Spatial variation in 2-methyl-4-chlorophenoxyacetic acid mineralization and sorption in a sandy soil at field level

The phenoxyacetic acid herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) is frequently detected in groundwater beneath Danish agricultural fields. We investigated spatial variation in microbial MCPA mineralization potential in a flat agricultural field of fine sandy soil (USDA classification: Humic Dystrudept) located on the Yoldia plains of Northern Jutland, Denmark. Samples for determination of MCPA mineralization and sorption were collected from the Ap and Bs horizons at 51 sampling sites located in a 200 x 220 m grid. Spatial variation in sorption was low in both horizons (distribution coefficient, 0.36-4.16 L kg(-1)). Sorption correlated strongly with soil organic carbon content in both horizons (CV, 93 and 83%, respectively) and negatively with soil pH. [Ring-(14)C]-MCPA mineralized readily in the Ap horizon, with 49 to 62% of the (14)C-MCPA being converted to (14)CO(2) during the 67-d incubation period. With the subsoil, mineralization of (14)C-MCPA varied considerably between samples (0.5-72.8%). At neither depth was there correlation between (14)C-MCPA mineralization and sorption, soil pH, organic carbon content, clay content, number of colony-forming units (CFU), pseudomonad CFU, or any of the four microbial activity parameters measured. The presence of microbial genes encoding for the TfdA enzyme was quantified using real-time polymerase chain reaction. No correlation was found between MCPA mineralization potential and the natural background number of tfdA genes present in the soil samples. The degradation kinetics suggests that the high (14)C-MCPA mineralization rate detected in soil samples was linked to growth of the MCPA-degrading soil microbial community.     

Field-scale variation in microbial activity and soil properties in relation to mineralization and sorption of pesticides in a sandy soil

Pesticides applied to agricultural soils are subject to environmental concerns because leaching to groundwater reservoirs and aquatic habitats may occur. Knowledge of field variation of pesticide-related parameters is required to evaluate the vulnerability of pesticide leaching. The mineralization and sorption of the pesticides glyphosate and metribuzin and the pesticide degradation product triazinamin in a field were measured and compared with the field-scale variation of geochemical and microbiological parameters. We focused on the soil parameters clay and organic carbon (C) content and on soil respiratory and enzymatic processes and microbial biomass. These parameters were measured in soil samples taken at two depths (Ap and Bs horizon) in 51 sampling points from a 4-ha agricultural fine sandy soil field. The results indicated that the spatial variation of the soil parameters, and in particular the content of organic C, had a major influence on the variability of the microbial parameters and on sorption and pesticide mineralization in the soil. For glyphosate, with a co-metabolic pathway for degradation, the mineralization was increased in soils with high microbial activity. The spatial variability, expressed as the CV, was about five times higher in the Bs horizon than in the Ap horizon, and the local-scale variation within 100 m(2) areas were two to three times lower than the field-scale variation within the entire field of about 4 ha.     

Transport and reduction of nitrate in clayey till underneath forest and arable land

Transport and reduction of nitrate in a typically macroporous clayey till were examined at variable flow rate and nitrate flux. The experiments were carried out using saturated, large diameter (0.5 m), undisturbed soil columns (LUC), from a forest and nearby agricultural sites. Transport of nitrate was controlled by flow along the macropores (fractures and biopores) in the columns. Nitrate reduction (denitrification) determined under active flow mainly followed first order reactions with half-lives (t(1/2)) increasing with depth (1.5-3.5 m) from 7 to 35 days at the forest site and 1-7 h at the agricultural site. Nitrate reduction was likely due to microbial degradation of accumulated organic matter coupled with successive consumption of O2 and NO3- in the macropore water followed by reductive dissolution of Fe and Mn from minerals along the macropores. Concentrations of total organic carbon measured in soil samples were near identical at the two study sites and consequently not useful as indicator for the observed differences in nitrate reduction. Instead the high reduction rates at the agricultural site were positively correlated with elevated concentration of water-soluble organic carbon and nitrate-removing bacteria relative to the forest site. After high concentrations of water-soluble organic carbon in the columns from the agricultural site were leached they lost their elevated reduction rates, which, however, was successfully re-established by infiltration of new reactive organics represented by pesticides. Simulations using a calibrated discrete fracture matrix diffusion (DFMD) model could reasonably reproduce the denitrification and resulting flux of nitrate observed during variable flow rate from the columns.