For a long time, farmers in the red soil region of southern China have returned crop residues to the soil, but how various crop residues influence nitrous oxide (N 2 O) emissions is not well understood. We compared the influence of returning different crop residues [rapeseed cake (RC), maize straw, rice straw and wheat straw (WS)] in combination with different levels of nitrogen (N) fertilizer (nil, low and high) on red soil N 2 O emissions. Results confirmed the inverse relationship between cumulative N 2 O emissions and residue C:N ratio in red soil under different levels of N fertilizer. However, Nfertilizer application did not significantly influence N 2 O emissions in the WS (which had the highest C:N ratio) and corresponding control treatments, while it enhanced N 2 O emissions in the RC (which had the lowest C:N ratio) treatment and displayed significantly higher cumulative N 2 O emissions with low N fertilizer application. This phenomenon may be attributed to the poor nutrient content in red soil, which leads to 'Liebig's Law of the Minimum' on available C. N fertilizer application provided sufficient available N, while the readily available C, which was mainly dependent on the degradability of the residue, became the crucial factor influencing N 2 O emissions. Additional experiments, which showed that the addition of glucose and sucrose could increase N 2 O emissions when N (NH þ 4 À N) was sufficient, confirmed this hypothesis. Thus, to reduce N 2 O emissions when returning residues to red soil, we suggest that both the residue C:N ratio and the quality should be considered when deciding whether to apply N fertilizer.

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