The Effects Of Constructing Mixed Forests On Soil Aggregate Stability And Nutrient Content After Clear-cutting Of Eucalyptus Grandis Pure Forest

Eucalyptus grandis is an important afforestation tree species for industrial raw material forests in southern China.However,due to the large planting areas of Eucalyptus grandis with multiple generations of continuous cultivation and short-cycle extensive rough management,a series of problems such as decreased soil nutrient content,soil acidification,and degradation of soil quality have been caused.Constructing mixed forests after clear-cutting of Eucalyptus grandis pure forests is one of the important ways to improve the soil quality.However,the effects of construction of mixed forests（excluding Eucalyptus grandis species）after clear-cutting of Eucalyptus grandis pure forests on the soil aggregate stability and nutrient change are still unclear.To fill this gap,We selected mixed forest of Cinnamomum camphora×Metasequoia glyptostroboides（the Cinnamomum camphora mixed forest for short）,mixed forest of Cunninghamia lanceolata×Phoebe zhennan×Prunus persica’Atropurpurea’（the Cunninghamia lanceolata mixed forest for short）and mixed forest of Prunus persica’Atropurpurea’×Malus×micromalus（the Prunus persica’Atropurpurea’mixed forest for short）as the research object,and selected pure forest of Eucalyptus grandis as the research control group at the demonstration site of Eucalyptus grandis pure forest replacement and transformation in Shawan District,Leshan City,Sichuan Province.We divided the soil samples into whole soil and aggregates with particle size of>5mm,5-2mm,2-0.25mm and<0.25mm after collecting intact soil by using the five-point sampling method to study the influence of the mixed forests on the stability of soil aggregates and nutrient content.The main control factors of soil physics and chemistry affecting the stability of soil water stable aggregates and nutrient content were explored.The main results are as follows:（1）Compared with the pure Eucalyptus grandis forest,all three mixed forests showed a trend of reducing soil bulk density.Only the Cinnamomum camphora mixed forest significantly increased the soil natural moisture content and soil total porosity.All three mixed forests generally increased soil p H,soil organic carbon（SOC）,total nitrogen（TN）,total phosphorus（TP）and total potassium（TK）.Soil p H and SOC content of the Cinnamomum camphora mixed forest and the Prunus persica’Atropurpurea’mixed forest were significantly higher than those of the pure Eucalyptus grandis forest.Soil TN content of the Prunus persica’Atropurpurea’mixed forest was significantly higher than that of the pure Eucalyptus grandis forest.Soil TP and TK content of the three mixed forests were significantly higher than those of the pure Eucalyptus grandis forest.（2）Compared with the pure Eucalyptus grandis forest,all three mixed forests generally increased the mass proportion of water stable aggregates of>5 mm and 5～2 mm size fractions and the values of mean weight diameter（MWD）,geometric mean diameter（GMD）,macroaggregate proportion(R_0.25,%)of soil water stable aggregates.They all generally decreased the mass proportion of aggregates of 2～0.25 mm and<0.25 mm size fractions and the values of fractal dimension（D）and percentage of aggregate disruption（PAD,%）.The mass proportion of water stable aggregates with particle size of>5 mm and MWD of the Cinnamomum camphora mixed forest and Prunus persica’Atropurpurea’mixed forest were significantly higher than those of the pure Eucalyptus grandis forest.The PAD of the Cinnamomum camphora mixed forest and Prunus persica’Atropurpurea’mixed forest were significantly lower than those of the pure Eucalyptus grandis forest.The mass proportion of aggregates with particle size of 5～2 mm of the Cunninghamia lanceolata mixed forest and the Cinnamomum camphora mixed forest were significantly higher than that of the pure Eucalyptus grandis forest.The mass proportion of aggregates with particle size of 2～0.25 mm and D of the three mixed forests were significantly lower than those of the pure Eucalyptus grandis forest.The mass proportion of aggregates with particle size of<0.25 mm of the Cinnamomum camphora mixed forest were significantly lower than those of the pure Eucalyptus grandis forest.The GMD and R_0.25 of the Cinnamomum camphora mixed forest were significantly higher than those of the pure Eucalyptus grandis forest.（3）Compared with the pure Eucalyptus grandis forest,the Cinnamomum camphora mixed forest and the Prunus persica’Atropurpurea’mixed forest generally significantly increased SOC content in the aggregates of each particle size,and they also was a trend to increase the TN content in the aggregates of each particle size fraction.SOC content and TN content in each particle size aggregate of the Cunninghamia lanceolata mixed forest were no significant different from those of the pure Eucalyptus grandis forest,except that TN content in aggregates with a particle size of 2～0.25 mm of the Cunninghamia lanceolata mixed forest was significantly lower than that of the pure Eucalyptus grandis forest.All three mixed forests significantly increased TK content in aggregates of different particle sizes,but had no significant effect on the TP content in aggregates of each particle size.（4）Pearson correlation analysis results showed that the contents of SOC,TN and TP in the whole soil were highly significantly positively correlation with WSA_{>5 mm},MWD,GMD,R_0.25,p H,SOCAS2～0.25 mm,SOCAS<0.25 mm,TNAS<0.25 mm,TPAS>5 mm and TPAS2～0.25 mm,and they were highly significantly negatively correlated with WSA_{2⁰.25 mm},WSA_{<0.25 mm},D and PAD.The TK content of the whole soil was significantly positively correlated with WSA_{>5 mm},MWD,p H,TP,TPAS>5 mm,TKAS>5 mm,TKAS 5～2 mm,TKAS 2～0.25 mm,GMD and TPAS 5～2 mm,and it was significantly negatively correlated with WSA_{5² mm} and WSA_{2⁰.25 mm}.MWD,GMD and R_0.25 were highly significantly positively correlated with WSA_{>5 mm},p H,SOC,TN,TP,the content of SOC in water stable aggregates of each particle size,TN_{AS>5 mm},TN_{AS2⁰.25 mm} and TN_{AS<0.25 mm},and they were highly significantly negatively correlated with WSA_{2⁰.25 mm} and WSA_{<0.25 mm}.D and PAD were highly significantly positively correlated with WSA_{2⁰.25 mm} and WSA_{<0.25 mm},and they were highly significantly negatively correlated with WSA_{>5 mm},p H,SOC,TN,TP,the content of SOC in water stable aggregates of each particle size,TN_{AS>5 mm},TN_{AS 2⁰.25 mm} and TNAS<0.25 mm.（5）The results of redundancy analysis（RDA）showed that organic carbon content in water stable aggregates of<0.25 mm size fraction(SOC_{AS<0.25 mm})and soil p H were the most significant factors affecting the nutrient content of the whole soil,with the interpretation degree of 48.5%and 40.5%respectively.Water stable aggregates of<0.25 mm size fraction(WSA_<0.25mm),SOC_{AS<0.25 mm} and water stable aggregates of>5 mm size fraction(WSA_{>5 mm})are the most significant factors affecting the stability of water stable aggregates,with the interpretation of88.1%,60.5%and 7.0%respectively.In summary,the three mixed forests increased the SOC,TN,TP and TK nutrient content by generally improving the stability of soil water stable aggregates and soil p H.The three mixed forests also improved the stability of water stable aggregates by generally promoting the transformation of small to large sized aggregates,increasing the mass proportion of large aggregates,increasing SOC and TN nutrient content in water stable aggregates,and increasing soil p H.Among the three mixed forests,the Cinnamomum camphora mixed forest（3Cinnamomum camphora:1 Metasequoia glyptostroboide）can be preferred for the transformation of the pure Eucalyptus grandis forest in the study area because of its more obvious role in improving soil physical properties,soil p H and nutrient content of the whole soil and aggregates,as well as enhancing aggregate stability.