| As important phenomenon of global changes,global warming and increasing deposition of atmospheric nitrogen(N)have large impacts on ecosystems and the entire biosphere and have attracted attention of the scientific community.The temperate semi-arid grassland ecosystems in northern China are sensitive to global changes and have large annual fluctuations of carbon(C)revenue and expenditure,and they may greatly change under global changes in the future.Grassland production and elements such as C,N,and phosphorus(P)play an important role in maintaining the stability of ecosystem function and structure,and they have important impacts on ecosystem C cycle.Previous studies on the effects of global changes on grassland production and C,N and P characteristics were mostly based on a single site,but few studies have conducted manipulative experiments simultaneously in several sites at a regional scale or along aridity gradient.How the responses of ecosystems to global changes vary with environmental gradient is still unclear.In this study,desert,typical,and meadow grasslands were selected along an aridity gradient from west to east in Inner Mongolia,and three study sites were set up in each grassland.Manipulative experiments with warming and N addition treatments were set up in each study site.The study aimed to investigate:(1)how plant production and C,N,and P characteristics in grassland ecosystems vary with an aridity gradient?(2)how warming and N addition affect plant production and C,N,and P characteristics in different grasslands?(3)how aridity affects the effects of warming and N addition?(4)are the results different depending on the approaches of manipulative experiment and gradient observation? The main results were as follows:(1)Along the aridity gradient,aboveground productions of grasses and forbs ranged from 37.2 to 128 and 13.2 to 149 g m-2,respectively.Total aboveground production and root biomass ranged from 58.9 to 259 and 725 to 2274 g m-2,respectively,and they both had contrary interannual variations due to the different effects of precipitations on biomass allocation.Root production and turnover ranged from 175 to 406 g m-2 yr-1 and 0.210 to 0.311 yr-1,respectively.Soil organic C,total N,and total P concentrations and root P concentration had no interannual variations,while interannual variations of root C and N and foliar C,N,and P concentrations varied with grassland types.These results were related to responses of soil nutrients and plant absorption to different precipitations.Total aboveground production was negatively correlated with aridity,but the effects of aridity on aboveground productions of grasses and forbs varied with years.Root biomass,root production,and root-shoot ratio were negatively correlated with aridity,while root turnover had no relationships with aridity.Soil organic C,total N,and total P concentrations were negatively correlated with aridity,but root N concentration were positively correlated with aridity,while aridity did not affect root P concentration.However,the relationships between root C or foliar element concentrations and aridity varied with years or functional groups.(2)Warming decreased aboveground productions of grasses and forbs by 36.6and 29.2% on average in dry year,respectively.However,due to the different demands of plant functional groups for water,warming decreased aboveground production of grasses by 30.2% but increased that of forbs by 23.1% on average in wet year.Warming significantly increased root-shoot ratio by 120% on average in dry year due to warming-induced decline in soil moisture.In contrast,warming reduced root production by 28.5% and root turnover by 28.2% on average,thus inhibiting root C input and nutrient cycle in grasslands.Ammonium N concentration in soils was increased by 37.7% on average under warming,indicating that warming significantly increased soil N mineralization.However,due to the different water conditions and plant absorptions in various grasslands,the effects of warming on nitrate N concentration varied with grassland types.Warming significantly increased available P concentration in soils by 11.7% on average,indicating that warming promoted the mineralization of organic P in soils.Warming tended to decrease soil organic C concentration,but C concentrations in roots and leaves were little affected.No significant changes of N concentrations in soils and roots were detected under warming,but warming can decrease N concentration in leaves of grasses while had no effects on N concentration in leaves of forbs.Warming obviously changed the coupling relationships between C and N or C and P in leaves or roots,and strengthened it between N and P in leaves,but the warming effects on the coupling relationship between N and P in roots varied with years.(3)N addition increased aboveground productions of grasses and forbs by 5.83 and 13.0% on average in dry year and 24.1 and 30.9% on average in wet year,respectively.N addition did not affect root production,but it increased root turnover by 13.2% on average in wet year.N addition tended to enhance ammonium N concentration in surface soil,and significantly increased nitrate N concentration by250% on average in soils,indicating that N input can stimulate N mineralization in soils.N addition decreased soil organic C concentration in dry grasslands and increased it in wet grasslands,but had little effects on C concentrations in roots and leaves,suggesting the high stability of C concentration in plants.N addition increased N concentration by 4.87% on average in the surface soil in dry year,but had no effects on soil N concentration or reduced it in meadow grassland in wet year due to the strong N absorption of plants in wet year.N addition significantly increased N concentrations in roots and leaves by 6.09 and 22.8% on average,respectively.Soil P concentration was increased in dry grasslands and reduced in wet grasslands by N addition in dry year,but was not affected in wet year.Effects of N addition on P concentrations in roots and forb leaves varied with grassland types,but N addition decrease P concentration in grass leaves by 4.77% on average.The reduced C:N ratio caused by N addition contributes to litter decomposition and nutrient cycle.N addition significantly changed the coupling relationships between C and N or C and P in leaves or roots,and strengthened it between N and P in leaves,but the effects of N addition on the coupling relationship between N and P in roots varied with years.(4)Along the aridity gradient,the effects of aridity on changes in aboveground production under warming or N addition varied with years,but aridity did not affect changes in root biomass and root-shoot ratio under warming.However,aridity promoted warming-induced but inhibited N addition-induced changes in root production and turnover.Aridity promoted the changes in organic C concentration in soils but had no significant effects on it in roots and leaves under warming or N addition,reflecting that the C concentrations in plants are relatively stable.Aridity had no significant effects on the changes in soil N concentration but inhibited the changes in root N concentration under warming or N addition.The changes in foliar N concentration were suppressed by aridity under N addition.Warming-induced changes in soil P concentration were increased by aridity,and warming or N addition-induced changes in root P concentration were inhibited by aridity,but the effects of aridity on changes in foliar P concentration induced by warming or N addition varied with years.Warming or N addition-induced changes in C:N ratios of soils,roots,and leaves were enhanced by aridity,but the changes in soil N:P ratio were suppressed and the changes in N:P ratios of leaves or roots were stimulated.(5)This study revealed that the responses of ecosystem production and C,N,and P characteristics to increases of temperature or soil N are largely different depending on the approach of manipulative experiment and gradient observation.That is,the short-term responses and long-term adaptability to global changes differed.Therefore,short-term manipulative experiments cannot really reflect the long-term adaptability of ecosystem production and C,N,and P characteristics to global changes,indicating that it is necessary to conduct long-term manipulative experiment.In summary,this study elucidated the effects of warming and N addition on grassland production and C,N,and P characteristics at a large regional scale by using methods of gradient observation and manipulative experiment.Warming and N addition directly or indirectly affected soil physicochemical properties,microbial activity,and plant photosynthesis,growth,or absorption by altering temperature,moisture,and nutrients,thus affecting grassland production and C,N,and P characteristics.Aridity gradient results in different vegetations and soil characteristics in various grasslands,thus regulating the effects of global changes.The methods of gradient observation and manipulative experiment exploring the responses of grassland ecosystems to environment factors can yield inconsistent results.Thus,it's necessary to conduct long-term positioning manipulative experiment.This study is helpful to deeply understand the processes of how global changes affect grassland ecosystems on a regional scale. |
PDF Full Text Request