Long-term fertilization effects on grain yield, water-use efficiency and soil fertility in the dryland of Loess Plateau in China

Wheat (Triticum aestivum L.) and corn (Zea mays L.) rotation system is important for food security in the Loess Plateau of China. Grain yield and water-use efficiency (WUE: grain yield per unit of water consumed) trends, and changes in soil properties during a 24-year fertilization experiment in Pingliang, Gansu, China, were recorded. Mean yields of wheat for the 16 years started in 1981 ranged from 1.29 t ha−1 for the unfertilized plots (CK) to 4.71 t ha−1 for the plots that received manure (M) annually with inorganic nitrogen (N) and phosphorus (P) fertilizers (MNP). Corn yields for the 6 years started in 1979 averaged 2.29 and 5.61 t ha−1 in the same treatments. Yields and WUEs declined significantly with lapse of time except CK and MNP for wheat. Wheat yields with the N and M declined at rate of 77 and 81 kg ha−1 year−1, but the decline of 57 kg ha−1 year−1 for NP was similar to that of 61 ha−1 year−1 for straw with N annually and P every second year (SNP). Likewise, the corn yields and WUEs declined from 160 to 250 kg ha−1 year−1 and from 0.01 to 0.03 kg m−3 year−1 among treatments, respectively. These declines were likely to loss of soil fertility and gradual dry weather. Yields were significantly correlated with seasonal evapotranspiration with slopes ranging from 0.5 to 1.27 kg m−3 for wheat and from 1.15 to 2.03 kg m−3 for corn. Soil organic carbon (SOC), total N (TN), and total P (TP) gradually built up 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. Soil available K declined rapidly without straw or manure. Balanced fertilization should be encouraged to ensure sustainable productivity in this intensive cropping system. The greatest SOC increases of about 160 mg ha−1 year−1 occurred in the SNP and MNP, suggesting that long-term additions of organic materials to soil could increase soil water-holding capacity which, in return, improves water availability to plants and arrests yield declines, and decrease CO2 emission from agricultural soils and sustain land productivity.