High importance of organic fertilizer N for grassland plant N nutrition in the years following fertilization

We applied ¹⁵N labelled cattle slurry over one year to pre-alpine grassland in order to study its importance for plant N nutrition not only in the year of application, but also in the four following years. This five-year ¹⁵N tracing study was combined with a space for time climate change experiment in order to assess long-term fertilizer N cycling under current and future climatic conditions. In the year of ¹⁵N fertilizer application, the recovery of ¹⁵N in harvested aboveground plant biomass was as low as 7-17%, while fertilizer ¹⁵N retention in the soil nitrogen pool was considerably higher (32-42%). In the year after its application, fertilizer was of equal importance for plant N nutrition compared to the year of application, as illustrated by a plant ¹⁵N recovery of 9-14%. ¹⁵N recovery in mowed plant biomass then only slowly declined in the following years and stayed significant over the entire 5 years monitored in this study.

After five years, the cumulative ¹⁵N recovery rate in mowed biomass was 33 to 41 %. Considering ¹⁵N recovery in soil and roots after 5 years revealed a total ¹⁵N tracer recovery of 66% for the climate change treatment and 77% for the climate control treatment. These results show a rapid cycling of nitrogen through soil organic matter until remineralization and plant uptake. Furthermore, we reveal a minimal contribution of recent fertilizer nitrogen to plant nutrition and the dominance of soil organic nitrogen over fertilizer nitrogen for plant nutrition in such grasslands. The findings reinforce the concept that fertilizing such grasslands is largely a fertilization of soils rather than a fertilization of plants, thereby replenishing mineralized soil organic nitrogen (SON) stocks that is exported by harvests. Particularly under climate change conditions, the low N recovery rates of plant nitrogen, high plant N export and the rapid remineralization of soil organic nitrogen led to negative nitrogen balances.