Soil Denitrification Potential Research Articles

Effect of Chemical Fertilizer and Manure Combined with Biochar on Denitrification Potential and Denitrifying Bacterial Community in Rhizosphere Soil

To clarify the effect of chemical fertilizer and manure combined with biochar on denitrifying microorganisms and denitrification potential of rhizosphere soil, a pot experiment growing lemon was conducted involving five treatments, namely no fertilization (CK), chemical fertilizer (CF), manure (M), chemical fertilizer combined with biochar (CFBC), and manure combined with biochar (MBC). We determined the characteristics of the rhizosphere soil nirS-, nirK-, and nosZ-type denitrifying bacteria populations; denitrification potential; and soil environmental factors to clarify the effects of chemical and manure combined with biochar on denitrification. Our results showed that compared with that in CK, the CF treatment reduced the rhizosphere soil denitrification potential by 47.7%, whereas the M and MBC treatments increased the denitrification potential by 2192.7% and 1989.9%, respectively. The M and MBC treatments increased the gene copy number of nirS and nosZ, the CF and CFBC treatments decreased the gene copy number of nirS and nosZ, and all four fertilization treatments increased the gene copy number of nirK. Stepwise regression analysis showed that pH was the main factor for the abundance of nirS-type denitrifying bacteria and SOM and NH+4-N were the main factors for the abundance of nirK-type denitrifying bacteria, whereas pH, NO-3-N, and N/P were main factors for the abundance of nosZ-type denitrifying bacteria. The results of partial least squares analysis indicated that the abundance of nirS-and nosZ-type denitrifying bacteria, pH, TN, and N/P were the main factors affecting rhizosphere denitrification potential. Therefore, in acidic purple soil, nirS- and nosZ-type denitrifying bacteria were the main drivers of the soil denitrification process in lemon rhizospheres under chemical fertilizer and pig manure combined with biochar, whereas fertilizer affected the rhizosphere soil denitrification process by regulating soil pH, TN, and N/P.

Denitrification Potential of Surface Soils of Constructed Wetlands in Newtown Creek, an Urban Superfund Site.

Denitrification, the anaerobic microbial conversion of nitrate (NO3 - ), a common water pollutant, to nitrogen (N) gases, is often high in the soil of natural wetlands. In areas where natural wetlands have been degraded or destroyed, constructed and restored wetlands have been used to restore ecosystem services like denitrification. Thus, denitrification in restored and constructed wetlands could play an important role in treating anthropogenic N sources such as combined sewer overflow (CSO) discharges which can be high in NO3 - . In this study, we measured denitrification potential using an anaerobic slurry assay and made a suite of ancillary measurements (soil moisture content, microbial biomass carbon (C) and N content, potential net N mineralization and nitrification, soil inorganic N pools and soil respiration) in four constructed salt marsh wetlands, and a series of wetland habitat basins in Newtown Creek, NY, an urban superfund site. Samples were also taken from natural salt marshes located at Paerdegat Basin, Jamaica Bay, NY. Our results show that constructed Spartina alterniflora marshes in ultra-urban Newtown Creek support denitrification potential equivalent to rates of natural marshes in Jamaica Bay and reference marshes in other urban estuaries. There were significant positive correlations between microbial biomass C and N content and organic matter content and denitrification potential. Results suggest that constructed wetlandscan support wetland vegetation, soils, and microbial life and contribute to N removal under ultra-urban conditions. This article is protected by copyright. All rights reserved.

Improving an existing proxy-based approach for floodplain denitrification assessment to facilitate decision making on restoration

Excess nitrogen (N) from agricultural sources is a major contributor to the water pollution of rivers in Europe. Floodplains are of tremendous importance as they can permanently remove nitrate (NO3) from the environment by releasing reactive N to the atmosphere in its gaseous forms (N2O, N2) during denitrification. However, the quantitative assessment of this ecosystem function is still challenging, particularly on the national level. In this study, we modeled the potential of NO3-N removal through microbial denitrification in soils of the active floodplains of the river Elbe and river Rhine in Germany. We combined laboratory measurements of soil denitrification potentials with straightforward modelling data, covering the average inundation duration from six study areas, to improve an existing Germany-wide proxy-based approach (PBAe) on NO3-N retention potential. The PBAe estimates this potential to be 30–150 kg NO3-N ha−1 yr−1. However, with soil pH and Floodplain Status Category identified as essential parameters for the proxies, the improved PBA (PBAi) yields a removal potential of 5–480 kg N ha−1 yr−1. To account for these parameters, we applied scaling factors using a bonus-malus system with a base value of 10–120 N ha−1 yr−1. Upscaling the determined proxies of the PBAi to the entire active floodplains of the river Elbe and river Rhine results in similarly high NO3-N retention sums of ~7000 t yr−1 in spite of very different retention area sizes, strengthening the argument for area availability as the primary objective of restoration efforts. Although PBAs are always subject to uncertainty, the PBAi enables a more differentiated spatial quantification of denitrification because local key controlling parameters are included. Hence, the PBAi is an innovative and robust approach to quantify denitrification in floodplain soils, supporting a better assessment of ecosystem services for decision-making on floodplain restoration.

Insight into the microbial nitrogen cycle in riparian soils in an agricultural region

Riparian zones are considered as an effective measure on preventing agricultural non-point source nitrogen (N) pollution. However, the mechanism underlying microbial N removal and the characteristics of N-cycle in riparian soils remain elusive. In this study, we systematically monitored the soil potential nitrification rate (PNR), denitrification potential (DP), as well as net N2O production rate, and further used metagenomic sequencing to elucidate the mechanism underlying microbial N removal. As a whole, the riparian soil had a very strong denitrification, with the DP 3.17 times higher than the PNR and 13.82 times higher than the net N2O production rate. This was closely related to the high soil NO3−-N content. In different profiles, due to the influence of extensive agricultural activities, the soil DP, PNR, and net N2O production rate near the farmland edge were relatively low. In terms of N-cycling microbial community composition, the taxa of denitrification, dissimilatory nitrate reduction, and assimilatory nitrate reduction accounted for a large proportion, all related to NO3−-N reduction. The N-cycling microbial community in waterside zone showed obvious differences to the landside zone. The abundances of N-fixation and anammox genes were significantly higher in the waterside zone, while the abundances of nitrification (amoA&B&C) and urease genes were significantly higher in the landside zone. Furthermore, the groundwater table was an important biogeochemical hotspot in the waterside zone, the abundance of N-cycle genes near the groundwater table was at a relative higher level. In addition, compared to different soil depths, greater variation in N-cycling microbial community composition was observed between different profiles. These results reveal some characteristics of the soil microbial N-cycle in the riparian zone in an agricultural region and are helpful for restoration and management of the riparian zone.

Soil nitrification and denitrification in Cunninghamia lanceolata plantations with different stand ages.

The variations in soil nitrification and denitrification processes, together with the abundances of functional microbes were investigated in Cunninghamia lanceolata plantations with different stand ages of 5, 8, 21, 27, and 40 years old. The results showed that the net nitrification rate fluctuated with increasing forest ages, with that of 8-year- and 27-year-old C. lanceolata plantations being significantly lower than other stand ages. The abundance of ammonia-oxidizing archaea (AOA) amoA in the 27-year-old plantation was significantly lower than that of the 40-year-old plantation, while there was no significant difference among the other stand ages. There was no significant difference in the abundance of AOB amoA gene, denitrifying functional genes or soil denitrification potential among different stand ages. The results of stepwise regression analysis showed that the abundance of AOA amoA gene was not significantly affected by soil physical and chemical properties. In addition, the abundance of AOB was positively associated with soil total carbon content and soil pH. The abundance of denitrifying functional genes including narG, nirK and nosZ increased with increasing soil pH. The abundance of nirK and nirS was influenced by soil total carbon. Stand age influenced soil net nitrification rate through the AOA amoA abundance. Moreover, soil denitrification potential was directly affected by stand age, or indirectly affected by stand age through soil microbial biomass carbon, soil pH and denitrifying gene abundance of narG and nirK. Compared with the denitrification process, soil nitrification and associated AOA amoA gene abundance were more sensitive to the development of C. lanceolata plantations. The rotation period sould be appropriately extended to reduce the risk of nitrogen losses resulting from soil nitrification.

Long-term nitrogen addition increases denitrification potential and functional gene abundance and changes denitrifying communities in acidic tea plantation soil

The response of soil denitrification to nitrogen (N) addition in the acidic and perennial agriculture systems and its underlying mechanisms remain poorly understood. Therefore, a long-term (12 years) field trial was conducted to explore the effects of different N application rates on the soil denitrification potential (DP), functional genes, and denitrifying microbial communities of a tea plantation. The study found that N application to the soil significantly increased the DP and the absolute abundance of denitrifying genes, such as narG, nirK, norB, and nosZ. The diversity of denitrifying communities (genus level) significantly decreased with increasing N rates. Moreover, the denitrifying communities composition significantly differed among the soils with different rates of N fertilization. Further variance partitioning analysis (VPA) revealed that the soil (39.04%) and pruned litter (32.53%) properties largely contributed to the variation in the denitrifying communities. Dissolved organic carbon (DOC) and soil pH, pruned litter's total crude fiber (TCF) content and total polyphenols to total N ratio (TP/TN), and narG and nirK abundance significantly (VIP >1.0) influenced the DP. Finally, partial least squares path modeling (PLS-PM) revealed that N addition indirectly affected the DP by changing specific soil and pruned litter properties and functional gene abundance. Thus, the findings suggest that tea plantation is a major source of N2O emissions that significantly enhance under N application and provide theoretical support for N fertilizer management in an acidic tea plantation system.

Variation in Soil Denitrification among Fertilization Regimes and Its Microbial Mechanism

A 27-year field experiment with various fertilization regimes was chosen for evaluating how fertilization regimes influence soil denitrification potential (SDP), to determine whether the microbial mechanism of SDP variation varies with fertilization regimes. The results showed that the compost (OM) fertilizer treatment presented the highest SDP followed by half compost plus half chemical fertilizer (NPKOM) treatment. Both SDPs were 3–25 times higher than those of the other treatments, and the SDP was higher in the OM than NPKOM, indicating an increase in SDP with the application of compost. The higher SDPs from OM and NPKOM were closely associated with higher abundances of functional genes, and enrichments of Rubrivivax gelatinosus and Azospirillum sp. TSO28-1, is mainly determined by the organic carbon (SOC), total nitrogen (TN), and dissolved organic carbon (DOC). For the treatments without compost, SDP mainly followed the order of chemical fertilizers of N and P (NP) > chemical fertilizers of N and K (NK) > unfertilized (Nil) > chemical fertilizers of N, P, and K (NPK) > chemical fertilizers of P and K (PK), in which the variations had a close association with different specific microbial species. That is, relative to Nil, the enhanced SDP in NP and NK was coupled with the enrichment of Ideonella sp. NC3L-43b and R. gelatinosus, likely due to their higher NO3 − contents, while the reduced SDP in NPK was linked to the depletions of R. gelatinosus and Azospira sp. NC3H-14, and that in PK to the depletion of R. gelatinosus and Azospirillum sp. TSO28-1, probably as a result of their lower N:P ratios. Microbial mechanisms for SDP differences varied with fertilization regimes, and R. gelatinosus can be regarded as a universal microbial indicator for SDP differences across the fertilization treatments.

Metagenomics reveals divergent functional profiles of soil carbon and nitrogen cycling under long-term addition of chemical and organic fertilizers in the black soil region

The long-term effects of different fertilization regimes on the microbial functional potential of soils involving nutrient cycling remain largely unknown. Here, metagenomic sequencing was applied to investigate the influences of long-term chemical and organic fertilization on soil microbial C and N cycling across southern, middle and northern sites of black soil region in Northeast China. The results showed that biogeographic distance induced the most influential on the microbial functional profiles of soil C and N cycling, and significant effects of manure fertilization were detected across three experimental sites. Organic fertilization enriched the relative abundances of Proteobacteria and Planctomycetes that carry C and N cycling genes, while inhibited the growth of oligotrophic groups such as Verrucomicrobia. Chemical fertilization increased the gene abundances involved in methane oxidation, but had little effect on soil C degradation and fixation. Contrarily, manure fertilization, particularly the combination of chemical and organic fertilizers (CFM), significantly decreased the abundance of cooC (reductive acetyl-CoA pathway) and coxS (CO oxidation) while enhanced the abundance of icd (rTCA cycle), which are involved in C fixation. Additionally, chemical fertilization enriched the gene abundance that involved in soil N degradation, nitrification and anammox, whereas manure fertilization was beneficial for the functional potentials of assimilatory and dissimilatory nitrate reductions across the black soils. However, CFM significantly promoted the soil denitrification potential, possibly due to excess N input, which might result in soil N loss via the emission of nitrogenous gas in this region. Furthermore, the substantial enhancement in soil P contents induced by manure addition predominantly affected the C and N cycling profiles, abundance of functional genes and microbial taxa. Moreover, diverse correlations between C and N cycling genes suggested the synergetic or antagonistic interactions of C and N metabolic potentials in the black soils. Overall, this study provided in-depth insights into distinct microbial functional potentials under long-term chemical and organic fertilization that may have predictable consequences for soil nutrient cycling in agroecosystems of black soil region.

WIDESPREAD CAPACITY FOR DENITRIFICATION ACROSS A BOREAL FOREST LANDSCAPE.

A warming climate combined with frequent and severe fires cause permafrost to thaw, especially in the region of discontinuous permafrost, where soil temperatures may only be a few degrees below 0 °C. Soil thaw releases carbon (C) and nitrogen (N) into the actively cycling pools, and whereas C emissions following permafrost thaw are well documented, the fates of N remain unclear. Denitrification could release N from ecosystems as nitrous oxide (N2O) or nitrogen gas (N2), but the contributions of these processes to the high-latitude N cycle remain uncertain. We quantified microbial capacity for denitrification and N2O production in boreal soils, lakes, and streams using anoxic C- and N-amended assays, and assessed correlates of denitrifying enzyme activity (DEA) in Interior Alaska. Riparian soils and stream sediments supported the highest potential rates of denitrification, upland soils were intermediate, and lakes supported lower rates, whereas deep permafrost soils supported little denitrification. Time since fire had no effect on denitrification potential in upland soils. Across all landscape positions, DEA was negatively correlated with ammonium pools. Within each landscape position, potential rate of denitrification increased with soil or sediment organic matter content. Widespread N loss to denitrification in boreal forests could constrain the capacity for N-limited primary producers to maintain C stocks in soils following permafrost thaw.

Seasonal and spatial characterisation of soil properties, nitrification and denitrification at the urban river-riparian interface with permeable revetments

Understanding the seasonal and spatial variation characteristics and formation mechanism of soil nitrification and denitrification on permeable revetment can provide basis for revetment design and protecting the ecological function of river-riparian interface. In this study, permeable imitation wooden pile revetments, commonly used in Shanghai, were selected. In spring, summer and autumn, soil samples were collected and tested at different distances from the river and depths in the river-riparian interface. The seasonal and spatial variation characteristics of nitrification and denitrification potentials their relationships with soil properties were analysed. The results indicated the seasonal change of soil structure indexes were not significant, but that of the material content related indexes were significant. The influence of soil depth on soil properties is greater than the distance from the river. The soil nitrification potentials during spring (4.43 ± 4.26 mg kg−1 d−1) and autumn (5.24 ± 4.54 mg kg−1 d−1) were significantly higher than that during summer (3.53 ± 2.66 mg kg−1 d−1), whereas the soil denitrification potential was significantly higher during summer (11.65 ± 9.20 mg kg−1 d−1) than during autumn (10.79 ± 8.33 mg kg−1 d−1) and spring (10.21 ± 7.11 mg kg−1 d−1). The seasonal variations of the soil nitrification and denitrification potentials in the occasionally flooded area (0–20 cm depth) were larger than in the frequently flooded area (25–45 cm depth) and the permanently flooded area (55–75 cm depth). The seasonal and spatial differences determine the state of soil submergence, resulting in the difference in soil’s properties, leading to the difference in nitrification and denitrification intensity. The results of this study can provide the guidelines for urban revetment construction for nitrogen removal in riparian zones.

Response of soil denitrification potential and community composition of denitrifying bacterial to different rates of straw return in north-central China

Denitrification is a key process for soil available nitrogen (N) loss and is greatly influenced by fertilization strategies; however, the effect of straw return rates on soil denitrification potential (DP) and the community composition of denitrifying bacteria remain unclear. A 30-year straw return experiment was used to study changes in soil DP, the abundance and community composition of nirK- and nirS-type denitrifying bacteria in north-central China. The experiment included five treatments: control (CK, without fertilizers and straw application), and the application of N and phosphorus (P) fertilizers combined with 0 (S0), 2250 (S1), 4500 (S2), and 9000 kg ha−1 (S3) maize straw, respectively. Relative to the CK treatment, the S0 greatly increased soil DP and the abundance of nirK and nirS genes, and these values further increased with the increasing straw rates; however, the chemical fertilization and straw rates had no effects on the α-diversity of two denitrifying communities. Principal component analysis showed that the chemical fertilizers and straw return rates significantly influenced the composition of nirS-type denitrifying community, but had less effect on nirK-type denitrifying community. Soil NO3−-N, total N, and organic matter were positive correlated with soil DP and the abundance of nirK- and nirS-type denitrifying bacteria across all treatments, and were the dominant factors affecting the composition of two nir denitrifying communities under the high rates of straw return. Our results indicated that the rate of straw return had different effects on the abundance and composition of nirK- and nirS-type denitrifying communities, and nirS-type denitrifying community was more sensitive to the increasing return rate of straw than nirK-type denitrifying community.

Soil Characteristics and Hydromorphological Patterns Control Denitrification at the Floodplain Scale

Nitrate pollution in aquatic ecosystems is still a major problem in Germany. There is a great potential to permanently remove nitrate from aquatic systems through denitrification as a relevant ecosystem function. However, the controlling factors and the dimension of the denitrification potential are still not fully understood due to the high complexity of the process. This study presents the combined assessment of potential soil denitrification rates, physical and chemical soil parameters, and hydrological parameters from six floodplains of four large German rivers, namely the Rhine, the Elbe, the Weser, and the Main. Based on multivariate statistics, results show that the denitrification potential of soil was almost solely controlled by soil pH. The lab assays showed mean soil denitrification potentials of 6.4–11.4 mg N m−2h−1(pH < 7) and 23.0–30.5 mg N m−2h−1(pH > 7). We contend that when upscaling these estimates to annual rates of potential denitrification, the duration of average inundation should be incorporated, as this accounts for water saturation and nutrient supply − the major controlling variables for denitrification. Results provide evidence that the denitrification potential can only be fully exploited in frequently inundated floodplains. Thus, despite favorable soil conditions for denitrification, floodplains that have suffered from anthropogenic impacts, lose their importance in nitrate removal for the river system. We conclude that pH and lateral hydrological connectivity are likely to be key factors that should be considered when estimating denitrification as an ecosystem function.

Interacting drivers and their tradeoffs for predicting denitrification potential across a strong urban to rural gradient within heterogeneous landscapes

Denitrification is a significant regulator of nitrogen pollution in diverse landscapes but is difficult to quantify. We examined relationships between denitrification potential and soil and landscape properties to develop a model that predicts denitrification potential at a landscape level. Denitrification potential, ancillary soil variables, and physical landscape attributes were measured at study sites within urban, suburban, and forested environments in the Gwynns Falls watershed in Baltimore, Maryland in a series of studies between 1998 and 2014. Data from these studies were used to develop a statistical model for denitrification potential using a subset of the samples (N = 188). The remaining measurements (N = 150) were used to validate the model. Soil moisture, soil respiration, and total soil nitrogen were the best predictors of denitrification potential (R2adj = 0.35), and the model was validated by regressing observed vs. predicted values. Our results suggest that soil denitrification potential can be modeled successfully using these three parameters, and that this model performs well across a variety of natural and developed land uses. This model provides a framework for predicting nitrogen dynamics in varying land use contexts. We also outline approaches to develop appropriate landscape-scale proxies for the key model inputs, including soil moisture, respiration, and soil nitrogen.

Contribution of Particulate and Mineral-Associated Organic Matter to Potential Denitrification of Agricultural Soils

Water-extractable organic carbon (WEOC) is considered as the most important carbon (C) source for denitrifying organisms, but the contribution of individual organic matter (OM) fractions (i.e., particulate (POM) and mineral-associated (MOM)) to its release and, thus, to denitrification remains unresolved. Here we tested short-time effects of POM and MOM on potential denitrification and estimated the contribution of POM- and MOM-derived WEOC to denitrification and CO2 production of three agricultural topsoils. Suspensions of bulk soils with and without addition of soil-derived POM or MOM were incubated for 24 h under anoxic conditions. Acetylene inhibition was used to determine the potential denitrification and respective product ratio at constant nitrate supply. Normalized to added OC, effects of POM on CO2 production, total denitrification, and its product ratios were much stronger than those of MOM. While the addition of OM generally increased the (N2O + N2)-N/CO2-C ratio, the N2O/(N2O + N2) ratio changed differently depending on the soil. Gas emissions and the respective shares of initial WEOC were then used to estimate the contribution of POM and MOM-derived WEOC to total CO2, N2O, and N2O + N2 production. Water-extractable OC derived from POM accounted for 53–85% of total denitrification and WEOC released from MOM accounted for 15–47%. Total gas emissions from bulk soils were partly over- or underestimated, mainly due to nonproportional responses of denitrification to the addition of individual OM fractions. Our findings show that MOM plays a role in providing organic substrates during denitrification but is generally less dominant than POM. We conclude that the denitrification potential of soils is not predictable based on the C distribution over POM and MOM alone. Instead, the source strength of POM and MOM for WEOC plus the WEOC’s quality turned out as the most decisive determinants of potential denitrification.

Effect of Rice Planting on Nitrous Oxide (N2O) Emission under Different Levels of Nitrogen Fertilization

Nitrogen (N) fertilization is one of the most effective practices to increase productivity, and has therefore had a fast global increase. Consequently, the effects of the application of N fertilizer on emissions of N2O have been widely studied, but the effect of rice planting on N2O emission was not adequately quantified. To evaluate the effect of rice cultivation on N2O emissions, different levels of N were applied in a typical temperate rice field, and the N2O fluxes were compared in rice-planted and non-planted soils. Seasonal N2O fluxes responded differently with respect to N fertilization in the two different soil conditions. In non-planted soils, seasonal N2O fluxes ranged within 0.31–0.34 kg N2O ha−1 under 0 kg N ha−1 fertilization, and significantly increased by increasing N fertilization rates, with an average rate of 0.0024 kg N2O kg−1 N for 3 years. In rice-planted soils, seasonal N2O fluxes were also increased by N fertilization but showed large negative N2O fluxes, irrespective of the N fertilization level. This study confirms that the rice reacted as a reducer of N2O emissions, not an emission source, in paddy fields, suggesting that N2O fluxes should be estimated by the static chamber planted with rice to obtain a more precise field environment. The differences of N2O fluxes between the rice-planted and non-planted soils might have been caused by the rice plant’s rhizospheric activities, which may have influenced the N2O consumption potential in the rice plants’ rhizosphere. The N2O consumption potential was significantly increased with increasing N fertilization rates and was highly correlated with rice biomass yields. Therefore, the decrease in N2O fluxes by N fertilization in rice-planted soils might have been caused by a decreasing denitrification potential in paddy soils.

Carbon availability limits the denitrification potential of sandy loam soil from corn agroecosystems with long-term tillage and residue management

Conservation tillage and crop residues should increase the soluble organic carbon and nitrate concentration in agricultural soil, which increases the denitrification potential. Basal denitrification (72 h laboratory incubation) was 2.1–2.7 times higher in a sandy loam soil under 15 yr of conservation tillage than conventional tillage and 1.8–2.0 times higher with high-residue (additional input 8.6–9.4 Mg dry matter·ha−1·yr−1) than low-residue inputs. Adding glucose and nitrate increased the soil denitrification potential 3- to 14-fold. Denitrification was limited by carbon availability, even in soil with 15 yr of conservation tillage and high-residue inputs.

Catchment topography and the distribution of electron donors for denitrification control the nitrate concentration in headwater streams of the Lake Hachiro watershed

ABSTRACT We examined the linkages between topography and electron donors for denitrification on in-stream NO3 − concentration in headwater catchments in the Lake Hachiro watershed having marine sedimentary rock, Japan. In 35 headwater catchments (0.07–16.9 km2), we sampled stream water every season in 2 years. The water samples were analyzed for NO3 –, dissolved nitrous oxide (dN2O), and SO4 2 – concentrations. Stream sediment was sampled once for the measurement of denitrification potential (DP). Water-extractable soil organic carbon (WESOC) and easily oxidizable sulfide (EOS) in the sediment, which can be considered the principal potential electron donors for denitrification, were measured. The topographical features of each catchment were calculated using a digital elevation model with 10-m grid cells. Stream NO3 – concentrations displayed large spatial variation among catchments, ranging from 0.06 to 0.52 mg N L–1, and were negatively correlated with topographic wetness index (TWI) (P < 0.01) and were positively correlated with catchment slope (P < 0.01), indicating that NO3 – concentrations decreased in wetter and gentle slope catchments. Sediment DP and the WESOC content in sediments were positively correlated with TWI, significantly. These results suggested denitrification was likely to occur in higher TWI catchments. Generalized linear model showed that TWI, slope aspect, and sediment DP significantly affected in-stream NO3 – concentration and WESOC was a significant explanatory variable for sediment DP. EOS content in riverbed sediments was not selected as a significant explanatory variable for either in-stream NO3 − concentrations or sediment DP. But higher soil DP with higher EOS was detected in the stream bank subsoil at the catchment where the higher EOS content in the riverbed sediment was observed, which suggested EOS in riverbed sediments can contain site-specific information about denitrification hotspot driven by sulfides. We conclude that catchment topography and the distribution of electron donors in riverbed sediment can be important factors to explain the spatial variation in in-stream NO3 – concentration and sediment DP.

Soil nitrogen accumulation, denitrification potential, and carbon source tracing in bioretention basins

Bioretention basins are one of the most commonly used green stormwater features, with the potential to accumulate significant levels of nitrogen (N) in their soil and to permanently remove it through denitrification. Many studies have investigated the N removal potential of bioretention basins through the assessment of inflow and outflow concentrations. However, their long-term N removal through soil accumulation and denitrification potential is less known. This study investigated the temporal variation of total N (TN) accumulation and denitrification potential in soils of 25 bioretention basins within a 13-year soil chronosequence, in a subtropical climate in Australia. The denitrification potential of a subset of seven bioretention basins was investigated in accompaniment with nutrient and soil characteristics. Additionally, stable isotopes (δ13C and δ15N) were used to assess temporal changes in the soil composition as well as to identify the sources of carbon (C) into these basins. Over 13 years of operation, TN accumulated faster in the top 5 cm of soil than deeper soils. Soil TN density was highest in the top 5 cm with an average of 1.4 kg N m−3, which was about two times higher than deeper soils. Site age and soil texture were the best predictors of soil TN density and denitrification (1 to 9.7 mg N m−2 h−1). The isotope values were variable among basins. Low δ15N values in young basins (-1.02‰) suggested fixation as the main source of N, while older basins had higher δ15N, indicating higher denitrification. Bioretention plants were the primary source of soil C; although the occurrence of soil amendment also contributed to the C pool. To improve the performance of these bioretention basins, we recommend increasing vegetation at initial years after construction, and enhancing more frequent anaerobic conditions in the high soil profile. These two conditions can improve denitrification potential, and thus the performance of these basins for improving water quality.

Denitrification Potential of Paddy and Upland Soils Derived From the Same Parent Material Respond Differently to Long-Term Fertilization

To investigate the effect of land use and fertilization regimes on soil denitrification potential (SDP), two more than 30-year long-term fertilization experiments derived from same parent material (paddy soil and upland soil) were selected. Generally, the SDP in paddy soil was 6.82 times higher than the upland soil, which was due to the higher abundances of narG, nirS, and nirK genes and nirS-denitrifying bacteria (Bradyrhizobium, Cupriavidus, and Herbaspirillum) in paddy soil. Inorganic fertilization regimes in upland soil did not significantly affect the SDP over Control, while the SDP in NPK and 2NPK of paddy soil decreased by 26.48% and 75.65%, respectively. Compared with Control, NPKOM consistently yielded the highest SDP in both two soils, with 2.47 times and 2.86 times higher for paddy soil and upland soil. The SDP of paddy soil showed correlation with narG and nirS genes mainly regulated by Alo, while the difference in the SDP of upland soil largely depended on the change of nirS-denitrifying bacteria at genus level (Herbaspirillum, Sulfuritalea, and Cupriavidus) and species level, which was mainly controlled by soil pH. PLS path modeling further demonstrated that direct effect of functional genes on the SDP was the greatest for paddy soil, while nirS-denitrifying bacterial communities for upland soil. The results presented herein represent a key step toward understanding the mechanisms that govern SDP under land use and long-term fertilization.

Differences in labile soil organic matter explain potential denitrification and denitrifying communities in a long-term fertilization experiment

Content and quality of organic matter (OM) may strongly affect the denitrification potential of soils. In particular, the impact of soil OM fractions of differing bioavailability (soluble, particulate, and mineral-associated OM) on denitrification remains unresolved. We determined the potential N2O and N2 as well as CO2 production for samples of a Haplic Chernozem from six treatment plots (control, mineral N and NP, farmyard manure - FYM, and FYM + mineral N or NP) of the Static Fertilization Experiment Bad Lauchstädt (Germany) as related to OM properties and denitrifier gene abundances. Soil OM was analyzed for bulk chemical composition (13C-CPMAS NMR spectroscopy) as well as water-extractable, particulate, and mineral-associated fractions. Soils receiving FYM had more total OM and larger portions of labile fractions such as particulate and water-extractable OM. Incubations were run under anoxic conditions without nitrate limitation for seven days at 25 °C in the dark to determine the denitrification potential (N2O and N2) using the acetylene inhibition technique. Abundances of nirS, nirK, and nosZ (I + II) genes were analyzed before and after incubation. The denitrification potential, defined as the combined amount of N released as N2O + N2 over the experimental period, was larger for plots receiving FYM (25.9–27.2 mg N kg−1) than pure mineral fertilization (17.1–19.2 mg N kg−1) or no fertilization (12.6 mg N kg−1). The CO2 and N2O production were well related and up to three-fold larger for FYM-receiving soils than under pure mineral fertilization. The N2 production differed significantly only between all manured and non-manured soils. Nitrogenous gas emissions related most closely to water-extractable organic carbon (WEOC), which again related well to free particulate OM. The larger contribution of N2 production in soils without FYM application, and thus, with less readily decomposable OM, coincided with decreasing abundances of nirS genes (NO2− reductase) and increasing abundances of genes indicating complete denitrifying organisms (nosZ I) during anoxic conditions. Limited OM sources, thus, favored a microbial community more efficient in resource use. This study suggests that WEOC, representing readily bioavailable OM, is a straightforward indicator of the denitrification potential of soils.