| The karst regions of southwest China are greatly suffered from rocky desertification. Soil system is an important part of karst rocky desertification system, and the deterioration of soil quality is the essence of rocky desertification. So it is necessary to study on the variation of soil characteristics under rocky desertification for the reasonable use of soil and the maintenance of karst ecosystem.Soil samples were collected from Libo and Puding in Guizhou province with typical rocky desertification characteristics. The content of soil water and nutrients, including ammonium (NH4+-N), nitrate (NO3--N) and available phosphorus, were measured in different rocky desertification stages, different soil layers and different time. The time and spatial variation of soil water and available nutrients was analyzed, as well as water-fertilizer coupling effect. The influences of rocky desertification on soil physical properties and NO3--N penetration were studied. The paper is to provide scientific data for the ecological recovery under soil degradation.The results were as follows:(1) The variation of soil basic parameters was studied, the results showed that:the soil bulk density of non-rocky desertification soil was low, and the structure coefficient reached as high as94.87%, with a strong ability to retain water and nutrients. As the development of rocky desertification, soil bulk density and clay content increased in the mass, and porosity, water stable aggregates and structure coefficient decreased. The correlation coefficient between the content of soil organic matter and bulk density, sandy particles, water-stable aggregates>5mm and soil structure coefficient was-0.9251,0.8489,0.9028and0.8524respectively, which was extremely significant.(2) The variation of NH4+-N and N03--N was studied from July,2007to May,2009. The results indicated that the sequence of soil nitrogen content was non-degraded rendzina>potential degraded rendzina>moderate degraded terra rossa>light degraded rendzina. The content of nitrogen was higher in surface soil layer than that in bottom layer. The nitrogen content was higher in summer and lower in winter, but irregularly in moderate degraded terra rossa. The content of soil organic matter had an extremely significant relation with NH4+-N and NO3--N in all soils except moderate degraded terra rossa, and the correlation coefficient was0.6704and0.9154respectively. The content of NH4+-N and NO3--N had a positive relationship as well, with the correlation coefficient reached to0.7601.(3) The variation of soil available phosphorus was studied during the experiment, and the results showed that:The sequence of available phosphorus content was non-degraded rendzina>potential degraded rendzina>moderate degraded terra rossa>light degraded rendzina. The content of available phosphorus was higher in surface soil layer than that in bottom layer. The content of soil available phosphorus was higher in both summer and winter, as a bimodal curve with a trend of first increased, then decreased and finally increased. But moderate degraded terra rossa was excluded. The content of available phosphorus had an extremely significant relationship with organic matter and clay content, the correlation coefficient was0.8008and-0.8852respectively.(4) By simulating soil column in laboratory, the NO3--N vertical transport characteristics in typical soils were studied. The results were as follows:vertical transport velocity of NO3--N decreased, and the breakthrough curves (BTCs) of NO3--N were more dispersed, in each horizon from the surface layer to the bottom in every soil profile. As rocky desertification processed, the BTCs rose and fell gently, and tailing was more obvious. An analytical solute transport model (CXTFIT2.0) was used to estimate BTCs of NO3--N. The correlation coefficient between the value of estimated and observed was as high as0.8, which indicated that CXTFIT2.0model was suitable in fitting NO3--N transport. The transport of NO3--N was mainly affected by the coefficient of soil structure. As soil structure coefficient decreased, NO3--N outflow was retarded, and the peak of concentration was reduced. Soil bulk density, the content of organic matter and clay also affected the vertical transport of NO3--N. Lower bulk density, clay content and higher organic matter content were each associated with faster nitrate transport.(5) The variation of soil water content was studied during the experiments, the sequence of water content in surface soil layer was non-degraded rendzina>potential degraded rendzina>light degraded rendzina>moderate degraded terra rossa. The content of water was higher in surface soil layer than that in bottom layer, but contrarily in moderate degraded terra rossa. The water content was higher in summer and lower in winter, which was synchronous with the rainfall. The water in the surface layer reached the highest value of589.1g·kg-1in July,2008in non-rocky desertification soil. The content of soil water had a significant relationship with NH4+-N and NO3--N, moderate rocky desertification soil not included, and an insignificant relation with available phosphorus. |
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