Effects of inorganic fertilizer inputs on grain yields and soil properties in a long-term wheat-corn cropping system in south China

A long-term double cropping wheat (Triticum aestivum L.) and corn (Zea mays L.) experiment was conducted at Qiyang, Hunan, China, to study the effects of inorganic fertilizers on grain yields and soil properties and to identify the possible causes of yield trends. Six treatments of unfertilized control, N, NP, NK, PK, and NPK were included. The treatments (N, NP, NK, and NPK) where inorganic nitrogen (N) was added showed significant (P < 0.05) yield declines of 76 to 114 kg ha(-1) yr(-1) for wheat and 94 to 260 kg ha(-1) yr(-1) for corn, respectively, except for corn yield in the NPK treatment in which the decline was not significant during a 15-yr (1990-2005) period. Comparatively, the decline amounts in corn were much higher than in wheat. The yields of wheat and corn remained unchanged in the PK treatment. The total organic carbon (C), total N, phosphorus (P), and available P, potassium (K), copper (Cu), and zinc (Zn) contents of soil were either increased or decreased (luring the study period. Both the decreases of exchangeable calcium (Ca2+) and magnesium (Mg2+) and increases of exchangeable hydrogen (H+) and Al3+ contents of soil in the treatments where inorganic N was applied were significant (P < 0.05). The same four treatments showed significant pH declines ranging from 0.07 to 0.12 yr(-1). Several lines of evidence point to decline of soil pH due to inorganic N fertilizer added as leading to the overall yield decline of wheat and corn. However, the yields of both crops increased significantly after lime application. In the long term, the farmers should be encouraged to use adequate lime based on a balanced approach to ensure sustainable productivity.