| The typical chestnut soil area is a fragile ecological area.The natural environment of this region is characterized by little rainfall and strong evaporation.The calcium carbonate in the soil is enriched in large quantities,forming a compact gray calcic horizon.Due to both natural factors and human disturbance,the ecological environment in this region has been seriously degraded.In order to improve the ecological environment of this region,lots of afforestation projects have been carried out.However,the results are not as expected.In order to explore the restrictive factors of afforestation in chestnut soil areas and improve the effectiveness of forestry ecological construction,this study chose the chestnut soil with deep excavation as the research object,testing the hypothesis of calcic horizon regeneration,and then explored the process and vertical distribution characteristics of calcic horizon regeneration by sampling from selected sites,in Wulanchabu City of Inner Mongolia Province.On the basis of calcic horizon regeneration,the evolution patterns of soil physical and chemical properties and water transport characteristics during the process of calcium carbonate deposition were systematically analyzed.The relevant mathematical model was established to determine the mechanism of calcic horizon regeneration in chestnut soil.Thirdly,according to the process of calcium carbonate deposition,this study tested afforestation techniques from different perspectives including the new forestry material—yeast extraction,aiming to reduce calcareous deposit regeneration.After the applications of afforestation techniques in field during afforestation process of chestnut soil,the effects of these techniques on calcium carbonate deposition,physical and chemical properties and moisture migration characteristics in chestnut soil were studied.The responses of afforested seedlings to soil improvement and new plant growth regulation technology were compared.The main conclusions are as follows:(1)Under the conditions of little rainfall and strong evaporation,the calcium carbonate deposition occurred again below the depth of about 20 cm in chestnut soil with deep excavation and removal of calcic deposits,which confirmed the occurrence of calcic horizon regeneration.Calcic deposition mainly occurred at the soil depth of 20-40 cm,and the average concentration of calcium carbonate increased from 192.25 mg/g to 289.87 mg/g in ten years after afforestation.The calcareous layer thickened with the increase of afforestation time from 8.5 cm to 35.7 cm,and the regeneration process evolved from shallow to deep layers in the vertical dimension(extending from about 20 cm to about 60 cm).Meanwhile,the calcic horizon regeneration approached to adjacent natural chestnut soil in terms of the amount of calcium carbonate and the vertical distribution.(2)During the regeneration process of calcic horizon,the physicochemical properties of soil at the depth of 20-60 cm showed a decrease in porosity,moisture content,organic matter,total nitrogen and total phosphorus contents,and an increase in pH and bulk density.The underlying mechanisms can be summarized as follows:during the regeneration of calcic horizon,Ca(HCO3)2 in soil capillary pores transferred by water turns into CaC03,which leads to the destruction of the physical structure of oil and nutrient transport is subsequently impeded.The results of linear regression showed that there was a significant negative correlation between soil calcium carbonate content and soil physical and chemical properties.(3)The deposition of calcium carbonate in chestnut soil could reduce the hydraulic conductivity of the deposition layer,and the water infiltration process was hindered by calcic horizon.More soil moisture was dispersed in the surface layer and more readily evaporated,and the water holding capacity was therefore reduced.The observations of moisture migration characteristics could fit with a Kostiakov power function model.The quantitative analysis of the relationship between moisture migration characteristics(wetting front migration,cumulative infiltration capacity,infiltration rate,and moisture variation)with the content of calcium carbonate in deposition layer were studied.(4)Calcic horizon regeneration could negatively affect the physical and chemical properties and moisture migration characteristics of chestnut soil,which resulting in low efficiency of afforestation.Therefore,reducing calcic horizon regeneration is the key to improve the effect of forestry ecological projects.In this study,effective afforestation techniques were further tested.Among them,the application of 30 g water retention agent,mulching straw blanket and unidirectional permeable film could effectively inhibit the calcic horizon regeneration,improve physical structure,increase soil nutrient content and reduce soil pH significantly.It’s worth noting that,seepage irrigation and water-collecting slope treatment had no significant effect on the physical and chemical properties of chestnut soil or on the inhibition of calcic horizon regeneration.Due to the mineralization reaction between polar amino acids and Ca2+,the inhibitory effect of yeast extraction on the deposition of calcium carbonate in 20-60 cm chestnut soil was not significant.However,with the sufficient nutrient supplement,the physicochemical properties of chestnut soil had significant responses to the application of 20g,30g,and 40g of yeast extraction around roots.(5)The responses of the characteristics of soil moisture migration in chestnut soil to the afforestation techniques were as follows:the application of water retaining agent slowed down the infiltration process;however,the water holding capacity increased.The four mulch materials form a barrier to infiltration.The infiltration process of soil with straw blanket covering was relatively faster;while the infiltration process of soil mulched plastic film was the slowest due to its impermeability.The water holding capacity was the highest after soil being covered with unidirectional permeable film,followed by plastic film treatment.Straw blanket could effectively inhibit soil moisture evaporation due to its shading effect.Rich organic matter in yeast extraction could increase soil aggregation and improve soil structure.With the increasing application amount,soil infiltration and water holding capacity increased gradually.(6)The survival rate,growth,root length,root surface area,root tips and leaf water characteristics of four kinds of common sivicultural seedlings in semi-arid chestnut soil area could be significantly improved by 30 g water retention agent,mulching straw blanket and unidirectional permeable film,and root application of yeast extraction.Foliar spray yeast extraction could significantly improve the growth and leaf water characteristics of the four seedlings which had a good response to the concentration of 1.5%and 2%.Since yeast extraction is rich in growth hormone and cytokinin which could effectively help seedlings to form chlorophyll,the growth of seedlings sprayed after leafing was significantly increased.The leaf water characteristics of container seedling were better when foliar sprayed after leafing;While the bare root seedlings were suitable for foliar spraying in growing period.In summary,calcic horizon regeneration still occurred after deep excavation in the chestnut soil,which could further alter soil physical and chemical properties and water transport characteristics.This might be the main reason for the low efficiency of forestry ecological projects in the semi-arid chestnut soil areas.It is proposed that the application of 30 g water retention agent,mulching straw blanket and unidirectional permeable film,root and foliar spray of yeast extraction could effectively inhibit calcic horizon regeneration,improving physical and chemical properties of chestnut soil.The results of this study can provide scientific basis to forestry ecological projects in typical chestnut soil areas. |
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