Fates of slurry-nitrogen applied to mountain grasslands: the importance of dinitrogen emissions versus plant N uptake

Intensive fertilization of grasslands with cattle slurry can cause high environmental nitrogen (N) losses in form of ammonia (NH₃), nitrous oxide (N₂O), and nitrate (NO₃⁻) leaching. Still, knowledge on short-term fertilizer N partitioning between plants and dinitrogen (N₂) emissions is lacking. Therefore, we applied highly ¹⁵N-enriched cattle slurry (97 kg N ha⁻¹) to pre-alpine grassland field mesocosms. We traced the slurry ¹⁵N in the plant-soil system and to denitrification losses (N₂, N₂O) over 29 days in high temporal resolution. Gaseous ammonia (NH₃), N₂ as well N₂O losses at about 20 kg N ha⁻¹ were observed only within the first 3 days after fertilization and were dominated by NH₃. Nitrous oxide emissions (0.1 kg N ha⁻¹) were negligible, while N₂ emissions accounted for 3 kg of fertilizer N ha⁻¹. The relatively low denitrification losses can be explained by the rapid plant uptake of fertilizer N, particularly from 0–4 cm depth, with plant N uptake exceeding denitrification N losses by an order of magnitude already after 3 days. After 17 days, total aboveground plant N uptake reached 100 kg N ha⁻¹, with 33% of N derived from the applied N fertilizer. Half of the fertilizer N was found in above and belowground biomass, while at about 25% was recovered in the soil and 25% was lost, mainly in form of gaseous emissions, with minor N leaching. Overall, this study shows that plant N uptake plays a dominant role in controlling denitrification losses at high N application rates in pre-alpine grassland soils.