Benjamin Wolf

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.

Interlaboratory assessment of nitrous oxide isotopomer analysis by isotope ratio mass spectrometry and laser spectroscopy: current status and perspectives.

RATIONALE In recent years, research and applications of the N2O site-specific nitrogen isotope composition have advanced, reflecting awareness of the contribution of N2O to the anthropogenic greenhouse effect, and leading to significant progress in instrument development. Further dissemination of N2O isotopomer analysis, however, is hampered by a lack of internationally agreed gaseous N2O reference materials and an uncertain compatibility of different laboratories and analytical techniques. METHODS In a first comparison approach, eleven laboratories were each provided with N2O at tropospheric mole fractions (target gas T) and two reference gases (REF1 and REF2). The laboratories analysed all gases, applying their specific analytical routines. Compatibility of laboratories was assessed based on N2O isotopocule data for T, REF1 and REF2. Results for T were then standardised using REF1 and REF2 to evaluate the potential of N2O reference materials for improving compatibility between laboratories. RESULTS Compatibility between laboratories depended on the analytical technique: isotope ratio mass spectrometry (IRMS) results showed better compatibility for δ(15)N values, while the performance of laser spectroscopy was superior with respect to N2O site preference. This comparison, however, is restricted by the small number of participating laboratories applying laser spectroscopy. Offset and two-point calibration correction of the N2O isotopomer data significantly improved the consistency of position-dependent nitrogen isotope data while the effect on δ(15)N values was only minor. CONCLUSIONS The study reveals that for future research on N2O isotopocules, standardisation against N2O reference material is essential to improve interlaboratory compatibility. For atmospheric monitoring activities, we suggest N2O in whole air as a unifying scale anchor.

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.

Isotopic evidence for nitrous oxide production pathways in a partial nitritation-anammox reactor

Nitrous oxide (N2O) production pathways in a single stage, continuously fed partial nitritation-anammox reactor were investigated using online isotopic analysis of offgas N2O with quantum cascade laser absorption spectroscopy (QCLAS). N2O emissions increased when reactor operating conditions were not optimal, for example, high dissolved oxygen concentration. SP measurements indicated that the increase in N2O was due to enhanced nitrifier denitrification, generally related to nitrite build-up in the reactor. The results of this study confirm that process control via online N2O monitoring is an ideal method to detect imbalances in reactor operation and regulate aeration, to ensure optimal reactor conditions and minimise N2O emissions. Under normal operating conditions, the N2O isotopic site preference (SP) was much higher than expected - up to 40‰ - which could not be explained within the current understanding of N2O production pathways. Various targeted experiments were conducted to investigate the characteristics of N2O formation in the reactor. The high SP measurements during both normal operating and experimental conditions could potentially be explained by a number of hypotheses: i) unexpectedly strong heterotrophic N2O reduction, ii) unknown inorganic or anammox-associated N2O production pathway, iii) previous underestimation of SP fractionation during N2O production from NH2OH, or strong variations in SP from this pathway depending on reactor conditions. The second hypothesis - an unknown or incompletely characterised production pathway - was most consistent with results, however the other possibilities cannot be discounted. Further experiments are needed to distinguish between these hypotheses and fully resolve N2O production pathways in PN-anammox systems.

Modeling N2O emissions from steppe in Inner Mongolia, China, with consideration of spring thaw and grazing intensity

AimsTemperate grassland is one of the major global biome types and is widely used as rangeland. Typically, cold winters are followed by a transition period with soil thawing that may last from days to weeks. Pulse N2O emissions during freeze-thaw events have been observed in a range of temperate ecosystem types and may contribute significantly to annual N2O emissions. It was shown recently that spring thaw pulse N2O emissions dominated annual N2O emissions in a steppe region of Inner Mongolia. Even though biogeochemical models are increasingly used for up scaling of N2O emissions from terrestrial ecosystems, they still need to be further developed to be capable of both simulating pulse N2O emission during spring thaw and accounting for the impact of grazing on soil N2O emissions in general.MethodsIn this study, we modified an existing biogeochemical model, Mobile-DNDC, to allow an improved simulation of plant production, snow height, and soil moisture for steppe in Inner Mongolia exposed to different grazing intensities. The newly introduced routines relate maximum snow height to end-of-season biomass (ESSB), to account for decreased plant productivity due to grazing and consider the increase of resistance (impedance) of soil ice on the soil hydraulic conductivity.ResultsThe implementation of the impedance concept, which means the consideration of decreased hydraulic conductivity in frozen soil, resulted in an improved simulation of soil water content and decreased simulated oxygen content in the top soil during freeze-thaw periods. Increased soil moisture and associated oxygen limitation stimulated N2O emission by enhanced denitrification. Based on observations in the field, maximum snow height was limited by ESSB, protecting snow against erosion by wind. Since grazing reduced biomass and thereby snow cover, water availability during spring thaw was smaller at grazed sites as compared to ungrazed sites. In agreement with field observations, lower water content and anaerobiosis resulted in decreased N2O emissions during spring thaw.ConclusionsThe introduction of the impedance concept into Mobile-DNDC is a major step forward in simulating pulse N2O emissions from soils during spring-thaw.

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.

Seasonality of soil microbial nitrogen turnover in continental steppe soils of Inner Mongolia

Annual estimates and seasonal patterns of gross nitrogen turnover in terrestrial soils are poorly understood due to the lack of experimental evidence. Based on year round sampling in wintergrazed and ungrazed steppe soils of Inner Mongolia, we show that measurements of net rates of ammonification (−9 to −6 kg N ha−1 year−1) or nitrification (19 to 31 kg N ha−1 year−1) do not at all reflect the pronounced dynamics of gross rates of ammonification (215–240 kg N ha−1 year−1) or nitrification (362–417 kg N ha−1 year−1). Four different seasons with characteristic functional patterns of N turnover were identified: (1) Growing season dynamics as characterized by drying/rewetting cycles and negatively correlated temporal courses of net microbial growth and periods with intensive gross ammonification, contributing 40–52% and 29–32% to cumulative annual gross ammonification and nitrification, respectively. Net N mineralization was almost exclusively observed during the growing season. (2) Microbial N dynamics during the autumn freeze-thaw period was characterized by a sharp decline in microbial biomass in conjunction with a peak of gross nitrification contributing 19–36% to cumulative annual fluxes. (3) During winter at constantly frozen soil, a net build-up of microbial biomass was observed, whereas gross N turnover rates were low, contributing 7–10% and 6–11% to cumulative annual gross ammonification or gross nitrification, respectively. (4) The spring freeze-thaw period showed extremely dynamic changes in gross N turnover and soil nitrate concentrations. This period contributed 34–44% and 21–46% to cumulative annual gross ammonification and nitrification, respectively. This study highlights that freeze-thaw cycles are key periods for understanding patterns and magnitudes of gross N turnover in semi-arid continental steppe ecosystems. The results further imply that the observed patterns of microbial biomass and gross N turnover dynamics are likely the consequence of a seasonal succession of microbial communities and turnover of microbial biomass. Our findings emphasize the necessity for high resolution studies on gross N turnover as a prerequisite to infer functioning and annual budgets of ecosystem N cycling.

Nitrate leaching and soil nitrous oxide emissions diminish with time in a hybrid poplar short-rotation coppice in southern Germany

Hybrid poplar short‐rotation coppices (SRC) provide feedstocks for bioenergy production and can be established on lands that are suboptimal for food production. The environmental consequences of deploying this production system on marginal agricultural land need to be evaluated, including the investigation of common management practices i.e., fertilization and irrigation. In this work, we evaluated (1) the soil‐atmosphere exchange of carbon dioxide, methane, and nitrous oxide (N2O); (2) the changes in soil organic carbon (SOC) stocks; (3) the gross ammonification and nitrification rates; and (4) the nitrate leaching as affected by the establishment of a hybrid poplar SRC on a marginal agricultural land in southern Germany. Our study covered one 3‐year rotation period and 2 years after the first coppicing. We combined field and laboratory experiments with modeling. The soil N2O emissions decreased from 2.2 kg N2O‐N ha−1 a−1 in the year of SRC establishment to 1.1–1.4 kg N2O‐N ha−1 a−1 after 4 years. Likewise, nitrate leaching reduced from 13 to 1.5–8 kg N ha−1 a−1. Tree coppicing induced a brief pulse of soil N2O flux and marginal effects on gross N turnover rates. Overall, the N losses diminished within 4 years by 80% without fertilization (irrespective of irrigation) and by 40% when 40–50 kg N ha−1 a−1 were applied. Enhanced N losses due to fertilization and the minor effect of fertilization and irrigation on tree growth discourage its use during the first rotation period after SRC establishment. A SOC accrual rate of 0.4 Mg C ha−1 a−1 (uppermost 25 cm, P = 0.2) was observed 5 years after the SRC establishment. Overall, our data suggest that SRC cultivation on marginal agricultural land in the region is a promising option for increasing the share of renewable energy sources due to its net positive environmental effects.

Disentangling gross N2O production and consumption in soil

The difficulty of measuring gross N2O production and consumption in soil impedes our ability to predict N2O dynamics across the soil-atmosphere interface. Our study aimed to disentangle these processes by comparing measurements from gas-flow soil core (GFSC) and 15N2O pool dilution (15N2OPD) methods. GFSC directly measures soil N2O and N2 fluxes, with their sum as the gross N2O production, whereas 15N2OPD involves addition of 15N2O into a chamber headspace and measuring its isotopic dilution over time. Measurements were conducted on intact soil cores from grassland, cropland, beech and pine forests. Across sites, gross N2O production and consumption measured by 15N2OPD were only 10% and 6%, respectively, of those measured by GFSC. However, 15N2OPD remains the only method that can be used under field conditions to measure atmospheric N2O uptake in soil. We propose to use different terminologies for the gross N2O fluxes that these two methods quantified. For 15N2OPD, we suggest using ‘gross N2O emission and uptake’, which encompass gas exchange within the 15N2O-labelled, soil air-filled pores. For GFSC, ‘gross N2O production and consumption’ can be used, which includes both N2O emitted into the soil air-filled pores and N2O directly consumed, forming N2, in soil anaerobic microsites.