The Responses Of Degraded Typical Steppe Vegetation And Soil To Nitrogen And Phosphorus Fertilizations In Inner Mongolia

Inner Mongolia locates in the northern China,which is rich in steppe resources.However,the steppes in Inner Mongolia have been severely degraded in recent decades because of increasing economic and population pressures,showing severe deleterious symptoms such as smaller plant height,lower vegetation cover,reduced aboveground and belowground biomass,and the replace of dominant and sub-dominant species.With the decreasing steppe quality,the steppe has lost their original functions of sand-fixation,water conservation,air purification and environment protection.This continuous degradation ultimately led Inner Mongolia steppe ecosystem to the brink of collapse.Therefore,it is urgent for us to take measures in recovering the degraded steppes.Fertilization,an agricultural measure that can rapidly improve the soil nutrient availability,has been gradually used in recovering the degraded steppe.A large number of research has proved that proper fertilization can promote the recovery of soil and vegetation of degraded steppe.However,the differences in types of steppe as well as the soil and climate conditions,particularly the precipitation,cause different response of steppes to fertilizations.In typical steppe with low precipitation,whether or not the fertilization can improve the grassland productivity is still uncertain.Understanding the responsive mechanisms of degraded typical steppe to nitrogen and phosphorus fertilization will provide basic theories and technical guidance for making reasonable grassland management measures.The current experimental site was located in Xilingol,Inner Mongolia.In June 2014,about 50 km2 of degraded steppe was fenced and freed from graze.A two-way completely randomized factorial design was employed.There were 27 plots in total with 400 m2(20m×20m)for each plot.The fertilizer was applied in July 2014,June 2015 and June 2016.The plant and soil samples were collected in August every year.We analyzed the aboveground and belowground biomass,plant composition,contents of nitrogen and phosphorus in dominant specie(Stipa krylovii Roshev),contents of soil nitrogen and phosphorus,and the enzyme activities.The main results were as follows:1.Overall,nitrogen fertilization promoted the aboveground biomass of steppe.However,the effects of fertilization varied among three years.The precipitation in 2015 was 412 mm,which was higher than that in 2014(255mm)and in 2016(198mm).The aboveground biomass increased with increased fertilization rates in 2015,whereas ?unimodal? relationship appeared between the nitrogen fertilization and aboveground biomass in 2014 and 2016,where the aboveground biomass in N1 was higher than that in N0 and N2.The results suggested that the nitrogen threshold was low in normal dry year and low nitrogen rate could meet the needs of plant for nitrogen,and the productivity will be reduced when excessive fertilization was applied.In current study,the nitrogen threshold was about 50kg/hm2 in normal dry year.The effect of fertilization on root biomass was not significant.2.The population structure and plant species proportion of degraded typical steppe were relative stable,with the proportion of dominant plant species insensitive to the fertilization.During the three years of fertilization,the proportion of gramineous plants remained above 70%,with Stipa krylovii Roshev accounted for above 60%.The proportions of other plant species were small and were not influenced much by fertilization.Overall,fertilization played a positive role in the recovery of degraded steppe in three years.Some plants which are typical in degraded steppe gradually disappeared,such as the proportion of Convolvulus ammannii changed from 7.8% in 2014 to 2.23% in 2015 and 0.23% in 2016.3.Fertilization significantly improved the forage quality.With the increasing rates of nitrogen and phosphorus fertilization,the contents of total nitrogen and phosphorus in Stipa krylovii Roshev was improved.Compared with 2014 and 2015,total nitrogen content in 2016 increased by 44.14% and 52.83%,total phosphorus was increased by 62.18% and 37.31%.With increasing phosphate fertilization rates,the stoichiometric characteristic value(the ratio of nitrogen and phosphorus)of Stipa krylovii Roshev reduced significantly,suggesting that the limitation of phosphate to plant gradually decreased.4.Fertilization positively improved soil fertilities.The contents of available nutrients improved with increased fertilization years.Compared with those in 2014 and 2015,the contents of alkali-hydrolysable nitrogen in 2016 increased by 17.77% and 8.63%,the contents of available phosphorus increased by 9.42% and 35.41%,the contents of organic matter increased by 30.18% and 56.99%.The contents of soil total nitrogen and phosphorus also showed an increased trend with insignificant changes.At 0-10 cm soil layer,the contents of total nitrogen was 1.253 g/kg,1.254 g/kg,1.297 g/kg,and the total phosphorus was 0.306 g/kg,0.341 g/kg and 0.393 g/kg,in 2014,2015 and 2016,respectively.The contents of soil nutrients in at 10-20 cm soil layer changed little.5.The effect of fertilizer application on soil enzyme activities was influenced by soil physical and chemical properties.Correlation analysis indicated that the contents of available phosphorus and organic matter were significantly negatively correlated with soil enzyme activity.In the first two years,there was a trend that the enzyme activity increased with the increase of fertilizer rates.In early stage,the soil nutrient availability was low,particularly the contents of available phosphorus and organic matter.The enzyme activity was sensitive to fertilization at low soil fertility.The substrate concentration of soil urease enzymes could be quickly improved by fertilization,so it made the enzymes active.The improvement of urease activities attributed to the increase of soil nutrient contents.In 2016,the soil urease enzyme activity showed reduced trend with increased fertilization rates.With the increasing of fertilization time,the content of soil nutrient was improved gradually.Soil urease catalytic capacity is close to saturation.The urease activity response to fertilization is not obvious.To the phosphatase,the fertilization increased its number and yield in the first two years,so the phosphatase activity was improved.However the effect of promotion was transformed into inhibition.