Grazing under Irrigation Affects N2O-Emissions Substantially in South Africa

Fertilized agricultural soils serve as a primary source of anthropogenic N2O emissions. In South Africa, there is a paucity of data on N2O emissions from fertilized, irrigated dairy-pasture systems and emission factors (EF) associated with the amount of N applied. A first study aiming to quantify direct N2O emissions and associated EFs of intensive pasture-based dairy systems in sub-Sahara Africa was conducted in South Africa. Field trials were conducted to evaluate fertilizer rates (0, 220, 440, 660, and 880 kg N ha&minus
1 year&minus
1) on N2O emissions from irrigated kikuyu&ndash
perennial ryegrass (Pennisetum clandestinum&ndash
Lolium perenne) pastures. The static chamber method was used to collect weekly N2O samples for one year. The highest daily N2O fluxes occurred in spring (0.99 kg ha&minus
1 day&minus
1) and summer (1.52 kg ha&minus
1). Accumulated N2O emissions ranged between 2.45 and 15.5 kg N2O-N ha&minus
1 and EFs for mineral fertilizers applied had an average of 0.9%. Nitrogen in yielded herbage varied between 582 and 900 kg N ha&minus
1. There was no positive effect on growth of pasture herbage from adding N at high rates. The relationship between N balance and annual N2O emissions was exponential, which indicated that excessive fertilization of N will add directly to N2O emissions from the pastures. Results from this study could update South Africa&rsquo
s greenhouse gas inventory more accurately to facilitate Tier 3 estimates.