Recovery of 15N-labeled urea and soil nitrogen dynamics as affected by irrigation management and nitrogen application rate in a double rice cropping system

A sophisticated soil microcosm system and ¹⁵N-labeled urea were used to investigate nitrogen (N) use efficiency and soil N dynamics in a rice monoculture system in two successive seasons. Topsoil (0 cm?C20 cm) and subsoil (20 cm?C50 cm) samples were collected from a traditional double rice cropping field in the Jiangxi Province, China, and these soil samples were derived from Quaternary red clay. Treatments were randomly assigned with two irrigation regimes and three N application rates (no application control, 80% traditional rate and 100% traditional rate noted as N0, N1 and N2, respectively). The levels of ¹⁵N recovery of plants, ¹⁵N and N remaining in soil were determined. Moreover, the N dynamic of soil solution from different layers of the soil profile was surveyed. The results showed that the effects of irrigation management and N application rate varied in different rice growing seasons. Irrigation regimes had remarkable effects on grain yield and chemical ¹⁵N fertilizers (CF-¹⁵N) uptake. When compared to flood irrigation (FI), the shallow water depth with wetting and drying (WD) increased grain yield up to 5.7%?C20%. Although the highest grain yield was obtained with reduced N application level, both N apparent recovery (NAR) and ¹⁵N use efficiency (the percentage of plant N uptake derived from applied N, %Ndfan) significantly decreased with increasing N inputs. However, the interaction between irrigation management and N application rate on grain yield and N use efficiency (NUE) of CF-¹⁵N were not significant. A survey of soil solutions every 5 days indicated that NH ₄ ⁺ -N was the main residual form of N, and high NH ₄ ⁺ -N leaching was observed. When compared to FI, WD decreased vertical NH ₄ ⁺ -N and TN leaching, especially at 10?C50 cm depths of soil profile in the second season. NH ₄ ⁺ -N was the main N residual form in the soil profile. Therefore, in this study, the WD irrigation regime and reduced rate (N1) was the optimal irrigation and fertilizer management strategy to increase the NUE of CF-N, increase the after effects of CF-¹⁵N, decrease leaching loss of CF-¹⁵N and minimize the shallow groundwater pollution risk, which were all beneficial for the ecological environment.     

Effects of macrophyte-associated nitrogen cycling bacteria on denitrification in the sediments of the eutrophic Gonghu Bay, Taihu Lake

Integrated Elodea nuttallii-immobilized nitrogen cycling bacteria (INCB) technology was used for ecological restoration in the eutrophic Gonghu Bay, Taihu Lake. Sediment denitrification was investigated through microcosm incubations with four different treatments: bare sediment core as control without restoration, sediment + E. nuttallii, sediment + E. nuttallii + INCB, and sediment + INCB. The sediments with E. nuttallii-INCB assemblage (E-INCB) had the highest denitrification rates among all the treatments, and the E-INCB increased the denitrification rate by 162% in the sediments. The presence of macrophytes yielded a penetration depth of O₂ to more than 20 mm below the sediment–water interface (SWI), while the depth was only 4 mm in the sediments without macrophytes. The quantity of denitrifier in E-INCB sediments (within ~2 cm below the SWI) showed a significant increasing trend during one-month incubation, which was one order of magnitudes higher than that in the sediments without INCB. Macrophytes caused deeper O₂ penetration and increased oxic-anoxic interface, which could stimulate the coupled nitrification–denitrification. The high denitrification rate of the E-INCB treatment may result from the increased inorganic nitrogen content in the vicinity of the SWI, causing more nitrate to reach the anoxic denitrification zone. The results showed that E-INCB assemblage could increase benthic N removal by stimulating denitrification via combined O₂ penetration and enhanced microbial N cycling processes. E-INCB might be used as a potential restoration method for controlling fresh water system eutrophication.     

Ozone pollution influences soil carbon and nitrogen sequestration and aggregate composition in paddy soils

Background and aims

Much attention has focused on the effects of tropospheric ozone (O₃) on terrestrial ecosystems and plant growth. Since O₃ pollution is currently an issue in China and many parts of the world, understanding the effects of elevated O₃ on soil carbon (C) and nitrogen (N) sequestration is essential for efforts to predict C and N cycles in terrestrial ecosystems under predicted increases in O₃. Thus the main objective of this study was to determine whether an increases in atmospheric O₃ concentration influenced soil organic C (SOC) and N sequestration.

Methods

A free-air O₃ enrichment (O₃-FACE) experiment was started in 2007 and used continuous O₃ exposure from March to November each year during crop growth stage in a rice (Oryza sativa L.)—wheat (Triticum aestivum L.) rotation field in the Jiangsu Province, China. We investigated differences in SOC and N and soil aggregate composition in both elevated and ambient O₃ conditions.

Results

Elevated atmospheric O₃ (18–80 nmol mol^?1 or 50 % above the ambient) decreased the SOC and N concentration in the 0–20 cm soil layer after 5 years. Elevated O₃ significantly decreased the SOC concentration by 17 % and 5.6 % in the 0–3 cm and the 10–20 cm layers, respectively. Elevated O₃ significantly decreased the N concentration by 8.2–27.8 % in three layers at the 20 cm depth. In addition, elevated O₃ influenced the formation and transformation of soil aggregates and the distribution of SOC and N in the aggregates across soil layer classes. Elevated O₃ significantly decreased the macro-sized aggregate fraction (16.8 %) and associated C and N (0.5 g kg^?1 and 0.32 g kg^?1, respectively), and significantly increased the silt+ clay-sized aggregate fraction (61 %) and associated C (1.7 g kg^?1) in the 0–3 cm layer. Elevated O₃ significantly decreased the macro-sized aggregate fraction (9.6 %) and associated C and N (1.4 g kg^?1 and 0.35 g kg^?1, respectively), and significantly increased the silt+ clay-sized aggregate fraction (41.8 %) and decreased the corresponding associated N (0.14 g kg^?1) in the 3–10 cm layer. Elevated O₃ did not significantly effect the formation and transformation of aggregates in the 10–20 cm layer, yet it did significantly increase the C concentration in the macro-sized fraction (1 g kg^?1) and decrease the N concentration in the macro- and micro-sized fractions (0.24 g kg^?1 and 0.16 g kg^?1, respectively).

Conclusion

Long-term exposure to elevated atmospheric O₃ negatively affected the physical structure of the soil and impaired soil C and N sequestration.     

Responses of soil chemical and biological properties to nitrogen addition in a Dahurian larch plantation in Northeast China

Soil microbial properties play a key role in belowground ecosystem functioning, but are not well understood in forest ecosystems under nitrogen (N) enrichment. In this study, soil samples from 0–10 cm and 10–20 cm layers were collected from a Dahurian larch (Larix gmelinii Rupr.) plantation in Northeast China after six consecutive years of N addition to examine changes in soil pH, nutrient concentrations, and microbial biomass and activities. Nitrogen addition significantly decreased soil pH and total phosphorus, but had little effect on soil total organic carbon (TOC) and total N (TN) concentrations. The NO ₃ ^? -N concentrations in the two soil layers under N addition were significantly higher than that in the control, while NH ₄ ⁺ -N concentrations were not different. After six years of N addition, potential net N mineralization and nitrification rates were dramatically increased. Nitrogen addition decreased microbial biomass C (MBC) and N (MBN), and MBC/TOC and MBN/TN in the 0–10 cm soil layer, but MBC/MBN was increased by 67% in the 0–10 cm soil layer. Soil basal respiration, microbial metabolic quotient (qCO₂), and β-glucosidase, urease, acid phosphomonoesterase and nitrate reductase activities in the two soil layers showed little change after six years of N addition. However, soil protease and dehydrogenase activities in the 0–10 cm layer were 41% and 54% lower in the N addition treatment than in the control, respectively. Collectively, our results suggest that in the mid-term N addition leads to a decline in soil quality in larch plantations, and that different soil enzymes show differentiated responses to N addition.     

Soil Organic Carbon is Increased in Mixed-Species Plantations of Eucalyptus and Nitrogen-Fixing Acacia

Soil organic carbon (soil C) sequestration in forests is often higher under nitrogen (N₂)-fixing than under non-N₂-fixing tree species. Here, we examined whether soil C could be increased using mixed-species plantations compared to monocultures, which are less productive aboveground than mixtures. In addition, we compared soil C sequestration under N₂-fixing trees with non-N₂-fixing trees that received N fertilizer. Monocultures of Eucalyptus globulus (E) and the N₂-fixing Acacia mearnsii (A) and mixtures of these species were planted in a replacement series: 100%E, 75%E + 25%A, 50%E + 50%A, 25%E + 75%A and 100%A. Soil samples were also collected from fertilized monoculture treatments (100%EFer) of E. globulus (250 kg N ha^?1). Total organic C, N and phosphorus were determined at age 8 years at two soil depths (0–10 cm and 10–30 cm) and three density fractions of soil organic matter (SOM) were quantified for 0–5 cm depth. Soil C was highest in the 50%E + 50%A mixed stand and was highly correlated with aboveground biomass, not to the percentage of A. mearnsii in mixtures. This was largely due to soil C at 10–30 cm because there were no treatment effects on soil C at 0–10 cm. All density fractions of SOM at 0–5 cm increased with the percentage of A. mearnsii. In E. globulus monocultures, N fertilization did not increase soil C when compared with unfertilized stands. These results indicate that the inclusion of N₂-fixing trees into eucalypt plantations may increase soil C stocks through increased productivity.     

Nitrogen dynamics in Lake Okeechobee: forms,functions, and changes

Total nitrogen (TN) in Lake Okeechobee, a large, shallow, turbid lake in south Florida, has averaged between 90 and 150 μM on an annual basis since 1983. No TN trends are evident, despite major storm events, droughts, and nutrient management changes in the watershed. To understand the relative stability of TN, this study evaluates nitrogen (N) dynamics at three temporal/spatial levels: (1) annual whole lake N budgets, (2) monthly in-lake water quality measurements in offshore and nearshore areas, and (3) isotope addition experiments lasting 3 days and using ¹⁵N-ammonium (¹⁵NH₄ ⁺) and ¹⁵N-nitrate (¹⁵NO₃ ⁻) at two offshore locations. Budgets indicate that the lake is a net sink for N. TN concentrations were less variable than net N loads, suggesting that in-lake processes moderate these net loads. Monthly NO₃ ⁻ concentrations were higher in the offshore area and higher in winter for both offshore and nearshore areas. Negative relationships between the percentage of samples classified as algal blooms (defined as chlorophyll a > 40 μg l⁻¹) and inorganic N concentrations suggest N-limitation. Continuous-flow experiments over intact sediment cores measured net fluxes (μmol N m⁻² h⁻¹) between 0 and 25 released from sediments for NH₄ ⁺, 0–60 removed by sediments for NO₃ ⁻, and 63–68 transformed by denitrification. Uptake rates in the water column (μmol N m⁻² h⁻¹) determined by isotope dilution experiments and normalized for water depth were 1,090–1,970 for NH₄ ⁺ and 59–119 for NO₃ ⁻. These fluxes are similar to previously reported results. Our work suggests that external N inputs are balanced in Lake Okeechobee by denitrification.     

Distribution of Metals (Cu,Zn, Pb,and Cd) in Sediments of the Anzali Lagoon,North Iran

Concentrations of four metals (Cu, Zn, Pb, and Cd) in the sediments of the Anzali Lagoon in the northern part of Iran were determined to evaluate the level of contamination and spatial distribution. The sediments were collected from 21 locations in the lagoon. At each lagoon site a core, 60 cm long, was taken. The ranges of the measured concentrations in the sediments are as follows: 17–140 mg kg^?1 for Cu, 20–113 mg kg^?1 for Zn, 1–37 mg kg^?1 for Pb and 0.1–3.5 mg kg^?1 for Cd in surficial (0-20 cm) and 16–87 mg kg^?1 for Cu, 28.5–118 mg kg^?1 for Zn, 3–20 mg kg^?1 for Pb and 0.1–3.5 mg kg^?1 for Cd in deep (40–60 cm) sediments. The results of the geoaccumulation index (I_geo) show that Cd causes moderate to heavy pollution in most of the study area. Environmental risk evaluation showed that the pollution in the Anzali Lagoon is moderate to considerable and the ranking of the contaminants followed the order: Cd > Cu > Pb > Zn. Some locations present severe pollution by metals depending on the sources, of which sewage outlets and phosphate fertilizers are the main sources of contaminants to the area.     

Estimation of nitrogen dynamics in a vertical-flow constructed wetland

The vertical-flow constructed wetland (VFCW) is a promising engineering technique for removal of excess nutrients and certain pollutants from wastewater and stormwater. The aim of this study was to develop a model using the STELLA software for estimating nitrogen (N) dynamics in an artificial VFCW (i.e., a substrate column with six zones) associated with a growing Cyperus alternifolius species under a wetting (wastewater) -to-drying ratio of 1:3. The model was calibrated by our experimental data with a reasonable agreement prior to its applications. Simulations showed that rates of NH₄⁺-N and NO₃⁻-N leaching decreased with increasing zone number (or column depth), although such a decrease was much more profound for NH₄⁺-N. Our simulations further revealed that rate of NH₄⁺-N leaching decreased with time within each zone, whereas rate of NO₃⁻-N leaching increased with time within each zone. Additionally, both the rates of NH₄⁺-N and NO₃⁻-N leaching through zones followed the water flow pattern: breakthrough during wetting period and cessation during drying period. In general, the cumulative amounts of total nitrogen (TN) were in the following order: leaching > denitrification > uptake > settlement. About 54% of the TN from the wastewater flowed out of the VFCW system, 18% of TN lost due to denitrification, 6% of TN was taken up by roots of a single plant (one hill), and the rest of 22% TN from the wastewater was removed from other mechanisms, such as volatilization, adsorption, and deposition. This study suggested that to improve the overall performance of a VFCW for N removal, prevention of N leaching loss was one of the major issues.     

Biological amelioration of subsoil acidity through managing nitrate uptake by wheat crops

Subsoil acidity occurs in many agricultural lands in the world, and is considered to be an irreversible constraint due to amelioration difficulties. This field study aimed to develop a biological method to ameliorate subsoil acidity through the root-induced alkalisation resulting from nitrate uptake. Aluminium (Al)-tolerant wheat variety Diamondbird and Al-sensitive variety Janz (Triticum aestivum L.) were grown at two contrasting field sites with mild and severe subsurface acidity, respectively, and were supplied with either Ca(NO₃)₂ at the soil surface, Ca(NO₃)₂ at 10 cm depth or urea at 10 cm depth. Application of nitrate increased rhizosphere pH up to 0.5 units and bulk soil pH to 0.3 units, and to a depth >30 cm in the Kandosol. The placement of nitrate at 10 cm increased subsoil pH more than the surface application. Nitrate application increased nitrate concentration in soil profiles as expected, whereas urea application increased NH ₄ ⁺ concentration which in turn favored acidification processes. Diamondbird generally produced more tillers and shoot biomass at anthesis but the two varieties did not differ in grain yield or rhizosphere alkalisation. Similar grain yields were achieved under supply of nitrate and urea. The results suggest that biological amelioration through managing nitrate uptake is possible as part of an integrated approach to combat subsoil acidity in farming systems.     

Interaction Between Nociceptin/Orphanin FQ and Adrenergic System on Food Intake in Neonatal Chicken

The information emerging from the studies demonstrates adrenergic system and nociceptin/orphanin FQ (N/OFQ) play a crucial role on appetite regulation but there is no information for their interaction. The purpose of this study was to examine the effects of intracerebroventricular (ICV) injection of prazosin (α₁ receptor antagonist), yohimbine (α₂ receptor antagonist), metoprolol (β₁ adrenergic receptor antagonist), ICI 118,551 (β₂ adrenergic receptor antagonist) and SR59230R (β₃ adrenergic receptor antagonist) on N/OFQ-induced hyperphagia by 3-h food-deprived neonatal broiler chicken. In experiment 1, chicken injected with saline, prazosin (10 nmol), N/OFQ (16 nmol) and co-injection of prazosin + N/OFQ. In experiment 2, ICV injection of saline, yohimbine (13 nmol), N/OFQ (16 nmol) and yohimbine + N/OFQ applied to the birds. In experiment 3, injections were saline, metoprolol (24 nmol), N/OFQ (16 nmol) and metoprolol + N/OFQ. In experiment 4, the birds received ICV injection of saline, ICI 118,551 (5 nmol), (C) N/OFQ (16 nmol) and co-administration of ICI 118,551 + N/OFQ. In experiment 5, chicken injected with saline, SR59230R (20 nmol), N/OFQ (16 nmol) and SR59230R + N/OFQ. Then, cumulative food intake was recorded until 120 min after injection. According to the results, ICV injection of N/OFQ significantly increased food intake (P < 0.001). The effect of N/OFQ significantly amplified by co-injection of N/OFQ + β₂ adrenergic receptor antagonist (P < 0.001). Also, administration of β₁ or β₃ adrenergic receptor antagonist had no effect on N/OFQ-induced hyperphagia (P > 0.05). These results suggest that the effect of N/OFQ on cumulative food intake is mediated via β₂ adrenergic receptors in neonatal chicken.     

Effects of litter types, microsite and root diameters on litter decomposition in Pinus sylvestris plantations of northern China

Background and aims

Litter decomposition is a major process in the carbon (C) flow and nutrient cycling of terrestrial ecosystems, but the effects of litter type, microsite, and root diameter on decomposition are poorly understood.

Methods

Litterbags were used to examine the decomposition rate of leaf litter and roots at three soil depths (5, 10 and 20 cm) over a 470-day period in Pinus sylvestris plantations in northern China.

Results

Leaves and the finest roots decomposed more quickly at 5 cm depth and coarser roots (>1-mm) decomposed more quickly at 10 and 20 cm depth. Roots generally decomposed more quickly than leaf litter, except at 5 cm deep; leaves decomposed more quickly than the coarsest roots (>5-mm). Root decomposition was strongly influenced by root diameter. Leaves experienced net nitrogen (N) immobilization and coarse roots (>2-mm) experienced more N release than fine roots. Significant heterogeneity was seen in N release for fine-roots (<2-mm) with N immobilization occurring in smaller (0.5–2-mm) roots and N release in the finest roots (<0.5-mm).

Conclusions

Soil depth of litter placement significantly influenced the relative contribution of the decomposition of leaves and roots of different diameters to carbon and nutrient cycling.     

Effects of Multi-nutrient Additions on GHG Fluxes in a Temperate Grassland of Northern China

Human activities have substantially enhanced the availability of important nutrient elements such as nitrogen (N), phosphorus (P), and potassium (K) in ecosystems worldwide. However, how the concurrent increase in all of these nutrients will affect greenhouse gas (that is, CO₂, N₂O, CH₄) levels remains unknown. In a temperate steppe of northern China, a 2-year field experiment was conducted to examine the effects of multi-nutrient additions on GHG fluxes from 2009 to 2010. Four levels of annual nutrient loads were mimicked: 0 g NPK (control), 15.5 g P m^?2 and 19.5 g K m^?2 as KH₂PO₄ (PK), 10 g N m^?2 as NH₄NO₃ plus PK (10N + PK), and 20 g N m^?2 plus PK (20N + PK) per year. The results show that multi-nutrient additions led to significant increases in net primary production (NPP) and soil temperature (ST), a significant decrease in soil moisture (SM) in 2010, and no significant changes in other soil parameters. Seasonal patterns differed greatly for different GHG fluxes in response to different nutrient treatments, largely as a result of differences in influential factors. The 10N + PK treatment significantly increased CO₂ uptake, whereas the 20N + PK treatment significantly decreased CO₂ uptake. The application of P and K without additional N significantly enhanced CH₄ uptake, whereas the two N + PK treatments significantly enhanced N₂O emissions. Significant positive, linear relationships were found between cumulative CO₂ uptake and soil total nitrogen (TN), microbial biomass carbon, and microbial biomass nitrogen, whereas significant negative, linear relationships were found with NPP, SM, and the C/N ratio. Significant positive, linear relationships were found between cumulative N₂O emission and ST, TN, NPP, and total organic carbon, whereas no relationships were found between cumulative CH₄ uptake and any soil parameters. CO₂ flux was related to N₂O flux temporally, to a certain extent, for all the treatments. In the control, N₂O flux showed a negative, linear relationship with CH₄ flux, whereas no regular relationships were detected between CO₂ and CH₄ fluxes in any treatment. Our findings imply that increasing nutrient deposition will change the magnitude, patterns, and relationships among GHG uptakes and emissions in the future.     

Plant growth regulator and nitrogen fertilizer effects on soil organic carbon sequestration in creeping bentgrass fairway turf

The objective of this study was to determine the effects of plant growth regulator (PGR) (no PGR, trinexapac-ethyl, and paclobutrazol) and N fertilizer (zero N, an average of 37 kg N ha^?1 month^?1, 6 and 12 kg N ha^?1 week^?1) on soil organic C (SOC) and soil N in creeping bentgrass (Agrostis stolonifera L.) fairway turf. After 4 years of field experiments soil samples were obtained from soil depths of 0–2.5, 2.5–5, 5–7.5, 7.5–10, 10–15, 15–20, and 20–30 cm. Soil bulk density, SOC, total N, NO ₃ ^? –N, and NH ₄ ⁺ –N concentrations were determined. Paclobutrazol and trinexapac-ethyl application increased SOC. The 37 kg N ha^?1 month^?1 application increased SOC at the 0–2.5 cm depth with both PGRs. When paclobutrazol was used, N fertilizer always increased SOC; however, the greatest increase was observed with the 12 kg N ha^?1 week^?1 application when compared to other rates, inversely related to the NH ₄ ⁺ –N concentration. Nitrogen application increased soil total N and NO ₃ ^? –N in the upper three depths. The application of PGRs and N fertilizer to creeping bentgrass fairway turf is an effective strategy for promoting C sequestration.     

Seasonal distribution of nitrifying bacteria and rates of nitrification in coastal marine sediments

The distribution of nitrification potential (NP) with depth in sediment and season was investigated in a shallow sandy sediment (0.5 m water) and a deeper muddy sediment (17m water). In both sediments, nitrifying bacteria were present in the anoxic strata (oxygen penetration was 5 mm below the surface). The NP at 6–8 cm depth in the sediment was 50% and 10% of the surface NP at the sandy and muddy sediment, respectively. It is suggested that bioturbation and physical disturbance of the sediment were the most likely reasons for this distribution. The NP increased as sediment temperature decreased. This effect was less marked in the muddy sediment. It is concluded that during the summer, the numbers or specific activity of nitrifying bacteria diminished for the following reasons: There was decreased O₂ penetration into the sediment and increased competition for O₂ by heterotrophs; there was increased competition for NH₄ ⁺ and there was inhibition by H₂S. These effects counteracted the potentially higher growth rates and increased rates of NH₄ ⁺ production at the elevated summer temperatures. The potential nitrification rates in the upper 1 cm, which were measured at 22°C, were converted to calculated rates at the in situ temperature (Q₁₀=2.5) and in situ oxygen penetration. These calculated rates were shown to closely resemble the measured in situ rates of nitrification. The relationship between the in situ rates of nitrification and the nitrification potential is discussed.     

Importance of transient storage zones for ammonium and phosphate retention in a sandy-bottom Mediterranean stream

1. The ability of hyporheic sediments to exchange water and retain ammonium and phosphate in the Riera Major stream ,North-East Spain, under different discharge conditions was measured by conducting short-term nutrient and chloride additions. 2. The mean exchange coefficients from free-flowing water to the storage zone (k₁) and vice versa (k₂) were 0.82 × 10–4 s^??1 and 7 × 10^??3 s^??1, respectively. The ratio of storage zone cross-sectional area to stream cross-sectional area (A_S/A) averaged 2.8 × 10–2 and was negatively correlated with discharge (r = –0.85, d.f. = 13, P < 0.001). 3. The percentage of hyporheic zone water which came from surface water varied as a function of discharge and hyporheic depth, ranging between 33% and 95% at 25 cm depth, and between 78% and 100% at 10 cm depth. 4. The nutrient retention efficiency in the hyporheic zone at 10 cm depth measured as uptake length (S_wh) was less than 3.3 cm for ammonium and 37 cm for phosphate. Higher nutrient retentions were measured in the sediments at 10 cm depth than at 25 cm, indicating that near-surface sediments were involved more actively in phosphate retention than the deeper hyporheic sediments. The lack of ammonium at any depth of the hyporheic zone showed that ammonium was very rapidly taken up in the surfacial sediments.     

Land cover change effects on soil chemical and biological properties after planting Mongolian pine (Pinus sylvestris var. mongolica) in sandy lands in Keerqin, northeastern China

We compared soil moisture content, pH, total organic carbon (C _org), total nitrogen (TN), total phosphorus (TP) and inorganic N (NH₄ ⁺–N, NO₃ ^?–N) concentrations, soil potential C and N mineralization rates, soil microbial biomass C (C _mic), soil metabolic quotient (qCO₂), soil microbial quotient (C _mic/C _org) and soil enzyme (urease and invertase) activities in semiarid sandy soils under three types of land cover: grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, and elm (Ulmus punila)–grass savanna in southeastern Keerqin, in northeast China. Soil C _org, TN and TP concentrations (0–10, 10–20, 20–40 and 40–60 cm) were lower while soil C/N and C/P ratios were higher in the plantation than in grassland and savanna. The effects of land cover change on NH₄ ⁺–N and NO₃ ^?–N concentrations, soil potential nitrification and C mineralization rates in the surface soil (0–10 cm) were dependent on sampling season; but soil potential N mineralization rates were not affected by land cover type and sampling season. The effects of land cover change on C _mic and qCO₂ of surface soil were not significant; but C _mic/C _org were significantly affected by land cover change and sampling season. We also found that land cover change, sampling season and land cover type?×?sampling season interaction significantly influenced soil enzyme (urease and invertase) activities. Usually soil enzyme activities were lower in the pine plantations than in grassland and savanna. Our results suggest that land cover change markedly influenced soil chemical and biological properties in sandy soils in the semiarid region, and these effects vary with sampling season.     

Remediation of Nitrogen-Contaminated Sediment Using Bioreactive,Thin-layer Capping with Biozeolite

In situ amendment of nitrogen-contaminated sediment using bioreactive, thin-layer capping (BTC) with biozeolite (i.e., zeolite with heterotrophic nitrifiers as well as aerobic denitrifiers attached) was studied herein. BTC with biozeolite for nitrogen-contaminated sediment management was evaluated through long-term (170 days) sediment incubation laboratory experiments. The results showed that BTC with relatively small dose rates (<10 kg m^?2) of biozeolite reduced the total nitrogen (TN) concentration in overlying water by over 90%, so it was effective in reducing the amount of N released from sediment. Higher-dose rates of biozeolite capping achieved an even higher removal efficiency. With the DO concentration of 1.5 ～ 6.5 mg L^?1 in overlying water, the reduction efficiency of TN in overlying water using BTC was higher than that less than 1 mg L^?1. In BTC systems, biological regeneration (i.e., heterotrophic nitrifiers attached to zeolite can regenerate the zeolite ion exchange capacity for ammonium) occurred in biozeolite which was saturated with ammonium during the nitrification period. In addition, TN contents in surface sediment in BTC systems were reduced at different levels after the experiment. These findings indicate that the BTC can be a feasible remedial approach to reduce N in overlying water and sediment in eutrophic water bodies. In the BTC, N load was reduced by the added biozeolite through adsorbing ammonium (NH₄⁺-N), converting NH₄⁺-N into nitrate nitrogen (NO₃^?-N) and nitrogen gas (N₂), and assimilating inorganic nitrogen.     

On the importance of quantifying bioavailable nitrogen instead of total nitrogen

Monitored and modeled data provided the basis for the establishment of two nitrogen (N) budgets covering the Kattegat-Belt Sea area in the period 2000–2009: one for total nitrogen (TN) and one for bioavailable nitrogen (N_bio). Our results show a significant difference between the two budgets, and we argue that N_bio is more important than TN for our understanding of the sources causing marine eutrophication. Consequently, an optimal strategy for abatement of eutrophication aims at minimizing N_bio rather than TN. The TN budget shows that advection from the adjacent seas is the dominant source of N to the Kattegat-Belt Sea area. The loadings from land and atmosphere only account for 14 and 9 % of the TN loadings, respectively. However, when the bioavailability of the different N sources is taken into account, the supply from land and atmosphere becomes relatively more important, now accounting for 21 and 16 %, respectively (37 % in total). The ecological relevance of land and atmosphere loadings is most likely even larger since a fraction of the input from the Skagerrak is exported again before it can support primary production. Water action plans have reduced the direct loadings of TN from land and atmosphere by about 35 % since the 1980s. The contributions from land and atmosphere accounted for 47 % of the N_bio loadings in the 1980s. Hence, loadings from land and atmosphere have only decreased by 10 % points since the 1980s: from 47 to 37 %. The largest sink of TN in the study areas is advection to the adjacent seas (71 %) whereas denitrification and burial only accounts for 17 and 11 %, respectively.     

Simultaneous nitrification–denitrification and microbial community profile in an oxygen-limiting intermittent aeration SBBR with biodegradable carriers

To enhance the startup and efficient simultaneous nitrification and denitrification for sewage treatment, sequencing batch biofilm reactors (SBBRs) partially coupled with rice husk were established and operated under various intermittent micro-aeration cycles (IMCs) and COD/N ratios under oxygen-limiting intermittent aeration conditions. Experimental results showed that the increase of IMCs with non-aeration/micro-aeration mode of (8 h/4 h)₁ to (2 h/1 h)₄ in a 12 h-cycle accelerated the startup performance and improved NH₄⁺–N and COD removal. NH₄⁺–N, TN and COD removal efficiencies were 98.7?±?0.9, 89.2?±?5.2 and 82.9?±?6.7% at COD/N ratio of 7.6 with the highest IMCs in SBBR, respectively. Higher TN removal efficiencies of 87.2?±?4.0 and 58.1?±?3.5% were also achieved at lower COD/N ratio of 5.6 and 2.8, respectively. In SBBRs with various IMCs, facultative denitrifier like genus Acinetobacter and solid-phase denitrifier belonging to Comamonadaceae family were enriched. However, aerobic denitrifiers with function of heterotrophic nitrification like Paracoccus were favored to enrich under higher IMCs condition, and more anoxic denitrifiers like sulfur-based autotrophic denitrifier Thiothrix and heterotrophic denitrifiers like Pseudomonas and Methyloversatilis were observed at lower IMCs condition. Autotrophic nitrifier (Nitrosomonas and Nitrosipra) and heterotrophic nitrifiers both contributed to the efficient nitrification.     

Microbial N turnover processes in three forest soil layers following clear cutting of an N saturated mature spruce stand

Microbial N turnover processes were investigated in three different forest soil layers [organic (O) layer, 0–10 cm depth (M₁), 10–40 cm depth (M₂)] after the clear cutting of a nitrogen (N) saturated spruce stand at the Höglwald Forest (Bavaria, Germany). The aim of the study was to provide detailed insight into soil-layer specific microbial production and the consumption of inorganic N within the main rooting zone. Furthermore, we intended to clarify the relevance of each soil layer investigated in respect of the observed high spatial variation of seepage water nitrate (NO ₃ ^? ) concentration at a depth of 40 cm. The buried bag and the ¹⁵N pool dilution techniques were applied to determine the net and gross N turnover rates. In addition, soil pH, C:N ratio, pool sizes of soil ammonium (NH ₄ ⁺ ) and NO ₃ ^? , as well as quantities of microbial biomass carbon (C_mic) and nitrogen (N_mic) were determined. The 40 cm thick upper mineral soil was found to be the main place of NO ₃ ^? production with a NO ₃ ^? supply or net nitrification three times higher than in the considerably thinner O layer. Nevertheless, O layer nitrification processes determined via in situ field experiments showed significant correlation with seepage water NO ₃ ^? . An improved correlation noted several months after the cut may result from a transport-induced time shift of NO ₃ ^? with downstream hydrological pathways. In contrast, the soil laboratory incubation experiments found no indication that mineral soil is relevant for the spatial heterogeneity of seepage water NO ₃ ^? . The results from our study imply that in situ experiments may be better suited to studies investigating N turnover in relation to NO ₃ ^? loss via seepage water in similar ecosystems in order to gain representative data.