Assessment of carbon sustainability under different tillage systems in a double rice cropping system in Southern China

Purpose

Adoption of the carbon (C)-friendly and cleaner technology is an effective solution to offset some of the anthropogenic emissions. Conservation tillage is widely considered as an important sustainable technology and for the development of conservation agriculture (CA). Thus, the objective of this study was to assess the C sustainability of different tillage systems in a double rice (Oryza sativa L.) cropping system in southern China.

Methods

The experiment was established with no-till (NT), rotary tillage (RT), and conventional tillage (CT) treatments since 2005. Emission of greenhouse gasses (GHG), C footprint (CF), and ecosystem service through C sequestration in different tillage systems were compared.

Result and discussion

Emission of GHG from agricultural inputs (Mg CO₂-eq ha^?1 year^?1) ranged from 1.81 to 1.97 for the early rice, 1.82 to 1.98 for the late rice, and 3.63 to 3.95 for the whole growing season, respectively. The CF (kg CO₂-eq kg^?1 of rice year^?1) in the whole growing seasons were 1.27, 1.85, and 1.40 [excluding soil organic carbon (SOC) storage] and 0.54, 1.20, and 0.72 (including SOC storage) for NT, RT, and CT, respectively. The value of ecosystem services on C sequestration for the whole growing seasons ranged from ¥3,353 to 4,948 ha^?1 year^?1 and followed the order of NT > CT > RT. The C sustainability under NT was better than that under RT for the late, but reversed for the early rice. However, NT system had better C sustainability for the whole cropping system compared with CT.

Conclusions

Therefore, NT is a preferred technology to reduce GHG emissions, increase ecosystem service functions of C sequestration, and improve C sustainability in a double rice cropping region of Southern China.     

Storage and sequestration potential of topsoil organic carbon in China's paddy soils

Carbon (C) storage and sequestration in agricultural soils is considered to be an important issue in the study of terrestrial C cycling and global climatic change. The baseline C stock and the C sequestration potential are among the criteria for a region or a state to adopt strategies or policies in response to commitment to the Kyoto Protocol. Paddy soils represent a large portion of global cropland. However, little information on the potential of C sequestration and storage is available for such soils. In this paper, an estimation of the topsoil soil organic carbon (SOC) pool and the sequestration potential of paddy soils in China was made by using the data from the 2nd State Soil Survey carried out during 1979–1982 and from the nationwide arable soil monitoring system established since then. Results showed that the SOC density ranged from 12 to 226 t C ha^?1 with an area‐weighted mean density of 44 t C ha^?1, which is comparable to that of the US grasslands and is higher than that of the cultivated dryland soils in China and the US. The estimated total topsoil SOC pool is 1.3 Pg, with 0.85 Pg from the upper plow layer and 0.45 Pg from the plowpan layer. This pool size is ～2% of China's total storage in the top 1 m of the soil profiles and ～4% of the total topsoil pool, while the area percentage of paddy soil is 3.4% of the total land. The C pool in paddy soils was found predominantly in southeast China geographically and in the subgroups of Fe‐accumulating and Fe‐leaching paddy soils pedogenetically. In comparison with dryland cultivation, irrigation‐based rice cultivation in China has induced significant enrichment of SOC storage (0.3 Pg) in paddy soils. The induced total C sequestration equals half of China's total annual CO₂ emission in the 1990s. Estimates using different SOC sequestration scenarios show that the paddy soils of China have an easily attainable SOC sequestration potential of 0.7 Pg under present conditions and may ultimately sequester 3.0 Pg. Soil monitoring data showed that the current C sequestration rate is 12 Tg yr^?1. The total C sequestration potential and the current sequestration rate of the paddy soils are over 30%, while the area of the paddy soils is 26% that of China's total croplands. Therefore, practicing sustainable agriculture is urgently needed for enhancing SOC storage to realize the ultimate SOC sequestration of rice‐based agriculture of China, as the current C sequestration rate is significantly lower than the potential rate.     

High emissions of greenhouse gases from grasslands on peat and other organic soils

Drainage has turned peatlands from a carbon sink into one of the world's largest greenhouse gas (GHG) sources from cultivated soils. We analyzed a unique data set (12 peatlands, 48 sites and 122 annual budgets) of mainly unpublished GHG emissions from grasslands on bog and fen peat as well as other soils rich in soil organic carbon (SOC) in Germany. Emissions and environmental variables were measured with identical methods. Site‐averaged GHG budgets were surprisingly variable (29.2 ± 17.4 t CO₂‐eq. ha^?1 yr^?1) and partially higher than all published data and the IPCC default emission factors for GHG inventories. Generally, CO₂ (27.7 ± 17.3 t CO₂ ha^?1 yr^?1) dominated the GHG budget. Nitrous oxide (2.3 ± 2.4 kg N₂O‐N ha^?1 yr^?1) and methane emissions (30.8 ± 69.8 kg CH₄‐C ha^?1 yr^?1) were lower than expected except for CH₄ emissions from nutrient‐poor acidic sites. At single peatlands, CO₂ emissions clearly increased with deeper mean water table depth (WTD), but there was no general dependency of CO₂ on WTD for the complete data set. Thus, regionalization of CO₂ emissions by WTD only will remain uncertain. WTD dynamics explained some of the differences between peatlands as sites which became very dry during summer showed lower emissions. We introduced the aerated nitrogen stock (N_air) as a variable combining soil nitrogen stocks with WTD. CO₂ increased with N_air across peatlands. Soils with comparatively low SOC concentrations showed as high CO₂ emissions as true peat soils because N_air was similar. N₂O emissions were controlled by the WTD dynamics and the nitrogen content of the topsoil. CH₄ emissions can be well described by WTD and ponding duration during summer. Our results can help both to improve GHG emission reporting and to prioritize and plan emission reduction measures for peat and similar soils at different scales.     

Carbon footprint of rice production under biochar amendment – a case study in a Chinese rice cropping system

As a controversial strategy to mitigate global warming, biochar application into soil highlights the need for life cycle assessment before large‐scale practice. This study focused on the effect of biochar on carbon footprint of rice production. A field experiment was performed with three treatments: no residue amendment (Control), 6 t ha^?1 yr^?1 corn straw (CS) amendment, and 2.4 t ha^?1 yr^?1 corn straw‐derived biochar amendment (CBC). Carbon footprint was calculated by considering carbon source processes (pyrolysis energy cost, fertilizer and pesticide input, farmwork, and soil greenhouse gas emissions) and carbon sink processes (soil carbon increment and energy offset from pyrolytic gas). On average over three consecutive rice‐growing cycles from year 2011 to 2013, the CS treatment had a much higher carbon intensity of rice (0.68 kg CO₂‐C equivalent (CO₂‐C_e) kg^?1 grain) than that of Control (0.24 kg CO₂‐C_e kg^?1 grain), resulting from large soil CH₄ emissions. Biochar amendment significantly increased soil carbon pool and showed no significant effect on soil total N₂O and CH₄ emissions relative to Control; however, due to a variation in net electric energy input of biochar production based on different pyrolysis settings, carbon intensity of rice under CBC treatment ranged from 0.04 to 0.44 kg CO₂‐C_e kg^?1 grain. The results indicated that biochar strategy had the potential to significantly reduce the carbon footprint of crop production, but the energy‐efficient pyrolysis technique does matter.     

Carbon Sequestration in Reclaimed Minesoils

Minesoils are drastically influenced by anthropogenic activities. They are characterized by low soil organic matter (SOM) content, low fertility, and poor physicochemical and biological properties, limiting their quality, capability, and functions. Reclamation of these soils has potential for resequestering some of the C lost and mitigating CO₂ emissions. Soil organic carbon (SOC) sequestration rates in minesoils are high in the first 20 to 30 years after reclamation in the top 15 cm soil depth. In general, higher rates of SOC sequestration are observed for minesoils under pasture and grassland management than under forest land use. Observed rates of SOC sequestration are 0.3 to 1.85 Mg C ha^{? 1} yr^{? 1} for pastures and rangelands, and 0.2 to 1.64 Mg C ha^{? 1} yr^{? 1} for forest land use. Proper reclamation and postreclamation management may enhance SOC sequestration and add to the economic value of the mined sites. Management practices that may enhance SOC sequestration include increasing vegetative cover by deep-rooted perennial vegetation and afforestation, improving soil fertility, and alleviation of physical, chemical and biological limitations by fertilizers and soil amendments such as biosolids, manure, coal combustion by-products, and mulches. Soil and water conservation are important to SOC sequestration. The potential of SOC sequestration in minesoils of the US is estimated to be 1.28 Tg C yr^?1, compared to the emissions from coal combustion of 506 Tg C yr^{? 1}.     

An agronomic assessment of greenhouse gas emissions from major cereal crops

Agricultural greenhouse gas (GHG) emissions contribute approximately 12% to total global anthropogenic GHG emissions. Cereals (rice, wheat, and maize) are the largest source of human calories, and it is estimated that world cereal production must increase by 1.3% annually to 2025 to meet growing demand. Sustainable intensification of cereal production systems will require maintaining high yields while reducing environmental costs. We conducted a meta‐analysis (57 published studies consisting of 62 study sites and 328 observations) to test the hypothesis that the global warming potential (GWP) of CH₄ and N₂O emissions from rice, wheat, and maize, when expressed per ton of grain (yield‐scaled GWP), is similar, and that the lowest value for each cereal is achieved at near optimal yields. Results show that the GWP of CH₄ and N₂O emissions from rice (3757 kg CO₂ eq ha^?1 season^?1) was higher than wheat (662 kg CO₂ eq ha^?1 season^?1) and maize (1399 kg CO₂ eq ha^?1 season^?1). The yield‐scaled GWP of rice was about four times higher (657 kg CO₂ eq Mg^?1) than wheat (166 kg CO₂ eq Mg^?1) and maize (185 kg CO₂ eq Mg^?1). Across cereals, the lowest yield‐scaled GWP values were achieved at 92% of maximal yield and were about twice as high for rice (279 kg CO₂ eq Mg^?1) than wheat (102 kg CO₂ eq Mg^?1) or maize (140 kg CO₂ eq Mg^?1), suggesting greater mitigation opportunities for rice systems. In rice, wheat and maize, 0.68%, 1.21%, and 1.06% of N applied was emitted as N₂O, respectively. In rice systems, there was no correlation between CH₄ emissions and N rate. In addition, when evaluating issues related to food security and environmental sustainability, other factors including cultural significance, the provisioning of ecosystem services, and human health and well‐being must also be considered.     

Response of soil carbon dioxide fluxes,soil organic carbon and microbial biomass carbon to biochar amendment: a meta‐analysis

Biochar as a carbon‐rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta‐analysis procedures to examine responses of soil carbon dioxide (CO₂) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO₂ fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO₂ fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N‐fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land‐use types. Responses of soil CO₂ fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO₂ fluxes and MBC content, respectively. Both soil CO₂ fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO₂ fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO₂ flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1–9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land‐use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.     

Proteomics reveals the adaptability mechanism of Brassica napus to short-term boron deprivation

The application of pyrogenic carbon, biochar, to agricultural soils is currently discussed as a win-win strategy to sequester carbon in soil, thus improving soil fertility and mitigate global warming. Our aim was to investigate if biochar may improve plant eco-physiological responses under sufficient water supply as well as moderate drought stress. A fully randomized greenhouse study was conducted with the pseudo-cereal Chenopodium quinoa Willd, using three levels of biochar addition (0, 100 and 200?t ha^?1) to a sandy soil and two water treatments (60% and 20% of the water holding capacity of the control), investigating growth, water use efficiency, eco-physiological parameters and greenhouse gas (GHG) fluxes. Biochar application increased growth, drought tolerance and leaf-N- and water-use efficiency of quinoa despite larger plant?Cleaf areas. The plants growing in biochar-amended soil accumulated exactly the same amount of nitrogen in their larger leaf biomass than the control plants, causing significantly decreased leaf N-, proline- and chlorophyll-concentrations. In this regard, plant responses to biochar closely resembled those to elevated CO₂. However, neither soil- nor plant?Csoil-respiration was higher in the larger plants, indicating less respiratory C losses per unit of biomass produced. Soil-N₂O emissions were significantly reduced with biochar. The large application rate of 200?t ha^?1 biochar did not improve plant growth compared to 100?t ha^?1; hence an upper beneficial level exists. For quinoa grown in a sandy soil, biochar application might hence provide a win-win strategy for increased crop production, GHG emission mitigation and soil C sequestration.     

Long‐term impacts of manure amendments on carbon and greenhouse gas dynamics of rangelands

Livestock manure is applied to rangelands as an organic fertilizer to stimulate forage production, but the long‐term impacts of this practice on soil carbon (C) and greenhouse gas (GHG) dynamics are poorly known. We collected soil samples from manured and nonmanured fields on commercial dairies and found that manure amendments increased soil C stocks by 19.0 ± 7.3 Mg C ha^?1 and N stocks by 1.94 ± 0.63 Mg N ha^?1 compared to nonmanured fields (0–20 cm depth). Long‐term historical (1700–present) and future (present–2100) impacts of management on soil C and N dynamics, net primary productivity (NPP), and GHG emissions were modeled with DayCent. Modeled total soil C and N stocks increased with the onset of dairying. Nitrous oxide (N₂O) emissions also increased by ~2 kg N₂O‐N ha^?1 yr^?1. These emissions were proportional to total N additions and offset 75–100% of soil C sequestration. All fields were small net methane (CH₄) sinks, averaging ?4.7 ± 1.2 kg CH₄‐C ha^?1 yr^?1. Overall, manured fields were net GHG sinks between 1954 and 2011 (?0.74 ± 0.73 Mg CO₂ e ha^?1 yr^?1, CO₂e are carbon dioxide equivalents), whereas nonmanured fields varied around zero. Future soil C pools stabilized 40–60 years faster in manured fields than nonmanured fields, at which point manured fields were significantly larger sources than nonmanured fields (1.45 ± 0.52 Mg CO₂e ha^?1 yr^?1 and 0.51 ± 0.60 Mg CO₂e ha^?1 yr^?1, respectively). Modeling also revealed a large background loss of soil C from the passive soil pool associated with the shift from perennial to annual grasses, equivalent to 29.4 ± 1.47 Tg CO₂e in California between 1820 and 2011. Manure applications increased NPP and soil C storage, but plant community changes and GHG emissions decreased, and eventually eliminated, the net climate benefit of this practice.     

Agricultural peatland restoration: effects of land‐use change on greenhouse gas (CO2 and CH4) fluxes in the Sacramento‐San Joaquin Delta

Agricultural drainage of organic soils has resulted in vast soil subsidence and contributed to increased atmospheric carbon dioxide (CO₂) concentrations. The Sacramento‐San Joaquin Delta in California was drained over a century ago for agriculture and human settlement and has since experienced subsidence rates that are among the highest in the world. It is recognized that drained agriculture in the Delta is unsustainable in the long‐term, and to help reverse subsidence and capture carbon (C) there is an interest in restoring drained agricultural land‐use types to flooded conditions. However, flooding may increase methane (CH₄) emissions. We conducted a full year of simultaneous eddy covariance measurements at two conventional drained agricultural peatlands (a pasture and a corn field) and three flooded land‐use types (a rice paddy and two restored wetlands) to assess the impact of drained to flooded land‐use change on CO₂ and CH₄ fluxes in the Delta. We found that the drained sites were net C and greenhouse gas (GHG) sources, releasing up to 341 g C m^?2 yr^?1 as CO₂ and 11.4 g C m^?2 yr^?1 as CH₄. Conversely, the restored wetlands were net sinks of atmospheric CO₂, sequestering up to 397 g C m^?2 yr^?1. However, they were large sources of CH₄, with emissions ranging from 39 to 53 g C m^?2 yr^?1. In terms of the full GHG budget, the restored wetlands could be either GHG sources or sinks. Although the rice paddy was a small atmospheric CO₂ sink, when considering harvest and CH₄ emissions, it acted as both a C and GHG source. Annual photosynthesis was similar between sites, but flooding at the restored sites inhibited ecosystem respiration, making them net CO₂ sinks. This study suggests that converting drained agricultural peat soils to flooded land‐use types can help reduce or reverse soil subsidence and reduce GHG emissions.     

Predicting soil carbon changes in switchgrass grown on marginal lands under climate change and adaptation strategies

The United States Great Lakes Region (USGLR) is a critical geographic area for future bioenergy production. Switchgrass (Panicum virgatum) is widely considered a carbon (C)‐neutral or C‐negative bioenergy production system, but projected increases in air temperature and precipitation due to climate change might substantially alter soil organic C (SOC) dynamics and storage in soils. This study examined long‐term SOC changes in switchgrass grown on marginal land in the USGLR under current and projected climate, predicted using a process‐based model (Systems Approach to Land‐Use Sustainability) extensively calibrated with a wealth of plant and soil measurements at nine experimental sites. Simulations indicate that these soils are likely a net C sink under switchgrass (average gain 0.87 Mg C ha^?1 year^?1), although substantial variation in the rate of SOC accumulation was predicted (range: 0.2–1.3 Mg C ha^?1 year^?1). Principal component analysis revealed that the predicted intersite variability in SOC sequestration was related in part to differences in climatic characteristics, and to a lesser extent, to heterogeneous soils. Although climate change impacts on switchgrass plant growth were predicted to be small (4%–6% decrease on average), the increased soil respiration was predicted to partially negate SOC accumulations down to 70% below historical rates in the most extreme scenarios. Increasing N fertilizer rate and decreasing harvest intensity both had modest SOC sequestration benefits under projected climate, whereas introducing genotypes better adapted to the longer growing seasons was a much more effective strategy. Best‐performing adaptation scenarios were able to offset >60% of the climate change impacts, leading to SOC sequestration 0.7 Mg C ha^?1 year^?1 under projected climate. On average, this was 0.3 Mg C ha^?1 year^?1 more C sequestered than the no adaptation baseline. These findings provide crucial knowledge needed to guide policy and operational management for maximizing SOC sequestration of future bioenergy production on marginal lands in the USGLR.     

An appraisal of the utility of biochar in a rotation involving tobacco–rice in southern China

The employment of biochar in crop production can not only improve soil quality, but also helps the field ecosystem to fix carbon and reduce emissions. Although the benefits of their application in crop production have been more and more confirmed, it is not clear when it comes to the acidic soil of tobacco and rice rotation. A tobacco–rice rotation experiment was conducted in southern China to probe the application value of biochar under these conditions. Three biochar application rates were employed in this experiment. BC0 (without biochar), BC25 (25 t ha⁻¹), and BC50 (50 t ha⁻¹). The findings show that biochar significantly boosted soil fertility and crop yields. Meanwhile, the soil organic carbon of tobacco rice rotation field with biochar increased by 31.76%. After a whole growth period of tobacco and rice, the cumulative emission reduction of CO₂ and N₂O from the soil by biochar were 15,944 kg ha⁻¹ and 1810 g ha⁻¹, respectively. The use of biochar not only significantly improved the bacterial diversity of tobacco and rice rotation soil, but also altered the original microbial community structure. The profusion of Proteobacteria and Acidobacteria was reduced and the abundance of Actinobacteria and Bacteroidetes was enhanced in the treatments with biochar. Among them, Sphingomonadales, Planctomycotes, and Ktedonobacteria, which are beneficial to plant growth and soil health, have become key phylotypes. The carbon balance analysis data show that the net carbon sequestration of the two treatments with biochar is positive, while that of the treatment without biochar is negative. In terms of economic benefit, the application of biochar increased the average of 2.055 CNY kg⁻¹ consumed energy (CE) in the whole tobacco–rice rotation system. The ecological benefit was 0.51 kg C kg⁻¹ CE. In conclusion, biochar can be effectively used in the practice of tobacco–rice rotation and acidic soil improvement in southern China.     

Woody biochar's greenhouse gas mitigation potential across fertilized and unfertilized agricultural soils and soil moisture regimes

Biochar has been widely researched as an important technology for climate smart agriculture, yet work is still necessary to identify the magnitude of potential greenhouse gas (GHG) mitigation and mechanisms involved. This study measured slow‐pyrolysis wood‐derived biochar's impact on GHG efflux, mineral N dynamics, and soil organic C in a series of two incubations across fertilized and unfertilized agricultural soils and soil moisture regimes. This research explored the magnitude of biochar's full GHG mitigation potential and drivers of such impacts. Results of this incubation indicate slow‐pyrolysis wood‐derived biochar has potential to provide annual emission reductions of 0.58–1.72 Mg CO₂‐eq ha^?1 at a 25 Mg ha^?1 biochar application rate. The greatest GHG mitigation potential was from C sequestration and nitrous oxide (N₂O) reduction in mineral N fertilized soils, with minimal impacts on N₂O emissions in unfertilized soils, carbon dioxide (CO₂) emissions, and methane (CH₄) uptake. Analysis of mineral N dynamics in the bulk soil and on biochar isolates indicated that neither biochar impacts on net mineralization and nitrification nor retention of ammonium () on biochar isolates could explain biochar's N₂O reduction. Instead, biochar amendments exhibited consistent N₂O emission reductions relative to the N₂O emission in the control soil regardless of soil type and fertilization. Results across a soil moisture gradient suggest that woody biochar may aerate soils shifting redox conditions and subsequent N₂O production. Understanding the magnitude of biochar's GHG reduction potential and the mechanisms driving these effects can help inform biochar modeling efforts, explain field results and identify agricultural applications that maximize biochar's full GHG mitigation potential.     

Sorghum biomass production in the continental United States and its potential impacts on soil organic carbon and nitrous oxide emissions

National scale projections of bioenergy crop yields and their environmental impacts are essential to identify appropriate locations to place bioenergy crops and ensure sustainable land use strategies. In this study, we used the process‐based Daily Century (DAYCENT) model with site‐specific environmental data to simulate sorghum (Sorghum bicolor L. Moench) biomass yield, soil organic carbon (SOC) change, and nitrous oxide emissions across cultivated lands in the continental United States. The simulated rainfed dry biomass productivity ranged from 0.8 to 19.2 Mg ha^?1 year^?1, with a spatiotemporal average of  Mg ha^?1 year^?1, and a coefficient of variation of 35%. The average SOC sequestration and direct nitrous oxide emission rates were simulated as  Mg CO₂e ha^?1 year^?1 and  Mg CO₂e ha^?1 year^?1, respectively. Compared to field‐observed biomass yield data at multiple locations, model predictions of biomass productivity showed a root mean square error (RMSE) of 5.6 Mg ha^?1 year^?1. In comparison to the multi State (n = 21) NASS database, our results showed RMSE of 5.5 Mg ha^?1 year^?1. Model projections of baseline SOC showed RMSE of 1.9 kg/m² in comparison to a recently available continental SOC stock dataset. The model‐predicted N₂O emissions are close to 1.25% of N input. Our results suggest 10.2 million ha of cultivated lands in the Southern and Lower Midwestern United States will produce >10 Mg ha^?1 year^?1 with net carbon sequestration under rainfed conditions. Cultivated lands in Upper Midwestern states including Iowa, Minnesota, Montana, Michigan, and North Dakota showed lower sorghum biomass productivity (average: 6.9 Mg ha^?1 year^?1) with net sequestration (average: 0.13 Mg CO₂e ha^?1 year^?1). Our national‐scale spatially explicit results are critical inputs for robust life cycle assessment of bioenergy production systems and land use‐based climate change mitigation strategies.     

Carbon Sequestration in Fine Roots and Foliage Biomass Offsets Soil CO2 Effluxes along a 19-year Chronosequence of Shrub Willow (Salix x dasyclados) Biomass Crops

Previous greenhouse gas (GHG) assessments for the shrub willow biomass crops (SWBC) production system lacked quantitative data on the soil CO₂ efflux (F_c). This study quantifies the mean annual cumulative F_c, the C sequestration in the above- and belowground biomass, and the carbon balance of the production system. We utilized four SWBC fields, which have been in production for 5, 12, 14, and 19 years. Two treatments were applied: continuous production (CP)—shrub willows were harvested, and stools were allowed to regrow, and tear-out (TO) (crop removal)—shrub willows were harvested, and stools were sprayed with herbicide following spring, crushed, and mixed into the soil. Mean annual cumulative F_c were measured using dynamic closed chambers (LI-8100A and LI-8150). Across different age classes, the mean cumulative F_c ranged from 27.2 to 35.5 Mg CO₂ ha^?1 year^?1 for CP and 26.5 to 29.3 Mg CO₂ ha^?1 year^?1 for TO. The combined carbon (C) sequestration of the standing above- and belowground biomass, excluding stems, ranged from 50.6 to 94.8 Mg CO₂ eqv. ha^?1. In the CP treatment, the annual C sequestration in the fine roots and foliage offsets the annual cumulative F_c. Across different age classes, C balances ranged from ?21.5 to ?59.3 Mg CO₂ ha^?1 for CP and 26.5 to 29.3 Mg CO₂ ha^?1 for TO. The GHG potential of SWBC is about ?36.3 Mg CO₂ eqv. ha^?1 at the end of 19 years, suggesting that the SWBC system sequesters C until termination of the crop.     

Carbon and nitrogen dynamics in bioenergy ecosystems: 2. Potential greenhouse gas emissions and global warming intensity in the conterminous United States

This study estimated the potential emissions of greenhouse gases (GHG) from bioenergy ecosystems with a biogeochemical model AgTEM, assuming maize (Zea mays L.), switchgrass (Panicum virgatum L.), and Miscanthus (Miscanthus × giganteus) will be grown on the current maize‐producing areas in the conterminous United States. We found that the maize ecosystem acts as a mild net carbon source while cellulosic ecosystems (i.e., switchgrass and Miscanthus) act as mild sinks. Nitrogen fertilizer use is an important factor affecting biomass production and N₂O emissions, especially in the maize ecosystem. To maintain high biomass productivity, the maize ecosystem emits much more GHG, including CO₂ and N₂O, than switchgrass and Miscanthus ecosystems, when high‐rate nitrogen fertilizers are applied. For maize, the global warming potential (GWP) amounts to 1–2 Mg CO₂eq ha^?1 yr^?1, with a dominant contribution of over 90% from N₂O emissions. Cellulosic crops contribute to the GWP of less than 0.3 Mg CO₂eq ha^?1 yr^?1. Among all three bioenergy crops, Miscanthus is the most biofuel productive and the least GHG intensive at a given cropland. Regional model simulations suggested that substituting Miscanthus for maize to produce biofuel could potentially save land and reduce GHG emissions.     

Biochar in Combination with Nitrogen Fertilizer is a Technique: To Enhance Physiological and Morphological Traits of Rice (Oryza sativa L.) by Improving Soil Physio-biochemical Properties

The over use of synthetic nitrogen (N) fertilizers is the major anthropogenic cause of low N-use efficiency and environmental damage in wetland rice production. Biochar (B) addition to soil is suggested as a climate change mitigation tool that supports carbon sequestration and reduces N losses and greenhouse gas emissions from the soil. Therefore, this study assessed the effect of four levels of B (0, 10, 20 and 30 t ha^?1) combined with two levels of N (135 and 180 kg ha^?1) on soil health, roots dynamics, physiological attributes, and yield components of rice. The addition of B at 30 t ha^?1 combined with 135 N kg ha^?1 increased chlorophyll content, net photosynthetic rate, biomass, and grain yield by 104%, 64%, 12%, and 30%, respectively, over control. Further, root traits such as total root length (TRL), total root volume (TRV), total root surface area (TRSA), and total average root diameter (TARD) were improved under 30 t ha^?1 combined with 135 N kg ha^?1 by 20%, 13%, 13%, and 25%, respectively, than non-biochar treatment under lower N application. Improvements in these traits resulted from higher N uptake due to improved soil physiochemical properties and soil microbial biomass combined with biochar. Interestingly, enhanced N metabolizing enzyme activities, including nitrate reductase (NR), glutamine synthetase (GS), and glutamine oxoglutarate aminotransferase (GOGAT) in biochar-treated plots, further supported the increases in these traits. Our results revealed that the integration of 30 t B ha^?1 with 135 kg N ha^?1 is a favorable option for enhancing soil health and rice grain yield.

     

Impact of biochar application on nitrogen nutrition of rice, greenhouse-gas emissions and soil organic carbon dynamics in two paddy soils of China

Aims

Two field microcosm experiments and ¹⁵N labeling techniques were used to investigate the effects of biochar addition on rice N nutrition and GHG emissions in an Inceptisol and an Ultisol.

Methods

Biochar N bioavailability and effect of biochar on fertilizer nitrogen-use efficiency (NUE) were studied by ¹⁵N-enriched wheat biochar (7.8803 atom% ¹⁵N) and fertilizer urea (5.0026 atom% ¹⁵N) (Experiment I). Corn biochar and corn stalks were applied at 12 Mg?ha^?1 to study their effects on GHG emissions (Experiment II).

Results

Biochar had no significant impact on rice production and less than 2 % of the biochar N was available to plants in the first season. Biochar addition increased soil C and N contents and decreased urea NUE. Seasonal cumulative CH₄ emissions with biochar were similar to the controls, but significantly lower than the local practice of straw amendment. N₂O emissions with biochar were similar to the control in the acidic Ultisol, but significantly higher in the slightly alkaline Inceptisol. Carbon-balance calculations found no major losses of biochar-C.

Conclusion

Low bio-availability of biochar N did not make a significantly impact on rice production or N nutrition during the first year. Replacement of straw amendments with biochar could decrease CH₄ emissions and increase SOC stocks.     

Potential Soil Carbon Sequestration and CO2 Offset by Dedicated Energy Crops in the USA

Energy crops are fast-growing species whose biomass yields are dedicated to the production of more immediately usable energy forms, such as liquid fuels or electricity. Biomass-based energy sources can offset, or displace, some amount of fossil-fuel use. Energy derived from biomass provides 2 to 3% of the energy used in the U.S.A.; but, with the exception of corn-(Zea mays L.)-to-ethanol, very little energy is currently derived from dedicated energy crops. In addition to the fossil-fuel offset, energy cropping might also mitigate an accentuated greenhouse gas effect by causing a net sequestration of atmospheric C into soil organic C (SOC). Energy plantations of short-rotation woody crops (SRWC) or herbaceous crops (HC) can potentially be managed to favor SOC sequestration. This review is focused primarily on the potential to mitigate atmospheric CO₂ emissions by fostering SOC sequestration in energy cropping systems deployed across the landscape in the United States. We know that land use affects the dynamics of the SOC pool, but data about spatial and temporal variability in the SOC pool under SRWC and HC are scanty due to lack of well-designed, long-term studies. The conventional methods of studying SOC fluxes involve paired-plot designs and chronosequences, but isotopic techniques may also be feasible in understanding temporal changes in SOC. The rate of accumulation of SOC depends on land-use history, soil type, vegetation type, harvesting cycle, and other management practices. The SOC pool tends to be enhanced more under deep-rooted grasses, N-fixers, and deciduous species. Carbon sequestration into recalcitrant forms in the SOC pool can be enhanced with some management practices (e.g., conservation tillage, fertilization, irrigation); but those practices can carry a fossil-C cost. Reported rates of SOC sequestration range from 0 to 1.6 Mg C ha^?1 yr^?1 under SRWC and 0 to 3 Mg C ha^?1 yr^?1 under HC. Production of 5 EJ of electricity from energy crops—a perhaps reasonable scenario for the U.S.A.—would require about 60 Mha. That amount of land is potentially available for conversion to energy plantations in the U.S.A. The land so managed could mitigate C emissions (through fossil C not emitted and SOC sequestered) by about 5.4 Mg C ha^?1 yr^?1. On 60 Mha, that would represent 324 Tg C yr^?1—a 20% reduction from current fossil-fuel CO₂ emissions. Advances in productivity of fast-growing SRWC and HC species suggest that deployment of energy cropping systems could be an effective strategy to reduce climate-altering effects of anthropogenic CO₂ emissions and to meet global policy commitments.     

Successive straw biochar application as a strategy to sequester carbon and improve fertility: A pot experiment with two rice/wheat rotations in paddy soil

Aims

A pot study spanning four consecutive crop seasons was conducted to compare the effects of successive rice straw biochar/rice straw amendments on C sequestration and soil fertility in rice/wheat rotated paddy soil.

Methods

We adopted 4.5 t ha^?1, 9.0 t ha^?1 biochar and 3.75 t ha^?1 straw for each crop season with an identical dose of NPK fertilizers.

Results

We found no major losses of biochar-C over the 2-year experimental period. Obvious reductions in CH₄ emission were observed from rice seasons under the biochar application, despite the fact that the biochar brought more C into the soil than the straw. N₂O emissions with biochar were similar to the controls without additives over the 2-year experimental period. Biochar application had positive effects on crop growth, along with positive effects on nutrient (N, P, K, Ca and Mg) uptake by crop plants and the availability of soil P, K, Ca and Mg. High levels of biochar application over the course of the crop rotation suppressed NH₃ volatilization in the rice season, but stimulated it in the wheat season.

Conclusions

Converting straw to biochar followed by successive application to soil is viable for soil C sequestration, CH₄ mitigation, improvements of soil and crop productivity. Biochar soil amendment influences NH₃ volatilization differently in the flooded rice and upland wheat seasons, respectively.