Zhisheng Yao

Grazing-induced reduction of natural nitrous oxide release from continental steppe

Atmospheric concentrations of the greenhouse gas nitrous oxide (N2O) have increased significantly since pre-industrial times owing to anthropogenic perturbation of the global nitrogen cycle, with animal production being one of the main contributors. Grasslands cover about 20 per cent of the temperate land surface of the Earth and are widely used as pasture. It has been suggested that high animal stocking rates and the resulting elevated nitrogen input increase N2O emissions. Internationally agreed methods to upscale the effect of increased livestock numbers on N2O emissions are based directly on per capita nitrogen inputs. However, measurements of grassland N2O fluxes are often performed over short time periods, with low time resolution and mostly during the growing season. In consequence, our understanding of the daily and seasonal dynamics of grassland N2O fluxes remains limited. Here we report year-round N2O flux measurements with high and low temporal resolution at ten steppe grassland sites in Inner Mongolia, China. We show that short-lived pulses of N2O emission during spring thaw dominate the annual N2O budget at our study sites. The N2O emission pulses are highest in ungrazed steppe and decrease with increasing stocking rate, suggesting that grazing decreases rather than increases N2O emissions. Our results show that the stimulatory effect of higher stocking rates on nitrogen cycling and, hence, on N2O emission is more than offset by the effects of a parallel reduction in microbial biomass, inorganic nitrogen production and wintertime water retention. By neglecting these freeze–thaw interactions, existing approaches may have systematically overestimated N2O emissions over the last century for semi-arid, cool temperate grasslands by up to 72 per cent.

Soil‐atmosphere exchange potential of NO and N2O in different land use types of Inner Mongolia as affected by soil temperature, soil moisture, freeze‐thaw, and drying‐wetting events

[1] Changes in precipitation and temperature in Asian continental steppelands may affect soil physical, chemical and biological processes that control the biosphere-atmosphere exchange of N-trace gases. The changes include regional desertification, global warming and strong El Nino events that impact the large steppe land area in China and Mongolia. The area is so large that feedbacks to the global greenhouse gas balance may occur. In this study we investigated how changes in soil moisture and temperature, and especially drying-rewetting and freeze-thaw events, affect nitric oxide (NO) and nitrous oxide (N 2 O) fluxes from large intact soil cores taken from representative land use/cover types in the region of the Xilin River catchment, Inner Mongolia. These soil cores were incubated under varying conditions with respect to temperature (ranging from -10 to 15°C) and simulated rainfall (25, 45 and 65 mm). Following drying-rewetting and freeze-thaw transitions, we observed pulses of NO and N 2 O emissions from the soils of typical steppe, mountain meadow, sand dune and marshland. A comparable trend in soil CO 2 emissions and soil air N 2 O concentrations indicated that the high substrate availability and rapid recovery of microbial activity after soil wetting and thawing resulted in high gas fluxes. Across the whole temperature range, NO and N 2 O fluxes from all soils, except for N 2 O emissions from marshland soils, showed a positive exponential relationship with soil temperature. A combination of soil temperature and soil moisture explained most of the observed variations in NO (up to 74-90%) and N 2 O (up to 67-89%) fluxes for individual soils. Spatial differences in NO emissions between land use/cover types could be explained by differences in soil organic carbon and pH, whereas spatial variations of N 2 O fluxes were primarily correlated with differences in soil microbial biomass. On the basis of the incubation under controlled conditions, the average annual flux, weighted by the areal extent of the different investigated land use/cover types in the region, was estimated at ~3.9 ± 1.1 kg N ha -1 yr -1 for NO and 0.53 ± 0.20 kg N ha -1 yr -1 for N 2 O, respectively. It is noteworthy that our measurements were conducted using soil cores without a vegetation cover, which probably resulted in an overestimation of N-trace gas fluxes. However, our results indicate that the rarely determined NO formation appears to be a significant pathway in the N cycle of semiarid steppe, which is highly sensitive to the climatic change taking place in these regions, especially an increase in intensity and frequency of drying-wetting and freeze-thaw cycles.

Feedback of grazing on gross rates of N mineralization and inorganic N partitioning in steppe soils of Inner Mongolia

Plant-microbe interactions are crucial regulators of belowground nitrogen cycling in terrestrial ecosystems. However, such interactions have mostly been excluded from experimental setups for the investigation of gross inorganic N fluxes and N partitioning to plants and microorganisms. Ungulate grazing is likely to feed back on soil N fluxes, and hence it is of special importance to simultaneously investigate grazing effects on both plant and microbial N fluxes in intact plant-soil systems, where plant-microbe interactions persist during the experimental incubation. Based on the homogenous 15NH4+ labelling of intact plant-soil monoliths we investigated how various stocking rates (0, 2.35, 4.8 and 7.85 sheep ha−1 grazing season−1) in steppe of Inner Mongolia feedback on gross rates of N mineralization and short-term inorganic N partitioning between plant, microbial and soil N pools. Our results showed that the effect of grazing on gross N mineralization was non-uniform. At low stocking rate gross N mineralization tended to decrease but increased with higher grazing pressure. Hence, there was no significant correlation between stocking rate and gross N mineralization across the investigated grazing intensities. Grazing decreased 15N recovery both in plant and microbial N pools but strongly promoted NO3− accumulation in the soil and thus negatively affected potential ecosystem N retention. This appeared to be closely related to the grazing-induced decline in easily degradable soil C availability at increasing stocking rate.

Annual methane uptake by typical semiarid steppe in Inner Mongolia

[1] Steppe ecosystems cover approximately 10% of the global land surface. Recent measurements have shown that steppe soils function as a significant sink for atmospheric methane (CH 4 ). However, precise quantification of the annual CH 4 uptake by steppe is challenged by infrequent measurements of exchange rates, which often only cover the growing season. In order to understand the annual dynamics and magnitude of CH 4 exchange, especially contribution of nongrowing season to the cumulative annual CH 4 exchange, we conducted year-round CH 4 flux measurements at high temporal resolution at two adjacent steppe sites. One was ungrazed and fenced since 1999 (UG99) and the other was grazed during the winter (WG01). The measurements were supplemented with observations of CH 4 concentrations in the soil profile. Sites were located in typical Leymus chinensis steppe in Inner Mongolia, China. The results show that the typical semiarid steppe functioned exclusively as a sink for atmospheric CH 4 throughout the entire year. Even during the spring soil thawing, a period with high water content in the top soil, CH 4 uptake was dominant. The seasonality of CH 4 uptake displayed a strong dependency on the seasonal variation in soil temperature. Soil moisture increased in importance when temperature was not the limiting factor. For example, CH 4 rates decreased sharply following summer rainfall events. The annual CH 4 uptake by the ungrazed UG99 and the winter-grazed WG01 sites was 3.7 and 2.1 kg C ha -1 , respectively. The contribution of the nongrowing season (October-April) to the cumulative annual CH 4 uptake was approximately 30% (25%-36%). Additionally, our data suggest that winter grazing significantly alters the capacity of steppe soils for CH 4 uptake. However, more measurements at paired ungrazed/grazed sites are needed to assess how grazing might affect the CH 4 uptake capacity of steppe soils at a larger regional or global scale.

Improving rice production sustainability by reducing water demand and greenhouse gas emissions with biodegradable films

In China, rice production is facing unprecedented challenges, including the increasing demand, looming water crisis and on-going climate change. Thus, producing more rice at lower environmental cost is required for future development, i.e., the use of less water and the production of fewer greenhouse gas (GHG) per unit of rice. Ground cover rice production systems (GCRPSs) could potentially address these concerns, although no studies have systematically and simultaneously evaluated the benefits of GCRPS regarding yields and considering water use and GHG emissions. This study reports the results of a 2-year study comparing conventional paddy and various GCRPS practices. Relative to conventional paddy, GCRPSs had greater rice yields and nitrogen use efficiencies (8.5% and 70%, respectively), required less irrigation (−64%) and resulted in less total CH4 and N2O emissions (−54%). On average, annual emission factors of N2O were 1.67% and 2.00% for conventional paddy and GCRPS, respectively. A cost-benefit analysis considering yields, GHG emissions, water demand and labor and mulching costs indicated GCRPSs are an environmentally and economically profitable technology. Furthermore, substituting the polyethylene film with a biodegradable film resulted in comparable benefits of yield and climate. Overall, GCRPSs, particularly with biodegradable films, provide a promising solution for farmers to secure or even increase yields while reducing the environmental footprint.

Annual N 2 O emissions from conventionally grazed typical alpine grass meadows in the eastern Qinghai–Tibetan Plateau

Annual nitrous oxide (N2O) emissions from high-altitude alpine meadow grasslands have not been effectively characterized because of the scarcity of whole-year measurements. The authors performed a year-round measurement of N2O fluxes from three conventionally grazed alpine meadows that represent the typical meadow landscape in the eastern Qinghai-Tibetan Plateau (QTP). The results showed that annual N2O emissions averaged 0.123±0.053 (2SD, i.e., the double standard deviation indicating the 95% confidence interval) kgNha-1yr-1 across the three meadow sites. N2O flux pulses during the spring freezing-thawing period (FTP) were observed at only one site, indicating a large spatial variability in association with soil moisture differences. Approximately 34-57% (mean: 46%) of the annual N2O emissions occurred in the non-growing season, highlighting the substantial importance of accurate flux observations during this period. The simultaneous observations showed conservative, marginal nitric oxide (NO) fluxes of 0.058±0.032 (2SD) kgNha-1yr-1. The N2O fluxes across the three field sites correlated negatively with the soil nitrate concentrations during the entire year-round period (P<0.05). Furthermore, a significant joint regulatory effect of topsoil temperature and moisture on the N2O and NO fluxes was observed during the relatively warm periods. Based on the results of the present and previous studies, a simple extrapolation roughly estimated the annual total N2O emission from Chinese grasslands to be 73±15 (2SD) GgNyr-1 (1Gg=109g). A linear dependence of the annual N2O fluxes on the aboveground net primary productivity (ANPP) was also found. This result may provide a simple approach for estimating the N2O emission inventories of frigid alpine or temperate grasslands that are ungrazed either in the summer or year round. However, further confirmation of this relationship with a wider ANPP range is still needed in the future studies.

Straw return reduces yield-scaled N2O plus NO emissions from annual winter wheat-based cropping systems in the North China Plain

Straw return in combination with synthetic N fertilizer is considered to be beneficial to soil fertility and crop yield. Such practice, however, can considerably modify soil microbial activity and relative C and N availability, both of which are known to regulate soil nitrous oxide (N2O) and nitric oxide (NO) emissions. Minimizing these emissions per unit of crop yield is a prerequisite to minimize the environmental footprint of agricultural production and thus, a policy objective. In our study, we quantified N2O and NO emissions and determined fertilizer-N use efficiencies (NUE) and crop yields of two double-cropping (summer maize/Welsh onion-winter wheat) systems with and without straw incorporation in the North China Plain. Relative to the fertilized treatment without straw amendments, straw incorporation showed a significant inhibitory effect on annual N2O emissions from the maize-wheat system (-31%), but no significant effect was observed for the Welsh onion-wheat system. However, straw return significantly reduced annual NO emissions by >30% for both systems. Meanwhile, straw return in both systems significantly increased the NUE and crop yields by 34-47% and 7-16%, respectively, as compared to the treatment without straw additions. Across the double-cropping systems, annual direct emission factors of N2O, NO and N2O+NO were 0.37-0.57%, 0.08-0.78% and 0.57-1.36%, respectively. Furthermore, a negative relationship between direct emission factors of N2O+NO and crop NUE was observed, highlighting the importance of optimizing NUE for reducing environmental risks of a cropping system. When expressing emissions on a yield basis, straw return significantly reduced annual yield-scaled N2O+NO emissions by 15-42% for both systems. Overall, our results show that the combined application of crop straw and synthetic N fertilizer is a promising N management strategy for maximizing crop yields while mitigating N-trace gas emissions.

Urea deep placement reduces yield-scaled greenhouse gas (CH 4 and N 2 O) and NO emissions from a ground cover rice production system

Ground cover rice production system (GCRPS), i.e., paddy soils being covered by thin plastic films with soil moisture being maintained nearly saturated status, is a promising technology as increased yields are achieved with less irrigation water. However, increased soil aeration and temperature under GCRPS may cause pollution swapping in greenhouse gas (GHG) from CH4 to N2O emissions. A 2-year experiment was performed, taking traditional rice cultivation as a reference, to assess the impacts of N-fertilizer placement methods on CH4, N2O and NO emissions and rice yields under GCRPS. Averaging across all rice seasons and N-fertilizer treatments, the GHG emissions for GCRPS were 1973 kg CO2-eq ha−1 (or 256 kg CO2-eq Mg−1), which is significantly lower than that of traditional cultivation (4186 kg CO2-eq ha−1or 646 kg CO2-eq Mg−1). Furthermore, if urea was placed at a 10–15 cm soil depth instead of broadcasting, the yield-scaled GHG emissions from GCRPS were further reduced from 377 to 222 kg CO2-eq Mg−1, as N2O emissions greatly decreased while yields increased. Urea deep placement also reduced yield-scaled NO emissions by 54%. Therefore, GCRPS with urea deep placement is a climate- and environment-smart management, which allows for maximal rice yields at minimal GHG and NO emissions.

Benefits of integrated nutrient management on N2O and NO mitigations in water-saving ground cover rice production systems

To cope with challenges of food security and water scarcity in rice production, water-saving ground cover rice production systems (GCRPSs) are increasingly adopted in China and globally. Reduced soil moisture as well as increased soil aeration and temperature under GCRPSs may promote soil N transformations, and in turn give rise to environmental challenges. These include emissions of the potent greenhouse gas nitrous oxide (N2O) and atmospheric pollutant nitric oxide (NO). Using conventional flooding rice cultivation as a reference, a three-year field experiment was conducted to investigate the performances of GCRPSs under inorganic (urea) or integrated nutrient management (a combination of synthetic and organic fertilizers), with regards to soil N2O and NO emissions as well as grain yields. N2O and NO emissions in GCRPSs exhibited high seasonal and interannual variations along with changes in soil inorganic N content and rainfall. When urea alone was applied, the average N2O and NO emissions from GCRPSs were 4.11 and 0.14 kg N ha-1, respectively. These emissions were significantly higher than those of conventional rice cultivation, with 1.47 and 0.052 kg N ha-1 for N2O and NO, respectively. When integrated nutrient management was performed for GCRPSs, N2O and NO emissions were reduced by approximately 77% and 50%, respectively, i.e., the emission magnitude comparable with N-trace gas losses from conventional rice cultivation. Moreover, GCRPSs with integrated nutrient management resulted in optimal grain yields, and thus, the yield-scaled N2O + NO emissions were the lowest compared to other treatments. Averaged over 3 years, the direct emission factors of N2O and NO for GCRPSs with urea alone were 2.58% and 0.064%, respectively. Those for GCRPSs with integrated nutrient management were 0.48% and 0.016%, respectively. The results of this study suggest that GCRPS with integrated nutrient management is an eco-friendly strategy for optimizing crop yields while mitigating N2O and NO emissions.

Net ecosystem carbon and greenhouse gas budgets in fiber and cereal cropping systems

To assess the contributions of fiber and cereal production on climate change, the net ecosystem exchange of carbon dioxide (CO2), main exchanges of non-CO2 carbon, and methane (CH4) and nitrous oxide (N2O) fluxes were continuously monitored throughout two year-round crop cycles (Y1 and Y2: 1st and 2nd year-round crop cycles, respectively) using eddy covariance, biometric observation, and static chamber methods in typical cotton and wheat-maize rotational cropping systems in China. The evaluation of net ecosystem carbon budgets (NECBs: considering net ecosystem CO2 exchange and non-CO2 carbon exchanges by fertilization, seeding, and harvest) and greenhouse gas budgets (GHGBs: adding CH4 and N2O fluxes to the NECBs based on CO2 equivalents) showed that the cotton cropping system persistently functioned as an intensive carbon (-1527 and -974 kg C ha-1 yr-1) and greenhouse gas (GHG) source (5618 and 3591 kg CO2-eq ha-1 yr-1) because of the large CO2 emissions during the long fallow periods (5748 and 5160 kg CO2 ha-1 in Y1 and Y2, respectively). The wheat-maize cropping system had high net ecosystem production (NEP) and low harvest index and therefore, served as a notable carbon sink (1461 kg C ha-1 yr-1 in Y2). Although high irrigation water and chemical fertilizer inputs stimulated N2O emissions, the wheat-maize cropping system still behaved as an important GHG sink (-4257 kg CO2-eq ha-1 yr-1 in Y2) because of the tremendous net carbon sequestration. However, in Y1 incidental wind damage lowered the NEP and turned the wheat-maize cropping system into a GHG source (2144 kg CO2-eq ha-1 yr-1). The NEP, NECBs, and GHGBs of the double cropping system generally exceeded those of the single cropping system. The traditional rotation between double and single cropping systems should be restored to maintain soil carbon storage and alleviate the radiative forcing effects of cotton production.