Effectiveness and trade-offs of leonardite and rock powder additives (zeolite, bentonite, lava rock dust, diabase, and basalt) for ammonia and greenhouse gas mitigation during cattle slurry storage

Ammonia (NH$_3$) and greenhouse gas emissions from slurry storage represent major environmental challenges in livestock production. Additives such as humic substances and rock powders have been proposed as mitigation options. However, results regarding their effectiveness remain inconsistent, and their implications for nitrogen trade-offs between NH$_3$ and nitrous oxide (N$_2$O) emissions and total greenhouse gas emissions during slurry storage are poorly constrained. This study evaluated a humic substance (leonardite) and five rock powder additives (zeolite, bentonite, diabase, lava rock dust, and basalt) in controlled slurry storage. Additives were mixed with diluted untreated slurry at an application rate of 5% (w/w), and NH$_3$ emissions were quantified by continuous acid trapping, while, N$_2$O, methane (CH$_4$), and carbon dioxide (CO$_2$) were measured once daily during 10-min measurement periods, using the final 3 min for evaluation, over a 22-day storage period. Leonardite significantly reduced cumulative NH3 emissions by 32%, followed by zeolite with a 29% reduction, whereas other additives did not significantly affect NH$_3$ emissions. ... mehrHowever, ammonia reduction by leonardite was accompanied by a 20% increase in N$_2$O emissions, suggesting potential nitrogen trade-offs, while zeolite reduced NH$_3$ without increasing N$_2$O emissions. Diabase and lava rock dust significantly reduced CO$_2$ emissions by 20% and 14%, respectively. Across all treatments, CH$_4$ contributed most to total greenhouse gas emissions expressed as CO$_2$ equivalents and increased by up to 25–31% for some additives, offsetting potential environmental benefits from NH$_3$, even after inclusion of indirect N$_2$O emissions associated with NH$_3$ volatilisation. Consequently, no treatment resulted in a significant reduction in total greenhouse gas emissions expressed as CO$_2$ equivalents. Overall, mitigation effects during slurry storage were limited and additive specific. These findings highlight the need for integrated, multi-gas assessments to ensure robust environmental benefits.