The effect of alternative forage species and gibberellic acid on nitrate leaching

Abstract

Nitrogen (N) leaching from soil is a significant concern for intensively grazed forage-based systems because it can cause a decline in water quality. Urine patches are the main source of N leaching in these systems. In New Zealand, increasing pressure to increase export earnings, while reducing N leaching loss from agriculture, poses a challenge for farmers and mitigation options are urgently needed. One approach is to increase the uptake of urine-N by forage plants, thereby reducing N leaching. The aim of this PhD programme was to increase our knowledge and understanding of the effects of alternative forages and gibberellic acid (GA) on N leaching from grazed agricultural soil.

Three lysimeter experiments were conducted. The first quantified the effect of forage type (perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) (RGWC), Italian ryegrass (Lolium multiflorum Lam.), and lucerne (Medicago sativa L.)) and GA application on N leaching, herbage dry matter (DM) yield, and N uptake. Lysimeters (0.5 m diam., 0.7 m deep) were treated with urine (700 kg N ha⁻¹ ), and ± GA (8 g GA ha⁻¹) in May 2014. A ¹⁵N balance was conducted to determine the fate of the applied urine-N. The second lysimeter experiment determined the N leaching loss, herbage DM yield, and N uptake from the urine patch of an Italian ryegrass, plantain (Plantago lanceolata L.), white clover mixture (Italian-Plantain Mix), compared with RGWC. Urine was applied in March 2015 and included a 700 kg N ha⁻¹ (Urine 700) treatment, and urine-N at the concentration excreted by cows grazing each forage type (Urine Actual). The third experiment used smaller lysimeters (0.18 m diam., 0.5 m deep) to determine the N leaching loss, herbage DM yield, and N uptake response of RGWC to an application of GA (8 g GA ha⁻¹) over a range of urine-N rates (0, 25, 50, 100, 200, 400, and 700 kg N ha-1) applied in April 2016.

A pot experiment was also conducted to determine whether there were any differences in the soil ammonia-oxidising bacteria and archaea beneath perennial ryegrass and Italian ryegrass, these were compared with bare soil. Pots (0.0144 m², 0.13 m deep) were destructively harvested 1, 15, 30, 61, and 90 days following urine application (700 kg N ha⁻¹) in May 2015.

Significant reductions in N leaching loss, were shown for Italian ryegrass (35%) and Italian-Plantain Mix (45-89%) forages, when compared with RGWC. The mechanisms behind this were a reduction in urine-N excretion (for Italian-Plantain Mix), and increased cool-season uptake of urine-N by the Italian ryegrass. This was reinforced when no difference in soil ammonia-oxidisers was shown between perennial ryegrass and Italian ryegrass which suggested that Italian ryegrass was not inhibiting nitrification. Gibberellic acid had no effect on N leaching. This research has clearly shown Italian ryegrass, and Italian-Plantain Mix as promising alternatives to RGWC, which could reduce N leaching losses from grazed systems. Lucerne is not recommended as an alternative to RGWC, as N leaching losses were higher under grazed conditions. Gibberellic acid had no direct effect on N leaching loss and so is not recommended as a direct mitigation tool for N leaching losses in grazed systems.