Pyrogenic conversion of rice straw and wood to biochar increases aromaticity and carbon accumulation in soil

Carbon use efficiency has recently been proposed as a central parameter that promotes soil organic carbon storage based on data assimilation with a global soil organic carbon database and a vertical, microbial explicit soil organic carbon model (Tao et al., 2023). In this research, we present a sensitivity study with a vertical soil organic carbon model, COMISSION v2.0 (Ahrens et al., 2020), that not only models microbial interactions explicitly but also represents organo-mineral interactions with a maximum capacity, Qmax, to form mineral-associated organic carbon (MAOC).The COMISSION model represents the formation of MAOC from microbial necromass and dissolved organic carbon analogous to Langmuir sorption. Empirical studies have provided Qmax parameterizations derived from quantile or boundary line regressions with clay and silt content. For the sensitivity study, we vary Qmax along the full range of observed Qmax values while simultaneously varying carbon use efficiency (CUE). Our results highlight that CUE and Qmax promote soil organic carbon storage to similar degrees along their respective observed ranges. The remaining parameters of the COMISSION model were kept at their calibrated values from a multi-site calibration with soil organic carbon, mineral-associated organic carbon, and radiocarbon profiles (Ahrens et al., 2020). While Qmax and CUE are of similar importance for promoting soil organic carbon storage, they also interact in promoting SOC storage. Higher Qmax values strengthen the promotion of soil organic carbon storage with higher CUE. This positive interaction results from higher microbial necromass with higher CUE and the subsequent association of microbial necromass on mineral surfaces mediated through Qmax. The sensitivity study revealed that CUE is the dominant driver for microbial biomass levels. Qmax affects microbial biomass only to a small degree through 'competition' between mineral surfaces and microbial biomass for dissolved organic carbon. While the effect of Qmax on microbial biomass is small, the relationship between Qmax and microbial biomass is generally negative. At the lower end of the tested range of carbon use efficiencies (CUE < 0.15), further model experiments reveal that imposing a stronger microbial limitation of depolymerization can lead to a negative relationship between CUE and soil organic carbon storage.Overall, our results highlight that in soil organic carbon models with microbial interactions and a limited capacity to form organo-mineral associations, both processes can be of similar importance in promoting soil organic carbon storage. The current debate in the observational realm, whether there is indeed an upper limit for mineral-associated organic carbon formation, can spark a similar debate in the modeling realm on how to represent mineral-associated organic carbon formation in models mechanistically. ReferencesAhrens B, Guggenberger G, Rethemeyer J et al. (2020) Combination of energy limitation and sorption capacity explains 14C depth gradients. Soil Biology and Biochemistry, 148, 107912.Tao F, Huang Y, Hungate BA et al. (2023) Microbial carbon use efficiency promotes global soil carbon storage. Nature, 618, 981-985.Funding acknowledgment: Bernhard Ahrens has received funding through the AI4SoilHealth project. The AI4SoilHealth project has received funding from the European Union's Horizon Europe research and innovation programme under grant agreement No. 101086179.

Microbial carbon use efficiency (CUE), describing the partitioning of microbe assimilated carbon into microbial growth and respiration, is commonly used in soil carbon models to link microbial activities with the consumption of soil organic carbon (SOC). However, the role of CUE in regulating SOC storage remains debated. Previous studies have reported that a higher CUE could not only favour SOC formation through microbial necromass accumulation, but also trigger SOC losses because an enhancement in enzyme production facilitates SOC decomposition. The former leads to a positive relationship between CUE and SOC, while the latter leads to a negative one. Temperature dependencies introduce additional uncertainties while exploring the SOC-CUE relationship since temperature affects both SOC decomposition and CUE. Based on the meta-analysis and numerical simulations with a mechanistic model (T&C), we examined the relationship between CUE, SOC storage and temperature. Numerical results recover the expected SOC storage decrease with increasing temperature when temperature effects are isolated; however, an increase of SOC storage with decreasing CUE is found once temperature effects are discounted, indicating that SOC storage increase with increasing CUE is likely a by-product of temperature dependencies. In addition, we show that CUE variability plays a more important role in affecting SOC storage at lower temperature. Our study helps refine the understanding of SOC responses in a warming climate.

ABSTRACTSalinization and alkalization contribute significantly to soil degradation. Rice ( Oryza sativa L.) cultivation is an effective approach to remediate saline‐alkali soil. However, how rice straw (RS), rice straw biochar (RSB), and rice straw ash (RSA) impact soil organic carbon (SOC) accumulation and stability in saline‐alkali soil remains unknown. Herein, SOC and SOC fractions contents in bulk soil and its particle‐ and aggregate‐size classes under RS, RSB, and RSA amendments and control with amendments (CK) were investigated by field experiment. Carbon‐13 nuclear magnetic resonance spectroscopy was used to evaluate bulk SOC chemical composition. The SOC and SOC fractions contents ranked as CK<RSA<RS<RSB. Aromatic C was higher whereas O‐alkyl C was lower in RSB relative to other treatments. The contents of SOC and SOC fractions in bulk soil were generally positively correlated with those in particle‐ and aggregate‐size classes as well as with aromatic C. Redundancy analysis showed that exchangeable sodium and electrical conductivity were the most significant factors in shaping SOC contents and chemical composition. The results indicated that RSB is more beneficial for SOC accumulation and stabilization as compared to RS and RSA. The primary mechanisms of SOC accumulation in RSB‐amended soil included physical protection afforded by aggregate classes, chemical protection mediated by silt and clay fractions, and biochemical protection with recalcitrant aromatic C. Our findings suggest that converting RS into RSB and the subsequent application of this biochar have the potential for improving soil quality in saline‐alkali paddy field.

Application of organic amendments is an effective approach for improving soil organic carbon and soil fertility. To investigate the effects of different organic amendments on soil organic carbon and its labile fraction content, a batch of incubation experiments was conducted on the fluvo-aquic soil in Dongting Lake region, Hunan Province. There were six treatments, including soil amended with rice straw, soil amended with Chinese milk vetch, soil amended with bio-organic fertilizer, soil amended with pig manure, and soil amended with rice straw-derived biochar, with unamended soil as control. Each treatment had the same amount of carbon input. After 180 days of incubation, application of organic amendments increased soil labile organic carbon content. Application of bio-organic fertilizer, pig manure and rice straw-derived biochar significantly increased soil organic carbon content by 26.1%, 9.7% and 30.7%, respectively. There was no significant change in soil organic carbon content in rice straw and Chinese milk vetch treatments which were more favourable to the accumulation of soil dissolved organic carbon and microbial biomass carbon. Pig manure was more favourable to the accumulation of soil dissolved organic carbon. Bio-organic fertili-zer could benefit the accumulation of soil microbial biomass carbon and readily oxidizable organic carbon. Rice straw-derived biochar could promote the accumulation of soil microbial biomass carbon and light fraction organic carbon. Compared with rice straw, soil carbon pool management index was increased by 31.8%, 111.6%, 62.2% and 50.7% in Chinese milk vetch, bio-organic fertilizer, pig manure and rice straw-derived biochar treatments, respectively. The performance of bio-organic fertilizer, pig manure, and rice straw biochar was better than rice straw and Chinese milk vetch from the perspective of soil carbon sequestration and soil carbon pool management index.

Successive addition of rice straw biochar enhances carbon accumulation in soil irrigated with saline or non-saline water

Long-term application of Jatropha press cake promotes seed yield by enhanced soil organic carbon accumulation, microbial biomass and enzymatic activities in soils of semi-arid tropical wastelands

Microbial carbon use efficiency during plant residue decomposition: Integrating multi-enzyme stoichiometry and C balance approach

Understanding how organic amendments affect microbial carbon use efficiency (CUE) and necromass C (MNC) is crucial for understanding soil organic C (SOC) formation and accrual in paddy fields, but the underlying mechanisms remain largely unclear. In this study, the microbial CUE, MNC, and microbial community composition, as well as SOC fractions and chemical composition, were measured under long-term organic amendments: rice straw (RS), green manure (GM), and pig manure (PM) in paddy soils. Four treatments were included: (1) chemical fertilizers (CF); (2) CF plus RS (CF + RS); (2) CF plus GM (CF + GM); and (4) CF plus PM (CF + PM). The CUE, MNC, and microbial community were determined by 18O-H2O incubation, amino sugars levels, and phospholipid fatty acids (PLFAs) content, respectively. Results showed that SOC, particulate organic C (POC), and mineral-associated organic C (MAOC) concentrations were significantly increased by organic amendments compared with chemical fertilization alone. The O-alkyl C decreased, but aromatic C increased with long-term organic amendments, suggesting enhanced SOC hydrophobicity. GM and PM inputs significantly enhanced microbial CUE, but straw return did not affect microbial CUE compared to CF. Microbial growth and C uptake increased by 25.2–42.4% and 19.8–30.0% under organic amendments relative with CF. Microbial respiration was increased by RS and GM amendments. Turnover time was more rapid in CF + RS and CF + GM than in CF and CF + PM. Compared to CF, organic amendments increased the MNC concentration due to the increase in microbial biomass. In addition, CF + RS and CF + GM enhanced the MNC contribution to SOC, but PM had no effect, suggesting that PM contributed more organic C from non-microbial sources. The SOC, POC, and MAOC increased with microbial CUE and MNC, indicating that microbial traits play a crucial role in SOC accrual. Higher microbial CUE and biomass explained the increased MNC accumulation under organic amendments. Our study highlights the crucial role of microbe-mediated processes in SOC accrual under long-term organic amendments in paddy soils. Our findings show that organic amendments are an effective management practice for accumulating more SOC in paddy soils.

Microbial carbon use efficiency of glucose varies with soil clay content: A meta-analysis

The impact of global warming on soil processes is a critical area of concern. Limited studies have investigated soil organic carbon (SOC) dynamics' adaptation to warming. This poses a great challenge in assessing and understanding terrestrial C cycle response to climate change. Carbon Use Efficiency (CUE), indicating the proportion of metabolized organic C allocated to microbial biomass growth, is a pivotal regulator governing the fate of soil C. Moreover, our understanding of fundamental drivers of microbial CUE is largely elusive and inconclusive, especially in tropical ecosystems.To address these knowledge gaps, we translocated top soil samples (10 cm deep soil cores) from two higher elevation sites (Vuria, 2000 m a.s.l, and Ngangao, 1800 m a.s.l) to a lower site (Macha, 1600 m a.s.l) along a moist montane rain forest gradient in Taita Hills, Kenya. Utilizing an 18O-water tracing approach, we examined the changes in microbial CUE in response to approximately three years of experimental warming. We also measured enzyme activities and conducted a 6-month laboratory incubation (15°C and 25°C) to study temperature sensitivity in native and translocated samples.Our hypotheses were: (i) Both microbial CUE and C related enzyme activities would decrease, however, N- and P- cycle enzyme would increase along an altitudinal gradient toward the top of the Taita Hills, primarily governed by soil C and N availability; (ii) passive warming by soil translocation would result in higher CUE in translocated soils compared to native soils; (iii) At lower temperatures, soil microbial CUE is expected to decrease due to microbes allocating increased energy towards synthesis of enzymes involved in nutrient acquisition, while reducing C investment towards their growth.Initial findings have revealed significant distinctions in enzyme activity profile due to elevation and temperature effects. Specifically, β-glucosidase and acid phosphatase activities increased and decreased along the elevation, respectively. Consistent with our hypothesis, enzyme activities and microbial CUE were higher in translocated soil than native soil. The six-month incubation had a similar effect on translocated soils and lower temperature increased the microbial CUE. In summary, our study indicates that passive warming alters microbial temperature adaption and underscores the influential role of soil enzyme activities in regulating microbial CUE. We suggest that soil microbiome at lower temperature indicates greater need for nutrients and energy. Our results highlight the need to investigate a wide variety of temperature influence on tropical soils in order to better understand and predict how the changing climate will affect C and nutrient cycling.

The organic carbon content of soils is a key parameter of soil fertility. Moreover, carbon accumulation in soils may mitigate the increase in atmospheric CO2 concentration. The principles of carbon accumulation in arable soils are well known. The inclusion of clover/alfalfa/grass within the rotation is a central instrument to increase soil organic carbon. In addition, the regular application of rotted or composted farmyard manure within the rotation can increase soil organic carbon contents much more than the separate application of straw and cattle slurry. Humic substances, as a main stable part of soil organic carbon, play a central role in the accumulation of soil carbon. A major effect of compost application on soil carbon may be the introduction of stable humic substances which may bind and stabilize labile organic carbon compounds such as amino acids, peptides, or sugars. From this point of view, a definite soil carbon saturation index may be misleading. Besides stable composts, commercially available humic substances such as Leonardite may increase soil organic carbon contents by stabilization of labile C sources in soil.

Divergence of microbial carbon use efficiency and soil organic carbon along a tidal flooding gradient in a subtropical coastal wetland.

With regard of the problems of soil acidification and soil degradation caused by high intensive planting in south China, a 2-year pot experiment consisting of six harvests under a rice–rice–vegetable rotation cropping system was conducted to assess the effects of NPK+ rice straw (RS) and combined application of RS with peanut bran, biochar, and organic fertilizer on soil chemical and microbial characteristics in paddy soil. The control treatment received chemical fertilizer alone. Results showed that RS and the combination of RS with organic ameliorants, especially NPK+ rice straw + biochar (RSBC) treatment led to the greatest improvement of soil pH, soil organic carbon, microbial biomass carbon, and total nitrogen (TN) content, and urease (UE), acid phosphatase (ACP) and catalase (CAT) activities concurrently without yield sacrificing, which inferred that RSBC treatment could be an effective measure to alleviate soil acidification, boost carbon sequestration and nutrients content as well as soil enzyme activities in rice-rice-vegetable rotation system. Besides, Pearson’s correlation analysis showed that soil mineral nitrogen (Nmin) content was negatively related to pH, and the available potassium (AK) content was positively related to UE and CAT activity but negatively related to ACP activity. Canonical correspondence analysis demonstrated the Nmin and AK explained 27.2% and 13.7% of the variation in microbial species, respectively. Therefore, it is believed that soil Nmin and AK content could be the primary factors of soil microbial properties under the rice-rice-vegetable rotation system.

IntroductionSusbtantial agricultural wastes are produced globally which need urgent management policies. To explore the effective utilization of agricultural waste in enhancing soil quality and carbon sequestration capacity, straw and its biochar can be applied as soil ameliorants.MethodsThis study was designed to investigate the impact of different return-to-field methods of rice straw on the transformation between different carbon components in the soil of Siraitia grosvenorii fields. We hypothesize that rice straw and its biochar, as soil amendments, can influence the transformation and cycling of different carbon components in the soil of S. grosvenorii fields through various return-to-field methods. Rice straw, rice straw biochar, and “rice straw + rice straw biochar” were applied as additives in a 2-year field experiment.ResultsThe results showed that the field application of rice straw and its biochar increased the content of soil organic carbon, the amount of organic carbon mineralization, particulate organic carbon, mineral-associated organic carbon, dissolved organic carbon, and readily oxidizable organic carbon content, while reducing the content of soil microbial biomass carbon. The combined application of rice straw and biochar in S. grosvenorii cultivation fields had a more significant effect on various soil carbon fractions compared to the use of either rice straw or biochar alone. The co-application of rice straw and its biochar to the soil increased the content of soil organic carbon by 117.4%, enhanced the mineralization of organic carbon by 100.0%, and reduced the content of soil microbial biomass carbon by 61.6%. The metabolic entropy and microbial entropy of rice straw and its biochar mixed application in the field were 5.2 and 0.18 times higher than of the control group, respectively.DiscussionIn summary, the return of rice straw and biochar to the field improves soil structure and the content of recalcitrant organic carbon, providing a habitat for microorganisms, thereby promoting the stability and cycling of soil organic carbon.

Comprehending the factors influencing microbial carbon use efficiency (CUE), and where CUE is most optimal to soil organic carbon (SOC) storage, are crucial for managing microbial roles in SOC sequestration and model prediction. Yet, establishing a direct mathematical relationship between CUE and SOC might be challenging, with global distributions and controls remaining unresolved, particularly in response to various global changes. Here, we leverage a global synthesis of CUE measurements by 18O-microbial DNA growth, and observed an average CUE across all biomes at 0.3, with the highest in temperate grasslands and deeper soils, and the lowest in tropical forests. Random forest analysis identified climates (aridity index and mean annual temperature: MAT) and soil properties (pH, bulk density and soil C:N ratio) as primary drivers influencing CUE. However, microbial biomass size overall exhibited a smaller effects on CUE, despite its substantial impact in each land use type. We then review how these drivers affecting CUE values may be altered by warming, soil fertilization, altered precipitation and elevated carbon dioxide. Notably, nitrogen additions plays a big role in increasing CUE and promoting SOC contents, while warming effects depend on time-scale, with long-term warming potentially leading to SOC losses with a lower CUE and growth. Moreover, we found that the CUE–SOC relationship varies across different climates, greatly driven by MAT and soil properties. Higher CUE promots SOC per fine fraction (clay+silt) across the major data points, contrasting with a negative relationship in a subarctic study, where pH is the primary determinate. Consequently, there might be no simple linear relationship between CUE and C in microbial biomass and soil. We conclude by discussing the integration of CUE into SOC models and the necessity of incorporating interactions between CUE and individual drivers for predicting soil carbon-climate change scenarios. Our study underscores the importance of considering microbial CUE and other microbial processes for improving projections of SOC dynamics.