Sulfur Nutrition Status Of Citrus And Effects Of Sulfur Enrichment On Soil Microorganism And Nutrient Leaching

Citrus is an important commercial crop which is traditionally considered as"chlorine-sensitive crop".The long-term application of sulfur-containing fertilizers such as potassium sulfate compound fertilizer and magnesium sulfate,etc.has caused sulfur accumulation in citrus orchard soil.Therefore,it is of great significance to clarify the supply status of soil sulfur in citrus orchard and its effect on soil environment and citrus growth.Soil and leaf sulfur status of 322 pomelo?Citrus grandis?orchards of Pinghe County and 31 ponkan?Citrus reticulata?orchards of Yongchun County in Fujian Province were measured and determined in this study.The effects of different sulfur levels on soil enzyme activity,microorganism and the leaching loss of nutrients were studied by soil incubation test and soil column leaching test.The pot test was used to explore the feasibility of replacing sulfur with chlorine on the condition of high sulfur level supplied and to analyze the potential risks.The purpose was to provide scientific evidence for the reasonable application of sulfur and chlorine containing fertilizer in citrus orchards.The main results are as follows:1.Sulfur nutritional status of soil and leaf in citrus orchard:?1?The available sulfur content of soil in the pomelo orchards ranged from14.59 to 113.20 mg·kg^-1,with an average value of 56.27 mg·kg^-1,and that of 86.65%samples exceeded high content(>30 mg·kg^-1).The available sulfur content of soil in the ponkan orchard was from 31.23 to 310.37mg·kg^-1,with the average value of 128.57 mg·kg^-1,and that of all of the samples exceeded high content.The available sulfur content of soil went up with the increase of total sulfur content,soil organic matter content,CEC and clay content.The available sulfur content of soil was increasing with the extension of planting years.The average available sulfur content of soil in the soil profile showed the trend like 0?20 cm<20?40 cm<40?60cm.These results proved that both of the pomelo orchard in Pinghe County and the ponkan orchard in Yongchun County had the problem of sulfur accumulation in soil.?2?The sulfur content of pomelo leaves ranged between 0.26 and 0.50%with an average of 0.34%while that of ponkan leaves ranged from 0.24 to0.84%,with an average of 0.31%.The sulfur content of leaves of all pomelo and ponkan was equal to or greater than optimum?i.e.,either between 0.2 and 0.4%or above 0.4%?,indicating that the sulfur nutrition of pomelo and ponkan were sufficient.2.The effects of different sulfur levels on soil enzyme activity and microorganism showed that with the increase of sulfur content,soil urease activity increased first followed by the decreased,protease activity decreased and acid phosphatase activity increased.The high-throughput sequencing results showed that high sulfur levels extremely significantly reduced the overall abundance and diversity of soil bacterial communities,among which the relative abundance of Bacteroidetes,Firmicutes and Spirochaetes increased,but six bacterial phylum levels including Proteobacteria reduced their relative abundance and reached extremely significant level.High sulfur levels also apparently reduced in the fungal community overall abundance,among which the relative abundance of Penicillium extremely significant increased,and the relative abundance of Trichoderma and Chaetomium reduced greatly,but the effect on the diversity of fungi was not significant.3.According to the result of the soil column leaching test which is applied to research the effects of different sulfur dosages on the leaching loss of soil nutrients,it proved that compared with the control treatment of equal nitrogen content,increasing sulfur content in soil could promote the leaching loss of sulfur(SO₄^2-),nitrogen?NH₄⁺-N and NO₃^--N?,phosphorus?DTP,DRP,DOP?and cations(K⁺,Ca²⁺,Mg²⁺).In the leachates of first three times leaching,the leaching loss amount of sulfur was significantly positively correlated with that of nitrogen?N?,phosphorus?P?,potassium?K?,calcium?Ca?and magnesium?Mg?.4.The results of substituting sulfur with chlorine on the condition of high sulfur levels indicated that on the condition of high sulfur levels sulfur replaced by chlorine would make ponkan seedlings infected with chlorine toxicity.Chlorine toxicity would significantly reduced the photosynthetic pigment content and PS?reaction center function of the seedlings,causing the decreased of the net photosynthetic rate.Besides,O₂·^-production rate,H₂O₂ content,MDA content and relative electrical conductivity of ponkan seedlings influenced by chlorine toxicity significantly increased,and leaf membrane lipid peroxidation was intensified.Those proved that there was the risk of chlorine toxicity by replacing sulfur with chlorine at high sulfur levels,and the feasibility needed further analysis.In conclusion,soil and plant in citrus orchard are rich in sulfur nutrients.High sulfur levels significantly affect the nitrogen-related enzyme activity,microbial overall abundance and diversity,the leaching loss of SO₄^2-,nitrogen,phosphorus and base ions in the soil.However,there was a risk of Cl-toxicity on ponkan seedlings by replacing sulfur with chlorine at high sulfur levels,and the feasibility needs to further analysis.