| Intensive agricultural production had to be used, because of the contradiction between population increasing and acreage reduction. Under the condition of artificial high intensity using and high load production, farmland soil quality occurred variations including macroscopic(degradation) and microscopic(fatigue) processes to different extent. So, study of soil quality evolvement trend and variation degree under model of modern intensive production was key content of soil science.Although soil was a gold resource, which was reclaiming action and some physiological function, all properties based on rational utilization. Overuse soil and mismanagement would result in damage of soil physiological function, especially appeared significantly degradation, lost the fertility at last. Soil fatigue appeared in process of blindly pursuing highyield, and it was a decline process of physiological function, and it was prelude of soil degradation. People paid more attention to soil degradation, but ignored soil fatigue. Studying farmland soil fatigue symptom, reason and mechanism, exploring material basis of soil anti-fatigue, which was urgent need of opening out soil quality evolvement and effect law under artificial intensive using, profound understanding soil properties, comprehensively mastering soil physiological function, searching for effective approach of soil permanent utilization and ensuring sustainable development of agriculture and ecological environment.The research analysed soil nutrient fatigue and its occurrence mechanism by taking farmland in semiarid area liable to humid as the research object and taking spatiotemporal soil nutrient status theory as the main goal, and it used the methods including located monitoring, indoor simulation and biological effect experiment by taking degree of calcium saturation in calcareous soil as index. The conclusion:1. The full concept of“soil nutrient status”which could embody crop nutrient supply ability was dynamic variation process of soil available nutrient in certain period of time(crop growth stage) and certain soil layers(root developoment layer). The seasonal and regional variation of farmland soil nutrient status showed the fertilization habit and the characteristics of nutrient transfer in dry land and biological absorption in Guanzhong Area.There were two different variation regions in soil available N spatiotemporal variation characteristics.“Vertical gradient”area was in 0~50 cm, which was significantly decreasing characteristics from up to down;“time gradient”area was in 50~100 cm, which embodied characteristics of crop root absorption and nutrient transfer process. And vertical gradient was not obvious in 0~20 cm soil layer, its content was above 70 mg.kg-1. Only some brevity time variation gradient was“available N abundant equivalent region”, obviously, it was not close with crop growth process, and it was not reliable as soil nutrient diagnosis area. The content of soil available N was 50~70 mg/kg in 20~50 cm soil layers, and its vertical gradient isoline was relatively dense, especially in 30~40 cm, so the region was“conduction region”of nitrogen deep moving. The content of available N in 50~100 cm soil layer was below 50 mg.kg-1 all the year round, the isoline in the area was vertical extension, the variation gradient of available N content embodied crop growth process, it was“activity region”.There were two significantly temporal variation characteristics in soil available P content variation. The gradient of spatial distribution of the soil available P was very obvious in wheat growth stage, especially in 0~50 cm, and soil available P consumed obviously below 60 cm from wheat turning green stage to mature stage. Then, the content of soil available P in maize growth stage was obviously temporal variation gradient. In deep layers, soil available P content was increasing in maize seedling stage, then decreasing was significant after seedling stage, especially in maize mature stage.The time variation gradient of soil available K was very obvious in 0~100 cm, and the gradient among different layers were not obvious. The soil available K content in 0~100 cm soil layer showed general decline trend in time domain, which decreased from over 140 mg.kg-1 in wheat turning green stage to 70 mg.kg-1 in maize mature stage. The annual variation process of soil available K was significant“steady and drop”alternative process. The soil available K content was about 150 mg.kg-1 from sowing stage to overwintering stage, which was“rich-K stable stage”; the soil available K content variation was“affluence rapid decline process”from wheat turning green stage to jointing stage; the soil available K content was maintaining about 100 mg.kg-1 from jointing stage to mature stage, which was“middle level stable stage”. And in maize growth stage, the soil available K content obviously decreased again, which was“stress decline stage”of potassium.The fertilizing custom by farmland soil available nutrient status in dry land, and the characteristics of“upside fertility and underside leanness, upside dry and underside wet”was very significant on soil profile. Low and unstable surface soil moisture limited to nutrient using, which was important environment factor of leading to soil fatigue.2. The horizontal space variation characteristics(the space variation of farmland soil fatigue) of main farmland soil fertility properties were obvious in Guanzhong area, which showed the local fertilizing custom and history.In history, farmer mainly cultured fertilizer by organic fertilizer before, which followed the principle of proximity, however, in current years, we mainly cultured fertilizer by chemical fertilizer, the soil organic matter content decreases more gradually as the distance from the village increases.in the horizontal direction, it showed a changeable tendency consistent with that of soil organic matter content, whilst this trend was opposite to that of soil available N content in 0~40 cm, which showed that slather inorganic nitrogen fertilizer lead to nitrogen accumulation on soil profile.Slather inorganic fertilizer would lead to the decline of degree of calcium saturation, even in calcareous soil, and the variation of farmland soil Ca saturation in Guanzhong was 55.76%~72.76%, the average was 61%. The Ca saturation variation trend was similar to soil organic matter and was opposite to that of soil available N content in 0~40 cm, and the soil surface layer variation was larger than others, which showed the effect of soil nutrient fatigue on slather inorganic nitrogen fertilizer. The decline of Ca saturation is a hidden process of soil degradation, and it is the main symptom of soil fatigue.3. Using different types and concentration nitrogen fertilizers could result in calcium degradation in calcareous soil, and the difference among nitrogen varieties showed accompany ionic effect.Calcareous soil exchangeable calcium content did go down after leached by equal volume of NH4Cl, 1/2(NH4)2SO4 and NH4NO3 (different NH4+ concentration). The influence decreased in this order: NH4NO3 < NH4Cl < (NH4)2SO4. Soil exchangeable calcium content was significantly negatively correlated negatively with incubation time during 7 days after adding carbamide into the soil and became stable in the following days. Overall, the results indicate that topsoil exchangeable calcium content gradually decreases with long-term heavy nitrogen fertilizer usage in a calcareous soil. Decalcification can make soil calcium saturation lower, down to 50% or less.4. Using phosphorus fertilizers could result in the decline of Ca saturation in calcareous soil. Taking KCl treatment as control, the soil exchangeable calcium content did go down after leached by different concentration KH2PO4 solution. There was a negative correlation between soil potent calcium(exchangeable Ca) and solution concentration, and the effect of KH2PO4 solution on soil calcium was very obvious. Excluding influence of potassium, PO43- concentration was higher, the effect on soil potent calcium and exchangeable Ca was larger. There was a negative correlation between soil CaCO3 content and solution concentration, too. When KH2PO4 solution concentration was above 0.2 mol/L, the decalcification was obvious. The increasing of PO43- concentration resulted in soil Ca saturation decreasing significantly. 5. Soil physics status was one of important factors of nutrient fatigue.The research of the influence of soil hardness to maize growth and calcium absorption showed there was no significant influence of soil hardness to maize growth in the first 15 days after seeding, however, inhibited aboveground maize growth in the following days, and maize height orders were R1.20 g.cm-3 >R1.10 g.cm-3 >R1.30 g.cm-3 >R1.40 g.cm-3 >R1.50 g.cm-3. And the bulk density of R1.10 g.cm-3 was too low, root could not contact well with soil, which was disadvantage to water and nutrient absorption, so its plant height was lower than R1.20 g.cm-3.Root dry mass and activity both decreased because of the high soil hardness which had a negative effect on calcium absorption. The calcium content in maize seedling, the lowest was 11.78 g.kg-1(R1.10 g.cm-3), the highest was 16.67 g.kg-1(R1.20 g.cm-3). The increase of soil hardness causes root activity decrease which lead to the decrease of calcium absorption, thus, lower crop resistance to disease and cause insenescence in advance. It was concluded that soil bulk density was too high or too low, which were disadvantage to crop growing. |
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