Similar distribution of 15N labeled cattle slurry and mineral fertilizer in soil after one year

Targeted use of animal manures as a nitrogen (N) fertilizer is challenging because of their poorly predictable N fertilizer value. An enhanced understanding of their N transformation processes in soil under field conditions is necessary to better synchronize N availability and crop N demand. N-15 labeled cattle slurry, produced by feeding a heifer with N-15 labeled ryegrass hay, was used in an on-farm trial on two neighboring fields, cropped with maize or grass-clover, in order to assess crop N uptake and N dynamics in the topsoil. Recovery of applied total N in plant biomass was higher for mineral fertilizer (Min) (45-48%) than for slurry (Slu) (17-22%) when applied at the same rate of mineral N. Also, N derived from fertilizer in plant biomass was higher for Min than for Slu, due to both greater NH3 emissions and greater initial immobilization of slurry N. Despite initial differences between the two in the relative distribution of residual fertilizer N in soil N pools, already in the following spring the majority (77-89%) of residual N from both fertilizers was found in the non-microbial organic N pool. Of the applied total N, 18-26% remained in the topsoil after the first winter for Min, compared to 32-52% for Slu. Thus, the proportion of fertilizer N not taken up by the first crop after application, enters the soil organic N pool and must be re-mineralized to become plant available.

Automated summary

The targeted use of animal manures as nitrogen fertilizers is challenging due to their unpredictable nitrogen fertilizer value. This study aimed to better understand the nitrogen transformation processes in soil under field conditions and synchronizing nitrogen availability and crop nitrogen demand. It was found that the recovery rate of applied total nitrogen in plant biomass was higher for mineral fertilizer than for slurry, and the majority of residual nitrogen from both fertilizers was found in the non-microbial organic nitrogen pool.