Nitrification inhibitor chlorate and nitrogen substrates differentially affect comammox Nitrospira in a grassland soil.

Introduction: Through the combined use of two nitrification inhibitors, Dicyandiamide (DCD) and chlorate with nitrogen amendment, this study aimed to investigate the contribution of comammox Nitrospira clade B, ammonia oxidizing bacteria (AOB) and archaea (AOA) to nitrification in a high fertility grassland soil, in a 90-day incubation study. Methods: The soil was treated with nitrogen (N) at three levels: 0 mg-N kg-1 soil, 50 mg-N kg-1 soil, and 700 mg-N kg-1 soil, with or without the two nitrification inhibitors. The abundance of comammox Nitrospira, AOA, AOB, and nitrite oxidising bacteria (NOB) was measured using qPCR. The comammox Nitrospira community structure was assessed using Illumina sequencing. Results and Discussion: The results showed that the application of chlorate inhibited the oxidation of both NH4+ and NO2- in all three nitrogen treatments. The application of chlorate significantly reduced the abundance of comammox Nitrospira amoA and nxrB genes across the 90-day experimental period. Chlorate also had a significant effect on the beta diversity (Bray-Curtis dissimilarity) of the comammox Nitrospira clade B community. Whilst AOB grew in response to the N substrate additions and were inhibited by both inhibitors, AOA showed litle or no response to either the N substrate or inhibitor treatments. In contrast, comammox Nitrospira clade B were inhibited by the high ammonium concentrations released from the urine substrates. These results demonstrate the differential and niche responses of the three ammonia oxidising communities to N substrate additions and nitrification inhibitor treatments. Further research is needed to investigate the specificity of the two inhibitors on the different ammonia oxidising communities.

Meta-analysis shows the impacts of ecological restoration on greenhouse gas emissions.

International initiatives set ambitious targets for ecological restoration, which is considered a promising greenhouse gas mitigation strategy. Here, we conduct a meta-analysis to quantify the impacts of ecological restoration on greenhouse gas emissions using a dataset compiled from 253 articles. Our findings reveal that forest and grassland restoration increase CH4 uptake by 90.0% and 30.8%, respectively, mainly due to changes in soil properties. Conversely, wetland restoration increases CH4 emissions by 544.4%, primarily attributable to elevated water table depth. Forest and grassland restoration have no significant effect on N2O emissions, while wetland restoration reduces N2O emissions by 68.6%. Wetland restoration enhances net CO2 uptake, and the transition from net CO2 sources to net sinks takes approximately 4 years following restoration. The net ecosystem CO2 exchange of the restored forests decreases with restoration age, and the transition from net CO2 sources to net sinks takes about 3-5 years for afforestation and reforestation sites, and 6-13 years for clear-cutting and post-fire sites. Overall, forest, grassland and wetland restoration decrease the global warming potentials by 327.7%, 157.7% and 62.0% compared with their paired control ecosystems, respectively. Our findings suggest that afforestation, reforestation, rewetting drained wetlands, and restoring degraded grasslands through grazing exclusion, reducing grazing intensity, or converting croplands to grasslands can effectively mitigate greenhouse gas emissions.

Soil organic carbon loss decreases biodiversity but stimulates multitrophic interactions that promote belowground metabolism.

Soil organic carbon (SOC) plays an essential role in mediating community structure and metabolic activities of belowground biota. Unraveling the evolution of belowground communities and their feedback mechanisms on SOC dynamics helps embed the ecology of soil microbiome into carbon cycling, which serves to improve biodiversity conservation and carbon management strategy under global change. Here, croplands with a SOC gradient were used to understand how belowground metabolisms and SOC decomposition were linked to the diversity, composition, and co-occurrence networks of belowground communities encompassing archaea, bacteria, fungi, protists, and invertebrates. As SOC decreased, the diversity of prokaryotes and eukaryotes also decreased, but their network complexity showed contrasting patterns: prokaryotes increased due to intensified niche overlap, while that of eukaryotes decreased possibly because of greater dispersal limitation owing to the breakdown of macroaggregates. Despite the decrease in biodiversity and SOC stocks, the belowground metabolic capacity was enhanced as indicated by increased enzyme activity and decreased enzymatic stoichiometric imbalance. This could, in turn, expedite carbon loss through respiration, particularly in the slow-cycling pool. The enhanced belowground metabolic capacity was dominantly driven by greater multitrophic network complexity and particularly negative (competitive and predator-prey) associations, which fostered the stability of the belowground metacommunity. Interestingly, soil abiotic conditions including pH, aeration, and nutrient stocks, exhibited a less significant role. Overall, this study reveals a greater need for soil C resources across multitrophic levels to maintain metabolic functionality as declining SOC results in biodiversity loss. Our researchers highlight the importance of integrating belowground biological processes into models of SOC turnover, to improve agroecosystem functioning and carbon management in face of intensifying anthropogenic land-use and climate change.

Global dataset of soil organic carbon in tidal marshes.

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha-1 in the top 30 cm and 231 ± 134 Mg SOC ha-1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

Non-native Brachiaria humidicola with biological nitrification inhibition capacity stimulates in situ grassland N2O emissions.

Introduction: Brachiaria humidicola, a tropical grass, could release root exudates with biological nitrification inhibition (BNI) capacity and reduce soil nitrous oxide (N2O) emissions from grasslands. However, evidence of the reduction effect in situ in tropical grasslands in China is lacking. Methods: To evaluate the potential effects of B. humidicola on soil N2O emissions, a 2-year (2015-2017) field experiment was established in a Latosol and included eight treatments, consisting of two pastures, non-native B. humidicola and a native grass, Eremochloa ophiuroide, with four nitrogen (N) application rates. The annual urea application rates were 0, 150, 300, and 450 kg N ha-1. Results: The average 2-year E. ophiuroides biomass with and without N fertilization were 9.07-11.45 and 7.34 t ha-1, respectively, and corresponding values for B. humidicola increased to 31.97-39.07 and 29.54 t ha-1, respectively. The N-use efficiencies under E. ophiuroide and B. humidicola cultivation were 9.3-12.0 and 35.5-39.4%, respectively. Annual N2O emissions in the E. ophiuroides and B. humidicola fields were 1.37 and 2.83 kg N2O-N ha-1, respectively, under no N fertilization, and 1.54-3.46 and 4.30-7.19 kg N2O-N ha-1, respectively, under N fertilization. Discussions: According to the results, B. humidicola cultivation increased soil N2O emissions, especially under N fertilization. This is because B. humidicola exhibited the more effective stimulation effect on N2O production via denitrification primarily due to increased soil organic carbon and exudates than the inhibition effect on N2O production via autotrophic nitrification. Annual yield-scaled N2O emissions in the B. humidicola treatment were 93.02-183.12 mg N2O-N kg-1 biomass, which were significantly lower than those in the E. ophiuroides treatment. Overall, our results suggest that cultivation of the non-native grass, B. humidicola with BNI capacity, increased soil N2O emissions, while decreasing yield-scaled N2O emissions, when compared with native grass cultivation.

Attenuation of Methane Oxidation by Nitrogen Availability in Arctic Tundra Soils.

CH4 emission in the Arctic has large uncertainty due to the lack of mechanistic understanding of the processes. CH4 oxidation in Arctic soil plays a critical role in the process, whereby removal of up to 90% of CH4 produced in soils by methanotrophs can occur before it reaches the atmosphere. Previous studies have reported on the importance of rising temperatures in CH4 oxidation, but because the Arctic is typically an N-limited system, fewer studies on the effects of inorganic nitrogen (N) have been reported. However, climate change and an increase of available N caused by anthropogenic activities have recently been reported, which may cause a drastic change in CH4 oxidation in Arctic soils. In this study, we demonstrate that excessive levels of available N in soil cause an increase in net CH4 emissions via the reduction of CH4 oxidation in surface soil in the Arctic tundra. In vitro experiments suggested that N in the form of NO3- is responsible for the decrease in CH4 oxidation via influencing soil bacterial and methanotrophic communities. The findings of our meta-analysis suggest that CH4 oxidation in the boreal biome is more susceptible to the addition of N than in other biomes. We provide evidence that CH4 emissions in Arctic tundra can be enhanced by an increase of available N, with profound implications for modeling CH4 dynamics in Arctic regions.

Comammox Nitrospira Clade B is the most abundant complete ammonia oxidizer in a dairy pasture soil and inhibited by dicyandiamide and high ammonium concentrations.

The recent discovery of comammox Nitrospira, a complete ammonia oxidizer, capable of completing the nitrification on their own has presented tremendous challenges to our understanding of the nitrification process. There are two divergent clades of comammox Nitrospira, Clade A and B. However, their population abundance, community structure and role in ammonia and nitrite oxidation are poorly understood. We conducted a 94-day microcosm study using a grazed dairy pasture soil amended with urea fertilizers, synthetic cow urine, and the nitrification inhibitor, dicyandiamide (DCD), to investigate the growth and community structure of comammox Nitrospira spp. We discovered that comammox Nitrospira Clade B was two orders of magnitude more abundant than Clade A in this fertile dairy pasture soil and the most abundant subcluster was a distinctive phylogenetic uncultured subcluster Clade B2. We found that comammox Nitrospira Clade B might not play a major role in nitrite oxidation compared to the role of canonical Nitrospira nitrite-oxidizers, however, comammox Nitrospira Clade B is active in nitrification and the growth of comammox Nitrospira Clade B was inhibited by a high ammonium concentration (700 kg synthetic urine-N ha-1) and the nitrification inhibitor DCD. We concluded that comammox Nitrospira Clade B: (1) was the most abundant comammox in the dairy pasture soil; (2) had a low tolerance to ammonium and can be inhibited by DCD; and (3) was not the dominant nitrite-oxidizer in the soil. This is the first study discovering a new subcluster of comammox Nitrospira Clade B2 from an agricultural soil.

Relationship between nitrapyrin and varying nitrogen application rates with nitrous oxide emissions and nitrogen use efficiency in a maize filed.

Reducing nitrogen losses can be accomplished by mixing fertilizers with nitrification inhibitors (NI). In some agricultural systems, increasing soil N supply capacity by the use of NI could lead to improved N use efficiency (NUE) and increased crop yields. This study examined the effect of different N rates and NI in maize in the north of Iran. The maize was fertilized with urea at three levels (69, 115 and 161 kg N.ha-1) alone or with nitrapyrin as NI. Increasing the N application rate resulted in a considerable rise in growing-season N2O emissions. When nitrapyrin was used, N2O emissions were dramatically reduced. NI treatment reduced N2O emissions in the growth season by 88%, 88%, and 69% in 69, 115, and 161 kg of N.ha-1, respectively. NI treatment reduced yield-scaled N2O emissions; the lowest quantity of yield-scaled N2O was found in 69 N + NI (0.09 g N2O-N kg-1 N uptake). Additionally, grain yield increased by 19%, 31% and 18.4% after applying NI to 69 N, 115 N, and N69, N115 and N161. Results showed that 115 N + NI and N69 treatments showed the highest (65%) and lowest (29%) NUEs, respectively. Finally, our findings show that NI can reduce N2O emissions while increasing NUE and yield, but that the application method and rate of nitrapyrin application need to be improved in order to maximize its mitigation potential.

Oxygen availability regulates the quality of soil dissolved organic matter by mediating microbial metabolism and iron oxidation.

Dissolved organic matter (DOM) plays a vital role in biogeochemical processes and in determining the responses of soil organic matter (SOM) to global change. Although the quantity of soil DOM has been inventoried across diverse spatio-temporal scales, the underlying mechanisms accounting for variability in DOM dynamics remain unclear especially in upland ecosystems. Here, a gradient of SOM storage across 12 croplands in northeast China was used to understand links between DOM dynamics, microbial metabolism, and abiotic conditions. We assessed the composition, biodegradability, and key biodegradable components of DOM. In addition, SOM and mineral-associated organic matter (MAOM) composition, soil enzyme activities, oxygen availability, soil texture, and iron (Fe), Fe-bound organic matter, and nutrient concentrations were quantified to clarify the drivers of DOM quality (composition and biodegradability). The proportion of biodegradable DOM increased exponentially with decreasing initial DOM concentration due to larger fractions of depolymerized DOM that was rich in small-molecular phenols and proteinaceous components. Unexpectedly, the composition of DOM was decoupled from that of SOM or MAOM, but significantly related to enzymatic properties. These results indicate that microbial metabolism exhibited a dominant role in DOM generation. As DOM concentration declined, increased soil oxygen availability regulated DOM composition and enhanced its biodegradability mainly through mediating microbial metabolism and Fe oxidation. The oxygen-induced oxidation of Fe(II) to Fe(III) removed complex DOM compounds with large molecular weight. Moreover, increased oxygen availability stimulated oxidase-catalyzed depolymerization of aromatic substances, and promoted production of protein-like DOM components due to lower enzymatic C/N acquisition ratio. As global changes in temperature and moisture will have large impacts on soil oxygen availability, the role of oxygen in regulating DOM dynamics highlights the importance of integrating soil oxygen supply with microbial metabolism and Fe redox status to improve model predictions of soil carbon under climate change.

Non-native plant invasion can accelerate global climate change by increasing wetland methane and terrestrial nitrous oxide emissions.

Approximately 17% of the land worldwide is considered highly vulnerable to non-native plant invasion, which can dramatically alter nutrient cycles and influence greenhouse gas (GHG) emissions in terrestrial and wetland ecosystems. However, a systematic investigation of the impact of non-native plant invasion on GHG dynamics at a global scale has not yet been conducted, making it impossible to predict the exact biological feedback of non-native plant invasion to global climate change. Here, we compiled 273 paired observational cases from 94 peer-reviewed articles to evaluate the effects of plant invasion on GHG emissions and to identify the associated key drivers. Non-native plant invasion significantly increased methane (CH4 ) emissions from 129 kg CH4 ha-1  year-1 in natural wetlands to 217 kg CH4 ha-1  year-1 in invaded wetlands. Plant invasion showed a significant tendency to increase CH4 uptakes from 2.95 to 3.64 kg CH4 ha-1  year-1 in terrestrial ecosystems. Invasive plant species also significantly increased nitrous oxide (N2 O) emissions in grasslands from an average of 0.76 kg N2 O ha-1  year-1 in native sites to 1.35 kg N2 O ha-1  year-1 but did not affect N2 O emissions in forests or wetlands. Soil organic carbon, mean annual air temperature (MAT), and nitrogenous deposition (N_DEP) were the key factors responsible for the changes in wetland CH4 emissions due to plant invasion. The responses of terrestrial CH4 uptake rates to plant invasion were mainly driven by MAT, soil NH4 + , and soil moisture. Soil NO3 - , mean annual precipitation, and N_DEP affected terrestrial N2 O emissions in response to plant invasion. Our meta-analysis not only sheds light on the stimulatory effects of plant invasion on GHG emissions from wetland and terrestrial ecosystems but also improves our current understanding of the mechanisms underlying the responses of GHG emissions to plant invasion.

Long-Term Compost Amendment Spurs Cellulose Decomposition by Driving Shifts in Fungal Community Composition and Promoting Fungal Diversity and Phylogenetic Relatedness.

Cellulose is the most abundant polysaccharide in plant biomass and an important precursor of soil organic matter formation. Fungi play a key role in carbon cycling dynamics because they tend to decompose recalcitrant materials. Here, we applied [12C]cellulose and [13C]cellulose to distinguish the effects of application of compost, nitrogen-phosphorus-potassium (NPK) fertilizer, and no fertilizer (control) for 27 years upon cellulose decomposition via RNA-based stable isotope probing (RNA-SIP). The loss ratio of added cellulose C in compost soil was 67.6 to 106.7% higher than in NPK and control soils during their 20-day incubation. Dothideomycetes (mainly members of the genus Cryptococcus) dominated cellulose utilization in compost soil, whereas the copiotrophic Sordariomycetes were more abundant in NPK and unfertilized soils. Compared with NPK and control soils, compost application increased the diversity of 13C-assimilating fungi. The 13C-labeled fungal communities in compost soil were more phylogenetically clustered and exhibited greater species relatedness than those in NPK and control soils, perhaps because of stringent filtering of narrow-spectrum organic resources and biological invasion originating from added compost. These changes led to an augmented decomposition capacity of fungal species for cellulose-rich substrates and reduced cellulose C sequestration efficiency. The RNA-SIP technique is more sensitive to responses of fungi to altered soil resource availability than DNA-SIP. Overall, long-term compost application modified fungal community composition and promoted fungal diversity and phylogenetic relatedness, accelerating the decomposition of substrate cellulose in soil. This work also highlights the RNA-SIP technique's value for comprehensively assessing the contributions of active fungi to the substrate decomposition process. IMPORTANCE Cellulose is a very rich component in plant biomass and an important precursor of soil organic matter formation. Fungal communities are known to be important drivers of organic carbon accumulation in arable soils. However, current understanding of responses of fungal species to cellulose amendment and the contributions of active fungi to substrate decomposition process is still very superficial. Here, we established a [13C]cellulose microcosm experiment with soils subjected to long-term application of compost, nitrogen-phosphorus-potassium (NPK) fertilizer, and no fertilizer (control). The novel 13C-RNA-SIP technique with subsequent high-throughput sequencing was used to investigate the linkages between active fungal taxa and cellulose decomposition. Our study demonstrated that Dothideomycetes dominated cellulose utilization in compost soil, whereas the copiotrophic Sordariomycetes were more enriched in both NPK and unfertilized soils. We also found that the compost amendment promoted fungal diversity and phylogenetic relatedness and strengthened the decomposition capacity of fungi for cellulose-rich substrates by enhancing synergistic interactions, thereby reducing cellulose C sequestration efficiency. Overall, our research has implications for our understanding of the role of active fungi in cellulose C transformation in soils undergoing different types of long-term nutrient management.

Fungal key players of cellulose utilization: Microbial networks in aggregates of long-term fertilized soils disentangled using 13C-DNA-stable isotope probing.

Long-term compost application accelerates organic carbon (C) accumulation and macroaggregate formation in soil. Stable aggregates and high soil organic C (SOC) content are supposed to increase microbiota activity and promote transformation of litter compounds (i.e., cellulose) into SOC. Here, we used 13C-DNA-stable isotope probing with subsequent high-throughput sequencing to characterize fungal succession and co-occurrence trends during 13C-cellulose decomposition in aggregate size classes in soils subjected to no fertilizer (control), nitrogen-phosphorus­potassium (NPK) fertilizers, and compost (Compost) application for 27 years. Ascomycota (mostly saprotrophic fungi) were always highly competitive for cellulose in all aggregate size classes at the early stages of cellulose decomposition (20 days). Compost-treated soil was enriched with Ascomycota compared to the control soil, wherein Sordariomycetes, the majority, strongly dominated the cellulose utilization (13C incorporation in DNA). 13C-labeled fungal communities converged in the Compost soil, with lower abundance and diversity compared with the NPK and control soils. Such convergence led to greater cellulose decomposition, indicating that compost amendment increased the capacity of a few dominant fungal taxa to decompose litter. Compost soil had more 13C-labeled fungal decomposers in microaggregates and lower fungal decomposers in macroaggregates when compared with the levels in the NPK and control soils. This implies that compost application facilitates fungal colonization towards smaller aggregates. Fungal interactions were reinforced in microaggregates (<250 µm), with more positive associations than those in macroaggregates (>250 µm), indicating greater fungal synergism for recalcitrant resource utilization in microaggregates. The keystone taxa in the co-occurrence networks were not related to cellulose decomposition in microaggregates, but did in macroaggregates. The findings advance a process-based understanding of cellulose utilization by fungal key players based on C and energy availability and the regulation of microbial activity at the aggregate level.

Combined biochar and double inhibitor application offsets NH3 and N2O emissions and mitigates N leaching in paddy fields.

The effects of combined biochar and double inhibitor application on gaseous nitrogen (N; nitrous oxide [N2O] and ammonia [NH3]) emissions and N leaching in paddy soils remain unclear. We investigated the effects of biochar application at different rates and double inhibitor application (hydroquinone [HQ] and dicyandiamide [DCD]) on NH3 and N2O emissions, N leaching, as well as rice yield in a paddy field, with eight treatments, including conventional urea N application at 280 kg N ha-1 (CN); reduced N application at 240 kg N ha-1 (RN); RN + 7.5 t ha-1 biochar (RNB1); RN + 15 t ha-1 biochar (RNB2); RN + HQ + DCD (RNI); RNB1 + HQ + DCD (RNIB1); RNB2 + HQ + DCD (RNIB2); and a control without N fertilizer. When compared with N leaching under RN, biochar application reduced total N leaching by 26.9-34.8% but stimulated NH3 emissions by 13.2-27.1%, mainly because of enhanced floodwater and soil NH4+-N concentrations and pH, and increased N2O emission by 7.7-21.2%, potentially due to increased soil NO3--N concentrations. Urease and nitrification inhibitor addition decreased NH3 and N2O emissions, and total N leaching by 20.1%, 21.5%, and 22.1%, respectively. Compared with RN, combined biochar (7.5 t ha-1) and double inhibitor application decreased NH3 and N2O emissions, with reductions of 24.3% and 14.6%, respectively, and reduced total N leaching by up to 45.4%. Biochar application alone or combined with double inhibitors enhanced N use efficiency from 26.2% (RN) to 44.7% (RNIB2). Conversely, double inhibitor application alone or combined with biochar enhanced rice yield and reduced yield-scaled N2O emissions. Our results suggest that double inhibitor application alone or combined with 7.5 t ha-1 biochar is an effective practice to mitigate NH3 and N2O emission and N leaching in paddy fields.

Divergent responses of wetland methane emissions to elevated atmospheric CO2 dependent on water table.

Elevated atmospheric CO2 may have consequences for methane (CH4) emissions from wetlands, yet the magnitude and direction remain unpredictable, because the associated mechanisms have not been fully investigated. Here, we established an in situ macrocosm experiment to compare the effects of elevated CO2 (700 ppm) on the CH4 emissions from two wetlands: an intermittently inundated Calamagrostis angustifolia marsh and a permanently inundated Carex lasiocarpa marsh. The elevated CO2 increased CH4 emissions by 27.6-57.6% in the C. angustifolia marsh, compared to a reduction of 18.7-23.5% in the C. lasiocarpa marsh. The CO2-induced increase in CH4 emissions from the C. angustifolia marsh was paralleled with (1) increased dissolved organic carbon (DOC) released from plant photosynthesis and (2) reduced (rate of) CH4 oxidation due to a putative shift in methanotrophic community composition. In contrast, the CO2-induced decrease in CH4 emissions from the C. lasiocarpa marsh was associated with the increases in soil redox potential and pmoA gene abundance. We synthesized data from worldwide wetland ecosystems, and found that the responses of CH4 emissions to elevated CO2 was determined by the wetland water table levels and associated plant oxygen secretion capacity. In conditions with elevated CO2, plants with a high oxygen secretion capacity suppress CH4 emissions while plants with low oxygen secretion capacity stimulate CH4 emissions; both effects are mediated via a feedback loop involving shifts in activities of methanogens and methanotrophs.

Effect of field-aged biochar on fertilizer N retention and N2O emissions: A field microplot experiment with 15N-labeled urea.

Biochar application is thought to improve crop yield and reduce N leaching and gas emissions; however, little is known about how field-aged biochar affects fertilizer N retention and N2O emissions. Here, a field microplot experiment is established in the North China Plain at maize season by applying 15N-labeled urea to the sandy loam soil both with (Biochar) and without (Control) application of 3-year field-aged biochar at 12 t ha-1. Overall, 25.6-26.2% of the urea N was taken up by maize aboveground biomass, field-aged biochar did not affect yield or fertilizer N recovery efficiency. After maize harvest, the residual ratio of applied N in the soil profile (0-40 cm) was 21.6 and 20.3% under Control and Biochar treatment, respectively, with an increase of 10.2% in the topsoil (0-20 cm) and decrease of 37.2% in the subsoil (20-40 cm) following biochar amendment, probably due to reduced NO3- leaching. Cumulative N2O emissions and urea N-induced N2O emissions under Control treatment were 2.06 and 0.78 kg N ha-1, and significantly decreased to 1.89 and 0.74 kg N ha-1 after Biochar treatment, respectively. N2O emissions derived from the applied N accounted for 38.0 and 39.4% of the total emissions under Control and Biochar treatment, respectively. N2O emissions from decomposition of soil organic N induced by the priming effect of the applied N was 0.69 and 0.56 kg N ha-1 under Control and Biochar treatment, respectively, contributing 33.7 and 29.7% of the total emissions. Overall, our results suggest that field-aged biochar increased the retention of fertilizer N in the topsoil by reducing NO3- leaching, while effectively reduced N2O emissions from fertilizer N and mineralization of organic N in the sandy loam soil.

Corn cobs efficiently reduced ammonia volatilization and improved nutrient value of stored dairy effluents.

Dairy farms produce considerable quantities of nutrient-rich effluent, which is generally stored before use as a soil amendment. Unfortunately, a portion of the dairy effluent N can be lost through volatilization during open pond storage to the atmosphere. Adding of covering materials to effluent during storage could increase contact with NH4+ and modify effluent pH, thereby reducing NH3 volatilization and retaining the effluent N as fertilizer for crop application. Here the mitigation effect of cover materials on ammonia (NH3) volatilization from open stored effluents was measured. A pilot-scale study was conducted using effluent collected at the Youran Dairy Farm Company Limited, Luhe County, Jiangsu, China, from 15 June to 15 August 2019. The study included seven treatments: control without amendment (Control), 30-mm × 25-mm corn cob pieces (CC), light expanded clay aggregate - LECA (CP), lactic acid (LA) and lactic acid plus CC (CCL), CP (CPL) or 20-mm plastic balls (PBL). The NH3 emission from the Control treatment was 120.1 g N m-2, which was increased by 38.1% in the CP treatment, possibly due to increased effluent pH. The application of CC reduced NH3 loss by 69.2%, compared with the Control, possibly due to high physical resistance, adsorption of NH4+ and effluent pH reduction. The lactic acid amendment alone and in combination with other materials also reduced NH3 volatilization by 27.4% and 31.0-46.7%, respectively. After 62 days of storage, effluent N conserved in the CC and CCL treatments were 21.0% and 22.0% higher than that in the Control (P < 0.05). Our results suggest that application of corn cob pieces, alone or in combination with lactic acid, as effluent cover could effectively mitigate NH3 volatilization and retain N, thereby enhancing the fertilizer value of the stored dairy effluent and co-applied as a soil amendment after two months open storage.

Methane and nitrous oxide have separated production zones and distinct emission pathways in freshwater aquaculture ponds.

Aquaculture systems receive intensive carbon (C) and nitrogen (N) loadings, and are therefore recognized as major anthropogenic sources of methane (CH4) and nitrous oxide (N2O) emissions. However, the extensively managed aquaculture ponds were identified as a hotspot of CH4 emission but just a weak N2O source. Here, we investigate annual CH4 and N2O fluxes from three earthen ponds used for crab culture, of different sizes, in southeast China. Our purposes are to identify the spatiotemporal variations of CH4 and N2O emissions and their components among ponds and to evaluate the zone for CH4 and N2O production. Static chamber-measured CH4 flux ranged from 0.03 to 64.7 mg CH4 m‒2 h‒1 (average: 9.02‒14.3 mg CH4 m‒2 h‒1), and temperature, followed by dissolved organic C (DOC) concentration, and redox potential, were the primary drivers of seasonal CH4 flux patterns. Annual mean diffusive CH4 flux was 1.80‒2.34 mg CH4 m‒2 h‒1, and that by ebullition was up to 7.20‒12.0 mg CH4 m‒2 h‒1 (79.1‒83.5% of the total CH4 flux). Annual CH4 emission was positively correlated with sediment DOC concentration but negatively (P < 0.05) correlated with water depth across ponds, with the highest CH4 emission occurred in a pond with low water depth and high DOC concentration. The calculated diffusive N2O flux by the gas transfer velocity was 0.32‒0.60 times greater than the measured N2O emission, suggesting that N2O in water column can not only evade as water-air fluxes but diffuse downwards and to be consumed in anaerobic sediments. This also indicates that N2O was primarily produced in water column. The highly reduced condition and depletion of NO3‒-N in sediments, can limit N2O production from both nitrification and denitrification but favor N2O consumption, leading the ponds to become a weak source of N2O annually and even a sink of N2O in summer. Our results highlight that the current global CH4 budget for inland waters is probably underestimated due to a lack of data and underestimation of the contribution of ebullitive CH4 flux in small lentic waters. The downwards N2O diffusion from the water column into sediment also indicates that the extensively-used model approach based on gas transfer velocity potentially overestimates N2O fluxes, especially in small eutrophic aquatic ecosystems.

Microbial decomposition of soil organic matter determined by edaphic characteristics of mangrove forests in East Asia.

Mangrove forests cover only 0.1% of the world's continental area; however, these are a substantial carbon sink owing to the high primary production and low rate of decomposition of soil organic matter (SOM). The extremely low decomposition rate of SOM in mangrove forests is believed to be caused by low oxygen and nutrient availability as well as recalcitrant biomass from mangrove. However, only a few studies have addressed the microbial mechanism that plays a key role in the decomposition of SOM. In this study, the decomposition of SOM were determined by conducting a field survey and an lab incubation experiment using soil samples from mangrove forests in three regions; Okinawa, Shenzhen, and Hong Kong. In particular, we examined the occurrence of the enzymic latch mechanism, which involves phenolic inhibition of enzymic decomposition, in mangrove forest soils that highlights the importance of phenol oxidase as a key controlling factor. The results clearly showed that enzymic latch involved in the accumulation of SOM in the mangroves of Shenzhen and Hong Kong, whereas the accumulation of SOM in Okinawa was controlled by other mechanisms, such as the iron gate mechanism, which involves stabilization of soil carbon in iron-SOM complexes. The characteristics of mangrove forests, such as iron concentration, were shown as substantial determination factors in the dynamics of SOM. We concluded that the decomposition of SOM were strongly affected by the characteristics of mangrove forests, and the occurrence of enzymic latch in mangrove forests has a potential application in geoengineering technology to enhance the carbon sequestration capacity of mangrove forests.