nghao Li

Ecosystem stability and compensatory effects in the Inner Mongolia grassland

Numerous studies have suggested that biodiversity reduces variability in ecosystem productivity through compensatory effects; that is, a species increases in its abundance in response to the reduction of another in a fluctuating environment. But this view has been challenged on several grounds. Because most studies have been based on artificially constructed grasslands with short duration, long-term studies of natural ecosystems are needed. On the basis of a 24-year study of the Inner Mongolia grassland, here we present three key findings. First, that January–July precipitation is the primary climatic factor causing fluctuations in community biomass production; second, that ecosystem stability (conversely related to variability in community biomass production) increases progressively along the hierarchy of organizational levels (that is, from species to functional group to whole community); and finally, that the community-level stability seems to arise from compensatory interactions among major components at both species and functional group levels. From a hierarchical perspective, our results corroborate some previous findings of compensatory effects. Undisturbed mature steppe ecosystems seem to culminate with high biodiversity, productivity and ecosystem stability concurrently. Because these relationships are correlational, further studies are necessary to verify the causation among these factors. Our study provides new insights for better management and restoration of the rapidly degrading Inner Mongolia grassland.

Rangeland degradation and restoration management in China

Rangelands of China have for centuries provided forage for livestock but now their role in water, soil, and biodiversity conservation is being recognised by Governments and people. However, much of the rangelands has recently degradedanddesertificationisnowawidespreadproblem.Thecauseofthedegradationisover-grazingandover-cultivation. Climatechangeisexacerbatingtheproblem.TheChineseGovernmentshavebeguntoaddressthesesevereproblemsthrough policyadjustmentsandprojects.Inparallel,someresearchanddevelopmentistakingplace.Therearemajorimpedimentsto addressing the problem; the importance of rangelands to China and its people are generally underestimated, legislative protectionisincompleteandoftenineffective,littleattentionispaidtoscientificknowledgefordevelopmentofmanagement of natural resources, there is insufficient technological support, and Governments are not able to invest sufficiently to effectively restore and develop rangeland natural resources. However, with this background we propose how the problems might be more effectively addressed in the future.

Nitrogen response efficiency increased monotonically with decreasing soil resource availability: a case study from a semiarid grassland in northern China

The concept of nutrient use efficiency is central to understanding ecosystem functioning because it is the step in which plants can influence the return of nutrients to the soil pool and the quality of the litter. Theory suggests that nutrient efficiency increases unimodally with declining soil resources, but this has not been tested empirically for N and water in grassland ecosystems, where plant growth in these ecosystems is generally thought to be limited by soil N and moisture. In this paper, we tested the N uptake and the N use efficiency (NUE) of two Stipa species (S. grandis and S. krylovii) from 20 sites in the Inner Mongolia grassland by measuring the N content of net primary productivity (NPP). NUE is defined as the total net primary production per unit N absorbed. We further distinguished NUE from N response efficiency (NRE; production per unit N available). We found that NPP increased with soil N and water availability. Efficiency of whole-plant N use, uptake, and response increased monotonically with decreasing soil N and water, being higher on infertile (dry) habitats than on fertile (wet) habitats. We further considered NUE as the product of the N productivity (NP the rate of biomass increase per unit N in the plant) and the mean residence time (MRT; the ratio between the average N pool and the annual N uptake or loss). The NP and NUE of S. grandis growing usually in dry and N-poor habitats exceeded those of S. krylovii abundant in wet and N-rich habitats. NUE differed among sites, and was often affected by the evolutionary trade-off between NP and MRT, where plants and communities had adapted in a way to maximize either NP or MRT, but not both concurrently. Soil N availability and moisture influenced the community-level N uptake efficiency and ultimately the NRE, though the response to N was dependent on the plant community examined. These results show that soil N and water had exerted a great impact on the N efficiency in Stipa species. The intraspecific differences in N efficiency within both Stipa species along soil resource availability gradient may explain the differences in plant productivity on various soils, which will be conducive to our general understanding of the N cycling and vegetation dynamics in northern Chinese grasslands.

Diversity‐dependent stability under mowing and nutrient addition: evidence from a 7‐year grassland experiment

Anthropogenic perturbations may affect biodiversity and ecological stability as well as their relationships. However, diversity-stability patterns and associated mechanisms under human disturbances have rarely been explored. We conducted a 7-year field experiment examining the effects of mowing and nutrient addition on the diversity and temporal stability of herbaceous plant communities in a temperate steppe in northern China. Mowing increased population and community stability, whereas nutrient addition had the opposite effects. Stability exhibited positive relationships with species richness at population, functional group and community levels. Treatments did not alter these positive diversity-stability relationships, which were associated with the stabilising effect of species richness on component populations, species asynchrony and portfolio effects. Despite the difficulty of pinpointing causal mechanisms of diversity-stability patterns observed in nature, our results suggest that diversity may still be a useful predictor of the stability of ecosystems confronted with anthropogenic disturbances.

Non-additive effects of water and nitrogen addition on ecosystem carbon exchange in a temperate steppe

Changes in precipitation and nitrogen (N) deposition can influence ecosystem carbon (C) cycling and budget in terrestrial biomes, with consequent feedbacks to climate change. However, little is known about the main and interactive effects of water and N additions on net ecosystem C exchange (NEE). In a temperate steppe of northern China, a field-manipulated experiment was conducted to evaluate the responses of NEE and its components to improve N and water availability from 2005 to 2008. The results showed that both water and N additions stimulated gross ecosystem productivity (GEP), ecosystem respiration (ER), and NEE. Water addition increased GEP by 17%, ER by 24%, and NEE by 11% during the experimental period, whereas N addition increased GEP by 17%, ER by 16%, and NEE by 19%. The main effects of both water and N additions changed with time, with the strongest water stimulation in the dry year and a diminishing N stimulation over time. When water and N were added in combination, there were non-additive effects of water and N on ecosystem C fluxes, which could be explained by the changes in species composition and the shifts of limiting resources from belowground (water or N) to aboveground (light). The positive water and N additions effects indicate that increasing precipitation and N deposition in the future will favor C sequestration in the temperate steppe. The non-additive effects of water and N on ecosystem C fluxes suggest that multifactor experiments are better able to capture complex interactive processes, thus improving model simulations and projections.

Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms.

• It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.

Response of the Abundance of Key Soil Microbial Nitrogen-Cycling Genes to Multi-Factorial Global Changes

Multiple co-occurring environmental changes are affecting soil nitrogen cycling processes, which are mainly mediated by microbes. While it is likely that various nitrogen-cycling functional groups will respond differently to such environmental changes, very little is known about their relative responsiveness. Here we conducted four long-term experiments in a steppe ecosystem by removing plant functional groups, mowing, adding nitrogen, adding phosphorus, watering, warming, and manipulating some of their combinations. We quantified the abundance of seven nitrogen-cycling genes, including those for fixation (nifH), mineralization (chiA), nitrification (amoA of ammonia-oxidizing bacteria (AOB) or archaea (AOA)), and denitrification (nirS, nirK and nosZ). First, for each gene, we compared its sensitivities to different environmental changes and found that the abundances of various genes were sensitive to distinct and different factors. Overall, the abundances of nearly all genes were sensitive to nitrogen enrichment. In addition, the abundances of the chiA and nosZ genes were sensitive to plant functional group removal, the AOB-amoA gene abundance to phosphorus enrichment when nitrogen was added simultaneously, and the nirS and nirK gene abundances responded to watering. Second, for each single- or multi-factorial environmental change, we compared the sensitivities of the abundances of different genes and found that different environmental changes primarily affected different gene abundances. Overall, AOB-amoA gene abundance was most responsive, followed by the two denitrifying genes nosZ and nirS, while the other genes were less sensitive. These results provide, for the first time, systematic insights into how the abundance of each type of nitrogen-cycling gene and the equilibrium state of all these nitrogen-cycling gene abundances would shift under each single- or multi-factorial global change.

The pattern between nitrogen mineralization and grazing intensities in an Inner Mongolian typical steppe

Ungulate grazing is known to play a crucial role in regulating energy flow and nutrient cycling in grassland ecosystems. However, previous studies of the effect of grazing on soil N dynamics have showed controversial results. Some studies indicate that grazing stimulates N mineralization while others report that grazing suppresses N mineralization. In order to reconcile these contrasting results, we investigated the response pattern of nitrogen transformation to multiple grazing intensities in an Inner Mongolian steppe. In our study, we measured net nitrogen mineralization rates and nitrification rates during a whole growing season in a 17-year field experiment that had five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha−1). Primarily because of changes in temperature and moisture conditions, net N mineralization rates varied substantially during the growing season with higher values occurring in late July. No consistent differences in net N mineralization rates were observed between grazing intensity treatments at the monthly time scale. Compared to mineralization rates, net nitrification rates were generally low with slightly higher values occurring in late July and late August. Ungulate grazing stimulated the cumulative net N transformations (mineralization, nitrification and ammonification) at the annual time scale, and the most stimulation occurred at a moderate grazing intensity of 4.00 sheep ha−1, whereas the highest grazing intensity of 5.33 sheep ha−1 and the lighter grazing intensity of 1.33 sheep ha−1 stimulated less. The general response of net N mineralization to grazing intensity gradient is roughly in the form of a normal distribution at the annual time scale. Our study demonstrated that grazing intensity in concert with soil moisture and temperature conditions imposed significant controls on soil N transformation and availability in this Inner Mongolian steppe.

Changes in nitrogen resorption traits of six temperate grassland species along a multi-level N addition gradient

Nitrogen (N) resorption from senescing leaves is an important mechanism of N conservation for terrestrial plant species, but changes in N-resorption traits over wide-range and multi-level N addition gradients have not been well characterized. Here, a 3-year N addition experiment was conducted to determine the effects of N addition on N resorption of six temperate grassland species belonging to three different life-forms: Stipa krylovii Roshev. (grass), Cleistogenes squarrosa (T.) Keng (grass), Artemisia frigida Willd. (semishrub), Melissitus ruthenica C.W.Wang (semishrub and N-fixer), Potentilla acaulis L. (forb) and Allium bidentatum Fisch.ex Prokh. (forb). Generally, N concentrations in green leaves increased asymptotically for all species. N concentrations in senescent leaves for most species (5/6) also increased asymptotically, except that the N concentration in senescent leaves of A. bidentatum was independent of N addition. N-resorption efficiency decreased with increasing N addition level only for S. krylovii and A. frigida, while no clear responses were found for other species. These results suggest that long-term N fertilization increased N uptake and decreased N-resorption proficiency, but the effects on N-resorption efficiency were species-specific for different temperate grassland species in northern China. These inter-specific differences in N resorption may influence the positive feedback between species dominance and N availability and thus soil N cycling in the grassland ecosystem in this region.

Grazing alters the biophysical regulation of carbon fluxes in a desert steppe

To determine the role of grazing on CO2 fluxes in a desert steppe, we used paired eddy-covariance systems to measure the net ecosystem exchange (NEE) and microclimate on adjacent pastures of grazed (GS) and ungrazed (FS) steppes on the Mongolian Plateau from 2010 to 2011. The first year was an average precipitation year, while the second year was a dry year. In 2010, there was 91% greater growing seasonal gross ecosystem production (GEP) and 55% greater ecosystem respiration (Re) in the GS than in the FS. As a result, the GS acted as a net carbon uptake of 20 g C m 2 while the FS was a small net carbon release of 10 g C m 2 . The greater GEP was mainly caused by the greater photosynthetic capacity due to the suitable environmental conditions and longer growing time in a day and in the growing period accompanied by the enhanced Re that seemed to be responsible for the increased NEE, which compensated for the lower leaf area in the GS. However, an inverse trend was detected in 2011. The seasonal cumulative GEP, Re and NEE were characterized with 92% greater GEP and similar Re in the FS compared with the GS. As a result, the FS acted as a small net carbon uptake of 5 g C m 2 , while the GS was a net carbon release of 59 g C m 2 . Although the GS had greater carbon uptake in 2010, the variation of daily NEE from both years was lower in the FS, suggesting that the FS has a greater resistance to the changing climate. This also means that future modeling effort should consider year-to-year differences in the carbon balance because relationships between fluxes and climatic regulators change annually in different land use change scenarios.