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
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warming. Overall, this study reveals a minimal contribution of recent fertilizer N to plant nutrition, but a sur-
prisingly 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.