| Paddy-upland rotation is a planting pattern that is conducive to the production of rice and dryland crops,which is mainly distributed in the Yangtze River Basin in China.In this region,there are rice-wheat,rice-oilseed rape,rice-vegetable and other planting patterns.As an important agricultural production base,the status of agricultural production in Yangtze River Basin is directly related to the food security supply of China.However,the demand for food is increasing,while the acreage of arable land is difficult to increase.Thence,more fertilizers were applied to the soil to increase yields,resulted in low crop nutrient utilization and serious agricultural non-point source pollution,which seriously damaged the sustainability of agricultural production.In Yangtze River Basin,the soil available phosphorus content was difficult to increase in proportion to the accumulation of soil phosphorus and the phosphorus utilization efficiency was low.In order to provide a scientific basis for the efficient utilization of phosphorus in farmland in this region,the availability of soil phosphorus and its influencing factors were explored in different crop rotation systems.The experiment was conducted at the Institute of Agricultural Sciences in Rugao County,Jiangsu Province from 2018 to 2020.Four paddy-upland rotation systems in the experiment included rice-wheat（R-W）,rice-oilseed rape（R-O）,rice-cabbage（R-C）,and rice-fallow（R-F）rotation.Three fertilization treatments under each rotation system were applied,including no fertilization treatment（CK）,no phosphate treatment（NK）,and NPK fertilization treatment（NPK）.By analyzing the crop yield and biomass,phosphorus uptake by aboveground crops,soil phosphorus fraction contents,soil microbial biomass and soil alkaline phosphatase activity under different phosphorus application conditions in dry season and rice season maturity,the variation patterns and main influencing factors of soil phosphorus balance and availability under different paddy and upland rotation systems were clarified.The key research finding are as follows:1.The rice-oilseed rape rotation system was conducive to maintaining the stable yield of rice crops and increasing the utilization of phosphorus fertilizer.The rice-oilseed rape rotation under the NK treatment increased the yield by 3.4%-4.3%compared with the rice-wheat and rice-fallow rotation in rice season.Different from the NK treatment,the rice-oilseed rape rotation under the NPK treatment had the smallest increase in yield compared with other crop rotations,which was only 6.6%.2.The rice-oilseed rape rotation system was conducive to maintaining the relative stability of the soil available phosphorus content.The imbalance of soil phosphorus in NK treatment was the most obvious.Under the NK treatment,the soil available phosphorus content in dry season of R-O rotation was significantly lower than that of other rotations,while the difference was no significant in rice season.Similarly,there was no significant difference between R-O rotation soil phosphorus removal and surplus and other rotations.The dynamic changes of soil available phosphorus showed that the decline rate of soil available phosphorus content in R-O rotation was smaller in the four rotations under CK treatment.3.The supplementary amount of soil labile phosphorus pool and the reduction of moderately labile phosphorus pool of rice-oilseed rape rotation system were higher than other rotations.Specifically,the relative content of soil labile phosphorus in R-O rotation in the first dry season under NK treatment was 5.7%-7.3%lower than other rotations,and the relative content of soil moderately labile phosphorus were 4.2%-6.4%higher than other rotations.However,the relative content of soil labile phosphorus in R-O rotations under NK treatment in rice season was 0%-1.5%lower than other rotations,and the soil moderately labile phosphorus was 0.5%-3.0%higher than other rotations.Subsequently,the relative content of soil labile phosphorus in R-O rotation in the second dry season under NK treatment was 3.0%-6.7%lower than other rotations,and the relative content of soil moderately labile phosphorus were 2.9%-4.2%higher than other rotations.The path analysis model showed that different active phosphorus pools all contributed to the soil available phosphorus,but the labile phosphorus pools contributed the most,and their total effects reached 0.60 and 0.75 in rice and dry season,respectively.In addition,Na HCO3-Pi,Na OH-Po,d.HCl-Pi,d.HCl-Pi and d.HCl-Po were the most important factors affecting the soil available phosphorus in rice season,while the soil available phosphorus in dry season were mainly regulated by Na HCO3-Pi,Na OH-Pi,Na OH-Po,c.HCl-Pi and Residual-P.4.The rice-oilseed rape rotation system can maintain a higher level of soil alkaline phosphatase activity in dry and rice seasons and reduce the soil microbial biomass C/P ratio in rice season.Compared with other rotations,the ratios of microbial biomass N/P and C/P of rice-oilseed rape rotation under NK treatment were significantly lower than those of rice-wheat rotation,while the soil alkaline phosphatase activity was 0.4%-25.7%higher than other rotations.The path analysis model showed that soil microbial biomass（0.56）and soil alkaline phosphatase（-0.51）contributed had the highest overall contribution to the soil available phosphorus in dry and rice seasons,respectively.And the soil alkaline phosphatase and microbial biomass stoichiometry had the greatest direct effects in the process of soil available phosphorus conversion in dry and rice seasons,respectively.In summary,when the soil phosphorus was relatively imbalance,rice-oilseed rape rotation released more alkaline phosphatase in dry season and regulated the soil microbial biomass C/P ratio in rice season,which was conducive to promoting the activation of the non-labile phosphorus by microorganisms to supplement the labile phosphorus,so as to ensure the soil available phosphorus content was relative stable without affecting crop yield and phosphorus output. |
