Environmental Impact of Rotationally Grazed Pastures at Different Management Intensities in South Africa

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. A three-year field study was conducted to investigate the yield response of different N-fertilizer treatments (0 (N0), 220 (N20), 440 (N40), 660 (N60) and 880 (N80) kg N ha−1 year−1) on grazed pastures and to calculate the carbon footprint (CF) of milk produced. Excessive N-fertilization (N60 and N80) did not increase herbage dry matter and energy yields from pastures. However, N80 indicated the highest N-yield but at the same time also the highest N surpluses at field level. A maximum fertilizer rate of 220 kg ha−1 year−1 (in addition to excreted N from grazing animals) appears sufficient to ensure adequate herbage yields (~20 t DM ha−1 year−1) with a slightly positive field-N-balance. This amount will prevent the depletion of soil C and N, with low N losses to the environment, where adequate milk yields of ~17 t ECM ha−1 with a low CF (~1.3 kg CO2 kg ECM−1) are reached. Methane from enteric fermentation (~49% ± 3.3) and N2O (~16% ± 3.2) emissions from irrigated pastures were the main contributors to the CF. A further CF reduction can be achieved by improved N-fertilization planning, low emission irrigation techniques and strategies to limit N2O emissions from pasture soils in South Africa.

Nitrous Oxide Emission from Grazing Is Low across a Gradient of Plant Functional Diversity and Soil Conditions

Nitrous oxide (N2O) emissions from pastures can vary significantly depending on soil and environmental conditions, nitrogen (N) input, as well as the plant’s ability to take up the N. We tested the hypothesis that legume-based N sources are characterized by significantly lower emission factors than mineral N based dairy systems. Therefore, this study monitored N2O emissions for a minimum of 100 days and up to two growing seasons across a gradient of plant species diversity. Emissions were measured from both grazed pastures and a controlled application of urine and dung using the static chamber method. About 90% of the simulated experiments’ accumulated N2O emissions occurred during the first 60–75 days. The average accumulated N2O emissions were 0.11, 0.87, 0.99, and 0.21 kg ha−1 for control, dung, urine patches, and grazed pastures, respectively. The N uptake efficiency at the excreta patch scale was about 70% for both dung and urine. The highest N2O-N emission factor was less than half compared with the IPCC default (0.3 vs. 0.77), suggesting an overestimation of N2O-N emissions from organically managed pastures in temperate climates. Plant diversity showed no significant effect on N2O emission. However, functional groups were significant (p < 0.05). We concluded that legume-containing pasture systems without a fertilizer addition generally appear capable of utilizing nitrogen inputs from excreta patches efficiently, resulting in low N2O emissions.

Partitioning of eddy covariance derived pasture N2O emissions for different sources and respective emission factors

&lt;p&gt;Nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) is a very potent greenhouse gas, and the majority of the emissions are associated with intensive livestock production. The magnitude of the emissions depends on the nitrogen (N) input to the soil, and on grazed pastures the largest share of the emissions is typically originating from the N applied via fertilization and excreta of the grazing animals. The uneven spatial distribution of the excretion leads to emission hot spots on grazing systems and makes the quantification of the gaseous emissions difficult. Micrometeorological methods like the eddy covariance (EC) that integrate emissions over a larger area method are well suited to quantify total field-scale N&lt;sub&gt;2&lt;/sub&gt;O emissions of grazed pastures. But the partioning of emissions for different sources and the determination of source-specific emission factors is still a challenge.&lt;/p&gt;&lt;p&gt;We present results of a 5-year field experiment carried out in western Switzerland. The investigated pasture was grazed by dairy cows in an intensive rotational management. The field was additionally fertilized with organic and mineral fertilizer each year, according to the N requirement of the grassland. The field-scale N&lt;sub&gt;2&lt;/sub&gt;O fluxes were quantified with the EC technique using a fast response Quantum cascade laser spectrometer for N&lt;sub&gt;2&lt;/sub&gt;O concentration measurements. The experimental setup and the environmental conditions resulted in high temporal and spatial dynamics of the N&lt;sub&gt;2&lt;/sub&gt;O fluxes with highest values typically occurring after mineral fertilization events in the summer month. Using N&lt;sub&gt;2&lt;/sub&gt;O background parametrizations retrieved from chamber measurements in one year and subtracting the background emission from the measured N&lt;sub&gt;2&lt;/sub&gt;O fluxes allowed us to calculate excreta-related emission factors (EFs) according to the IPCC guidelines. EFs for fertilizer N input were calculated using a pre-defined time window after the fertilizer was applied. The subtracted background emissions during the fertilization events were calculated from the EC measurements outside this time window. We attribute the observed emissions to the different N inputs and discuss potential reasons for the supposedly higher emissions after mineral fertilizer applications in comparison to organic fertilizer emissions.&lt;/p&gt;