| Land degradation caused by soil and water loss is one of the main reasons for ecosystem degradation and soil impoverishment in the red soil area of southern China.After long-term governance,soil and water loss has been effectively curtailed and vegetation has been initially restored.However,the comprehensive and systematic understanding of soil nutrient evolution in the red soil erosion area is still insufficient.Therefore,evaluating soil nutrient status scientifically,exploring the spatiotemporal evolution characteristics and influencing factors of soil nutrients,and revealing the limiting factors is the key to accelerate the restoration of vegetation and the improvement of soil fertility in the red soil erosion area in southern China.In this study,based on the soil data from a comprehensive and systematic sample survey in 2008 in Changting County,a typical red soil erosion area,classical statistics,geostatistics and GIS technology were used to study the soil nutrient status and spatial distribution characteristics.Soil nutrient data of Changting County in 1980 were collated and integrated,and the spatiotemporal variation and influencing factors of soil nutrient were discussed by comparing the data of two periods.In 2019,intensive sampling was conducted in the severely eroded Hetian area,and the data in 1980 and 2008 were compared to analyze the spatiotemporal changes of soil nutrients.Based on the superposition of spatial and temporal dimensions of soil nutrients,the temporal and spatial variation of soil nutrients in the process of ecological restoration in the eroded red soil region of South China was discussed in order to provide scientific basis for ecological restoration and ecological civilization construction in Changting and the red soil region of South China.The main conclusions are the following:(1)The results of comprehensive sampling data analysis in 2008 showed that the overall level of soil nutrients in Changting County was low.The content of soil organic carbon and available phosphorus were moderate,and the content of total nitrogen,total phosphorus,total potassium,available nitrogen,and available potassium were low.The soil p H was acidic.Soil organic carbon content varied from 0.07 to 56.86 g/kg,with an average value of 15.85 g/kg,ranking at the third level of the second national soil survey nutrient classification standard(6 levels).The total nitrogen content was 0.03-3.50 g/kg,the total phosphorus content was 0.02-1.59 g/kg,and the total potassium content was1.77-62.51 g/kg,with the mean values of 0.98 g/kg,0.35 g/kg and 14.45 g/kg,respectively,belonging to the fourth,sixth and fourth grade.The p H value was 3.84-7.66,clay content was 5-56%.Available nutrient content varied greatly,the content of alkali-hydrolyzable nitrogen was 5.88-243.67 mg/kg,available phosphorus was0.10-199.60 mg/kg,available potassium was 5.80-554.79 mg/kg,the mean values were81.53 mg/kg,16.17 mg/kg,and 84.72 mg/kg,respectively,in level 4,3,4.(2)The spatial structure characteristics of soil nutrients in Changting County were analyzed by using geostatistical variogram.Soil total potassium,available phosphorus,available potassium,p H and clay had strong spatial correlation,while organic carbon,total nitrogen,total phosphorus and alkali hydrolyzable nitrogen had moderate spatial correlation.The spatial autocorrelation distance of soil nutrients varied significantly,ranging from 0.54 km to 45.99 km,and the correlation distance of soil alkali-hydrolyzed nitrogen and total phosphorus was larger.(3)Based on the ordinary Kriging interpolation method,the spatial estimation of soil nutrients was carried out,and the digital soil map was drawn according to the soil nutrient classification standard.The contents of soil nutrient indexes in the study area were mostly at medium or low levels.The high values were mainly distributed in the western mountainous areas with good vegetation and weak soil erosion,such as Gucheng Town,Datong Town and Tiechang Town,while the low values were mainly located in the central areas with severe soil and water loss,such as Xinqiao Town,Hetian Town,Sanzhou Town and Tufang Town.Soil organic carbon was mainly 6-23 g/kg(level 2-4),and total nitrogen content was mainly < 1.5 g/kg(3-6 level),and available nitrogen was mainly30-120 mg/kg(level 3-5).The distribution of soil carbon and nitrogen was consistent,showing a decreased trend from the outside to the center,basically corresponding to the topography of Changting County.Soil total phosphorus content was low,mainly less than0.4 g/kg(level 6),and increased spatially from east to west.The low-value area was in the center of the county,and the highest value was in the northwest.Available phosphorus was mainly 5-20 mg/kg(level 3-4),and its spatial distribution was more complex.The total potassium content of soil was mainly 10-20 g/kg(level 3-4),and the available potassium content was mainly 30-150 mg/kg(level 3-5),both of which were distributed in patches,and the high value areas were all located in Hongshan Town and Sidu Town in the southwest corner of the county.The soil p H was ranged from 4 to 6,with strong acidity.It was divided into two parts along the middle of the county,high in the west and low in the east.The spatial distribution of clay was relatively uniform,mostly between20-30%,which was more suitable for plant growth.Only two low-value patches were found at the junction of Hetian and Cewu Town,and Sanzhou and Tufang Town in the middle of the county.(4)Using the GIS spatial analysis module as a tool,the results of correlation coefficient and one-way ANOVA showed that the soil nutrients in Changting County were significantly affected by the structural factors such as topography and erodibility,while the random factors such as land use were relatively weak.The soil nutrients increased with the increase of altitude and decreased with the increase of soil erodibility.(5)The spatial and temporal variation characteristics of soil nutrients(organic carbon,alkali-hydrolyzated nitrogen,available phosphorus,available potassium and p H)in Changting County in 1980 and 2008 were studied by combining statistical data with spatial distribution superposition analysis.The variation trend of soil nutrient contents in Changting County was different from 1980 to 2008.Soil carbon,phosphorus and potassium contents showed an increasing trend,with the order as follows: available phosphorus > available potassium > organic carbon.Soil alkali-hydrolyzable nitrogen and p H decreased in different degrees.The regional differences of soil nutrients between towns were enhanced,and the spatial distribution pattern of soil nutrients was developing from homogenization to differentiation and fragmentation,and the spatial distribution trend was more complex.On the whole,the growth rate in the northwest of the county was higher than that in the central region,and the initial value of soil nutrients significantly affected the evolution of its spatial pattern.The nutrient in Anjie Town and Tufang in the north of Changting County had increased rapidly,while Hetian Town and Sanzhou Town in the central areas had a deteriorating trend,making regional differences more significant.However,whether the nutrient contents were increased or decreased,the soil nutrient levels of the central part of Changting County had increased slowly,and some areas showed a downward trend,indicating that the restoration degree of soil nutrients was different in the process of the restoration of the degraded red soil erosion.(6)Based on the study of the factors influencing the spatial and temporal changes of soil nutrients in Changting County from 1980 to 2008,it was found that the change of soil erosion degree was an important factor,and the change of land use patterns,such as fertilization management and planting mode,and the economic and social development also had an effect.The spatial distribution of soil nutrients corresponded to the intensity of soil erosion,and the areas with intense erosion had relatively poor nutrient content.Soil nutrients were negatively correlated with rainfall erosivity and decreased with rainfall erosivity increasing.The transformation of land use was under a complex effect on the temporal and spatial variability of soil nutrients.Soil nutrient contents had the smallest increase and the largest decrease when converted to bare land.The increase of soil carbon,nitrogen and potassium content was greater when converted to forest land.Phosphorus content increased considerably when converted to cultivated land and grassland.Soil p H had decreased greatly when it shifts to woodland and bare land.(7)Based on the soil nutrient data of three time(1980,2008 and 2019)in the severely eroded and degraded Hetian area in the past 40 years,the temporal and spatial variation of the soil nutrients were analyzed by superimposed analysis of statistical characteristics and spatial changes.The results showed that the average value of soil nutrients was mainly increased in general,but fluctuated.The contents of soil organic carbon and alkali-hydrolyzable nitrogen decreased slightly and then increased,the available potassium continued to increase,the available phosphorus increased and then decreased slightly,and the p H continued to decrease.The spatial distribution pattern of soil nutrients in the three periods was significantly different,and the initial value of soil nutrients in Hetian area was relatively low,which showed a spatial evolution from homogenization to complexity.The improvement of soil nutrients was slow from 1980 to2008.From 2008 to 2019,the soil nutrients in the spatial level remained stable and increased to a certain extent,and the growth rate was as follows: organic carbon >alkaline hydrolysis nitrogen > available phosphorus > available potassium > p H,among which the soil organic carbon increased faster,while the content and level of available nutrients increased slowly.(8)The overall level of soil nutrients in the study area was low,and showed fluctuation in both time and space.The problem of soil acidification was prominent.Based on the changes of soil nutrients in the two periods of Changting County and the three periods of Hetian area,the spatial and temporal changes of soil nutrients fluctuated and increased in the process of ecological restoration of erosion degraded land.The risk of "secondary degradation" should be vigilant.In recent 40 years,soil p H showed a continuous decline trend,which indicated that the acidity of forest soil should also be taken seriously in the restoration process with masson pine as the dominant species.In the future ecological management process,grass and shrub planting and application of organic microbial fertilizer can be used to promote soil improvement.And improving forest quality and enhance the stability of forest ecosystem by tree species optimization to maintain the effective restoration and sustainable development of ecosystem in eroded and degraded land.This study was conducive to a systematic and comprehensive understanding of regional soil resources,was the practical basis for promoting soil quality improvement,was the key to the completion of vegetation restoration and ecological reconstruction in the red soil region of southern China,was an important part in the management of mountain-water-forest-farmland-lake-grass-sand system,and was of great significance to the construction of a harmonious ecological environment. |
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