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.

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.

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.

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.

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.

Combined application of biochar with urease and nitrification inhibitors have synergistic effects on mitigating CH4 emissions in rice field: A three-year study.

Biochar and inhibitors applications have been proposed for mitigating soil greenhouse gas emissions. However, how biochar, inhibitors and the combination of biochar and inhibitors affect CH4 emissions remains unclear in paddy soils. The objective of this study was to explore the effects of biochar application alone, and in combination with urease (hydroquinone) and nitrification inhibitors (dicyandiamide) on CH4 emissions and yield-scaled CH4 emissions during three rice growing seasons in the Taihu Lake region (Suzhou and Jurong), China. In Suzhou, N fertilization rates of 120-280 kg N ha-1 increased CH4 emissions compared to no N fertilization (Control) (P < 0.05), and the highest emission was observed at 240 kg N ha-1, possibly due to the increase in rice-derived organic carbon (C) substrates for methanogens. Biochar amendment combined with N fertilization reduced CH4 emissions by 13.2-27.1% compared with optimal N (ON, Suzhou) and conventional N application (CN-J, Jurong) (P < 0.05). This was related to the reduction in soil dissolved organic C and the increase in soil redox potential. Addition of urease and nitrification inhibitor (ONI) decreased CH4 emissions by 15.7% compared with ON treatment. Combined application of biochar plus urease, nitrification and double inhibitors further decreased CH4 emissions by 22.2-51.0% compared with ON and CN-J treatment. ON resulted in the highest yield-scaled CH4 emissions, while combined application of biochar alone and in combination with the inhibitors decreased yield-scaled CH4 emissions by 12.7-54.9% compared with ON and CN-J treatment (P < 0.05). The lowest yield-scaled CH4 emissions were observed under combined application of 7.5 t ha-1 biochar with both urease and nitrification inhibitors. These findings suggest that combined application of biochar and inhibitors could mitigate total CH4 and yield-scaled CH4 emissions in paddy fields in this region.

Nitrous oxide emissions from China's croplands based on regional and crop-specific emission factors deviate from IPCC 2006 estimates.

Calculated N2O emission factors (EFs) of applied nitrogen (N) fertilizer are currently based upon a single, universal value advocated by the IPCC (Inter-governmental Panel on Climate Change) even though EFs are thought to vary with climate and soil types. Here, we compiled and analyzed 151 N2O EF values from agricultural fields across China. The EF of synthetic N applied to these croplands was 0.60%, on average, but differed significantly among six climatic zones across the country, with the highest EF found in the north subtropical zone for upland fields (0.93%) and the lowest in the middle subtropical zone for paddy fields (0.20%). Precipitation and soil pH, which showed non-linear relationships with EF, are among the factors governing it, explaining 7.0% and 8.0% of the regional variation in EFs, respectively. Annual precipitation was the key factor regulating N2O emissions from synthetic N fertilizers. Among crop types, legume crops had the highest EFs, which were significantly (P < 0.05) higher than those of cereals. Total soil N2O emissions from fertilized croplands with maize, rice, wheat, and vegetables in China, calculated using the climatic zone (regional) EFs, were estimated to be 239 Gg N yr-1 with an uncertainty of 21%. Importantly, this value was substantially (33%) lower than that (357 Gg N yr-1) derived from the IPCC default EF but close to the 253 Gg N yr-1 estimated using crop-specific EFs. N2O emissions from applied synthetic N fertilizer accounted for 66.5% of the total annual N2O emissions from China's maize, rice, wheat and vegetable fields. Taken together, our study's results strongly suggest that regional EFs should be included for accurate N2O inventories from croplands across China.

Responses of manure decomposition to nitrogen addition: Role of chemical composition.

Understanding the interactions among organic manure chemical composition, decomposition and nitrogen (N) fertilization is critical for sustainable agriculture management. Six organic manures were incubated in a cultivated black soil with or without N addition for one year, and carbon dioxide (CO2) emissions from these organic manures were monitored. Chemical compositions of the organic manures were determined by elemental analysis, proximate chemical analysis, and carbon (C)-13 nuclear magnetic resonance spectroscopy, and evaluated after cupric-oxide oxidation for lignin biomarkers. During the experimental period, 19-44% of manure C was decomposed without N addition, which decreased to 17-35% with N addition, except for the composted furfural residue with rice dregs. However, during different decomposition stages, N effect changed from stimulation to inhibition, or behaved as increasing inhibition. During stage 1 (days 0-100) when N stimulation effect reached a maximum, CO2 emissions from manure had positive relationships with labile C fraction indicators, including total sugars, soluble polyphenols, and lignin cinnamyl/vanillyl ratio regardless of N addition. N effect on manure decomposition was related to the C/N ratio and labile organic C content. During stage 2 (days 101-267), N effect shifted to inhibition, with CO2 emissions from manure negatively related to lignin vanillyl-units content. The magnitude of N inhibition increased linearly with the aromaticity of dissolved organic C, and was strengthened by nitrate in manure. Finally, N inhibition effect reached a maximum during stage 3 (days 268-365), increasing with higher aromatic C in manure. Critical factors for manure decomposition shifted from total sugars, soluble polyphenols, and lignin cinnamyl-units to recalcitrant lignin vanillyl-units and aromatic C fraction, which mediated the type and magnitude of N effect on decomposition. Our results suggested that the potential for enhancing soil C sequestration with organic manures would magnify under combined application with N fertilizer in the long term.

Extreme rainfall and snowfall alter responses of soil respiration to nitrogen fertilization: a 3-year field experiment.

Extreme precipitation is predicted to be more frequent and intense accompanying global warming and may have profound impacts on soil respiration (Rs) and its components, that is, autotrophic (Ra) and heterotrophic (Rh) respiration. However, how natural extreme rainfall or snowfall events affect these fluxes are still lacking, especially under nitrogen (N) fertilization. In this study, extreme rainfall and snowfall events occurred during a 3-year field experiment, allowing us to examine their effects on the response of Rs, Rh, and Ra to N supply. In normal rainfall years of 2011/2012 and 2012/2013, N fertilization significantly stimulated Rs by 23.9% and 10.9%, respectively. This stimulation was mainly due to the increase of Ra because of N-induced increase in plant biomass. In the record wet year of 2013/2014, however, Rs was independent on N supply because of the inhibition effect of the extreme rainfall event. Compared with those in other years, Rh and Ra were reduced by 36.8% and 59.1%, respectively, which were likely related to the anoxic stress on soil microbes and decreased photosynthates supply. Although N supply did not affect annual Rh, the response ratio (RR) of Rh flux to N fertilization decreased firstly during growing season, increased in nongrowing season and peaked during spring thaw in each year. Nongrowing season Rs and Rh contributed 5.5-16.4% to their annual fluxes and were higher in 2012/2013 than other years due to the extreme snowfall inducing higher soil moisture during spring thaw. The RR of nongrowing season Rs and Rh decreased in years with extreme snowfall or rainfall compared to those in normal years. Overall, our results highlight the significant effects of extreme precipitation on responses of Rs and its components to N fertilization, which should be incorporated into models to improve the prediction of carbon-climate feedbacks.

Accumulation of organic C components in soil and aggregates.

To explore soil organic carbon (SOC) accumulation mechanisms, the dynamics of C functional groups and macroaggregation were studied synchronously through aggregate fractionation and (13)C NMR spectroscopy in sandy loam soil following an 18-year application of compost and fertilizer in China. Compared with no fertilizer control, both compost and fertilizer improved SOC content, while the application of compost increased macroaggregation. Fertilizer application mainly increased the levels of recalcitrant organic C components characterized by methoxyl/N-alkyl C and alkyl C, whereas compost application mainly promoted the accumulation of methoxyl/N-alkyl C, phenolic C, carboxyl C, O-alkyl C and di-O-alkyl C in bulk soil. The preferential accumulation of organic C functional groups in aggregates depended on aggregate size rather than nutrient amendments. These groups were characterized by phenolic C and di-O-alkyl C in the silt + clay fraction, carboxyl C in microaggregates and phenolic C, carboxyl C and methoxyl/N-alkyl C in macroaggregates. Thus, the differences in accumulated organic C components in compost- and fertilizer-amended soils were primarily attributable to macroaggregation. The accumulation of methoxyl/N-alkyl C in microaggregates effectively promoted macroaggregation. Our results suggest that organic amendment rich in methoxyl/N-alkyl C effectively improved SOC content and accelerated macroaggregation in the test soil.

Thirty-year amendment of horse manure and chemical fertilizer on the availability of micronutrients at the aggregate scale in black soil.

PURPOSE: This study evaluates manure and chemical fertilizer effects on micronutrient (Fe, Mn, Cu, and Zn) content and availability in crops. METHODS: Seven treatments were selected, including three conventional fertilization treatments (NP, horse manure (M), and NP plus M (NPM)), three corresponding double rate fertilization (N2P2, M2, and N2P2M2), and a CK. Soil samples were collected and separated into four aggregates by wet-sieving in September 2009. Corn samples were collected and analyzed simultaneously. RESULTS: Treatment N2P2 increased DTPA extractable Fe, Mn, and Cu in soil by 732%, 388%, and 42%, whereas M2 decreased the corresponding values by 26%, 22%, and 10%, respectively, compared to CK. DTPA extractable Zn in soil and Zn in corn grain were higher in the M and M2 treatments than in the other treatments, and DTPA Zn was significantly correlated with soil organic carbon (SOC) in large macroaggregate, microaggregate, and silt + clay fractions. The Mn concentrations in corn stalks and grain were significantly correlated with DTPA extractable Mn in bulk soil and microaggregates, and Zn in stalks were significantly correlated with DTPA Zn in bulk soil, microaggregates, and large macroaggregates. CONCLUSIONS: Long-term application of horse manure could increase soil Zn availability and uptake by corn, possibly due to its activation by SOC. In contrast, chemical fertilizer application increased DTPA extractable Fe, Mn, and Cu in soil by reducing soil pH. Our results also suggest that Mn uptake by corn originated mainly in microaggregates, whereas Zn in crops was primarily sourced from large macroaggregates and microaggregates.