TECHNICAL REPORTS Denitrification below the Crop Rooting Zone as Influenced by Surface Tillage

in recent years, there has been increased concern over pollution of groundwater by NOj-, 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 30and 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 era. From 180 cm to the top of the water table total bacterial numbers were very low (< 100 organisms/gram). No 2 production activity was observed below the 180-cm depth in unamended soil incubated aerobically at 25°C for 24 h. Anaerobic ncubations with added NO~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 NO~loss below the crop rooting zone at this location. Surface tillage practices had little influence on microbial activity below the root-zone. N ENRICHMENT of groundwater resulting from agricultural practices is a topic of increasing concern, yet microbial N transformations below the crop rooting zone are poorly understood. Biological denitrification of NO3leached below the rooting zone may be a process that ameliorates groundwater contamination. Indirect evidence that subsoil denitrification may be an important process in the lower coastal plain soils of North Carolina has been provided by Gilliam et al. (1974). This early evidence was corroborated by later work in a poorly drained soil in the lower coastal plain, which documented decreased NO3-/C1ratios with increasing soil depth; indicating that denitrification was an important process in reducing NO~loading to the groundwater (Gambrell et al., 1975). Similar declines NO3-/CIratios with depth were observed by Gast et al. (1974) for a Webster soil (fine, loamy, mixed, mesic typic Haplaquolls) in Minnesota. However, subsequent N balance work suggested that denitrification was of minor importance in removing fertilizer N (Gast et al., 1978). Although indirect evidence suggests that denitrification below the rooting zone may be an important process in some systems, there have been few direct measurements of denitrifier numbers or denitrification activity below Both authors, Building 007, Room 229, BARC-WEST, Beltsville, MD 20705. Contribution from the USDA-Agricultural Research Service. Received 13 Jan. 1988. *Corresponding author. Published in J. Environ. Qual. 18:12-16 (1989). the crop rooting zone. In a study of the vadose zone above the Chalk aquifer in England, it was found that NO~-reducing bacteria were present at high levels down to 10 m at an unfertilized grassland site (Whitelaw and Rees, 1980). Balance sheet calculations of N and S in a European aquifer have indicated that the aquifer is a NO~sink and that Thiobacillus denitrificans used reduced S compounds in the denitrification of NO~(Kolle et al., 1986). Denitrifying bacteria have also been isolated from limestone material obtained from two drilling sites in England and from this information, along with data on the relative concentrations of NO~and thermonuclear tritium, it was concluded that denitrification in the groundwater was a likely possibility (Foster et al., 1985). Due to the diversity of subsoil environments and the paucity of data, it is difficult to generalize or predict the importance of subsurface denitrification at a given site. More studies are needed to provide a better data base to increase our understanding of microbial processes in the vadose zone. This study was initiated to (i) obtain information on the physico-chemical characteristics and microbial activities of the subsoil environment, (ii) determine the importance of denitrification in the vadose zone and the upper region of the water table in reducing NOjloading to groundwater, and (iii) determine if surface tillage practices influence denitrification and microbial activity below the root zone. MATERIALS AND METHODS Study Site and Sample Collection The study site was located in the Eastern Shore region of Maryland at the Wye Research and Education center near Queenstown, MD. The soil was a well-drained Matapeake silt loam (fine-silty, mixed, mesic Typic Hapludult) underlaid sand. Field plots (4.6 m by 12.2 m) had been cropped to continuous corn for 4 yr using either conventional moldboard plow tillage or no-tillage practices. Four replicate plots were established in each tillage treatment. Fertilizer N had been applied to all plots in 1980 through 1984 at a rate of 270 kg N/ha. In 1985 plots were fertilized at a rate of 180 kg N/ha. Soil samples down to the water table (ca. 420 cm) were collected in April 1986 at the end of the groundwater recharge period, and before the soil had been tilled, fertilized, or planted. A crucial aspect of this study was to obtain subsoil samples uncontaminated by surface soil material. The 0to 30-cm sample was taken with a 2.4-cm diam. hand probe, then a larger 7.6-cm diam. core was removed and a plastic tube (35 cm long by 7.6 cm diam.) was inserted in the resulting hole. This plastic liner protruded ca. 5 cm above the soil surface and minimized contamination due to surface soil material falling into the open hole when deeper samples were collected. Samples from the 30to 180-cm depths were collected using a 5-cm diam., 180 cm long coring tube. Stacked inside this tube were six 30-cm lengths of polycarbonate tubing, which served