Soil Engineering Effects Of Farming Plots Reconstruction In Hilly-mountainous Region Of Chongqing

Under the severe stress of population, resources and environment, soil engineer ing is introduced to ensure food safety and agricultural sustainable development. Modern soil engineering is an integrated technology including protecting, managing, innovating and regulating soil resource. It has a broad scope, which contains reconstruction of farming plots. The farming plots are the fields surrounded by ditches, canals, roads and field ridges, and are the basic units of field work. Reconstruction of farming plots refers to land levelling of plots and field ridges building according to certain designing standards. The purpose of the engineering is to improve cultivated land fragmentation, increase areas of effective arable land and promote soil quality. Since reco nstruction of farming plots breaks the original natural conditions, economic environment and social attribute, it will affect the plots, soil and ecological environment in the engineer ing district. And these influence are likely to hinder the implementation of the objectives. In recently, researches on soil science have gradually converted focus from previous qualitative description to quantitative aspect. Considering with the complex of soil properties, soil scientists have experimented on changes of soil physical characteristics and soil nutrient characteristics under the reconstruction of farming plots, but the recent researches are still less and not enough. Soil engineer ing process is actually a kind of artificial soil forming process, and also a kind of soil reconstructed process directed by people towards human needs. Within the special geological landform, relatively poor land resources, and scattered and numerous plots, the hilly-mountainous region of Chongqing is at a low level of agricultural scaled and mechanization production. So this region is one area that needs most focus on reconstruction of farming plots in China. Thus, taken the reconstruction of farming plots in hilly-mountainous region of Chongqing as the breakthrough point, plots characteristics, soil profiles properties, soil physical and chemical properties, soil genesis process and erosion characteristics of new reconstructed soil before and after the engineering w ere comparative studies. Based on that, the soil engineering effects of farming plots reconstruction were explored to provide the bas is for regional agricultural land consolidation and agricultural scaled and mechanization production in hilly-mountainous region of Chongqing. The main results obtained as follows: 1. Changes of plot characteristics under farming plots reconstructionBy analyzing the field characteristics in six reconstruction districts of different local geomorphic typies areas(plain district, hill district and low mountain district) of Chongqing before and after the farming plots reconstruction, the following conclusions were gained. Farming plots reconstruction improved the most on the land fragment in plain district, followed by that in hill district and the low mountain district least.Through reconstructed engineering, the average plot scales had been enhanced in different degrees in the six engineer ing districts, plain district gaining the largest improvement of 2 to 4 times and hill district the minimum. Consequently, local plain district had the greater potential in improving plot scales. Reconstructed engineering for farming plots could significantly cut down the quantity of plots with a scale <0.1 hm2, nevertheless, the reconstructed plots seemed still difficult to meet plains mechanized farming requirements in hilly-mountainous region of Chongqing. The variation coefficient of plot scales showed a decreasing trend in plain district and hill district and that of low mountain district was enlarged on the contrary, illustrating that reconstructed engineering, with the weaker capacity to merge all the small fragmentized plots into a larger plot, was restricted in low mountain district. Most of the plots were originally cambered while slightly got structured after the reconstruction, appearing mostly in rectangular, trapezoidal and polygon. Plots in plain district became the most structured while that in low mountain district turned into the most irregular relatively. The intensity of human disturbance on the plain district was the largest, but low mountain district, with the larger difficulty in plots-structuring for the greater topographic inequality, received less influence from the human disturbance relatively. Simpon index was relatively high in the six study areas, showing the severe land fragment in hilly-mountainous region of Chongqing. Simpon index decreased and land fragment was improved after the reconstruction. Spatial autocorrelation analys is results of the plots area indicated that the plots distribution was mainly correlated positively in each reconstruction area. Moran’s I value were enhanced through reconstructed engineering in each study area and spatial autocorrelation was improved to the largest extent in plain district.Given the limited plot scale to be reached in hilly areas, we can set up the largest plots scale that could satisfy the requirement of the regional agr iculture large-scale production and then determined a cultivated land fragmentation index with the reference of one of the characteristic values, which enabled us to better reflect the changes of the area of land fragmentation degree. 2. Changes of soil properties under the reconstruction of farming plotsThrough the analysis of soil profile properties before and after the reconstructed engineering of farming plots, the main conclusions were as follows. The reconstructed engineering of farming plots to some extent changed the soil layer structure and soil physical and chemical properties in different short engineering periods(1 month, 7 months and 19 months).On the analysis of soil properties in study area before the reconstruction, the results showed that soil physical and chemical properties were better in paddy field soil profile developed from purple sand-shale than the same soil profile types of paddy field developed from purple sediments. Those soil profiles developed from purple sediment were located nearby the flood discharge trench, which resulted in a higher ground water level and the long-time flooded condition and finally produced negative effects on the soil aggregate properties and organic matter content. Under the same parent material, the levels of soil basic physicochemical characteristics in different land use types could be sorted as: traditional dryland for years < dryland transformed from paddy field < traditional paddy field for years. The single correlation of soil aggregate index, soil fractal dimens ion and soil basic physicochemical characteristics of dry land soil w as weaker than paddy soil. The relationships between the fractal dimens ion of soil particles and soil bas ic physicochemical characteristics were stronger than that between the fractal dimens ion of soil micro-aggregates and soil basic physicochemical characteristics.After the reconstruction of farming plots, the soil genetic layers of reconstructed profile were less clear than that before the reconstruction, and there were many changes in soil profile structure. The number of soil genetic layers in certain profile decreased after the reconstruction. There were four types of soil profile structure compared with the soil profiles before the reconstruction, namely, without changes, changes in number of soil genetic layers, new soil layer instead of old soil layer, and changes in order of soil genetic layers. Signif icantly changes of the soil properties were observed after the farming plots reconstruction engineering in topsoil(layer A). In reconstruction district(1 month), soil thickness and bulk density signif icantly increased 69.18% and 14.63%. Silt content and organic matter content signif icantly reduced 14.16% and 24.95%. PAD signif icant increased 58%, available water capacities signif icantly increased 92.36%, and soil available nutrient capacities increased slightly. In reconstruction district(7 months), average thickness of topsoil signif icantly increased to 25 cm, and gravels content was significantly increased to 5.64 ‰. Soil available water capacities signif icantly increased 102.96% than that before the reconstruction, while soil available nutrient capacities increased, too. The topsoil sand content was significantly reduced by 22.4% in reconstruction district(19 months). Soil silt and clay content were significantly increased by 26.39% and 28.76%, respectively. And topsoil organic matter content and CEC were signif icantly higher 9.59% and 13.84% than that before the reconstruction. PAD was significantly reduced by 33.22%, MWD significantly increased by 97.08%, and soil available nutrient capacities increased. Within 1 month engineering history, there were more adverse effects of farming plots reconstruction on soil properties. Within 7 months engineering history, the spatial variability of topsoil properties changed little, but the majority of soil properties indexes were not significantly different from that before the reconstruction. Within 19 months engineering history, the changes in soil properties had a tendency to positive effects. The correlation of topsoil properties in 3 regions had changed after the reconstruction, but this change in short-term reconstruction history did not have a regular ity. The effects of farming plots reconstruction on the deeper soil properties were lower than that of topsoil. The most significant effect on the deeper soil layers was that with the reconstruction history advancing, soil aggregate properties became worse firstly and then increased, and soil available phosphorus capacity increased.Under the cooperation of farming plots reconstruction and foreign soil engineer ing(here in after referred as “Foreign Soil Reconstructed Engineering”), the changes of soil physicochemical properties and genesis characteristics were analyzed, and the results were as follows. In topsoil of cover areas, the Foreign Soil Reconstructed Engineering improved the soil physicochemical properties and structure, changed the composition of the soil main elements, and changed the soil chemical weathering status although without changes in soil chemical weathering degrees.After the reconstruction, the soil horizon thickness was improved which remained stably(24 cm) within five years, the clay content increased, the sand content decreased, the soil nutrient content were increased, but the influence of reconstruction on soil p H was relatively small. Within f ive years after the reconstruction, the soil physicochemical properties were improved by the Foreign Soil Reconstructed Engineering. When the farming plots had been reconstructed about 1 year, the improvement of soil physicochemical properties was very obvious with the deep impact of the reconstruction; when over 5 years, this impact was weaken gradually, because soil physicochemical properties improved more slowly than before under the influence of tillage. When the farming plots had been reconstructed about 1 year, the soil element content were between the content of the origin soil and that of foreign soil; when over 5 years, some changes occurred in the soil of cover areas. The contents of Si O2 and Mg O decreased, while other soil elements increased slightly. The value of CIA could reflect the soil weathering status most accurately than others, and the indication of WIP was the worst. When the farming plots had been reconstructed about 1 year, the soil chemical weathering degree did not change, which was still the stage of plagioclase weathering and Na, Ca, Mg leaching with moderately weathered. However, the chemical weathering status was changed into the state between that of origin topsoil and foreign topsoil; when over 5 years, the value of CIA decreased slightly, the value of Na/K increased slightly, and the chemical weathering status was similar to status of origin soil. These results indicated that, the effects of the Foreign Soil Reconstructed Engineer ing on the origin soil chemical weathering state was weakened with the increasing engineering history.The Foreign Soil Reconstructed Engineering could alter the soil-forming process, and it would provide some new ideas for soil classifications. Although WRB has proposed the Anthrosols and Technosols, it does not contain the soil engineer ing and foreign soil engineer ing. Therefore, the scope and classification of Anthrosols should be further explored. The effects of the farming plots reconstruction and foreign soil engineer ing may be taken into consideration in future study of soil classifications. 4. Erosion characteristic of the new reconstructed soil under the reconstruction of farming plotsJust after the reconstruction of farming plots, soil was in an unstable state, so it could be named as “new reconstructed soil”. Six experiment plots were set to represent six different field width. The slope of all exper iments plots was 10 degree. By analyzing the soil erosion of the new reconstructed soil in different experiment plots under the natural rainfall, we got following conclusions. Compared to stable soil, the process of the runoff and sediment yield of the new reconstructed soil under 3. Changes of soil properties under t he cooperation of far ming plots reconstr uction and f oreign soil engineering different field width were more complex, and the single correlations between factors were weak.The influence of rainfall intens ity on runoff amount and soil loss amount was especially strong. Under high rainfall intensity, the runoff and sediment yield in experiment plots dramatically increased which was far greater than that under low rainfall intens ity. Therefore, it was very unfavorable for the new reconstructed soil in the period of soil instability to encountering with high intensity rainfall events. In the new reconstructed soil, “runoff degradation of soil erosion phenomena” was occurred in conditions of low rainfall, low rainfall intensity and wet soil. This phenomena showed that the runoff amount and soil loss amount in 30 m and 40 m plots were lower than that in 20 m and 50 m plots.The soil properties of the new reconstructed soil in every experiment plot had changed after a flood season. In the upper slope, sand content of topsoil increased by 9%~20%, whereas clay content decreased significantly; In the lower slope, clay content increased by 3% ~18%, while sand content obviously declined; In the middle slope, the changes of soil particle composition were the smallest. Macro aggregate were damaged and micro aggregate increased accordingly. The increment of soil bulk density was between 0.02 g/cm3 and 0.13 g/cm3, and the increment in the upper slope was higher than that in the lower slope. The variation range of soil available water was about-0.61 mm ~-1.92 mm, and the change rule of available water was in contrast with that of bulk density. Rainfall erosion on soil had a negative effect on soil organic matter. As the field width increased, the soil organic matter showed a decrease trend; The change rule of organic matter decreased as follows: upper slope > middle slope > lower slope. The available nitrogen, available phosphorus, and rapidly available potassium of soil all decreased after the flood season. The soil available nutr ients decreased as follows: upper slope > middle slope > lower slope. The characteristics of particle composition and nutrition of lost sediment had good regularity, along with the change of field widths. In sediment particle composition, 0.02-0.002 mm silt content was dominated, and the range was from 40.76% to 58.29%; Except for the experiment plot with 40 m field width, sand content showed an increase trend and silt content showed a decrease trend with the increasing of field width. In each experiment plot, the main component of micro aggregate of lost sediment was 0.02-0.002 mm, and the ratio of 0.02-0.002 mm micro aggregate was the highest one in the experiment plot with 5m width(66.08%) and lowest in experiment plot with 50 m width. With the increase of field width, the available nitrogen, available phosphorus, and rapidly available potassium of lost sediment declined firstly and then increased, and the values of them were the highest in the experiment plot with 5m width and the lowest in the experiment plot with 30 m.To sum up, plots characteristics, soil physical and chemical properties, soil genesis process and erosion characteristics of new reconstructed soil were all affected by farming plots reconstruction in hilly-mountainous region of Chongqing. Land fragment was improved, and plain district had the most profound effect. The most soil engineering effects of traditional farming plots reconstruction with 1 month engineering history were negative, while that with 19 months engineering history were positive. Cooperating with foreign soil engineering, farming plots reconstruction significantly impacted soil properties with one year engineer ing history, however this influence was greatly weakened after five years engineering history. After the reconstruction of farming plots, the erosion of new reconstructed soil was strongly affected by heavy rain events. And there was a turning point of soil and water loss in farming plot width with 30m-40 m.