NITROUS OXIDE ON PASTURES GRAZED BY DAIRY COWS.

The main greenhouse gases (GHG) contributing to global warming potential from the agricultural sector are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Nitrous oxide has a 121-year life span in the atmosphere and a global warming potential of 265 times that of CO2, if compared over a 100-year period. This makes the control of N2O from the agricultural sector crucial to assist in mitigation of GHG emissions. Chemical fertilizers are the largest user of reactive nitrogen (N), but the use of N fertilizer in agricultural systems is not always efficient and effective. Sometimes less than 50% of the N applied is utilized by plants. As a consequence, the inefficient use of N fertilizer has contributed to a rise in N2O emissions. Furthermore, N fertilization is one of the most expensive inputs in pasture production as commonly between 200 and 600 kg of N per ha per year are used, contributing substantially to economic losses. Thus, both from a GHG emissions perspective and an economical point of view, N fertilization on pastures should be optimized. It is therefore of concern that current N fertilizer guidelines are maybe excessive as yield response is often not observed. If too high, the risk of nitrate leaching and other forms of environmental pollution is increased. Too high levels may also create imbalances but provide opportunities to reduce surplus inputs while reducing major N losses and focus on useful cycling of N to improve the nitrogen-use efficiency. Consequently, in the study cited below the authors aimed to obtain quantitative values for N2O emissions from pasture-based dairy production systems in the southern Cape. In their paper, the authors present results to address the following questions: (1) What would be the response of N2O emissions to N fertilization, under irrigation and intensive grazing practices, if N levels exceed 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 N2O emissions? (3) Could the estimated factor (EF), calculated from N2O emission values from these pastures, accurately predict N2O emissions compared to the suggested EF as set by the IPCC Tier 1 default value for N fertilizers?

Field trials were conducted to evaluate chemical fertilizer rates of 0, 220, 440, 660 and 880 kg N per ha per year on N2O emissions from irrigated kikuyu–perennial ryegrass pastures in the southern Cape. The static chamber method was used to weekly collect N2O samples for one year.

The highest daily N2O fluxes occurred in spring (0.99 kg per ha per day) and summer (1.52 kg per ha per day). The total N2O emissions ranged between 2.45 and 15.5 kg N2O-N per ha per year and EFs for chemical fertilizers applied showed an average of 0.9%, which is lower than the IPCC Tier 1 value. Nitrogen in the yielded herbage varied between 582 and 900 kg N per ha. 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 N fertilization at the higher rates added to the N2O emissions from the pastures at a progressively increased rate.  

The results showed that the excessive use of chemical fertilizers on intensive rotational grazed pastures in the southern Cape resulted in high N surpluses of more than 400 kg N per ha per year and the fertilizers provided the majority of the N in the system. The results also indicated that the relationship between N2O-N losses and N input, when plant N demand is considered, can best be described by a nonlinear exponential function rather than a linear function. Furthermore, the suggested EF of the IPCC Tier 1 value for grazing systems led to an overestimation of the N2O emissions from these pastures. It was recommended that a better approach would be to replace the EF of the IPCC Tier 1 value with regional EF values which are dependent on the N balance. This should result in more accurate N2O emissions from managed soils at a regional scale, where other threats such as groundwater pollution and eutrophication are addressed at the same time. 

Reference:

H. P.J. Smit, T. Reinsch, P. A. Swanepoel, C. Kluß & F. Taube, 2020. Grazing under Irrigation Affects N2O-Emissions Substantially in South Africa. Atmosphere 2020, 11, 925; doi:10.3390/atmos11090925.