The effect of pasture species composition and a nitrification inhibitor on nitrate leaching losses

Abstract

The aim of this research programme was to determine the effects of pasture species composition and a nitrification inhibitor on nitrate leaching losses, and root architecture during the winter period. A lysimeter and field trial was conducted for two years. A total of 32 lysimeters (0.2 m², 70 cm deep) were collected from a pasture site at the Lincoln University Research Dairy Farm (LURDF), Canterbury. Sixteen main field plots (2.1x6 m) were also established at LURDF for destructive soil/root sampling. The soil type was a Templeton fine sandy loam soil (NZ classification: Immature Pallic soil). In year one (2010/11), the lysimeters and field plots were sown with four different pasture species: (i) perennial ryegrass, white clover (P. ryegrass WC); (ii) tall fescue, WC (T. fescue WC); (iii) Italian ryegrass, WC (It. ryegrass WC) and (iv) a mixed pasture consisting of P. ryegrass, It. ryegrass, WC, red clover, chicory and plantain (Diverse). In year two (2011/12), the lysimeters were collected from untreated areas within each main field plot to determine leaching losses beneath well-established pasture. Natural cow urine was applied to all lysimeters at a rate of 1000 kg N ha⁻¹, and half the treatments received DCD as a fine particle suspension at 10 kg DCD ha⁻¹. Artificial cow urine was applied to 1 m² sub-plots within each main field plots. All the field sub-plots were intensively soil sampled between 0 and 80 cm depth during the spring following treatment application, and the roots within each sample were removed from the soil and various architectural parameters were measured by the computer scanner and software package WinRHIZO. In both years, nitrate leaching losses from the lysimeters were lowest beneath It. ryegrass WC, being 24-54% lower than the other pasture species (P < 0.05). The T. fescue WC pasture leached up to 116% more nitrate than other pasture species in year one, and differences between P. ryegrass WC, T. fescue WC and Diverse in year two were not statistically significant. The application of the nitrification inhibitor, DCD, significantly (P < 0.001) reduced nitrate leaching by 27-40% in the first year and 25-41% in the second. In both years, the DCD-treated It. ryegrass WC treatments had the lowest leaching losses. Total dry matter yields in year one were between 11 and 58% higher in the It. ryegrass WC compared to the other pasture species. In year two, total annual dry matter production ranged from 20921 kg DM ha⁻¹ (T. fescue WC) to 25135 kg DM ha⁻¹ (P. ryegrass WC). In year one, the root architecture of the pasture species was similar, indicating that T. fescue WC roots were relatively undeveloped. In year two, significantly (P < 0.05) higher root length densities were observed beneath the T. fescue WC pasture species. Despite a relatively shallow root system, the active winter growth of It. ryegrass WC enabled a greater amount of soil N uptake during the leaching period, which resulted in lower nitrate leaching losses compared to the other pasture species. The lysimeter and field trials indicated that seasonal growth pattern (especially high winter growth) was more important than the presence of deep roots in terms of reducing nitrate leaching losses. To understand and quantify this conceptual finding more precisely, a ¹⁵N uptake trial comparing the It. ryegrass WC and T. fescue WC pasture species was conducted. Enriched ¹⁵N urea (10 atom %) was injected at a rate of 300 kg N ha⁻¹ into the side walls of 48 lysimeters (18 cm diameter; 70 cm deep) at three different depths: 0, 250 and 450 mm. The lysimeters were installed inside a climate-controlled growth chamber under simulated winter conditions. Water was applied to half of the lysimeters to initiate leaching (leached treatments), while the remaining lysimeters were irrigated to just below field capacity (non-leached treatments). Herbage samples were taken and analysed for ¹⁵N recovery, while leachate samples were diffused and analysed for ¹⁵N by an isotope ratio mass spectrometer. At the end of the trial, root samples were obtained from selected lysimeters and analysed by the WinRHIZO scanner and software package to determine root architecture. ¹⁵N recovery was consistently higher for It. ryegrass WC compared with T. fescue WC, despite T. fescue WC having significantly (P < 0.05) higher root length densities. In all of the 450 mm depth N injection treatments (leached and non-leached), the T. fescue WC pasture species contained significantly (P < 0.05) greater root length densities than It. ryegrass WC. Dry matter yields of It. ryegrass WC were again higher than T. fescue WC. The results clearly indicate that deep T. fescue WC roots are relatively inactive during the winter period, and that winter growth is a vital plant characteristic that is more important than the presence of deep roots in reducing nitrate leaching losses. Growing Italian ryegrass-based pastures in intensively grazed pasture systems is a viable option for farmers, and may have additional benefits because of its high winter growth rates. The conclusion of this research programme is that the strategic use of Italian ryegrass in grazed pasture systems in combination with the nitrification inhibitor, DCD, can significantly reduce nitrate leaching losses.