Soil copper (Cu) nutrient status and its influencing factors in pomelo orchards in Pinghe county, Fujian province

Abstract:【Objective】Citrus is the leading fruit crop in the world, and citrus production and consumption have grown significantly. In 2014, there was approximately 2.5×106 hm2 of citrus with a production of 3.6×107 t in 19 provinces in China. Pomelo (Citrus grandis) is the third largest citrus with a yield of3.2×106 t. In China, the most famous area for pomelo production is Pinghe county (24°02′-24°35′ N, 116°53′-117°31′ E) , in south Fujian province with an average annual rainfall of over 1 600 mm. Currently, the quality of‘Guanximiyou'pomelo is declining. Quality and yield depends upon integrated nutrient management, which would not be possible without the knowledge of soil fertility. Copper (Cu) is one of the essential microelements in plant. In the citrus production, spraying Cu bactericides or fun-gicides is general considered a standard measure for controlling canker disease. Both Cu deficiency and toxicity cause adverse effects on yield and quality in citrus, and there is limited information available about copper fertilizer application and factors influencing soil Cu content. The aim of our study was to understand soil Cu content and relevant impact factors in Pinghe.【Methods】Our study adopted the data from analyses of soil copper (Cu) content and soil physical and chemical properties. 315 soil samples at depth of 0-40 cm were collected from representative‘Guanximiyou'pomelo orchards in ten townships (Wenfeng, Qiling, Luxi, Jiufeng, Nansheng, Guoqiang, Banzai, Shan'ge, Xiazhai and Xiaoxi) in Pinghe county. And 48 soil samples at depths of 0-20 cm, 20-40 cm and 40-60 cm for measuring soil Cu leaching traits in old orchards (>15 year) with severe soil copper enrichment in Xiaoxi town and Xiazhai town. Most of the soil samples were from 10 to 15-year-old pomelo orchards and collected from September to October at harvest season. Each soil sample consisted of 10 sub-samples (two sub-samples per plant, five plants per orchard) taken near the canopy drip line. After air drying, the soil samples were gently ground, sieved (0.850 mm and 0.150 mm) and properly stored before analysis. In addition, 55 samples of commercial organic fertilizers were collected from a fertilizer distributor in Pinghe for test of total Cu content based on dry weight.【Results】In pomelo orchards, the mean values of soil available N, P and K were 104.17 g· kg-1, 185.79 g· kg-1 and 214.30 g· kg-1, respectively, and the mean values of soil p H, soil organic matter (OM) and soil cation exchange capacity (CEC) were 4.34, 23.20 g· kg-1 and 1.64 cmol· kg-1, respectively. According to the classification standard of soil nutrients, the soil was acidic with high fertility. However, the soil physical and chemical properties of the basal soil in the pomelo orchards in different towns had significant differences. The mean values of soil total Cu content (with a range of 3.0-176.9 mg · kg-1) and available Cu content (with range 0.01-29.62 mg · kg-1) were30.15 mg· kg-1 and 5.20 mg· kg-1, respectively. The percentage of soil samples with high Cu availability reached 28.89% and only 0.63% of the soil samples had excessive total Cu content. The mean values of soil total Cu content and available Cu content were higher in Luxi town and Xiaoxi town than in the other towns. Total soil Cu content was significantly positively correlated with soil available Cu content (r =0.588**) . The soil available Cu content gradually decreased with soil depth, the percentage of soil samples with high available Cu content was 95.83%, 60.42% and 39.58% in 0-20 cm, 20-40 cm and 40-60 cm soil depths, respectively. Meanwhile, our results also showed that the soil available Cu content increased with the increases of planting years, soil CEC and soil OM content in pomelo orchards. Additionally, the total Cu content in commercial organic fertilizers was 63.77 mg · kg-1 in average with a range of 4.3-291.0 mg· kg-1. 21.82% of the organic fertilizer samples had excessive copper, while there was no excessive copper in inorganic fertilizer samples.【Conclusion】In conclusion, due to long-term unreasonable use of Cu fertilizers and related materials, there is increasing problem of excessive enrichment in soils in pomelo orchard in Pinghe county. To prevent the risk of Cu pollution, attention should by paid upon the inspection of copper content and proper management of Cu containing fertilizers and materials for the sustainable development of pomelo production in Pinghe county.