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Denitrification below the Crop Rooting Zone as Influenced by Surface Tillage

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^* Corresponding author.

ABSTRACT

In recent years, there has been increased concern over pollution of groundwater by NO₃^–, yet little is known about microbial N transformations below the crop rooting zone. This study investigated the importance of microbial denitrification below the crop rooting zone of conventional and no-till corn plots. The soil was a well-drained Matapeake silt loam located in the Atlantic Coastal Plain. Plots were sampled in 30- and 60-cm depth increments down to the water table, which occurred at ca. 420 cm. Total viable bacteria and numbers of denitrifying bacteria decreased exponentially with increasing soil depth down to 150 cm. From 180 cm to the top of the water table total bacterial numbers were very low (< 100 organisms/gram). No CO₂ production activity was observed below the 180-cm depth in unamended soil incubated aerobically at 25°C for 24 h. Anaerobic incubations with added NO₃^– and glucose showed no denitrifying activity below the 180-cm depth. These results indicate that C levels below the rooting zone in this well-drained, low organic matter soil are too low to support anaerobic conditions necessary for denitrification or to sustain a microbial population. Therefore, denitrification is not a mechanism of significant NO₃^– loss below the crop rooting zone at this location. Surface tillage practices had little influence on microbial activity below the root-zone.

Contribution from the USDA-Agricultural Research Service.

Received for publication January 13, 1988.

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