| The karst ecosystem in southwest China is very vulnerable because of the special geological conditions and geographical background. The unreasonable interference of human activities lead to the destruction of vegetation, soil erosion, land degradation, a large area exposed bedrock landscape. Rock desertification developing and soil quality declining can endanger the function of karst ecosystem. Therefore, Restoring vegetation and improving soil quality is the key to the control of rock desertification developing. Soil organic carbon\nitrogen not only is the key of soil quality, but also the basic role in the soil fertility system. Thus, it is very important to study the character of soil organic carbon\nitrogen accumulation and nitrogen mineralization, and discuss their influential mechanism in vegetation recovery process. The study could provide a scientific basis for soil nutrient regulation and management in vulnerable karst ecological system.In this study the Shiqing village in Nanchuan and Zhongliang Mountain in Beibei were chosen as study site. In this area,15 typical sample plots were selected based on the field investigation, and soil samples were collected according to pedogenetic layer. Studied organic carbon\nitrogen distribution law in soil aggregate-size classes, elucidated the key aggregate-size in organic\nitrogen accumulation, analyzed soil nitrogen nutrient supply situation and crucial influence factor in vegetation recovery, and studied the rich soil calcium of influence on soil nitrogen mineralization. The results showed that:1. Vegetation recovery could increase soil organic carbon and nitrogen obviously.(1) With the progress of vegetation restoration, the contents of soil organic carbon (TOC), labile organic carbon (LOC), total nitrogen (TN), and available nitrogen (AN) increased, but decreased with soil depth. Soil dissolve organic carbon (DOC) content in different sample plots showed abandoned land≈ brush land< shrub-arbor land< arbor forest land-grass land, and in grass land soil was significantly higher than others. The DOC content decreased with soil depth, and had evidently seasonal variation, summer or autumn> spring or winter. The content of soil mineral nitrogen (MN) in different sample plots showed grass land< abandoned land< brush land< shrub-arbor land< arbor forest land, also decreased with soil depth. There was also obvious seasonal variation in DOC content, winter or spring> summer or autumn. The compositions of mineral nitrogen were different in these sample plots. Among them, ammonium nitrogen was dominant composition in grass land, brushland and arbor forest, whereas in abandoned land and bush-arbor land nitrate nitrogen was dominant composition. In addition, the improving effect of artificial forest on soil carbon and nitrogen was better than Lonicera japonica land.(2) The content of TOC, LOC, LFOC, TN, AN, LFON, MN in soil aggregates had an increasing tendency along with the descending of aggregates particle size, and the highest content was investigated in<0.25 mm. The content of carbon and nitrogen for all aggregate-classes increased in the order of abandoned land, grass land, brush land, shrub-arbor land, arbor land in study area. Artificial forest lands have more effects on the improvement of the soil nitrogen than Lonicera japonica land.(3)The organic carbon\nitrogen content of <0.25 mm aggregate-size was influenced greatest in the process of vegetation recovery. And its content could be increased obviously with vegetation succession, among them the active organic carbon\nitrogen content was enhanced more than total organic carbon\nitrogen. The C/N ratio of aggregate increased with the decrease of aggregate-size, at the same time, the proportion of active organic carbon to total organic carbon was the largest in the aggregate-size classes. Furthermore, there was significantly positive relationship between the light fraction organic carbon\nitrogen content of the soil aggregate of 0.25-1 mm\<0.25 mm classes. Accordingly, the light fraction organic carbon\nitrogen was preference of the process to smaller eraggregate-size in leaf litter decomposition period. So in the process of accumulation, active organic carbon and nitrogen were in preference to the smaller sizes of aggregate, and the maller sizes aggregate played an important role in this process.(4) The variation coefficient of organic carbon\nitrogen was higher among the aggregate-size classes at the early stage of vegetation restoration. It showed organic carbon\nitrogen increased quickly in smaller aggregate-size classes. While the variation coefficient became low among the aggregate-size classes at the semi-late period of vegetation restoration, it showed that organic carbon\nitrogen increased in smaller aggregate-size classes, and the others also increased significantly. Organic carbon\nitrogen was evenly distributed relatively among soil aggregate-size classes.(5) The results showed that 5-10 mm and 2-5 mm soil aggregate-size accounts for absolute advantage, Compared with abandoned land, natural vegetation recovery could increase obviously the amount of 5-10 mm aggregate-size. The contributions of aggregate carbon\nitrogen content to total soil carbon\nitrogen content differed in all aggregate-classes, in which the content of carbon\nitrogen in 5-10 mm and 2-5 mm classes of soil aggregates were the biggest. And it meant that soil nutrient mainly was stored in large size aggregates. Large size aggregates had an important significance to the storage of soil nutrient. Moreover the soil aggregate size distribution pattern in artificial vegetation recovery was similar to abandoned land, namely the content of aggregate carbon and nitrogen had smaller increment. While the function of which artificial forest land improved carbon and nitrogen in soil aggregate was more than Lonicera japonica land.2. In process of vegetation recovery, soil nitrogen nutrient supply situation.(1) Vegetation recovery had important effect on soil net nitrogen mineralization rate, ammonium rate, nitrification rate. Net nitrogen mineralization rate during vegetation recovery showed grass land< abandoned land< brush land< shrub-arbor land< arbor land. Ammonium rate showed, shrub-arbor land< abandoned land< grass land< brush land< arbor land. Nitrification rate showed, grass land< brush land< abandoned land< arbor land< shrub-arbor land. Meanwhile, soil net nitrogen mineralization rate showed seasonal variation evidently. On the whole, it showed that mineralization rate in summer or spring was than in winter. Besides, soil net nitrogen mineralization rate in artificial forest land was a bit higher than Lonicera japonica land.(2) The first-order kinetics equation could fit the mineralization process of soil nitrogen well. Vegetation recovery could improve soil nitrogen mineralization potential in varying extent, specifically, it showed abandon land< Lonicera japonica land< Cinnamomum camphora tree land< grassland< brush land< shrub-abor land< arbor land.(3) Temperature and moisture had obvious interaction to soil nitrogen mineralization process. In the low moisture condition, nitrogen mineralization rate increase with temperature rise. High moisture could cause the inhibition to soil nitrogen mineralization, and the soil net nitrogen mineralization rate increase with the decrease of temperature within high moisture. In the low moisture condition, Net nitrogen mineralization rate increases with humidity increase, and in the high moisture, the phenomena was opposite. Under 35℃and 30% mass water content condition, soil nitrogen mineralization rate reaches its maximum, namely abandoned land< grass land< brush land < shrub-arbor land< arbor land. Additionally, temperature sensitivity coefficient showed an opposite trend of variation against the soil nitrogen mineralization rate under different sample plots.(4) In the total organic nitrogen storage of each aggregate-size, the classes of 0.25-1 mm,5-10 mm and 2-5 mm contributed more significantly than the others to soil net nitrogen mineralization quantity. With the positive vegetation succession, the weight percentage of >5 mm aggregate-size classes was improved. Accordingly, the capacity of soil supply mineral nitrogen and storage organic nitrogen were intensified.(5) At the same time, there also were significantly positive correlation between net nitrogen mineralization rate and the content of soil organic carbon\nitrogen, and negatively correlation between net nitrogen mineralization rate and light C/N ratio of fraction organic matter. The soil nitrogen mineralization rate increased with the process of vegetation restoration. Even though the season, temperature, moisture, and soil calcium had important influence on soil nitrogen mineralization, at the approximately same condition the soil net mineralization rate still increased with the process of vegetation restoration. So soil organic matter was the most important influence factor on mineral nitrogen supply.3. Rich soil calcium content could promote soil organic nitrogen mineralization.(1) The results showed that there were significantly positive correlation between soil net nitrogen mineralization rate and content of soil calcium of all forms expect residue form and acid-soluble. Accordingly, soil condition of calcium-rich was beneficial to promotion the soil nitrogen mineralization.(2) The simulating experiments on calcium carbonate addition soil showed there was priming effect to soil nitrogen mineralization process. The increment amount of net nitrogen mineralization caused by priming effect had variance in different sample soils. It showed abandoned land< grass land< brush land< shrub-arbor land< arbor land. However, mineral nitrogen increasing rate caused by priming effect showed opposite trend among these different sample soils.(3) The increase rate of mineralization nitrogen by priming effect adding carbonate calcium had variance in karst soil, yellow soil and purple soil. Among them yellow soil of low calcium content had obvious priming effect, purple soil was the most weak, the karst soil was in between. Adding carbonate calcium could promote soil nitrogen mineralization amount, but increase rate of mineralization nitrogen had decrease trend with soil calcium content increase.(4) Adding organic matter had a greater fixation effect on soil mineral nitrogen; among them the fixation effect in arbor forest soil was relatively weak. Adding calcium carbonate and organic matter could relieve the microbial immobilization of mineral nitrogen to a certain extent. During the later incubation period, The mineral nitrogen content of adding organic matter treatment soil samples had increasing trend with incubation time, and in the whole incubation period, net nitrogen mineralization rate showed a rising trend.4. In process of vegetation recovery, leaf litter decomposition character and the correlation between leaf litters decompose and soil organic\nitrogen.(1) The coefficient k of leaf litter decomposition varied from0.73 to 1.33, and in different sample lands, it showed grass land< brush land< arbor land< brush-arbor land.The k value of Cinnamomum camphora tree land was greater than arbor land and less shrub-arbor land. With the extension of vegetation recovery, leaf litter decomposition rate was increased. At the same time, and organic carbon of leaf litter was releasing status during the decomposition period, and release rate of organic carbon, nitrogen increased with process of vegetation positive succession. The study result also displayed that the leaf litter decomposition rate and organic carbon\nitrogen release rate mainly was controlled by the initial nitrogen content and C/N ratio of leaf litter.(2) The results of correlation analysis showed that soil organic carbon\nitrogen and nitrogen mineralization were related with leaf litter decomposition rate and organic carbon\nitrogen release rate positively.5. The coupling relationship between Soil urease catalyzing reaction character and soil nitrogen mineralization.(1) The kinetics and thermodynamic parameters of soil urease reaction were influence during vegetation recovery period. The soil quality was improved which could intensify the affinity of enzyme and substrate, reduce the active energy of soil urease reaction, enhance the substrate order array in the active center of urease. These were beneficial to enzyme reaction proceeding.(2) Soil net nitrogen mineralization rate were positively correlated with activity of urease and parameters of kinetics and thermodynamics urease reaction, especially ammonium rate showed significantly positive correlation. Vegetation recovery could improve condition of Soil urease reaction, which could enhance soil nitrogen mineralization and capacity of soil nitrogen supply.In summary, the vegetation restoration could improve soil organic carbon\nitrogen accumulation. In the process of accumulation, active organic carbon and nitrogen were in preference to the smaller sizes of aggregate, and the maller sizes aggregate played an important role in this process. With the hierarchical formation of aggregates, soil organic carbon\nitrogen mainly was stored in the bigger aggregate, and it had a significant role to the nitrogen supply. Soil organic matter was the material basis of soil nitrogen mineralization. With soil organic matter accumulating, soil mineral nitrogen supply capacity was enhanced. Although soil nitrogen mineralization process could be affected by environmental factors, rich calcium content of soil and so on during vegetation recovery, under the same conditions, nitrogen mineralization rate was increased with the vegetation succession. So soil organic matter was the most important influence factor on mineral nitrogen supply. At the same time, with soil organic matter accumulating during vegetation recovery, soil enzyme catalyzing reaction circumstance was improved, and it could promote the nitrogen supply capacity enhancement. At fact, in the process of ecological restoration, vegetation and soil were interaction, and both of them were joint development; soil nutrient elements were provided sufficiently by accumulating organic matter, which could promote vegetation better developing. |
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