Abstract: 【Objective】Relying on fertile black soil resources, northeastern China has developed the biggest grain bases. Clarifying the characteristic of black soil fertility nutrients is essential for soil fertility improvement and maintenance in Northeast China. A long-term cultivated land quality monitoring experiment established in the 1980s was used to investigate the changes in soil nutrients and fertility in the black soil region.【Method】A temporal variation descriptive statistics analysis was conducted to explore the changes in soil organic matter (SOM), total nitrogen (TN), alkaline-hydrolyzable nitrogen (AN), available phosphorus (AP) and available potassium (AK), for three time intervals, including the first (1988-1997), middle (1998-2003) and final (2004-2013) monitoring period, during the study. The changes in soil carbon, nitrogen and carbon-nitrogen ratio (C/N) were analyzed so as to investigate relationship between the carbon and nitrogen budgets. Finally, principal component analysis was used to determine factors contributing to soil fertility, and the general scores of soil fertility at three monitoring periods were calculated. 【Result】 All soil nutrient concentrations showed increased with time and overall soil fertility improved after 10 to 26 years. The SOM, TN, AN, AP and AK significantly increased between the first (1988-1997) and final (2004-2013) monitoring period by 33.9%, 43.9%, 27.6%, 90.3% and 11.8%, respectively. Further analysis indicated that the AP concentrations of 71.4% monitoring sites maintained at a level of 15.0 to 50.0 mg·kg^-1, which meets crop nutrients requirement and would not be expected cause the ground or surface water pollution. However, the concentrations of AP at 28.6% of the monitoring sites exceeded the environmental threshold and some measures should be taken to reduce P fertilizer inputs in these sites. The ratio of C/N decreased with time from 10.3 in 1988 to 9.6 in 2013, decreased by 6.8%. Principal component analysis indicated that SOM and TN were the main factors contributing to soil fertility of the monitoring sites and the 5 fertility factors displayed an order of SOM＞TN＞AN＞AP＞AK. The average general scores of soil fertility at the first and middle monitoring periods were -1.099 and -0.541, respectively, while the black soil fertility has improved significantly and the score increased to 0.5888 at the final monitoring period.【Conclusion】Current farm fertilizer management improved black soil fertility, however a small proportion (28.6%) required better P fertilizer management so as to decrease the risk of ground or surface water pollution. More organic materials should be applied to the black soil to maintain the balance of soil C/N ratio and improve soil fertility.

Key words: black soil, conventional fertilization, long-term monitoring, available phosphorus, soil C/N ratio