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 Sep- tember （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 volatili- zation 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
Aims Nitrogen(N)and phosphorus(P)are limiting nutrients to life across a variety of ecosystems.N:P stoichiometry,concerning the balance of these two elements,has recently received great attention.However,little is known about the nature of N:P stoichiometry in obligate mutualism.Methods N:P stoichiometry of Ficus racemosa and its pollinating wasp Ceratosolen fusciceps,an example of coevolving obligate mutualism,was investigated,and the N:P stoichiometric traits of male versus female wasps were compared.Important Findings Nutrient concentrations in C.fusciceps were much higher than in its host.N enrichment in fig wasp was evidently stronger than phosphorus.N concentrations of male fig wasps were significantly higher than those of females,while P concentrations of female fig wasps were remarkably higher than those ofmale ones.Therefore,N:P ratios inmale fig wasps were significantly greater than in female fig wasps.N:P ratio in fig-pollinating wasp displayed linear functions to fig N contents,suggesting that N limitation in fig wasps may dominate the nutritional relationship between fig pollinator and its host.Fig wasp population size had significant influences on N concentrations in host fig and female wasp per se.Driven by the nutritional stress of pollinating and parasite insects,fig fruit preferred increasing its diameter first but not nutrient richness.Values forNand P contents of fig pollinators showed seasonal differenceswith greater N:P ratios in dry season than in rainy season.The observations suggest that tropical climate change would result in more severe N limitation to fig-pollinating wasp and may further influence the stability of fig–fig wasp mutualism.
Nitrogen deposition has dramatically altered biodiversity and ecosystem functioning on the earth; however, its effects on soil bacterial community and the underlying mechanisms of these effects have not been thoroughly examined. Changes in ecosystems caused by nitrogen deposition have traditionally been attributed to increased nitrogen content. In fact, nitrogen deposition not only leads to increased soil total N content, but also changes in the NIL^-N content, NO3--N content and pH, as well as changes in the heterogeneity of the four indexes. The soil indexes for these four factors, their heterogeneity and even the plant community might be routes through which nitrogen deposition alters the bacterial community. Here, we describe a 6-year nitrogen addition experiment conducted in a typical steppe ecosystem to investigate the ecological mechanism by which nitrogen deposition alters bacterial abundance, diversity and composition. We found that various characteristics of the bacterial community were explained by different environmental factors. Nitrogen deposition decreased bacterial abundance that is positively related to soil pH value. In addition, nitrogen addition decreased bacterial diversity, which is negatively related to soil total N content and positively related to soil NOa--N heterogeneity. Finally, nitrogen.addition altered bacterial composition that is significantly related to soil NH4＋-N content. Although nitrogen deposition significantly altered plant biomass, diversity and composition, these characteristics of plant community did not have a significant impact on processes of nitrogen deposition that led to alterations in bacterial abundance, diversity and composition. Therefore, more sensitive molecular technologies should be adopted to detect the subtle shifts of microbial community structure induced by the changes of plant community upon nitrogen deposition.
