Abstract: A long-term (1982-2001) field experiment was conducted on a calcareous soil at Zhangye, Gansu under wheat (Triticum aestivum L.)-wheat(Triticum aestivum L.)-corn (Zea mays L.) rotation system in order to determine the effects of long-term fertilization on crop yield, content and accumulation of nitrate-N in soil profiles. Twenty-four plots in a split-plot include eight treatments [nitrogen (N), phosphorus (P), potassium (K) and farmyard manure (M) application] and 3 replicates were designed. Main treatments were M and no M, and the sub-treatments were no-fertilizer (CK), N, NP and NPK. In P and K were part of treatments, their ratio to N was 1N﹕0.22P﹕0.42K. All M, P and K fertilizers were applied as the basal dressing. The crops were harvested for grain yield in all years and for straw yields from 1988 to 1997. After crop harvest in 2000, the soil was sampled from the 0-20, 20-60, 60-100, 100-140 and 140-180 cm depths to determine NO3--N content. The twenty-year grain yield decreased in the order of MNPK≥MNP＞NPK＞MN＞NP＞M＞N＞CK. Corn yield of CK in 2000 was only 28.2% of that in 1984, and wheat in 2001 was 25.7% of that in 1982. Average impact of fertilizers on grain yield decreased in the order of N＞M＞P＞K. Yield response to N and P fertilizers increased with progression of the experiment. K fertilizer showed no effects on increase of grain yield during the initial 6 years (1982-1987), moderate effects in the next 5 years (1988-1992), and dramatic effects in the last 9 years (1993-2001). The straw yield trend was similar to grain yield. Accumulation and distribution of NO3--N in soil was significantly affected by annual fertilizations. Chemical fertilizers (NP and NPK) caused NO3--N accumulation in most subsoil layers, with major impact in the 20-140 cm depth. The combination of chemical fertilizers and M (MNP and MNPK) reduced soil NO3--N accumulation as compared to chemical fertilizers alone. In conclusion, long-term fertilization significantly improved grain and straw yield in this rotation. The findings suggest that it is important to balance the application of chemical fertilizers and manure in order to protect soil and underground water from potential NO3--N pollution while keeping high productivity in the Oasis agro-ecosystem.

Key words: Long-term fertilization, Zhangye oasis, Crop yields, Accumulation of nitrate N, N balance sheet