Abstract: A field experiment was conducted to study the characteristics of paddy field nitrogen (N) leakage and runoff under rice-duck farming (MRD), conventio nal farming (MR), and conventional farming with flooding (CK). Comparing with th at under MR, the paddy field under MRD had a notable decrease of N (especially N O3--N) concentration in its leaked liquid; but this concentration was tended to be increased, compared with that under CK. After 7〖KG-*2〗-〖KG-*7〗9 days o f fertilization, the NH4+-N and NO3--N concentrations in paddy field su rface water were higher under MRD than under MR. However, owing to the no draini ng and the higher band, the paddy field under MRD had a notable reduction of drainage, resulting in a marked decr ease of N runoff than that under MR. Comparing with MR, the paddy field under MR D had an addition of nitrogen supply from duck dung, a reduction of N leakage an d runoff, a lesser application of chemical nitrogen fertilizer, and more nitroge n uptake by rice plant. Both the reduction of N input and that of N output in ri ce-duck farming system were nearly equal in quantity.

Key words: carbon isotopes, photosynthetic gas exchange., chlorophyll fluorescence, mesophyll conductance, photosynthesis, isotope ratio mass spectrometry