Ammonia (NH3) emission and redeposition play a major role in terrestrial nitrogen (N) cycles and can also cause environmental problems, such as changes in biodiversity, soil acidity, and eutrophication. Previous field grazing experiments showed inconsistent (positive, neutral, and negative) NH3 volatilization from soils in response to varying grazing intensities. However, it remains unclear whether, or to what extent, NH3 emissions from soil are affected by increasing grazing intensities in Inner Mongolian grasslands. Using a 5-year grazing experiment, we investigated the relationship between NH3 volatilization from soil and grazing pressure (0.0, 3.0, 6.0, and 9.0 sheep/hm2 ) from June to September of 2009 and 2010 via the vented-chamber method. The results show that soil NH3 volatilization was not significantly different at different grazing intensities in 2009, although it was higher at the highest stocking rate during 2010. There was no significant linear relationship between soil NH3 volatilization rates and soil NH4+-N, but soil NH3 volatilization rates were significantly related to soil water content and air temperature. Grazing intensities had no significant influence on soil NH3 volatilization. Soil NH3 emissions from June to September (grazing period), averaged over all grazing intensities, were 9.6±0.2 and 19.0±0.2 kg N/hm2 in 2009 and 2010, respectively. Moreover, linear equations describing monthly air temperature and precipitation showed a good fit to changes in soil NH3 emissions (r=0.506, P=0.014). Overall, grazing intensities had less influence than that of climatic factors on soil NH3 emissions. Our findings provide new insights into the effects of grazing on NH3 volatilization from soil in Inner Mongolian grasslands, and have important implications for understanding N cycles in grassland ecosystems and for estimating soil NH3 emissions on a regional scale.
YunHai ZHANGNianPeng HEGuangMing ZHANGJianHui HUANGQiBing WANGQingMin PANXingGuo HAN
Estimation of gross primary production(GPP)from remote sensing data is an important approach to study regional or global carbon cycle.However,for a given algorithm,it usually has its limitation on applications to a wide range of vegetation types and/or under diverse environmental conditions.This study was conducted to compare the performance of two remote sensing GPP algorithms,the MODIS GPP and the vegetation photosynthesis model(VPM),in a semiarid temperate grassland ecosystem.Methods The study was conducted at a typical grassland site in Ujimuqin of Inner Mongolia,North China,over 2 years in 2006 and 2007.Environmental controls on GPP measured by the eddy covariance(EC)technique at the study site were first investigated with path analysis of meteorological and soil moisture data at a daily and 8-day time steps.The estimates of GPP derived from the MODIS GPP and the VPM with site-specific inputs were then compared with the values of EC measurements as ground truthing at the site.Site-specific emax(a)was estimated by using rectangular hyperbola function based on the 7-day flux data at 30-min intervals over the peak period of the growing season(May to September).Important Findings Between the two remote sensing GPP algorithms and various estimates of the fraction of absorbed photosynthetic active radiation(FPAR),the VPM based on FPAR derived from the enhanced vegetation index(EVI)works the best in predicting GPP against the ground truthing of EC GPP.A path analysis indicates that the EC GPP in this semiarid temperate grassland ecosystem is controlled predominantly by both soil water and temperature.The site water condition is slightly better simulated by the moisture multiplier in the VPM than in the MODIS GPP algorithm,which is a most probable explanation for a better performance of the VPM than MODIS GPP algorithm in this semiarid grassland ecosystem.
Jianfeng LiuOsbert Jianxin SunHongmei JinZhiyong ZhouXingguo Han
Aims Elevated atmospheric CO_(2)has the potential to enhance the net primary productivity of terrestrial ecosystems.However,the role of soil microorganisms on soil C cycling following this increased available C remains ambiguous.This study was conducted to determine how quality and quantity of plant litter inputs would affect soil microorganisms and consequently C turnover.Methods Soil microbial biomass and community structure,bacterial community-level physiological profile,and CO_(2)emission caused by different substrate C decomposition were investigated using techniques of biological measurements,chemical and stable C isotope analysis,and BIOLOG-ECO microplates in a semiarid grassland ecosystem of northern China in 2006 and 2007 by mixing three contrasting types of plant materials,C_(3)shoot litter(SC_(3)),C_(3)root litter(RC_(3)),and C4 shoot litter(SC4),into the 10-to 20-cm soil layer at rates equivalent to 0(C0),60(C60),120(C120)and 240 g C m2(C240).Important Findings Litter addition significantly enriched soil microbial biomass C and N and resulted in changes in microbial structure.Principal component analysis of microbial structure clearly differentiated among zero addition,C_(3)-plant-derived litter,and C4-plant-derived litter and among shoot-and root-derived litter of C_(3)plants;soilmicroorganismsmainly utilized carbohydrates without litter addition,carboxylic acids with C_(3)-plant-derived litter addition and amino acidswith C4-plant-derived litter addition.We also detected stimulated decomposition of older substratewith C4-plant-derived litter inputs.Our results showthat both quality and quantity of belowground litter are involved in affecting soil microbial community structure in semiarid grassland ecosystem.
Aims Our aim was to study how diversity and dominance of plant species and plant functional types(PFTs)change and covary across three dune fixation stages in the Chinese steppe zone.Methods In the Chinese steppe zone,we measured coverage,mean height and density of each plant species in three types of dunes(mobile,semi-fixed and fixed dunes)in four sites(Mu Us,Otindag,Hulunbeir and Horqin).Plant species were grouped into 24 PFTs according to their lifespan,photosynthetic pathway,reproductive mode and life form.Dominance of each plant species and PFT were determined,and species diversity and PFT diversity were quantified using Shannon–Wiener index.Important Findings PFT diversity was positively related to plant species diversity in each dune stage,but PFT diversity increased more with increasing plant species diversity in the mobile and semi-fixed dunes than in the fixed dunes.Dune fixation stage explained 87.2%of the variation in plant species diversity and 84.8%of the variation in PFT diversity.Dominant species and PFTs differed among the three dune fixation stages;the more fixed the dunes were,the more perennial,shrubby,clonal and C3 species co-dominated.Specifically,in mobile dunes annual C4 non-clonal herbs were the most dominant,and in semi-fixed and fixed dunes perennial C3 clonal shrubs were most dominant.
