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The major aims of this study were, firstly, to analyse the grazing-induced steppe degradation process and, secondly, to identify an efficient and sustainable grazing management system for the widely degraded Inner Mongolian typical steppe ecosystem. From 2005-2008 a grazing experiment was conducted to compare two grazing management systems, the Mixed System (MS) and the Traditional System (TS), along a gradient of seven grazing intensities, i.e. ungrazed (GI0), very-light (GI1), light (GI2), light-moderate (GI3), moderate (GI4), heavy (GI5), and very-heavy (GI6). Each grazing intensity treatment was considered a production unit comprising two adjacent plots, one for hay-making (single-cut system) and one for grazing. Hay-making and grazing alternated annually in the MS, while in the TS the same plots were used either for hay-making or for grazing. Effects of management system, grazing intensity, and year on end-of-season standing biomass (ESSB), aboveground net primary production (ANPP), relative difference in ANPP between 2005 and 2008 (ANPPDiff), relative growth rate (RGR), and sward characteristics (litter accumulation, soil coverage) were analysed. Litter accumulation of production units was affected by grazing intensity (P < 0.001) and decreased from GI0 to GI6 by 83%. Correspondingly, soil coverage decreased (P < 0.001) from GI0 to GI6 by 43%, indicating an increased vulnerability to soil erosion. We found varying compensatory growth responses to grazing intensity among years, probably because of temporal variability in precipitation. The ability of plants to partially compensate for grazing damage was enhanced in years of greater seasonal precipitation. The ANPP of production units was negatively affected by grazing intensity and decreased from GI0 to GI6 by 37, 30, and 55% in 2006 (P < 0.01), 2007 (P < 0.05), and 2008 (P < 0.001), respectively. The effect of management system × grazing intensity interaction on ANPP (P < 0.05) and ANPPDiff (P < 0.05) suggested greater grazing resilience of the MS as compared to the TS at GI3 to GI6.

The fungal loop model of semiarid ecosystems integrates microtopographic structures and pulse dynamics with key microbial processes. However limited data exist about the composition and structure of fungal communities in these ecosystems. The goal of this study was to characterize diversity and structure of soil fungal communities in a semiarid grassland. The effect of long-term nitrogen fertilization on fungi also was evaluated. Samples of rhizosphere (soil surrounding plant roots) and biological soil crust (BSC) were collected in central New Mexico, USA. DNA was amplified from the samples with fungal specific primers. Twelve clone libraries were generated with a total of 307 (78 operational taxonomic units, OTUs) and 324 sequences (67 OTUs) for BSC and rhizosphere respectively. Approximately 40% of soil OTUs were considered novel (less than 97% identity when compared to other sequences in NCBI using BLAST). The dominant organisms were dark-septate (melanized fungi) ascomycetes belonging to Pleosporales. Effects of N enrichment on fungi were not evident at the community level; however the abundance of unique sequences, sampling intensity and temporal variations may be uncovering the effect of N in composition and diversity of fungal communities. The fungal communities of rhizosphere soil and BSC overlapped substantially in composition, with a Jaccard abundance similarity index of 0.75. Further analyses are required to explore possible functions of the dominant species colonizing zones of semiarid grassland soils.

Background and Aims Knowledge about the effect of land use change on soil organic carbon (OC) in semiarid grassland is essential for understanding C cycles and for forecasting ecosystem C sequestration. Our objectives were (1) to study the effect of land use change on aggregate size distribution, aggregate-associated OC concentrations, and aggregate-associated stocks in a semiarid grassland area and (2) to relate changes in the aggregate fractions to changes in total soil OC. Methods Cropland and shrubland plots were established in a semiarid grassland area in 1982. We collected soil samples from adjacent grassland, cropland, and shrubland plots 27 years later and measured OC concentrations in the macroaggregate (>0.25 mm), microaggregate (0.25–0.053 mm) and silt+clay (<0.053 mm) fractions. Results Total soil OC concentrations and stocks decreased significantly after the grassland was converted to cropland or shrubland. Soil microbial biomass C, root biomass, and root C also declined. The proportion of soil in the macroaggregate fraction decreased after conversion to cropland or shrubland. Decreases in macroaggregate-associated OC stocks accounted for more than half of the OC losses that occurred when grassland was converted to cropland. The decreases in macroaggregate-associated OC stocks were due to declines in both macroaggregation and macroaggregate-associated OC concentrations after conversion to cropland. In contrast, decreases in microaggregate-associated OC stocks accounted for more than half of the OC losses when grassland was converted to shrubland. The declines in microaggregate-associated OC stocks were primarily due to a decrease in microaggregate-associated OC concentrations after conversion to shrubland. Conclusions Land use changed caused significant decreases in soil OC stocks. Conversion to cropland soil resulted in large decreases in macroaggregate-associated OC stocks whereas conversion to shrubland resulted in large decreases in microaggregate-associated OC stocks. Any changes in land use in semiarid grasslands could cause the grassland soil to become a source of atmospheric CO2; therefore extreme caution should be taken to avoid this hazard.

Increased available soil nitrogen can increase biomass, lower species richness, alter soil chemistry and modify community structure in herbaceous ecosystems worldwide. Although increased nitrogen availability typically increases aboveground production and decreases species richness in mesic systems, the impacts of nitrogen additions on semiarid ecosystems remain unclear. To determine how a semiarid grassland responds to increased nitrogen availability, we examined plant community structure and above-and belowground net primary production in response to long-term nitrogen addition in a desert grassland in central New Mexico, USA. Plots were fertilized annually (10 g N m⁻²) since 1995 and NPP measured from 2004 to 2009. Differences in aboveground NPP between fertilized and control treatments occurred in 2004 following a prescribed fire and in 2006 when precipitation was double the long-term average during the summer monsoon. Presumably, nitrogen only became limiting once drought stress was alleviated. Belowground NPP was also related to precipitation, and greatest root growth occurred the year following the wettest summer, decreasing gradually thereafter. Belowground production was unrelated to aboveground production within years and unrelated to nitrogen enrichment. Species richness changed between years in response to seasonal precipitation variability, but was not altered by nitrogen addition. Community structure did respond to nitrogen fertilization primarily through increased abundance of two dominant perennial grasses. These results were contrary to most nitrogen addition studies that find increased biomass and decreased species richness with nitrogen fertilization. Therefore, factors other than nitrogen deposition, such as fire or drought, may play a stronger role in shaping semiarid grassland communities than soil fertility.

A hypothesis has been advanced that the incursion of woody plants into world grasslands over the past two centuries has been driven in part by increasing carbon dioxide concentration, [CO₂], in Earth's atmosphere. Unlike the warm season forage grasses they are displacing, woody plants have a photosynthetic metabolism and carbon allocation patterns that are responsive to CO₂, and many have tap roots that are more effective than grasses for reaching deep soil water stores that can be enhanced under elevated CO₂. However, this commonly cited hypothesis has little direct support from manipulative experimentation and competes with more traditional theories of shrub encroachment involving climate change, management, and fire. Here, we show that, although doubling [CO₂] over the Colorado shortgrass steppe had little impact on plant species diversity, it resulted in an increasingly dissimilar plant community over the 5-year experiment compared with plots maintained at present-day [CO₂]. Growth at the doubled [CO₂] resulted in an [almost equal to]40-fold increase in aboveground biomass and a 20-fold increase in plant cover of Artemisia frigida Willd, a common subshrub of some North American and Asian grasslands. This CO₂-induced enhancement of plant growth, among the highest yet reported, provides evidence from a native grassland suggesting that rising atmospheric [CO₂] may be contributing to the shrubland expansions of the past 200 years. Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers; this process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock grazing.

Under the aim of searching for a more sustainable grazing management system, a mixed management system (grazing and haymaking alternate annually) was proposed and tested against traditional management system (used consistently either for grazing or haymaking) in the semiarid grassland of Inner Mongolia with a field manipulation experiment. The responses of aboveground biomass to the two grazing management systems were examined across different levels of organization (i.e., species, plant functional group, and community) and in five consecutive years from 2005 to 2009. The effects of the two systems on seed production potential of four dominant species (Leymus chinensis, Stipa grandis, Agropyron cristatum, Cleistogenes squarrosa) were also investigated. Our results demonstrate that, in the traditional system, aboveground biomass production across all the levels of organization was reduced by grazing. In mixed system, however, no significantly negative relationship between the biomass response and stocking rate was detected at all organization levels. Precipitation fluctuation had strong influence on biomass responses, and compared to the traditional system the slope of the biomass-precipitation relationship tends to be higher in the mixed system. This effect might be attributed to the more positive response of L. chinensis and A. cristatum to increase in precipitation. In the traditional system, both the ratio and the density of reproductive tillers of the grazing subplots were significantly reduced compared to the haymaking or ungrazed control plots. In the mixed system, there was no significant difference between the haymaking subplots and the ungrazed control plots, regardless of the grazing pressures imposed on the haymaking subplots in the previous growing season. Our findings suggest that the mixed system mitigates the sheep grazing-induced species shift and it tends to be more responsive to increasing precipitation as compared to the traditional system. Therefore, replacement of the traditional grazing strategy with the mixed system could provide an important contribution to sustainable land-use of the Inner Mongolia grasslands.

BACKGROUND AND AIMS: Knowledge about the effects of water and fertilizer on soil CO₂ efflux (SCE) and Q ₁₀ is essential for understanding carbon (C) cycles and for evaluating future global C balance. A two-year field experiment was conducted to determine the effects of water, fertilizer, and temperature on SCE in semiarid grassland in northern China. METHODS: SCE, as well as environmental factors was measured in two grasslands, one with bunge needlegrass (BNE, Stipa bungeana) and one with purple alfalfa (ALF, Medicago sativa), with four treatments: CK (unwatered and unfertilized); W (50 mm water addition yr⁻¹); F (50 kg phosphorus (P) fertilizer ha⁻¹ yr⁻¹ for ALF, 100 kg nitrogen (N) + 50 kg P fertilizer ha⁻¹ yr⁻¹ for BNE); and W + F. RESULTS: During the 11-month experimental period from July 2010 to October 2011, the addition of water consistently stimulated mean SCE in BNE and ALF, and the positive effects were relatively stronger during dry seasons. P fertilization consistently enhanced SCE in ALF, and the positive effect was strongly dependent on the availability of soil water. The effects of N plus P fertilization on SCE in BNE varied seasonally from significant increases to small reductions to no response. Water addition increased the Q ₁₀ of SCE in ALF by 11 % but had no effect in BNE. Fertilization, however, reduced the Q ₁₀ of SCE by 21 % and 13 % for BNE and ALF, respectively. Models that rely only on Q ₁₀ underestimated the emissions of soil CO₂ by 8–15 % at the study site, which was mediated by species and treatment. CONCLUSIONS: Responses of SCE and its temperature sensitivity to water and fertilizer may vary with species and depend on the period of measurement. Models of SCE need to incorporate the availability of ecosystemic water and nutrients, as well as species, and incorporate different environmental factors when determining the impact of water, nutrients, and species on SCE.

We studied the diversity of arbuscular mycorrhizal fungi (AMF) in semiarid grassland and the effect of long-term nitrogen (N) fertilization on this fungal community. Root samples of Bouteloua gracilis were collected at the Sevilleta National Wildlife Refuge (New Mexico, USA) from control and N-amended plots that have been fertilized since 1995. Small subunit rDNA was amplified using AMF specific primers NS31 and AM1. The diversity of AMF was low in comparison with other ecosystems, only seven operational taxonomic units (OTU) were found in B. gracilis and all belong to the genus Glomus. The dominant OTU was closely related to the ubiquitous G. intraradices/G. fasciculatum group. N-amended plots showed a reduction in the abundance of the dominant OTU and an increase in AMF diversity. The greater AMF diversity in roots from N-amended plots may have been the result of displacement of the dominant OTU, which facilitated detection of uncommon AMF. The long-term implications of AMF responses to N enrichment for plant carbon allocation and plant community structure remain unclear.

Soil properties (i.e. soil organic carbon, SOC; soil organic nitrogen, SON; and soil C/N ratio) and vegetation in a semiarid grassland of Inner Mongolia, northern China, were studied with the method of geostatistical analysis. We examined the spatial heterogeneity of soil and plants, and possible impacts of land use on their heterogeneity and on the relationship between soil resources and plant richness. Land use affected small scale spatial heterogeneity in plants and soil. SOC, SON and C/N ratio displayed autocorrelation over a range of ~2 m under most circumstances on sites where livestock grazing had been excluded. The uncontrolled grazing site (UG, i.e. unregulated grazing by excessive livestock) displayed an increased range of spatial autocorrelation and the total amount of variability in soil nitrogen over the other land use types. Plant life forms and plant species exhibited spatial autocorrelation over a range of about 2 m on the grazing exclusion (GE) and mowed (MW) sites, while pattern of spatial autocorrelation for several less common species on the UG site were difficult to predict. Plant species richness was positively related with spatial heterogeneity of SOC, SON and C/N on both GE and MW sites, and with only SOC heterogeneity on the UG site. These suggest that spatial soil heterogeneity plays an active role in maintaining plant species richness. However, we call for caution in generalization of the control of spatial soil heterogeneity over plant richness when multiple modes of disturbances are present, as we found in this study that higher total amount of variation in soil nitrogen and C/N ratio on the over-grazed UG site did not lead to increased plant species richness, and that land use had apparent effects on the patterns of spatial heterogeneity in both vegetation and soil.

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.