| The implementation of the Grain for Green Project in the Loess Plateau not only helps to promote regional vegetation restoration,but also reduce soil erosion.As the tree species planted in the early stage were too single,a series of ecological problems also appeared with the increase of vegetation restoration years,including serious degradation of artificial vegetation and slow growth of biomass.Since the 12 th Five Year Plan,different tree species have been added in the Loess Plateau to adjust the single stand structure and replace the original pure stand with mixed stand.The pure forests of Populus tomentosa and Prunus armeniaca and the mixed forest soils of Populus tomentosa-5-year-old Pinus tabulaeformis,Populus tomentosa-10-year-old Pinus tabulaeformis,Prunus armeniaca-Pinus sylvestris and Prunus armeniaca-Picea asperata in Longtan Basin was proposed to be the project of this paper.Through sampling and experimental analysis for three consecutive years,interannual and seasonal changes of soil carbon and water has been analyzed,the effect of degraded forest structural adjustment on soil carbon and water has been revealed with the changes of litter,root system,soil physicochemical and biological properties and other influencing factors The main results are as follows:(1)After the structural adjustment of degraded forest,the soil total carbon and organic carbon of mixed forest increased as a whole,but with the increase of vegetation restoration years,the increase of soil total carbon and organic carbon of mixed forest was not linear.The trends of soil total carbon and organic carbon were similar.The average content of soil organic carbon in Prunus armeniaca-Pinus sylvestris mixed forest from 2017 to 2019 was2018(7.41 g/kg)> 2019(7.08 g/kg)>2017(5.95 g/kg).The average content of soil organic carbon in Prunus armeniaca-Picea asperata mixed forest from 2017 to 2019 was 2018(7.00 g/kg)>2019(6.87g/kg)>2017(6.70 g/kg).The average content of soil organic carbon in Populus tomentosa-5-year-old Pinus tabulaeformis mixed forest from 2017 to 2019 was 2019(5.21 g/kg)>2018(5.10 g/kg)>2017(4.45 g/kg).The average content of soil organic carbon in Populus tomentosa-10-year-old Pinus tabulaeformis mixed forest from2017 to 2019 was 2017(6.86 g/kg)>2018(6.45 g/kg)>2019(6.01 g/kg).In the three years of continuous monitoring,the 5-year-old mixed forest increased,while the10-year-old mixed forest decreased year by year.In the early stage of recovery,it increases first,then decreases,and then increases.The soil total carbon and organic carbon storage of mixed forest were higher than that of pure forest in 2017-2019,but the change trend of carbon storage of each forest was different with time.In addition,the performance of different stands is different in different seasons.The proportion of recombined organic carbon of mixed forest was higher than that of pure forest in most soil layers,and all exceeded 60%.In vertical soil profile,the total carbon and organic carbon content of Populus tomentosa,Prunus armeniaca and its mixed forest decreased with the increase of soil depth in 2017-2019,there is a phenomenon of surface aggregation.There was significant difference between shallow and deep layers(P < 0.05).(2)The soil moisture content of mixed forest formed by the adjustment of stand structure is higher than that of the corresponding pure forest in the overall and stratified results,and the soil moisture content of mixed forest and its corresponding pure forest is basically consistent with the decrease of soil depth.The performance of water content is different in different years.The 0-80 cm soil layer in 2017 and 0-60 cm soil layer in 2018 were the highest in Populus tomentosa-10-year-old Pinus tabulaeformis mixed forest.In 2017,the range of 80-100 cm,2018,60-100 cm and2019,0-100 cm,the Populus tomentosa-5-year-old Pinus tabulaeformis mixed forest was the highest.The soil moisture of Prunus armeniaca-Picea asperata mixed forest was the highest in 2017 and 2019,and that of Prunus armeniaca-Pinus sylvestris mixed forest was the highest in 2018.The change of soil water content was different in different seasons.In May and September,the soil water content within 1m depth increased first and then decreased with the increase of soil depth,while in November,the soil water content continued to decrease with the increase of soil depth.(3)There was a significant positive correlation between soil carbon and water(P < 0.01).However,the correlation between soil carbon and water was different in different years.There was a significant positive correlation between soil total carbon,soil organic carbon and soil water content in 2017 and 2018(P < 0.05),but there was no significant correlation between soil carbon and water in 2019.There was no significant correlation between soil inorganic carbon and soil moisture content in2017-2019.The correlation between soil organic carbon and soil water content was different in different seasons,and showed a very significant positive correlation in November(P < 0.01),but in May and August,the correlation between soil organic carbon and water content was not significant.(4)Soil total carbon content,soil organic carbon content and soil water content were significantly positively correlated with soil total nitrogen,soil total phosphorus,soil microbial biomass carbon,soil microbial biomass nitrogen,fine root biomass density,fine root length density and fine root area density(P < 0.01),and negatively correlated with soil p H and bulk density(P < 0.01).There was a significant positive correlation between soil inorganic carbon content and soil total nitrogen(P < 0.01),and a significant negative correlation between soil inorganic carbon content and soil total phosphorus,soil microbial biomass carbon and soil microbial biomass nitrogen.Redundancy analysis showed that the contribution rate and interpretation rate of soil total nitrogen were the highest among the driving factors of soil carbon and water in the range of 0-1 m soil depth. |
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