Optimizing grazing exclusion duration for carbon sequestration in grasslands: Incorporating temporal heterogeneity of aboveground biomass and soil organic carbon

Grasslands account for approximately one-third of the global terrestrial carbon stocks. However, a limited understanding of the impact of grazing exclusion on carbon storage in grassland ecosystems hinders progress towards restoring overgrazed grasslands and promoting carbon sequestration. In this study, we conducted a comprehensive meta-analysis to investigate the effects of grazing exclusion on aboveground biomass (AGB) and soil organic carbon (SOC) in four grasslands: alpine grasslands (AP), tropical savannas (TS), temperate subhumid grasslands (TG), and a semi-desert steppe (SD). Our meta-analysis indicated that grazing exclusion significantly enhanced carbon sequestration in grassland ecosystems, and the benefits of carbon sequestration were most pronounced in the AP, followed by the TG, SD, and TS. Grazing exclusion duration (DUR) was a significant factor associated with the response of aboveground biomass (AGB) and soil organic carbon (SOC) to grazing exclusion. Moreover, the relationships between AGB and DUR were nonlinear, with existence thresholds of 18, 21, 12, 19, and 23 years in global grasslands (ALL), AP, TS, TG, and SD, respectively. However, the relationship between SOC and DUR was linear, with SOC continuing to increase as DUR increased (up to 40 years). The multi-objective optimization indicated that the optimal duration of grazing exclusion for grassland carbon sequestration was 18-20, 21-23, 12-14, 19-21, and 23-25 years for ALL, AP, TS, TG, and SD, respectively. Our study contributes to the enhancement of grazing management and offers better options for increasing carbon sequestration in grasslands.

Grazing exclusion alters denitrification N2O/(N2O + N2) ratio in alpine meadow of Qinghai-Tibet Plateau

Grazing exclusion has been implemented worldwide as a nature-based solution for restoring degraded grassland ecosystems that arise from overgrazing. However, the effect of grazing exclusion on soil nitrogen cycle processes, subsequent greenhouse gas emissions and underlying mechanisms remain unclear. Here, we investigated the effect of four-year grazing exclusion on plant communities, soil properties, and soil nitrogen cycle-related functional gene abundance in an alpine meadow on the Qinghai-Tibet Plateau. Using an automated continuous-flow incubation system, we performed an incubation experiment and measured soil-borne N2O, N2, and CO2 fluxes to three successive "hot moment" events (precipitation, N deposition, and oxic-to-anoxic transition) between grazing-excluded and grazing soil. Higher soil N contents (total nitrogen, NH4+, NO3-) and extracellular enzyme activities (β-1,4-glucosidase, β-1,4-N-acetyl-glucosaminidase, cellobiohydrolase) are observed under grazing exclusion. The aboveground and litter biomass of plant community was significantly increased by grazing exclusion, but grazing exclusion decreased the average number of plant species and microbial diversity. The N2O + N2 fluxes observed under grazing exclusion were higher than those observed under free grazing. The N2 emissions and N2O/(N2O + N2) ratios observed under grazing exclusion were higher than those observed under free grazing in oxic conditions. Instead, higher N2O fluxes and lower denitrification functional gene abundances (nirS, nirK, nosZ, and nirK + nirS) under anoxia were found under grazing exclusion than under free grazing. The N2O site-preference value indicates that under grazing exclusion, bacterial denitrification contributes more to higher N2O production compared with under free grazing (81.6 % vs. 59.9 %). We conclude that grazing exclusion could improve soil fertility and plant biomass, nevertheless it may lower plant and microbial diversity and increase potential N2O emission risk via the alteration of the denitrification end-product ratio. This indicates that not all grassland management options result in a mutually beneficial situation among wider environmental goals such as greenhouse gas mitigation, biodiversity, and social welfare.

Temperate grassland vegetation restoration influenced by grazing exclusion and climate change

Grasslands are one of the most important terrestrial biomes, supporting a wide range of ecological functions and services. Grassland degradation due to overgrazing is a severe issue worldwide, especially in developing regions. However, observations from multiple sources have shown that temperate grasslands in China have significantly increased during the past two decades. It remains controversial what factors have driven the vegetation restoration in this region. In this study, we combined remote-sensing images and field survey datasets to quantify the contributions of different factors to vegetation restoration in six temperate grasslands in northern China. Across the six grasslands, the Normalized Difference Vegetation Index (NDVI) increased by 0.003-0.0319 year-1. The average contributions of grazing exclusion and climate change to the NDVI increase were 49.23 % and 50.77 %, respectively. Precipitation change was the primary climate factor driving vegetation restoration, contributing 50.76 % to the NDVI variance. By contrast, climate warming tended to slow vegetation restoration, and atmospheric CO2 concentration change contributed little to the NDVI increase in the temperate grasslands. These results emphasize the significant contributions of both climate change and human management to grassland vegetation restoration.

Temporal and vertical dynamics of carbon accumulation potential under grazing-excluded grasslands in China: The role of soil bulk density

Despite the progress made in understanding relevant carbon dynamics under grazing exclusion, previous studies have underestimated the role of soil bulk density (BD), and its implications for potential accumulation of soil organic carbon (SOC), especially at regional scale over long term. In this study, we first constructed a database covering a vast majority of the grasslands in northwestern China based on 131 published literatures. A synthesis was then conducted by analyzing the experimental data to comprehensively investigate the mechanisms of vegetation recovery, carbon-nitrogen coupling, and the importance of changed soil BD in evaluating SOC sequestration potential. The results showed that although the recovery of vegetation height and cover were both critical for improving vegetation biomass, vegetation height required a longer recovery period. While the SOC accumulation was found to be greater in surface layers than deeper ones, it exhibited a reduced capacity for carbon sequestration and an increased risk of SOC loss. Grazing exclusion significantly reduced soil BD across different soil profiles, with the rate of change influenced by soil depth, time, geographical and climatic conditions. The potential for SOC accumulation in the top 30 cm of soil based on data of 2003-2022 was 0.78 Mg ha-1 yr-1 without considering BD effects, which was significantly underestimated compared to that of 1.16 Mg ha-1 yr-1 when BD changes were considered properly. This suggests that the efficiency of grazing exclusion in carbon sequestration and climate mitigation may have been previously underreported. Furthermore, mean annual precipitation represented the most relevant environmental factor that positively correlated to SOC accumulation, and a wetter climate may offer greater potential for carbon accumulation. Overall, this study implies grazing exclusion may play an even more critical role in carbon sequestration and climate change mitigation over long-term than previously recognized, which provides essential scientific evidence for implementing stepwise ecological restoration in grasslands.

Spatiotemporal evolution of soil water erosion in Ningxia grassland based on the RUSLE-TLSD model

Accurate assessment of grassland soil erosion before and after grazing exclusion and revealing its driving mechanism are the basis of grassland risk management. In this study, the long-term soil erosion in Ningxia grassland was simulated by integrating and calibrating the transport limited sediment delivery (TLSD) function with the revised universal soil loss equation (RUSLE) model. The differential mechanisms of soil loss were explored using the GeoDetector method, and the relative effects of precipitation changes (PC) and human activities (HA) on grassland soil erosion were investigated using double mass curves. The measured sediment discharges from six hydrological stations verified that the RUSLE-TLSD model could reliably simulate water erosion in Ningxia. From 1988 to 2018, the water erosion rate of grassland in Ningxia ranged from 74.98 to 14.98 t⋅ha-1⋅a-1, showing an overall downward trend. July to September is the period with the highest of water erosion. The slope is the dominant factor influencing the spatial distribution of water erosion. After grazing exclusion, the net water erosion rate in Ningxia grassland and sub-regions decreased significantly. The double mass curves results show that human activities were the main driver of net erosion reduction. The focus of water erosion control in Ningxia is to control soil erosion in different terrains and protect grassland with slopes greater than 10°.

Effect of grazing exclusion on emission of greenhouse gases and soil organic carbon turnover in alpine shrub meadow

Grazing exclusion (GE) is a management option used widely to restore degraded grassland and improve grassland ecosystems. However, the impacts of GE on soil properties and greenhouse gas emissions of alpine shrub meadow are still unclear, especially long-term GE of more than ten years. To fill part of this gap, we examined the effects of long-term GE of alpine shrub meadow on soil nutrients, soil properties, greenhouse gas emissions (CO₂ and CH₄) and soil organic carbon (SOC) turnover. When compared to grazed grassland (GG), long-term GE resulted in: 1) greater SOC, nitrogen (N), and phosphorous (P) content, especially in the 20-30 cm soil layer; 2) greater soil C:N, C:P and N:P ratios in the 20-30 cm depth; 3) greater soil CO₂, but lesser CH₄ emission during the growing season; and 4) much faster SOC turnover time (0-30 cm). GE of more than ten years can increase grassland C reserves and improve the C sequestration capacity of the ecosystem. Results from this study can have important implications in developing future grassland management policies on soil nutrient balances, restoration of degraded grassland and controlling shrub expansion.

Reconsidering the efficiency of grazing exclusion using fences on the Tibetan Plateau

Grazing exclusion using fences is a key policy being applied by the Chinese government to rehabilitate degraded grasslands on the Tibetan Plateau (TP) and elsewhere. However, there is a limited understanding of the effects of grazing exclusion on alpine ecosystem functions and services and its impacts on herders' livelihoods. Our meta-analyses and questionnaire-based surveys revealed that grazing exclusion with fences was effective in promoting aboveground vegetation growth for up to four years in degraded alpine meadows and for up to eight years in the alpine steppes of the TP. Longer-term fencing did not bring any ecological and economic benefits. We also found that fencing hindered wildlife movement, increased grazing pressure in unfenced areas, lowered the satisfaction of herders, and rendered substantial financial costs to both regional and national governments. We recommend that traditional free grazing should be encouraged if applicable, short-term fencing (for 4-8 years) should be adopted in severely degraded grasslands, and fencing should be avoided in key wildlife habitat areas, especially the protected large mammal species.

Grazing exclusion promotes grasses functional group dominance via increasing of bud banks in steppe community

To understand the bud banks response to grazing exclusion, we conducted a demographic experiment in long-term grazing exclusion (20 year and 30 year) typical steppe. Results showed that grass functional group constituted the vast majority of the aboveground vegetation and belowground bud bank in all treatments. Long-term grazing exclusion significantly increased total aboveground biomass (2.5 and 2.6 times in 20y and 30y grazing exclusion grasslands, respectively), and decreased total stem density (31% and 37% in 20y and 30y grazing exclusion grasslands, respectively). Grazing exclusion for 20 and 30 years increased grass aboveground biomass respectively by 6.0 and 8.0 times, and decreased grass stem density by 38% and 33%. Grazing exclusion had different effects on belowground bud density of grass and forb functional group. Long-term grazing exclusion significantly increased plant buds and bud bank size (25% and 37% in 20y and 30y grazing exclusion grasslands, respectively), especially for grass functional group (49% and 95% in 20y and 30y grazing exclusion grasslands, respectively), but had no significant effects on forb bud density. Changes of aboveground community were significantly related to changes of belowground bud bank under both grazing and grazing exclusion grasslands. The bud bank density of grass functional group was significantly positive with total (R² = 0.33, P < 0.05) and grass aboveground biomass (R² = 0.36, P < 0.01), while negative related with total (R² = -0.27, P < 0.05) and grass stem density (R² = -0.22, P < 0.05). Grazed grasslands, 20y and 30y grazing exclusion grasslands all were not meristem limited and had large reserve bud banks, which would completely replace the aboveground stem population during the growing season. These findings indicate that grazing exclusion could not only improve a large bud bank for grassland restoration but also improve the dominance of grass functional group by increasing grass belowground bud banks in typical steppe community. We propose that the belowground bud bank might be a good approach to indicating potential succession direction of aboveground community.

Land sharing in South Patagonia: Conservation of above-ground beetle diversity in forests and non-forest ecosystems

Land-sharing strategies, as variable retention silvicultural proposals, are useful to mitigate harmful effects of economic activities on forest biodiversity; benefits have been reported worldwide for several organisms. However, we suggest that this approach could be useful to improve beetle conservation not only in forests but also in other ecosystem types, based on the results from Southern Patagonia (Argentina). We studied above-ground beetle communities using pitfall traps in Nothofagus pumilio forests, Mulguraea tridens shrublands, and magellanic steppes. The forests were located in Tierra del Fuego Province, while the shrublands and the steppes were in Santa Cruz Province. In forests and shrublands, we compared retention approaches (aggregated/dispersed retention harvesting in forests, and managed cut and retention strips in shrublands) vs. control situations (without harvesting/cuttings). In dry and humid steppes, both impacted by livestock, we evaluated grazed and exclusion paddocks, comparable to structural retentions (reference areas without grazing do not exist). Richness, abundance, frequency, Shannon-Wiener diversity and Pielou evenness indices, and similarity among assemblages were evaluated using univariate and multivariate statistical tests. In forests and shrublands, retention approaches (aggregated/dispersed and strips) allowed the partial or total maintenance of beetle community richness, preserving them similar to natural and non-impacted ecosystems. In dry and humid steppes, exclusion areas presented significantly different richness, abundance and diversity of arthropod assemblages, but with inverse trends: lower values in grazed areas than in exclusions in dry steppe, and higher values in grazed areas than in exclusions in humid steppe. We concluded that land-sharing could be implemented in forests and non-forest ecosystems to preserve beetle communities, being the variable retention approaches and the grazing exclusion areas good alternatives for private or public lands. Likewise, we consider that legislation to promote conservation (like National Law 26331) should not be only applicable for and implemented in forests, but also in non-forest ecosystems.

Effects of grazing exclusion on the grassland ecosystems of mountain meadows and temperate typical steppe in a mountain-basin system in Central Asia's arid regions, China

Grazing exclusion has been proposed as a method of restoring degraded grassland ecosystems. However, its effectiveness remains poorly understood in mountain-basin grasslands in arid regions. Thus, we investigated the plant community characteristics, C and N storage levels, and soil organic carbon and total nitrogen concentrations and storage within the upper 0-40 cm soil layer in a grazed grassland (GG) and a fenced grassland (FG) with grazing exclusion in mountain meadow (MM) and temperate typical steppe (TTS) habitats in a mountain-basin ecosystem in an arid region of Central China, which are both vital grassland resources for livestock grazing and ecological conservation. In MM, our investigation revealed that grazing exclusion was beneficial to the productivity, coverage, height, diversity, and C and N storage of aboveground plants. However, grazing exclusion was not an effective option for soil C and N sequestration. In TTS, grazing exclusion effectively improved the plant productivity, coverage, height, plant and soil C and N sequestration, although it was not beneficial for maintaining plant diversity. Our findings suggest that reduced or rotational grazing may be a better choice than grazing exclusion in MM. In addition, considering the trade-off between biomass productivity and species diversity in TTS, short-term grazing exclusion should be considered. Additionally, grazing exclusion should be combined with other appropriate measures rather than operating on a standalone basis.

Comparison of evapotranspiration components and water-use efficiency among different land use patterns of temperate steppe in the Northern China pastoral-farming ecotone

Water-use efficiency (WUE), which links carbon and water cycles, is an important indicator of assessing the interactions between ecosystems and regional climate. Using chamber methods with and without plant removal treatments, we investigated WUE and evapotranspiration (ET) components in three ecosystems with different land-use types in Northern China pastoral-farming ecotone. In comparison, ET of the ecosystems with grazing exclusion and cultivating was 6.7 and 13.4 % higher than that of the ecosystem with free grazing. The difference in ET was primarily due to the different magnitudes of soil water evaporation (E) rather than canopy transpiration (T). Canopy WUE (WUEc, i.e., the ratio of gross primary productivity to T) at the grazing excluded and cultivated sites was 17 and 36 % higher than that at the grazing site. Ecosystem WUE (WUEnep, i.e., the ratio of net ecosystem productivity to ET) at the cultivated site was 34 and 28 % lower in comparison with grazed and grazing excluded stepped, respectively. The varied leaf area index (LAI) of different land uses was correlated with microclimate and ecosystem vapor/carbon exchange. The LAI changing with land uses should be the primary regulation of grassland WUE. These findings facilitate the mechanistic understanding of carbon-water relationships at canopy and ecosystem levels and projection of the effects of land-use change on regional climate and productivity.

Grazing exclusion reduced soil respiration but increased its temperature sensitivity in a Meadow Grassland on the Tibetan Plateau

Understanding anthropogenic influences on soil respiration (R_s) is critical for accurate predictions of soil carbon fluxes, but it is not known how R_s responds to grazing exclusion (GE). Here, we conducted a manipulative experiment in a meadow grassland on the Tibetan Plateau to investigate the effects of GE on R_s. The exclusion of livestock significantly increased soil moisture and above‐ground biomass, but it decreased soil temperature, microbial biomass carbon (MBC), and R_s. Regression analysis indicated that the effects of GE on R_s were mainly due to changes in soil temperature, soil moisture, and MBC. Compared with the grazed blocks, GE significantly decreased soil carbon release by 23.6% over the growing season and 21.4% annually, but it increased the temperature sensitivity (Q₁₀) of R_s by 6.5% and 14.2% for the growing season and annually respectively. Therefore, GE may reduce the release of soil carbon from the Tibetan Plateau, but under future climate warming scenarios, the increases in Q₁₀ induced by GE could lead to increased carbon emissions.

Is grazing exclusion effective in restoring vegetation in degraded alpine grasslands in Tibet, China?

Overgrazing is considered one of the key disturbance factors that results in alpine grassland degradation in Tibet. Grazing exclusion by fencing has been widely used as an approach to restore degraded grasslands in Tibet since 2004. Is the grazing exclusion management strategy effective for the vegetation restoration of degraded alpine grasslands? Three alpine grassland types were selected in Tibet to investigate the effect of grazing exclusion on plant community structure and biomass. Our results showed that species biodiversity indicators, including the Pielou evenness index, the Shannon-Wiener diversity index, and the Simpson dominance index, did not significantly change under grazing exclusion conditions. In contrast, the total vegetation cover, the mean vegetation height of the community, and the aboveground biomass were significantly higher in the grazing exclusion grasslands than in the free grazed grasslands. These results indicated that grazing exclusion is an effective measure for maintaining community stability and improving aboveground vegetation growth in alpine grasslands. However, the statistical analysis showed that the growing season precipitation (GSP) plays a more important role than grazing exclusion in which influence on vegetation in alpine grasslands. In addition, because the results of the present study come from short term (6-8 years) grazing exclusion, it is still uncertain whether these improvements will be continuable if grazing exclusion is continuously implemented. Therefore, the assessments of the ecological effects of the grazing exclusion management strategy on degraded alpine grasslands in Tibet still need long term continued research.