Carbon budgets of wetland ecosystems in China Xiao, Derong; Deng, Lei; Kim, Dong‐Gill ...
Global change biology,
June 2019, 2019-Jun, 2019-06-00, 20190601, Volume:
25, Issue:
6
Journal Article
Peer reviewed
Wetlands contain a large proportion of carbon (C) in the biosphere and partly affect climate by regulating C cycles of terrestrial ecosystems. China contains Asia's largest wetlands, accounting for ...about 10% of the global wetland area. Although previous studies attempted to estimate C budget in China's wetlands, uncertainties remain. We conducted a synthesis to estimate C uptake and emission of wetland ecosystems in China using a dataset compiled from published literature. The dataset comprised 193 studies, including 370 sites representing coastal, river, lake and marsh wetlands across China. In addition, C stocks of different wetlands in China were estimated using unbiased data from the China Second Wetlands Survey. The results showed that China's wetlands sequestered 16.87 Pg C (315.76 Mg C/ha), accounting for about 3.8% of C stocks in global wetlands. Net ecosystem productivity, jointly determined by gross primary productivity and ecosystem respiration, exhibited annual C sequestration of 120.23 Tg C. China's wetlands had a total gaseous C loss of 173.20 Tg C per year from soils, including 154.26 Tg CO2‐C and 18.94 Tg CH4‐C emissions. Moreover, C stocks, uptakes and gaseous losses varied with wetland types, and were affected by geographic location and climatic factors (precipitation and temperature). Our results provide better estimation of the C budget in China's wetlands and improve understanding of their contribution to the global C cycle in the context of global climate change.
China's wetlands have total C stocks of 16.87 Pg C, Herein, plant C stocks is 0.22 Pg C and soil C stocks is 16.65 Pg C. Net ecosystem productivity, jointly determined by gross primary productivity and ecosystem respiration, exhibited annual C sequestration of 120.23 Tg C. Moreover, C stocks, uptakes and gaseous losses varied with wetland types. Marsh wetlands have the highest C stocks of 10.20 Pg C, following by lake, river and coastal wetlands, with the values of 4.20, 1.92 and 0.54 Pg C, respectively.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The effects of nitrogen (N) deposition on soil organic carbon (C) and greenhouse gas (GHG) emissions in terrestrial ecosystems are the main drivers affecting GHG budgets under global climate change. ...Although many studies have been conducted on this topic, we still have little understanding of how N deposition affects soil C pools and GHG budgets at the global scale. We synthesized a comprehensive dataset of 275 sites from multiple terrestrial ecosystems around the world and quantified the responses of the global soil C pool and GHG fluxes induced by N enrichment. The results showed that the soil organic C concentration and the soil CO2, CH4 and N2O emissions increased by an average of 3.7%, 0.3%, 24.3% and 91.3% under N enrichment, respectively, and that the soil CH4 uptake decreased by 6.0%. Furthermore, the percentage increase in N2O emissions (91.3%) was two times lower than that (215%) reported by Liu and Greaver (Ecology Letters, 2009, 12:1103–1117). There was also greater stimulation of soil C pools (15.70 kg C ha−1 year−1 per kg N ha−1 year−1) than previously reported under N deposition globally. The global N deposition results showed that croplands were the largest GHG sources (calculated as CO2 equivalents), followed by wetlands. However, forests and grasslands were two important GHG sinks. Globally, N deposition increased the terrestrial soil C sink by 6.34 Pg CO2/year. It also increased net soil GHG emissions by 10.20 Pg CO2‐Geq (CO2 equivalents)/year. Therefore, N deposition not only increased the size of the soil C pool but also increased global GHG emissions, as calculated by the global warming potential approach.
The plants input more C into the soil, causing the ecosystem to become a net CO2 sink under the N enrichment (deposition), which negatively contributed to global warming. N enrichment (deposition) suppressed the CH4 uptake but stimulated CH4 emissions, which increased atmospheric CH4 and N enrichment (deposition) increased N2O emissions, which positively contributed to global warming. Globally, N deposition increased the terrestrial soil C sink by 6.34 Pg CO2/year. It also increased net soil GHG emissions by 10.20 Pg CO2‐Geq (CO2 equivalents)/year, as calculated by the global warming potential approach.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Natural vegetation restoration can enhance soil organic carbon (SOC) sequestration, but the mechanisms and control factors underlying SOC sequestration are still unknown. The objectives of the study ...are to quantify the temporal variation of soil and aggregate‐associated organic carbon (OC) and identify factors controlling the variation following natural vegetation restoration after farmland abandonment. We collected soils from sites having 5, 30, 60, 100, and 160 years of a natural vegetation restoration chronosequence after farmland abandonment in the Loess Plateau, China. The results showed that natural vegetation restoration increased macroaggregates (0.25–2 mm; 46.6% to 73.9%), SOC (2.27 to 9.81 g kg−1), and aggregate OC (7.33 to 36.98 g kg−1) in the top 20‐cm soil compared with abandoned farmland, and the increases mainly occurred in the early stage (<60 years). The increase of SOC was contributed by OC accumulated in macroaggregates (0.25–2 mm) rather than microaggregates (≤0.25 mm). Moreover, SOC sequestration in the topsoil (0–10 cm) was mainly determined by fine root biomass (FR), labile organic carbon (LOC), and microbial biomass carbon (MBC). And in the subsoil (10–20 cm), SOC sequestration was mainly determined by the proportion of macroaggregates. The results suggest that natural vegetation restoration increased SOC and aggregate OC, and FR, MBC, LOC, and the physical protection of aggregates played important roles in regulating SOC and aggregate OC.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Nitrification inhibitors (NI) retain nitrogen (N) in the ammonium (NH
4
+
) form longer in soil providing more time for plant uptake of NH
4
+
. They can also reduce production of the greenhouse gas ...nitrous oxide (N
2
O) by inhibiting nitrification and subsequent denitrification processes. However, this extended retention of N in the NH
4
+
form in the soils treated with NI can increase ammonia (NH
3
) emission. Studies conducted so far provide conflicting results on the effect of NI treatment on NH
3
emissions. Here we have collated results available to date from peer-reviewed literature (46 data set from 21 studies from 1970 to 2010) and categorized the reported results into three groups—increase, no change, and decrease in % applied N lost as NH
3
(hereafter NH
3
loss) in NI treatments. Significant increase in NH
3
loss in NI treatment was observed in both pasture and cropping soils and from both applied urine and urea with NI (e.g., dicyandiamide (DCD), ATC 4-amino- 1.2,4-triazole). This increase in NH
3
loss was between 0.3 and 25.0 % (
n
= 26, mean 6.7 ± standard error 1.3 %). No change in NH
3
loss with DCD was also observed in some soils (
n
= 14), while a small number of studies reported a decrease which was between −0.3 and −4.1 % (
n
= 6, −1.3 ± 0.6 %). Overall, the soils with higher pH and lower cation exchange capacity (CEC) lost more NH
3
with NIs irrespective of land use and type of N input. The combined addition of both NI and urease inhibitor reduced NH
3
loss compared to sole NI application (
n
= 4, −5.9 ± 1.3 %). Collectively, the analysed results from the small number of available data sets reported suggest that NH
3
loss significantly increases with NI application, depending on soil properties such as soil pH and CEC. More studies are needed both to quantitatively determine the effect of NIs on NH
3
loss and to mitigate the loss.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
► Dependency of direct N2O emission on N input examined using 26 published datasets. ► N2O response to increased N additions was non-linear in more cases than linear. ► Direct N2O emission factor ...remains constant or changes nonlinearly with N input. ► We propose a relationship describing N2O response to increasing N input rates.
Rising atmospheric concentrations of nitrous oxide (N2O) contribute to global warming and associated climate change. It is often assumed that there is a linear relationship between nitrogen (N) input and direct N2O emission in managed ecosystems and, therefore, direct N2O emission for national greenhouse gas inventories use constant emission factors (EF). However, a growing body of studies shows that increases in direct N2O emission are related by a nonlinear relationship to increasing N input. We examined the dependency of direct N2O emission on N input using 26 published datasets where at least four different levels of N input had been applied. In 18 of these datasets the relationship of direct N2O emission to N input was nonlinear (exponential or hyperbolic) while the relationship was linear in four datasets. We also found that direct N2O EF remains constant or increases or decreases nonlinearly with changing N input. Studies show that direct N2O emissions increase abruptly at N input rates above plant uptake capacity. The remaining surplus N could serve as source of additional N2O production, and also indirectly promote N2O production by inhibiting biochemical N2O reduction. Accordingly, we propose a hypothetical relationship to conceptually describe in three steps the response of direct N2O emissions to increasing N input rates: (1) linear (N limited soil condition), (2) exponential, and (3) steady-state (carbon (C) limited soil condition). In this study, due to the limited availability of data, it was not possible to assess these hypothetical explanations fully. We recommend further comprehensive experimental examination and simulation using process-based models be conducted to address the issues reported in this review.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Agroforestry is an agricultural diversification practice that purposefully grows trees together with crops and/or animals. Nitrogen (N) plays a pivotal role in the sustainability and productivity of ...these systems. In this study, we synthesize our current understanding and data availability of N dynamics in agroforestry systems, providing the first comprehensive synthesis on major N pathways in agroforests. We review five categories of N pools and fluxes: N input, internal N cycle, N sequestration, N loss, and N output. Within these categories, we compile data from worldwide measurements of N input and N gaseous emissions. The N inputs among agroforestry systems are from N
2
fixation through leguminous trees (global average: 240 kg N ha
–1
yr
–1
), organic amendments and inorganic fertilizers (36 kg N ha
–1
yr
–1
), and, possibly, undocumented N deposition. Various pathways are identified in internal N cycles: direct transfer of fixed N from tree to crop, N mineralization of organic matter originating from trees, pumping up subsoil N by tree roots, and ammonia captured by trees. Agroforestry practices almost consistently show reductions in N loss, via soil erosion, runoff, and leaching. Agroforestry systems release gaseous N as nitrous oxide (N
2
O) (~4.7 kg N
2
O–N ha
–1
yr
–1
) and nitric oxide (NO). While N
2
O emissions in agroforestry are either higher (improved fallows) or lower (riparian buffers) in comparison to monocropping, NO emissions are similar in agroforestry (improved fallows) and monocropping. The N sequestration rates in agroforestry soil pools (98–154 kg N ha
−1
yr
−1
) are three times higher than biomass pools (36–43 kg N ha
−1
yr
−1
). Since agroforestry practices can increase crop yield and N content in crops, this translates into higher N output in agroforestry systems. Studies are required to accurately quantify sub-components of N pools and fluxes and identify mitigation measures to manage N loss in agroforestry systems.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Extreme droughts have serious impacts on the pools, fluxes and processes of terrestrial carbon (C) and nitrogen (N) cycles. A deep understanding is necessary to explore the impacts of this extreme ...climate change events. To investigate how soil C and N pools and fluxes respond to drought and explore their mechanisms we conducted a meta-analysis synthesizing the responses of soil C and N cycles to droughts (precipitation reduction experiments) in three main natural ecosystems: forests, shrubs and grasslands. Data were collected from 148 recent publications (1815 sampling data at 134 sites) with the drought experiments from 1 to 13 years across the globe. Drought reduced soil organic C content (-3.3%) mainly because of decreased plant litter input (-8.7%) and reduced litter decomposition (-13.0%) across all the three ecosystem types in the world. Drought increased mineral N content (+31%) but reduced N mineralization rate (-5.7%) and nitrification rate (-13.8%), and thus left total N unchanged. Compared with the local precipitation, drought increased the accumulation of dissolved organic C and N contents by +59% and +33%, respectively, due to retarded mineralization and higher stability of dissolved organic matter. Among the three ecosystem types, forest soils strongly increased litter C (+64%, n=8) and N content (+33%, n=6) as well as microbial CO2 (+16%, n=55), whereas total CO2 emission remains unaffected. Drought decreased soil CO2 emission (-15%, n=53) in shrubs due to reduction of microbial respiration and decreased root biomass. The 98% (n=39) increase of NH4+ concentration in forest soils corresponds to 11% (n=37) decrease of NO3- and so, it reflected the increase of N mineralization rate, but the decrease of nitrification. For shrubs and grasslands, however, stabilized or decreased N mineralization and nitrification mean less N uptake by plants under drought. Overall, the effects of drought on soil C and N cycles were regulated by the ecosystem type, drought duration and intensity. The drought intensity and duration intensify all effects, especially on the decreasing total CO2 emission. However, the most studies mainly focused on the short-term droughts, and there is a lack of comprehensive understanding of how drought effects in a long-term consequences. So, future studies should strengthen drought frequency impacts on ecosystem C and N dynamics in the long-term sequence (> 10 years) in order to face the impacts of global change.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Agriculture is the backbone of the Ethiopian economy, and the agricultural sector is dominated by smallholder farming systems. The farming systems are facing constraints such as small land size, lack ...of resources, and increasing degradation of soil quality that hamper sustainable crop production and food security. The effects of climate change (e.g., frequent occurrence of extreme weather events) exacerbate these problems. Applying appropriate technologies like climate-smart agriculture (CSA) can help to resolve the constraints of smallholder farming systems. This paper provides a comprehensive overview regarding opportunities and challenges of traditional and newly developed CSA practices in Ethiopia, such as integrated soil fertility management, water harvesting, and agroforestry. These practices are commonly related to drought resilience, stability of crop yields, carbon sequestration, greenhouse gas mitigation, and higher household income. However, the adoption of the practices by smallholder farmers is often limited, mainly due to shortage of cropland, land tenure issues, lack of adequate knowledge about CSA, slow return on investments, and insufficient policy and implementation schemes. It is suggested that additional measures be developed and made available to help CSA practices become more prevalent in smallholder farming systems. The measures should include the utilization of degraded and marginal lands, improvement of the soil organic matter management, provision of capacity-building opportunities and financial support, as well as the development of specific policies for smallholder farming.
Effects of exclosures on restoring degraded lands may vary with soil type, exclosure age, and conditions before the establishment of exclosures. Yet, studies investigating the effectiveness of ...exclosures in restoring degraded lands under different environmental conditions are lacking. This study aims at investigating the changes in woody species richness and diversity, and ecosystem carbon stocks after implementing exclosures in the Central Rift Valley, Ethiopia. Vegetation and soil data were gathered from 120 nested plots established in exclosures of eight and 30‐years‐old and adjacent grazing lands. Results showed that exclosures contained a higher number of economically important woody species compared to their respective adjacent grazing lands. However, the exclosures and respective adjacent grazing lands did not differ significantly in the diversity of tree and shrub species, and both the exclosures and adjacent grazing lands were dominated by few tree and shrub species. The older exclosure (30 years old) displayed significantly (p < 0.01) higher soil organic carbon and soil total nitrogen content and stocks than the adjacent grazing land, whereas the youngest exclosure (8 years old) did not show a significant difference in these variables. The results suggest that a longer time (e.g., ≥10 years) is needed to detect significant differences in soil organic carbon and total soil nitrogen. However, exclosures could bring considerable changes in woody species density in a relatively shorter period (e.g., ≤10 years) and support to restore degraded native woody species.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Aims
This study was conducted to assess the effects of shifting cultivation and its conversion to mono-cropping on soil organic carbon (SOC) and total nitrogen (STN).
Methods
We compared soil pH, ...texture, bulk density and SOC and STN contents and stocks (0–100 cm) in natural forest (NF), adjacent shifting cultivation (SC) areas (> 100 years old) having three (SC-3Y), five (SC-5Y) and seven (SC-7Y)-year-old fallowing, and 10 year-old mono-cropping field (MCF) converted from shifting cultivation in Western Ethiopia.
Results
There was no significant difference in soil pH in NF and all shifting cultivation areas. However, MCF had lower soil pH compared to SC-3Y and SC-5Y. There was no or very little difference in soil texture and bulk density across the study sites. Shifting cultivation did not affect SOC and STN stocks. However, conversion of shifting cultivation to mono-cropping decreased SOC (45–50% over 10 years; loss of 11.6 ± 0.2 Mg C ha
−1
yr.
−1
) and STN stocks (18–45% over 10 years; loss of 0.6 ± 0.1 Mg N ha
−1
yr.
−1
).
Conclusions
While shifting cultivation maintained SOC and STN, its conversion to mono-cropping decreased them, potentially contributing to global warming and decreasing soil fertility.
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BFBNIB, DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NMLJ, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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