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Background and aims To test the hypothesis that dominant plant species could acquire different nitrogen (N) forms over a spatial scale and they also have the ability to compete for available N with microbes. Methods A short-term ¹⁵N labeling experiment was conducted in the temperate grassland ecosystem of North China in July of 2013. Three N forms ($NO_3^ - $, $NH_4^ + $ and glycine) labeled with ¹⁵N were injected into the two soil depths (0-5 and 5-15 cm) surrounding each plant to explore N acquisition by plants and microbes. Three dominant plant species (Artemisia frigida, Cleistogenes squarrosa and Artemisia capillaris) were investigated. Results Two hours after ¹⁵N labeling, all three dominant plant species absorbed both organic and inorganic N, but different patterns were observed at two soil depths. Uptake of $NO_3^ - $ was significantly higher at 0-5 cm than at 5-15 cm soil depth among all the dominant plant species. ¹⁵N recovery by microbes was significantly higher than plants. However, ¹⁵N recovery by plants showed different patterns over soil depths. Conclusions Dominant plant species in the temperate grassland have different patterns in acquisition of N added to soil in organic form and absorption of inorganic N, and microbes were more effectively than plants at competing for N in a short-term period.

It is important to accurately evaluate ecosystem respiration (RE) in the alpine grasslands of the Tibetan Plateau and the temperate grasslands of the Inner Mongolian Plateau, as it serves as a sensitivity indicator of regional and global carbon cycles. Here, we combined flux measurements taken between 2003 and 2013 from 16 grassland sites across northern China and the corresponding MODIS land surface temperature (LST), enhanced vegetation index (EVI), and land surface water index (LSWI) to build a satellite-based model to estimate RE at a regional scale. First, the dependencies of both spatial and temporal variations of RE on these biotic and climatic factors were examined explicitly. We found that plant productivity and moisture, but not temperature, can best explain the spatial pattern of RE in northern China’s grasslands; while temperature plays a major role in regulating the temporal variability of RE in the alpine grasslands, and moisture is equally as important as temperature in the temperate grasslands. However, the moisture effect on RE and the explicit representation of spatial variation process are often lacking in most of the existing satellite-based RE models. On this basis, we developed a model by comprehensively considering moisture, temperature, and productivity effects on both temporal and spatial processes of RE, and then, we evaluated the model performance. Our results showed that the model well explained the observed RE in both the alpine (R2 = 0.79, RMSE = 0.77 g C m−2 day−1) and temperate grasslands (R2 = 0.75, RMSE = 0.60 g C m−2 day−1). The inclusion of the LSWI as the water-limiting factor substantially improved the model performance in arid and semi-arid ecosystems, and the spatialized basal respiration rate as an indicator for spatial variation largely determined the regional pattern of RE. Finally, the model accurately reproduced the seasonal and inter-annual variations and spatial variability of RE, and it avoided overestimating RE in water-limited regions compared to the popular process-based model. These findings provide a better understanding of the biotic and climatic controls over spatiotemporal patterns of RE for two typical grasslands and a new alternative up-scaling method for large-scale RE evaluation in grassland ecosystems.

AIM: National and international policy frameworks, such as the European Union's Renewable Energy Directive, increasingly seek to conserve and reference ‘highly biodiverse grasslands’. However, to date there is no systematic global characterization and distribution map for grassland types. To address this gap, we first propose a systematic definition of grassland. We then integrate International Vegetation Classification (IVC) grassland types with the map of Terrestrial Ecoregions of the World (TEOW). LOCATION: Global. METHODS: We developed a broad definition of grassland as a distinct biotic and ecological unit, noting its similarity to savanna and distinguishing it from woodland and wetland. A grassland is defined as a non‐wetland type with at least 10% vegetation cover, dominated or co‐dominated by graminoid and forb growth forms, and where the trees form a single‐layer canopy with either less than 10% cover and 5 m height (temperate) or less than 40% cover and 8 m height (tropical). We used the IVC division level to classify grasslands into major regional types. We developed an ecologically meaningful spatial catalogue of IVC grassland types by listing IVC grassland formations and divisions where grassland currently occupies, or historically occupied, at least 10% of an ecoregion in the TEOW framework. RESULTS: We created a global biogeographical characterization of the Earth's grassland types, describing approximately 75% of IVC grassland divisions with ecoregions. We mapped 49 IVC grassland divisions. Sixteen additional IVC grassland divisions are absent from the map because of the fine‐scale distribution of these grassland types. MAIN CONCLUSIONS: The framework provided by our geographical mapping effort provides a systematic overview of grasslands and sets the stage for more detailed classification and mapping at finer scales. Each regional grassland type can be characterized in terms of its range of biodiversity, thereby assisting in future policy initiatives.

Given that plant growth is often water-limited in grasslands, it has been proposed that projected increases in precipitation could increase plant productivity and carbon sequestration. However, the existing evidence for this hypothesis comes primarily from observational studies along natural precipitation gradients or from short-term manipulative experiments. It remains unclear whether long-term increased precipitation persistently stimulates grassland productivity. In the world's largest remaining temperate grassland, we found that experimentally increased precipitation enhanced net primary production, soil-available nitrogen and foliar nitrogen concentrations during the first six years, but it ceased to do so in the following four years, unless nitrogen was simultaneously added with water. The ¹⁵N enrichment of plant and soil nitrogen pools in later years indicates increased nitrogen losses, which exacerbated nitrogen limitation and ended the stimulation of productivity by increased precipitation. Changes in species abundance might have contributed little to the changes in water treatment effects. Our study demonstrates that the long-term response of grassland ecosystems to increased precipitation will be mediated by nitrogen availability. Our results also point to a shift from co-limitation by water and nitrogen early to perhaps limitation by nitrogen only later in this temperate grassland, highlighting significant variations in the type of resource limitation induced by climate change.

1. Characterizing patterns of variation in plant traits across species and environmental gradients is critical for understanding performance of species in ecosystems. One-dimensional pattern of variation has been demonstrated in leaf traits, which is known as the leaf economic spectrum. However, it is unclear whether such a spectrum exists for root traits. 2. For roots of 15 species from temperate grasslands, we determined respiration rate, relative growth rate, life span and 10 morphological, chemical and anatomical root traits. We further evaluated pairwise and multiple-trait relationships by Pearson's correlation and principle component analysis including phylogenetic contrasts. 3. We found that root functions were related to three clusters of variation. Root respiration rate and relative growth rate were positively correlated with average root diameter (AD), but they were negatively correlated with specific root length (SRL). In contrast, root life span was not correlated with AD, but it was positively correlated with SRL. These results are inconsistent with the presumption of the root economic spectrum. 4. The principle components analysis revealed a multi-dimensional pattern of variation in root traits among the 15 coexisting herbaceous species. Moreover, species within the same phylogenetic clades tended to have similar root trait syndromes. Most of the root traits exhibited a significant phylogenetic signal. 5. Synthesis. Our results do not support a one-dimensional root economic spectrum in the coexisting herbaceous species of temperate grasslands. In contrast, the pattern of variation in root traits was multi-dimensional. We further demonstrated that species in different phylogenetic clades possess diverse root trait syndromes for efficient resource acquisition. Our findings provide a next step in understanding root functions and plant strategies in temperate grasslands.

Plasticity of root traits plays an important role in plant growth and survival under changing climate. Shift in precipitation is one of the most pertinent global change factors driving changes in structure and function of grasslands. However, few studies have investigated intraspecific variation of root traits in response to precipitation change under field conditions. We conducted a 10‐year simulated increased precipitation experiment in a temperate grassland and a 700‐km regional scale transect along a precipitation gradient ranging from 144.23 to 412.29 mm in northern China. The morphological, chemical and anatomical traits of the first two‐order roots were determined on 15 common herbaceous species in the manipulation experiment and two regionally common species (Leymus chinensis, Artemisia frigida) along a precipitation gradient. We found that most of the root traits of the herbaceous species exhibited no significant responses to water addition. The two regionally common species adjusted their root traits at sites with the annual precipitation lower than certain value, that is 250 and 160 mm for L. chinensis and A. frigida, respectively. These results indicate that root traits of the herbaceous species exhibit little plasticity in response to precipitation change and that the adjustment of root traits occurs when the range of annual precipitation exceeds a certain threshold. Root traits of L. chinensis and A. frigida varied differently both in manipulation experiment and along the precipitation gradient. Root traits of L. chinensis were relatively constant, while A. frigida adjusted their morphological root traits in response to water addition. Moreover, L. chinensis showed higher specific root length (SRL) and area, and root N contents at sites with annual precipitation lower than c. 250 mm. In contrast, A. frigida displayed thicker roots with lower SRL and area at sites with annual precipitation lower than c. 160 mm. Our results showed that root traits of herbaceous species in temperate grasslands exhibited little plasticity and that different species have evolved diverse adaptive strategies in response to precipitation change. These novel findings provide valuable information to predict responses of temperate grasslands to future climate change. 摘要 根系性状的种内变异反映了植物对变化环境的适应策略，是植物在变化环境中生存和生长的关键因素。降水变化是影响草地生态系统结构和功能重要的全球变化因子之一。然而，很少有研究探讨在野外条件下草原植物根系性状对降水变化的响应。本研究利用在内蒙古温带草原开展的野外长期（10年）增加降水控制实验和沿着从东到西的自然降水梯度(412.3–144.2 mm)设置的长约700 km的样带，分别选取了15种常见草本物种和2种区域广布种为研究对象，对其根系形态性状、化学性状和解剖结构进行测定，以期探讨草原植物对降水变化的适应策略。我们的研究结果发现：1）内蒙古典型草原植物的大部分根系性状对增加降水无显著响应，少数几个物种通过改变某几个根性状来适应降水的改变。沿着自然降水梯度，只有当降水量改变到一定程度之后，物种的表型性状才会有适应性调整。这些结果表明，内蒙古典型草原植物根系性状对降水变化响应的敏感性较小，可以适应一定范围的降水变化。2）在增加降水实验中，羊草的根系性状对增加降水无显著响应，而增加降水使冷蒿的比根长和比表面积降低而组织密度升高。沿着自然降水梯度，羊草在降水量低的样点有较高的比根长、比表面积和根N浓度，有较低的组织密度和C：N比，而冷蒿在降水量低的样点有较高的根系直径，和较低的比根长和比表面积。此结果表明，不同的物种通过多样的根系性状调整策略来适应降水的变化。 综上所述，内蒙古温带草原植物的根系性状对降水变化的响应敏感性较小，且不同植物种通过不同的表型调整策略来适应降水的变化。本研究为理解和预测温带草原植物对未来气候变化的响应提供了重要的理论基础。 A free Plain Language Summary can be found within the Supporting Information of this article. A free Plain Language Summary can be found within the Supporting Information of this article.

Aim Intensification of land use strongly impacts plant communities by causing shifts in taxonomic and functional composition. Mechanisms of land use‐induced biodiversity losses have been described for temperate grasslands, but a quantitative assessment of species‐specific occurrence optima and maximum tolerance (niche breadth) to land‐use intensity (LUI) in Central European grasslands is still lacking. Location Temperate, managed permanent grasslands in three regions of Germany. Methods We combined extensive field work with a null model–randomization approach, defined a “habitat niche” for each plant species based on occurrence and abundance across 150 grassland sites differing in LUI (i.e., amount of fertilizer, mowing/grazing intensity and a compound index of these), and assessed their realized niche breadth (tolerance). Underlying mechanisms driving species’ responses to LUI were assessed by relating plant functional traits, Ellenberg indicator values (EIV), Grime's ecological strategies (CSR) and Briemle utilization numbers. Results Out of 151 plant species, 34% responded negatively, whereas 10% responded positively to high LUI. This pattern was mainly driven by species’ response to fertilization and mowing frequency; grazing intensity response was less pronounced. Positively reacting species, displaying broader niches, were associated with competition‐related functional traits, high EIV for nutrient supply and moisture and high mowing tolerance under spatiotemporally variable conditions. Negatively responding species, displaying relatively narrow niches confined to spatiotemporally homogeneous low LUI sites, were associated with a nutrient‐retentive strategy, under nutrient‐poor, base‐rich soil conditions. Conclusion Our analyses of individual species’ reactions clearly demonstrate that species responding negatively to high LUI display little tolerance towards intensive fertilization and mowing, leading to plant diversity loss; whereas grazing partly thwarts these effects by creating new habitat niches and promoting ruderal species. Our approach can be applied to other habitat types and biogeographical regions in order to quantify local specific response or tolerance, adding to existing knowledge about local vegetation dynamics. Employing a combined null model–randomization approach based on species’ occurrence and abundance in temperate grasslands, we calculated plant species‐specific agricultural habitat niches and niche breadths and characterized response‐driving mechanisms by using plant functional and ecological traits. Our approach may be applied to any other habitat type for explaining and predicting community assembly and species coexistence in response to current land‐use practices.

To investigate large-scale patterns of above-ground and below-ground biomass partitioning in grassland ecosystems and to test the isometric theory at the community level. Northern China, in diverse grassland types spanning temperate grasslands in arid and semi-arid regions to alpine grasslands on the Tibetan Plateau. We investigated above-ground and below-ground biomass in China's grasslands by conducting five consecutive sampling campaigns across the northern part of the country during 2001-05. We then documented the root : shoot ratio (R/S) and its relationship with climatic factors for China's grasslands. We further explored relationships between above-ground and below-ground biomass across different grassland types. Our results indicated that the overall R/S of China's grasslands was larger than the global average (6.3 vs. 3.7). The R/S for China's grasslands did not show any significant trend with either mean annual temperature or mean annual precipitation. Above-ground biomass was nearly proportional to below-ground biomass with a scaling exponent (the slope of log-log linear relationship between above-ground and below-ground biomass) of 1.02 across various grassland types. The slope did not differ significantly between temperate and alpine grasslands or between steppe and meadow. Our findings support the isometric theory of above-ground and below-ground biomass partitioning, and suggest that above-ground biomass scales isometrically with below-ground biomass at the community level.

The temperate grasslands in China play a vital part in regulating regional carbon cycle and climate change. Net primary productivity (NPP) is a crucial index that reflects ecological function of plants and the carbon sequestration capacity of grassland ecosystem. Climate change can affect NPP by changing vegetation growth, but the effects of climate change on the NPP of China’s temperate grasslands remain unclear. Based on MODIS data and monthly climate data during 2000–2020, this study explored the spatiotemporal changes in grassland NPP and its response to climate change in temperate grasslands of China. We found that the annual NPP over the entire China’s temperate grasslands increased significantly by 4.0 gC/m 2 /year from 2000 to 2020. The annual NPP showed increasing trends for all the different grassland vegetation types, with the smallest increase for temperate desert steppe (2.2 gC/m 2 /year) and the largest increase for temperate meadow (5.4 gC/m 2 /year). The correlation results showed that increased annual precipitation had a positive relationship with the NPP of temperate grasslands. Increased summer and autumn precipitation could increase grassland NPP, particularly for the temperate meadow. With regard to the effects of temperatures, increased temperature, particularly the summer maximum temperature, could decrease annual NPP. However, increased spring minimum temperature could increase the NPP of temperate desert steppe. In addition, this study found, for the first time, an asymmetric relationship between summer nighttime and daytime warming and the NPP of temperate meadow. Specifically, nighttime warming can increase NPP, while daytime warming can reduce NPP in temperate meadow. Our results highlight the importance of including seasonal climate conditions in assessing the vegetation productivity for different grassland types of temperate grasslands and predicting the influences of future climate change on temperate grassland ecosystems.

Aims Soil organic nitrogen (N) turnover is significantly influenced by elevated N deposition, precipitation and human-caused disturbances, but the underlying mechanism remains unclear. Identifying the relationships among the soil organic N fractions and N-mineralizing enzymes activities may advance our knowledge of the dynamics of soil organic N. Methods A field experiment was conducted in a semi-arid steppe and an abandoned cropland in northern China to investigate the effects of elevated N deposition and precipitation on soil organic N fractions and their relationships with N-mineralizing enzymes, i.e., protease, amidase, urease and N-acetyl-β-D-glucosaminidase (NAG) activities. Results The concentrations of N in various fractions were consistently lower in the abandoned cropland compared with the steppe. Nitrogen addition consistently decreased amino acid N content and activities of urease, protease and amidase in both sites but increased amino sugar N content and NAG activity in the steppe. Water addition decreased hydrolysable ammonium N content but increased amino sugar N content and activities of protease and NAG in both sites. Furthermore, urease and NAG activities were significantly positively correlated with the proportions of amino acid N and amino sugar N and, explained significant proportions of the variations in soil organic N fractions in the steppe. However, soil organic carbon (C), rather than N-mineralizing enzymes, explained greatest proportion of the variations in soil organic N fractions in the abandoned cropland. Conclusions The concurrent increase of N deposition and precipitation could promote the recovery of soil N (and C) losses in the abandoned cropland resulting from previous agriculture. Furthermore, in the steppe where NH4+ was available at relative high concentrations, enzymatic mineralization was the dominant route involved in potential soil organic N turnover. However, the direct route may be favored over the enzymatic mineralization route with decreasing availability of C relative to N in the abandoned cropland, which is driven by the need for C. These findings confirmed that the forms of N available, and the relative availability of C and N determine N uptake pathways both through enzymatic mineralization route and direct uptake route in the semi-arid grasslands.