Effect of Chemical Weed Control on Soil Bio-Chemical Indices- A Review
Abstract
The assessment and monitoring of soil life and soil health can be used to develop more sustainable and productive farming systems. Hence, the consequence of herbicide application on soil health is always a concern for the research community. In view of this, the findings available from India in respect to the impact of herbicides on the non-target organisms and important soil bio-chemical processes are reviewed in this paper. There is great variation among the reports showing short term transient depressing to non-inhibitory or even stimulatory effects of herbicides on total soil microbial count and different soil bio-chemical indices. The impact differed depending upon the soil type, experimental conditions, herbicide in question and its dose, and the sensitivity of the non-target species or strains. No severe ill effect on soil flora, soil bio-chemical indices and soil fauna has been observed sofar at recommended dose of herbicide under field conditions. However, the available information is based on the short term experiments and there is need to develop database on long-term field application basis. The paper concludes with some suggested areas for future research requiring urgent attention.
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Introduction
Soils contain microorganisms viz. bacteria, fungi, yeasts, photosynthetic organisms including algae and macroorganisms such as protozoa, nematodes, mites, springtails, spiders, insects and earthworms. The functions of this complex array of biota are diverse, and include residue decomposition, nutrient storage and release, soil structure and stability, resistance against disease and degradation or immobilisation of soil pollutants. A minimum of species is necessary to carryout essential tasks. It is believed that high biodiversity leads to a higher soil functional stability and thereby, a greater capacity to recover from perturbation and maintaining environmental sustainability.
Weed control in agricultural and non-agricultural lands is rapidly shifting towards chemical methods because of its time, labour and cost advantages. Although herbicides are meant for plants, possibility of a direct effect on other organisms can not be ruled out as a number of basic and universal biochemical processes essential for all forms of life are alike. Direct impacts on sensitive organisms can occur when the chemical reaches the soil due to targeted deposition of pre-emergent herbicides, or through unintentional deposition from spray and spray drift, dripping from plant material, and contaminated plant material falling to the soil.
A decrease in the population of sensitive species may cause an increase in the population of resistant soil microorganisms due to relatively lesser competition. Thus, their application may have impacts on organisms that benefit the wider agro-ecosystem. Such concerns from the research community and general population were well documented in June 1992 at the United Nations Conference on the Environment and Development (UNCED) in Rio de Janeiro, which is referred to as the Convention on Biological Diversity.
Infield conditions, herbicides may also greatly influence soil biota populations indirectly by their effects on vegetation which provide habitat and food for many of them. The soil organisms may respond differentially due to the changes in vegetation rather than to direct herbicide effect (Grossbard and Davies 1976, Haugland 1994). Influence of herbicides on soil biota population and agriculturally important soil bio-chemical processes have been reviewed in this paper in light of the findings reported from India.
Conclusion
: Herbicides being toxic to plants may exert some kind of impact on other lifeforms in soil by their direct chemical action and by changing the soil ecosystem as result of changes in vegetation cover. Overall, the experimental results showed that the population of soil microflora are stimulated or depressed by herbicides, depending upon the chemical nature, preparation, its dose, and sampling time and soil type. It is difficult to draw conclusions from such varied results on the counts of the microorganisms of the soil vis-a-vis herbicide application. However, at recommended rate of herbicide application, often a reversible change in the equilibrium of the population of micro-flora and fauna takes place in soil for a short period of time under field conditions.
It may be kept in mind that to effectively evaluate the relative effects of different agricultural practices in the long-term it is necessary to sample until the ecosystem has achieved some degree of equilibrium rather than monitoring only initial cropping cycles (Yeates et al. 1999). If herbicide application is to remain a viable practice in sustainable farming systems, evaluation of herbicide effects from repeated and long-term use is essential to ensure optimum nutrient availability and plant growth. However, the literature available sofar is based on either laboratory experiments or short-term field experiments. Report on the basis of well-planned long-term field experiment is not available to draw any conclusion regarding the environmental implications of herbicide. Therefore, the steps should urgently betaken to generate data on long-term application basis. Changes in the many vital soil processes become visible in along term, for example changes in soil organic C content. Some processes show “transition phenomenon”, that is an impact may continue for years without any visible changes in the measured soil characteristics; and after a certain transition time the characteristics change at rapid rate. For example nitrate leaching from grasslands due to mineral nitrogen fertilization did not vary much during the initial years, but after several years it increased suddenly in spite of that the mineral N fertilization remained the same. Feasibility of such “transition phenomenon” in the microbially mediated important soil processes in respect to the soil fertility and productivity should not be ignored, especially in light of the reports showing the differential effect by the different group of microbes, even strains, to a given herbicide. Long-term experiments and database is needed to foresee such probabilities and to derive suitable remedial measure. Most of the information generated sofar are of superficial in nature and dealt primarily about total counts. There is a dearth of information regarding the herbicide effect on the changes in microbial diversity, nitrification, denitrification, sulfur oxidation, mineralization of plant nutrients, crop residue decomposition and its consequence upon quantitative and qualitative aspect of soil organic matter equilibrium. In depth study in respect to the herbicide effect on biological nitrogen fixation is also meager. Future research is very much warranted in these directions. No serious effort has yet been made to study the dynamics of various groups of soil fauna in the fields receiving herbicide application, and it needs more attention.
