Distribution And Transformation Characteristics Of Phosphorus Fractions In Acid And Limestone Forest Soil With Soil Aggregate Structure

Soil parent material,vegetation type as well as the pedogenesis stage together effect the amounts and speciation of total phosphorus（TP）,inorganic phosphorus（P_I）and organic phosphorus（P_O）in soils;whereas the processes of precipitation-dissolution as well as adsorption-desorption of P_I,mineralization-immobilization of P_O control the nutrient supply for plant growth and the potential for P loss leaching to surface water in the natural forest ecosystem.Limestone soil is rich in calcium and magnesium which has strong bondings towards P at higher p H value;whereas acid red soil is rich in iron and aluminum which most of P are bonded by strongly adsorption fixation and deposition processes.In this study,a karst limestone soil and an acid red soil from South China which were selected and the characteristic of concentration of P_I and P_O in the bulk soil and soil aggregates,which from O/A and AB horizons,were determined using continuous extraction apporoaches.Here,P_I includes Exchangeable P（Ex-P）,Aluminum P（Al-P）,Iron P（Fe-P）,Occluded P（Oc-P）,Autologous P（Au-P）,Detritus P（De-P）and Po includes labile organic P（LP_O）,moderately labile organic P（MLP_O）,stable organic P（SP_O）and microbial biomass phosphorus（MBP）.In addition,the impact of environmental factors on the distribution of P species were explored in the bulk soil and soil aggregates.Meanwhile,the transformation mechanism among different fractionations of P and the availability of different soil P were examined through determining the activities of phosphatase and phytase.Moreover,the characteristics of P adsorption were estimated using Langmuir and Freundlich fitting equations,combining with phosphorus sorption index（PSI）and degree of P sorption saturation（DPS）,the risk of P leaching in the two soils were evaluated.The main findings of this study are as follows:1)Soil types have great impact on the distribution of P in bulk soil and soil aggregates.In general,limestone soil had more abundant phosphorus storage than the red soil aggregates.In red soil and limestone soils of O/A and AB horizons,residual phosphorus（Res-P）was the main form in two soil horizons.The limestone soil had higher Ex-P content than the red soil,suggesting a higher bioavailable P content in the former.The soil aggregates of the limestone soil possessed higher contents of TP,P_I,P_O and lower content of MBP,indicating that the stable soil structure in this soil could protect P within soil aggregates.Compared with limestone soil,the contents of P_Ifractions were lower in the soil aggregates of the red soil except for Fe-P;among the P_O fractions,the content of MBP were higher and the content of SP_O equivalent to limestone soil,whereas MLPo were lower,indicating that limestone soil aggregates have more abundant P storage than red soil aggregates,as well as the important promoting roles of P_O and MBP in the P supply in red soil.2)The relationship among different P_I fractions were weaker in the red soil relative to the limestone soil,whereas P_I fractions had close correlation with P_Ofractions,suggesting MBP,LP_O,MLP_Oand Al-P could be direct source of Ex-P.In addition,phytase activity was strongly positively correlated with P_O fractions,indicating P_O as the potential source of bioavailable P in red soil.The strategy of P acquisition by plants and microorganisms is mainly through the enhancement of phosphatase and phytase activities,promoting rapid mineralization of P_O and accelerating P turnover between red soil-plant-microorganisms adapted to phosphorus-limited ecosystems.Whereas in the limestone soil of developed on phosphorus-rich bedrock,the different P_I fractions were closely correlated,whereas the relation between P_I and P_O fractions were much weaker,indicating that the stable P_I（mainly the calcium-bound P）could be the potential sourse of transformation of bioavailable P.3)The maximun phosphorus adsorption（Q_m）value in the bulk soil of the O/A horizons of these two soils were similar,but the red soil had greater phosphorus adsorption constant（k _L）and maximum buffering capacity（MBC）values,therefore had a stronger buffer than the limestone soil.In the AB horizon,the limestone soil had a greater Q_m but a smaller k _L;it suggests a lower adsorption strength in spite of more adsorption sites,leading to a lower MBC.In the red soil aggregates,the Q_m value was generally lower in the micro-aggregates,indicating that the macro-aggregates play in more important role in soil P adsportion.Whereas in the limestone soil,except for o.25-0.5 mm aggregate with the lowest Q_m value,the Q_m value of the other aggregates has the same level,indicating that the different sizes aggregates has a strong phosphorus adsorption ability.The maximum value of k _L occurred at 0.25-0.5 mm aggregates for all soils.As a result,MBC increased and then decreased with the decrease of soil aggregate size.It indicates that the 0.25-0.5mm aggregates has the greatest contribution to P buffering in the soils.4)The PSI values of the bulk soils of these two soils were similar and all the DPS values were below 1.1%,suggesting that due to little human disturbance in the natural forest soils,the DPS is low.In the O/A horizon,the available P associated DPS(DPS_M3and DPS_Olsen)and the Fe associated DPS(DPS_citrare)were greater in the red soil than the limestone soil.In the AB horizon,different DPSs were similar except for DPS_M3 and DPS_citrare which weer higher in the red soil.In the soil aggregates,PSI had the trend to increase and then decrease with the decrease of soil aggregate size,with the maximum value at 0.25-0.5 mm aggregate,futher inferring the importance of the aggregate in soil P adsorption.In general,PSI in the soil aggregates of the limestone soil was greater than of the red soil,but the difference was only significant in the<0.106 mm aggregate of the AB horizon.The results suggest that the capacity to adsorb P in these two soils is generally similar with a slight lower value in the red soil.The<0.106 mm aggregate of the red soil had the greater DPS values(except DPS_M3)than other soil aggregates of this soil and those of the limestone soil.Considering red soil<0.106 mm aggregate constituted the greatest proportion of soil aggregates in the red soil,its higher DPS suggests a potentially higher P leaching risk in this soil aggregate.All the together,comparing these two soils,the limestone soil has more sufficient P storage and higher P bioavailability,suggesting a better P supply.Although the red soil with smaller P pools,the link between organic and inorganic P was closer.Together with higher enzyme activities,the organic P could be mineralized to alleviate the lack of bioavailable P in soil.The P leaching risk was low in both soils,with 0.25-0.5 mm aggregates having stronger P fixation,while in the red soil,<0.016 mm aggregates had a higher potential P leaching risk due to soil structural instability and weak P adsorption and buffering capacity.