| Atmospheric nitrogen deposition has been one of the most attracted problems in the field of global change currently.With the aggravating of nitrogen deposition,it can alter the grassland ecosystem characteristics,which will make a definite effect on its function of soil and water conservation subsequently.Bothriochloa ischaemum was selected for this study,which was considered as the representative grass species of the gully area of Loess Plateau.Based on nitrogen addition to artificial planting B.ischaemum community for three years in succession,simulated rainfall experiments were conducted to systematically study how does the increasing soil available nitrogen influence the plant community characteristics and soil properties,as well as how does the reciprocity between the two influenced the loss processes of runoff,sediment.The main conclusions are as the following:(1)There seemed to be a threshold for effects of nitrogen application on the inflorescence number and plant height,in which the growth promotion under 2.5g N m-2yr-1 level was the most notable.The biomass and coverage of B.ischaemum community enhanced with the increase of nitrogen application.When the amount of nitrogen addition is high,root system could meet its growth needs with small absorbing area,showing that total surface area,specific surface area and root shoot ratio decreased with the augment of nitrogen application.And the root biomass density of B.ischaemum was the largest at 5g N m-2 yr-1 addition level.(2)As for the growth period of B.ischaemum,soil organic matter(SOM)had no clear changes at the early stage,and then increased significantly.With the growth of B.ischaemum,the ammonium nitrogen(NH4+-N)showed an increasing trend,but nitrate nitrogen(NO3--N)declined in the overall,to some extent,which also brought about no notable change in total nitrogen(TN).There were no prominent differences in total phosphorus(TP)over time,while available phosphorus(AvP)had an increasing trend.With regard to soil depth,the content of SOM,TN and NH4+-N decreased with soil depth increasing.The NO3--N content in soil surface was lower than that in deep layer,on the contrary,NH4+-N was not leached into deeper soil easily,making higher content in the surface layer.With the increase of soil depth,the content of AvP increased gradually.When it comes to different nitrogen treatments,with the increase of nitrogen application,the content of SOM increased at first and then decreased,with AvP content decreasing trend on the whole.In addition,the content of NH4+-N declined significantly.There was no significant difference in TN,TP and NO3--N among all treatments.(3)Characteristics of B.ischaemum community could affect the process of the slope runoff and sediment erosion markedly.The initial runoff time was closely related to the coverage and rainfall intensity.The initial runoff delayed with the coverage increasing and rainfall intensity decreasing(77~195s).Under the rainfall intensity of 60mm/h,the cumulative runoff and runoff rate of other nitrogen treatments were less than that of N0 and CK.With the augment of rainfall intensity,the cumulative runoff volume and the average runoff rate increased,nevertheless,no significant differences were found between different treatments.The cumulative sediment yield,sediment yield rate and concentration increased with the amplifying of rainfall intensity with all treatments,being 5.8g,0.07g/min and 0.05g/L,respectively,which were lower than other treatments at the 5g N m-2yr-1 level.The cumulative sediment yield increased with the runoff volume,and the growing rate in turn was N0>N2.5>N10>N5.The cumulative sediment yield cut down with the vegetation cover and the root biomass density increasing,and it can be expressed as the power function of the two factors and root biomass density(R2=0.85,p<0.01).(4)B.ischaemum communities had obvious seasonal variation characteristics,which would have an apparent impact on the runoff and sediment yield process.During the whole growing season(from June to September),the initial runoff time increased gradually(50~190s).There was no distinct difference in the cumulative runoff volume and runoff rate from July to September,but all were significantly higher than those in June.The cumulative runoff volume and runoff rate of high nitrogen treatments(N10 and N5)were higher than those of low nitrogen treatment(N2.5 and N0)in the whole growth period.Cumulative sediment yield,sand rate and concentration reduced with the growth time of B.ischaemum,and overall,the N0 treatment was the smallest,N2.5 treatment was the largest.The cumulative sediment yield was a power function growth along with the cumulative runoff volume increase throughout the plants’ growth,but the increase rates were not the same.And the N2.5 treatment had the largest increases,which increased by 24.4% to 92.1% than that of N0 treatment(the minimum).For different nitrogen treatments,the runoff and sediment yield had obvious inter-annual variability characteristics in B.ischaemum communities.When they had grown for three years,the cumulative runoff yield and stable rate increased by 4.1% and 8.2% compared with the second year,however,cumulative sediment yield and stable rate decreased by 75.0% and 68.7% than those of last year.(5)Nitrogen application would make a remarkable difference on biological crust types,showing that high nitrogen level could promote the development of algae crust,inhibiting the growth of moss crust.And the biological crust type could make further differences on its water stability.Biological crust can significantly affect the process of the runoff and sediment processes.Compared to the slope with biological soil crusts(BSCs),the initial runoff time decreased by 27.4%~59.6%.Regardless of presence of BSCs,there were no notable differences for cumulative runoff and runoff rate,with 4.3% reduction in total,compared to the slope with biological crusts.In addition,the cumulative sediment yield,average sediment rate increased by 97.2%,concentration increasing by 87.3% than the condition with BSCs.In general,the cumulative sediment yield was closely related to the biological crust thickness,vegetation coverage and runoff rate,which was a power function decrease along with the increase of biological crust thickness and vegetation coverage. |
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