Study Of Nitrogen Transformation And Its Influencing Factors Under Vegetation Restoration In Loess Hilly Region

In Loess Hilly Region,vegetation restoration can effective control soil erosion and improve ecological environment,but also affect the soil nutrient element fluxion between in plant-soil system.Soil nitrogen is also considered the limiting nutritional factor in the productivity of plants in terrestrial ecosystems,and generally very small but rapidly change during the plant growing season.However,the dynamic changes of different function nitrogen components with different vegetation restoration patterns are still poorly understood.Therefore,we studied the dynamic change of soil nitrogen components from April to October under five conversion lands: Robinia pseudoacacia(RP),Caragana korshinskii(CK),R.pseudoacacia and Platycladus orientalis mixed forest(RP + PO),R.pseudoacacia and Prunus davidiana mixed forest(RP + PD),and abandoned farmland(AF),which have been converted from slope farmland(SF)for 15 years in the Loess Hilly Region.Meanwhile,we set a laboratory incubation aim to understand the effect of key factors such as litter input,moisture and temperature on nitrogen transformation.The main conclusions are as follows:(1)Dynamic characteristicsThe average densities of nitrate nitrogen(NO3--N),soluble organic nitrogen(SON),microbial biomass nitrogen(MBN),mineral nitrogen and soluble total nitrogen(STN)changed significantly from April to October in the 0—30 cm soil layer(P < 0.05),and varied significantly with soil depth only occurred in NO3--N and MBN(P < 0.05).During the sampling period(April to October),the average NO3--N and NH4+-N density ranged from 0.13 g·m-2 to 1.71 g·m-2,from 0.17 g·m-2 to 0.74 g·m-2,reaching a minimum value on October and maximum on April.While the average density of SON varied significantly from 0.29 g·m-2(on April)to 2.92 g·m-2(on August),the MBN varied from 0.71 g·m-2(on June)to 5.97 g·m-2(on October).Based on correlation analyses,we found that the dynamic change of NO3--N,SON,STN,MBN were significantly affected by soil moisture and temperature(P < 0.05,P < 0.01).(2)Difference of soil nitrogen componentsAfter long-term vegetation recovery,the average densities of NO3--N,NH4+-N,MBN,STN and Min-N in vegetation recovery lands was significantly higher than that of the slope farmland in the 0—30 cm soil layer(P < 0.05),the rate of soil nitrogen components increased in the following order: MBN(increase rate was 90%—198%)> NO3--N and Min-N(35%—201%,25%—237% respectively)> NH4+-N(24%—73%)> STN(14%—33%).From the perspective of vegetation recovery pattern,the improvement trend was as follows: RP > CK > RP + PD,RP + PO > AF,and it was also observed that converting SF to RP and CK forests is an efficient method of nitrogen activation in the Loess Hilly Region of China.(3)The relevance between soil physical,chemical properties and soil nitrogen componentsBased on correlation analyses,we found that the dynamic change of NO3--N,SON,MBN were significantly affected by soil pH,total nitrogen(TN)and organic matter(SOM)(P < 0.01),among these factors,the nutrient components had a higher influence than soil physical properties.From another perspective,the NO3--N was more sensitive to the change of soil physical and chemical properties than others.(4)Effects of litter input on soil nitrogen mineralizationLitter input had a significant influence on soil mineral nitrogen accumulation during the incubation period,the change of NO3--N accumulation is the most significant.The performance of different treatments showed the following order: CK > RP,RP + PO(9:1)> RP + PD(1:1)> AF.The net nitrification rate was significant change in the CK,RP and RP + PO(9:1)litter input treatments.Moreover,the lower litter biomass C/N,the more mineral nitrogen accumulation and the more significant net mineralization rate change.(5)Effects of soil moisture and temperature on soil nitrogen mineralizationThe change of NO3--N accumulation is most sensitive to soil moisture and temperature change during the incubation period,and increased with the rise in temperature(38? > 25? > 12?)and moisture(60% > 30%).Moreover,the change of net nitrification rate increased with the incubation temperature rise,while there was no significant difference between different moisture treatments.