Effects Of Soil Moisture Alteration And Organic Carbon Addition On Phosphorus Transformation And Rice Growth Characteristics In Paddy Soils

【Object】As an essential and growth-limited nutrient element in agroecosystems,phosphorus（P）plays important roles in microbial metabolism,primary productivity,and crop quality.In general,P cycling in agroecosystem is closely related to some key issues,such as global food security and environmental pollution.Therefore,how to activate soil legacy P,improve the availability of P,reduce environmental pollution and avoid the depletion of P resources have attracted growing concerns in current.In recent years,the frequent occurrence of global extreme climate changed in precipitation pattern,resulted in agricultural soil suffering frequent soil moisture fluctuation and drying/wetting alteration.The significant fluctuation of soil water content easily happens in rice cultivated paddy fields due to agriculture water management practices in rice-upland crop rotation systems,especially in south China.The significant change in soil moisture influenced soil physicochemical and biological properties,it also significantly affected phosphorus transformation and availability,consequently impacted the growth and yield of rice.Organic carbon（OC）addition can not only activate soil non-labile P through competing adsorption sites and chelating with soil metal ions,but also provide carbon source for microorganisms and regulate P-cycling related microbial community to.However,it is not clear how OC addition affects soil physicochemical properties and P transformation under soil moisture alteration in paddy fields.In this study,field investigated experiment,microcosmic experiment and pot experiment（soil-plant system）were establisned,three paddy fields were selected in typical rice producing areas in China,the influences of iron reduction mediated by Fe RB community on P availability under soil moisture alteration and organic carbon addition were deeply studied.【Methods】In this study,three paddy fields（HN,ZJ,XJ）were selected in typical rice-producing areas in China.The responses of soil P transformation and related microorganisms to soil moisture alteration and carbon addition were studied via field investigation,soil microcosmic incubation experiment and pot experiment（soil-plant system）.Firstly,the effects of land conversion from paddy to upland soil)on P transformation,microbial metabolic limitation and soil quality were explored through theory of ecological enzyme stoichiometry and vector model.Secondly,two-factor experiment was performed（soil moisture×carbon addition）,the responses of soil P transformation and availability to soil moisture alteration（dry-wet cycles with fast frequency,HDW,140-20%WHC;dry-wet cycles with slow frequency,LDW,140-20%WHC）and carbon addition(pig manure,1.0 mg C g^-1soil)were investigated via soil P sequential extraction technique under 25±0.5℃condition.Thirdly,in pot experiment,the effects of soil moisture alteration（（1）continuously flooded,CF,180%WHC;（2）periodically flooded,PF,180-70%WHC）and carbon addition（（1）without anything addition,CK;（2）applied N fertilizer without C addition,N;（3）applied N fertilizer with C addition,CN）on P transformation and availability mediated by Fe through different Fe fractions extraction approach.Meanwhile,quantitative real-time polymerase chain reaction（q PCR）and high-throughput sequencing technology were employed in pot experiment to determine the responses of microorganisms related to P transformation（phoD gene community,Fe RB/Fe OB communities）;rice growth to soil moisture alteration and carbon addition were also revealed.Finally,the key factors（physicochemical and biological factors）and dominant microbial strains/taxa driving the variations of soil P availability and rice yield were identified.【Results】（i）Land conversion from paddy to upland soil significantly decreased soil total organic P（T-Po）,available P（Olsen-P）,microbial biomass（MBP,MBN,MBC）and C/N/P-requiring enzyme activities,while land conversion has no significant effect on soil total P.Soil microbial relative metabolic limitations of P（4.2%）and C（12.7%）were significantly aggravated by land conversion.Compared with the paddy fields,soil structural stability index（SSI）,soil biological activity（SBA）,soil quality index（SQI）and ecosystem multifunctionality（EMF）in upland soil were reduced by 18.7%,5.2%,17.5%and 17.6%,respectively.（ii）Soil pH in acidic soils（HN,ZJ）was decreased,while it was increased in alkaline soil（XJ）under the condition of dry-wet alteration.The reduction of soil moisture（140→20%WHC）significantly decreased the contents of Fe²⁺,DOC and MBP.Compared with the LDW treatment,the contents of Fe²⁺,DOC and MBP in the HDW treatment were significantly lower.Pig manure addition significantly increased MBP and DOC by 15.4%and 21.7%,respectively.However,it decreased the content of Fe²⁺in paddy soil.（iii）The reduction in soil moisture（140→20%WHC）significantly decreased soil labile P（Ex-Pi,Al-Pi,L-Po）and moderately labile P（Fe-Pi,ML-Po）,while increased soil occluded Pi（Oc-Pi）.Compared with the LDW treatment,the treatment of HDW significantly increased soil labile inorganic P（Ex-Pi,Al-Pi）and moderately labile organic P（ML-Po）,while decreased soil occluded Pi（Oc-Pi）and labile organic P（L-Po）.Pig manure addition significantly increased soil labile and moderately labile P across the three tested soils,while it decreased soil non-labile inorganic P（Ca-Pi）and organic P（NL-Po）by 27.3%and16.9%,respectively.（iv）Compared with the LDW treatment,the treatment of HDW reduced acid phosphatase（ACP）activity by 17.8%in acid soil,and decreased alkaline phosphatase（ALP）activity by 13.2%in alkaline soil.Irrespective to soil types,pig manure addition significantly increased ACP activity and ALP activity.Additionally,the key factors in influencing soil P availability were Fe²⁺,pH and L-Po in HN,ZJ and XJ sites,respectively.（v）In contrast to the CF treatment,soil pH value in the periodically flooded treatment（PF）tended to be lower in acid soil but higher in alkaline soil.Meanwhile,the PF treatment significantly decreased soil DOC（10.1%,averagely）,Fe²⁺（32.8%,averagely）and microbial biomass（MBP,MBN,MBC）when compared with the CF treatment.Pig manure addition significantly increased soil DOC and microbial biomass（MBC,MBN,MBP）,while decreased soil Fe²⁺content.（vi）Compared with the CF treatment,the PF treatment significantly decreased soil labile inorganic P（Na HCO₃-Pi）,moderately labile inorganic P（Na OH-Pi）and labile organic P（Na HCO₃-Po）,while it significantly increased non-labile organic P（con-HCl-Po）.Pig manure addition could counteract the decrease of soil labile inorganic P and organic P caused by soil moisture decline,suggesting that the reduction of soil available P by the PF treatment could be compensate by the feedback mechanisms of soil enzyme and nutrient such as the increases in soil ACP and ALP activity under pig manure addition（p<0.05）.（vii）The PF treatment increased Chao1 index of phoD gene communities by 22.0%,15.3%and 31.2%in HN,ZJ and XJ sites,respectively.Soil moisture alteration and pig manure addition significantly impactedβ-diversity of phoD community.Meanwhile,compared with the CF treatment,the PF treatment significantly promoted soil iron conversion from non-labile Fe fractions（Fe＿mag,Fe＿prs）to oxidized Fe fractions（Fe＿ox）,while pig manure addition promoted Fe transformation from oxidized Fe fractions to non-labile Fe fractions.The PF treatment increased the Shannon index and microbial network stability of Fe RB/Fe OB community,and positively affected the complexity of Fe OB,but it negatively influenced the complexity of Fe RB community.Pig manure addition increased the complexity of Fe RB/Fe OB network.Moreover,regardless soil types（HN,ZJ,XJ）,the taxa of Anaeromyxobacter,Bacillus and Clostridium were the dominate strains for Fe RB communities,respectively,and Anaerolinea was the dominate strain for Fe OB communities across all treatments.In addition,the gene copy number of phoD in rice tillering stage of rice（53 DAS）was lower than in panicle initiation stage（136 DAS）in PF treatment.（viii）In soil-planting system,PF treatment significantly reduced the plant height（4.6%）,dry matter（10.8%）,P uptake（11.8%）and yield（6.1%）of rice in these three soils compared with CF treatment.However,compared with CF treatment,the root/shoot ratio（R/S）of rice in PF treatment increased by31.6%,28.3%and 42.3%in HN,ZJ and XJ soils,respectively.In addition,pig manure addition could compensate for the limitation of rice growth and reduction of yield caused by water stress and nutrient（P）limitation.【Conclusion】（i）Land conversion from paddy to upland soil significantly reduced soil total Po and soil P availability,but aggravated microbial relative P and C metabolic limitations.The change in different P fractions proportion was mainly regulated by the decrease of microbial biomass（C/N/P）and P-cycling related enzyme activities caused by soil moisture alteration.At the same time,soil quality and soil ecosystem multifunctionality were significantly degraded under this land conversion system.（ii）Dry-wet alteration and periodically flooded changed soil pH.In acidic soils pH was decreased after dry-wet cycles,while soil pH in alkaline soil（XJ）was increased.Pig manure addition had no significant effect on soil pH in these three soils.In addition,both the HDW and PF treatments significantly reduced soil DOC,Fe²⁺and microbial biomass C/N/P,indicating that frequent soil moisture alteration promoted the rapid decomposition of soil DOC and adversely affected microbial growth.Pig manure addition increased soil DOC by additional C supplementing,this strengthened iron oxidation by promoting microbial metabolism and enhanced Fe²⁺adsorption,resulting in a decrease in Fe²⁺content.（iii）Compared with the low frequency dry-wet alteration treatment（LDW）,high frequency dry-wet（HDW）improved soil P availability（Ex-Pi,Al-Pi）by reducing the soil occluded P（Oc-Pi）,and promoted soil organic P（ML-Po）by reducing the enzyme activities related to P transformation and soil MBP.The PF treatment significantly decreased labile inorganic P（Na HCO₃-Pi）,moderately labile inorganic P（Na OH-Pi）and labile organic P（Na HCO₃-Po）,but it increased soil non-labile P（Pi,Po）.The decrease in moderately labile inorganic P（Na OH-Pi）was closely associated with the conversion of inactive Fe（Fe＿mag,Fe＿prs）to oxidized Fe in the PF treatment.In addition,pig manure addition off-set the decrease of soil P availability caused by the low frequency dry-wet alteration（LDW）and periodically flooded treatments（PF）.（iv）The decrease in soil P availability under periodically flooded treatment（PF）was positively responded to the increases in P-related enzymes activity,phoD gene copies,α-diversity of phoD gene community and stability of Fe RB/Fe OB network.Pig manure addition counteracted the limitation of soil P availability by regulating phoD gene community composition,increasing P-related enzymes activity,the gene copies andα-diversity of phoD gene community and complexity of Fe RB/Fe OB network.（v）Compared with the CF treatment,the PF treatment significantly reduced rice height,1000-grain weight,dry matter,P uptake and rice yield,which was related to higher root/shoot ratio（R/S）and lower soil P availability under soil water stress in PF treatment.Pig manure addition counteracted the limitation of rice growth and yield reduction caused by water stress and P limitation.Therefore,our findings highlighted that organic substance addition plays a significant effect on regulating soil P availability in paddy fields,and it could compensate for P limitation and rice yield reduction caused by soil moisture alteration.