Glyphosate and Soil Health

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Impacts on Soil Health

Title Findings
Glyphosate toxicity and the effects of long-term vegetation control on soil microbial communities (2001) Soil microbial communities from ponderosa pine were tested using concentrations of up to 100x greater than expected for a single field application. Long-term, repeated applications had a minimal effect on seasonal microbial characteristics, and instead was a function of the time of year and site quality.
Glyphosate effects on plant mineral nutrition, crop rhizophere microbiota, and plant disease in glyphosate-resistant crops (2012) The majority of literature indicates that mineral nutrition in GR crops is not affected by the GR trait nor by glyphosate application. Data largely indicates that the GR transgene and glyphosate do not increase crop disease, nor are there nutrition and disease issues specific to these crops.

GLYPHOSATE TOXICITY AND THE EFFECTS OF LONG-TERM VEGETATION CONTROL ON SOIL MICROBIAL COMMUNITIES (2001)

We assessed the direct and indirect effect of the herbicide glyphosate on soil microbial communities from ponderosa pine (Pinus ponderosa) plantations of varying site quality. Direct, toxic effects were tested using culture media and soil bioassays at glyphosate concentrations up to 100-fold greater than expected following a single field application. Indirect effects on microbial biomass, respiration, and metabolic diversity (Biolog and catabolic response profile) were compared seasonally after 9–13 years of vegetation control using repeated glyphosate applications in a replicated field study.
Three pine plantations were selected to provide a range of soil characteristics associated with glyphosate binding (clay, Fe and Al oxide content) and site growing potential from the lowest to the highest in northern California. Glyphosate was toxic to bacteria and fungi from each plantation when grown in soil-free media. Culturable populations were reduced, as was the growth rate and metabolic diversity of surviving bacteria, by increasing concentrations of glyphosate. This toxicity was not expressed when glyphosate was added directly to soil, however. Microbial respiration was unchanged at expected field concentrations (5–50 μg g−1), regardless of soil, and was stimulated by concentrations up to 100-fold greater. Increased microbial activity resulted from utilization of glyphosate as an available carbon substrate. Estimated N and P inputs from glyphosate were inconsequential to microbial activity.
Long-term, repeated applications of glyphosate had minimal affect on seasonal microbial characteristics despite substantial changes in vegetation composition and growth. Instead, variation in microbial characteristics was a function of time of year and site quality. Community size, activity, and metabolic diversity generally were greatest in the spring and increased as site quality improved, regardless of herbicide treatment.(2012)
Our findings suggest that artificial media assays are of limited relevance in predicting glyphosate toxicity to soil organisms and that field rate applications of glyphosate should have little or no affect on soil microbial communities in ponderosa pine plantations.

http://www.sciencedirect.com/science/article/pii/S0038071701001031


GLYPHOSATE EFFECTS ON PLANT MINERAL NUTRITION , CROP RHIZOSPHERE MICROBIOTA, AND PLANT DISEASE IN GLYPHOSATE-RESISTANT CROPS (2012)

Claims have been made recently that glyphosate-resistant (GR) crops sometimes have mineral deficiencies and increased plant disease. This review evaluates the literature that is germane to these claims. Our conclusions are: (1) although there is conflicting literature on the effects of glyphosate on mineral nutrition on GR crops, most of the literature indicates that mineral nutrition in GR crops is not affected by either the GR trait or by application of glyphosate; (2) most of the available data support the view that neither the GR transgenes nor glyphosate use in GR crops increases crop disease; and (3) yield data on GR crops do not support the hypotheses that there are substantive mineral nutrition or disease problems that are specific to GR crops.

http://pubs.acs.org/doi/abs/10.1021/jf302436u


THE EFFECT OF GLYPHOSATE APPLICATION ON SOIL MICROBIAL ACTIVITIES IN AGRICULTURAL LAND (2013)

ABSTRACT
In recent years, intensive use of herbicides has increasingly become a matter of environmental concern partially because of the effects of these chemicals on soil microorganisms. Glyphosate [N- (phosphonomethyl) glycine] (GP) is a broad-spectrum, non-selective, post emergence herbicide that is widely used in agriculture.
In this study, glyphosate effects as N, P and C nutrient sources on microbial population and the effect of different concentration of it on dehydrogenease activity and soil respiration were investigated.
The results show that in a soil with a long historical use of glyphosate (soil 1), the hetrotrophic bacterial population was significantly (p<0.05) increased. Also, by increase in the bacterial population, the herbicide existence as the possible nutrient source is enhanced. According to results, bacterial populations in the presence of glyphosate as P source was significantly (p<0.01) higher than N and C sources.
The application of GP to the soil led to a significant increase in dehydrogenase activity with respect to untreated control soil samples. Also, respiration rates increased with increasing glyphosate application up to 50 mM but in 500 mM the inhibitory effect of glyphosate was observed. It can be concluded that glyphosate application may alter (increase) soil microbial activity and population. Increased microbial activity may be beneficial or detrimental toward plant growth, soil microbial ecology, and soil quality.

http://www.ajol.info/index.php/ajb/article/view/99062


IMPACT OF GLYPHOSATE RESISTANT CORN, GLYPHOSATE APPLICATIONS, AND TILLAGE ON SOIL NUTRIENT RATIOS, EXOENZYME ACTIVITIES AND NUTRIENT ACQUISITION RATIOS (August 2016)

ABSTRACT
BACKGROUND
We report results of the last two years of a 7-year field experiment designed to test the null hypothesis: applications of glyphosate on glyphosate resistant (GR) and non-resistant (nonGR) corn (Zea mays L.) under conventional tillage and no-till would have no effect on soil exoenzymes and microbial activity.


RESULTS
Bulk soil (BS) and rhizosphere soil (RS) macronutrient ratios were not affected by either GR or nonGR corn, or glyphosate applications.
Differences observed between exoenzyme activities were associated with tillage rather than glyphosate applications.
In 2013 nutrient acquisition ratios for bulk and rhizosphere soils indicated P limitations, but sufficient assimilable N. In 2014 P limitations were observed for bulk and rhizosphere soils, in contrast to balanced C and N acquisition ratios in rhizosphere soils. Stoichiometric relationships indicated few differences between glyphosate and non-glyphosate treatments. Negative correlations between C:P and N:P nutrient ratios and nutrient acquisition ratios underscored the inverse relation between soil nutrient status and microbial community exoenzyme activities.
CONCLUSIONS
Inconsistent relationships between microbial community metabolic activity and exoenzyme activity indicated an ephemeral effect of glyphosate on BS exoenzyme activity. Except for ephemeral effects, glyphosate applications appeared not to affect the function of the BS and RS exoenzymes under conventional tillage or no-till.

http://onlinelibrary.wiley.com/doi/10.1002/ps.4413/abstract

Glyphosate effects on soil rhizosphere-associated bacterial communities (2105)

In this study, rhizosphere soil was sampled from rhizoboxes following 4 growth periods, and bacterial community composition was compared between glyphosate treated and untreated rhizospheres using next-generation barcoded sequencing. In the presence or absence of glyphosate, corn and soybean rhizospheres were dominated by members of the phyla Proteobacteria, Acidobacteria, and Actinobacteria. Proteobacteria (particularly gammaproteobacteria) increased in relative abundance for both crops following glyphosate exposure, and the relative abundance of Acidobacteria decreased in response to glyphosate exposure. Given that some members of the Acidobacteria are involved in biogeochemical processes, a decrease in their abundance could lead to significant changes in nutrient status of the rhizosphere. Our results also highlight the need for applying culture-independent approaches in studying the effects of pesticides on the soil and rhizosphere microbial community.

http://www.sciencedirect.com/science/article/pii/S004896971530989X