Sulfur nutrition status in trees and soils of ‘Guanximiyou’ pomelo orchards

Abstract:【Objective】‘Guanximiyou’pomelo is a product of national geographical indication in Pinghe county (24°02′-24°35′ N，116°53′-117°31′ E), Zhangzhou city, Fujian province. The planting area ofhoney pomelo in Pinghe county was approximately 4.67×104 hm2 with a yield about 120×104 t in 2017. The planting area, yield and export volume all rank the first in the country. Sulfur is an essential nutri- ent element for plant growth and development. It participates in many important physiological and metabolic processes in plants. Citrus is considered as“Clsensitive crop”, therefore, farmers have used po- tassium sulfate, magnesium sulphate and other sulfur-containing fertilizers for a long time. The fertilizer application amount for honey pomelo in Pinghe county ranks the first among the main citrus producing counties in China. Long-term excessive fertilization has caused problems such as soil acidification, nutrient enrichment, and nutritional imbalances, which seriously affect the yield and quality of honey pomelo. Therefore, integrated nutrient management in honey pomelo orchards is very important. In order to provide theoretical and practical basis for the rational application of sulfur-containing fertilizers, soil sulfur content status, soil sulfur distribution characteristics and factors affecting sulfur content in  honey pomelo orchards were analyzed. The relationships of sulfur enrichment with soil acidification, and with calcium and magnesium leaching were discussed.【Methods】314 representative honey pomelo orchards for soil and leaf sample collection were selected from 10 major pomelo producing towns (Wen- feng, Qiling, Luxi, Jiufeng, Nansheng, Guoqiang, Banzai, Shange, Xiazhai and Xiaoxi) in Pinghe coun- ty, Fujian province. In order to explore the distribution characteristics of available sulfur in the soil pro- file, 27 honey pomelo orchards with planting years of 15 or more were selected, and soil samples from three soil layers (0-20 cm, 20-40 cm and 40-60 cm) were collected at 10 cm inward from the canopy drip line for avoiding fertilization ditch. A total of 81 soil profile samples from honey pomelo orchards were collected. 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 col- lected from the market in Pinghe county for test of total sulfur content on dry weight basis【. Results】Soil available sulfur content in the honey pomelo orchards was in a range of 10.98-116.65 mg·kg-1 and the average value was 55.63 mg·kg-1. The soil available sulfur content was classified into the high (>30 mg·kg-1), the moderate (16-30 mg·kg-1) and the low (<16 mg·kg-1) levels, which were 83.44%, 13.06% and 3.50%, respectively. The average content of soil available sulfur of pomelo orchards in all towns ex- ceeded the high (30 mg·kg-1) level, and the proportion of high (>30 mg·kg-1) samples exceeded 70%. Soil available sulfur content increased with the increases in soil organic matter content, cation exchange capacity (CEC) and clay content. The results also showed that soil available sulfur content increased with the ages of orchard. There was a significant negative correlation between soil available sulfur con- tent and soil pH in the honey pomelo orchards. The average value of soil available sulfur content in the soil layers at 40-60 cm, 20-40 cm and 0-20 cm was 72.50 mg·kg-1, 97.81 mg·kg-1 and 141.70 mg·kg-1, respectively. The average value of soil available sulfur content in the 40-60 cm soil layer was 95.45% and 44.87% higher compared with the 0-20 cm and the 20-40 cm soil layers, respectively. Correlation analysis showed that the content of soil exchangeable calcium and exchangeable magnesium were ex- tremely significantly negatively correlated with the content of available sulfur. The sulfur content in the leaves ranged from 0.24% to 0.53%, with an average value of 0.34%. The leaf sulfur content of all hon- ey pomelo orchards was higher than 0.2%, and the proportion of the moderate (0.2%-0.4%) and the high (>0.4%) levels were 86.94% and 13.06%, respectively. The coefficient of variation and the proportion of samples with high soil available sulfur content were 3.00 times and 6.39 times higher than that of leaf sulfur content, respectively, indicating that excessive application of sulfur fertilizer affected sul- fur absorption by honey pomelo.【Conclusion】There was a problem of excessive sulfur in the soil and leaves in honey pomelo orchards in Pinghe county. Soil available sulfur enrichment led to soil acidification. Sulfur is easily leached in the soil, which promotes the leaching of calcium and magnesium in the soil. The use of sulfur-containing fertilizers should be controlled in Pinghe county. It is necessary to carry out research on the effect of partial chloride- sulfur substitution on yield and quality of pomelo.