soil organic carbon

physical properties

soil nitrogen fractions

Soil microbial biomass carbon

optimal fertilizer treatment

soil microbial biomass

carbon

Physicochemical parameters

Degradation

Sustainable development

We studied the effects of the long-term (17 years) application of different chemical fertilizer regimes on soil N and its inorganic fractions in relation to the soil physical properties in a sloping cropland in the hilly Loess Plateau. Seven treatments comprised of two factors were arranged within a randomized complete block design. As expected, crop yield increased 2-4-fold, but soil structure did not degrade. Under long-term fertilization, the portion of the small aggregates (<2 mm) and sand content were significantly decreased while the large aggregates (>2 mm) and the silt content increased by 276% and 7.4%, respectively, as compared with those in areas without fertilization. Moreover, the various continuous chemical fertilization treatments increased SOC content by 12.9% and total N by 12.4%, on average, compared with unfertilized plots. The SOC, total N, and shoot C, shoot N had close relationships with the large aggregates (>2 mm) and the clay content. The results suggest that, in this setting, the long-term addition of both N and P may sustain soil quality of an infertile sloping cropland in this region, compared to agriculture without fertilizer or applications of N alone.