Denitrification in the vadose zone and in surficial groundwater of a sandyentisol with citrus production

A portion of nitrate (NO3-), a final breakdown product of nitrogen (N) fert ilizers, applied to soils and/or that produced upon decomposition of organi c residues in soils may leach into groundwater. Nitrate levels in water exc ess of 10 mg L-1 (NO3-N) are undesirable as per drinking water quality stan dards. Nitrate concentrations in surficial groundwater can vary substantial ly within an area of citrus grove which receives uniform N rate and irrigat ion management practice. Therefore, differences in localized conditions whi ch can contribute to variations in gaseous loss of NO3- in the vadose zone and in the surficial aquifer can affect differential concentrations of NO3- N in the groundwater at different points of sampling. The denitrification c apacity and potential in a shallow vadose zone soil and in surficial ground water were studied in two large blocks of a citrus grove of `Valencia' oran ge trees (Citrus sinensis (L.) Obs.) on Rough lemon rootstock ( Citrus jamb hiri (L.)) under a uniform N rate and irrigation program. The NO3-N concent ration in the surficial groundwater sampled from four monitoring wells (MW) within each block varied from 5.5- to 6.6-fold. Soil samples were collecte d from 0 to 30, 30 to 90, or 90 to 150 cm depths, and from the soil/groundw ater interface (SGWI). Groundwater samples from the monitoring wells (MW) w ere collected prior to purging (stagnant water) and after purging five well volumes. Without the addition of either C or N, the denitrification capaci ty ranged from 0.5 to 1.53, and from 0.0 to 2.25 mg N2O-N kg(-1) soil at th e surface soil and at the soil/groundwater interface, respectively. The den itrification potential increased by 100-fold with the addition of 200 mg kg (-1) each of N and C. The denitrification potential in the groundwater also followed a pattern similar to that for the soil samples. Denitrification p otential in the soil or in the groundwater was greatest near the monitor we ll with shallow depth of vadose zone (MW3). Cumulative N2O-N emission (deni trification capacity) from the SGWI soil samples and from stagnant water sa mples strongly correlated to microbial most probable number (MPN) counts (r (2) = 0.84 - 0.89), and dissolved organic C (DOC) (r(2) = 0.96 - 0.97). Den itrification capacity of the SGWI samples moderately correlated to water-fi lled pore space (WFPS) (r(2) = 0.52). However, extractable NO3-N content of the SGWI soil samples poorly (negative) correlated to denitrification capa city (r(2) = 0.35). However, addition C, N or both to the soil or water sam ples resulted in significant increase in cumulative N2O emission. This stud y demonstrated that variation in denitrification capacity, as a result of d ifferences in denitrifier population, and the amount of readily available c arbon source significantly (at 95% probability level) influenced the variat ion in NO3-N concentrations in the surficial groundwater samples collected from different monitoring wells within an area with uniform N management.