Potential Nitrogenase Activity Of Biological Soil Crusts From Different Erosion Regions On The Loess Plateau, China

Biological soil crusts （biocrusts） that constituted by pioneer organismes such as algae,cyanobactira, mosses and lichens were extensively formed in the Loess Plateau region,China, which cover up70%soil surface in rehabilitated grasslands after the “Grain forGreen” eco-project was implementated in the region., Biocrusts may be an important partof nitrogen source for its nitrogen fixed organisms. In the paper, the study was conductedto determine the potential nitrogenase activity （NA） of biocrusts from the revegetatedgrasslands in different erosion type regions （water erosion region, wind-water crisscrosserosion regions and the wind erosion region） on the Loess Plateau, as well as theirresponses to variation in moisture and temperature. Then the statistical analysis andsimulation calculation were used to estimate the amount of nitrogen fixation by biocrusts.The purpose was to reveal the nitrogen contribution to soil of the biological soil crust,provide scientific basis for vegetation restoration in this area. The results are as follows:（1）Biocrusts are ubiquitous living covers in all the three erosion type regions inthe Loess Plateau region, NAs of the biocrusts showed siginificant differences betweenthe three three erosion type regions.The NAs of biocrusts of the three erosion type regions were highly variable. Underthe given the proper temperature and moisture conditions, the average of NA of biocrustsfrom water erosion, water-wind crisscross erosion and wind erosion regions were126.4,37.4and5.6μmol C-22H4·m·h^-1.（2）The NAs for biocrusts of the three erosion type regions and its responses tohydrothermic factors were different from each other.The NAs for biocrusts of the three erosion type regions and its responses tohydrothermic factors were different when measured under different hydrothermicalconditions. No significant difference was found in NAs for biocrusts from water erosionregions under100%～40%field water-holding capacity. However, the NA show a dramastic decline when the moisture decreased to20%field water-holding capacity. TheNA of biocrusts from the wind-water crisscross erosion regions showed a dramasticincrease when soil moisture was varied between60%and40%field water-holding capacity,while it showed unsignificant difference for soil moisture100%～80%and20%fieldwater-holding capacity. However, NAs of biocrusts in the wind erosion region showed adramatic decline when soil moisture was lower than100%field water-holding capacity, thenitrogen fixation of biocrusts was suspended when the moisture decreased to20%fieldwater-holding capacity.The optimal temperatures for biocrusts nitrogen fixation of the three erosion typeregions were respectively35℃（water erosion region）,25℃（wind-water crisscross erosionregions） and15℃（wind erosion region） when measured under adequate moistureconditions.（3） Soil nitrogen content was significantly improved for the formation ofbiocrusts, but the distribution of nitrogen in soil profiles in the three fifferent erosionstypes regions were different.The contents of soil total nitrogen (1.16～1.29g·kg^-1), alkali-hydrolyzable nitrogen(80.3～115.8mg·kg^-1), ammonium nitrogen (3.05～9.58mg·kg^-1) and microbial nitrogen(425.5～462.3mg·kg^-1) in the biocrusts layers was significantly higher than that in the0^-10cm layers in the three different erosion type region; The nitrate (0.52～2.76mg·kg^-1)content of biocrusts layers have no significant difference with the0^-10cm soil layer.Soil total nitrogen, alkali-hydrolyzable nitrogen and microbial nitrogen contents ofbiocrusts did not show significantly difference between before the rainy season（April-May）, rainy season （July-August） and the end of the rainy season （October-November） in the wind-water crisscross erosion region. And the soil total nitrogen,alkali-hydrolyzable nitrogen and microbial nitrogen contents of every soil layers amongthe three erosion type region were not significantly different.（4） The distribution pattern of the nitrogen forms were different between thethree erosion regions. No significant difference was found in soil total nitrogen,alkali-hydrolyzable nitrogen and microbial nitrogen contents in biocrusts layersbetween the the three erosion types regions. Soil nitrate nitrogen and ammoniumnitrogen in biocrusts layers and in the soil beneath showed a significant difference betweenthe three erosion regions.At the regional scale, the difference of the total nitrogen, alkali-hydrolyzable nitrogenand microbial nitrogen contents of biocrusts layers among the three erosion types area was not significant, the total nitrogen, alkali-hydrolyzable nitrogen and microbial nitrogencontents in the wind erosion type region were significantly lower than that in thewind-water crisscross erosion region and water erosion type region. While the soil nitratenitrogen and ammonium nitrogen is not the case, the nitrate nitrogen contents of biocrustslayer in wind erosion type region was higher than that in wind-water crisscross erosionregion and water erosion type region, but the nitrate nitrogen contents of0^-10cm soillayers in the wind erosion crisscross region and wind-water crisscross erosion region werehigher than that in the wind erosion region; Meanwhile, the ammonium nitrogen contentsof biocrusts layers in wind-water crisscross erosion region was lower than that in watercrisscross erosion region and water erosion type region, but the ammonium nitrogencontents of0^-10cm soil layers were completely opposite.（5） Biocrusts plays an importang role on soil nitrogen accumulaiton.Biocrusts-fixed nitrogen in the three different erosion types region were up to0.83,6.51,10.39kg·ha^-1·a^-1, while soil total nitrogen increase rate were5.36,3.54,4.90kg·ha^-1·a^-1.