RELATIONSHIP OF ECO-ENVIONMENTAL CHANGE WITH NATURAL EROSION AND ARTIFICIALLY ACCELERATED EROSION

The natural landscape of the Loess Plateau was changed by severe soil erosion. The Ziwuling forest area provides research base for tracing back eco environmental change related to natural erosion and artificially accelerated erosion. Using methods of typical region investigations, in situ experimental study and chemical analysis of samples, impact of vegetation destruction and rehabilitation on soil erosion, characteristics of natutal erosion under conditions of natural ecological balance and artificially accelerated erosion resulting from vegetation destruction in forest area, and the processes of artificially accelerated erosion and soil degradation have been analyzed and discussed.

Effects of Accelerated Soil Erosion on Soil Nutrient Loss After Deforestation on the Loess Plateau

Soil erosion and nutrient losses on newly-deforested lands in the Ziwuling Region on the Loess Plateau of China were monitored to quantitatively evaluate the effects of accelerated soil erosion, caused by deforestation, on organic matter, nitrogen and phosphorus losses. Eight natural runoff plots were established on the loessial hill slopes representing different erosion patterns of dominant erosion processes including sheet, rill and shallow gully (similar to ephemeral gully). Sediment samples were collected after each erosive rainfall event. Results showed that soil nutrients losses increased with an increase of erosion intensity. Linear relations between the losses of organic matter, total N, NH4-N, and available P and erosion intensity were found. Nutrient content per unit amount of eroded sediment decreased from the sheet to the shallow gully erosion zones, whereas total nutrient loss increased. Compared with topsoil, nutrients in eroded sediment were enriched, especially available P and NH4-N. The intensity of soil nutrient losses was also closely related to soil erosion intensity and pattern with the most severe soil erosion and nutrient loss occurring in the shallow gully channels on loessial hill slopes. These research findings will help to improve the understanding of the relation between accelerated erosion process after deforestation and soil quality degradation and to design better eco-environmental rehabilitation schemes for the Loess Plateau.

Developmetn stage threshold of watershed Landforms in Loess Plateau and separation of erosion mechanism

Threshold is a limit and marginal point when a qualitative change takes place. Since threshold principle was firstly used in landform research by S.A.Schumm in the 1970s, the quantitative research on watershed landform development stage has come true. Davis proposed a three-stage model on landform evolution: young, mature and old stages. Thereafter Strahler quantified this model by hypsometric analysis method. The authors thought that the material movement stage cannot be expressed by hypsometric method in watershed landform at development stage, because of the uncertainty on stage delimitation. To meet this shortcoming, this paper presents an integral erosion value method. A clear delimitation on landform development stage in the Loess Plateau region has been tested by this method. The result shows that gullied loessial hilly area is at the mature stage, and gullied loessial tableland area is at the young stage. It is estimated that from the point of erosion related sediment yield, natural erosion accounts for 70% of the total erosion amount, and artificial accelerating erosion is 30%. Therefore soil and water conservation is very crucial for the Loess Plateau.

Soil conservation and ecosystem services

Accelerated soil erosion,driven by anthropogenic activities such as conversion of natural ecosystems to agroecosystems and mechanical tillage,has numerous adverse impacts on ecosystem services.In addition to degrading soil quality and reducing agronomic/biomass productivity on-site through a decrease in use-efficiency of inputs,off-site impacts of accelerated erosion include eutrophication and contamination,sedimentation of reservoirs and waterways,and emissions of greenhouse gases(e.g.,CO_(2),CH_(4) and N_(2)O).While advancing food and nutritional security,adoption of restorative land use and recommended management practices are important to strengthening numerous ecosystem services such as improving water quality and renewability,increasing below and above-ground biodiversity,enhancing soil resilience to climate change and extreme events,and mitigating climate change by sequestering C in soil and reducing the emission of CO_(2),CH_(4) and N_(2)O.An effective control of accelerated erosion is essential to sustainable development and improving the environment.