Organic amendments modify action of pesticides

José Pichel Andrés/DICYT The low content of organic matter of some agricultural soils makes the farmers use organic residues to increase their fertility and obtain better harvests. They need to make use of pesticides to fight insects, fungi or weed simultaneously. The Institute of Natural Resources and Agrobiology of Salamanca (IRNASA, center of CSIC) has studied the effect of combining different organic amendments with many pesticides which may cause contamination problems if not applied correctly.

The project has included the study of three types of organic amendments: mushroom after-farming substrate, sewage sludge and grape marcs, composed by bunch remains, skin and seeds. The scientists have analyzed the behavior of an herbicide (linuron), an insecticide (diazinon) and a fungicide (miclobutanil) in each agricultural soil amended with one of these three organic residues. “We have studied the three most important processes which pesticides go through when they are applied, soil adsorption, mobility and degradation, Sonia Rodríguez explains to DiCYT. She is a researcher of IRNASA who has published this work’s last results in the magazine Science of the Total Environment.

The three pesticides have different physical-chemical characteristics and it is reflected in the results. For example, the researchers have paid attention to these compounds’ average life expectancy, that is, the time 50% of the applied compound takes to degrade. The herbicide, for example, has decreased when the amendments were applied, while the effect on the insecticide and the fungicide was the opposite, it has increased their average life expectance. In this way, they have demonstrated that pesticides “show different degradation mechanisms”, says Jesús Marín Benito, IRNASA scientist who has participated in the research.

Nevertheless, the parameters which may be measured are numerous and this type of organic amendment used has also influence. “If this pesticide has a tendency to be adsorbed by soluble organic matter, it may be more bioavailable to be attacked by microorganisms and it is degraded before, while if it is adsorbed by the soil, the average life expectancy is increased”, the experts emphasize.

A great deal of information in this project has been obtained due to the technique of marking the compounds with radioactive isotope carbon 14, since “we are able to know which processes the pesticide has been through before it was degraded”, if it changes to CO2 (mineralization), if it is available (removable fraction) or if it is held in the soil (bound fraction). Specifically, it has been applied to know the diazinon and the linuron dissipation mechanisms and analyze their mobility. “Without carbon 14, you may think that the pesticide has disappeared, but it is really adsorbed by the soil”, they point out.

A lot of variables at stake

In conclusion, the organic matter characteristics as well as the pesticide are essential; however, they are not the only variables one must take into account in this complex process. IRNASA also studies the microorganisms, especially to check how they are affected when the organic matters and the pesticides are applied. “A fungicide is designed to attack a fungus which is harmful for the farming, but it may also affect other life forms”, and so “studying the microorganisms is a way of calculating what happens in the agricultural soil”. Curiously, the presence of amendments as well as of pesticides increases the activity of these microscopic life forms, thus “they are not toxic compounds in the doses we have applied”.

This research results are comparable to another type of amendments and pesticides with similar characteristics, since the information offered by these researches allows to be used in mathematical models. If one takes into consideration the data about pesticides’ adsorption, mobility and degradation, the model allows predictions about their behavior under specific climate conditions, for example.

Practical conclusions for farmers

The most important thing is that this new knowledge may be the basis to obtain practical conclusions for farmers about the use of different compounds. An herbicide or a fungicide must stay for a certain time to attack the weeds or the fungi which damage the plants; however, if they are persistent and spread a lot, they may cause problems, so finding the ideal point is not easy. “It is important to know if using an amendment has any effect on pesticides’ behavior, particularly to prevent the contamination of underground water”, Sonia Rodríguez points out.

In this way, the study also contributes to value residues which come from grapes and mushroom farming, the last one especially in La Rioja, where it is the second most important farming after the vineyard. That is why the University of La Rioja has also participated in the research. The possibility of taking advantage of these organic residues not only solves a problem of residue accumulation, but also contributes to increasing the fertility of some agricultural soils with little organic matter.

	Bibliography
	Jesús M. Marín-Benito, Eliseo Herrero-Hernández, M. Soledad Andrades, María J. Sánchez-Martín, M. Sonia Rodríguez-Cruz. Effect of different organic amendments on the dissipation of linuron, diazinon and myclobutanil in an agricultural soil incubated for different time periods. Science of the Total Environment 476-477:611-21. doi: 10.1016/j.scitotenv.2014.01.052