Impacts of pasture species and ruminant urine on N₂O emissions and nitrogen transforming microbial communities in soil mesocosms

Multispecies grasslands can alter nitrogen (N) cycling processes. A mesocosm experiment was performed comparing bare soil, soil planted with perennial ryegrass (Lolium perenne), white clover (Trifolium repens), plantain (Plantago lanceolata), and a mixed sward with all three species. We aimed to assess the plant effects on: ruminant urine-induced dry matter yields, plant N uptake, soil N2O emissions, and genes encoding nitrifier and denitrifier N transformations. After 35 days urine application increased both dry matter in the plantain (11%) and mixed (19%) swards, and N uptake in the ryegrass (63%), plantain (120%) and mixed (103%) swards. Urine increased cumulative N2O emissions, and while there was a trend for greater N uptake to cause lower emissions this was not significant. Urine addition elevated inorganic-N concentrations with few differences between plant treatments. Ammonia oxidising bacteria increased under urine but plantain did not inhibit this response. On days 3–10, increases in nirK and nosZI were indicative of enhanced nitrifier-denitrification and N2O scavenging by denitrifiers, respectively. The elevation of nosZI under urine-affected soil warrants further investigation in relation to potential pasture species effects. Consistent with previous in situ studies this study showed no evidence of plantain affecting soil inorganic-N concentrations via biological nitrification inhibition.