Objectives We studied the effects of substituting chemical fertilizer with organic fertilizer on growth, nutrient accumulation and yield of Ponkan, and soil microbial properties under equal nutrient supply. We determined the appropriate quantity of organic fertilizer that can be substituted for chemical fertilizer, providing the theoretical basis for reducing the latter and enhancing the efficiency of agricultural technology.

Methods Using 15-year-old Taiwan Ponkan (Citrus reticulata Blanco cv. Ponkan) grafted on Poncirus trifoliata (L.) as test material, field experiments were conducted in 2019 and 2020 in Chongqing. The six treatments included no fertilizer application i.e., control (CK), pure chemical fertilizer application (FP), N, P and K substitution ratio of 10%, 25%, and 18% (T1), substitution ratio of 15%, 38%, and 27% (T2), substitution ratio of 20%, 51%, and 36% (T3), and substitution ratio of 30%, 76%, and 54% (T4). Except for CK, the treatments’ N, P and K nutrient input was consistent. The growth, nutrient accumulation, yield and fertilizer contribution to yield and soil microbial properties were analyzed.

Results 1) The root activity and growth of branches with organic fertilizer were higher than FP. Organic fertilizer application (P<0.05) increased the proportion of fine roots (0–0.5 mm). Compared with FP, SPAD, P_n, PI_abs and PI_total were promoted to a certain extent, increasing with organic fertilizer application at first and subsequently decreasing. Moreover, T2 and T3 recorded higher Ponkan growth than other treatments. 2) In 2019 and 2020, branch accumulation and fruit N, P and K in the organic fertilizer substitution treatments was higher than FP. However, there were fluctuations in the recorded increase. Compared with FP, the yield in each organic fertilizer substitution treatment increased by 7.2%–26.4% and 2.0%–36.9%, respectively, compared with FP, and the fertilizer contribution rate increased by 5.2%–45.4% and 3.3%–54.9%. With increasing organic fertilizer application, fruit yield and fertilizer contribution rate fluctuate, with T2 and T3 recording higher values. 3) Sucrase and urease enzyme activity and the number of cultivatable bacteria and actinomycetes treated with organic fertilizer were higher than FP, reaching the highest in July and increasing with organic fertilizer application. The number of cultivatable soil fungi increased at first and subsequently decreased, with T3 and T4 having higher soil microorganisms.

Conclusions Substituting chemical fertilizer with organic fertilizer under equal nutrient supply can promote the growth and development of lateral roots, especially fine roots and spring and autumn shoots, enhance the photosynthesis of leaves, improve nutrient accumulation in leaves, twigs and fruits, especially yield of Ponkan and promote soil microbial characteristics. Overall, the optimum substitution ratio was 15% to 20% for N, 38% to 51% for P, and 27% to 36% for K.