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Nitrogen (N) addition can increase the vegetative growth, improve the plant production, and restore the degraded terrestrial ecosystems. But, it simultaneously aggravates the soil phosphorus (P) limitation for plant growth, thus affecting its positive effects on ecosystems. However, how plants and soil microorganisms will change under conditions of high P content in soil is still unknown. In this study, we explored the effects of three levels of N addition (0, 7.5, and 15 g.N.m–2.year–1) on plants and microorganisms at the high P addition level (13.09 g.P.m–2.year–1) in the alpine steppe. We found that the soil microbial community composition had no significant difference between different N addition levels, and the soil AN and AP had a significant effect on the phospholipid fatty acid (PLFA) composition. The abundance of the core PLFAs (i.e., 16:1ω7c, 16:0, a17:1, i17:0, 18:1ω9c, and 18:1ω7c) also remained unchanged after N addition, and microbes at individual, population, and community levels were all correlated with SOM, AK, AN, and pH. Conversely, plant biomass and nutrient content showed linear trends with increasing N addition, especially the dominant functional groups. Specifically, the biomass and plant tissue N content of Gramineae, and the total N content of aboveground biomass were all improved by N addition. They were correlated with soil ammonium and AP. The structural equation modeling (SEM) demonstrated that N addition had a direct negative effect on soil microbial biomass, but an indirect positive effect on aboveground biomass via soil ammonium. These findings clarify the importance of N-amendment in regulating plants and microorganisms under high P conditions and provide a better understanding of the N-added effects in the alpine steppe.

Low temperature is widely regarded as the most important limiting factor in the alpine meadow ecosystem.The alpine meadow ecosystem represents a huge carbon pool that is witnessing rapid increases in air temperature.While warming may alleviate the low-temperature limitation to primary production,warming may decrease soil moisture by increasing the evaporation level and may stimulate ecosystem respiration.In addition,N may become a limiting factor if primary production enhances.This study focuses on how warming affects the H2O and N cycles,and how changes in these cycles affects carbon balance.First,the Biome-BGC model(V.4.2) is parameterized and validated using real time data from the Haibei Alpine Meadow Ecosystem Research Station,Chinese Academy of Sciences and then this model is used to assess the effects of warming.In the model increasing air temperature was set at 1.2—1.7℃ based on the previous results of the free-air temperature enhancement(FATE) experiment at the station.The simulated results showed warming increases evapotranspiration and decreased soil moisture in the growing seasons,while slightly increasing soil moisture in non-growing season.Warming slightly but significantly increases the litter's decomposition rate,and increases the annual concentration of inorganic N in soil by 7.9%.Warming promoteed both gross primary production(GPP) and heterotrophic respiration(RH).The increases of SLAI(specific leaf area index) and assimilation rates leaded to increased GPP.The GPP increased by up to 34.3% every year.Because of the enhancement of microbial activity and increased litter input,the warming increased average annual RH by 17.2%.RH increased by 24.9% during the non-growing seasons,whereas it only increases by 12.3% due to decreasing water content in soils during the growing seasons.The GPP increase surpassed RH during the growing seasons.Warming increased annual mean NEE(net absorption) by 29.6%.The results suggest that in the future warming scenario,the alpine meadow ecosystem may remain a weak carbon sink.Warming plays the most important role in the change of carbon flux.Carbon flux is also influenced by water loss caused by warming,and the role of water loss in some processes was prominent.The balance between the effects of warming on litter decomposition and on plant absorbing resulted in increase of soil inorganic N.N availability and H2O are not limiting factors on plant growth in the short term,but this may change in the long-term;this should be studied further in the future.Comparisons with newly published simulation results related to the Tibetan Plateau are also discussed.

Stomatal characteristics are used as proxies of paleo-environment. Only a few model species have been used to study the mechanisms of genetic and environmental effects on stomatal initiation. Variation among species has not been quantified. In this paper, results from an in situ reciprocal transplant experiment along an elevation gradient in the northeast Tibetan Plateau are reported, in which the relative effects of genetics (original altitude) and environment (transplant altitude) on stomatal density (SD) and length (SL) were quantified. In Thalictrum alpinum, only the environment significantly influenced SD, with the variance component ([Formula: see text]) of the environment found to be much greater than that of genetics ([Formula: see text]) ([Formula: see text]). In Kobresia humillis, only genetics significantly influenced SD and SL, with the genetics variance component found to be greater than that of the environment ([Formula: see text], for SD). These results suggest that the extent to which genetics and the environment determine stomatal initiation and development is species-specific. This needs to be considered when studying genetic or environmental controls of stomatal initiation, as well as when SD and SL are used as proxies for ancient climate factors (e.g., CO(2) concentration).

Soil organic carbon (SOC) was considered to be a key index in evaluation of soil degradation and soil C sequestration. To discuss the spatial-temporal dynamics of SOC in arable layer in reversed desertification area, a case study was conducted in Yulin City, Shaanxi Province, China. Data of SOC were based on general soil survey in 1982 and repeated soil sampling in 2003. Soil organic carbon content (SOCC) was determined by K2Cr3O7-FeSO4 titration method, and soil organic carbon density (SOCD) was calculated by arithmetic average and area weighted average method, respectively. On average, SOCC and SOCD of the arable layer in the study area from 1982 to 2003 had increased 0.51g/kg and 0.16kg/m2, respectively. Considering main soil types, the widest distributed Arid-Sandic Entisols had lowest values and increments of SOCC and SOCD during the study period; while the second widest Los-Orthic Entisols had higher values and increments of SOCC and SOCD, compared to the mean values of the whole region. The results indicated that reversed desertification process was due to the modification of land use and management practices, such as natural vegetation recovery, planting grass, turning arable land to grassland, and soil and water conservation etc., which can improve SOCC and SOCD and thus enhance soil C sequestration.