Effect of glyphosate application on soil quality and health under natural and zero tillage field conditions

Abstract

Agriculture is a primary source of income in several countries, including Argentina. Among the many agrochemicals used, glyphosate-based herbicides raised controversy, encouraging research to clarify if the benefits of their use outweigh their alleged harmfulness. In this spirit, this study assessed soil quality indicators on glyphosate-sprayed fields under natural (NC) and zero tillage conditions (ZT) in Northwest Argentina, to analyze the effect of the herbicide application on soil degradation. The ZT soils underwent five years of continuous spraying (2-4 times a year) after land clearing, while the NC soil, without any laboring practices, was subjected to two consecutive applications. Among the measured indicators (physical, chemical, and biological), water-stable aggregates (WSA), particulate organic matter (POM) and dehydrogenase activity (DHA) showed quality differences between ZT and NC samples. The highest values were found in NC (WSA 72%; POM 4.9%; DHA 1460 mg TPF/gsoil/d) and the lowest in ZT (WSA 13%; POM 1.69%; DHA 180 mg TPF/gsoil/d); showing a lower quality in ZT regarding structure stability, nutrient availability and microbial activity. A Discriminant Analysis revealed that as glyphosate application increased, the overall soil quality decreased within the NC samples, resembling that of ZT. Thus, soil health deterioration was attributed solely to glyphosate spraying in NC. Furthermore, multivariate analysis allowed identification of chemical indicators as of higher sensitivity to the short-term response after application, and biological indicators as more sensitive to long-term changes. The quality decline in time in the NC soil, caused by the use of glyphosate-based herbicides, could endanger the soils sustainability.