Denitrification potential in subsoils: A mechanism to reduce nitrate leaching to groundwater

Understanding subsurface denitrification potential will give greater insights into landscape nitrate (NO3-) delivery to groundwater and indirect nitrous oxide (N2O) emissions to the atmosphere. Potential denitrification rates and ratios of N2O/(N2O + N-2) were investigated in intact soil cores collected from 0-0.10, 0.45-0.55 and 1.20-1.30 m depths representing A, B and C soil horizons, respectively from three randomly selected locations within a single intensively managed grazed grassland plot in south eastern Ireland. The soil was moderately well drained with textures ranging from loam to clay loam (gleysol) in the A to C horizon. An experiment was carried out by amending soils from each horizon with (i) 90 mg NO3--N as KNO3, (ii) 90 mg NO3--N + 150 mg glucose-C, (iii) 90 mg NO3--N + 150 mg DOC (dissolved organic carbon, prepared using top soil of intensively managed grassland) kg(-1) dry soil. An automated laboratory incubation system was used to measure simultaneously N2O and N-2, at 15 degrees C, with the moisture content raised by 3% (by weight) above the moisture content at field capacity (FC), giving a water-filled pore space (WFPS) of 80, 85 and 88% in the A, B and C horizons, respectively. There was a significant effect (p < 0.01) of soil horizon and added carbon on cumulative N2O emissions. N2O emissions were higher from the A than the B and C horizons and were significantly lower from soils that received only nitrate than soils that received NO3- (+) either of the C sources. The two C sources gave similar N2O emissions. The N-2 fluxes differed significantly (p < 0.05) only between the A and C horizons. During a 17-day incubation, total denitrification losses of the added N decreased significantly (p < 0.01) with soil depth and were increased by the addition of either C source. The fraction of the added N lost from each horizon were A: 25, 61, 45%; B: 12, 29, 28.5% and C: 4, 20, 18% for nitrate, nitrate + glucose-C and nitrate + DOC, respectively. The ratios of N2O to N2O + N-2 differed significantly (p < 0.05) only between soil horizons, being higher in the A (0.58-0.75) than in the deeper horizons (0.10-0.36 in B and 0.06-0.24 in C), clearly indicating the potential of subsoils for a more complete reduction of N2O to N-2. Stepwise multiple regression analysis revealed that N2O flux increased with total organic C and total N but decreased with NO3--N which together explained 88% of the variance (p < 0.001). The N-2 flux was best explained (R-2 = 0.45, p < 0.01) by soluble organic nitrogen (SON) (positive) and with NO3--N (negative). Stepwise multiple regression revealed a best fit for total denitrification rates which were positive for total C and negative for NO3--N with the determination coefficient of 0.76 (p < 0.001). The results suggest that without C addition, potential denitrification rate below the root zone was low. Therefore, the added C sources in subsoils can satisfactorily increase nitrate depletion via denitrification where the mole fraction of N2O would be further reduced to N-2 during diffusional transport through the soil profile to the atmosphere and/or to groundwater. Subsoil denitrification can be ccelerated either through introducing C directly into permeable reactive barriers and/or indirectly, by irrigating dirty water and manipulating agricultural plant composition and diversity. (C) 2011 Elsevier B.V. All rights reserved.