Long-term fertilizer and water availability effects on cereal yield and soil chemical properties in Northwest China

Wheat (Triticum aestivum L.) and corn (Zea mays L.) rotation is important for the region's food security in Northwest China. Grain yield and water-use efficiency (WUE: grain yield/estimated evapotranspiration [ET]) trends, and changes in soil properties during a 24-yr rainfed fertilization experiment in Pingliang, Gansu, China, were recorded. Mean wheat yields for the 16 yr ranged from 1.29 Mg ha(-1) for the unfertilized plots (CK) to 4.71 Mg ha(-1) for the plots that received manure (M) annually with inorganic N and P fertilizers (MNP). Corn yields for the 6 yr averaged 2.29 and 5.61 Mg ha(-1) in the same treatments. Yields and WUEs declined with years except for the CK and MNP treatments for wheat. Wheat yields for the N and M treatments declined about 80 kg ha(-1) yr(-1), compared with about 60 kg ha(-1) yr(-1) for the NP treatment and the N plus straw treatment receiving P every second year (SNP). Likewise, the corn yields and WUEs declined significantly for all treatments. Grain yield-ET relationships were linear with slopes ranging from 0.51 to 1.27 kg ha(-1) m(-1) for wheat and 1.15 to 2.03 kg ha(-1) m(-1) for corn. Soil organic C (SOC), total N (TN), and total P (TP) gradually increased with time except the CK, in which TN and TP remained unchanged but SOC and available P (AP) decreased. Soil AP decreased in the N treatment. Soil available K declined rapidly without straw or manure additions. The greatest SOC increases of about 160 mg kg(-1) yr(-1) occurred in SNP and MNP treated soils, suggesting that long-term additions of organic materials could increase water-holding capacity that, in return, improves water availability to plants and arrests grain yield declines, and sustains productivity.