FEATURE
Eco-efficiency of irrigated dairy pastures in South Africa :
N 2
O Emissions from managed soils and its implication for carbon footprint calculations
A synopsis of two published articles : Smit et al . ( 2020 ). Grazing under irrigation affects N 2
O-emissions substantially in South Africa . Atmosphere , 11 ( 9 ), p . 925 . Smit et al . ( 2021 ) Environmental impact of rotationally grazed pastures at different management intensities in South Africa . Animals , 11 ( 5 ), p . 1214 .
Hendrik P . J . Smit 1 , Thorsten Reinsch 1 , Pieter A . Swanepoel 2 , Christof Kluß 1 and Friedhelm Taube 1 , 3
Current Address : 1 Institute of Crop Science and Plant Breeding , Grass and Forage Science / Organic Agriculture , Christian-Albrechts-University Kiel , D-24118 Kiel , Germany . 2 Department of Agronomy , Stellenbosch University , Stellenbosch , 7600 , South Africa . 3 Grass Based Dairy Systems , Animal Production Systems Group , Wageningen University ( WUR ), Wageningen 6700 HB , The Netherlands .
Introduction
Nitrogen fertilization , irrigation and concentrate feeding are important factors in rotational pasture management for dairy farms in South Africa . The extent to which these factors affect environmental efficiency is subject to current and intense debate among scientists . Dairy farmers generally use high amounts of mineral fertilizer to promote herbage growth and maximize herbage yield per hectare , despite the high amounts of manure that is available . However , increments in herbage yield are not increasing linearly with additionally applied nitrogen ( Viljoen et al . 2020 ). Fertilized agricultural soils serve as a primary source of anthropogenic
N 2
O emissions , a greenhouse gas with a global warming potential 265 times that of CO 2
. In South Africa , there is a paucity of data on N 2
O emissions from fertilized , irrigated dairy-pastures and emission factors ( EF ) associated with the amount of N fertilized . However , it is important to create region-specific EFs to quantify emission hotspots and subsequently promote mitigation strategies to ensure climate-smart dairy production . The growing concern over GHG and the effect of dairy production on the environment has led to the need to express the total emissions associated with milk . Product carbon footprint ( CF ) analysis has become broadly accepted as an assessment method . The N-fertilizer management is an important parameter to consider when calculating the CF of milk ( Henriksson et al . 2011 ). This study could update South Africa ´ s greenhouse gas inventory more accurately and can promote ways to produce more environmentally friendly milk with low associated greenhouse gas emissions and therefore a lower carbon footprint . Accordingly , important questions were addressed : ( 1 ) What would be the response of N 2
O emissions under managed soils in the southern Cape region of South Africa to N fertilization , under irrigation and intensive grazing practices , because of N levels exceeding plant requirements ? ( 2 ) Can high stocking rates on intensively managed , highly fertilized and irrigated dairy pastures lead to high amounts of N returned through excreta to the soil and result in a high N surplus which could underestimate predicted N 2
O emissions ? ( 3 ) To what extent does mineral N-fertilizer usage in intensive rotationally stocked dairy pasture systems in South Africa affect the milk yield , CF , N-balance and N-footprint per hectare and per kg of milk produced ?
Materials and Methods
Field trials were laid out as a randomized block design to evaluate five N fertilizer rates ( 0 , 220 , 440 , 660 and 880 kg N ha -1 year -1 ) as treatments on N 2
O emissions from irrigated kikuyu-perennial ryegrass ( Pennisetum clandestinum-
11 Grassroots Vol 21 No 3 November 2021