NH₃, N₂O and CH₄ emissions during passively aerated composting of straw-rich pig manure

Abstract

Straw-rich manure from organic pig farming systems was composted in passively aerated static piles to estimate the effect of monthly turning on organic matter degradation and NH₃, N₂O and CH₄ emissions. Turning enhanced the rate of drying and degradation. The four-month treatment degraded 57 ± 3% of the initial organic matter in the turned piles, while only 40 ± 5% in the static piles. The turned piles showed low ammonia and N₂O emissions, 3.9 ± 0.2% and 2.5 ± 0.1% of total initial nitrogen, respectively. Static piles gave low ammonia (2.4 ± 0.1% N_initial), but high (9.9 ± 0.5% N_initial) N₂O emissions. Prevalence of anaerobic regions in the static system was supported by the higher CH₄ emissions, 12.6 ± 0.6% VS_degraded for the static vs. 0.4 ± 0.0% VS_degraded for the turned system. It was shown, that straw-rich pig manure with very low C/N ratios could be composted directly without significant NH₃ and N₂O emissions if turned on a monthly basis.

Introduction

Modern livestock activities may result in large emissions of ammonia, nitrous oxide and odours from buildings, manure storage and following application of manure to land (Burton, 1997). These emissions lead to a large number of environmental problems (El-Ahraf and Willis, 1996). Animal manures are also a valuable resource as they can be used to recycle nutrients and organic matter to soil, thus reducing the use of artificial fertilisers and peat (Richard and Choi, 1999). Manure management is increasingly viewed as a mechanism for reducing the negative environmental impacts associated with livestock production, and a wide range of technologies are being developed and evaluated for that purpose (Burton, 1997).

Composting is among the most promising techniques for treating the solid fraction of animal manures, as it is inexpensive and can produce a valuable end product (Lopez-Real and Baptista, 1996). The composting process results in breakdown and stabilisation of organic matter, mass and volume reduction through CO₂ evolution and H₂O evaporation and destruction of pathogens and weed seeds. Correctly managed, the resulting compost can be used as an organic fertiliser in agriculture without negative impacts on plant growth and the environment (Epstein, 1997).

Commonly applied composting strategies of forced aeration and windrow turning, so-called intensive composting systems, result in short composting times but will also lead to high ammonia emissions (Witter and Lopez-Real, 1988, Mahimairaja et al., 1994). Composting of animal manures in passively aerated static piles, so-called extensive composting, gives a significant reduction in ammonia emissions (Lopez-Real and Baptista, 1996). Although these reduced NH₃ emissions might be assumed to result in greater conservation of nitrogen in the final product, research indicates that substantial quantities of N can instead be lost as nitrogen gas or nitrous oxide via nitrification and denitrification (Czepiel et al., 1996, Veeken et al., 2002). Compaction of the compost bed in time results in decreased air permeabilities of the structure, consequently it reduces natural convection (Das and Keener, 1997). Therefore compaction reduces aerobic degradation and may lead to increased presence of anaerobic regions, promoting emissions of methane and nitrous oxide, both harmful greenhouse gases (Czepiel et al., 1996, Hellmann et al., 1997). Rynk et al. (1992) recommend periodic turning of the compost pile to restore the structure and porosity of the pile. In this way the composting process is accelerated and the removal of pathogens is improved.

This study examines the hypothesis that periodic turning might also reduce the emissions of greenhouse gases, nitrous oxide and methane. Straw-rich pig manure from an organic farming system was subjected to passively aerated composting for 4 months. Two piles were not disturbed (static system) and two piles were monthly turned (turned system). The composting process was evaluated with respect to drying, organic matter degradation, and the quality of composition changes.