Effects Of Different Long-term Cropping Systems On Phosphorus Forms And Availability Of Red Soil In Panxi Area

Phosphorus（P）is the main factor limiting crop production in red soils.The P fertilizer was applied in regions with red soil to maintain the high crop yield.However,the supply of bioavailable P for crop plants is insufficient due to the strong adsorption and fixing behaviors in red soil.In this study,we explored the P fractions and availability in red soil under different long-term cropping systems.We also analyzed that how the cropping systems affect P forms,adsorption-desorption characteristics and bacterial community structure of a P mineralising gene（pho D）via changing soil physicochemical properties under the experimental conditions.This study could provide a scientific reference for improving the P availability of red soil and sustainable development of agriculture.The main results were as follows:（1）The total P,available P contents and P activation coefficient in topsoil were higher than those in subsoil.The value of relative P parameters of pea-corn rotation in topsoil was the highest among all cropping systems.The organic P was the main kind of P fraction in red soil,and the contents of high effective water-soluble P（H₂O-P）and sodium bicarbonate extractable P（Na HCO₃-P）were lower than other P fractions.Except for pea-corn rotation soil,the sodium hydroxide extractable P（Na OH-P）was the main kind of P fraction of other cropping systems,which was the largest potential P source in red soil.In addition,p H,cation exchange capacity,Fe/Al oxides and soil texture were significantly correlated to the available P content（P<0.05）,which were the main factors affecting the P availability in red soil.（2）The capacity of P adsorption and desorption increased constantly with increasing exogenous P concentration;however,the change in the percentage of desorbed P had no clear trend.The soils under pea–corn rotation and taro continuous cropping had lower P adsorption capacities and higher P desorption capacities compared to those under the other cropping systems.The maximum P adsorption capacities of pea–corn rotation and taro continuous cropping soils were 16.1%and 32.4%of abandoned farmland soil,while the corresponding desorption coefficients were 6.9 and 1.6 times higher than that of abandoned farmland soil,respectively.In addition,the zeta potential of soil colloids decreased with soil p H increasing.Among the cropping systems,the absolute value of zeta potential was lowest in tobacco–walnut intercropping,while its isoelectric point was the highest,resulting in the highest P adsorption capacity.The main factors affecting P adsorption were available P,cation exchange capacity,free iron oxides and sand content,and the main factors affecting P desorption were available P,sand and clay content.（3）The available P content and pho D abundance were the highest in pea-corn rotation soil,while alkaline phosphatase（ALP）activity was the lowest in this soil.And soil available P was significantly positively correlated with pho D abundance,while it was significantly negatively correlated with ALP activity（P<0.05）.Moreover,the pho D-harboring bacterial community was dominated by the phyla Proteobacteria and Actinobacteria,with Cupriavidus and Bradyrhizobium being the dominant genera.Some bacterial species have multiple functions,indicating the coupling of P processes with other nutrient（e.g.,C and N）cycles.The RDA model selection procedure revealed that ALP,soil organic carbon and N:P ratio were the significant predictors for the pho D-harboring bacterial community structure across all samples.Meanwhile,structure equation model（SEM）showed that the effects of total N and available P were stronger than SOC on soil pho D-harboring bacteria and soil texture was also one of the main factors affecting the pho D-harboring bacterial community structure,which had a significant up-regulation effect on the abundance and diversity of pho D community and indirectly regulate gene compisition by affecting available P.（4）The different long-term cropping systems affect the P fractions and availability in red soils by changing physicochemical properties,such as SOC,cation exchange capacity and soil texture.Among the eight long-term cropping systems,pea-corn rotation soil had higher contents of total P,available P,H₂O-P and Na HCO₃-P.Moreover,in pea-corn rotation soil,the capacity of P adsorption was low,the capacity of desorption was high.The abundance and diversity of pho D-harboring bacterial community were the highest in pea-corn rotation soil,which was conducive to the P activation.In conclusion,pea-corn rotation was the most suitable system to improve P availability of red soil in Panxi area.