The role of gross and net N transformation processes and NH4 + and NO3 − immobilization in controlling the mineral N pool of a temperate mixed deciduous forest soil

In many forests of Europe and north-eastern North America elevated N deposition has opened the forest N cycle, resulting in NO₃ ^? leaching. On the other hand, despite this elevated N deposition, the dominant fate of NO₃ ^? and NH₄ ⁺ in some of these forests is biotic or abiotic immobilization in the soil organic matter pool, preventing N losses. The environmental properties controlling mineral N immobilization and the variation and extent of mineral N immobilization in forest soils are not yet fully understood. In this study we investigated a temperate mixed deciduous forest, which is subjected to an average N deposition of 36.5 kg N ha^?1 yr^?1, but at the same time shows low NO₃ ^? concentrations in the groundwater. The aim of this study was to investigate whether the turnover rate of the mineral N pool could explain these low N leaching losses. A laboratory ¹⁵N pool dilution experiment was conducted to study gross and net N mineralization and nitrification and mineral N immobilization in the organic and uppermost (0–10 cm) mineral layer of the forest soil. Two locations, one at the forest edge (GE) and another one 145 m inside the forest (GF1), were selected. In the organic layers of GE and GF1, the gross N mineralization averaged 10.9 and 11.1 mg N kg^?1 d^?1, the net N mineralization averaged 6.1 and 6.8 mg N kg^?1 d^?1 and NH₄ ⁺ immobilization rates averaged 3.8 and 3.6 mg N kg^?1 d^?1. In the organic layer of GE and GF1, the average gross nitrification was 3.8 and 4.6 mg N kg^?1 d^?1, the average net nitrification was ?25.2 and ?31.3 mg N kg^?1 d^?1 and the NO₃ ^? immobilization rates averaged 29.0 and 35.9 mg N kg^?1 d^?1. For the mineral (0–10 cm) layer the same trend could be observed, but the N transformation rates were much lower for the NH₄ ⁺ pool and not significantly different from zero for the NO₃ ^? pool. Except for the turnover of the NH₄ ⁺ pool in the mineral layer, no significant differences were observed between location GE and GF1. The ratio of NH₄ ⁺ immobilization to gross N mineralization, gross N mineralization to gross nitrification, and NO₃ ^? immobilisation to gross nitrification led to the following observations. The NH₄ ⁺ pool of the forest soil was controlled by N mineralization and NO₃ ^? immobilization was importantly controlling the forest NO₃ ^? pool. Therefore it was concluded that this process is most probably responsible for the limited NO₃ ^? leaching from the forest ecosystem, despite the chronically high N deposition rates.     

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.     

Soil-mixing effects on inorganic nitrogen production and consumption in forest and shrubland soils

Soils that are physically disturbed are often reported to show net nitrification and NO₃⁻ loss. To investigate the response of soil N cycling rates to soil mixing, we assayed gross rates of mineralization, nitrification, NH₄⁺ consumption, and NO₃⁻ consumption in a suite of soils from eleven woody plant communities in Oregon, New Mexico, and Utah. Results suggest that the common response of net NO₃⁻ flux from disturbed soils is not a straightforward response of increased gross nitrification, but instead may be due to the balance of several factors. While mineralization and NH₄⁺ assimilation were higher in mixed than intact cores, NO₃⁻ consumption declined. Mean net nitrification was 0.12 mg N kg⁻¹ d⁻¹ in disturbed cores, which was significantly higher than in intact cores (−0.19 mg N kg⁻¹ d⁻¹). However, higher net nitrification rates in disturbed soils were due to the suppression of NO₃⁻ consumption, rather than an increase in nitrification. Our results suggest that at least in the short term, disturbance may significantly increase NO₃⁻ flux at the ecosystem level, and that N cycling rates measured in core studies employing mixed soils may not be representative of rates in undisturbed soils.     

Influence of hydrological connectivity of riverine wetlands on nitrogen removal via denitrification

Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from <5 to 1,133 μM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35–1,888 μmol N m^?2 h^?1) were up to two orders of magnitude higher than rates measured in isolated wetlands (2–231 μmol N m^?2 h^?1), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9–1,888 μmol N m^?2 h^?1) than in winter (2–365 μmol N m^?2 h^?1). Denitrification supported by water column nitrate (D_W) accounted for 60–100% of total denitrification (Dtot); denitrification coupled to nitrification (D_N) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.     

Gross Nitrogen Turnover of Natural and Managed Tropical Ecosystems at Mt. Kilimanjaro,Tanzania

In our study at Mt. Kilimanjaro, East Africa, we quantified gross rates of ammonification, nitrification, nitrogen immobilization, and dissimilatory nitrate reduction to ammonium in soils across different land uses, climate zones (savanna, montane forest ecosystems, extensive agroforest homegarden, and intensively managed coffee plantation), and seasons (dry, wet, and transition from dry to wet season) to identify if and to what extent conversion of natural ecosystems to cultivated land has affected key soil microbial nitrogen turnover processes. Overall variation of gross soil nitrogen turnover rates across different ecosystems was more pronounced than seasonal variations, with the highest turnover rates occurring at the transition between dry and wet seasons. Nitrogen production and immobilization rates positively correlated with soil organic carbon and total nitrogen concentrations as well as substrate availability of dissolved organic carbon and nitrogen r > 0.67, P < 0.05), but did not correlate with soil ammonium and nitrate concentrations. Soil nitrogen turnover rates were highest in the montane Ocotea forest (ammonification 29.84, nitrification 12.67, NH₄ ⁺ immobilization 38.92, NO₃ ^? immobilization 10.74, and DNRA 1.54 µg N g^?1 SDW d^?1) and progressively decreased with decreasing annual rainfall and increasing land-use intensity. Using indicators of N retention and characteristics of soil nutrient status, we observed a grouping of faster, but tighter N cycling in the (semi-) natural savanna and Ocotea forest. This contrasted with a more open N cycle in managed systems (the homegarden and coffee plantation) where N was more prone to leaching or gaseous losses due to high nitrate production rates. The partly disturbed (selected logging) lower montane forest ranged between these two groups.     

Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe

Combined measurements of nitrification activity and N₂O emissions were performed in a lowland and a montane tropical rainforest ecosystem in NE-Australia over a 18 months period from October 2001 until May 2003. At both sites gross nitrification rates, measured by the BaPS technique, showed a strong seasonal pattern with significantly higher rates of gross nitrification during wet season conditions. Nitrification rates at the montane site (1.48?±?0.24–18.75?±?2.38 mg N kg^?1 day^?1) were found to be significantly higher than at the lowland site (1.65?±?0.21–4.54?±?0.27 mg N kg^?1 day^?1). The relationship between soil moisture and gross nitrification rates could be described best by O’Neill functions having a soil moisture optimum of nitrification at app. 65% WFPS. At the lowland site, for which continuous measurements of N₂O emissions were available, nitrification was positively correlated with N₂O emission. Nitrification contributed significantly to N₂O formation during dry season (app.85%) but less (app. 30%) during wet season conditions. In average 0.19‰ of the N metabolized by nitrification was released as N₂O. The N₂O fraction loss for nitrification was positively correlated with changes in soil moisture and varied slightly between 0.15 and 0.22‰. Our results demonstrate that combined N₂O emission and microbial N turnover studies covering prolonged observation periods are needed to clarify and quantify the role of the microbial processes nitrification and denitrification for annual N₂O emissions from soils of terrestrial ecosystems.     

Nutrient Biogeochemistry During the Early Stages of Delta Development in the Mississippi River Deltaic Plain

Nutrient biogeochemistry associated with the early stages of soil development in deltaic floodplains has not been well defined. Such a model should follow classic patterns of soil nutrient pools described for alluvial ecosystems that are dominated by mineral matter high in phosphorus and low in carbon and nitrogen. A contrast with classic models of soil development is the anthropogenically enriched high nitrate conditions due to agricultural fertilization in upstream watersheds. Here we determine if short-term patterns of soil chemistry and dissolved inorganic nutrient fluxes along the emerging Wax Lake delta (WLD) chronosequence are consistent with conceptual models of long-term nutrient availability described for other ecosystems. We add a low nitrate treatment more typical of historic delta development to evaluate the role of nitrate enrichment in determining the net dinitrogen (N₂) flux. Throughout the 35-year chronosequence, soil nitrogen and organic matter content significantly increased by an order of magnitude, whereas phosphorus exhibited a less pronounced increase. Under ambient nitrate concentrations (>60 μM), mean net N₂ fluxes (157.5 μmol N m^?2 h^?1) indicated greater rates of gross denitrification than gross nitrogen fixation; however, under low nitrate concentrations (<2 μM), soils switched from net denitrification to net nitrogen fixation (?74.5 μmol N m^?2 h^?1). As soils in the WLD aged, the subsequent increase in organic matter stimulated net N₂, oxygen, nitrate, and nitrite fluxes producing greater fluxes in more mature soils. In conclusion, soil nitrogen and carbon accumulation along an emerging delta chronosequence largely coincide with classic patterns of soil development described for alluvial floodplains, and substrate age together with ambient nitrogen availability can be used to predict net N₂ fluxes during early delta evolution.     

Nitrification and denitrification in a midwestern stream containing high nitrate: in situ assessment using tracers in dome-shaped incubation chambers

The extent to which in-stream processes alter or remove nutrient loads in agriculturally impacted streams is critically important to watershed function and the delivery of those loads to coastal waters. In this study, patch-scale rates of in-stream benthic processes were determined using large volume, open-bottom benthic incubation chambers in a nitrate-rich, first to third order stream draining an area dominated by tile-drained row-crop fields. The chambers were fitted with sampling/mixing ports, a volume compensation bladder, and porewater samplers. Incubations were conducted with added tracers (NaBr and either ¹⁵N[NO₃ ^?], ¹⁵N[NO₂ ^?], or ¹⁵N[NH₄ ⁺]) for 24–44 h intervals and reaction rates were determined from changes in concentrations and isotopic compositions of nitrate, nitrite, ammonium and nitrogen gas. Overall, nitrate loss rates (220–3,560 μmol N m^?2 h^?1) greatly exceeded corresponding denitrification rates (34–212 μmol N m^?2 h^?1) and both of these rates were correlated with nitrate concentrations (90–1,330 μM), which could be readily manipulated with addition experiments. Chamber estimates closely matched whole-stream rates of denitrification and nitrate loss using ¹⁵N. Chamber incubations with acetylene indicated that coupled nitrification/denitrification was not a major source of N₂ production at ambient nitrate concentrations (175 μM), but acetylene was not effective for assessing denitrification at higher nitrate concentrations (1,330 μM). Ammonium uptake rates greatly exceeded nitrification rates, which were relatively low even with added ammonium (3.5 μmol N m^?2 h^?1), though incubations with nitrite demonstrated that oxidation to nitrate exceeded reduction to nitrogen gas in the surface sediments by fivefold to tenfold. The chamber results confirmed earlier studies that denitrification was a substantial nitrate sink in this stream, but they also indicated that dissolved inorganic nitrogen (DIN) turnover rates greatly exceeded the rates of permanent nitrogen removal via denitrification.     

Net sediment N2 fluxes in a southern New England estuary: variations in space and time

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N₂ across the sediment–water interface over an annual cycle using the N₂/Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10–90 μmol N₂–N m^?2 h^?1 over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (?5 to ?650 μmol N₂–N m^?2 h^?1) was measured at three sites and only observed in summer (June–August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.     

Effects of forest management on soil N cycling in beech forests stocking on calcareous soils

The effects of forest management (thinning) on gross and net N conversion, the balance of inorganic N production and consumption, inorganic N concentrations and on soil microbial biomass in the Ah layer were studied in situ during eight intensive field measuring campaigns in the years 2002–2004 at three beech (Fagus sylvatica L.) forest sites. At all sites adjacent thinning plots (“T”) and untreated control plots (“C”) were established. Since the sites are characterized either by cool-moist microclimate (NE site and NW site) or by warm-dry microclimate (SW site) and thinning took place in the year 1999 at the NE and SW sites and in the year 2003 at the NW site the experimental design allowed to evaluate (1) short-term effects (years 1–2) of thinning at the NW site and (2) medium-term effects (years 4–6) of thinning under different microclimate at the SW and NE site. Microbial biomass N was consistently higher at the thinning plots of all sites during most of the field campaigns and was overall significantly higher at the SWT and NWT plots as compared to the corresponding untreated control plots. The size of the microbial biomass N pool was found to correlate positively with both gross ammonification and gross nitrification as well as with extractable soil NO₃⁻ concentrations. At the SW site neither gross ammonification, gross nitrification, gross ammonium (NH₄⁺) immobilization and gross nitrate (NO₃⁻) immobilization nor net ammonification, net nitrification and extractable NH₄⁺ and NO₃⁻ contents were significantly different between control and thinning plot. At the NET plot lower gross ammonification and gross NH₄⁺ immobilization in conjunction with constant nitrification rates coincided with higher net nitrification and significantly higher extractable NO₃⁻ concentrations. Thus, the medium-term effects of thinning varied with different microclimate. The most striking thinning effects were found at the newly thinned NW site, where gross ammonification and gross NH₄⁺ immobilization were dramatically higher immediately after thinning. However, they subsequently tended to decrease in favor of gross nitrification, which was significantly higher at the NWT plot as compared to␣the␣NWC plot during all field campaigns after␣thinning except for April 2004. This increase␣in␣gross nitrification at the NWT plot (1.73 mg N kg⁻¹ sdw day⁻¹ versus 0.48 mg N kg⁻¹ sdw day⁻¹ at the NWC plot) coincided with significantly higher extractable NO₃⁻ concentrations (4.59 mg N kg⁻¹ sdw at the NWT plot versus 0.96 mg N kg⁻¹ sdw at the NWC plot). Pronounced differences in relative N retention (the ratio of gross NH₄⁺ immobilization + gross NO₃⁻ immobilization to gross ammonification + gross nitrification) were found across the six research plots investigated and could be positively correlated to the soil C/N ratio (R = 0.94; p = 0.005). In sum, the results obtained in this study show that (1) thinning can lead to a shift in the balance of microbial inorganic N production and consumption causing a clear decrease in the N retention capacity in the monitored forest soils especially in the first two years after thinning, (2)␣the resistance of the investigated forest ecosystems to disturbances of N cycling by thinning may vary with different soil C contents and C/N ratios, e. g. caused by differences in microclimate, (3) thinning effects tend to decline with the growth of understorey vegetation in the years 4–6 after thinning.     

Temporal patterns of net soil N mineralization and nitrification through secondary succession in the subtropical forests of eastern China

Linking temporal trends of soil nitrogen (N) transformation with shifting patterns of plants and consequently changes of litter quality during succession is important for understanding developmental patterns of ecosystem function. However, the successional direction of soil N mineralization and nitrification in relation to species shifts in the subtropical regions remains little studied. In this study, successional patterns of net soil N mineralization and nitrification rates, litter-fall, forest floor litter, fine root and soil properties were quantified through a successional sequence in the subtropical forests of eastern China. Net N mineralization rate was ‘U-shaped’ through succession: highest in climax evergreen broad-leaved forest (CE: 1.6?±?0.2 mg-N kg^?1 yr^?1) and secondary shrubs (SS: 1.4?±?0.2 mg-N kg^?1 yr^?1), lowest in conifer and evergreen broad-leaved mixed forest (MF: 1.1?±?0.1 mg-N kg^?1 yr^?1) and intermediate in conifer forest (CF: 1.2?±?0.1 mg-N kg^?1 yr^?1) and sub-climax forest (SE: 1.2?±?0.2 mg-N kg^?1 yr^?1). Soil nitrification increased with time (0.02?±?0.1, 0.2?±?0.1, 0.5?±?0.1, 0.2?±?0.1, and 0.6?±?0.1 mg-N kg^?1 yr^?1 in SS, CF, MF, SE and CE, respectively). Annual production of litter?fall increased through succession. Fine root stocks and total N concentration, soil total N, total carbon (C) and microbial biomass C also followed ‘U?shaped’ temporal trends in succession. Soil bulk density was highest in MF, lowest in CE, and intermediate in SS, CF and SE. Soil pH was significantly lowest in CE. Temporal patterns of soil N mineralization and nitrification were significant related to the growth of conifers (i.e. Pinus massoniana) and associated successional changes of litter-fall, forest floor, fine roots and soil properties. We concluded that, due to lower litter quality, the position of Pinus massoniana along the succession pathway played an important role in controlling temporal trends of soil N transformation.     

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.     

Nutrient Removal as a Function of Corn Stover Cutting Height and Cob Harvest

One-pass harvest equipment has been developed to collect corn (Zea mays L.) grain, stover, and cobs that can be used as bioenergy feedstock. Nutrients removed in these feedstocks have soil fertility implication and affect feedstock quality. The study objectives were to quantify nutrient concentrations and potential removal as a function of cutting height, plant organ, and physiological stage. Plant samples were collected in 10-cm increments at seven diverse geographic locations at two maturities and analyzed for multiple elements. At grain harvest, nutrient concentration averaged 5.5 g?N kg^?1, 0.5 g?P kg^?1, and 6.2 g?K kg^?1 in cobs, 7.5 g?N kg^?1, 1.2 g?P kg^?1, and 8.7 g?K kg^?1 in the above-ear stover fraction, and 6.4 g?N kg^?1, 1.0 g?P kg^?1, and 10.7 g?K kg^?1 in the below-ear stover fraction (stover fractions exclude cobs). The average collective cost to replace N, P, and K was $11.66 Mg^?1 for cobs, $17.59 Mg^?1 for above-ear stover, and $18.11 Mg^?1 for below-ear stover. If 3 Mg ha^?1 of above-ear stover fraction plus 1 Mg of cobs are harvested, an average N, P, and K replacement cost was estimated at $64 ha^?1. Collecting cobs or above-ear stover fraction may provide a higher quality feedstock while removing fewer nutrients compared to whole stover removal. This information will enable producers to balance soil fertility by adjusting fertilizer rates and to sustain soil quality by predicting C removal for different harvest scenarios. It also provides elemental information to the bioenergy industry.     

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.     

气候变化对草甸草原土壤氮素矿化作用影响的实验研究

 用盖顶PVC管法,将锡林河流域中1469m高海拔处的草甸草原原状土柱分别移植到海拔1187m、960m的低海拔处培养,用以研究温度变化对土壤氮素的净氨化速率、净硝化速率和净矿化速率的可能影响。经过一个生长季培养后的测定结果表明：从高海拔到低海拔,实验所选择的3个地点的年均气温分别为-0.5℃、2.2℃和4.4℃,受此不同气温的影响,移植到这3个地点的草甸草原土壤氮素的净氨化速率分别为0.05 mgN·kg-1·m-1,0.13mgN·kg-1·m-1和1.09mgN·kg-1·m-1;净硝化速率分别为0.05mgN·kg-1·m-1,0.76mgN·kg-1·m-1和0.26mgN·kg-1N·m-1;净矿化速率分别为0.10mgN·kg-1·m-1,0.89mgN·kg-1·m-1和1.35mgN·kg-1·m-1。由此可推断未来气候变化将促进草甸草原土壤氮素的净矿化作用。     

Accelerated microbial organic matter mineralization following salt-water intrusion into tidal freshwater marsh soils

The impact of salt-water intrusion on microbial organic carbon (C) mineralization in tidal freshwater marsh (TFM) soils was investigated in a year-long laboratory experiment in which intact soils were exposed to a simulated tidal cycle of freshwater or dilute salt-water. Gas fluxes [carbon dioxide (CO₂) and methane (CH₄)], rates of microbial processes (sulfate reduction and methanogenesis), and porewater and solid phase biogeochemistry were measured throughout the experiment. Flux rates of CO₂ and, surprisingly, CH₄ increased significantly following salt-water intrusion, and remained elevated relative to freshwater cores for 6 and 5 months, respectively. Following salt-water intrusion, rates of sulfate reduction increased significantly and remained higher than rates in the freshwater controls throughout the experiment. Rates of acetoclastic methanogenesis were higher than rates of hydrogenotrophic methanogenesis, but the rates did not differ by salinity treatment. Soil organic C content decreased significantly in soils experiencing salt-water intrusion. Estimates of total organic C mineralized in freshwater and salt-water amended soils over the 1-year experiment using gas flux measurements (18.2 and 24.9 mol C m^?2, respectively) were similar to estimates obtained from microbial rates (37.8 and 56.2 mol C m^?2, respectively), and to losses in soil organic C content (0 and 44.1 mol C m^?2, respectively). These findings indicate that salt-water intrusion stimulates microbial decomposition, accelerates the loss of organic C from TFM soils, and may put TFMs at risk of permanent inundation.     

The contribution of nitrogen transformation processes to total N2O emissions from soils used for intensive vegetable cultivation

The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total N₂O emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to N₂O production from the intensive vegetable fields and to identify the affecting factors, a ¹⁵N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with ¹⁵NH₄NO₃ or NH ₄ ¹⁵ NO₃. The average N₂O emission rate was 24.2 ng N?kg^?1 h^?1 in WF soil, but it ranged from 69.6 to 507 ng N?kg^?1 h^?1 in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3–31.4%, 25.4–54.4% and 22.5–57.7% of the N₂O emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ?≥?5.7), the increased N₂O emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the N₂O product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH?≤?4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced N₂O product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high N₂O emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO ₃ ^- or NH ₄ ⁺ ) could also significantly affect the sources and rates of N₂O emission.     

Interaction of benthic microalgae and macrofauna in the control of benthic metabolism, nutrient fluxes and denitrification in a shallow sub-tropical coastal embayment (western Moreton Bay, Australia)

Benthic biogeochemistry and macrofauna were investigated six times over 1 year in a shallow sub-tropical embayment. Benthic fluxes of oxygen (annual mean ?918 μmol O₂ m^?2 h^?1), ammonium (NH₄ ⁺), nitrate (NO₃ ^?), dissolved organic nitrogen, dinitrogen gas (N₂), and dissolved inorganic phosphorus were positively related to OM supply (N mineralisation) and inversely related to benthic light (N assimilation). Ammonium (NH₄ ⁺), NO₃ ^? and N₂ fluxes (annual means +14.6, +15.9 and 44.6 μmol N m^?2 h^?1) accounted for 14, 16 and 53 % of the annual benthic N remineralisation respectively. Denitrification was dominated by coupled nitrification–denitrification throughout the study. Potential assimilation of nitrogen by benthic microalgae (BMA) accounted for between 1 and 30 % of remineralised N, and was greatest during winter when bottom light was higher. Macrofauna biomass tended to be highest at intermediate benthic respiration rates (?1,000 μmol O₂ m^?2 h^?1), and appeared to become limited as respiration increased above this point. While bioturbation did not significantly affect net fluxes, macrofauna biomass was correlated with increased light rates of NH₄ ⁺ flux which may have masked reductions in NH₄ ⁺ flux associated with BMA assimilation during the light. Peaks in net N₂ fluxes at intermediate respiration rates are suggested to be associated with the stimulation of potential denitrification sites due to bioturbation by burrowing macrofauna. NO₃ ^? fluxes suggest that nitrification was not significantly limited within respiration range measured during this study, however comparisons with other parts of Moreton Bay suggest that limitation of coupled nitrification–denitrification may occur in sub-tropical systems at respiration rates exceeding ?1,500 μmol O₂ m^?2 h^?1.     

Gross nitrogen transformations in soils from uncut and cut boreal upland and peatland coniferous forest stands

Gross and net nitrogen (N) ammonification and nitrification were measured in soils from an uncut and recently cut upland and peatland conifer stand in northwestern Ontario, Canada. Rates of gross total inorganic N immobilization were similar to gross mineralization, resulting in low net mineralization rates in soils from all four upland and peatland conifer stands. Gross ammonification rates were variable but similar in soils from uncut and cut peatland hollows (18–19mgNkg^–1day^–1) and upland forest floor soils (14–19mgNkg^–1day^–1). Gross ammonium ( ) immobilization rates were also variable but similar to ammonification rates. Median gross nitrification rates were within 0–2mgNkg^–1day^–1 in soils from all four upland and peatland cut and uncut stands, although rates were consistently higher for the soils from the cut stands. Large variability in gross nitrification rates were observed in peatland soils, however the highest gross nitrification rates were measured in saturated peatland soils. Net rates remained low in the soils from all four stands due to high nitrate ( ) immobilization and very fast turnover (<0.2 day). Our results suggest that potential losses may be negated by high immobilization in uncut and cut boreal forest stands. This study reveals the potential for the interaction of N production and consumption processes in regulating N retention in upland and peatland conifer forests, and the resilience of the boreal forest to disturbance.     

Potential nitrogen and carbon processing in a landscape rich in milldam legacy sediments

Recent identification of the widespread distribution of legacy sediments deposited in historic mill ponds has increased concern regarding their role in controlling land–water nutrient transfers in the mid-Atlantic region of the US. At Big Spring Run in Lancaster, Pennsylvania, legacy sediments now overlay a buried relict hydric soil (a former wetland soil). We compared C and N processing in legacy sediment to upland soils to identify soil zones that may be sources or sinks for N transported toward streams. We hypothesized that legacy sediments would have high nitrification rates (due to recent agricultural N inputs), while relict hydric soils buried beneath the legacy sediments would be N sinks revealed via negative net nitrification and/or positive denitrification (because the buried former wetland soils are C rich but low in O₂). Potential net nitrification ranged from 9.2 to 77.9 g m^?2 year^?1 and potential C mineralization ranged from 223 to 1,737 g m^?2 year^?1, with the highest rates in surface soils for both legacy sediments and uplands. Potential denitrification ranged from 0.37 to 21.72 g m^?2 year^?1, with the buried relict hydric soils denitrifying an average of 6.2 g m^?2 year^?1. Contrary to our hypothesis, relict hydric layers did not have negative potential nitrification or high positive potential denitrification rates, in part because microbial activity was low relative to surface soils, as indicated by low nitrifier population activity, low substrate induced respiration, and low exoenzyme activity. Despite high soil C concentrations, buried relict hydric soils do not provide the ecological services expected from a wetland soil. Thus, legacy sediments may dampen N removal pathways in buried relict hydric soils, while also acting as substantial sources of NO₃ ^? to waterways.