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.