Reduction of nitrogen losses by manipulating carbon inputs and pasture composition : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

The nitrogen (N) cycle is very important to New Zealand agriculture, with N being a key nutrient needed for plant growth. However, there are major environmental concerns relating to the loss of N from agricultural systems. These include nitrate (NO₃⁻) leaching and the emission of nitrous oxide (N₂O). Nitrate leaching is of concern due to its potential human health risks and eutrophication of fresh water ways; while N₂O is a potent greenhouse gas and an ozone depleting substance. There is also a significant economic loss to farmers, as lost N needs to be replaced through fertilisers. Pastoral grazing systems have a high risk of NO₃⁻ leaching and N₂O emissions due to the high N loading rates of urine patches (approximately 613 kg N ha⁻¹ for cattle). Carbon (C) has many important interactions with the N cycle, these include immobilisation/mineralisation, nitrification, and denitrification. These interactions are of importance due to their potential to be manipulated in a way that reduces N losses from agricultural systems. Carbon inputs can be manipulated through growing different pasture/crop types with contrasting traits (rooting depth, carbon allocation, root exudates, winter growth), adding urine from animals fed on different diets and applying artificial C inputs. There is however, a lack of detailed knowledge on how C affects the N cycle in New Zealand grazed pastoral systems, on shallow stony soils. The aim of this research was to improve our understanding of these interactions and help develop techniques where C inputs can be manipulated to reduce N losses. To do this three lysimeter trials were conducted at Lincoln University’s Ashley Dene Research Development Station (ADRDS). These lysimeters contained Balmoral stony silt loam and were treated with cattle urine. Lysimeter experiment 1 was split into two trials; the objective of Trial 1 was to determine the effect of artificial inputs of readily available C on the N cycle; the objective of Trial 2 was to determine the effect of different urine compositions and crop types on the N cycle. Trial 1 discovered that applying readily available C to soil significantly reduced N leaching losses, without causing an increase in N₂O emissions. This was attributed to the added C increasing immobilisation of N in the soil. Trial 1 also revealed that the perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) (PRG/WC) pasture leached 58% less N than the lucerne (Medicago sativa) crop. This reduction was attributed to the higher winter plant growth and N uptake of the PRG/WC pasture, reducing the amount of mineral N available to be leached. Trial 2 discovered that under PRG/WC pasture, urine from cows feed a diet of fodder beet (FB) (Beta vulgaris) leached 64% less NO₃⁻-N than urine from cows feed a diet of PRG/WC; even at the same urine N loading rate. Soil under the FB urine had significantly lower ammonia oxidising bacteria (AOB) amoA gene abundance (P = 0.005) and amount of soil NO₃⁻-N (P = 0.026), suggesting that the FB urine has a biological nitrification inhibitor (BNI) effect. Trial 2 also discovered that the PRG/WC pasture leached 65-84% less N than the bare fallow FB soil. This reduction was attributed to the higher winter plant growth and N uptake of the PRG/WC pasture, reducing the amount of mineral N available to be leached. Lysimeter experiment 2 discovered that by lowering the urine-N loading rate by 28%, from 700 kg N ha⁻¹ to 500 kg N ha⁻¹, N2O emissions were reduced by 38%, and total N leaching losses were reduced by 39%. This demonstrates that farm management practices that reduce urine-N rate could be an effective way of reducing N losses from grazing systems. Lysimeter experiment 3 discovered that Italian ryegrass (Lolium multiforum)/white clover/plantain (Plantago lanceolate) (IRG/WC/P) and perennial ryegrass/white clover/plantain (PRG/WC/P) significantly reduced N leaching losses by 24% and 14%, respectively, compared with traditional PRG/WC. The reasons for these reductions were: (i) the higher plant N uptake, which decreased the soil mineral N content and subsequently reduced the amount of N available to be leached and; (ii) the greater herbage yield which increased transpiration, thus reducing drainage volume. This research programme has highlighted the importance of manipulating C inputs through winter plant growth and the manipulation of cattle diet in reducing N losses. Increasing plant N uptake over cooler months is potentially an economically viable and effective way of reducing farm N losses. This can be achieved by sowing more winter active crops/pastures, reducing time cattle spend on bare fallow soil and/or through the use of effective catch crops. Manipulating cattle diet by feeding stock low N feed and subsequently reducing the urine N loading rate is potentially an economically viable and effective way of reducing farm N leaching losses; especially if that low N feed such as fodder beet, can provide a BNI effect in the cattle urine.