Nitrous oxide emissions from feces and synthetic urine of cattle grazing forage grass fertilized with hog slurry

Abstract

Effects of hog slurry application to fertilized grassland on emissions of nitrous oxide (N₂O) from feces and urine excreted by cattle grazing was determined. Feces from cattle grazing forage grass fertilized with two rates of plant available N from a hog slurry, (i) hog slurry as Split application in fall and spring, each 72 kg N/ha (Split), and (ii) hog slurry each spring at 148 kg N/ha (Single), and zero slurry (Control), were monitored in field experiments in 2004 and 2005. The slurry treatments resulted in total N of feces patches of 84 (Control), 86 (Split), and 106 g N/m² (Single). Three synthetic urine treatments of (i) 136 g N/m² (Low), (ii) 233 g N/m² (Medium), and (iii) 329 g N/m² (High) and no urine (Background) applied to simulate the range in levels possible for cattle grazing the grass fertilized with the slurry treatments were examined in 2005. Nitrous oxide emissions from feces of cattle grazing the Split and Single treatments were higher than Control. Cumulative N₂O emissions from feces additions in the studies were 7, 31, and 91 mg N/m² for the Control, Single and Split treatments, respectively. Nitrous oxide emissions from feces increased with feces NO₃⁻ and soil NH₄⁺ concentrations, and decreased with feces and soil moisture. Cumulative N₂O emissions from urine treatments were much higher than from feces, being 165, 534 and 694 mg/m². Nitrous oxide emissions from urine increased with soil NO₃⁻, NH₄⁺ and NO₂⁻ concentrations and decreased with soil moisture. Nitrous oxide emissions estimated for excreta of grazing cattle were 27 g/kg of direct soil emissions for grassland receiving no slurry (Control) and 104 and 73 g/kg of direct emissions of the Split and Single slurry treatments. Findings indicate direct N₂O emission estimates for hog cattle grazing production systems are enhanced more by hog slurry treatment than are emissions from excreta.

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.