Being the kernel of the traditional agriculture of China, and one of the major practices in developing ecological agriculture, intercropping still occupies an important position in modern agriculture in China as well as other parts of the world. One of the advantages of intercropping are higher yields relative to monocultures, which consequently removes more soil nutrients. However, in respect of high nutrient removal, few researches have been done on intercropping as on monocultures on a long-term scale.

To find a solution to this issue, a long-term field experiment was established in 2009 at Baiyun, Wuwei City, Gansu Province, Northwest China, to investigate changes in soil chemical properties in response to phosphorus (P) application and cropping system. The experiment was laid out in a two factorial design with three replicates. The first factor was the P application rate, designed to have three rates (0, 40 and 80 kg ha^-2 of triple superphosphate). The second factor was the cropping system: maize & fava bean, maize& soybean, maize & chickpea, maize & rapeseed and their corresponding monocultures. In 2013 and 2014, yields were measured and some soil chemical properties (soil organic matter, soil total N, Olsen P, available K, and pH) were examined after the crops were harvested to explore changes in soil fertility as affected by cropping system.

Results are as follow: 1) the grain yield of chickpea & maize, fava bean & maize, soybean & maize, and rapeseed & maize were 38.2%, 32.6%, 34.0% and 38.4% higher than their respective monocultures treatment regardless of the fertilization rate; 2) intercropping increased significantly the content of soil organic matter; 3) neither P application nor cropping pattern had any effect on soil total nitrogen concentration; 4) in 2013, intercropping decreased soil Olsen P by 5.2% under the chickpea & maize treatment, by 6.9% in fava bean & maize treatment, by 15.9% in soybean & maize treatment and by 11.3% in rapeseed & maize treatment, while it did not affect much in 2014; 5) intercropping significantly reduced available K (10.3% under chickpea & maize, by 14.1% under treatment fava bean & maize, by 8.5% under soybean & maize and by 13.2% under rapeseed & maize) over the two years: 6) there were no significant differences in soil pH between P applications and cropping systems in 2013 and 2014. To sum up, intercropping enhances productivity for at least five to six years. Regardless of P application, it can increase grain yield. This effect is intensified with P application rate up to 80 kg ha^-2. Moreover, intercropping tended to increase soil organic matter, reduce soil Olsen P and available K. But neither P application nor intercropping had much effect on soil total nitrogen or pH in 2013 and 2014. In conclusion, intercropping enhances not only crop productivity over time, but also improves stability and sustainability of soil fertility if coupled with proper applications of organic manure, P and K fertilizers.