Fate of ¹⁵N-labelled urea as affected by long-term manure substitution

Quantifying the fate of fertilizer nitrogen (N) is essential to develop more sustainable agricultural fertilization practices. However, the fate of chemical fertilizer N, particularly in long-term manure substitution treatment regimes, is not fully understood. The present study aimed to investigate the fate of ¹⁵N-labelled urea in a chemical fertilizer treatment (CF, 240 kg ¹⁵N ha^-1) and N manure 50 % substitution treatment (1/2N + M, 120 kg ¹⁵N ha^-1 + 120 kg manure N ha^-1) in two continuous crop seasons, based on a 10-year long-term experiment in the North China Plain (NCP). The results showed that manure substitution greatly enhanced ¹⁵N use efficiency (¹⁵NUE) (39.9 % vs. 31.3 %) and suppressed ¹⁵N loss (6.9 % vs. 7.5 %) compared with the CF treatment in the first crop. However, the N₂O emissions factor in the 1/2N + M treatment was increased by 0.1 % (0.5 kg ¹⁵N ha^-1 for CF vs. 0.4 kg ¹⁵N ha^-1 for 1/2N + M) compared with the CF treatment, although N leaching and NH₃ volatilization rates decreased by 0.2 % (10.8 kg ¹⁵N ha^-1 for CF vs. 5.1 kg ¹⁵N ha^-1 for 1/2N + M) and 0.5 % (6.6 kg ¹⁵N ha^-1 for CF vs. 2.8 kg ¹⁵N ha^-1 for 1/2N + M), respectively. In which, only NH₃ volatilization presented significantly difference between treatments. It is important to note that in the second crop, the residual ¹⁵N in soil (0-20 cm) remained mostly in the soil for the CF (79.1 %) and the 1/2N + M treatment (85.3 %), and contributed less to crop N uptake (3.3 % vs. 0.8 %) and leached losses (2.2 % vs. 0.6 %). This proved that manure substitution could enhance the stabilization of chemical N. These results suggested that long-term manure substitution effectively increases NUE, suppresses N loss, and improves N stabilization in soil, but negative impacts such as N₂O emissions due to climate change should be investigated further.