Since the late 1950s, governmental rangeland policies have changed the grazing management on the Tibetan Plateau (TP). Increasing grazing pressure and, since the 1980s, the privatization and fencing of pastures near villages has led to land degradation, whereas remote pastures have recovered from stronger overgrazing. To clarify the effect of moderate grazing on the carbon (C) cycle of the TP, we investigated differences in below-ground C stocks and C allocation using in situ 13 CO 2 pulse labeling of (i) a montane Kobresia winter pasture of yaks, with moderate grazing regime and (ii) a 7-year-old grazing exclosure plot, both in 3440 m asl. Twenty-seven days after the labeling, 13 C incorporated into shoots did not differ between the grazed (43% of recovered 13 C) and ungrazed (38%) plots. In the grazed plots, however, less C was lost by shoot respiration (17% vs. 42%), and more was translocated below-ground (40% vs. 20%). Within the below-ground pools, <2% of 13 C was incorporated into living root tissue of both land use types. In the grazed plots about twice the amount of 13 C remained in soil (18%) and was mineralized to CO 2 (20%) as compared to the ungrazed plots (soil 10%; CO 2 9%). Despite the higher contribution of root-derived C to CO 2 efflux, total CO 2 efflux did not differ between the two land use types. C stocks in the soil layers 0-5 and 5-15 cm under grazed grassland were significantly larger than in the ungrazed grassland. However, C stocks below 15 cm were not affected after 7 years without grazing. We conclude that the larger below-ground C allocation of plants, the larger amount of recently assimilated C remaining in the soil, and less soil organic matter-derived CO 2 efflux create a positive effect of moderate grazing on soil C input and C sequestration.

With 450,000 kmKobresia (syn. Carex) pygmaea dominated pastures in the eastern Tibetan highlands are the world's largest pastoral alpine ecosystem forming a durable turf cover at 3000-6000 m a.s.l. Kobresia's resilience and competitiveness is based on dwarf habit, predominantly below-ground allocation of photo assimilates, mixture of seed production and clonal growth, and high genetic diversity. Kobresia growth is co-limited by livestock-mediated nutrient withdrawal and, in the drier parts of the plateau, low rainfall during the short and cold growing season. Overstocking has caused pasture degradation and soil deterioration over most parts of the Tibetan highlands and is the basis for this man-made ecosystem. Natural autocyclic processes of turf destruction and soil erosion are initiated through polygonal turf cover cracking, and accelerated by soil-dwelling endemic small mammals in the absence of predators. The major consequences of vegetation cover deterioration include the release of large amounts of C, earlier diurnal formation of clouds, and decreased surface temperatures. These effects decrease the recovery potential of Kobresia pastures and make them more vulnerable to anthropogenic pressure and climate change. Traditional migratory rangeland management was sustainable over millennia, and possibly still offers the best strategy to conserve and possibly increase C stocks in the Kobresia turf.

The carbon- and nitrogen-rich soils of montane grasslands are exposed to above-average warming and to altered precipitation patterns as a result of global change. To investigate the consequences of climatic change for soil nitrogen turnover, we translocated intact plant-soil mesocosms along an elevational gradient, resulting in an increase of the mean annual temperature by approx. 2 °C while decreasing precipitation from approx. 1500 to 1000 mm. Following three years of equilibration, we monitored the dynamics of gross nitrogen turnover and ammonia-oxidizing bacteria (AOB) and archaea (AOA) in soils over an entire year. Gross nitrogen turnover and gene levels of AOB and AOA showed pronounced seasonal dynamics. Both summer and winter periods equally contributed to cumulative annual N turnover. However, highest gross N turnover and abundance of ammonia oxidizers were observed in frozen soil of the climate change site, likely due to physical liberation of organic substrates and their rapid turnover in the unfrozen soil water film. This effect was not observed at the control site, where soil freezing did not occur due to a significant insulating snowpack. Climate change conditions accelerated gross nitrogen mineralization by 250% on average. Increased N mineralization significantly stimulated gross nitrification by AOB rather than by AOA. However, climate change impacts were restricted to the 2-6 cm topsoil and rarely occurred at 12-16 cm depth, where generally much lower N turnover was observed. Our study shows that significant mineralization pulses occur under changing climate, which is likely to result in soil organic matter losses with their associated negative impacts on key soil functions. We also show that N cycling processes in frozen soil can be hot moments for N turnover and thus are of paramount importance for understanding seasonal patterns, annual sum of N turnover and possible climate change feedbacks.

No abstract

Summary Montane grasslands of Central Europe are expected to be exposed to strong warming and to altered precipitation patterns, suggesting that biosphere–atmosphere–hydrosphere exchange of carbon (C) and nitrogen (N) compounds may be vulnerable to future climatic conditions. By transferring small lysimeters along an altitudinal gradient, we assessed the impact of climate change conditions on soil–atmosphere exchange of methane (CH4) and nitrous oxide (N2O) as well as on ammonium (NH4+) and nitrate (NO3−) in soil water in extensively managed montane grassland in southern Germany. Lysimeter transfer to lower altitude increased air and soil temperatures by more than 2°C and reduced summer precipitation as well as soil moisture throughout the year compared with a control transfer within the high altitude site. This simulation of climate change conditions almost doubled the CH4 sink strength from −0.11 to −0.19 g C m−2 year−1, which appeared to be mainly related to improved gas diffusion after reduced soil moisture. Mean NH4+ and NO3− concentrations in soil water (0.05 mg NH4+–N l−1 and 0.08 mg NO3−–N l−1) and N2O emissions (approximately 0.03 g N m−2 year−1) remained small and unaffected by climate change simulation. Our study suggests that expected climate change conditions will have positive effects on the non‐CO2 greenhouse gas balance in extensively managed montane grassland because of increased net CH4 uptake in soil. For N2O emission, we conclude that potential effects of management changes may override the small effects of simulated climate change on N2O emissions observed in this study.