Fate of 15N-labelled urea when applied to long-term fertilized soils of varying fertility

Quantifying the fate of nitrogen (N) fertilizer is essential to develop more sustainable agricultural N management practices. However, our understanding of N losses, particularly in low fertility soils remains incomplete. We evaluated the fate and N use efficiency of N fertilizer under different long-term fertilization regimes, i.e., no N; synthetic N; manure plus synthetic N in a calcareous Cambisol in the North China Plain. A standard rate (160 kg N ha−1) of 15 N-labelled urea was applied to the above treatments in summer maize (first crop) and the same amount of unlabelled urea was applied to winter wheat (second crop). We found the manure plus synthetic N treatment had a significantly higher fertilizer N use efficiency (56%) with lower residual fertilizer N in soil (47 kg N ha−1) than the synthetic N treatment (46% and 64 kg N ha−1, respectively), due to the better synchrony of fertilizer N supply and crop demand in the manure plus synthetic N treatment. Surprisingly, compared with the synthetic N treatment, application of N fertilizer to the N-deficient treatment increased fertilizer N use efficiency significantly to 68%, and reduced the residual fertilizer N in soil (31 kg N ha−1). Fertilizer N losses accounted for 11–16% of applied 15N-labelled urea with no significant differences between treatments. We found that fertilizer N use efficiency was increased in the high fertility soil supplied with manure compared with the low fertility soil supplied with synthetic N fertilizer, which emphasized the importance of recycling the manure or crop residues to soil.