The effect of diverse forage species and irrigation management on plant nitrogen uptake and nitrate leaching losses

Abstract

Reducing the loss of nitrogen (N) through nitrate (NO₃⁻) leaching from cow deposited urine patches is one of the greatest challenges facing the New Zealand dairy industry. The objective of this PhD research programme was therefore to determine the effect of diverse forage species and irrigation management on plant N uptake and NO₃⁻ leaching from the urine patch.

The first experiment was conducted using lysimeters collected from a free-draining Paparua fine sandy loam soil located on the Lincoln University Research Dairy Farm (LURDF), Canterbury, New Zealand. The lysimeters were collected from two pre-existing grazed forages: (i) a ‘standard’ forage containing perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.); and (ii) a ‘diverse forage’ containing perennial ryegrass, white clover, red clover (Trifolium pratense L.), plantain (Plantago lanceolata L.), chicory (Cichorium intybus L.) and prairie grass (Bromus willdenowii L.). Dairy cow urine was applied to both sets of lysimeters in late spring at a loading rate of either 500 or 700 kg N ha⁻¹. Following the urine application, irrigation was applied at a rate of 18 mm every three days (‘optimum’) or 9 mm every three days (‘deficit’) from November to April. The herbage N uptake and N leaching in drainage were measured thereafter using standard methods. Compared with deficit irrigation, NO₃⁻ leaching losses were 88–97% lower under optimum irrigation at a urine application rate of 700 kg N ha⁻¹. Leaching losses from the 500 kg N ha⁻¹ urine treatment were below 4 kg N ha⁻¹ for both forage types and there was no significant difference between the irrigation treatments. The differences in NO₃⁻ leaching losses were attributed to greater herbage growth and N uptake during the summer period by forages that were supplied with sufficient water under optimum irrigation. Forage type had no effect on herbage N uptake or NO₃⁻ leaching losses when applied at 700 kg N ha⁻¹.

The second and third experiments were conducted using lysimeters and companion soil blocks which were also collected from a free-draining Paparua fine sandy loam soil located on the LURDF. The lysimeters and soil blocks were collected from two pre-existing irrigated plots: (i) a standard forage containing perennial ryegrass and white clover; and (ii) a diverse forage containing perennial ryegrass, white clover and plantain. Dairy cow urine, at a N loading rate of 700 kg N ha⁻¹, was applied to one set of 30 lysimeters in December (early summer). A second set of 30 lysimeters and 24 soil blocks received a dairy cow urine application in February (late summer). Following the urine application, irrigation was applied to all lysimeters from December to April using one of the following systems: (i) pivot irrigation at a rate of 15 mm every three days; (ii) rotorainer irrigation at a rate of 45 mm every nine days; and (iii) flood irrigation at a rate of 90 mm every 18 days. The herbage N uptake and N leaching in drainage from the lysimeters were measured thereafter using standard methods. Soil cores were taken from the soil blocks and were used to measure ammonia oxidizing bacteria (AOB) and archaea (AOA) abundance and nitrification rates under these cow urine patches. Compared with the standard forage, NO₃⁻ leaching losses were 82% lower under the diverse forage containing plantain when urine was applied in December and 74% lower when urine was applied in February. The companion soil blocks showed that compared with the soil under the standard forage, AOB abundance was lower under the diverse forage containing plantain. Consequently, soil NH₄⁺–N concentrations remained higher under the diverse forage while the NO₃⁻–N concentrations were lower. The differences in NO₃⁻ leaching losses were attributed to a combination of nitrification inhibition (likely due to a biological nitrification inhibitor released from the plantain) and reduced drainage losses under the diverse forage. Irrigation type had no effect on herbage N uptake or NO₃⁻ leaching losses.

In conclusion, the strategic use of diverse forages containing plantain, is a viable mitigation option to reduce NO₃⁻ leaching losses from urine patch areas. Furthermore, results have demonstrated that diverse forages can perform well under a range irrigation types in New Zealand when irrigation is applied using best management practices.