Effectiveness of different mitigation strategies to reduce nitrous oxide emissions from pig manure amended soils

Developing effective mitigation strategies for reducing nitrous oxide (N₂O) emissions from manured soils requires a better understanding of the microorganisms and mechanisms involved (Barton et al. 2013; Banning et al. 2015). Previous work has indicated that nitrifying microorganisms at the surface (0–10 cm) were largely responsible for N₂O emissions in Western Australian semi-arid soils and these microorganisms responded to targeted mitigation strategies for reducing N₂O (Barton et al. 2013). However, the effect of adding pig manure to these soils on the N₂O emitting microbial populations and mitigation remains largely unknown. The aim of this study was to evaluate the effectiveness of different pig manure types (stockpiled, composted and pelletised manure) and application methods (broadcast or incorporated into the soil) at reducing N₂O emissions following manure amendment. It was hypothesised that the amount of nitrified-N₂O could be reduced by a) incorporating manure at depth to avoid ammonia oxidisers in the topsoil, and b) composting or pelletising manure to decrease availability of ammonium (VanderZaag et al. 2011; Barton et al. 2013).

A soil microcosm experiment having a 2 × 5 × 2 factorial arrangement of treatments in triplicate was conducted using 557-mL glass jars, with factors being sandy or clayey soil (clay contents of 1.6 and 8.2% respectively) (collected from UWA Future Farm, Pingelly), five different amendments applied at 100 kg of N/ha (unamended, inorganic fertiliser, stockpiled, composted or pelletised manure), and two application methods (broadcast or incorporated). The microcosms were adjusted to 40% water holding capacity and incubated at 25°C for 2 weeks. The glass jars were unsealed, except during gas flux measurements when they were sealed with an air-tight lid fitted with a septum to trap the expired gases for 2 hours. The N₂O flux was analysed at 0, 2, 6, 24, 48, 72, 96, 120, 168 and 336 h by gas chromatography.

The N₂O emissions ranged from 0.002 to 0.85 kg/ha/d but were most pronounced in the clayey soil (Fig. 1b) and for the stockpiled manure amendment (Fig. 1). Incorporating stockpiled manure in sandy soils caused a 2-fold decrease in N₂O flux compared to broadcast (Fig. 1a), but this benefit was lost in the clay soils (Fig. 1b). Although the composted manure had the overall lowest emissions on both soils (Fig. 1), the pelletised manure reduced the emissions relative to stockpiled manure and probably offers the best mitigation option for semi-arid soils since it avoids emissions during the composting process and is easier to handle, transport and apply. Composting is more suitable for larger or mixed (piggery and grain) enterprises where there are multiple waste streams to manage. In conclusion, the effectiveness of the greenhouse gas mitigation method depends on both manure type and soil type. Mitigation methods that decrease nitrification and availability of ammonium and nitrate, such as composting, pelletising or incorporating manure, have the greatest potential to reduce N₂O emissions in semi-arid cropping systems.

Fig. 1.  Nitrous oxide (N₂O) flux during the first 48 hours following the broadcast application or incorporation of different manure types to sandy (a) and clayey (b) soils (mean ± SEM; n = 3).The treatments are as follows : unamended control (□), mineral fertiliser (■), stockpiled manure broadcast (▵) or incorporated (▲), composted manure broadcast (◇) or incorporated (♦) and pelletised manure (●).