Effects Of Nitrogen Fixing Tree Species On Po Composition And Transformation Of Soil Aggregates In A Subtropical Plantation

Phosphorus(P)has always been regarded as one of the most critical limiting nutrient factors for the improvement of productivity and ecological service function of tropical and subtropical forests,especially artificial forests,which seriously hindered the sustainable development of plantations in this area.Therefore,making full use of the potential P sources in soil and looking for new forest management measures and methods to improve the retention and effectiveness of P in subtropical soil is one of the most important conditions to improve the productivity and ecosystem service function of plantation.Taking the key scientific problem of how to improve the soil P supply potential and availability of artificial forest as the research goal,this study selected Eucalyptus urophylla plantation(PP1),E.urophylla/Acacia mangium plantation(MP1),Pinus massoniana plantation(PP2)and P.massoniana and E.fordii Oliv.plantation(MP2)as research objects,using the method of field monitoring and indoor analysis.The structure and function of soil microbial community were characterized by phospholipid fatty acid and soil enzyme activity respectively,and combined with environmental factors,the soil and soil aggregates were studied after after long-term mixed planting of N₂-fixing tree species introduced into Eucalyptus urophylla plantation or Pinus massoniana plantation:(1)the variation of P content and its components,(2)the variation of microbial community structure and function(soil enzyme activity),and(3)the relationship between Po transformation and microbial and environmental factors.Through the study of this problems,this paper expounds the main regulation mechanisms and key control factors of the effects of mixed planting of N₂-fixing tree species on soil P retention and availability in Eucalyptus urophylla plantation or Pinus massoniana plantation from the ecosystem level,which provides an important scientific basis for the application and popularization of N₂-fixing tree species in improving soil quality and soil nitrogen(N)and P availability in subtropical plantations.In order to provide theoretical and technical support for the establishment of mixed forests with higher productivity,better economic value and stronger ecosystem services to replace the original low-quality and low-efficiency Eucalyptus or Pinus massoniana plantation.The main results are as follows:(1)The mixed planting of N₂-fixing tree species in Eucalyptus or Pinus massoniana plantation significantly increased the N content in bulk soil and all aggregate-sized fractions,and the content was the highest in<0.25mm aggregate size fraction.Compared with PP,TN content in the MP soil increased significantly in the bulk soil in bulk soil,>2 mm,1-2 mm,0.25-1 mm and<0.25mm size fractions,respectively,39.53%,30.01%,50.33%,52.06%and 67.48%.Compared with PP2,MP2 significantly increased by 32.35%,15.93%,15.20%,11.97%and 31.97%in bulk soil and all aggregate-sized fractions,respectively.The content of SOM in MP increased significantly by 122.38%,29.89%,41.95%,33.12%,33.34%in bulk soil and all aggregate-sized fractions,respectively.But,there was no significant difference in SOM between MP2 and PP2.(2)Compared with PP1,the content of labile P in MP1 increased significantly in bulk soil and>2mm,1-2mm and<0.25mm aggregates,but the content of highly recalcitrant P decreased significantly in bulk soil and soil aggregates.The change trend of Po content in MP1 increased with the decrease of aggregate size,and the content was the highest in micro-aggregates,and there was a positive correlation between labile P with mean weight diameter(MWD).However,compared with PP2,only highly recalcitrant P in MP2 decreased significantly in bulk soil and soil aggregates.Except that there was a significant negative correlation between moderately labile P and MWD in soil aggregates with>2mm aggregate,the other P components had no significant correlation with MWD in bulk soil or soil aggregates.(3)Microbial biomass C(MBC),N(MBN)and P(MBP)in MP1 were significantly higher than those in PP1,in bulk soil and all soil aggregateswith particle size decreasing,and showed a significant positive correlation with MWD.Although MBN and MBP in MP2 were significantly higher than those in PP2,there was no significant correlation between them and MWD.(4)Compared with PP1,the soil enzyme activities in MP1 increased significantly,in which NAG,ACP and PDE increased significantly in the original soil and all aggregate-sized fractions,while LAP only increased in the bulk soil,>2mm,0.25-1mm and<0.25mm size fractions.The activity of NAG,LAP,ACP and PDE of Eucalyptus plantation increased with the decrease of particle size,and the activity was the highest in micro-aggregates,and there was a significant positive correlation between NAG,ACP and PDE with MWD.However,compared with PP2,only NAG,LAP and PDE in MP2 changed significantly,in which NAG increased significantly in bulk soil,>2 mm and 1-2mm size fractions,LAP increased significantly in bulk soil,>2 mm and 0.25-1mm size fractions,while PDE increased significantly in bulk soil and all aggregate-sized fractions.Moreover,in Pinusmassoniana plantation,most of the enzyme activities related to N and P transformation were not significantly correlated with MWD in bulk soil and soil aggregates.(5)In Eucalyptus plantation,the results of structural equation model showed that the introduction of N₂-fixing tree species promoted the growth of AMF and finally improved the transformation of soil Po by increasing soil SOC content.However,in Pinus massoniana plantation,the introduction of N₂-fixing tree species could not significantly increase the content of soil SOC,and there was no significant change in labile P,moderately labile P and moderately resistant P and ACP.