The importance of denitrification for N2O emissions from an N-saturated forest in SW China: results from in situ 15N labeling experiments

Long-term elevated atmogenic deposition (~5 g m^?2 year^?1) of reactive nitrogen (N) causes N saturation in forests of subtropical China which may lead to high nitrous oxide (N₂O) emissions. Recently, we found high N₂O emission rates (up to 1,730 μg N₂O–N m^?2 h^?1) during summer on well-drained acidic acrisols (pH = 4.0) along a hill slope in the forested Tieshanping catchment, Chongqing, southwest China. Here, we present results from an in situ ¹⁵N–NO₃ ^? labeling experiment to assess the contribution of nitrification and denitrification to N₂O emissions in these soils. Two loads of 99 at.% K¹⁵NO₃ (equivalent to 0.2 and 1.0 g N m^?2) were applied as a single dose to replicated plots at two positions along the hill slope (at top and bottom, respectively) during monsoonal summer. During a 6-day period after label application, we found that 71–100 % of the emitted N₂O was derived from the labeled NO₃ ^? pool irrespective of slope position. Based on this, we assume that denitrification is the dominant process of N₂O formation in these forest soils. Within 6 days after label addition, the fraction of the added ¹⁵N–NO₃ ^? emitted as ¹⁵N–N₂O was highest at the low-N addition plots (0.2 g N m^?2), amounting to 1.3 % at the top position of the hill slope and to 3.2 % at the bottom position, respectively. Our data illustrate the large potential of acid forest soils in subtropical China to form N₂O from excess NO₃ ^? most likely through denitrification.     

An in-depth look into a tropical lowland forest soil: nitrogen-addition effects on the contents of N2O, CO2 and CH4 and N2O isotopic signatures down to 2-m depth

Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha^?1 year^?1) to a seasonal lowland forest affected depth distribution and contents of soil nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄), as well as natural abundance isotopic signatures of N₂O, nitrate (NO₃ ^?) and ammonium (NH₄ ⁺). In the control plots during dry season, we deduced limited N₂O production by denitrification in the topsoil (0.05–0.40 m) as indicated by: ambient N₂O concentrations and ambient ¹⁵N-N₂O signatures, low water-filled pore space (35–60%), and similar ¹⁵N signatures of N₂O and NO₃ ^?. In the subsoil (0.40–2.00 m), we detected evidence of N₂O reduction to N₂ during upward diffusion, indicating denitrification activity. During wet season, we found that N₂O at 0.05–2.00 m was mainly produced by denitrification with substantial further reduction to N₂, as indicated by: lighter ¹⁵N-N₂O than ¹⁵N-NO₃ ^? throughout the profile, and increasing N₂O concentrations with simultaneously decreasing ¹⁵N-N₂O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between ¹⁸O-N₂O and ¹⁵N-N₂O site preferences. Long-term N addition did not affect dry-season soil N₂O-N contents, doubled wet-season soil N₂O-N contents, did not affect ¹⁵N signatures of NO₃ ^?, and reduced wet-season ¹⁵N signatures of N₂O compared to the control plots. These suggest that the increased NO₃ ^? concentrations have stimulated N₂O production and decreased N₂O-to-N₂ reduction. Soil CO₂-C contents did not differ between treatments, implying that N addition essentially did not influence soil C cycling. The pronounced seasonality in soil respiration was largely attributable to enhanced topsoil respiration as indicated by a wet-season increase in the topsoil CO₂-C contents. The N-addition plots showed reduced dry-season soil CH₄-C contents and threshold CH₄ concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH₄ uptake rates remained similar between treatments possibly because diffusive CH₄ supply from the atmosphere largely limited CH₄ oxidation.     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.     

Indirect N2O emissions from shallow groundwater in an agricultural catchment (Seine Basin, France)

Production and accumulation of nitrous oxide (N₂O), a major greenhouse gas, in shallow groundwater might contribute to indirect N₂O emissions to the atmosphere (e.g., when groundwater flows into a stream or a river). The Intergovernmental Panel on Climate Change (IPCC) has attributed an emission factor (EF_5g) for N₂O, associated with nitrate leaching in groundwater and drainage ditches—0.0025 (corresponding to 0.25% of N leached which is emitted as N₂O)—although this is the subject of considerable uncertainty. We investigated and quantified the transport and fate of nitrate (NO₃ ^?) and dissolved nitrous oxide from crop fields to groundwater and surface water over a 2-year period (monitoring from April 2008 to April 2010) in a transect from a plateau to the river with three piezometers. In groundwater, nitrate concentrations ranged from 1.0 to 22.7 mg NO₃ ^?–N l^?1 (from 2.8 to 37.5 mg NO₃ ^?–N l^?1 in the river) and dissolved N₂O from 0.2 to 101.0 μg N₂O–N l^?1 (and from 0.2 to 2.9 μg N₂O–N l^?1 in the river). From these measurements, we estimated an emission factor of EF_5g = 0.0026 (similar to the value currently used by the IPCC) and an annual indirect N₂O flux from groundwater of 0.035 kg N₂O–N ha^?1 year^?1, i.e., 1.8% of the previously measured direct N₂O flux from agricultural soils.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

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.     

The effect of nitrogen addition on soil organic matter dynamics: a model analysis of the Harvard Forest Chronic Nitrogen Amendment Study and soil carbon response to anthropogenic N deposition

Recent observations indicate that long-term N additions can slow decomposition, leading to C accumulation in soils, but this process has received limited consideration by models. To address this, we developed a model of soil organic matter (SOM) dynamics to be used with the PnET model and applied it to simulate N addition effects on soil organic carbon (SOC) stocks. We developed the model’s SOC turnover times and responses to experimental N additions using measurements from the Harvard Forest, Massachusetts. We compared model outcomes to SOC stocks measured during the 20th year of the Harvard Forest Chronic Nitrogen Amendment Study, which includes control, low (5 g N m^?2 yr^?1) and high (15 g N m^?2 yr^?1) N addition to hardwood and red pine stands. For unfertilized stands, simulated SOC stocks were within 10 % of measurements. Simulations that used measured changes in decomposition rates in response to N accurately captured SOC stocks in the hardwood low N and pine high N treatment, but greatly underestimated SOC stocks in the hardwood high N and the pine low N treatments. Simulated total SOC response to experimental N addition resulted in accumulation of 5.3–7.9 kg C per kg N following N addition at 5 g N m^?2 yr^?1 and 4.1–5.3 kg C per kg N following N addition at 15 g N m^?2 yr^?1. Model simulations suggested that ambient atmospheric N deposition at the Harvard Forest (currently 0.8 g N m^?2 yr^?1) has led to an increase in cumulative O, A, and B horizons C stocks of 211 g C m^?2 (3.9 kg C per kg N) and 114 g C m^?2 (2.1 kg C per kg N) for hardwood and pine stands, respectively. Simulated SOC accumulation is primarily driven by the modeled decrease in SOM decomposition in the Oa horizon.     

Simulation of N2O emissions and nitrate leaching from plastic mulch radish cultivation with LandscapeDNDC

Radish is one of the major dry field crops in Asia commonly grown with plastic mulch and high rates of N fertilization, and potentially harming the environment due to N₂O emissions and nitrate leaching. Despite the widespread use of plastic mulch, biogeochemical models so far do not yet consider impacts of mulch on soil environmental conditions and biogeochemistry. In this study, we adapted and successfully tested the LandscapeDNDC model against field data by simulating crop growth, C and N turnover and associated N₂O emissions as well as nitrate leaching for radish cultivation with plastic mulch and in conjunction with different rates of N fertilization (465–765 kg N ha^?1 year^?1). Due to the sandy soil texture and monsoon climate, nitrate leaching with rates up to 350 kg N ha^?1 year^?1 was the dominant reason for overall low nitrogen use efficiency (32–43 %). Direct or indirect N₂O emissions (calculated from simulated nitrate leaching rates and IPCC EF_ind = 0.0075) ranged between 2 and 3 kg N ha^?1 year^?1, thus contributing an equal amount to total field emissions of about 5 kg N ha^?1 year^?1. Based on our results, emission factors for direct N₂O emissions ranged between 0.004 and 0.005. These values are only half of the IPCC default value (0.01), demonstrating the need of biogeochemical models for developing site and/or region specific EFs. Simulation results also revealed that changes in agricultural management by applying the fertilizer only to the rows would be an efficient mitigation strategy, effectively decreasing field nitrate leaching and N₂O emissions by 50–60 %.     

Improving N management through intercropping alleviates the inhibitory effect of mineral N on nodulation in pea

Background and aims

Symbiotic N₂ fixation is essential in the development of sustainable agriculture, but the nodulation of legumes is usually inhibited by N fertilization. Here, the intercropping of maize and pea in strips under various N managements was used as a means to alleviate the inhibitory effect of mineral N on pea nodulation and N₂ fixation and to improve system performance.

Methods

N natural abundance (δ ¹⁵N) analysis was employed to quantify N₂ fixation in the 3 years (2012 to 2014) of field experiment in Hexi Corridor of Northwestern China. Four N management systems with N rate of 0 kg N ha^?1 (the control), 90?+?45 kg N ha^?1 (base N plus topdressing N), 90?+?90 kg N ha^?1, and 90?+?135 kg N ha^?1 were implemented in the maize/pea strip intercropping to form different ratios of base N to topdressing N.

Results

Intercropped pea improved nodule biomass per plant by 99 %, increased nitrogen derived from the atmosphere (Ndfa) by 35 %, and promoted aboveground plant tissue N accumulation by 35 % as compared with sole pea, averaged across the four N treatments. Compared to the highest N fertilizer treatment, a reduction of topdressing to 45 kg N ha^?1 increased the nodule biomass of intercropped pea by 116 %, Ndfa by 35 %, and grain yield by 6 %.

Conclusions

Adaptation of suitable N management in cereal/legume intercropping systems will allow an effective conversion of atmospheric N₂ into crop available N and thus maximizing the system productivity.

     

Changes to the N cycle following bark beetle outbreaks in two contrasting conifer forest types

Outbreaks of Dendroctonus beetles are causing extensive mortality in conifer forests throughout North America. However, nitrogen (N) cycling impacts among forest types are not well known. We quantified beetle-induced changes in forest structure, soil temperature, and N cycling in Douglas-fir (Pseudotsuga menziesii) forests of Greater Yellowstone (WY, USA), and compared them to published lodgepole pine (Pinus contorta var. latifolia) data. Five undisturbed stands were compared to five beetle-killed stands (4–5 years post-outbreak). We hypothesized greater N cycling responses in Douglas-fir due to higher overall N stocks. Undisturbed Douglas-fir stands had greater litter N pools, soil N, and net N mineralization than lodgepole pine. Several responses to disturbance were similar between forest types, including a pulse of N-enriched litter, doubling of soil N availability, 30–50 % increase in understory cover, and 20 % increase in foliar N concentration of unattacked trees. However, the response of some ecosystem properties notably varied by host forest type. Soil temperature was unaffected in Douglas-fir, but lowered in lodgepole pine. Fresh foliar %N was uncorrelated with net N mineralization in Douglas-fir, but positively correlated in lodgepole pine. Though soil ammonium and nitrate, net N mineralization, and net nitrification all doubled, they remained low in both forest types (<6 μg N g soil^?1 NH₄ ⁺or NO₃ ^?; <25 μg N g soil^?1 year^?1 net N mineralization; <8 μg N g soil^?1 year^?1 net nitrification). Results suggest that beetle disturbance affected litter and soil N cycling similarly in each forest type, despite substantial differences in pre-disturbance biogeochemistry. In contrast, soil temperature and soil N–foliar N linkages differed between host forest types. This result suggests that disturbance type may be a better predictor of litter and soil N responses than forest type due to similar disturbance mechanisms and disturbance legacies across both host–beetle systems.     

Effects of Litter Inputs on N<Subscript>2</Subscript>O Emissions from a Tropical Rainforest in Southwest China

Litter inputs are expected to have a strong impact on soil N₂O efflux. This study aimed to assess the effects of the litter decomposition process and nutrient efflux from litter to soil on soil N₂O efflux in a tropical rainforest. A paired study with a control (L) treatment and a litter-removed (NL) treatment was followed for 2 years, continuously monitoring the effects of these treatments on soil N₂O efflux, fresh litter input, decomposed litter carbon (LCI) and nitrogen (LNI), soil nitrate (NO₃ ^?–N), ammonium (NH₄ ⁺–N), dissolved organic carbon (DOC), and dissolved nitrogen (DN). Soil N₂O flux was 0.48 and 0.32 kg N₂O–N ha^?1 year^?1 for the L and NL treatments, respectively. Removing the litter caused a decrease in the annual soil N₂O emission by 33%. The flux values from the litter layer were higher in the rainy season as compared to the dry season (2.10 ± 0.28 vs. 1.44 ± 0.35 μg N m^?2 h^?1). The N₂O fluxes were significantly correlated with the soil NO₃ ^?–N contents (P < 0.05), indicating that the N₂O emission was derived mainly from denitrification as well as other NO₃ ^? reduction processes. Suitable soil temperature and moisture sustained by rainfall were jointly attributed to the higher soil N₂O fluxes of both treatments in the rainy season. The N₂O fluxes from the L were mainly regulated by LCI, whereas those from the NL were dominated jointly by soil NO₃ ^? content and temperature. The effects of LCI and LNI on the soil N₂O fluxes were the greatest in the 2 months after litter decomposition. Our results show that litter may affect not only the variability in the quantity of N₂O emitted, but also the mechanisms that govern N₂O production. However, further studies are still required to elucidate the impacting mechanisms of litter decomposition on N₂O emission from tropical forests.     

Nitrous oxide emissions during the non-rice growing seasons of two subtropical rice-based rotation systems in southwest China

Background and aims

High nitrous oxide (N₂O) emissions may occur during the non-rice growing season of Chinese rice-upland crop rotation systems. However, our understanding of N₂O emission during this season is poor due to a scarcity of available field N₂O measurements.

Methods

Using the static manual chamber-GC technique, seasonal N₂O emissions during the non-rice growing season were simultaneously measured at two adjacent rice-wheat and rice-rapeseed fields in southwest China for three consecutive annual rotation cycles (May 2005 to May 2008).

Results

Compared to the control, N fertilizer applications significantly enhanced soil N₂O emissions from both wheat and rapeseed systems. Seasonal cumulative N₂O fluxes from wheat systems were on average 2.6 kg N ha^?1 for the recommended practice (RP [150 kg N ha^?1]) and 5.0 kg N ha^?1 for the conventional practice (CP [250 kg N ha^?1]). Lower N₂O emissions were observed from the adjacent rapeseed systems. Average cumulative seasonal N₂O fluxes from rapeseed were 1.5 and 2.2 kg N ha^?1 for the RP and CP treatments, respectively. The first 3 weeks after N fertilization were the “hot moment” of N₂O emissions for both the wheat and rapeseed systems. The lowest yield-scaled N₂O fluxes for wheat were obtained at the RP treatment (mean: 0.81 kg N Mg^?1) while for rapeseed the CP treatment produced the lowest yield-scaled fluxes (mean: 0.79 kg N Mg^?1). On average, the direct N₂O emission factors (EF_d) for the wheat system (1.76 %) were over two times higher than for the rapeseed system (0.73 %).

Conclusions

Intercropping of rapeseed tends to result in lower N₂O emissions than wheat for rice-upland crop rotation systems of southwest China, indicating that either the N fertilization or the cropping system need to be considered not only for improving the estimate of regional and/or national N₂O fluxes but also for proposing the climate-smart agricultural management practice to reduce N₂O emissions from agricultural soils.     

Quantifying the contribution of land use to N<Subscript>2</Subscript>O,NO and CO<Subscript>2</Subscript> fluxes in a montane forest ecosystem of Kenya

Increasing demand for food and fibre by the growing human population is driving significant land use (LU) change from forest into intensively managed land systems in tropical areas. But empirical evidence on the extent to which such changes affect the soil-atmosphere exchange of trace gases is still scarce, especially in Africa. We investigated the effect of LU on soil trace gas production in the Mau Forest Complex region, Kenya. Intact soil cores were taken from natural forest, commercial and smallholder tea plantations, eucalyptus plantations and grazing lands, and were incubated in the lab under different soil moisture conditions. Soil fluxes of nitrous oxide (N₂O), nitric oxide (NO) and carbon dioxide (CO₂) were quantified, and we approximated annual estimates of soil N₂O and NO fluxes using soil moisture values measured in situ. Forest and eucalyptus plantations yielded annual fluxes of 0.3–1.3 kg N₂O–N ha^?1 a^?1 and 1.5–5.2 kg NO–N ha^?1 a^?1. Soils of commercial tea plantations, which are highly fertilized, showed higher fluxes (0.9 kg N₂O–N ha^?1 a^?1 and 4.3 kg NO–N ha^?1 a^?1) than smallholder tea plantations (0.1 kg N₂O–N ha^?1 a^?1 and 2.1 kg NO–N ha^?1 a^?1) or grazing land (0.1 kg N₂O–N ha^?1 a^?1 and 1.1 kg NO–N ha^?1 a^?1). High soil NO fluxes were probably the consequence of long-term N fertilization and associated soil acidification, likely promoting chemodenitrification. Our experimental approach can be implemented in understudied regions, with the potential to increase the amount of information on production and consumption of trace gases from soils.     

Responses of nitrous oxide fluxes and soil nitrogen cycling to nutrient additions in montane forests along an elevation gradient in southern Ecuador

Tropical montane forests are commonly limited by N or co-limited by N and P. Projected increases in N deposition in tropical montane regions are thought to be insufficient for vegetation demand and are not therefore expected to affect soil N availability and N₂O emissions. We established a factorial N- and P-addition experiment (i.e., N, P, N + P, and control) across an elevation gradient of montane forests in Ecuador to test these hypotheses: (1) moderate rates of N and P additions are able to stimulate soil-N cycling rates and N₂O fluxes, and (2) the magnitude and timing of soil N₂O-flux responses depend on the initial nutrient status of the forest soils. Moderate rates of nutrients were added: 50 kg N ha^?1 year^?1 (in the form of urea) and 10 kg P ha^?1 year^?1 (in the form of NaH₂PO ₄ ^. 2H₂O) split in two equal applications. We tested the hypotheses by measuring changes in net rates of soil–N cycling and N₂O fluxes during the first 2 years (2008–2009) of nutrient manipulation in an old-growth premontane forest at 1,000 m, growing on a Cambisol soil with no organic layer, in an old-growth lower montane forest at 2,000 m, growing on a Cambisol soil with an organic layer, and an old-growth upper montane rainforest at 3,000 m, growing on a Histosol soil with a thick organic layer. Among the control plots, net nitrification rates were largest at the 1,000-m site whereas net nitrification was not detectable at the 2,000- and 3,000-m sites. The already large net nitrification at the 1,000-m site was not affected by nutrient additions, but net nitrification became detectable at the 2,000- and 3000-m sites after the second year of N and N + P additions. N₂O emissions increased rapidly following N and N + P additions at the 1,000-m site whereas only smaller increases occurred at the 2,000- and 3,000-m sites during the second year of N and N + P additions. Addition of P alone had no effect on net rates of soil N cycling and N₂O fluxes at any elevation. Our results showed that the initial soil N status, which may also be influenced by presence or absence of organic layer, soil moisture and temperature as encompassed by the elevation gradient, is a good indicator of how soil N cycling and N₂O fluxes may respond to future increases in nutrient additions.     

Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways

Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO₂ and N₂O immediately following wetting were among the highest ever published (up to 1,588 mg C m^?2 h^?1 and 3,121 μg N m^?2 h^?1). Mean post-wetting CO₂ and N₂O fluxes were significantly higher in the loam and sandy loam channels (286 and 194 mg C m^?2 h^?1; 168 and 187 μg N m^?2 h^?1) than in the sand channels (45 mg C m^?2 h^?1 and 7 μg N m^?2 h^?1). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources.     

Do “hot moments” become hotter under climate change? Soil nitrogen dynamics from a climate manipulation experiment in a post-harvest forest

Whole-tree forest harvest can increase soil nitrous oxide (N₂O) effluxes and leaching of nitrogen (N) from soils. These altered N dynamics are often linked to harvesting effects on microclimate, suggesting that this “hot moment” for N cycling may become hotter with climate change. We hypothesized that increases in temperature and precipitation during this post-harvest period would increase availability of soil mineral N and soil-atmosphere N₂O efflux. To test this hypothesis we implemented a climate manipulation experiment after a forest harvest, and measured soil N₂O fluxes and inorganic N accumulating on ion exchange resins. Climate treatments were: control (A, ambient), heated (H, +2.5 °C), wetted (W, +23 % precipitation), and a two-factor treatment (H+W). For all treatments, the first year after harvest had highest N₂O efflux and resin N. Wetting significantly increased cumulative soil N₂O fluxes, but only when soils were not heated too. The cumulative soil-to-atmosphere N₂O efflux from W (5.8 mg N₂O–N m^?2) was significantly higher than A (?1.9 mg N₂O–N m^?2), but H+W (~0 mg N₂O–N m^?2) was similar to A. Regardless of wetting, heating increased resin N, but only on certain dates. Cumulative resin N was on average 125 % greater in the H plots than non-heated plots. Thus, changes in temperature and precipitation each impart distinct changes to the soil N cycle. Heating increased resin N regardless of water inputs, while wetting increasing N₂O but not when combined with heating. Our results suggest that climate change may exacerbate soil N losses from whole-tree harvest in the future, but the form and quantity of N loss will depend on how the future climate changes.     

Nitrogen-15 signals of leaf-litter-soil continuum as a possible indicator of ecosystem nitrogen saturation by forest succession and N loads

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of ¹⁵N (??¹⁵N) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, N₂O emission and inorganic N leaching as well as N concentration and ?? ¹⁵N of leaves, litters and soils. We found that foliar N concentration and ??¹⁵N were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly increased foliar ?? ¹⁵N (p < 0.05). Also, N addition decreased the ?? ¹⁵N of top soil in the N-poor pine and mixed forests and significantly increased that of organic and mineral soils in N-rich broadleaf forests (p < 0.05). In addition, the soil N₂O emission flux and inorganic N leaching rate increased with increasing N addition and were positively correlated with the ¹⁵N enrichment factor (?? _p/s) of forest ecosystems. Our study indicates that ?? ¹⁵N of leaf, litter and soil integrates various information on plant species, forest stand age, exogenous N input and soil N transformation and loss, which can be used to monitor N availability and N dynamics in forest ecosystems caused by increasing N deposition in the future.     

Optimal C:N ratio for the production of red pigments by Monascus ruber

The carbon-to-nitrogen (C:N) ratio in the biomass of microfungi tends to be quite different (e.g. 10–15) compared with the C:N ratio in the red pigments (e.g. >20) of the fungus Monascus ruber. Therefore, determining an optimal C:N ratio in the culture medium for maximizing the production of the pigments is important. A culture medium composition is established for maximizing the production of the red pigment by the fungus M. ruber ICMP 15220 in submerged culture. The highest volumetric productivity of the red pigment was 0.023 AU L^?1 h^?1 in a batch culture (30 °C, initial pH of 6.5) with a defined medium of the following composition (g L^?1): glucose (10), monosodium glutamate (MSG) (10), MgSO₄·7H₂O (0.5), KH₂PO₄ (5), K₂HPO₄ (5), ZnSO₄·7H₂O (0.01), FeSO₄·7H₂O (0.01), CaCl₂ (0.1), MnSO₄·H₂O (0.03). This medium formulation had a C:N mole ratio of 9:1. Under these conditions, the specific growth rate of the fungus was 0.043 h^?1 and the peak biomass concentration was 6.7 g L^?1 in a 7-day culture. The biomass specific productivity of the red pigment was 1.06 AU g^?1 h^?1. The best nitrogen source proved to be MSG although four other inorganic nitrogen sources were evaluated.     

Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation

To evaluate nitrogen (N) saturation in xeric environments, we measured hydrologic N losses, soil N pools, and microbial processes, and developed an N-budget for a chaparral catchment (Sierra Nevada, California) exposed to atmospheric N inputs of approximately 8.5 kg N ha^?1 y^?1. Dual-isotopic techniques were used to trace the sources and processes controlling nitrate (NO₃ ^?) losses. The majority of N inputs occurred as ammonium. At the onset of the wet season (November to April), we observed elevated streamwater NO₃ ^? concentrations (up to 520 µmol l^?1), concomitant with the period of highest gaseous N-loss (up to 500 ng N m^?2 s^?1) and suggesting N-saturation. Stream NO₃ ^? δ¹⁵N and δ¹⁸O and soil N measurements indicate that nitrification controlled NO₃ ^? losses and that less than 1% of the loss was of atmospheric origin. During the late wet season, stream NO₃ ^? concentrations decreased (to <2 µmol l^?1) as did gaseous N emissions, together suggesting conditions no longer indicative of N-saturation. We propose that chaparral catchments are temporarily N-saturated at ≤8.5 kg N ha^?1 y^?1, but that N-saturation may be difficult to reach in ecosystems that inherently leak N, thereby confounding the application of N-saturation indicators and annual N-budgets. We propose that activation of N sinks during the typically rainy winter growing season should be incorporated into the assessment of ecosystem response to N deposition. Specifically, the N-saturation status of chaparral may be better assessed by how rapidly catchments transition from N-loss to N-retention.     

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO₃ ^??CN, NH₄ ⁺?CN and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15?C0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural ¹⁵N abundance in NO₃ ^? during the passage of rain water through the ecosystem and bulk ??¹⁵N values in soil to detect N transformations. Depletion of ¹⁵N in NO₃ ^? and increased NO₃ ^??CN fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO₃ ^? concentrations progressively decreased and were enriched in ¹⁵N but did not reach the ??¹⁵N values of solid phase organic matter (??¹⁵N = 5.6?C6.7??). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the ??¹⁵N value of NO₃ ^? in litter leachate was smaller (??¹⁵N = ?1.58??) than in the other quarters (??¹⁵N = ?9.38 ± SE 0.46??) probably because of reduced mineralization and associated fractionation against ¹⁵N. Nitrogen isotope fractionation of NO₃ ^? between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO₃ ^??CN from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO₃ ^? gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.