High importance of organic fertilizer nitrogen applied to temperate grassland for plant nitrogen uptake in the years following fertilization

We applied 15 N labelled cattle slurry over one year to large grassland lysimeters in Southern Germany to study its importance for plant nitrogen (N) nutrition not only in the year of application, but also in the following four years, and for both current and future climatic conditions. In the year of 15 N fertilizer application, the recovery of 15 N in harvested aboveground plant biomass was as low as 16 %, with several fold larger 15 N recovery in soil organic nitrogen (SON). In the first year after its application, the labelled 15 N was of equal importance for plant N nutrition compared to the year of application, as illustrated by a plant 15 N recovery of 9–14 %. The recovery of 15 N fertilizer in mowed plant biomass remained relatively stable and only gradually declined in the following years, ultimately resulting in a cumulative 15 N recovery rate of 33–37 % in mowed biomass after five years. Total 15 N recovery in plant and soil after five years was 67.6 ± 3.8 % for the climate change treatment and 75.6 ± 10.8 % for the climate control treatment, suggesting increased long-term fertilizer N losses under warming. Overall, this study reveals a minimal contribution of recent fertilizer N to plant nutrition, but a surprisingly quick recycling via SON and re-allocation to plants. The findings support a concept that fertilizing such grasslands is largely a fertilization of soils rather than a direct fertilization of plants, with a fast turnover of fertilizer-derived SON promoting the risk of soil N mining. • 15 N labelled cattle slurry was traced over 5 years in grassland lysimeters. • In the year of fertilizer application, soil is fertilized rather than plants. • Plant N uptake largely based on soil organic N. • Fast turnover of fertilizer-derived soil organic N upon allocation to plants. • Climate change increases fertilizer N losses and soil N mining.