Global Ecosystem Nitrogen Cycling Reciprocates Between Land-Use Conversion and Its Reversal.

Anthropogenic land-use practices influence ecosystem functions and the environment. Yet, the effect of global land-use change on ecosystem nitrogen (N) cycling remains unquantified despite that ecosystem N cycling plays a critical role in maintaining food security. Here, we analysed 2430 paired observations globally to show that converting natural to managed ecosystems increases ratios of autotrophic nitrification to ammonium immobilisation and nitrate to ammonium, but decreases soil immobilisation of mineral N, causing increased N losses via leaching and gaseous N emissions, such as nitrous oxide (e.g., via denitrification), resulting in a leaky N cycle. Changing land use from intensively managed to one that resembles natural ecosystems reversed N losses by 108% on average, resulting in a more conservative N cycle. Structural equation modelling revealed that changes in soil organic carbon, pH and carbon to N ratio were more important than changes in soil moisture content and temperature in predicting ecosystem N retention capacities following land-use conversion and its reversion. The hotspots of leaky N cycles were mostly in equatorial and tropical regions, as well as in Western Europe, the United States and China. Our results suggest that whether an ecosystem exhibits a conservative N cycle after land-use reversion depends on management practices.

Resource availability enhances positive tree functional diversity effects on carbon and nitrogen accrual in natural forests.

Forests harbor extensive biodiversity and act as a strong global carbon and nitrogen sink. Although enhancing tree diversity has been shown to mitigate climate change by sequestering more carbon and nitrogen in biomass and soils in manipulative experiments, it is still unknown how varying environmental gradients, such as gradients in resource availability, mediate the effects of tree diversity on carbon and nitrogen accrual in natural forests. Here, we use Canada's National Forest Inventory data to explore how the relationships between tree diversity and the accumulation of carbon and nitrogen in tree biomass and soils vary with resource availability and environmental stressors in natural forests. We find that the positive relationship between tree functional diversity (rather than species richness) and the accumulation of carbon in tree biomass strengthens with increasing light and soil nutrient availability. Moreover, the positive relationship between tree functional diversity and the accumulation of carbon and nitrogen in both organic and mineral soil horizons is more pronounced at sites with greater water and nutrient availabilities. Our results highlight that conserving and promoting functionally diverse forests in resource-rich environments could play a greater role than in resource-poor environments in enhancing carbon and nitrogen sequestration in Canada's forests.

Rising atmospheric carbon dioxide concentrations increase gaps of rice yields between low- and middle-to-high-income countries.

The rising carbon dioxide concentrations are expected to increase future rice yields. However, variations in the CO2 fertilization effect (CFE) between rice subspecies and the influence of concurrent global warming introduce uncertainty in future global rice yield projections. Here we conducted a meta-analysis of rising carbon dioxide field experiments and employed crop modelling to assess future global rice yields for the top 14 rice producing countries. We found a robust parabolic relationship between rice CFE and temperature, with significant variations between rice subspecies. Our projections indicate that global rice production in the 2050s is expected to increase by 50.32 million tonnes (7.6%) due to CFE compared with historical production. Because low-income countries will experience higher temperatures, the gaps (difference of Δyield) between middle-to-high-income and low-income countries are projected to widen from the 2030s to the 2090s under elevated carbon dioxide. These findings underscore the critical role of CFE and emphasize the necessity to increase investments in research and technology for rice producing systems in low-income countries.

Biochar amendment alleviates soil microbial nitrogen and phosphorus limitation and increases soil heterotrophic respiration under long-term nitrogen input in a subtropical forest.

Nitrogen (N) and carbon (C) inputs substantially affect soil microbial functions. However, the influences of long-term N and C additions on soil microbial resource limitation and heterotrophic respiration-fundamental microbial functional traits-remain unclear, impeding the understanding of how soil C dynamics respond to global change. In this study, the responses of soil microbial resource limitation and heterotrophic respiration (Rh) to 7-year N and biochar (BC) additions in a subtropical Moso bamboo (Phyllostachys edulis) plantation were investigated. We used eight treatments: Control, no N and BC addition; N30, 30 kg N (ammonium nitrate)·hm-2·a-1; N60, 60 kg N·hm-2·a-1; N90, 90 kg N·hm-2·a-1; BC20, 20 t BC (originating from Moso bamboo chips) hm-2; N30 + BC20, 30 kg N·hm-2·a-1 + 20 t BC hm-2; N60 + BC20, 60 kg N·hm-2·a-1 + 20 t BC hm-2; and N90 + BC20, 90 kg N·hm-2·a-1 + 20 t BC hm-2. Soil microbes were co-limited by N and phosphorus (P) and not limited by C in the control treatments. Long-term N addition enhanced soil microbial N and P limitation but significantly reduced soil Rh by 15.1 %-20.0 % relative to that in the control treatments. BC amendment alleviated soil microbial N and P limitation and significantly decreased C use efficiency by 10.9 %-42.1 % but increased Rh by 33.6 %-91.6 % in the long-term N-free and N-supplemented treatments (P < 0.05). Soil C- and N-acquisition enzyme activities were the dominant drivers of soil microbial resource limitation. Furthermore, microbial resource limitation was a more reliable predictor of Rh than soil resources or microbial biomass. The results suggested that long-term N and BC additions affect Rh by regulating microbial resource limitation, highlighting its significance in understanding soil C cycling under environmental change.

Optimizing tradeoff strategies of soil microbial community between metabolic efficiency and resource acquisition along a natural regeneration chronosequence.

The tradeoff between community-level soil microbial metabolic efficiency and resource acquisition strategies during natural regeneration remains unclear. Herein, we examined variations in soil extracellular enzyme activity, microbial metabolic quotient (qCO2), and microbial carbon use efficiency (CUE) along a chronosequence of natural regeneration by sampling secondary forests at 1, 10, 20, 30, 40, and 100 years after rubber plantation (RP) clearance. The results showed that the natural logarithms of carbon (C)-, nitrogen (N)-, and phosphorus (P)-acquiring enzyme activities were 1:1.68:1.37 and 1:1.54:1.38 in the RP and secondary forests, respectively, thus demonstrating that microbial metabolism was co-limited by N and P. Moreover, the soil microbial C limitation initially increased (1-40 years) and later decreased (100 years). Overall, the qCO2 increased, decreased, and then increased again in the initial (< 10 years), middle (10-40 years), and late (100 years) successional stages, respectively. Except for specific P-acquiring enzyme activities, the changes in other indicators with natural regeneration were consistent in the dry and wet seasons. Both qCO2 and CUE were mainly predicted by microbial community composition and physiological traits. These results indicate that soil microbial communities could employ tradeoff strategies between metabolic efficiency and resource acquisition to cope with variations in resources. Our findings provide new information on tradeoff strategies between metabolic efficiency and resource acquisition during natural regeneration.

Soil enzyme profile analysis for indicating decomposer micro-food web.

Highly diverse exoenzymes mediate the energy flow from substrates to the multitrophic microbiota within the soil decomposer micro-food web. Here, we used a "soil enzyme profile analysis" approach to establish a series of enzyme profile indices; those indices were hypothesized to reflect micro-food web features. We systematically evaluated the shifts in enzyme profile indices in relation to the micro-food web features in the restoration of an abandoned cropland to a natural area. We found that enzymatic C:N stoichiometry and decomposability index were significantly associated with substrate availability. Furthermore, the higher Shannon diversity index in the exoenzyme profile, especially for the C-degrading hydrolase, corresponded to a greater microbiota community diversity. The increased complexity and stability of the exoenzyme network reflected similar changes with the micro-food web networks. In addition, the gross activity of the enzyme profile as a parameter for soil multifunctionality, effectively predicted the substrate content, microbiota community size, diversity, and network complexity. Ultimately, the proposed enzymic channel index was closely associated with the traditional decomposition channel indices derived from microorganisms and nematodes. Our results showed that soil enzyme profile analysis reflected very well the decomposer food web features. Our study has important implications for projecting future climate change or anthropogenic disturbance impacts on soil decomposer micro-food web features by using soil enzyme profile analysis.

Emerging contaminants: A One Health perspective.

Environmental pollution is escalating due to rapid global development that often prioritizes human needs over planetary health. Despite global efforts to mitigate legacy pollutants, the continuous introduction of new substances remains a major threat to both people and the planet. In response, global initiatives are focusing on risk assessment and regulation of emerging contaminants, as demonstrated by the ongoing efforts to establish the UN's Intergovernmental Science-Policy Panel on Chemicals, Waste, and Pollution Prevention. This review identifies the sources and impacts of emerging contaminants on planetary health, emphasizing the importance of adopting a One Health approach. Strategies for monitoring and addressing these pollutants are discussed, underscoring the need for robust and socially equitable environmental policies at both regional and international levels. Urgent actions are needed to transition toward sustainable pollution management practices to safeguard our planet for future generations.

Soil metabolomics: Deciphering underground metabolic webs in terrestrial ecosystems.

Soil metabolomics is an emerging approach for profiling diverse small molecule metabolites, i.e., metabolomes, in the soil. Soil metabolites, including fatty acids, amino acids, lipids, organic acids, sugars, and volatile organic compounds, often contain essential nutrients such as nitrogen, phosphorus, and sulfur and are directly linked to soil biogeochemical cycles driven by soil microorganisms. This paper presents an overview of methods for analyzing soil metabolites and the state-of-the-art of soil metabolomics in relation to soil nutrient cycling. We describe important applications of metabolomics in studying soil carbon cycling and sequestration, and the response of soil organic pools to changing environmental conditions. This includes using metabolomics to provide new insights into the close relationships between soil microbiome and metabolome, as well as responses of soil metabolome to plant and environmental stresses such as soil contamination. We also highlight the advantage of using soil metabolomics to study the biogeochemical cycles of elements and suggest that future research needs to better understand factors driving soil function and health.

Distinct effects of canopy vs understory and organic vs inorganic N deposition on root resource acquisition strategies of subtropical Moso bamboo plants.

Atmospheric nitrogen (N) deposition inevitably alters soil nutrient status, subsequently prompting plants to modify their root morphology (i.e., adopting a do-it-yourself strategy), mycorrhizal symbioses (i.e., outsourcing strategy), and root exudation (i.e., nutrient-mining strategy) linking with resource acquisition. However, how N deposition influences the integrated pattern of these resource-acquisition strategies remains unclear. Furthermore, most studies in forest ecosystems have focused on understory N and inorganic N deposition, neglecting canopy-associated processes (e.g., N interception and assimilation) and the impacts of organic N on root functional traits. In this study, we compared the effects of canopy vs understory, organic vs inorganic N deposition on eight root functional traits of Moso bamboo plants. Our results showed that N deposition significantly decreased arbuscular mycorrhizal fungi (AMF) colonization, altered root exudation rate and root foraging traits (branching intensity, specific root area, and length), but did not influence root tissue density and N concentration. Moreover, the impacts of N deposition on root functional traits varied significantly with deposition approach (canopy vs. understory), form (organic vs. inorganic), and their interaction, showing variations in both intensity and direction (positive/negative). Furthermore, specific root area and length were positively correlated with AMF colonization under canopy N deposition and root exudation rate in understory N deposition. Root trait variation under understory N deposition, but not under canopy N deposition, was classified into the collaboration gradient and the conservation gradient. These findings imply that coordination of nutrient-acquisition strategies dependent on N deposition approach. Overall, this study provides a holistic understanding of the impacts of N deposition on root resource-acquisition strategies. Our results indicate that the evaluation of N deposition on fine roots in forest ecosystems might be biased if N is added understory.

Biochar affects compressive strength of Portland cement composites: a meta-analysis.

One strategy to reduce CO2 emissions from cement production is to reduce the amount of Portland cement produced by replacing it with supplementary cementitious materials (SCMs). Biochar is a potential SCM that is an eco-friendly and stable porous pyrolytic material. However, the effects of biochar addition on the performances of Portland cement composites are not fully understood. This meta-analysis investigated the impact of biochar addition on the 7- and 28-day compressive strength of Portland cement composites based on 606 paired observations. Biochar feedstock type, pyrolysis conditions, pre-treatments and modifications, biochar dosage, and curing type all influenced the compressive strength of Portland cement composites. Biochars obtained from plant-based feedstocks (except rice and hardwood) improved the 28-day compressive strength of Portland cement composites by 3-13%. Biochars produced at pyrolysis temperatures higher than 450 °C, with a heating rate of around 10 C min-1, increased the 28-day compressive strength more effectively. Furthermore, the addition of biochar with small particle sizes increased the compressive strength of Portland cement composites by 2-7% compared to those without biochar addition. Biochar dosage of < 2.5% of the binder weight enhanced both compressive strengths, and common curing methods maintained the effect of biochar addition. However, when mixing the cement, adding fine and coarse aggregates such as sand and gravel affects the concrete and mortar's compressive strength, diminishing the effect of biochar addition and making the biochar effect nonsignificant. We concluded that appropriate biochar addition could maintain or enhance the mechanical performance of Portland cement composites, and future research should explore the mechanisms of biochar effects on the performance of cement composites. Supplementary Information: The online version contains supplementary material available at 10.1007/s42773-024-00309-2.

Microplastics affect the ecological stoichiometry of plant, soil and microbes in a greenhouse vegetable system.

Microplastic (MP) pollution is a growing global issue due to its potential threat to ecosystem and human health. Low-density polyethylene (LDPE) MP is the most common type of plastics polluting agricultural soils, negatively affecting soil-microbial-plant systems. However, the effects of LDPE MPs on the carbon (C): nitrogen (N): phosphorus (P) of soil-microbial-plant systems have not been well elucidated. Thus, we conducted a pot experiment with varying LDPE MP concentrations (w/w) (control without MPs; 0.2 % MPs (PE1); 5 % MPs (PE2); and 10 % MPs (PE3)) to study their effects on soil-microbial-plant C-N-P stoichiometry. Soil C:N ratio increased 2.3 and 3.4 times in PE2 and PE3, respectively. Soil C:P ratio increased 2.2 and 3.6 times in PE2 and PE3, respectively. Soil microbial C:N ratios decreased by 46.2 % in PE1, while C:P ratios decreased by 59.2, 38.6, and 67.9 % in PE1, PE2, and PE3, respectively. Soil microbial N:P ratio decreased in PE1 (17.2) and PE3 (59.1 %). MPs increased shoot C content and C:N ratios, particularly at the 5 % MP addition rate. MP addition altered dissolved organic C, N, and P concentrations, depending on the MP addition rate. Microbial community responses to MP exposure were complex, leading to variable effects on different microbial groups at different MP addition rates. Structural equation modeling showed that MP addition had a direct positive effect (ß = 0.96) on soil C-N-P stoichiometry and a direct negative effect (ß = -1.34) on microbial C-N-P stoichiometry. These findings demonstrate the complex interactions between MPs, soil microorganisms, and nutrient dynamics, highlighting the need for further research to better understand the ecological implications of MP pollution in terrestrial ecosystems.

Soil stoichiometric imbalances constrain microbial-driven C and N dynamics in grassland.

Grassland restoration leads to excessive soils with carbon (C) and nitrogen (N) contents that are inadequate to fulfill the requirements of microorganisms. The differences in the stoichiometric ratios of these elements could limit the activity of microorganisms, which ultimately affects the microbial C, N use efficiencies (CUE, NUE) and the dynamics of soil C and N. The present study was aimed at quantifying the soil microbial nutrient limitation and exploring the mechanisms underlying microbial-induced C and N dynamics in chrono-sequence of restored grasslands. It was revealed that grassland restoration increased microbial C, N content, microbial C, N uptake, and microbial CUE and NUE, while the threshold elemental ratio (the C:N ratio) decreased, which is mainly due to the synergistic effect of the microbial biomass and enzymatic stoichiometry imbalance after grassland restoration. Finally, we present a framework for the nutrient limitation strategies that stoichiometric imbalances constrain microbial-driven C and N dynamics. These results are the direct evidence of causal relations between stoichiometric ratios, microbial responses, and soil C, N cycling.

Carbon trade-off and energy budgeting under conventional and conservation tillage in a rice-wheat double cropping system.

Amid rising energy crises and greenhouse gas (GHG) emissions, designing energy efficient, GHG mitigation and profitable conservation farming strategies are pertinent for global food security. Therefore, we tested a hypothesis that no-till with residue retaining could improve energy productivity (EP) and energy use efficiency (EUE) while mitigating the carbon footprint (CF), water footprint (WF) and GHG emissions in rice-wheat double cropping system. We studied two tillage viz., conventional and conservation, with/without residue retaining, resulting as CT0 (puddled-transplanted rice, conventional wheat -residue), CTR (puddled-transplanted rice, conventional wheat + residue), NT0 (direct seeded rice, zero-till wheat -residue), and NTR (direct seeded rice, zero-till wheat + residue). The overall results showed that the NTR/NT0 had 34% less energy consumption and 1.2-time higher EP as compared to CTR/CT0. In addition, NTR increased 19.8% EUE than that of CT0. The grain yield ranged from 8.7 to 9.3 and 7.8-8.5 Mg ha-1 under CT and NT system, respectively. In NTR, CF and WF were 56.6% and 17.9% lower than that of CT0, respectively. The net GHG emissions were the highest (7261.4 kg CO2 ha-1 yr-1) under CT0 and lowest (4580.9 kg CO2 ha-1 yr-1) under NTR. Notably, the carbon sequestration under NTR could mitigate half of the system's CO2-eq emissions. The study results suggest that NTR could be a viable option to offset carbon emissions and water footprint by promoting soil organic carbon sequestration, and enhancing energy productivity and energy use efficiency in the South Asian Indo-Gangetic Plains.

Biochar and soil properties affect remediation of Zn contamination by biochar: A global meta-analysis.

Zinc (Zn) is one of the most common heavy metals that pollute soils and can threaten both environmental and human health. Biochar is a potential solution for remediating soil Zn contamination. This meta-analysis investigates the effect of biochar application on the remediation of Zn-contaminated soils and the factors affecting the remediation efficiency. We found that biochar application in Zn-contaminated soils reduced Zn bioavailability by up to 77.2% in urban soils, 55.1% in acidic soils, and 50.8% in coarse textured soils. Moreover, the remediation efficiency depends on the biochar production condition, with crop straw and sewage sludge feedstocks, high pyrolysis temperature (450-550 °C), low heating rate (<10 °C min-1), and short residence time (<180 min) producing high performing biochars. Biochar affects soil Zn bioavailability by changing soil pH and organic carbon, as well as through its high surface area, ash content, and O-containing surface functional groups. Our findings highlight the role of biochar as a promising and environmentally friendly material for remediating Zn contamination in acidic and/or coarse textured soils. We conclude that soil properties must be considered when selecting biochars for remediating soil Zn contamination.

Biochar modulating soil biological health: A review.

Biochar can be used for multifunctional applications including the improvement of soil health and carbon storage, remediation of contaminated soil and water resources, mitigation of greenhouse gas emissions and odorous compounds, and feed supplementation to improve animal health. A healthy soil preserves microbial biodiversity that is effective in supressing plant pathogens and pests, recycling nutrients for plant growth, promoting positive symbiotic associations with plant roots, improving soil structure to supply water and nutrients, and ultimately enhancing soil productivity and plant growth. As a soil amendment, biochar assures soil biological health through different processes. First, biochar supports habitats for microorganisms due to its porous nature and by promoting the formation of stable soil micro-aggregates. Biochar also serves as a carbon and nutrient source. Biochar alters soil physical and chemical properties, creating optimum soil conditions for microbial diversity. Biochar can also immobilize soil pollutants and reduce their bioavailability that would otherwise inhibit microbial growth. However, depending on the pyrolysis settings and feedstock resources, biochar can be comprised of contaminants including polycyclic aromatic hydrocarbons and potentially toxic elements that can inhibit microbial activity, thereby impacting soil health.

Effects of co-applied biochar and plant growth-promoting bacteria on soil carbon mineralization and nutrient availability under two nitrogen addition rates.

In the background of climate warming, the demand for improving soil quality and carbon (C) sequestration is increasing. The application of biochar to soil has been considered as a method for mitigating climate change and enhancing soil fertility. However, it is uncertain whether the effects of biochar application on C-mineralization and N transformation are influenced by the presence or absence of plant growth-promoting bacteria (PGPB) and soil nitrogen (N) level. An incubation study was conducted to investigate whether the effects of biochar application (0 %, 1 %, 2 % and 4 % of soil mass) on soil respiration, N status, and microbial attributes were altered by the presence or absence of PGPB (i.e., Sphingobium yanoikuyae BJ1) under two soil N levels (N0 and N1 soils as created by the addition of 0 and 0.2 g kg-1 urea- N, respectively). The results showed that biochar, BJ1 strain and their interactive effects on cumulative CO2 emissions were not significant in N0 soils, while the effects of biochar on the cumulative CO2 emissions were dependent on the presence or absence of BJ1 in N1 soils. In N1 soils, applying biochar at 2 % and 4 % increased the cumulative CO2 emissions by 141.0 % and 166.9 %, respectively, when BJ1 was absent. However, applying biochar did not affect CO2 emissions when BJ1 was present. In addition, the presence of BJ1 generally increased ammonium contents in N0 soils, but decreased nitrate contents in N1 soils relative to the absence of BJ1, which indicates that the combination of biochar and BJ1 is beneficial to play the N fixation function of BJ1 in N0 soils. Our results highlight that biochar addition influences not only soil C mineralization but also soil available N, and the direction and magnitude of these effects are highly dependent on the presence of PGPB and the soil N level.

Low-density polyethylene microplastics alter chemical properties and microbial communities in agricultural soil.

Microplastic (MP) pollution in agricultural soils, resulting from the use of plastic mulch, compost, and sewage sludge, jeopardizes the soil microbial populations. However, the effects of MPs on soil chemical properties and microbial communities remain largely unknown. Here, we investigated the effects of different concentration levels (0, 0.1, 1, 3, 5, and 7%; w:w) of low-density polyethylene (LDPE) MPs on the chemical properties and bacterial communities of agricultural soil in an incubation study. The addition of LDPE MPs did not drastically change soil pH (ranging from 8.22 to 8.42). Electrical conductivity increased significantly when the LDPE MP concentrations were between 1 and 7%, whereas the total exchangeable cations (Na+, K+, Mg2+, and Ca2+) decreased significantly at higher LDPE MP concentrations (3-7%). The highest available phosphorus content (2.13 mg kg-1) was observed in 0.1% LDPE MP. Bacterial richness (Chao1 and Ace indices) was the lowest at 0.1% LDPE MP, and diversity indices (Shannon and Invsimpson) were higher at 0 and 1% LDPE MP than at other concentrations. The effect of LDPE MP concentrations on bacterial phyla remained unchanged, but the bacterial abundance varied. The relative abundance of Proteobacteria (25.8-33.0%) was the highest in all treatments. The abundance of Acidobacteria (15.8-17.2%) was also high, particularly in the 0, 0.1, and 1% LDPE MPs. With the increase in LDPE MP concentration, the abundance of Actinobacteria gradually increased from 7.80 to 31.8%. Our findings suggest that different MP concentration levels considerably alter soil chemical properties and microbial composition, which may potentially change the ecological functions of soil ecosystems.

Surface-modified graphene oxide-based composites for advanced sequestration of basic blue 41 from aqueous solution.

Advanced materials for the efficient treatment of textile wastewater need to be developed for the sustainable growth of the textile industry. In this study, graphene oxide (GO) was modified by the incorporation of natural clay (bentonite) and mixed metal oxide (copper-cobalt oxide) to produce GO-based binary and ternary composites. Two binary composites, GO/bentonite and GO/Cu-Co Ox (oxide), and one ternary composite, GO/bentonite/Cu-Co Ox, were characterized by Fourier transform-infrared spectroscopy (FTIR), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and Brunauer-Emmett-Teller (BET) analysis. The adsorption efficiency of these composites was evaluated against a cationic dye, Basic Blue 41 (BB41). The composites had several surface functional groups, and the ternary composite had tubular porous structures formed by the cross-linking of the bentonite and GO planes. The BET surface area of the ternary composite was 50% higher than that of the GO. The BB41 removals were 92, 89, 80, and 69% for GO/bentonite/Cu-Co oxide, GO/bentonite, GO and GO/Cu-Co oxide, respectively. The pseudo-2nd-order and intraparticle diffusion models best describe the kinetics results, indicating chemisorption and slow pore diffusion-controlled adsorption processes. The Langmuir isotherm-derived adsorption capacity of GO/bentonite/Cu-Co oxide was 351.1 mg/g, which was very close to the measured value. After five consecutive cycles, the ternary composite retained 90% BB41 removal efficiency compared to its 1st cycle. Electrostatic interaction and pore diffusion were predicted to be the controlling mechanisms for the adsorption of the BB41. The GO-based ternary composite can be a feasible and scalable adsorbent for BB41 in wastewater treatment.

Pulp mill biosolids mitigate soil greenhouse gas emissions from applied urea and improve soil fertility in a hybrid poplar plantation.

Pulp mill biosolids (hereafter 'biosolids') could be used as an organic amendment to improve soil fertility and promote crop growth; however, it is unclear how the application of biosolids affects soil greenhouse gas emissions and the mechanisms underlying these effects. Here, we conducted a 2-year field experiment on a 6-year-old hybrid poplar plantation in northern Alberta, Canada, to compare the effects of biosolids, conventional mineral fertilizer (urea), and urea + biosolids on soil CO2, CH4 N2O emissions, as well as soil chemical and microbial properties. We found that the addition of biosolids increased soil CO2 and N2O emissions by 21 and 17%, respectively, while urea addition increased their emissions by 30 and 83%, respectively. However, the addition of urea did not affect soil CO2 emissions when biosolids were also applied. The addition of biosolids and biosolids + urea increased soil dissolved organic carbon (DOC) and microbial biomass C (MBC), while urea addition and biosolids + urea addition increased soil inorganic N, available P and denitrifying enzyme activity (DEA). Furthermore, the CO2 and N2O emissions were positively, while the CH4 emissions were negatively associated with soil DOC, inorganic N, available phosphorus, MBC, microbial biomass N, and DEA. In addition, soil CO2, CH4 and N2O emissions were also strongly associated with soil microbial community composition. We conclude that the application of the combination of biosolids and chemical N fertilizer (urea) could be a beneficial approach for both the disposal and use of pulp mill wastes, by reducing greenhouse gas emissions and improving soil fertility.