Application of sorbers to mitigate greenhouse gas emissions from land-applied pig litter

Nitrous oxide is the foremost greenhouse gas (GHG) generated by land-applied manures and chemical fertilisers (Australian Government 2013). This research project was part of the National Agricultural Manure Management Program and investigated the potential for sorbers (i.e. specific naturally-occurring minerals) to decrease GHG emissions from spent piggery litter (as well as other manures) applied to soils. The sorbers investigated in this research were vermiculite and bentonite. Both are clays with high cation exchange capacities, of approximately 100–150 cmol/kg (Faure 1998). The hypothesis tested in this study was that the sorbers bind ammonium in soil solution thereby suppressing ammonia (NH₃) volatilisation and in doing so, slowing the kinetics of nitrate formation and associated nitrous oxide (N₂O) emissions.

A series of laboratory, glasshouse and field experiments were conducted to assess the sorbers’ effectiveness. The laboratory experiments comprised 64 vessels containing manure and sorber/manure ratios ranging from 1 : 10 to 1 : 1 incorporated into a sandy Sodosol via mixing. The glasshouse trial involved 240 pots comprising manure/sorber incubations placed 5 cm below the soil surface, two soil types (sandy Sodosol and Ferrosol) and two different nitrogen (N) application rates (50 kg N/ha and 150 kg N/ha) with a model plant (kikuyu grass). The field trial consisted of 96, 2 m × 2 m plots on a Ferrosol site with digit grass used as a model plant. Manure/sorber mixtures were applied in trenches (5 cm below surface) to these plots at increasing sorber levels at an N loading rate of 200 kg/ha. Gas produced in all experiments was plumbed into a purpose-built automated gas analysis (N₂O, NH₃, CH₄, CO₂) system. In the laboratory experiments, the sorbers showed strong capacity to decrease NH₃ emissions (up to 80% decrease). Ammonia emissions were close to the detection limit in all treatments in the glasshouse and field trial. In all experiments, considerable N₂O decreases (>40%) were achieved by the sorbers. As an example, mean N₂O emission decreases from the field trial phase of the project are shown in Fig. 1a.

Fig. 1.  Average: (a) N₂O emissions; and (b) dry matter yield (DMY) from spent litter applied to Ferrosol in the field trial; upper and lower standard error values included. Vermiculite and bentonite results are combined in the figures. Highest sorber addition level corresponds to 1 : 1 ratio to dry weight manure mass. Differences are significant at P < 0.05 for N₂O decreases and DMY increases.

The decrease in GHG emissions brought about by the clays did not negatively impact agronomic performance. Both vermiculite and bentonite resulted in a significant increase in dry matter yields in the field trial (Fig. 1b). Continuing work will optimise the sorber technology for improved environmental and agronomic performance across a range of soils (Vertosol, Dermosol in addition to Ferrosol and Sodosols) and environmental parameters (moisture, temperature, porosity, pH).