| As the main agricultural soil type, purple soil(Eutric Regosol, FAO Soil Taxonomic System) accounts for 85% of the Sichuan Basin in China, its fertility and crop production are directly related to food security of 0.13 billion population in this region. However, on the one hand, the known soil fertility was from the Second National Soil Survey between 1980 s and 1990s; while on the other hand, what changes in the fertility of purple soil in the past 30 years is unkonwn. In this study, using data sets of soil fertility and corp yield from the National Soil Testing and Fertilization projects that were complemeted in 2005, statistical analyses were employed to understand the current status of soil fertility and the relationship between soil fertility and crop yield in purple soil. In doing so, firstly, data were collected from a total of 252,275 soil samples and 18,646 crop yield surveys in 114 counties(cities) of the Sichuan Basin. Secondly, studies on the effect of carbon-based soil amendments on purple soil fertility were conducted with field, pot and incubation experiments to understand changes in the soil fertility. The generated results, such as effects of different carbon-based soil amendments on aggregate structure, water storage capacity, nutrient availability, and other physical and chemical properties of purple soil, could provide technical support for the improvement of soil fertility and crop production in the purple soil regions.The obtained major results were as follows. The averaged contents of soil organic matter, total nitrogen(N), available phosphorus(P), available potassium(K) in the Sichuan Basin were 16.4 g·kg-1, 1.02 g·kg-1, 14.9 mg·kg-1, 91.9 mg·kg-1, respectively. In general, the soil in Sichuan Basin lacks of organic matter and available K, and the contents of total N and available P was in a medium level. As compared to the Second National Soil Survey, except little changes in soil available K after nearly 30 years, their contents of organic matter, total N and available P were improved by 28.1%, 21.4% and 200%, respectively. However, the proportions of soils lacking of soil organic matter, total N, available P and available K accounted for 75.7%, 55.6%, 49.8% and 65.4% of the total soil areas across the Sichuan Basin, respectively. After 30 years, the soil p H, which showed substantial varitions among different purple soils, as an increase of 0.32 unit in the acid soil but a decrease of 0.54 and 0.43 p H unit in the neutral and calcareous soil, respectively.Analyses of using the averaged yield classification for crop production levels in the purple soil region showed the following distinctive resuts for a variety of crops. The proportions of medium and low crop yield levels by potato, rapeseed, wheat, maize were 49.6%, 67.6%, 67.0% and 79.0%, respectively, indicating that a great potential to enhance these rainfed crop yields in the purple soil region. Crop yields significantly related to soil basic productivity in purple soils, which contributed a range of 48.7% to 62% for different crops. These results sugguested that the improvement of soil fertility could be an important way to enhance crop production. The principal component analyses showed that soil organic matter and total N were the primary factors to increase soil fertility, and thus the crop yield.Results of yield increases from two years of field experiments using different carbon-based amendments(1,500 kg·hm-2 season-1) were as follows. Compared with the control, the yield of mustard and maize was increased by 8.6%-30% and 7.2%-21.7%, respectively. Meanwhile, after a 2 years(4 crop seasons) of amendments the content of >5 mm field soil mechanical-stable and water-stable aggregates was significantly enhanced by 5.38-18.29% and 4.66-11.35%, compared with control, respectively. In addition, the mean mass diameter of soil mechanical-stable and water-stable aggregates, soil aggregate stability index, soil soil porosity, soil water holding capacity, storage capacity and soil porosity, were all significantly increased, except a decrease in the soil bulk density.The biomass production and uptake of N, P, and K in mustard were increased with the application of carbon-based amendments in both the pot and incubation experiments. Comparing with the control treatment, with an input of 1% carbon-based amendments into soil, the four different amendment treatments decreased the soil bulk density by 2.5 to 5.8 % while increased the soil porosity by 0.9 to 2.7 %, water-holding capacity, soil water holding capacity by 13.7% to 21.4%, and soil organic matter by 6.3% to 14.4%, respectively. Meanwhile, both the total N, total P, soil available N, P and K, were significantly increased, by carbon-based amendments, except the total K. In addition, greater treatmental effects of on soil availiable N, avaliable P and available K ranked as organic fertilizer> amendment II> amendment I> biochar> control after a period of 120-d treatment application. The soil cation exchange capacity, exchangeable magnesium and K, and soil buffer capacity were significantly increasedby the applied amendments.In summary, soil organic matter and nutrients were increased in the purple rainfed region over a 30-year long-term period, though the soil fertility was still in a low level, while the application of carbon-based amendments showed a promising strategy to improve soil fertility. As a result, the application of carbon-based amendments under an optimal fertilization condition would be a promising strategy to improve soil fertility and crop production. |
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