Global Grassland Research Articles

Grazing intensity is key to global grassland carbon sequestration potential

Grasslands are coming under ever-increasing pressure worldwide. Many grasslands are degraded due to overgrazing and inappropriate land management. This is impacting belowground biology and soil bio...

Nitrogen deposition increases global grassland N2O emission rates steeply: A meta-analysis

Nitrous oxide (N2O) emissions in grasslands contribute considerably to global warming and are influenced substantially by nitrogen deposition. However, the impacts of N deposition impact on soil N2O emission rates across global grassland remain unclear due to spatial heterogeneity. Here, we synthesized data from 34 published studies to clarify the response of N2O emission rates to N deposition across global grassland. The priming effect size of N deposition on global grassland N2O emission rate was 1.64 ± 0.14 (95% confidence interval 1.36–1.92, P < 0.0001). Categorical variable analysis revealed that various N types influenced effect size significantly, in the orders of urine > slurry > dung > NH4SO4 > NH4NO3 > urea. Furthermore, explained heterogeneity analysis revealed that air temperature, nitrogen dose, precipitation and soil bulk density was positively associated with effect size. In addition, air temperature, altitude, pH and nitrogen dose were the major factors influencing effect size. Path analysis also indicated that N dose took a significant role on direct effect (P < 0.01), the indirect effects were relative higher of soil pH, air temperature and altitude through soil bulk. We provide a key insight that continuous increase in N deposition would significantly stimulate grasslands N2O emission. Controlling greenhouse effect and soil acidification would mitigate the priming effect availably.

Effects of manipulated precipitation on aboveground net primary productivity of grassland fields: Controlled rainfall experiments in Inner Mongolia, China

AbstractRecovery from grassland degradation caused by climate change and human disturbance is critical to the sustainable development of drylands. Grassland in China accounts for 13% of the global grassland area, and more than half of it is still recovering from degradation. How climate change will contribute to the recovery of degraded grassland has not been clarified through field experiments. Here, we report the effects of manipulated precipitation regimes on above ground net primary productivity (ANPP) in five grassland fields along a degradation gradient using a 3‐year controlled rainfall manipulation experiment in Inner Mongolia, China. We found that differences of approximately 20–30% in growing season precipitation have limited impacts on the ANPP of grasslands, and the responses can depend crucially on the degradation status indicated by vegetation and soil features. ANPP sensitivity to spring drought increased with increasing degradation level, which was mainly determined by vegetation composition. ANPP was generally insensitive to summer drought except for mature grassland, in which drought led to severe soil water deficit, mainly mediated by soil water holding capacity. Our results suggest that the recovery of degraded grassland is expected to be delayed by increasing drought early in the growing season, and mature grassland would be more vulnerable to extreme summer drought. These findings highlight the negative impacts of seasonal drought on degraded grassland and emphasize the importance of accounting for climate change and degradation status in future grassland ecosystem restoration and management policies.

Global patterns and climatic drivers of above- and belowground net primary productivity in grasslands.

Understanding patterns and determinants of net primary productivity (NPP) in global grasslands is ongoing challenges, especially for belowground NPP (BNPP) and its fraction (fBNPP). By developing a comprehensive field-based dataset, we revealed that, along with gradients of mean annual precipitation, actual evapotranspiration, and aridity, aboveground NPP (ANPP), BNPP, and total NPP (TNPP) exhibited hump-shaped patterns, whereas fBNPP showed an opposite trend. ANPP and TNPP showed positive correlations with mean annual temperature, but fBNPP was negatively correlated with it. The relationship between BNPP and climatic factors was considerably weak, indicating that BNPP was relatively stable regardless of the climate conditions. We also observed that the sensitivities of ANPP and BNPP to interannual temperature variability and those of BNPP to interannual precipitation fluctuations exhibited large variations among different study sites, and differed from those at the spatial scale. In contrast, the temporal sensitivities of ANPP to interannual precipitation variability were highly similar across all the individual sites and much smaller than those at the spatial scale. Overall, these results highlight that precipitation, temperature and evapotranspiration all play vital roles in shaping ANPP pattern and its partitioning to belowground and that the patterns of BNPP along climatic gradients do not mirror those of the ANPP.

Special Issue on Dynamics of the Global Savanna and Grasslands Biomes

Savanna and grassland biomes cover more of the earth’s surface than any other biome type, and yet they are still largely understudied [...]

Global impacts of fertilization and herbivore removal on soil net nitrogen mineralization are modulated by local climate and soil properties.

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin ), that is, the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory-based) and realized (field-based) soil net Nmin . Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin . However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin , in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site-specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is an important context-dependent knowledge for grassland management worldwide.

Greenhouse gas balance in global pasturelands and rangelands

Grassland ecosystems play an essential role in climate regulation through carbon (C) storage in plant and soil. But, anthropogenic practices such as livestock grazing, grazing related excreta nitrogen (N) deposition, and manure/fertilizer N application have the potential to reduce the effectiveness of grassland C sink through increased nitrous oxide (N2O) and methane (CH4) emissions. Although the effect of anthropogenic activities on net greenhouse gas (GHG) fluxes in grassland ecosystems have been investigated at local to regional scales, estimates of net GHG balance at the global scale remains uncertain. With the data-model framework integrating empirical estimates of livestock CH4 emissions with process-based modeling estimates of land CO2, N2O and CH4 fluxes, we examined the overall global warming potential (GWP) of grassland ecosystems during 1961–2010. We then quantified the grassland-specific and regional variations to identify hotspots of GHG fluxes. Our results show that, over a 100-year time horizon, grassland ecosystems sequestered a cumulative total of 113.9 Pg CO2-eq in plant and soil, but then released 91.9 Pg CO2-eq to the atmosphere, offsetting 81% of the net CO2 sink. We also found large grassland-specific variations in net GHG fluxes, with pasturelands acting as a small GHG source of 1.52 ± 143 Tg CO2-eq yr−1 (mean ± 1.0 s.d.) and rangelands a strong GHG sink (−442 ± 266 Tg CO2-eq yr−1) during 1961–2010. Regionally, Europe acted as a GHG source of 23 ± 10 Tg CO2-eq yr−1, while other regions (i.e. Africa, Southern Asia) were strong GHG sinks during 2001–2010. Our study highlights the importance of considering regional and grassland-specific differences in GHG fluxes for guiding future management and climate mitigation strategies in global grasslands.

Global pattern and drivers of nitrogen saturation threshold of grassland productivity

Abstract Ecosystem productivity usually exhibits first increase and then saturated response to increasing nitrogen (N) additions, yet the broad‐scale pattern and potential drivers of the N saturation threshold are little investigated. By synthesizing N addition experiments with at least four N‐input levels from the global grasslands, we applied the quadratic‐plus‐plateau model to fit the above‐ground net primary productivity (ANPP)–N rate relationship, and estimated the saturation threshold for N rate (critical N rate, NCR) and ANPP (maximum ANPP, ANPPmax) from the inflection point where ANPP no longer statistically increased with N rate for individual experiments. Based on these estimations, we investigated the spatial pattern and driving factors of NCR and ANPPmax. The mean NCR and ANPPmax were 15.0 and 477.0 g m−2 year−1, respectively, but varied substantially among single‐site experiments. Management strategies (e.g. biomass harvest, different N forms and addition frequencies) minimally influenced both parameters. Structural equation models demonstrated that the spatial differences in NCR and ANPPmax were mainly explained by aridity index, and soil carbon (C)/N ratio also predicted the variation in NCR. Given that grasslands are important not only for the trend and variability of the land C sink but also for the maintenance of pasture yield, the pattern and controls of NCR and ANPPmax, as revealed by the current study, are crucial for constructing robust predictions of C sink capacity and improving N fertilizer management in grasslands.

An improved estimation of net primary productivity of grassland in the Qinghai-Tibet region using light use efficiency with vegetation photosynthesis model

Accurate estimation of grassland Net Primary Productivity (NPP) is one of the most important steps in monitoring and further analysing the status of the grassland ecosystem. However, the quality of regional or even global grassland NPP estimations are commonly limited by the two most important input datasets: Photosynthetic Active Radiation (PAR) and climatic data, which make the NPP estimate unrepresentative or even inaccurate. This paper uses a Light Use Efficiency with Vegetation Photosynthesis Model (LUE-VPM) to estimate the high-quality grassland NPP in Zeku. By using the PAR data derived from high resolution DEM products and the improved land surface temperature data, the NPP produced by LUE-VPM shows that it is statistically the same as in situ converted NPP with a p-value of t-test equalling 0.90 (the RMSE between the two is 97.77 gC/m2), while MODIS NPP in Zeku is statically different from the all the in-situ sampling results with a p-value of t-text smaller than 0.03. The results of this paper provided a better estimate than MODIS NPP, and LUE-VPM can be used for the high-quality regional grassland NPP estimation when the quality of PAR data and climatic data improved.

Microbial processing of plant remains is co-limited by multiple nutrients in global grasslands.

Microbial processing of aggregate-unprotected organic matter inputs is key for soil fertility, long-term ecosystem carbon and nutrient sequestration and sustainable agriculture. We investigated the effects of adding multiple nutrients (nitrogen, phosphorus and potassium plus nine essential macro- and micro-nutrients) on decomposition and biochemical transformation of standard plant materials buried in 21 grasslands from four continents. Addition of multiple nutrients weakly but consistently increased decomposition and biochemical transformation of plant remains during the peak-season, concurrent with changes in microbial exoenzymatic activity. Higher mean annual precipitation and lower mean annual temperature were the main climatic drivers of higher decomposition rates, while biochemical transformation of plant remains was negatively related to temperature of the wettest quarter. Nutrients enhanced decomposition most at cool, high rainfall sites, indicating that in a warmer and drier future fertilized grassland soils will have an even more limited potential for microbial processing of plant remains.

Pasture availability as a spatial indicator of grassland root turnover time on a global scale

Abstract Root turnover time is jointly predicted by climate factors and soil properties in global grasslands with different vegetation types. Pasture availability is directly related to the belowground dynamics of soil conditions on spatial scales. However, few studies have examined whether pasture availability could be a spatial indicator of grassland root turnover time on a global scale. In the present study, geographically weighted regression (GWR) modelling, coupled with the ordinary least squares (OLS) method, was used to develop an indicator of root turnover time in global grasslands based on pasture availability. The results revealed a significant relationship between pasture availability and root turnover time in global grasslands. Furthermore, this relationship may depend on the type of vegetation and could be affected by animal grazing (including buffaloes, cattle, horses, goats, pigs, and sheep). Based on regression modelling, significantly positive relationships were detected between pasture availability and root turnover time for temperate grassland and meadows (P

Estimation of Global Grassland Net Ecosystem Carbon Exchange Using a Model Tree Ensemble Approach

AbstractUnderstanding the net ecosystem CO2 exchange (NEE) between terrestrial ecosystems and the atmosphere is crucial for accurate estimation of carbon budgets, which remains unclear for grassland ecosystems. Here we upscaled site‐level NEE from 44 grassland flux towers (1,457 site‐months) to the global scale by using a model tree ensemble approach that considers the management activities (grazing and cutting) (MTE‐GM). Cross‐validation showed that MTE‐GM performs reasonably well in terms of among‐site variability and seasonal variation, with a Nash‐Sutcliffe efficiency of 0.90 and 0.86 and an R2 of 0.91 and 0.86, respectively. Radiation (shortwave and longwave), temperature, leaf area index, and fraction of absorbed photosynthetic active radiation had the highest relative explanatory power in predicting NEE. Based on MTE‐GM, mean annual NEE of global grassland was 72 ± 4 g C m−2 year−1 (1.9 ± 0.11 Pg C year−1) during 1982–2011, suggesting that the grassland ecosystems have been acting as a small carbon source during the past three decades. However, grasslands in temperate and continental regions had the largest carbon sink of −61.9 ± 5.7 and −51.8 ± 7.9 g C m−2 year−1, respectively. Moreover, we found that elimination of grassland management effect resulted in an extra emission of 1.7% CO2 to the atmosphere (CO2 sink from the management is 0.03 Pg C year−1). From the 1980s to the 2000s, 38% (22%) and 17% (18%) of pixels showed an increased (decreased) carbon uptake and decreased (increased) carbon release, respectively. Uncertainty assessment suggested that there would be higher confidence in NEE estimates in most parts of middle‐ to high‐latitude regions in the Northern Hemisphere.

High Below-Ground Productivity Allocation of Alpine Grasslands on the Northern Tibet.

The allocation of net primary production (NPP) between above- and belowground components is a key step of ecosystem material cycling and energy flows, which determines many critical parameters, e.g., the fraction of below ground NPP (BNPP) to NPP (fBNPP) and root turnover rates (RTR), in vegetation models. However, direct NPP estimation and partition are scarcely based on field measurements of biomass dynamics in the alpine grasslands on the Northern Tibetan Plateau (NTP). Consequently, these parameters are unverifiable and controversial. Here, we measured above- and belowground biomass dynamics (monthly from May to September each year from 2013 to 2015) to estimate NPP dynamics and allocations in four typical alpine grassland ecosystems, i.e., an alpine meadow, alpine meadow steppe, alpine steppe and alpine desert steppe. We found that NPP and its components, above and below ground NPP (ANPP and BNPP), increased significantly from west to east on the NTP, and ANPP was mainly affected by temperature while BNPP and NPP were mainly affected by precipitation. The bulk of BNPP was generally concentrated in the top 10 cm soil layers in all four alpine grasslands (76.1% ± 9.1%, mean ± SD). Our results showed that fBNPP was significantly different among these four alpine grasslands, with its means in alpine meadow (0.93), alpine desert steppe (0.92) being larger than that in the alpine meadow steppe (0.76) and alpine steppe (0.77). Both temperature and precipitation had significant and positive effects on the fBNPP, while their interaction effects were significantly opposite. RTR decreased with increasing precipitation, but increased with increasing temperature across this ecoregion. Our study illustrated that alpine grasslands on the NTP, especially in the alpine meadow and alpine desert steppe, partitioned an unexpected and greater NPP to below ground than most historical reports across global grasslands, indicating a more critical role of the root carbon pool in carbon cycling in alpine grasslands on the NTP.

Global negative effects of livestock grazing on arbuscular mycorrhizas: A meta-analysis

Livestock grazing activities substantially contribute to worldwide grassland degradation and potentially alter the growth of arbuscular mycorrhizal fungi. The global patterns of arbuscular mycorrhizal fungi in response to grazing, especially grazing intensity, are still unclear. In this study, we performed a meta-analysis of grazing intensity experiments to examine the grazing intensity effects on arbuscular mycorrhizal fungi across global grasslands. The results showed that heavy or moderate grazing consistently reduced arbuscular mycorrhizal fungal abundance by 34.38% and 9.14%, but light grazing had no significant effect. Arbuscular mycorrhizal fungal abundance was reduced with longer duration of grazing (−22.91%) and lower annual precipitation (-17.43%). Grazing decrease the abundance of arbuscular mycorrhizal fungal was possibly attributedto the reduction of above-ground biomass, in agreement with the carbon limitation hypothesis. It suggests the inhibition of arbuscular mycorrhizal fungal abundance by grazing at the cost of plant above-ground biomass. These findings highlight the negative effect of heavy grazing on arbuscular mycorrhizaes across worldwide grassland, and which may contribute to understand the effects of livestock grazing activities on symbiotic relationships between host plants and arbuscular mycorrhizal fungi.

Degraded patch formation significantly changed microbial community composition in alpine meadow soils

Approximately half of global grasslands are degraded. Although soil microbes play a key role in ecosystem functioning, their response to grassland degradation has not been fully investigated. In particular, degraded patch formation is the main feature of alpine meadow degradation, but little is known about its effect on soil microbes. In this study, soils were collected from three patch-degraded Tibetan alpine meadows to examine the effects of degraded patch formation on soil microbial communities. The alpine meadows at the three sites were in the third to fifth stages of degradation, respectively. Soil microbial abundance and community structure were determined through real-time PCR and MiSeq sequencing, respectively. The results showed that the degraded patch formation significantly decreased microbial respiration rates, changed the interaction patterns among microbial taxa, and increased fungal diversity, but did not significantly affect microbial abundance. Additionally, both prokaryotic and fungal community composition was significantly altered by the degraded patch formation. The functional predictions based on FAPROTAX and FUNGuild suggested that degraded patch formation significantly increased the proportion of nitrifiers, plant pathogenic fungi, and saprotrophic fungi, especially when mattic epipedons were eroded. The increased relative abundance of nitrifiers and pathogenic fungi can aggravate the risk of nitrogen leaching and plant diseases, respectively. Therefore, degraded patch formation can impede the recovery of degraded alpine meadows by changing soil microbial community composition.

Assessing the Dynamics of Grassland Net Primary Productivity in Response to Climate Change at the Global Scale

Understanding the net primary productivity (NPP) of grassland is crucial to evaluate the terrestrial carbon cycle. In this study, we investigated the spatial distribution and the area of global grassland across the globe. Then, we used the Carnegie-Ames-Stanford Approach (CASA) model to estimate global grassland NPP and explore the spatio-temporal variations of grassland NPP in response to climate change from 1982 to 2008. Results showed that the largest area of grassland distribution during the study period was in Asia (1737.23 × 104 km2), while the grassland area in Europe was relatively small (202.83 × 104 km2). Temporally, the total NPP increased with fluctuations from 1982 to 2008, with an annual increase rate of 0.03 Pg C/yr. The total NPP experienced a significant increasing trend from 1982 to 1995, while a decreasing trend was observed from 1996 to 2008. Spatially, the grassland NPP in South America and Africa were higher than the other regions, largely as a result of these regions are under warm and wet climatic conditions. The highest mean NPP was recorded for savannas (560.10 g C/(m2·yr)), whereas the lowest was observed in open shrublands with an average NPP of 162.53 g C/(m2·yr). The relationship between grassland NPP and annual mean temperature and annual precipitation (AMT, AP, respectively) varies with changes in AP, which indicates that, grassland NPP is more sensitive to precipitation than temperature.

Assessing the Impacts of Drought on Grassland Net Primary Production at the Global Scale

Quantitatively assessing the impacts of drought on grassland has significant implications to understand the degradation mechanism and prevention degraded grassland. In this study, we analyzed the relationship between grassland drought and grassland Net Primary Productivity (NPP) based on the self-calibrated Palmer Drought Severity Index (scPDSI) from 1982 to 2008. The results showed that the global grassland scPDSI value had a slightly increasing trend with the rate of 0.0119 per year (R2 = 0.195), indicating that the global grassland drought lighter to some extent during study period. Moreover, the correlation coefficient between annual grassland NPP and scPDSI was from −0.83 to 0.92. The grassland NPP decreased under mild drought from 1992 to 1996. Additionally, the correlation coefficient between scPDSI and NPP for each grassland type was: Closed Shrublands > Non-woody grassland > Savannas > Open Shrublands > Woody Savannas, indicating that drought had difference influences on the different grassland types. Our results might provide the underlying insights needed to be guide for the effects of extreme weather events on grassland NPP.

Precipitation-use efficiency may explain net primary productivity allocation under different precipitation conditions across global grassland ecosystems

Conclusions made regarding plant biomass allocation, trade-off patterns and aboveground/belowground biomass without considering the existence of perennials in grassland ecosystems are under fierce debate. This study focused on this issue and brings clarity to allocation patterns; 217 samples were collected across different grassland ecosystems worldwide that contained the mean values of aboveground and belowground net primary productivity (ANPP and BNPP). Based on the relationships between BNPP to ANPP ratio and mean annual precipitation (MAP), the change point was found to divide the habitats into precipitation-limited and precipitation-unlimited conditions. The results show that overall benefits pointed to ANPP (0.04) and pointed to BNPP (0.13) under different precipitation conditions, respectively. Our findings add BNPP to the calculation of precipitation-use efficiency (PUE) and confirm that PUE may explain the net productivity allocation patterns and trade-offs of ANPP and BNPP under precipitation-limited conditions. The study highlights production allocation and trade-off patterns under unlimited/limited precipitation backgrounds across global grassland ecosystems and connects PUE with the distribution of productivity, which should benefit our understanding of the complicated mechanisms of ecological processes.

Assessing the effects of climate variation and human activities on grassland degradation and restoration across the globe

Abstract While quantitative assessment of ecosystems degradation is quite addressed in the literature, quantitatively separating the relative contribution of climate variation and human activities to grassland ecosystem degradation is still limited, although well recognized as a key knowledge for a better understanding of the degradation mechanisms and for more efficient control of degradation processes. In this study, actual net primary productivity (ANPP) was selected as an indicator to investigate the grassland dynamics from 2000 to 2013. In addition, potential NPP (PNPP) and human-induced NPP (HNPP), which was calculated as the difference between PNPP and ANPP, were used to differentiate the relative impacts of climate variation and human activities on grassland degradation/restoration, respectively. Results showed that 60.94% of the total global area of grassland experienced restoration over the study period, with an increase in NPP by 1864.81 Tg C across the globe. The restored areas induced by climate variation and human activities accounted for 45.73% and 34.99% with an increases in NPP by 852.75 Tg C and 652.50 Tg C, respectively. By contrast, nearly 39.06% of the global grassland area experienced degradation. This degradation was associated with a noticeable decrease in ANPP by 325.53 Tg C. While 45.45% of the degraded grassland areas were influenced by climate variation, human activities were accounted for around 40.11% of this degradation. The relative effect of the two factors (i.e. climate variation and human activities) on grassland changes was found to vary greatly among the studied six continents. While human activities exerted larger impacts than climate variation on grassland degradation in North America and Europe, climate variation was the dominant factor for grassland restoration in Asia, Europe, Oceania and North America, which can be mainly attributed to the increase in precipitation during the study period. The findings of the current study improved our understanding of the relative impacts of climate variation and human activities on grassland degradation and restoration at the global scale.

Vegetation type controls root turnover in global grasslands

AbstractAimRoot turnover is an important process determining carbon and nutrient cycling in terrestrial ecosystems. It is an established fact that root turnover is jointly regulated by climatic, edaphic and biotic factors. However, the relative importance of these forces in determining the global patterns of root turnover time is far from clear.LocationGlobal.Time period1946–2017.Major taxa studiedGrasslands.MethodsWe compiled a database of 141 sites with 433 observations on root turnover time and applied structural equation modelling (SEM) to investigate the relative contribution of climate, soil properties and vegetation type to the observed variations in root turnover time.ResultsRoot turnover time was 3.1 years on average across the global grasslands and differed significantly among grassland types (tropical grassland and savanna, temperate grassland and meadow, alpine grassland and meadow, tundra and desert). It decreased with mean annual temperature, mean annual precipitation and Palmer drought severity index but increased with soil organic carbon content, total nitrogen content and carbon : nitrogen ratio. Soil bulk density and soil texture also significantly affected root turnover time, with clay content negatively correlating to root turnover time and explaining more variations than bulk density and sand content. The SEM showed that climatic factors had dominant effects on root turnover time when vegetation type was not considered. Vegetation type became the primary driver when it was included in the SEM.Main conclusionsOur results indicate that the influences of climatic and edaphic factors on root turnover time are predominantly manifested through vegetation type. The critical role of precipitation as revealed for the first time in this study challenges our current understanding of climate impacts on root turnover time. The findings necessitate accurate representation of vegetation type in Earth system models to predict root function dynamics under global change.