Effects Of Different Planting Patterns Under Walnut Forest On Soil Organic Carbon Pools

Walnut trees are widely planted all over the world because of their high economic value,long life and drought tolerance.With the increase of walnut cultivation area,there have the stronger competition among farmland and forestland.More and more attention has been paid to the construction of suitable planting patterns under walnut forests.Underwood planting pattern of walnut plays an important role in which solve the problem of the contradiction between them,making full use of water,fertilizer,light and heat resources,and improving land productivity.What’s more,planting under forest can bring short-term gains.In this study,five planting models of HLL,HLT,HDD,NCK and HCK were constructed,and the effects of walnut under forest planting on soil organic carbon pool were studied.It can select the suitable planting mode for the walnut plantation in the research area,develop the local under-forest economy and increase the income of farmers.The main results are as follows:（1）The transformation of farmland into walnut forest and the development of planting under walnut forest can promote the formation of soil aggregates,improve the structural stability,reduce the rate of structural damage and improve the soil properties.（2）Pure walnut forest and its under-forest planting mode reduced soil bulk density by 4.6-17.6%,while soil non-capillary porosity,capillary porosity and total porosity increased by 2.4-14.8%,2.8-29.1%,2.6-23.1%and 1.3-24.5%,respectively.What’s more,under-forest planting mode improved soil physical properties better than pure walnut forest.The results showed that planting under walnut forest could improve soil tightness,regulate soil porosity and aeration.（3）Soil organic carbon in pure walnut forest and its under-forest planting pattern was higher than that in blank control,and the order of soil organic carbon was HLL>HLT>HDD>HCK>NCK.Organic carbon content increased with the decrease of particle size firstly and then decreases.The maximum value appears in the diameter less than 0.25 mm and the minimum value in the diameter of2-1 mm.Among various planting patterns,the proportion of organic carbon in aggregates with particle sizes>5 mm and<0.25 mm was higher.The results showed that planting under walnut forest could promote the formation of large aggregates,improve the stability of aggregates and increase the organic content of aggregates,which was beneficial to the improvement of soil fertility and carbon fixation.（4）Soil microbial biomass carbon（MBC）content and its proportion in organic carbon,water soluble organic carbon（WSC）content and its proportion in organic carbon,labile organic carbon（LC）content and its proportion in organic carbon and stable carbon content were higher in walnut forest and under-forest planting mode than in farmland.The effect of planting pattern under forest on soil organic carbon content was better than that of pure walnut forest.It is concluded that the development of walnut planting under forests is conducive to increasing the content of soil organic components and promoting the mitigation of global warming.（5）Soil carbon pool management index（CMI）were higher in walnut forest and under-forest planting mode than in farmland,and under-forest planting mode was higher than that in pure walnut forest.The correlation analysis showed that the carbon components and carbon pool management index were significantly correlated with soil physical properties and CMI significantly correlated with carbon fractions.The results showed that CMI was closely related to soil physical properties and organic carbon content.In summary,all models can increase the stability of soil aggregates,improve the physical properties of soil,increase the content of soil organic carbon and its components,and improve the management index of soil carbon pool.The development of walnut under-forest planting in the dry-hot valley of Dadu River has a good effect on improving the function of soil carbon pool,increasing carbon storage and slowing down the increase of atmospheric CO₂concentration.In particular,cabbage is the most suitable vegetable to plant in walnut plantation,it could be planted two times in one year.It has very important function to increase the total income of the walnut plantation.The results provide a reference for improving soil fertility and carbon sink function by developing Underwood planting and selecting suitable planting patterns.