Effect of glyphosate on soil microbial activity and biomass

Abstract

Herbicides applied to soils potentially affect soil microbial activity. Quantity and frequency of glyphosate application have escalated with the advent of glyphosate-tolerant crops. The objective of this study was to determine the effect of increasing glyphosate application rate on soil microbial biomass and activity. The soil used was Weswood silt loam. The isopropylamine salt of glyphosate was added at rates of 47, 94, 140, and 234 µg ai g−1 soil based on an assumed 2-mm glyphosate–soil interaction depth. Glyphosate significantly stimulated soil microbial activity as measured by C and N mineralization but did not affect soil microbial biomass. Cumulative C mineralization, as well as mineralization rate, increased with increasing glyphosate rate. Strong linear relationships between mineralized C and N and the amount of C and N added as glyphosate (r 2 = 0.995, 0.996) and slopes approximating one indicated that glyphosate was the direct cause of the enhanced microbial activity. An increase in C mineralization rate occurred the first day following glyphosate addition and continued for 14 d. Glyphosate appeared to be directly and rapidly degraded by microbes, even at high application rates, without adversely affecting microbial activity.

Influence of repeated applications of glyphosate on its persistence and soil bioactivity

“…Besides its biological degradation in soil and water, at least transiently, glyphosate also stimulates the soil basal respiration, increases the microbial biomass (Wardle et al, 1994;Haney et al, 2000), and reduces the growth of earthworms after repeated applications (Springett & Gray, 1992). Souza et al (1999aSouza et al ( , 1999b verified that glyphosate leaching was very small in two different Brazilian soils and the soil microorganisms used the herbicide as C source, but the process was extremely dependent of soil moisture.…”

“…Nowadays it is one of the most popular herbicides not only in agriculture but also in forestry, because of its effective control of competitive vegetation and low mammalian toxicity (Busse et al, 2001). It acts by enzymatic inhibition of the shikimic acid pathway, resulting in depletion of essential aromatic amino acids needed for plant growing and survival (Haney et al, 2000;Environmental Protection Agency, 2003). Most living organisms do not have this enzyme, which is however, ubiquitous in microorganisms (Busse et al, 2001).…”

“…Although glyphosate is not recommended for direct application in soil, a significant amount may reach the soil during early-season or preplant applications (Haney et al, 2000). The soil half-life of glyphosate varies from few days to two or three months, and mostly, is smaller than one growing season; but, there are some reports of soil persistence for hundred and thousand days, and persistence of the phytotoxic activity for more than 19 weeks after the application (Heinonen-Tanski, 1989;Feng & Thompson, 1990;Weber, 1994).…”

“…It is known that glyphosate adsorbs strongly in the soil, through hydrogen bonds in organic matter and by anion exchange processes in the clay minerals, iron and aluminium oxides (Aubin & Smith, 1992;Haney et al, 2000;Veiga et al, 2001). The role of adsorption on the glyphosate bioavailability and biodegradation is not completely conclusive, although Veiga et al (2001) state that the adsorbed glyphosate is more persistent in the soil, because its degradation is slower than that of the free herbicide.…”

Action of pesticides to Metarhizium anisopliae in soil

“…Although in the present study trifluralin, glyphosate and ametrin affected microbial activity, this effect was only observed over a short period of time, a fact not sufficient to consider these products toxic to the fungus. Haney et al (2000) concluded that glyphosate seems to be rapidly and directly degraded by soil microorganisms. In another study (Haney et al 2002), these authors demonstrated that the product significantly stimulated soil microbial activity and biomass, in addition to being easily degraded irrespective of the amount of organic matter or soil type.…”