Effects Of Farmland Poplar Shelterbelts On Soil Multiple Properties And Underlying Mechanism Analysis In Northeast China

A comprehensive understanding of the changes in multiple functional indicators and physicochemical properties of soil after afforestation of farmland shelterbelts is a prerequisite for the management of existing shelterbelts and the renovation of degraded shelterbelts,and can also provide a theoretical basis for the evaluation of the impact of reforestation projects on the soil in China.So,we collected soil samples from the poplar shelterbelts and adjacent farmland in the Northeast Plain and measured multiple soil indicators.Firstly,the change mechanism of phosphorus（P）and carbon（C）in poplar shelterbelt soils compared to farmland soils was analyzed.Secondly,the difference and formation mechanism of soil properties between poplar shelterbelt and farmland were explored through paired analysis and positional analysis.Thirdly,farmland-shelterbelts differences with various soil characteristic indicators were comprehensively analyzed in 750 soil samples on a large regional scale.Finally,the management suggestions of farmland shelterbelts were proposed based on our results.The results showed as follows:1.The changes of soil P and its components:the P content in three particle size aggregates（0.25-2 mm:macroaggregates,0.053-0.25 mm:microaggregates,and<0.053 mm:silt+clay）and nine sequential leachable fractions（water-extractable inorganic P:H₂O-Pi,Na HCO₃extractable inorganic P:Na HCO₃-Pi,Na HCO₃ extractable organic P:Na HCO₃-Po,Na OH extractable inorganic P:Na OH-Pi,Na OH extractable organic P:Na OH-Po,HCl extractable inorganic P:HCl-Pi,concentrated HCl extractable inorganic P:conc.HCl-Pi,concentrated HCl extractable organic P:conc.HCl-Po,and Residue P:Re-P）was determined in farmlands and shelterbelts in the 0-40 cm soil layer.Our data revealed that poplar shelterbelts had 27.75%lower soil TP than neighboring farmlands,mainly occurring in macroaggregates（0.25–2 mm）with no changes in microaggregates and silt+clay.In leachable P fractions,the eight P fractions content of poplar shelterbelts were lower than those of farmlands,but it did not reach a significant level,and reduced TP was caused by changes in HCl-Pi（low plant-available）,and Na OH-Pi（medium plant-available）contributed 47.8%and 34.2%,respectively.Unlike other fractions,the Na OH-Po content increased by 23.34%.Combining soil fungal data,we found that the macroaggregates P reduction was mainly driven by fungal community changes,while the fraction-P changes were more closely related to soil p H and tree growth of poplar trees.2.Soil organic carbon（SOC）and inorganic carbon（SIC）changes and isotopic analysis:in order to explore the soil C changes and mechanisms after poplar shelterbelts construction,we measured the SOC and SIC content and isotopic(δ¹³SOC andδ¹³SIC)characteristics of the bulk soil and three aggregates.The results showed that at the 0-40 cm layer,poplar afforestation significantly reduced the mass proportion and SOC content of the silt+clay,but significantly increased the SIC content.Poplar afforestation reduced theδ¹³SOC value in the bulk soil and aggregates,and the proportion of new SOC in macroaggregates（21%）was higher than that of microaggregates（20%）and silt+clay（19%）.The proportion of new SOC in the bulk soil was positively correlated with the proportion of silt+clay.The proportion of new SOC in silt+clay was significantly positively correlated with MAP and theδ¹³SIC in bulk soil and aggregates.Theδ¹³SIC characteristics of poplars did not conform to pure plant soil-forming secondary carbonates,and the new SIC input ratio had not been calculated.Afforestation changes the influence factors theδ¹³ value of SOC and SIC.The SIC content of bulk soil and aggregates was significantly negatively correlated with nitrification,denitrification,phototrophy,and was also significantly correlated with Ectomycorrhizal,Dung Saprotroph-Plant Saprotroph,Arbuscular Mycorrhizal.The correlation between the above microbial functions andδ¹³SOC in macroaggregates was opposite to that of SIC content.In the 40-100cm soil layer,the proportion of three soil aggregates and SOC and SIC changed little.3.The overall influence and formation mechanism of shelterbelt construction in Northeast Plain:in five regions of the Northeast Plain,480 samples of 1 m deep 5-layer（each layer was20 cm thick）were collected for two times（positioning）in poplar shelterbelts and neighboring farmlands（pairing）,and 21 soil indexes were determined,and repeated pairing analysis and location analysis were performed.The results showed that repeat the pairing method to analyze the results of the two assays found that the changes in poplar shelterbelt compared with farmland soil had a high consistency.The consistency in the 0-20 cm layer and the 0-100 cm was high（76.92%-84.62%）,while the consistency in the 60-80 cm and the 80-100 cm was slightly low（46.15%-61.53%）.Short-term fixed plot sampling,which can explain the changes of some indicators of paired plots,including BD,TP,SOC,SIC and TK content/storage in the0-20 cm.Among them,the consistency of poplar compared with farmland TP and BD decreased because the TP and BD of poplar soil decreased faster than that of farmland soil.Due to some constraints in our short-term fixed plot sampling,some results have not successfully explained the change of paired plots.Overall,paired plots were more suitable for exploring the change of poplar compared to farmland.In addition,annual rainfall,longitude and tree density had significant effects on soil changes in poplars compared with farmlands.Based on the above,we expanded the study area to 15 regions 1 m deep and analyzed the differences of 16 soil characteristic indexes and their geoclimatic-microbial mechanisms in 750soil samples.The results showed that the poplar shelterbelts significantly improved the soil physical properties and and increased SIC content,p H and EC,but reduced TP and NO₃^--N content compared with farmlands in the 0-40 cm soil layer.Poplar afforestation decreased microbialαdiversity,the undefined saprotroph-fungi abundance but increased ectomycorrhizal fungi dominance compared with farmlands.The complexity and stability of the soil microbial network in poplar shelterbelt soils were slightly higher than those of farmland soils,and the key species of the networks are mostly fungi.Compared with farmland soils,the microbial nitrification and denitrification were decreased（9.25%-11.67%）in shelterbelt soils.Afforestation also leads to a decrease in NO₃^--N and WC in the 40-100 cm layer.Poplar shelterbelts growth had the greatest effect on increasing EC,p H,Porosity,SIC（0.700）and reducing NO₃^--N,TP,BD（-0.763）,among which the indirect influence of microbial function increased by about 40%of the direct impact of poplars,and the above effects were enhanced in areas with high precipitation and temperature areas.The influence of poplar shelterbelts on soil properties was more significant in the 0-40 cm layer than in the 40-100 cm layer.4.Based on the results of this study,the main suggestions for future shelter forest research were:first,fully considering the relationship between soil fungi and soil P may be more conducive to managing soil nutrients in agroforestry complex ecosystems,and more soil physicochemical,fertility and soil aggregates characteristic indicators need to be considered when evaluating the impact of farmland shelterbelts.Second,the evaluation of soil C sink function of shelterbelts needs to consider both SOC and SIC,and the latter changes even more significantly in Northeast China.Third,the shallow trench partition（about 40 cm）may be conducive to the absorption of NO₃^--N and TP leakage from farmland deep soil by farmland shelterbelts.These provide a new basis and data support for the management of existing farmland shelterbelts and the transformation of degraded shelterbelts.