A Study On Affecting Factors Of Soil Aggregate Movement And Nutrient Loss On Slopping Farmlands In The Black Soil Region Of Northeast China

The northeastern black soil area is one of the important regions for crop production andcommodity grains in China, which plays the key role in keeping national food security. Inrecent decades, the black soil region is facing some serious problems such as soil erosion anddegeneration which largely affect the production capacity. Therefore, preventing soil erosionand protecting valuable black soil resources have become an important issue. Based on thefield investigation, simulated rainfall experiments and laboratory analysises, the effects ofraindrop impact, rainfall intensities, slope gradient and straw mulch cover on soil aggregatemovement and nutrient loss on hillslope farmlands were investigated. The conclusions wereas follows:1. The effects of raindrop impact, rainfall intensiy, slope gradient and straw mulch coveron soil erosion in the slopping farmlands of the northeastern black soil region were studied.After placing nylon net cover above soil pans to eliminate raindrop kinetic energy, total runoffand erosion rate were reduced by6.7%and78.3%. When implementing straw mulch cover,the total runoff and erosion rate were reduced by16.3%and97.8%, compared to thebare-fallow treatment. The results showed that the raindrop impact was the crucial agent oferosion in sloping farmlands. Total runoff and erosion rate increased with an increase ofrainfall intensity or slope gradient. When slope gradient increased from5°to10°, erosion ratewas increased by63.4%to427.2%; and when rainfall intensity change from50mm/h to100mm/h, erosion rate was increased by260%.2. The effects of raindrop impact, rainfall intensity, slope gradient and straw mulch coveron aggregate movement were investigated. After eliminating raindrop kinetic energy, eachsize fraction of aggregate loss was significantly reduced, and the proportion of>0.25mmaggregates loss showed an increasing trend. Compared to the bare-fallow treatment, each sizefraction of aggregate loss was decreased by at least54.7%under the straw mulch cover.Micro-aggregate loss was dominated under the different rainfall intensities and slop gradients; and the loss from micro-aggregate accounted for more than60%of the total soil aggregateloss. The loss ratio of each aggregate size fraction was reduced by70%with an increase ofrainfall intensity, while the0.25²mm size fraction was sharply decreased. Each size fractionof aggregate loss and the loss proportion of>0.25mm size fraction were increased with anincrease of slope gradient. When slope gradient increased from5°to10°, the loss proportionof>0.25mm aggregate was increased by10.3times. The results showed that the slopegradient was the major influencing factor on the macro-aggregate movement. The mostobvious difference size fraction of aggregate loss was>5mm and0.25⁰.5mm under thedifferent slop gradients, the loss from>5mm and0.25⁰.5mm was increased by26.6timesand14.1times, compared to5°slope.3. The indices of the soil aggregates movement features were analyzed. Under threetreatments, MWD and GMD of lossed aggregates were decreased in order: the straw mulchcover> the nylon net cover> the bare land. The MWD and GMD of lossed aggregates underthe straw mulch cover were1.5times larger than that under the bare-fallow treatment, whichwas because the nylon net cover eliminated more than90%of the raindrops impact, and itwas also weaken the ability of raindrop impact to crush the soil aggregates. Compared to the50mm/h, the MWD and GMD of lossed aggregates were separately decreased by52.8%and27.4%under the100mm/h. The results showed that the slop gradient had the greater impacton the soil aggregates movement. By selecting indicators of aggregate loss and analysing therelationship among the various indicators, it was fund that the GMD was better describedaggregate loss and proposed that the GMD was positive correlated with MWD, and there wasnegatively correlated between MWSSA and D.4. The effects of raindrop impact, rainfall intensity, slope gradient and straw mulch coveron nutrient loss were explored. The nutrient loss was decreased under the nylon net cover orstraw mulch cover. After placing nylon net cover above soil pans to eliminate raindrop kineticenergy, the concentrations of NO₃-N, NH₄-N and HPO₄-P from runoff were decreased by15.5%,44.6%, and60.8%, respectively; the concentrations of NH₄-N and HPO₄-P fromeroded sediment were decreased by40.1%and31.7%, respectively. The nutrient loss fromslopping farmlands increased with an increase of rainfall intensity or slope gradient. Nutrientloss from sediment played an important part in total nutrient loss, which accounted for89%intotal nutrient loss. The main nutrient loss in eroded sediment was from nutrient content in<0.25mm aggregates. NH₄-N and HPO₄-P concentrations were enrichment in <0.25mm sizefraction.5. The main influencing factors of hillslope soil erosion and aggregates movement wereanalyzed. The mulch cover treatment was the key factor affecting the hillope soil erosion. The MWD from the aggregate loss showed that the slope gradient and mulch cover treatment hadgreater impacts, compared with rainfall intensity. Therefore, no-tillage measures caneffectively reduce the soil erosion in the black soil, and it can also eliminate the raindropimpact on the destruction of soil aggregates.