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

Abstract

A portion of nitrate (NO 3 − ), a final breakdown product of nitrogen (N) fertilizers, applied to soils and/or that produced upon decomposition of organic residues in soils may leach into groundwater. Nitrate levels in water excess of 10 mg L−1 (NO3–N) are undesirable as per drinking water quality standards. Nitrate concentrations in surficial groundwater can vary substantially within an area of citrus grove which receives uniform N rate and irrigation management practice. Therefore, differences in localized conditions which can contribute to variations in gaseous loss of NO 3 − 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 capacity and potential in a shallow vadose zone soil and in surficial groundwater were studied in two large blocks of a citrus grove of ‘Valencia’ orange trees (Citrus sinensis (L.) Obs.) on Rough lemon rootstock ( Citrus jambhiri (L.)) under a uniform N rate and irrigation program. The NO3–N concentration in the surficial groundwater sampled from four monitoring wells (MW) within each block varied from 5.5- to 6.6-fold. Soil samples were collected from 0 to 30, 30 to 90, or 90 to 150 cm depths, and from the soil/groundwater interface (SGWI). Groundwater samples from the monitoring wells (MW) were collected prior to purging (stagnant water) and after purging five well volumes. Without the addition of either C or N, the denitrification capacity ranged from 0.5 to 1.53, and from 0.0 to 2.25 mg N2O–N kg−1 soil at the surface soil and at the soil/groundwater interface, respectively. The denitrification 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 potential in the soil or in the groundwater was greatest near the monitor well with shallow depth of vadose zone (MW3). Cumulative N2O–N emission (denitrification capacity) from the SGWI soil samples and from stagnant water samples strongly correlated to microbial most probable number (MPN) counts (r2 = 0.84 – 0.89), and dissolved organic C (DOC) (r2 = 0.96 – 0.97). Denitrification capacity of the SGWI samples moderately correlated to water-filled pore space (WFPS) (r2 = 0.52). However, extractable NO3-N content of the SGWI soil samples poorly (negative) correlated to denitrification capacity (r2 = 0.35). However, addition C, N or both to the soil or water samples resulted in significant increase in cumulative N2O emission. This study demonstrated that variation in denitrification capacity, as a result of differences in denitrifier population, and the amount of readily available carbon source significantly (at 95% probability level) influenced the variation in NO3–N concentrations in the surficial groundwater samples collected from different monitoring wells within an area with uniform N management.