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We examined the effects of three fertilization treatments (CK, NPK, and NPKM) on soil pH, soil organic carbon (SOC), total nitrogen (TN), C/N ratio, and available nutrients [alkali extractable-N (AN), available phosphorus (AP), exchangeable K (EK)] in the plow layer (0–40 cm) in an apple orchard from 1988 to 2016 in North China, as well as the mechanisms of nitrogenous fertilizer action on soil acidification through a three-year experiment. The application of NPK and NPKM fertilizers improved SOC, TN, and available nutrients compared with the CK. The average C/N ratio decreased with the number of years of fertilization, and reached lowest in NPK treatment (6.80). By 2016, the soil pH in CK, NPK, and NPKM treatments was 6.17, 5.29, and 5.59, which were 0.16, 1.04 and 0.74, respectively, units lower than initial values. Compared with the previous 14 years (1988–2002), the latter 14 years (2002–2016) showed greater change rates of SOC, TN, C/N ratio, AN, AP, EK, and pH in all treatments. Further analysis showed that the highest net H+ production rate was found in the N900 treatment (136.8 kmol ha-1 y-1), followed by the N600 treatment (88.6 kmol ha-1 y-1), and the lowest appeared in the N300 treatment (50.6 kmol ha-1 y-1). Thus, long-term fertilization can remarkably increase SOC, TN, and available nutrients, especially in NPKM treatment; however, it also clearly decreased soil C/N ratio and pH due to the continuous excessive application of N fertilizers.

Long-term impact of fertilization on soil pH and fertility in an apple production system