Long-term fertilization increases soil nutrient accumulations but decreases biological activity in navel orange orchards of subtropical China

With its high economic benefits, navel orange (Citrus sinensis) has been widely planted and fertilizer has been increasingly applied in the subtropical China in the last 30 years. Comprehensive assessments are needed to explore the long-term fertilization impacts on soil chemical and biological properties in the navel orange orchards. Through a large number of soil and leaf samples from the young, middle-aged, and mature navel orange orchards, this study examined the impacts of stand age (corresponding to the fertilization year using compound chemical fertilizer) on seasonal variations in major soil properties and leaf nutrients in the subtropical China. Soil total nitrogen (N) and mineral N were significantly higher in the middle-aged and mature orchards than in the young orchard. Total phosphorus (P), available P, labile P, slow P, occluded P, weathered mineral P, total exactable P, and residual P generally increased with fertilization years (P < 0.05), and the increasing percentages for soil P fractions were much higher than those for N variables. The total N and P use efficiencies (plant uptake/soil input) were 20-34 and 10-15 %, respectively. Soil microbial biomass, invertase, urease, and acid phosphatase activities showed significant seasonal variations and decreased with fertilization years. Leaf N concentration significantly decreased with fertilization years, but no difference was found for P. Soil self-fertilization was impeded, and less fertilizer amount should be applied especially in the older navel orange orchards since N and P accumulations do not increase leaf nutrients but worsen soil biological quality.