| In Guanzhong area of Shaanxi Province has superior natural conditions, soil terrain, soil deep, loose soil, plentiful rain and is suitable for a variety of crop growth. Since ancient times, Guanzhong area is major grain, cotton, oil, fruit and vegetables producing areas. However, since the implementation of the land contracting system for more than 30 years time, adjusting the industrial structure of agriculture and seting intensive management level continuously rising, land use model and intensity, fertilization and tillage and crop rotation of Guanzhong farmland as the main content of soil management measures have profoundly changed, farmyard manure and traditional “loess move” fertilization aiming to recover calcium are out of use, because of the shallow tillage replaces deep tillage farming,” effect of calcium recovering is no longer present, then calcium balance mechanism of soil surface layer is destroyed. Instead, organisms in the soil respiration gathering a large amount of CO2 accelerate the decalcified soil, nitrogen denitrification in chemical fertilizers, the use of phosphate fertilizer to carry into the free acids enhance soil lime reaction which promote the process of soil demineralization. Among of a series of degradation of soil quality, the degradation of soil calcium is becoming more and more obvious. So far, soil calcium deficiency do not directly showing, however, decreasing trend of calcareous soil calcium is undeniable fact, which need early warning and to give greater attention. based on the actual soil management in Guanzhong farmland and combined with development and evolution of soil, which decided to select without disturbance of farmland as the research object, the village as the center in the limited space level range, selected the same the parent material, climate, geology and land use types, soil calcium seem to be not enough sensitive element as quality index, to explore the invisible degradation law of calcareous soil, from spatial and temporal scales to explore mechanism of agricultural management measures and strength utilization measures on soil calcium evolution, to relieve or control the effect of high strength intensive agricultural measures on soil quality, warning people pay close attention to modern agricultural technology measures on soil health and influence. The results are as follows:1. The soil organic matter is the indicator of the level of human activity and soil management, which indirectly drives the change of soil calcium. In the only interval 600 m horizontal spatial scales, the study showed that the average content was 19.42~26.20 g·kg-1, the variation coefficient was 0.12 in 0-20 cm soil organic matter; the average content change was 10.49~18.53 g·kg-1, the coefficient of variation was 0.20 in 20-40 cm soil organic matter. Soil organic matter content between 0-20 cm and 20-40 cm in Guanzhong area presented obvious spatial change trend that more far resulted to lower content from the village, especially from the village 0-200 m performance was more obvious, and changes in the scope of 200-600 m was more gentle. The spatial variation of soil organic matter content is the evidence of the spatial difference of soil management, suggesting that it was a good material to explore the influence of human activities on the content of soil calcium carbonate.2. The 0- 20 cm soil carbonate calcium content between 69.44g·kg-1and 101.59g·kg-1, the average content was 77.97g·kg-1; between 20 and 40 cm soil carbonate calcium content were between 77.39g·kg-1~109.12g·kg-1, the average content was 89.99g·kg-1. The distribution characteristics of soil calcium carbonate content were obvious at different distances in the farmland. The village as the center, soil calcium carbonate content showed a trend in fluctuation increasing gradually from the village at different distances, soil calcium carbonate content change was not very obvious in 0- 40 cm soil layer from the village 0-400 m range, but in 400-600 m increased significantly, which is contrary to organic matter content of the horizontal distribution.Although 0-40 cm belongs to the accumulation of loess in different historical periods, there was characteristic of 0-20 cm higher than the 20-40 cm at the each level of soil calcium carbonate content. The content of calcium carbonate in 0-40 cm of soil profile at 50 m from the village was lower than that of 500 m, and much lower than that in the same layer of the soil profile during the survey in 1982. The content of calcium carbonate was significantly increased in the soil below 40 cm. The total calcium content of 0-100 cm of soil profile at the 50 m horizontal distance was significantly higher than that of 500 m. These are strong evidences that the human activities affected soil calcium carbonate at process of leaching and deposition in the vertical scale.3. The amplitude of variation of soil exchange calcium content was 3.19~4.04 g·kg-1 and the average value was 3.74 g·kg-1 in 0-20 cm of Guanzhong farmland, coefficient of variation was 7.19% belonging to the weak variation degree; The amplitude of variation of soil exchange calcium content was 3.06~3.74 g·kg-1 and the average value was 3.44 g kg-1, coefficient of variation was 6.98% belonging to the weak variation degree in 20- 40 cm soil layer. The exchangeable calcium content in 0-20 cm soil layer is always higher than that of 20-40 cm soil layer, which is directly related to the fertilization site, the spatial distribution of plant root system and water content.4. The amplitude of variation of soil water soluble calcium content was 0.14~0.25 g kg-1 and the average value was 0.21 g kg-1 in 0-20 cm of Guanzhong farmland, coefficient of variation was 16.04% belonging to belonging to the moderate degree of variation; The amplitude of variation of soil water soluble calcium content was 0.13~0.22 g·kg-1 and the average value was 0.18 g kg-1 in 20-40 cm of Guanzhong farmland, coefficient of variation was 17.77% belonging to belonging to the moderate degree of variation. The content of water soluble calcium in 0-20 cm soil layer was slightly higher than that of 20-40 cm. The farther away from the village, the soil water soluble calcium content showed a downward trend along with the farther distance from the village. Water soluble calcium is absorbed directly by the plants and is easy to leach losing.5. Soil colloidal calcium saturation is one of the quality indicators of soil health, healthy soil saturation degree of calcium generally not less than 60~65%, gardening soil saturation degree of calcium should be above 80%, saturation degree of calcium changes were between 66.93% and 78.88% in 0-40 cm of Guanzhong farmland. It had obvious variation at space scale, but now soil remains healthy condition at temporary. There was also a part of the soil near the low threshold value of the health soil, which need to alert its changing trend..6. Through the location of the observation, there was obvious seasonal variation process of the content of calcium carbonate and water soluble calcium content in the anniversary. In the soil profile, soil calcium carbonate content presented distinctly spatial gradient that showed the trend of the deep leaching and deposition of calcium carbonate during the growing season of wheat. Layer of CaCO3 eluviation and deposition was relatively shallow during growth period of summer maize.During prolonging the growth period of wheat, the soil water soluble calcium carbonate was obviously increasing trend in 0-100 cm profile and the time gradient difference was obvious, and the water soluble calcium content decreased obviously in 0-30 cm soil layer during the growing period of maize. Verify the deep roots of wheat in the growth period was conducive to help calcium ion of deep soil transferred back to the soil surface, which contributed shallow root crop absorption and utilization so-called plant “ion pump” during summer. Shallow and root crop rotation helped to balance calcium nutrition.Together, through studying temporal and spatial changes of the calcareous soil calcium carbonate, exchangeable calcium and water soluble calcium, showing the modern agricultural production technology system, human activities have significantly accelerated the farmland soil CaCO3 eluviation and deposition. The part of soil calcium saturation closed to low the threshold of the farmland soil quality. Although the present circumstances, the soil water soluble calcium content and exchangeable calcium content were not enough threat to crop production, however, to for stealth characteristics of invisible degradation process that calcium carbonate declined in farmland has been very obvious, which is necessary to warn people to pay more attention to the function and effect of modern agricultural technology and measures on soil health. |
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