Low‐dose Herbicide Effects on Tree Establishment and Soil Nitrogen Biogeochemistry within Pine Savannas

Abstract

Core Ideas Imazapyr‐treated pines were larger than pines applied with other herbicides. Imazapyr‐treated pines had better survival when the treatment is applied during the second season. Imazapyr soils produced higher ammonification rates compared to the control. Sulfometuron methyl‐treated pines were smaller than control plot trees. Sulfometuron methyl‐treated soils had lower ammonification rates and microbial biomass. Longleaf pine (Pinus palustris P. Mill) survival and growth, net nitrogen mineralization, and soil microbial biomass, were evaluated after four growing seasons in a Florida wet flatwoods site following chemical vegetation control during the first year or second year after planting, or during both years. The four herbicide treatments included sulfometuron methyl at 0.26 ai kg ha−1, hexazinone (0.56 ai kg ha−1), sulfometuron (0.26 ai kg ha−1) plus hexazinone (0.56 ai kg ha−1) mix, and imazapyr at 0.21 ai kg ha−1. Imazapyr was the only treatment to significantly improve growth over the control in a single application. Consecutive annual applications of imazapyr and hexazinone on seedlings also improved growth rates compared to the control. Sulfometuron methyl‐treated pine trees had lower survival rates and were smaller than pines growing in the control plots after a single application. The survival and growth rates of imazapyr‐treated seedlings were improved when the chemical was applied during the second growing season after planting, instead of the first year. Imazapyr and hexazinone applications increased net nitrogen mineralization rates, but imazapyr was the only treatment to increase ammonification; compared to the control. Microbial and fungal biomass carbon showed no differences between treatments. The results did show that microbial biomass significantly increased with two consecutive years of herbicide applications over a single application. Imazapyr applied during the second growing season proved to be the best treatment for improving pine growth, controlling competitive vegetation, minimizing pine mortality, and to remain effective when soils are saturated.