Nitrous oxide emissions from feces and synthetic urine of cattle grazing forage grass fertilized with hog slurry
Introduction
Grazing by cattle (Bos primigenius taurus) consumes large quantities of forage over extensive areas, and returns a majority of the nutrients as feces and urine to patches on the soil. The high concentration of labile organic C and N in cattle feces and urine patches create “hot-spots” of emissions of nitrous oxide (N2O), a molecule which is a potent greenhouse gas which has been implicated in global warming (IPCC, 2007). As cattle feces and urine patches have N levels which exceed plant demand within or around the patch (Haynes and Williams, 1993), excess N is readily lost to the environment via physical, chemical and biological processes. Two of the processes, nitrification and denitrification, can be responsible for emissions of N2O from feces and urine patches (Allen et al., 1996, Müller et al., 1997, Carter, 2007).
It is difficult to approximate the importance of cattle grazing on global N2O emissions. According to Oenema et al. (1997), 5–30 g/kg of N excreted by grazing cattle is evolved as N2O, with an overall mean of 20 g/kg, but it can be as low as 0.5–3.3 g/kg (Wachendorf et al., 2008). Globally, grazing by livestock is estimated to contribute 1.5 Tg of N2O–N, which is higher than 100 Gg/Tg of its global annual flux due to anthropogenic activities (IPCC, 2007, Oenema et al., 1997, Oenema et al., 2005). Of the 1.5 Tg of N2O–N, feces and urine deposited by livestock grazing contributes 410 Gg/Tg, indirect sources 270 Gg/Tg, animal waste in housing and storage 190 Gg/Tg, application of the waste to land 100 Gg/Tg and burning of feces 30 Gg/Tg (Oenema et al., 1997, Oenema et al., 2005). Nitrous oxide emissions from urine patches are higher than from feces patches because the majority of N in urine is in the form of urea and other readily transformable organic N species. In addition, urine N concentrations can inhibit complete nitrification and increase soil nitrite (NO2−) concentrations. Soil NO2− is susceptible to loss as N2O (Tenuta and Beauchamp, 2000), and this may be more severe in coarse textured soils with basic pH (Monaghan and Barraclough, 1992).
To improve their productivity, grazed grassland is often fertilized with animal manure or synthetic fertilizer. Effects of fertilisation on N2O emissions from grasslands have received attention in recent years. Mkhabela et al. (2009) and Tenuta et al. (2010) reported that fertilisation of grassland with hog (Sus scrofa) slurry increased N2O emissions. Fertilizing grass based paddocks with manure increased yield, and the crude protein (CP) level of grass (Reid et al., 1966). In south-eastern Manitoba (Canada), application of hog slurry to grassland increased standing forage biomass from 1.2 to 2.6 t dry matter (DM)/ha and CP concentration from 95 to 178 g/kg DM (Wilson et al., 2010). Little research has been conducted to determine if fertilisation affects N2O emissions from excreta (i.e., feces and urine) voided by cattle grazing fertilized grass forage. Consumption of grass or legume hays with high N content increased daily urine N excretion from beef steers (Archibeque et al., 2001, Basurto-Gutierrez et al., 2003). Therefore, high levels of C and N in excrement of cattle grazing fertilized grasslands should increase N2O emissions from feces and urine patches, in particular N2O. Despite the potential for N2O contributions from grazed grassland systems, this has not been fully explored.
Results of two experiments are presented. The first examined the extent to which fertilisation of grass with hog slurry impacted emissions of N2O from feces of cattle grazing the slurry fertilized grass. The second experiment compared N2O emissions from synthetic urine which simulated a range of chemical compositions of urine from cattle grazing grass having received hog slurry.
Section snippets
Experimental site and layout
This study was part of a larger one which evaluated forage yield and quality, cattle performance, enteric CH4 emissions (Wilson et al., 2010) and soil N2O emissions (Tenuta et al., 2010) following application of hog slurry to grass. The study site was the University of Manitoba's La Broquerie Pasture and Hog Manure Research Site, and the experimental design was a factorial combination of three hog slurry treatments and two grass utilization treatments with two replicates for each combination of
Weather conditions
During the period for the Feces experiment in 2004, mean daily air temperature and total rain were 3 °C above and 29 mm below normal for September. During the Feces experiment in 2005, air temperature was 1 °C below normal for August and 2 °C above normal for September while precipitation was 27 and 41 mm below normal, respectively. The urine experiment was conducted during July and August 2005 with air temperature being normal and rain 12 mm less than normal.
Feces characteristics
In 2004, the single manure treatment
Feed characteristics
Wilson et al. (2010) reported the nutritional profile of the forage consumed by the steers to yield the feces used in our study. The CP was 95 g/kg DM for the Control and rose to 162 and 178 g/kg DM for the Split and Single treatments and equivalent to 15.2, 25.9, and 28.5 g N/kg DM, respectively. The NDF and ADF decreased in the order of Control > Split > Single with the ratio of NDF/ADF also having decreased in that same order with ratios of 1.80, 1.85 and 1.90, respectively. The composition of the
Conclusions
N2O emissions increased from feces and urine excreted by cattle grazing grassland fertilized with hog slurry. Cattle feces from the Single hog slurry treatment resulted in higher N2O emissions than from the Control treatment of no hog slurry. Urine application increased N2O emissions compared to the Background, with Medium and High urine N treatments having higher emissions than the Low treatment. Fecal N2O emissions were positively correlated with feces NO3− concentration and soil NH4+
Acknowledgements
This study was funded by Manitoba Conservation's Sustainable Development Innovations Fund, the Canada Research Chair Program in Applied Soil Ecology (M.T.), the Natural Sciences and Engineering Research Council (NSERC) Discovery Grant Program (M.T.), Canada's Greenhouse Gas Mitigation Fund, the NSERC Post-graduate Scholarship Program (D.G.T.), Manitoba Pork Council, Manitoba Livestock Manure Management Initiative, Manitoba Cattle Producers Association, Beef Council Research of Canada, and
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