Biogeosciences

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ISSN / EISSN : 1726-4170 / 1726-4189
Published by: Copernicus GmbH (10.5194)
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Published: 24 June 2022
Biogeosciences, Volume 19, pp 3051-3071; https://doi.org/10.5194/bg-19-3051-2022

Abstract:
Permafrost thaw in northern peatlands often leads to increased methane (CH4) emissions, but the underlying controls responsible for increased emissions and the duration for which they persist have yet to be fully elucidated. We assessed how shifting environmental conditions affect microbial communities and the magnitude and stable isotopic signature (δ13C) of CH4 emissions along a thermokarst bog transect in boreal western Canada. Thermokarst bogs develop following permafrost thaw when dry, elevated peat plateaus collapse and become saturated and dominated by Sphagnum mosses. We differentiated between a young and a mature thermokarst bog stage (∼ 30 and ∼ 200 years since thaw, respectively). The young bog located along the thermokarst edge was wetter, warmer, and dominated by hydrophilic vegetation compared to the mature bog. Using high-throughput 16S rRNA gene sequencing, we show that microbial communities were distinct near the surface and converged with depth, but fewer differences remained down to the lowest depth (160 cm). Microbial community analysis and δ13C data from CH4 surface emissions and dissolved gas depth profiles show that hydrogenotrophic methanogenesis was the dominant pathway at both sites. However, mean δ13C-CH4 signatures of both dissolved gas profiles and surface CH4 emissions were found to be isotopically heavier in the young bog (−63 ‰ and −65 ‰, respectively) compared to the mature bog (−69 ‰ and −75 ‰, respectively), suggesting that acetoclastic methanogenesis was relatively more enhanced throughout the young bog peat profile. Furthermore, mean young bog CH4 emissions of 82 mg CH4 m−2 d−1 were ∼ 3 times greater than the 32 mg CH4 m−2 d−1 observed in the mature bog. Our study suggests that interactions between the methanogenic community, hydrophilic vegetation, warmer temperatures, and saturated surface conditions enhance CH4 emissions in young thermokarst bogs but that these favourable conditions only persist for the initial decades after permafrost thaw.
Published: 22 June 2022
Biogeosciences, Volume 19, pp 3021-3050; https://doi.org/10.5194/bg-19-3021-2022

Abstract:
Knowledge of the effects of climate change on agro-ecosystems is fundamental to identifying local actions aimed to maintain productivity and reduce environmental issues. This study investigates the effects of climate perturbation on the European crop and grassland production systems, combining the findings from two specific biogeochemical models. Accurate and high-resolution management and pedoclimatic data were employed. Results have been verified for the period 1978–2004 (historical period) and projected until 2099 with two divergent intensities: the Intergovernmental Panel on Climate Change (IPCC) climate projections, Representative Concentration Pathway (RCP) 4.5 and RCP8.5. We have provided a detailed overview of productivity and the impacts on management (sowing dates, water demand, nitrogen use efficiency). Biogenic greenhouse gas balance (N2O, CH4, CO2) was calculated, including an assessment of the gases' sensitivity to the leading drivers, and a net carbon budget on production systems was compiled. Results confirmed a rise in productivity in the first half of the century (+5 % for croplands at +0.2 t DM ha−1 yr−1, +1 % for grasslands at +0.1 t DM ha−1 yr−1; DM denotes dry matter), whereas a significant reduction in productivity is expected during the period 2050–2099, caused by the shortening of the length of the plant growing cycle associated with rising temperatures. This effect was more pronounced for the more pessimistic climate scenario (−6.1 % for croplands and −7.7 % for grasslands), for the Mediterranean regions and in central European latitudes, confirming a regionally distributed impact of climate change. Non-CO2 greenhouse gas emissions were triggered by rising air temperatures and increased exponentially over the century, often exceeding the CO2 accumulation of the explored agro-ecosystems, which acted as potential C sinks. The emission factor for N2O was 1.82 ± 0.07 % during the historical period and rose to up to 2.05 ± 0.11 % for both climate projections. The biomass removal (crop yield, residues exports, mowing and animal intake) converted croplands and grasslands into net C sources (236 ± 107 Tg CO2 eq. yr−1 in the historical period), increasing from 19 % to 26 % during the climate projections, especially for RCP4.5. Nonetheless, crop residue restitution might represent a potential management strategy to overturn the C balance. Although with a marked latitudinal gradient, water demand will double over the next few decades in the European croplands, whereas the benefit in terms of yield (+2 % to +10 % over the century) will not contribute substantially to balance the C losses due to climate perturbation.
Siqi Li, Wei Zhang, , , , Rui Wang, Kai Wang, Zhisheng Yao, Chunyan Liu, Chong Zhang
Published: 22 June 2022
Biogeosciences, Volume 19, pp 3001-3019; https://doi.org/10.5194/bg-19-3001-2022

Abstract:
Accurate simulation of ammonia (NH3) volatilization from fertilized croplands is crucial to enhancing fertilizer-use efficiency and alleviating environmental pollution. In this study, a process-oriented model, CNMM–DNDC (Catchment Nutrient Management Model–DeNitrification–DeComposition), was evaluated and modified using NH3 volatilization observations from 44 and 19 fertilizer application events in cultivated uplands and paddy rice fields in China, respectively. The major modifications for simulating NH3 volatilization from cultivated uplands were primarily derived from a peer-reviewed and published study. NH3 volatilization from cultivated uplands was jointly regulated by wind speed, soil depth, clay fraction, soil temperature, soil moisture, vegetation canopy, and rainfall-induced canopy wetting. Moreover, three principle modifications were made to simulate NH3 volatilization from paddy rice fields. First, the simulation of the floodwater layer and its pH were added. Second, the effect of algal growth on the diurnal fluctuation in floodwater pH was introduced. Finally, the Jayaweera–Mikkelsen model was introduced to simulate NH3 volatilization. The results indicated that the original CNMM–DNDC not only performed poorly in simulating NH3 volatilization from cultivated uplands but also failed to simulate NH3 volatilization from paddy rice fields. The modified model showed remarkable performances in simulating the cumulative NH3 volatilization of the calibrated and validated cases, with drastically significant zero-intercept linear regression of slopes of 0.94 (R2 = 0.76, n = 40) and 0.98 (R2 = 0.71, n = 23), respectively. The simulated NH3 volatilization from cultivated uplands was primarily regulated by the dose and type of the nitrogen fertilizer and the irrigation implementation, while the simulated NH3 volatilization from rice paddy fields was sensitive to soil pH; the dose and depth of nitrogen fertilizer application; and flooding management strategies, such as floodwater pH and depth. The modified model is acceptable to compile regional or national NH3 emission inventories and develop strategies to alleviate environmental pollution.
Chen Yang, Yue Shi, Wenjuan Sun, Jiangling Zhu, Chengjun Ji, Yuhao Feng, Suhui Ma, Zhaodi Guo,
Published: 21 June 2022
Biogeosciences, Volume 19, pp 2989-2999; https://doi.org/10.5194/bg-19-2989-2022

Abstract:
China is one of the major forest countries in the world, and the accurate estimation of its forest biomass carbon (C) pool is critical for evaluating the country's C budget and ecosystem services of forests. Although several studies have estimated China's forest biomass using national forest inventory data, most of them were limited to the period of 2004–2008. In this study, we extended our estimation to the most recent period of 2014–2018. Using datasets of eight inventory periods from 1977 to 2018 and the continuous biomass expansion factor method, we estimated that the total biomass C pool and average biomass C density in Chinese forests increased from 4717 Tg C (1 Tg = 1012 g) in the period of 1977–1981 to 7975 Tg C in the period of 2014–2018 and 38.2 Mg C ha−1 to 45.8 Mg C ha−1 (1 Mg = 106 g), respectively, with a net increase of 3258 Tg C and an annual sink of 88.0 Tg C yr−1. Over the most recent 10 years (2009–2018), the average national forest biomass C density and C sink were 44.6 Mg C ha−1 and 154.8 Tg C yr−1, respectively, much larger than those of 39.6 Mg C ha−1 and 63.3 Tg C yr−1 in the period 1977–2008. These pronounced increases were largely attributed to afforestation practices, forest growth, and environmental changes. Our results have documented the importance of ecological restoration practices, provided an essential basis for assessing ecosystem services, and helped to achieve China's C neutrality target.
, , , Nathalie Lefèvre, , Sabrina Speich, , Matthieu Labaste, Christophe Noisel, Markus Ritschel, et al.
Published: 21 June 2022
Biogeosciences, Volume 19, pp 2969-2988; https://doi.org/10.5194/bg-19-2969-2022

Abstract:
The key processes driving the air–sea CO2 fluxes in the western tropical Atlantic (WTA) in winter are poorly known. WTA is a highly dynamic oceanic region, expected to have a dominant role in the variability in CO2 air–sea fluxes. In early 2020 (February), this region was the site of a large in situ survey and studied in wider context through satellite measurements. The North Brazil Current (NBC) flows northward along the coast of South America, retroflects close to 8 N and pinches off the world's largest eddies, the NBC rings. The rings are formed to the north of the Amazon River mouth when freshwater discharge is still significant in winter (a time period of relatively low run-off). We show that in February 2020, the region (5–16 N, 50–59 W) is a CO2 sink from the atmosphere to the ocean (−1.7 Tg C per month), a factor of 10 greater than previously estimated. The spatial distribution of CO2 fugacity is strongly influenced by eddies south of 12 N. During the campaign, a nutrient-rich freshwater plume from the Amazon River is entrained by a ring from the shelf up to 12 N leading to high phytoplankton concentration and significant carbon drawdown (∼20 % of the total sink). In trapping equatorial waters, NBC rings are a small source of CO2. The less variable North Atlantic subtropical water extends from 12 N northward and represents ∼60 % of the total sink due to the lower temperature associated with winter cooling and strong winds. Our results, in identifying the key processes influencing the air–sea CO2 flux in the WTA, highlight the role of eddy interactions with the Amazon River plume. It sheds light on how a lack of data impeded a correct assessment of the flux in the past, as well as on the necessity of taking into account features at meso- and small scales.
, , Charlotte Blasi, Olivier Crouzet, Jean-Christophe Lata, Isabelle Lamy
Published: 20 June 2022
Biogeosciences, Volume 19, pp 2953-2968; https://doi.org/10.5194/bg-19-2953-2022

Abstract:
Continental biogeochemical models are commonly used to predict the effect of land use, exogenous organic matter input or climate change on soil greenhouse gas emission. However, they cannot be used for this purpose to investigate the effect of soil contamination, while contamination affects several soil processes and concerns a large fraction of land surface. For that, in this study we implemented a commonly used model estimating soil nitrogen (N) emissions, the DeNitrification DeCompostion (DNDC) model, with a function taking into account soil copper (Cu) contamination in nitrate production control. Then, we aimed at using this model to predict N2O-N, NO2-N, NO-N and NH4-N emissions in the presence of contamination and in the context of changes in precipitations. Initial incubations of soils were performed at different soil moisture levels in order to mimic expected rainfall patterns during the next decades and in particular drought and excess of water. Then, a bioassay was used in the absence or presence of Cu to assess the effect of the single (moisture) or double stress (moisture and Cu) on soil nitrate production. Data of nitrate production obtained through a gradient of Cu under each initial moisture incubation were used to parameterise the DNDC model and to estimate soil N emission considering the various effects of Cu. Whatever the initial moisture incubation, experimental results showed a NO3-N decreasing production when Cu was added but depending on soil moisture. The DNDC-Cu version we proposed was able to reproduce these observed Cu effects on soil nitrate concentration with r2 > 0.99 and RMSE < 10 % for all treatments in the DNDC-Cu calibration range (> 40 % of the water holding capacity) but showed poor performances for the dry treatments. We modelled a Cu effect inducing an increase in NH4-N soil concentration and emissions due to a reduced nitrification activity and therefore a decrease in NO3-N, N2O-N and NOx-N concentrations and emissions. The effect of added Cu predicted by the model was larger on N2-N and N2O-N emissions than on the other N species and larger for the soils incubated under constant than variable moisture. Our work shows that soil contamination can be considered in continental biogeochemical models to better predict soil greenhouse gas emissions.
Zhibo Shao,
Published: 16 June 2022
Biogeosciences, Volume 19, pp 2939-2952; https://doi.org/10.5194/bg-19-2939-2022

Abstract:
Non-cyanobacterial diazotrophs may be contributors to global marine N2 fixation, although the factors controlling their distribution are unclear. Here, we explored what controls the distribution of the most sampled non-cyanobacterial diazotroph phylotype, Gamma A, in the global ocean. First, we represented Gamma A abundance by its nifH quantitative polymerase chain reaction (qPCR) copies reported in the literature and analyzed its relationship to climatological biological and environmental conditions. There was a positive correlation between the Gamma A abundance and local net primary production (NPP), and the maximal observed Gamma A abundance increased with NPP and became saturated when NPP reached ∼ 400 mg C m−2 d−1. Additionally, an analysis using a multivariate generalized additive model (GAM) revealed that the Gamma A abundance increased with light intensity but decreased with increasing iron concentration. The GAM also showed a weak but significant positive relationship between Gamma A abundance and silicate concentration, as well as a substantial elevation of Gamma A abundance when the nitrate concentration was very high ( 10 µM). Using the GAM, these climatological factors together explained 43 % of the variance in the Gamma A abundance. Second, in addition to the climatological background, we found that Gamma A abundance was elevated in mesoscale cyclonic eddies in high-productivity (climatological NPP > 400 mg m−2 d−1) regions, implying that Gamma A can respond to mesoscale features and benefit from nutrient inputs. Overall, our results suggest that Gamma A tends to inhabit ocean environments with high productivity and low iron concentrations and therefore provide insight into the niche differentiation of Gamma A from cyanobacterial diazotrophs, which are generally most active in oligotrophic ocean regions and need a sufficient iron supply, although both groups prefer well-lit surface waters. More sampling on Gamma A and other non-cyanobacterial diazotroph phylotypes is needed to reveal the controlling mechanisms of heterotrophic N2 fixation in the ocean.
Matthias Volk, , Anne-Lena Wahl, Seraina Bassin
Published: 15 June 2022
Biogeosciences, Volume 19, pp 2921-2937; https://doi.org/10.5194/bg-19-2921-2022

Abstract:
Climate change is associated with a change in soil organic carbon (SOC) stocks, implying a feedback mechanism on global warming. Grassland soils represent 28 % of the global soil C sink and are therefore important for the atmospheric greenhouse gas concentration. In a field experiment in the Swiss Alps we recorded changes in the ecosystem organic carbon stock under climate change conditions, while quantifying the ecosystem C fluxes at the same time (ecosystem respiration, gross primary productivity, C export in plant material and leachate water). We exposed 216 grassland monoliths to six different climate scenarios (CSs) in an altitudinal transplantation experiment. In addition, we applied an irrigation treatment (+12 % to 21 % annual precipitation) and an N deposition treatment (+3 and +15 kg N ha−1 yr−1) in a factorial design, simulating summer-drought mitigation and atmospheric N pollution. In 5 years the ecosystem C stock, consisting of plant C and SOC, dropped dramatically by about −14 % (-1034±610 g C m−2) with the CS treatment representing a +3.0 C seasonal (April–October) warming. N deposition and the irrigation treatment caused no significant effects. Measurements of C fluxes revealed that ecosystem respiration increased by 10 % at the +1.5 C warmer CS site and by 38 % at the +3 C warmer CS site (P≤0.001 each), compared to the CS reference site with no warming. However, gross primary productivity was unaffected by warming, as were the amounts of exported C in harvested plant material and leachate water (dissolved organic C). As a result, the 5-year C flux balance resulted in a climate scenario effect of -936±138 g C m−2 at the +3.0 C CS, similar to the C stock climate scenario effect. It is likely that this dramatic C loss of the grassland is a transient effect before a new, climate-adjusted steady state is reached.
, Jaan Laanemets, Taavi Liblik, Māris Skudra, Oliver Samlas, Inga Lips,
Published: 14 June 2022
Biogeosciences, Volume 19, pp 2903-2920; https://doi.org/10.5194/bg-19-2903-2022

Abstract:
The Gulf of Riga is a relatively shallow bay connected to the deeper central Baltic Sea (Baltic Proper) via straits with sills. The decrease in the near-bottom oxygen levels from spring to autumn is a common feature in the gulf, but in 2018, extensive hypoxia was observed. We analyzed temperature, salinity, oxygen, and nutrient data collected in 2018, along with historical data available from environmental databases. Meteorological and hydrological data from the study year were compared with their long-term means and variability. We suggest that pronounced oxygen depletion occurred in 2018 due to a distinct development of vertical stratification. Seasonal stratification developed early and was stronger in spring–summer 2018 than on average due to high heat flux and weak winds. Dominating northeasterly winds in early spring and summer supported the inflow of saltier waters from the Baltic Proper that created an additional deep pycnocline restricting vertical transport between the near-bottom layer (NBL) and the water column above. The estimated oxygen consumption rate in the NBL in spring–summer 2018 was about 1.7 mmolO2m-2h-1 , which exceeded the oxygen input to the NBL due to advection and vertical mixing. Such a consumption rate leads to near-bottom hypoxia in all years when vertical mixing in autumn reaches the seabed later than on average according to the long-term (1979–2018) meteorological conditions. The observed increase in phosphate concentrations in the NBL in summer 2018 suggests a significant sediment phosphorus release in hypoxic conditions counteracting the mitigation measures to combat eutrophication. Since climate change projections predict that meteorological conditions comparable to those in 2018 will occur more frequently, extensive hypoxia would be more common in the Gulf of Riga and other coastal basins with similar morphology and human-induced elevated input of nutrients.
, , Denise Müller-Dum, , Justus Notholt, Thorsten Warneke
Published: 13 June 2022
Biogeosciences, Volume 19, pp 2855-2880; https://doi.org/10.5194/bg-19-2855-2022

Abstract:
Southeast Asian peatlands represent a globally significant carbon store that is destabilized by land-use changes like deforestation and the conversion into plantations, causing high carbon dioxide (CO2) emissions from peat soils and increased leaching of peat carbon into rivers. While this high carbon leaching and consequentially high DOC concentrations suggest that CO2 emissions from peat-draining rivers would be high, estimates based on field data suggest they are only moderate. In this study, we offer an explanation for this phenomenon by showing that carbon decomposition is hampered by the low pH in peat-draining rivers. This limits CO2 production in and emissions from these rivers. We find an exponential pH limitation that shows good agreement with laboratory measurements from high-latitude peat soils. Additionally, our results suggest that enhanced input of carbonate minerals increases CO2 emissions from peat-draining rivers by counteracting the pH limitation. As such inputs of carbonate minerals can occur due to human activities like deforestation of river catchments, liming in plantations, and enhanced weathering application, our study points out an important feedback mechanism of those practices.
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