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

Understanding subsurface denitrification potential will give greater insights into landscape nitrate (NO ₃ ^-) delivery to groundwater and indirect nitrous oxide (N ₂O) emissions to the atmosphere. Potential denitrification rates and ratios of N ₂O/(N ₂O+N ₂) were investigated in intact soil cores collected from 0-0.10, 0.45-0.55 and 1.20-1.30m 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) 90mg NO ₃ ^--N as KNO ₃, (ii) 90mg NO ₃ ^--N+150mg glucose-C, (iii) 90mg NO ₃ ^--N+150mg 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 N ₂O and N ₂, at 15°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 N ₂O emissions. N ₂O 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 NO ₃ ^-+either of the C sources. The two C sources gave similar N ₂O 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 N ₂O to N ₂O+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 N ₂O to N ₂. Stepwise multiple regression analysis revealed that N ₂O flux increased with total organic C and total N but decreased with NO ₃ ^--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 NO ₃ ^--N (negative). Stepwise multiple regression revealed a best fit for total denitrification rates which were positive for total C and negative for NO ₃ ^--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 N ₂O 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 accelerated either through introducing C directly into permeable reactive barriers and/or indirectly, by irrigating dirty water and manipulating agricultural plant composition and diversity.