Considering their contribution to global warming, the sources and sinks of methane (CH4) should be accounted when undertaking a greenhouse gas inventory for grazed rangeland ecosystems. The aim of this study was to evaluate the mitigation potential of current ecological management programs implemented in the main rangeland regions of China. The influences of rangeland improvement, utilization and livestock production on CH4 flux/emission were assessed to estimate CH4 reduction potential. Results indicate that the grazed rangeland ecosystem is currently a net source of atmospheric CH4. However, there is potential to convert the ecosystem to a net sink by improving management practices. Previous assessments of capacity for CH4 uptake in grazed rangeland ecosystems have not considered improved livestock management practices and thus underestimated potential for CH4 uptake. Optimal fertilization, rest and light grazing, and intensification of livestock management contribute mitigation potential significantly.
Under the Kyoto Protocol of 1997, signatory countries can elect to report global greenhouse gas (GHG) emissions from managed lands (e.g., forest, cropland, grazing land and other managed lands), and can use verified emission reductions from managed lands to fulfill their emission reduction commitments. Continental rangelands are a widespread form of grazing land, which play an important role in the GHG budget. Methane (CH4) is the second most important long-living, anthropogenically-modified GHG after carbon dioxide (CO2)1,2. CH4 sources and sinks in managed grazing lands are primarily influenced by farming and rangeland management practices3. However, quantitative estimates of CH4 sources and sinks in managed continental rangelands are particularly uncertain because of high variation across different temporal and spatial scales4,5. Thus, the contribution of changes in management practices in grazed rangeland ecosystems that produce and sequester CH4 remains uncertain.
There are 492.8 million ha of rangelands in China, of which 313.4 million ha are grazed. These rangelands are mostly distributed in Inner Mongolia, Xinjiang, Ningxia, and the Qinghai-Tibet plateau6. China's rangelands provide ecological services of global significance. However, provision of many of these services has been impaired over the past 60 years. Human activities, including uncontrolled livestock grazing, wood harvesting, and cultivation in semiarid and arid rangeland regions, are implicated as causes of rangeland degradation and declining ecological service provision. There is now widespread agreement that overgrazing over the past half century has contributed to degradation of more than 90% of Chinese rangelands7. To conserve rangeland ecology, mitigate degradation and desertification, and promote economic development in pastoral regions, since the end of the 20th century the Central Government has implemented a series of policies and programs to restore rangeland ecosystem functions (Supplementary Table S1). To evaluate the effects of these policies and programs on CH4 emissions and uptake in grazed rangeland ecosystems, the overall CH4 budget was quantified by developing an area-weighted average for year-round CH4 fluxes in the main continental rangeland ecosystems of China (i.e., Inner Mongolia, Xinjiang and Ningxia autonomous regions, and the Qinghai-Tibet plateau). The CH4 budget for livestock production was then deduced at the national scale. We then quantify the CH4 mitigation effects of the changes in management practice that are commonly promoted in the major policies and programs. The management considered include rangeland improvement (reseeding, irrigation, fertilization and grazing prohibition), rangeland utilization (rest from grazing, light grazing and moderate grazing), and livestock production (intensive feeding systems) (Table 1). These practices may affect CH4 emission and uptake through two pathways: 1) an increase in CH4 uptake associated with an increase in soil-atmospheric exchange due to rangeland improvement and improved utilization of degraded rangelands; 2) a reduction in CH4 emissions from livestock associated with a decrease in the livestock population and/or an increase in livestock production.
Reseeding, irrigation, fertilization and grazing prohibition have been used to restore degraded rangelands in Ningxia autonomous region. Changes in CH4 fluxes in the growing season under different treatments are shown in Supplementary Fig. S1. Measured CH4 uptake under reseeding, irrigation, fertilization and grazing prohibition treatments were 5.61, 6.05, 6.23 and 5.27 kg ha−1 y−1, respectively, which represent CH4 mitigation potentials of 12.2%, 21.0%, 24.6% and 5.4% compared with the control treatment (5.00 kg ha−1 y−1), respectively (Table 2, management 1a, 1b, 1c, 1d and 1e).
Common practices promoted in government programs include rest from grazing, light grazing and moderate grazing. Dynamics of CH4 fluxes during the growing season in three studied regions (Sichuan, Xinjiang and Inner Mongolia) are shown in Supplementary Fig. S2. The CH4 flux of a heavily grazed area was also measured as a control for comparison with the improved grazing management practices. The measured CH4 uptakes under rest from grazing, light grazing and moderate grazing were 5.04, 5.22 and 5.41 kg ha−1 y−1, respectively, which represent CH4 mitigation potentials of 107.4%, 114.8% and 122.6% compared with control treatment (2.43 kg ha−1 y−1), respectively (Table 2, management 2a, 2b and 2c rangeland). CH4 emissions from livestock under rest from grazing, light grazing, moderate grazing and heavy grazing were 2.73, 2.83, 5.49 and 8.23 kg ha−1 y−1, respectively, in Sichuan province; 2.82, 2.75, 5.41 and 7.59 kg ha−1 y−1 in Xinjiang autonomous region; and 2.89, 2.81, 5.31 and 8.38 kg ha−1 y−1 in Inner Mongolia autonomous region (Fig. 1). Average CH4 emissions from livestock under rest from grazing, light grazing and moderate grazing were 2.81, 2.80 and 5.40 kg ha−1 y−1, respectively, which represent CH4 mitigation potentials of 65.2%, 65.3% and 33.1% compared with emissions under heavy grazing (8.07 kg ha−1 y−1), respectively (Table 2, management 2a, 2b and 2c livestock). Total annual CH4 fluxes in the improved grazing ecosystems were −0.01 to −2.42 kg ha−1 y−1, showing that grazing ecosystems under improved management can sequester CH4.
CH4 emissions from livestock production under intensive management were compared with emissions under extensive management in Sichuan, Xinjiang and Inner Mongolia. The average estimated CH4 uptakes in rangelands under intensive management were 5.41 kg ha−1 y−1, which represent a CH4 mitigation potential of 7.8% compared with extensive management (5.02 kg ha−1 y−1) (Table 2, management 3a rangeland). Estimated monthly CH4 output from livestock in three study regions (Sichuan, Xinjiang and Inner Mongolia) are shown in Supplementary Fig. S3. Annual average CH4 emissions from livestock under intensive management in Sichuan, Xinjiang and Inner Mongolia were 4.23, 1.88 and 5.56 kg ha−1 y−1, respectively (Fig. 2), which represent an average CH4 mitigation potential of 26.2% compared with extensive management (Table 2, management 3a livestock). The estimate of total annual net emissions (i.e., considering both soil-atmosphere exchange and livestock emissions) indicate that under intensive management the grazing rangeland ecosystem could become a net CH4 sink (−1.52 kg ha−1 y−1).
Measures to restore degraded rangelands (i.e., reseeding, irrigation, fertilization, and grazing prohibition) are widely applied in China. The main objective of promoting these management practices is to stimulate an increase in net primary productivity, improve soil nutrients, and restore rangeland ecosystem functions. In the process, these managements can increase microbial activity, CH4 oxidation and soil-atmospheric exchange of CH48,9,10,11. In terms of CH4 fluxes, adoption of these practices can increase soil CH4 uptake by 0.27 kg ha−1 y−1 to 1.23 kg ha−1 y−1 (Table 2 management 1a–d), representing a 5.4%–24.6% increase in CH4 uptake compared with conventional practices. Moreover, these practices often increase plant photosynthesis and thus sequester atmospheric CO212, which is also important in managing soil C stocks in rangelands. On the other hand, restoration of degraded rangelands may also lead to an increase in the organic matter digestibility of edible forage ingested by grazing livestock, which can increase livestock performance and reduce CH4 production from grazing livestock when expressed in terms of CH4 production per unit of livestock product output or per unit of daily weight gain13.
Optimal stocking rates are required to maintain sustainable utilization of natural rangeland resources and are beneficial to the restoration of degraded rangelands. Compared to heavy grazing, a reduction in the number of grazing livestock per ha can significantly reduce CH4 production under moderate and light grazing and under rest from grazing, as well as increase livestock productivity13. This study found that light and moderate grazing actively promote CH4 sequestration by rangeland soils. The combined direct and indirect effects of change in rangeland utilization may be to transform rangeland-based grazing ecosystems from a CH4 source into a sink. This strategy has additional mitigation potential if accompanied by livestock breed improvement14.
Although livestock production is increasingly important in China's food system, livestock respiration is considered to be a net source of GHG. Increasing demand for livestock products must be met while addressing the challenge of balancing livelihoods and environmental protection in rangeland areas. Recent estimates suggest that livestock contribute about 14.5% of global anthropogenic greenhouse gas emissions15. Although intensive livestock production is expanding across the world, there are still vast rangeland areas in developing countries where extensive livestock production continues under traditional feeding systems, accompanied by high CH4 emissions from ruminant livestock14. CH4 emission from ruminant livestock is an unavoidable and inefficient product associated with the specifics of the ruminant digestive system, but it can be significantly affected by regulating energy intake and the quantity and quality of dietary intake, thus reducing CH4 emissions per unit of product16. Other techniques can also be deployed to reduce the activity of methanogenic bacteria and protozoa in rumen and to increase the digestibility of the diet, so that CH4 emissions per unit of product are reduced17. In our study, CH4 emissions from intensive livestock production presented a mitigation potential of 26.2% compared with extensive livestock production (Table 2, management 3a livestock), and also altered total CH4 fluxes from a net source to a net sink in the grazed rangeland ecosystem.
Overall, present-day CH4 uptake is underestimated and CH4 emissions from livestock are overestimated, because the effects of rangeland improvement and intensive management promoted by ecological projects are not considered. This leads to uncertainty in the CH4 budget for the agriculture sector. There is a growing interest in science-based solutions for reducing CH4 emissions through improved rangeland and livestock management practices. There is no question that livestock in grazed rangeland ecosystems are a major CH4 source. In this study, the estimated CH4 production rates for all cattle, sheep and goats in China were 4.08, 1.00 and 0.89 Tg y−1, respectively in 2011, which is different from the total production rates estimated using the IPCC inventory method (Table 3). Of 264 pastoral and agro-pastoral counties in China, more than 50% are overstocked18. It is, therefore, obvious that reducing livestock numbers may be an effective management for mitigating CH4 emissions. However, implementation is a challenge since farmers' and herders' livelihoods depend on livestock. Therefore, continued improvement in the production efficiency of livestock while limiting CH4 emissions is one potential pathway for balancing these diverse and sometimes conflicting objectives.
The integrated CH4 mitigation potential of ecological conservation programs is mainly associated with stocking rate and the production system in grazed rangelands. Rangeland utilization currently has a large effect on environmental CH4 balance, and is driven by growth in demand for livestock products and rangeland resource scarcity. Improvements in livestock performance and rangeland management practices will contribute to more effective regulation of CH4 emissions. Data on the CH4 budget of grazed rangeland ecosystems can support evaluations of the GHG mitigation effects of policies and programs in managed lands, and are a high priority for climate research.
Surveyed representative rangeland regions
The temperate typical steppe site is located in Xinbaer Right Banner (N 47°36′, E 115°31′), Inner Mongolia. The site has an elevation of 610 m and a temperate continental climate. Annual average temperature is −2 ~ 1°C with a frost-free period of 128 days. Annual mean precipitation is 350 mm. Government programs support improved rangeland utilization and livestock production (Management 2 and 3 in Table 1). The temperate desert steppe site is located in Linwu County (N 37°46′, E 106°43′), Ningxia. The site has an elevation of 1250 m with a continental monsoon climate. Annual average temperature is 8–9°C with a frost-free period of 157 days. Annual mean precipitation is 206 mm. Because grazing is completely banned in Ningxia autonomous region, the practices evaluated include only grassland improvement (Management 1 in Table 1). The alpine meadow site is located in Hongyuan County (N 33°56′, E 102°35′), Sichuan. The site has an elevation of 3600 m and a temperate monsoon climate. Annual average temperature is 1–2°C with no absolute frost-free period. Annual mean precipitation is 753 mm. Government programs promote improved rangeland utilization and livestock production (Management 2 and 3 in Table 1). The temperate desert site is located in Fuhai County (N 46°01′, E 87°53′), Xinjiang. This site has an elevation of 1400 m and a mid-temperate continental climate. Annual average temperature is 6°C with a frost-free period of 150 days. Annual mean precipitation is 110 mm. Government programs promote improved rangeland utilization and livestock production (management 2 and 3 in Table 1).
Experimental design and sampling of the surveyed rangeland improvement areas
The surveyed areas for management practices 1a–e were approximately 30 ha each, with three replications. The survey procedure was to identify and record the latitude and longitude of the central location of each study site using GPS, mark the location with a stake, and design a transect bearing of either 120°, 240°, or 360° (Supplementary Fig. S4). CH4 fluxes of soil atmospheric exchanges were measured at 15 meters along each of the 3 transects lines. CH4 fluxes of soil atmospheric exchange were measured using the closed static chamber method. The chamber had a dimension of 50×50×50 cm made of stainless steel. The chamber was placed on a steel base frame driven 10 cm into each site one month prior to the start of the experiment. The base frame had a channel in which the chamber was inserted and the channel was filled with water to seal the chamber atmosphere. A 9 VDC fan was fixed in to the top wall of each chamber to mix the chamber atmosphere. The chamber was covered with a shroud made of camel hair, aluminum foil and white canvas to limit heating of the chamber atmosphere during sampling. During gas flux determination, a disposable syringe (100 ml) with a 3-way valve was used to collect 200 ml of chamber atmosphere in a sample gas bag at 10 min intervals over a 30 min period. The CH4 concentrations in the gas samples were analyzed using a wavelength scanning spectrophotometer (Picarro G1301, Santa Clara, USA). Gas fluxes were calculated using the following equation: where F is the flux (mg/m2/h) of CO2 or CH4; ρ is the density of 1 mole CH4 gas (kg/m3); Δc Δt−1 is the rate of change in gas concentration h−1; V and A are the volume (m3) and the chamber base area (m2), respectively.
Experimental design and sampling of the surveyed rangeland utilization areas
The design of grazing study was modeled after a biosphere where grazing impacts radiate in a diminishing response away from the centre, which was the location of the holding pen and water source (Supplementary Fig. S5). The piosphere grazing gradient was sampled along three replicate transects radiating from the center. The boundaries of the three grazing intensity zones (rest from grazing, light grazing, moderate grazing and heavy grazing) were defined along each transect19. The zones and their boundaries were defined by sampling species composition and vegetation coverage along the transects at 50 m intervals, using a single 20 cm × 50 cm quadrat, and grouping the plots into one of the three grazing intensity zones using cluster analysis. The method for measurement of CH4 fluxes of soil atmospheric exchange was the same as described for rangeland improvement areas above. CH4 emissions from livestock were estimated using a model described below.
Method for estimation of emissions from livestock production
We used a model to estimate CH4 emission from extensive and intensive livestock production systems20. The model was designed to analyze annual livestock feed supply and demand, livestock production, management practices, and CH4 emissions from livestock and excrement on a typical farm in each production system. The parameters of the model included rangeland area, livestock number, bodyweight, forage growth rate, forage digestibility, and supplementary feeding. Primary data for the model came from farm surveys in each study site. In each site, three representative farms were selected to represent intensive and extensive management. Farm survey data were obtained 3–4 times through the year. The experimental animals were used with the approval of the Experimental Animal Committee of the Chinese Academy of Sciences.
The area-weighted average annual CH4 budget was calculated by adding emissions (removals) from soil atmosphere CH4 uptake and emissions from livestock and livestock excrement. The amount of annual CH4 emission from livestock per ha was estimated and calculated by multiplying emissions per head by the stocking rate. The data of CH4 emissions from livestock production estimated using the model was compared with the data calculated using methods outlined in IPCC 2006 Guidelines for National Greenhouse Gas Inventories21. In order to calculate the mitigation potential, data on soil-atmospheric CH4 exchange were collected from a control area (non-measure, CK) and compared with data from sites under rangeland improvement and utilization. Data from heavily grazed (HG) areas and under extensive management were used as controls for comparison with rangeland utilization and intensive management, respectively.
This research was funded by the Chinese National Natural Science Foundation (31160109, 31270502), Inner Mongolia Palmary Youth Project (2010JQ04), the Research Project of Chinese Ministry of Education (213006A), the National Commonweal Special Project (Agriculture: 201003019, 200903060), State Key Lab of Grassland Resource, and Innovative Research Team Supported by the Ministry of Education of P. R. China.