Abstract
Dairy farms need to improve their environmental performance to justify continued consumption of dairy products. Previous studies have demonstrated improved re-use of dairy slurry nitrogen (N) and phosphorous (P) by separating solid and liquid fractions. This study was conducted to examine the long-term effects of applying whole dairy slurry (WS), separated liquid fraction (SLF), and mineral fertilizer on emission of nitrous oxide (N2O) from a grass sward in a moderate climate, and to identify methods to mitigate emissions. The results show that emission occurs in sharp spikes within a week of N application as well as after some wet/dry cycles and freeze/ thaw cycles even in this moderate climate. Greatest emissions occurred from late spring and summer applications. At the high application rate of total-N emissions were similar for WS and SLF, at the lower rate SLF exceeded WS, whereas at equivalent mineral-N, emissions were greater from WS than SLF. On the basis of crop N-uptake, SLF had similar or lower emissions than WS at comparable N rates, and the same was true on basis of yield except for lower emissions with WS than SLF at the low total-N rate (400 kg ha-1). By not finding pollution swapping, the study helps to support the use of slurry manure separation for its agronomic and environmental benefits. This multi-year field study supports current IPCC default values as cautious emission factors (EF) for dairy slurry and commercial fertilizer on intensively managed grass, though a lower EF may be justified for lower input grassland (<50 kg mineral-N ha-1 per dose). Our results suggest that N2O emissions can be reduced by transferring summer slurry application to early spring but on farms where this is not possible, mitigating summertime peaks should be targeted, possibly with the use of a nitrification inhibitor.
Highlights
Several recent reports have argued that food derived from livestock contributes disproportionately to environmental degradation compared to plant derived food (Westhoek et al, 2014; Sheppard and Bittman, 2015)
In Canada, as in many industrialized countries, most dairy manure is handled as a slurry which is stored beneath the barns, or in uncovered below-grade pits, above ground tanks, or large lagoons (Sheppard et al, 2011)
We found about a 50% difference between Fert400 and WS400 whereas Velthof et al (1997) reported larger differences between cattle slurry and calcium ammonium nitrate
Summary
Several recent reports have argued that food derived from livestock contributes disproportionately to environmental degradation compared to plant derived food (Westhoek et al, 2014; Sheppard and Bittman, 2015). Because of the increasing interest in the benefits of slurry separation, the long term effects of applications on grassland of whole and decanted liquid fraction on year-round N2O emissions needs to be assessed The objectives of this 5 year field study, conducted on long term grass- slurry manure plots (described by Bittman et al, 2011; Neufeld et al, 2017; Zhang et al, 2018), were to (1) quantify N2O emissions after multiple slurry applications on grass over the growing season, (2) quantify emission spikes due to wet/dry and freeze/thaw events, (3) compare emissions from applications of whole slurry (WS), separated liquid fraction (SLF), and mineral fertilizer (FERT) at comparable rates of mineral and total N, 4) identify methods that may mitigate N2O emissions from intensively fertilized grassland
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