Soil Microbial Diversity and N Cycling Processes in Response to Indaziflam Application.

Indaziflam is a recently-introduced herbicide for the control of weeds in pecan (Carya illinoinensis) orchards, an important crop in arid southern New Mexico, USA. Data are lacking however with regard to its impact on the biology and sustainability of soils supporting pecan production. A greenhouse experiment, using pecan saplings grown in soil-filled pots, was conducted to evaluate the effect of indaziflam application rates and application methods on soil microbial diversity and activity linked to N cycling processes. Experimental factors include: 2 indaziflam application rates (25 and 50 g ai ha^-1) and 2 application methods (homogenous and manual tillage to 5 cm depth). Also included were untreated pots serving as controls, and pots treated with pindarGT (another herbicide) for comparison purposes. Each treatment (6) was applied in triplicate and distributed in a randomized design. Pots were treated with indaziflam in August 2014, July 2015 and August 2015. Soil samples were collected 7 days after herbicide application. Results showed only a slight decrease in microbial diversity between control (H index: 4±0.1) compared to the treatments (H = 3.5±0.3, averaged across treatments). No significant effect of herbicide treatment on soil respiration, microbial biomass C and N mineralization was detected. After the July application of indaziflam, soil mineral N was dominated by NH₄⁺, as illustrated by NH₄/NO₃ ratios several-fold higher in the treatments (13) compared to the control (3). A marked decrease (3.5-fold lower) in glucosaminidase activity with herbicide treatment was also observed. These results suggest that indaziflam application had limited effect on total soil microbial activity, but may have repressed nitrification activity, a process carried out by a narrow subset of the soil microbial community and known to be sensitive to various environmental toxicants.