Ecoenzymatic stoichiometry reveals stronger microbial carbon and nitrogen limitation in biochar amendment soils: A meta-analysis

Soil quality and ecosystem multifunctionality after 13-year of organic and nitrogen fertilization

Organic and mineral fertilization increase crop productivity, but their combined effects on soil quality index (SQI) and ecosystem multifunctionality (EMF, defined as the capacity of soils to simultaneously provide multiple functions and services) are not clear. We conducted a 13-year field trial in North China Plain to examine how five maize-derived organic fertilizers (straw, manure, compost, biogas residue, and biochar) at equal C input rate (3.2 t C ha-1), with or without nitrogen (N) fertilization influenced topsoil (0-15 cm) physico-chemical properties, activities of enzymes responsible for carbon (C), N, and phosphorus (P) cycling, as well as SQI and soil EMF. Organic fertilizers with or without N increased SQI by 51-187 % and EMF by 31-351 % through the enhancement of soil physical (mean weight diameter of soil aggregates) and chemical properties (C, N, and P contents) as well as C, N, and P acquisition enzyme activities, albeit the biochar effects were of minor importance. N application increased EMF compared to soil without N. Soil quality increased with EMF. Random forest analysis revealed that microbial biomass C and N, available P, permanganate oxidizable C, dissolved organic C and N, mean weight diameter of aggregates, hot water extractable C, and electrical conductivity were the main contributions to soil EMF. We conclude that application of maize-derived organic fertilizers, especially compost and straw, with optimal N fertilization is a plausible strategy to increase SQI and EMF under a wheat/maize system.

Biochar addition promotes soil organic carbon sequestration dominantly contributed by macro-aggregates in agricultural ecosystems of China

Soil aggregates play pivotal roles in soil organic carbon (SOC) preservation and climate change. Biochar has been widely applied in agricultural ecosystems to improve soil physicochemical properties. However, the underlying mechanisms of SOC sequestration by soil aggregation with biochar addition are not well understood at a large scale. Here, we conducted a meta-analysis of 2335 pairwise data from 45 studies to explore how soil aggregation sequestrated SOC after biochar addition in agricultural ecosystems of China. Biochar addition markedly enhanced the proportions of macro-aggregates and aggregate stability, and the production of organic binding agents positively facilitated the formation of macro-aggregates and aggregate stability. Soil aggregate-associated organic carbon (OC) indicated a significantly increasement by biochar addition, which was attributed to direct and indirect inputs of OC from biochar and organic residues, respectively. Biochar stimulated SOC sequestration dominantly contributed by macro-aggregates, and it could be interpreted by a greater improvement in proportions and OC protection of macro-aggregates. Furthermore, the SOC sequestration of soil aggregation with biochar addition was regulated by climate conditions (mean annual temperature and precipitation), biochar attributes (biochar C/N ratio and pH), experimental practices (biochar addition level and duration), and agronomic managements (land type, cropping intensity, fertilization condition, and crop type). Collectively, our synthetic analysis emphasized that biochar promoted the SOC sequestration by improving soil aggregation in agricultural ecosystems of China.

Global synthesis on the response of soil microbial necromass carbon to climate-smart agriculture

Climate-smart agriculture (CSA) supports the sustainability of crop production and food security, and benefiting soil carbon storage. Despite the critical importance of microorganisms in the carbon cycle, systematic investigations on the influence of CSA on soil microbial necromass carbon and its driving factors are still limited. We evaluated 472 observations from 73 peer-reviewed articles to show that, compared to conventional practice, CSA generally increased soil microbial necromass carbon concentrations by 18.24%. These benefits to soil microbial necromass carbon, as assessed by amino sugar biomarkers, are complex and influenced by a variety of soil, climatic, spatial, and biological factors. Changes in living microbial biomass are the most significant predictor of total, fungal, and bacterial necromass carbon affected by CSA; in 61.9%-67.3% of paired observations, the CSA measures simultaneously increased living microbial biomass and microbial necromass carbon. Land restoration and nutrient management therein largely promoted microbial necromass carbon storage, while cover crop has a minor effect. Additionally, the effects were directly influenced by elevation and mean annual temperature, and indirectly by soil texture and initial organic carbon content. In the optimal scenario, the potential global carbon accrual rate of CSA through microbial necromass is approximately 980 Mt C year-1, assuming organic amendment is included following conservation tillage and appropriate land restoration. In conclusion, our study suggests that increasing soil microbial necromass carbon through CSA provides a vital way of mitigating carbon loss. This emphasizes the invisible yet significant influence of soil microbial anabolic activity on global carbon dynamics.

Understanding phosphorus mobilization mechanisms in acidic soil amended with calcium-silicon-magnesium-potassium fertilizer

Calcium-silicon-magnesium-potassium fertilizer (CSMP) is usually used as an amendment to counteract soil acidification caused by historical excessive nitrogen (N) applications. However, the impact of CSMP addition on phosphorus (P) mobilization in acidic soils and the related mechanisms are not fully understood. Specifically, a knowledge gap exists with regards to changes in soil extracellular enzymes that contribute to P release. Such a knowledge gap was investigated by an incubation study with four treatments: i) initial soil (Control), ii) urea (60 mg kg-1) addition (U); iii) CSMP (1%) addition (CSMP) and iv) urea (60 mg kg-1) and CSMP (1%) additions (U + CSMP). Phosphorus mobilization induced by different processes was distinguished by biologically based P extraction. The Langmuir equation, K edge X-ray absorption near-edge structure spectroscopy, and ecoenzyme vector analysis according to the extracellular enzyme activity stoichiometry were deployed to investigate soil P sorption intensity, precipitation species, and microbial-driven turnover of organophosphorus. Results showed that CaCl2 extractable P (or citric acid extractable P) content increased by 63.4% (or 39.2%) in the soil with CSMP addition, compared with the study control. The accelerated mobilization of aluminum (Al)/iron (Fe)-bound P after CSMP addition, indicated by the reduction of the sum of FePO4·2H2O and AlPO4 proportion, contributed to this increase. The decrease of P sorption capacity can also be responsible for it. The CSMP addition increased enzyme extractable P in the soil nearly 7-fold and mitigated the limitations of carbon (C) and P for soil microorganisms (indicated by the enzyme stoichiometry and ecoenzyme vector analysis), suggesting that microbial turnover processes also contribute to P mobilization in amended acidic soil. These findings indicate that the P mobilization in CSMP amended acidic soil not only attributed to both decreasing P sorption capacity and dissolving phosphate precipitation, but also to the increase of the microbial turnover of the organophosphorus pool.

Pyrolytic and hydrothermal carbonization affect the transformation of phosphorus fractions in the biochar and hydrochar derived from organic materials: A meta-analysis study

Carbonized organic materials are widely used to achieve soil improvement and alleviate soil pollution. The carbonization process significantly changes the total phosphorus (P) content and the P form in the solid phase derived from organic materials, which in turn has a significant impact on the P fertilizer effect in soils. In the present study, a meta-analysis with 278 observational data was conducted to detect the impact of the carbonization process (including pyrolytic carbonization and hydrothermal carbonization) on the transformation of P fractions in biochar or hydrochar derived from different organic materials. The results showed that the carbonization process significantly increased the total P content of the solid phase by 67.9%, and that the rate of P recovery from raw materials stayed high with a mean value of 86.8%. Among them, the impact of sludge-derived char was smaller when compared to the manure-derived char and biomass-derived char. The increase of total P in the biochar (or hydrochar) produced at >500 °C (or >200 °C) was more notable than that at <500 °C (or <200 °C). Simultaneously, the carbonization process significantly decreased the proportion of available P pool in the solid phase by 51.7% on average and increased the proportion of stable P pool in the solid phase by 204%. Appropriate production temperature helps to adjust the proportion of stable P pool in the solid phase. This meta-analysis pointed out that the carbonized solid phase recovers most of the P in the feedstock and that it promotes a significant transformation of available P pool in the feedstock to stable P in the carbonized solid phase. These findings provide useful information for the rational use of carbonization technology, the development of corresponding field management strategies, and the potential value of carbonized solid phase utilization.

Biochar carbon sequestration potential rectification in soils: Synthesis effects of biochar on soil CO2, CH4 and N2O emissions

Biochar production and its soil sequestration are promising ways to mitigate global warming. Effects of biochar on soil CO2, CH4 and N2O release have been studied extensively. In contrast, few studies have comprehensively quantified and synthesized the effect of biochar on soil greenhouse gas (GHG) emission and coupled it to the calculation of carbon sequestration potential. This study obtained the influence coefficient of biochar on soil GHG release relative to biochar carbon storage potential in soils under different environmental conditions, by literature statistics and data transformations. Our results showed that the overall average effect of biochar on soil CO2, CH4, N2O and CO2e release observed in our databases would compensate the potential of biochar soil carbon storage by -2.1 ± 3.3 %, 13.1 ± 9.8 %, -1.6 ± 8.6 % and 5.3 ± 11.4 %, respectively. By combining biochar induced soil GHG emission reduction mechanism and results from our literature statistics, some specific application environmental scenarios (such as biochar with high pyrolysis temperature of 500-600 °C, application in flooded soils, application in straw-return scenarios, etc.) were recommended, which could increase the actual carbon sequestration potential of biochar by an average of about 43.3 ± 30.2 % relative the amount of carbon buried. Our findings provide a scientific basis for developing a precise application strategy towards large scale adoption of biochar as a soil amendment for climate change mitigation.

Characterization and Adsorption Capacity of Modified Biochar for Sulfamethylimidine and Methylene Blue in Water

In this study, a composite of pond mud and lanthanum- and nano-zero valent iron-modified-biochar was investigated for its ability to adsorb methylene blue (MB) and sulfamethazine (SMZ). La-modified attapulgite and nano-zero valent iron (surface area enhanced by 43.7% via Brunauer-Emmett-Teller analysis) were successfully loaded onto the straw-sediment biochar (BC) surface. With the increase in pyrolysis temperature, the biocompatibility yield, the H, O, and N content, and the ratio of carbon elements decreased, while the pH value, surficial micropores, C element, and ash content increased. The biocarbon small molecules were gradually and tightly ordered, and the organic groups such as hydroxyl, carboxyl groups, and carbon oxygen double bonds were gradually lost or disappeared. The original Fe-BC had more phenolic hydroxyl groups forming an intermolecular hydrogen bond than others with a higher adsorption capacity possibly through the Schiff base reaction. The effect of various pH (2-9), temperature (15-35 °C), and initial concentration (1-25 mg L-1) on adsorption was investigated. pH and temperature were the main factors governing the adsorption process. The maximum adsorption capacity was observed at pH 4. The adsorption performances for MB followed the order Fe-BC > La-BC > BC, and the maximum removal rate was over 98.45% with pH = 7. The three types of BC dosages between 0.2 (6.67 g L-1) and 0.4 g showed a removal rate of 99% for MB. The adsorption capacity of Fe-BC, La-BC, and BC for MB was 2.201, 1.905, and 2.401 mg L-1 with pH = 4, while 4.79, 4.58, and 5.55 mg g-1 were observed with BC dosage at 0.025 g. For SMZ, the higher the temperature, the better the adsorption effect, and it reaches saturation at approximately 25 °C. To further evaluate the nature of adsorption, Langmuir/Freundlich/Temkin models were tested and the adsorption capacities were evaluated on the surface of the BC composite. The three modified materials were physisorbed to SMZ, while MB was chemisorbed. For MB, the adsorption performance of BC is the best < 0.2 g (6.67 g L-1) at pH 7.0 at 35 °C. The Elovich model was more suitable for MB, while the Freundlich and Temkin models could better fit the adsorption process of MB. The preparatory secondary dynamics equation and Langmuir equation were more compliant for SMZ, and the saturated adsorption capacities of straw-modified, La-BC, and Fe-BC reached 5.699, 6.088, and 5.678 mg L-1, respectively.

Biochar decreases and nitrification inhibitor increases phosphorus limitation for microbial growth in a wheat-canola rotation

Agricultural management practices affect microbial populations and ecoenzymatic activities; however, the effect of these practices on ecological stoichiometry relating the elemental ratio of resources to microbial biomass is poorly understood. In a 2-year field study, we assessed the effects of biochar and nitrapyrin (a commonly used nitrification inhibitor (NI)) on the ecological stoichiometry and microbial nutrient limitation in a wheat (Triticum aestivum L.)-canola (Brassica juncea L.) rotation. This study used a 3 × 2 factorial design that included two treatments: (i) biochar with three levels: no biochar addition (BC0), and biochar added at 10 (BC10) and 20 t ha^-1 (BC20), and (ii) NI with two levels: without (NI0) and with NI (NI1). Soil microbial biomass carbon (C), nitrogen (N) and phosphorus (P) were increased by biochar application, regardless of the application rate, but were not affected by NI application. Biochar increased and NI decreased β-1,4-glucosidase, β-1,4-N-acetyl glucosaminidase and acid phosphatase (P < 0.05) with subsequent changes in ecoenzymatic stoichiometry. Ecoenzymatic stoichiometry analysis showed microbial P limitation relative to N in the studied area irrespective of the treatment, with contrasting effects of biochar (decreasing) and NI (increasing) on the vector angle of ecoenzymatic stoichiometry (P = 0.037 and 0.043, respectively). Biochar applied at 20 t ha^-1 decreased the threshold elemental ratio of C:P at which microbial growth switches between nutrient and C limitations, suggesting a shift towards C relative to nutrient (P) limitation. This study concludes that biochar produced from manure compost can be useful in increasing microbial growth by alleviating P limitations in a wheat-canola rotation.