Environmental Effects Of Soil Erosion & Impacts And Physical Mechanism Of Plant Roots On Loess Soil Environment

The study includes two parts, one of which studies on the environmental effects of soil erosion, especially on the soil organic carbon redistribution as affected by soil erosion, another part of which studies on the impacts of pines and grasses roots on mineral weathering and elements transport of Loess soil.Part A:The fate of soil organic carbon (SOC) transported and redistributed by soil erosion over the steep agricultural landscapes is uncertain. Studying on spatial and temporal variation in soil organic carbon (SOC) is of great importance because of global environmental concerns. Our objectives were to (â…°) test the potential of ¹³⁷Cs and ²¹⁰pb_ex for directly assessing SOC redistribution on sloping land as affected by tillage, (â…±) determine the spatial patterns of both tillage and water erosion-induced SOC redistribution, (â…²) evaluate the compensating effects of tillage-induced soil redistribution on SOC loss due to water erosion, and (â…³) quantify changes of SOC between 1898-1954 and 1954-1998 periods.The main conclusions were drawn from this study as follows:(1) The amounts of SOC, ¹³⁷Cs and ²¹⁰pb_ex of sampling soil profiles increased in the following order: lower＞middle＞upper positions on the control plot. Cesium-137 and ²¹⁰Pb_ex in soil profile were significantly linearly correlated with SOC with R² of 0.81, 0.86 (P＜0.01) on the control plot, and with R² of 0.90 and 0.86 (P＜0.01) on the treatment plot. Our results evidenced that ³⁷Cs and ²¹⁰pb_ex, and SOC moved on the sloping land by the same pathway during tillage operations, indicating that fallout ¹³⁷Cs and ²¹⁰Pb_ex could be used directly for quantifying dynamic SOC redistribution as affected by tillage erosion.(2) The hillslope soil had an 89% decrease of ¹³⁷Cs inventories for the last 45 years and a 55% decrease of ²¹⁰Pb_ex inventories for the last 100 years. Significant increase of SOC amounts at the lower field boundary on concave slopes of the summit and sharp decrease on convex slopes of the backslope resulted from tillage-induced soil redistribution by moldboard plowing whereas the overall losses of SOC over the entire hillslope are attributed to severe water erosion. Tillage-induced soil redistribution could increase SOC and compensated for 8 %-14 % of the SOC losses due to water erosion during 1898-1998, but on the whole soil erosion reduced SOC pool over the steep cultivated hillslope of the Loess Plateau. During the period 1898-1954, net SOC loss from the entire study hillslope was 1.65 t C ha^-1 at the rate of 30.01 kg C ha^-1 yr^-1. Within the period 1954-1998, the net SOC loss was 10.65 t C ha^-1 at the rate of 236.72 kg C ha^-1 yr^-1.(3) Soil loss rates due to crop harvesting (SLCH) can be comparable to water and tillage erosion rates. However, little is known about soil losses caused by manually harvested crops in China. The study investigated SLCH for potato and sugar beet plots on farmer's fields spread over four regions in northeast China where harvesting is carded out by hand. Soil losses for sugar beet were on average 1.0 Mg/ha/harvest, ranging from 0.2 to 1.9 Mg/ha/harvest and SLCH for potato ranged from 0.2 to 3.0 Mg/ha/harvest with an average of 1.2 Mg/ha/harvest. Soil moisture content, average root mass and plant density could explain 45 % to 67 % of the variability of SLCH for sugar beet. Although SLCH is not the dominant soil erosion process in NE China, it contributes to overall soil loss rates, which have already exceeded their critical tolerance limits in this region. Part B:Plant roots have potential impacts on elements transport and minerals weathering and leaching in soil, but the physical mechanism of these impacts has seldom been studied. Our objective is to classify physical mechanism of plant roots affecting weathering and leaching of loess soil. The large-size dug profile was used for measuring root density and an undisturbed monolith soil infiltration device was used for measuring elements transport flux in the soil. The multi-statistic analysis was conducted to quantify the relationship between effects of plant roots on soil environment and enhance of plant roots on soil weathering rates, in hilly and gully regions of the Chinese Loess Plateau.The results drawn from this study are as follows:(1) Differences among elements movement in soils were not depended on the amount of soil elements but on the distribution of plant roots of≤1mm in diameters. The increased effects of roots on soil elements movement in soil profile decreased as the soil depth increased. The effects of plant roots on soil element movement in soil profiles increased in the following order: forestland＞grassland＞farmland. The critical soil depth of different plant roots affecting elements transport in soil profiles was 30cm for forestry and 10cm for grassland. The movement of nutritious elements in soil profiles increased in the following order: Ca＞Na＞Mg＞K＞Si＞Al for the normal elements, and Cu＞Mn＞Fe for micro-elements.(2) The minerals weathering rates decreased with the increase of the soil depth. For the same vegetation type, minerals weathering rates and differences of the dominant weathering minerals type in soil profiles were mainly depended on the distribution of plant roots of≤1mm in diameters. Intensifying effect of root systems of pines on minerals weathering rate in soil profiles was greater than that of grass species. The soil depths of plant roots significantly intensifying minerals weathering rates were 0-45cm for pines and 0-30cm for grasses. The effective root density of pine-lands and grasslands on minerals weathering rates of soil were 17 roots per 100 cm² and 60 roots per 100 cm² in soil profile.(3) As for the pine-land during the heavy rainstorm, elements output flux and minerals weathering rates in the soil layer of 0-60cm decreased significantly with the increase of the soil depth; as for the farmland, there are no significant differences for the whole soil profile. The impacts tended sharply to lighter in the soil layer of 0-30cm for tree-land as the increase of soil depth, but approached slowly to smaller in the soil layer below 30cm. During the rainfall of 2.0mm/min, intensifying effects of pine roots on minerals weathering in the loess soil increased with the following order: Sodium Chloride＜Dolomite＜Silicate＜Calcite.(4) The improvement effect of the soil environment by plants roots are mainly controlled by the density and weight of the effective roots with the diameters less than 1mm. Plant roots have a stronger improvement effect on the physical properties of soil than on chemical properties. The dominant factors controlling soil weathering and leaching rates are, from high to low, infiltration enhancement, increase of bioactive substance in soil and stabilization of soil structure as affected by plant roots. Mathematic models were developed on the basis of a comprehensive and quantitative study on the dominant factors that affect the chemical elements transport and mineral weathering of soil.