| Biological soil crust(biocrusts)is a bio-ground material formed extensively in the Loess Plateau region,China,with a coverage of 60% to 70%,which significantly affects the soil nitrogen level.Disturbance can significantly affect the coverage,composition and biomass of biocrusts,which may cause changes in soil nitrogen accumulation and its cycle.However,little research was conducted on the influence of the disturbance on the soil nitrogen level and thetransformation of biocrusts,with the effects of nitrogen cycling on biocrusts.In the dissertation,the experiment was carried out on mature biocrusts that developed for more than ten years on there vegetated grasslands on the Loess Plateau.The effects of different grade and seasonal disturbance on the coverage,species composition,biomass and biomass of nitrogen fixation organisms were investigated.Based on the results of the studies mentioned above,the effects of disturbance on the nitrogen cycle of biocrusts,including the nitrogen input and loss of biocrusts wer estimated.Purposes of the study was to reveal the effect of disturbance on nitrogen cycle in soil with biocrusts covered,which would likely provide scientific basis for land management in the Hilly Loess Plateau region.The main results are as follows.(1)Disturbance alters the coverage,species composition,biomass,biomass of nitrogen fixation organisms,water,temperature and soil organic material contents,the change depended on the grade and slope and season of disturbance.The coverage of biocrusts was reduced by 15.1% at undisturbed,14.3% and 16.6% disturbance intensity,and the coverage of cyanobacteria biocrusts was increased by 9.2% at 8.6%,19.6% and 30.4% disturbance intensity.The stability of the algae-moss-lichen mixed crust was changed to the light cyanobacteria.With the increase of the disturbance grade,the cyanobacteria and moss biomass decreased significantly.At 19.6% disturbance intensity,15 ° slope,algae and moss crust biomass was significantly higher than the 25 ° slope 5.32 mg·g-1,0.10g·m-2.After the rainy season,the rate of biocrusts recovery was 6% ·a-1,and biocrusts recovered 3.5% in the rainy season and-4.4% in the dry season at the same recovery time(January 2016~July 2017).The biomass of nitrogen fixation Nostoc spp.and Collema spp.were decreased by 38.5% and 31.9%.With the increase of disturbance grade,the annual mean temperature of biocrusts decreased by 1.17~2.42?,and the average annual water content increased by 0.04%~3.70%.The soil organic material decreased by 4.5g·kg-1 after one-year disturbance.(2)The soil nitrogen content of biocrusts was reduced by disturbance,mainly concentrated in the surface 0~2cm.The degree of the reduction was depending on the grade of disturbance,land slope and season variation.Total nitrogen and mineralized nitrogen in the biocrustal layer was decreased by 0.13~0.40 g·kg-1 and 11.45~32.68 mg·kg-1 respectively after one year disturbance compared to the undisturbed treatment.The content of microbial biomass nitrogen in the biocrustal layer was reduced by 69.99~330.97 mg·kg-1,whereas the contents was increased by 25.51~352.17 mg·kg-1 in soil of 0~2cm layer.The change of nitrogen accumulation induced was depending on the grade of disturbance,and slight variation was observed in the nitrogen accumulation in biocrustal layer at 14.3% and 16.6% disturbance intensity,while significant reduction was fund in the 19.6% and 30.4% disturbance intensity.In 15 ° slope,nitrogen content of biocrusts was significantly higher than 25 ° at 19.6% disturbance intensity.In rainy season,nitrogen content of biocructs was higher than that in the dry season.(3)Biocrusts nitrogen transformation was affected by disturbance,urease activity,the number of ammonifying bacteria and nitrifying bacteria decreased,whereas the number of denitrifying bacteria increased,meanwhile soil nitrogen mineralization rate of biocrusts decreased,the degree of reduction was related to the disturbance grade.Biocrustal urease activity reduced about 1.08 to 1.61 mg·g-1·24h-1 after disturbance,decreased significantly at 19.6% and 30.4% disturbance intensity.The number of ammonifier and nitrifier of biocrustal layer were significantly higher than undisturbed treatment of 120.4%~2335.7%,1960.3%~4171.7% respectively,the number of denitrifier was lower 72.3%~97.3% compared with undisturbed treatment.The number of denitrifier was higher 97.3%~573.6% than that undisturbed treatment in soil of 0~2cm layer.The soil nitrogen mineralization of biocrusts mainly occurred in the surface 0~2cm layer(including biocrustal layer),and soil nitrogen mineralization rate decreased by 50% to 61%.(4)Disturbance affects soil nitrogen level of biocrusts,mainly influencing the nitrogen input by changing cyanobacteria coverage and nitrogen fixation time,and increasing the nitrogen loss by erosion.After one-year disturbance,biocrusts nitrogen input decreased by 2.33 kg N·ha-1·a-1 at undisturbed treatment,increased by an average of 2.90 kg N·ha-1·a-1at 8.6%,19.6% and 30.4% disturbance intensity,decreased by 3.78 kg N·ha-1·a-1at 14.3% and 16.6% disturbance intensity,which was related to cyanobacteria coverage and nitrogen fixation time.After the disturbance,soil nitrogen loss was increased,the loss of nitrate nitrogen was the most serious,and the loss of nitrate nitrogen in runoff and sediment was 1.22mg·m-2and 0.04mg·m-2 respectively. |
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