Hydraulically active biopores stimulate pesticide mineralization in agricultural subsoil

Soil biopores can serve as preferential flow paths for downward transport of inorganic nutrients and organic compounds. Pesticides may also be transported down through the subsoil in biopores, thereby posing a threat to the groundwater resource. However, biopores may also constitute hot spots for microbially-mediated pesticide mineralization, thereby reducing the risk of pesticide leaching. To investigate this we identified hydraulically active biopores in a test plot of an agricultural field by percolating brilliant blue through the soil. Small portions of soil (500 mg) were sampled at approx. 1-cm distances along a transect covering 10 biopores and adjacent matrix soil at two depths: 30 cm below ground surface (b.g.s.; transition zone below the plough layer) and 55 cm b.g.s. (subsoil). The general microbial community was characterized by culturable heterotrophic bacteria (CFU), respiratory activity (CO2 production rate), and growth activity ([3H]leucine incorporation). Specific pesticide degrader populations (bromoxynil and 4-chloro-2-methyl-phenoxyacetic acid (MCPA)) were enumerated by the most probable number (MPN) method, and pesticide mineralization was quantified by 14C-mineralization assays. Compared to the matrix soil, increased density of heterotrophic bacteria, respiratory activity, growth activity, and bromoxynil mineralization was observed in the biopores in the subsoil layer, but not in the transition zone. By contrast, the density of MCPA degraders and MCPA mineralization activity were highly stimulated in the transition zone biopores, whereas the density of MCPA degraders was significantly lower in the subsoil, where no MCPA mineralization occurred. We conclude that hydraulically active biopores may constitute hot spots for pesticide mineralization, but that this biopore effect is compound-specific and likely inadequate to prevent pesticide leaching, especially, in subsoil with low densities of degrader populations.