudomonads in soya bean. Tropical Agriculture 79: 173-179.
11. Shukla AK and Mishra RR. 1997. Effect of herbicide butachlor on nitrogen transformation and soil microbes. Journal of the Indian Society of Soil Science 45: 571-574.
12. Mohammed G. 1987. Effect of herbicides upon dynamics of nematode population in soybean var. “Bragg”. Pesticides 21(11): 30-31.