Long-term glyphosate application and its effects on soil total nitrogen and microbial composition two years after application stopped in biochar-amended soil

Glyphosate is widely used in horticultural land management practices, but its environmental risks, especially in biochar-amended soil, remain poorly understood. This study aimed to evaluate the effects of repeated glyphosate applications on biochar-amended- soils, focusing on changes in soil nitrogen (N) cycling, soil microbial diversity, and community structure. The experiment was conducted in a macadamia orchard where wood-based biochar had been applied for five years prior to our study. Simultaneously, glyphosate (Roundup®) had been applied at the recommended label rate (4 L ha−1) with active ingredient of glyphosate of 360 g L−1, in a strip under tree canopy for 12 years, until its use was stopped two years prior to our sample collection. Thus, biochar and glyphosate were applied concurrently for three years before glyphosate use was stopped. Soil samples were collected from three areas, including: under the tree canopy with and without a history of glyphosate application; and areas outside the tree canopy with no history of glyphosate application. Changes in foliar and soil total carbon (TC), total nitrogen (TN) and soil N isotope composition (δ15N), and microbial community composition, were subsequently assessed. Our findings revealed that soil TN and δ15N were significantly higher under tree canopy with a history of glyphosate application compared with areas without glyphosate application history likely due to the die back of weed because the other sections were mechanically managed. Glyphosate residues were also found under and outside tree canopy where no glyphosate was applied. Therefore, higher TN and δ15N under tree canopy could not directly be attributed to glyphosate application. Overall, neither glyphosate nor biochar influenced the soil microbial diversity and community structure. This study suggested that glyphosate application and farm management practices may have long-term implications for soil N cycling, even after application has stopped.