Author
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Kremer, Robert |
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Submitted to: Weed Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 11/6/2013 Publication Date: 4/21/2014 Citation: Kremer, R.J. 2014. Environmental implications of herbicide resistance: soil biology and ecology. Weed Science. 62(2):415-426. DOI: 10.1614/WS-D-13-00114.1. Interpretive Summary: Many weeds that were routinely and effectively controlled using recommended herbicides applied to major crops (corn, soybean, cotton, wheat) have now developed resistance to those herbicides. Herbicide-resistant (HR) weeds present in crop fields require farmers to develop alternative strategies to control weed resistance to ensure acceptable crop productivity. HR weeds often occur in the field as actively growing infestations during the season and escape control or remain until they can be eliminated with an alternative strategy. These infestations not only affect crop growth and yields by competing for light, nutrients and water, but also affect soil ecosystem services by altering soil microbial processes responsible for making nutrients available for plants, production of soil organic matter, and promoting growth and health of crop plants. However, little is known about effects of weeds in general, and even less about effects of HR weeds, on soil biology and ecology compared with what is known about their interactions with crops. Thus the potential impacts of HR weeds on soil biological and ecological properties were reviewed based on previous studies with crops, weeds, and invasive plants. It was found that HR weeds likely behave similarly to their herbicide-susceptible counterparts by establishing extensive root systems in soil, which release various plant-derived chemical compounds resulting in suppression of some soil microorganisms and selection of others that specifically benefit weed growth. Some weeds also disrupt beneficial root-associated microorganisms such as mycorrhizae, which help plants absorb nutrients such as phosphorus from soil sources, and rhizobia, that form structures (nodules) on certain plant roots where atmospheric nitrogen is captured and changed to plant-available forms. A few HR weed species may alter the amount of carbon substrates, or food sources, released into soil for microbial growth. This may be due to indirect effects of resistance genes the weed developed after exposure to herbicides. Herbicides applied to HR weeds may enter the soil after being transported to roots and released to exert various effects on microbial community structure and function. Overall, the impacts of HR weeds on soil biology and ecology are not well known. Efforts need to be undertaken to better understand potential impacts of HR weeds on soil ecology and biology because it is important in developing effective weed management focused on controlling HR weeds in crop production systems. Conclusions and management recommendations from this review are important to farmers, research and agrochemical scientists, economists, and educators for developing and implementing new or adapted technologies for understanding, monitoring and minimizing potential risks from HR weeds to both the soil ecosystem and to crop productivity. Technical Abstract: Soil microbial community structure and activity are clearly linked to plant communities established in natural and agricultural ecosystems. A limited number of studies confirm that weeds alter their soil environment and select for specific microbial communities in the rhizosphere. Such rhizosphere modification is well documented for many crop and horticultural plants. However, the impact of weeds, especially those in agroecosystems, on soil biology and ecology has received less attention because effective weed management practices were developed to minimize the impacts of these plants on crop production. The recent development of herbicide resistance (HR) in several economically important weeds leading to unexpected and widespread infestations in crop fields treated with a single herbicide has prompted a re-evaluation of the effects of weed growth on soil biology and ecology. The objective of this paper is to review the potential impacts of herbicide-resistant weeds on soil biological and ecological properties based on previous and on-going studies on crops, weeds and invasive plants. Persistent weed infestations likely establish extensive root systems and release various plant metabolites into soil through root exudation. Many of these exudates are selective for specific soil microbial groups that mediate certain biochemical and nutrient acquisition processes. The exudate chemicals may stimulate development of microbial groups beneficial to weed growth and detrimental to crop growth or beneficial to both crop and weed. Changes in the interactions with symbiotic and associative microorganisms are known, especially for arbuscular mycorrhizal fungi (AMF) that are important in aiding the plant in uptake of nutrients and water, and in protection from soilborne pathogens. Mechanisms used by weeds to potentially disrupt AMF symbioses with adjacent crop plants are not clearly described. However, many herbicide-resistant weeds including Amaranthus and Chenopodium species do not support AMF symbioses and may reduce the density of AMF propagules in soil necessary for establishment of the symbiosis with crop plants in current and subsequent seasons. Impacts of herbicides applied to control herbicide-resistant weeds may compound the effects of weeds on the soil microbial community. Systemic herbicides released through weed roots may select additional microbial groups that mediate detrimental processes such as nutrient immobilization or serve as opportunistic pathogens. An understanding of the complex interactions of weeds with soil microorganisms under extensive and persistent infestations is important in developing effective management systems for controlling herbicide-resistant weeds. |
