Effect of Different Agricultural Practices on the N Cycle in a Clay Soil Degraded by Prolonged Irrigation with Treated Wastewater

Long-term irrigation of clay soils with treated wastewater (TWW) has led to structural damage, periodic lack of oxygen in the root zone and decreased yields. A field study was conducted in a drip-irrigated Has Avocado orchard planted on a wastewater-degraded montmorillonite clay soil in northern Israel. The impact of irrigation frequency (very 2 or 3 days), water quality (TWW, fresh, 50%TWW+50% fresh) and Scoria filled aeration trenches (30 x 30 cm) on the N cycle in the root zone (35 cm) was studied. All treatments received similar weekly N and water loads. Porewater and N₂O emission fluxes were sampled every 2-4 weeks using porous cups and static gas chambers. Results showed nitrate to be dominant N form in the root zone. Porewater nitrate followed a seasonal pattern of increasing concentrations at the start of the fertigation season (April), which stabilized at ~50 mg-N L^-1, reflecting the fertilizer concentrations. At the end of the fertigation season (November), the concentrations decreased to below 10 mg-N L^-1 and remained that way throughout the winter. A similar pattern was observed in the N₂O emission fluxes, with high fluxes during the fertigation season (April-November) and low fluxes during the winter (December-March). Irrigation every other day with TWW or with TWW+fresh lead to higher N₂O emissions compared to irrigation every 3 days with TWW (4.12 & 3.87 vs. 2.51 g m^-2 y^-1, respectively). On the other hand, irrigation every other day with fresh water resulted in the lowest daily and cumulative N₂O flux (0.31 g m^-2 y^-1) of al treatments. Highest daily N₂O emissions were measured from the scoria filled aeration trenches both through the fertigation season when it was irrigated daily with TWW (~0.03 μg cm^-2 min^-1) and in the winter following intensive rain events when water accumulates at the scoria-clay interface. Throughout the fertigation season nitrite concentrations were mainly low (< 1 mg L^-1), with occasional high concentration (>15 mg L^-1) immediately following fertigation events. No correlations were found between the N₂O flux and the nitrite concentrations. Our findings show that the irrigation water quality and the irrigation frequency effect the soil N cycle and N₂O emission, and that future attempts to mitigate N₂O emissions must take it into account. Furthermore, we show that scoria filled aeration trenches may serve as hotspots for N₂O emission, especially following intensive rain events. Further research is needed in order to evaluate the overall costs vs. the benefits of such agricultural practices aimed to improve soil aeration.