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Effects of Long-Term 2,4-D Field Applications on Soil Biochemical Processes¹

ABSTRACT

The effects of annual applications of 2,4-D (2,4-dichlorophenoxyacetic acid) on the soil biosphere were studied. Plots established on a Udic Boroll at Indian Head, Saskatchewan, that have received 2,4-D amine or ester, at rates of 1.12 kg ha^–1 for 35 consecutive years were sampled over a 9-month period to determine the effect of the herbicide on some biochemical activities related to soil fertility. Treated and control plots of stubble-seeded wheat (Triticum aestivum L.) were sampled just before and at 2 and 16 weeks after the annual 2,4-D application to determine temporary and long-term herbicide effects. In the top 2.5 cm of soil (the prime site of 2,4-D deposition), biomass, N mineralization rate, and potentially mineralizable N (N_o) were not affected by the amine or ester, but urease was temporarily depressed by both formulations. Nitrification and activity of phosphatases were reduced temporarily by the ester formulation. Dehydrogenase activity was depressed weakly but persistently by the ester formulation. Respiration was enhanced temporarily after amine application. There was a distinct tendency for the ester to exert a more depressive effect than the amine formulation on the measured variables. Although some herbicide-induced changes were significant (P <0.05), all depressive and stimulatory effects detected were relatively small (<35% change from control) and most were short-lived. The results were interpreted by comparing the duration and magnitude of the biochemical stress caused by the herbicide with stresses induced by naturally occurring adversities. From these comparisons we concluded that the effects of long-term 2,4-D applications were neither ecologically significant nor did they interfere with nutrient cycling to adversely affect soil fertility.

Key Words: herbicide • dimethylamine salt • isooctyl ester • microbial biomass • N transformations • soil enzyme activities

¹ Joint contribution from the Agric. Canada Research Stations at Swift Current, Sask. and at Regina, Sask.

² Soil scientists, Research Station, Research Branch, Agriculture Canada, Swift Current, Saskatchewan, S9H 3X2 and research chemist, Research Station, Research Branch, Agriculture Canada, Regina, Sask., S4P 3A2, respectively.

Received for publication October 22, 1986.