Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N₂O) and nitric oxide (NO) are not well understood. We examined emissions of N₂O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N₂O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N₂O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha⁻¹· y⁻¹). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha⁻¹· y⁻¹), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha⁻¹· y⁻¹). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified. Received 22 February 2000; accepted 6 September 2000.     

Microbial N Turnover and N-Oxide (N<Subscript>2</Subscript>O/NO/NO<Subscript>2</Subscript>) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N₂O, NO and NO₂ were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg⁻¹ soil dry weight day⁻¹. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N₂O and NO _x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N₂O fluxes varied between 0.39 and 1.60 μg N₂O-N m⁻² h⁻¹, equivalent to 0.03–0.14 kg N₂O-N ha⁻¹ y⁻¹, and were considerably lower than previously reported for the same region. NO _x flux rates ranged between 0.16 and 0.48 μg NO _x -N m⁻² h⁻¹, equivalent to 0.01–0.04 kg NO _x -N ha⁻¹ y⁻¹, respectively. N₂O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.     

The Impact of Nitrogen Placement and Tillage on NO, N2O, CH4 and CO2 Fluxes from a Clay Loam Soil

To evaluate the impact of N placement depth and no-till (NT) practice on the emissions of NO, N₂O, CH₄ and CO₂ from soils, we conducted two N placement experiments in a long-term tillage experiment site in northeastern Colorado in 2004. Trace gas flux measurements were made 2–3 times per week, in zero-N fertilizer plots that were cropped continuously to corn (Zea mays L.) under conventional-till (CT) and NT. Three N placement depths, replicated four times (5, 10 and 15 cm in Exp. 1 and 0, 5 and 10 cm in Exp. 2, respectively) were used. Liquid urea–ammonium nitrate (UAN, 224 kg N ha⁻¹) was injected to the desired depth in the CT- or NT-soils in each experiment. Mean flux rates of NO, N₂O, CH₄ and CO₂ ranged from 3.9 to 5.2 μg N m⁻² h⁻¹, 60.5 to 92.4 μg N m⁻² h⁻¹, −0.8 to 0.5 μg C m⁻² h⁻¹, and 42.1 to 81.7 mg C m⁻² h⁻¹ in both experiments, respectively. Deep N placement (10 and 15 cm) resulted in lower NO and N₂O emissions compared with shallow N placement (0 and 5 cm) while CH₄ and CO₂ emissions were not affected by N placement in either experiment. Compared with N placement at 5 cm, for instance, averaged N₂O emissions from N placement at 10 cm were reduced by more than 50% in both experiments. Generally, NT decreased NO emission and CH₄ oxidation but increased N₂O emissions compared with CT irrespective of N placement depths. Total net global warming potential (GWP) for N₂O, CH₄ and CO₂ was reduced by deep N placement only in Exp. 1 but was increased by NT in both experiments. The study results suggest that deep N placement (e.g., 10 cm) will be an effective option for reducing N oxide emissions and GWP from both fertilized CT- and NT-soils.     

Greenhouse Gas Budget of a Cool-Temperate Deciduous Broad-Leaved Forest in Japan Estimated Using a Process-Based Model

A terrestrial ecosystem model, called the Vegetation Integrative Simulator for Trace gases model (VISIT), which fully integrates biogeochemical carbon and nitrogen cycles, was developed to simulate atmosphere–ecosystem exchanges of greenhouse gases (CO₂, CH₄, and N₂O), and to determine the global warming potential (GWP) taking into account the radiative forcing effect of each gas. The model was then applied to a cool-temperate deciduous broad-leaved forest in Takayama, central Japan (36°08′N, 137°25′E, 1420 m above sea level). Simulations were conducted at a daily time step from 1948 to 2008, using time-series meteorological and nitrogen deposition data. VISIT accurately captured the carbon and nitrogen cycles of this typical Japanese forest, as validated by tower and chamber flux measurements. During the last 10 years of the simulation, the model estimated that the forest was a net greenhouse gas sink, having a GWP equivalent of 1025.7 g CO₂ m⁻² y⁻¹, most of which (1016.9 g CO₂ m⁻² y⁻¹) was accounted for by net CO₂ sequestration into forest biomass regrowth. CH₄ oxidation by the forest soil made a small contribution to the net sink (11.9 g CO₂-eq. m⁻² y⁻¹), whereas N₂O emissions were a very small source (3.2 g CO₂-eq. m⁻² y⁻¹), as expected for a volcanic soil in a humid climate. Analysis of the sensitivity of GWP to changes in temperature, precipitation, and nitrogen deposition indicated that warming temperatures would decrease the size of the sink, mainly as a result of increased CO₂ release due to increased ecosystem respiration.     

Carbon, nitrogen and Greenhouse gases budgets over a four years crop rotation in northern France

Croplands mainly act as net sources of the greenhouse gases carbon dioxide (CO₂) and nitrous oxide (N₂O), as well as nitrogen oxide (NO), a precursor of troposheric ozone. We determined the carbon (C) and nitrogen (N) balance of a four-year crop rotation, including maize, wheat, barley and mustard, to provide a base for exploring mitigation options of net emissions. The crop rotation had a positive net ecosystem production (NEP) of 4.4?±?0.7 Mg C ha^-1 y^-1 but represented a net source of carbon with a net biome production (NBP) of -1.3?±?1.1 Mg C?ha^-1 y^-1. The nitrogen balance of the rotation was correlated with the carbon balance and resulted in net loss (?24?±?28 kg N ha^-1 y^-1). The main nitrogen losses were nitrate leaching (?11.7?±1.0 kg N ha^-1 y^-1) and ammonia volatilization (?9 kg N ha^-1 y^-1). Dry and wet depositions were 6.7?±?3.0 and 5.9?±0.1 kg N ha^-1 y^-1, respectively. Fluxes of nitrous (N₂O) and nitric (NO) oxides did not contribute significantly to the N budget (N₂O: -1.8?±?0.04; NO: -0.7?±?0.04 kg N ha^-1 y^-1) but N₂O fluxes equaled 16% of the total greenhouse gas balance. The link between the carbon and nitrogen balances are discussed. Longer term experiments would be necessary to capture the trends in the carbon and nitrogen budgets within the variability of agricultural ecosystems.     

Effect of Increased Carbon Dioxide and Temperature on Runoff Chemistry at a Forested Catchment in Southern Norway (CLIMEX Project)

CLIMEX (Climate Change Experiment) is an integrated, whole-ecosystem research project that focuses on the response of forest ecosystems at the catchment scale to increased CO₂ and temperature. KIM catchment (860 m²) is completely enclosed by a transparent greenhouse, receives deacidifed “clean” rain, and has elevated CO₂ (560 ppmv) and elevated air temperature (3°–5°C above ambient). The uppermost 20% of the catchment is partitioned off, is not subject to changed CO₂ or temperature, and serves as an untreated control. Fluxes of nitrate and ammonium in runoff from KIM catchment increased from 2 mmol m^{− 2} y^{− 1} each in the 3 years before treatment to 6 and 3 mmol m^{− 2} y^{− 1}, respectively, in the 3 years after treatment (May 1994–April 1997), despite a 15 mmol m^{− 2} y^{− 1} decrease in N dry deposition due to the sealing of the walls to the enclosure. N flux in runoff from three reference catchments and the control section did not change. The net loss of inorganic N was thus about 20 mmol m^{− 2} treated soil y^{− 1}. There were no changes in organic N or total organic carbon in runoff. The ecosystem switched from a net sink to a net source of inorganic nitrogen (N). The increased loss of N may be due to accelerated decomposition of soil organic matter induced by higher temperature. Due to many decades of N deposition from long-range transported pollutants, the ecosystem prior to treatment was N saturated. If global change induces persistent losses of inorganic N on a regional scale, the result may be a significant increase in nitrate concentrations in fresh waters and N loading to coastal marine ecosystems. In regions with acid sensitive waters, such as southern Norway, the increased nitrate release caused by global change may offset improvements achieved by reduced sulfur and N deposition. Received 15 October 1997; accepted 18 November 1997.     

The Importance of Termites to the CH<Subscript>4</Subscript> Balance of a Tropical Savanna Woodland of Northern Australia

Termites produce methane (CH₄) as a by-product of microbial metabolism of food in their hindguts, and are one of the most uncertain components of the regional and global CH₄ exchange estimates. This study was conducted at Howard Springs near Darwin, and presents the first estimate of CH₄ emissions from termites based on replicated in situ seasonal flux measurements in Australian savannas. Using measured fluxes of CH₄ between termite mounds and the atmosphere, and between soil and the atmosphere across seasons we determined net CH₄ flux within a tropical savanna woodland of northern Australia. By accounting for both mound-building and subterranean termite colony types, and estimating the contribution from tree-dwelling colonies it was calculated that termites were a CH₄ source of +0.24 kg CH₄-C ha⁻¹ y⁻¹ and soils were a CH₄ sink of −1.14 kg CH₄-C ha⁻¹ y⁻¹. Termites offset 21% of CH₄ consumed by soil resulting in net sink strength of −0.90 kg CH₄-C ha⁻¹ y⁻¹ for these savannas. For Microcerotermes nervosus (Hill), the most abundant mound-building termite species at this site, mound basal area explained 48% of the variation in mound CH₄ flux. CH₄ emissions from termites offset 0.1% of the net biome productivity (NBP) and CH₄ consumption by soil adds 0.5% to the NBP of these tropical savannas at Howard Springs.     

Nitrogen Transformations in Flowpaths Leading from Soils to Streams in Amazon Forest and Pasture

The modification of large areas of tropical forest to agricultural uses has consequences for the movement of inorganic nitrogen (N) from land to water. Various biogeochemical pathways in soils and riparian zones can influence the movement and retention of N within watersheds and affect the quantity exported in streams. We used the concentrations of NO₃ ⁻ and NH₄ ⁺ in different hydrological flowpaths leading from upland soils to streams to investigate inorganic N transformations in adjacent watersheds containing tropical forest and established cattle pasture in the southwestern Brazilian Amazon Basin. High NO₃ ⁻ concentrations in forest soil solution relative to groundwater indicated a large removal of N mostly as NO₃ ⁻ in flowpaths leading from soil to groundwater. Forest groundwater NO₃ ⁻ concentrations were lower than in other Amazon sites where riparian zones have been implicated as important N sinks. Based on water budgets for these watersheds, we estimated that 7.3–10.3 kg N ha⁻¹ y⁻¹ was removed from flowpaths between 20 and 100 cm, and 7.1–10.2 kg N ha⁻¹ y⁻¹ was removed below 100 cm and the top of the groundwater. N removal from vertical flowpaths in forest exceeded previously measured N₂O emissions of 3.0 kg N ha⁻¹ y⁻¹ and estimated emissions of NO of 1.4 kg N ha⁻¹ y⁻¹. Potential fates for this large amount of nitrate removal in forest soils include plant uptake, denitrification, and abiotic N retention. Conversion to pasture shifted the system from dominance by processes producing and consuming NO₃ ⁻ to one dominated by NH₄ ⁺, presumably the product of lower rates of net N mineralization and net nitrification in pasture compared with forest. In pasture, no hydrological flowpaths contained substantial amounts of NO₃ ⁻ and estimated N removal from soil vertical flowpaths was 0.2 kg N ha⁻¹ y⁻¹ below the depth of 100 cm. This contrasts with the extent to which agricultural sources dominate N inputs to groundwater and stream water in many temperate regions. This could change, however, if pasture agriculture in the tropics shifts toward intensive crop cultivation.     

Fluxes of greenhouse gases from Andosols under coffee in monoculture or shaded by <Emphasis Type="Italic">Inga densiflora</Emphasis> in Costa Rica

The objective of this study was to evaluate the effect of N fertilization and the presence of N₂ fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄), related soil parameters (temperature, water-filled pore space, mineral nitrogen content, N mineralization potential) and litterfall in two highly fertilized (250 kg N ha⁻¹ year⁻¹) coffee cultivation: a monoculture (CM) and a culture shaded by the N₂ fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N₂O emissions with 84% of the annual N₂O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH₄ uptakes. The higher annual N₂O emissions from the shaded plantation (5.8 ± 0.3 kg N ha⁻¹ year⁻¹) when compared to that from the monoculture (4.3 ± 0.1 kg N ha⁻¹ year⁻¹) was related to the higher N input through litterfall (246 ± 16 kg N ha⁻¹ year⁻¹) and higher potential soil N mineralization rate (3.7 ± 0.2 mg N kg⁻¹ d.w. d⁻¹) in the shaded cultivation when compared to the monoculture (153 ± 6.8 kg N ha⁻¹ year⁻¹ and 2.2 ± 0.2 mg N kg⁻¹ d.w. d⁻¹). This confirms that the presence of N₂ fixing shade trees can increase N₂O emissions. Annual CO₂ and CH₄ fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO₂ ha⁻¹ year⁻¹, −1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH₄ ha⁻¹ year⁻¹, respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season.     

Biometric Based Carbon Flux Measurements and Net Ecosystem Production (NEP) in a Temperate Deciduous Broad-Leaved Forest Beneath a Flux Tower

Biometric based carbon flux measurements were conducted over 5 years (1999–2003) in a temperate deciduous broad-leaved forest of the AsiaFlux network to estimate net ecosystem production (NEP). Biometric based NEP, as measured by the balance between net primary production (including NPP of canopy trees and of forest floor dwarf bamboo) and heterotrophic respiration (RH), clarified the contribution of various biological processes to the ecosystem carbon budget, and also showed where and how the forest is storing C. The mean NPP of the trees was 5.4 ± 1.07 t C ha⁻¹ y⁻¹, including biomass increment (0.3 ± 0.82 t C ha⁻¹ y⁻¹), tree mortality (1.0 ± 0.61 t C ha⁻¹ y⁻¹), aboveground detritus production (2.3 ± 0.39 t C ha⁻¹ y⁻¹) and belowground fine root production (1.8 ± 0.31 t C ha⁻¹ y⁻¹). Annual biomass increment was rather small because of high tree mortality during the 5 years. Total NPP at the site was 6.5 ± 1.07 t C ha⁻¹ y⁻¹, including the NPP of the forest floor community (1.1 ± 0.06 t C ha⁻¹ y⁻¹). The soil surface CO₂ efflux (RS) was averaged across the 5 years of record using open-flow chambers. The mean estimated annual RS amounted to 7.1 ± 0.44 t C ha⁻¹, and the decomposition of soil organic matter (SOM) was estimated at 3.9 ± 0.24 t C ha⁻¹. RH was estimated at 4.4 ± 0.32 t C ha⁻¹ y⁻¹, which included decomposition of coarse woody debris. Biometric NEP in the forest was estimated at 2.1 ± 1.15 t C ha⁻¹ y⁻¹, which agreed well with the eddy-covariance based net ecosystem exchange (NEE). The contribution of woody increment (Δbiomass + mortality) of the canopy trees to NEP was rather small, and thus the SOM pool played an important role in carbon storage in the temperate forest. These results suggested that the dense forest floor of dwarf bamboo might have a critical role in soil carbon sequestration in temperate East Asian deciduous forests.     

Rates and pathways of nitrous oxide production in a shortgrass steppe

Most of the small external inputs of N to the Shortgrass steppe appear to be conserved. One pathway of loss is the emission of nitrous oxide, which we estimate to account for 2.5–9.0% of annual wet deposition inputs of N. These estimates were determined from an N₂O emission model based on field data which describe the temporal variability of N₂O produced from nitrification and denitrification from two slope positions. Soil water and temperature models were used to translate records of air temperature and precipitation between 1950 and 1984 into variables appropriate to drive the gas flux model, and annual N₂O fluxes were estimated for that period. The mean annual fluxes were 80 g N ha^–1 for a midslope location and 160 g N ha^–1 for a swale. Fluxes were higher in wet years than in dry, ranging from 73 to 100 g N ha^–1y^–1at the midslope, but the variability was not high. N₂O fluxes were also estimated from cattle urine patches and these fluxes while high within a urine patch, did not contribute significantly to a regional budget. Laboratory experiments using C₂H₂ to inhibit nitrifiers suggested that 60–80% of N₂O was produced as a result of nitrification, with denitrification being less important, in contrast to our earlier findings to the contrary. Intrasite and intraseasonal variations in N₂O flux were coupled to variations in mineral N dynamics, with high rates of N₂O flux occurring with high rates of inorganic N turnover. We computed a mean flux of 104 g N ha^–1 y^–1 from the shortgrass landscape, and a flux of 2.6 × 109 g N y^– from all shortgrass steppe (25 × 106 ha).     

N2O, CH4 and CO2 emissions from seasonal tropical rainforests and a rubber plantation in Southwest China

The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N₂O, CO₂ and CH₄ soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N₂O emission in these regions on a landscape level. The mean N₂O emission at site PF was 6.0 ± 0.1 SE μg N m⁻² h⁻¹. Fluxes of N₂O increased from <5 μg N m⁻² h⁻¹ during dry season conditions to up to 24.5 μg N m⁻² h⁻¹ with re-wetting of the soil by the onset of first rainfall events. Comparable fluxes of N₂O were measured in the SF and RP sites, where mean N₂O emissions were 7.3 ± 0.7 SE μg N m⁻² h⁻¹ and 4.1 ± 0.5 SE μg N m⁻² h⁻¹, respectively. The dependency of N₂O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N₂O emission peaks, not the total amount of N₂O emitted. For all sites, significant positive correlations existed between N₂O emissions and both soil moisture and soil temperature. Mean CH₄ uptake rates were highest at the PF site (−29.5 ± 0.3 SE μg C m⁻² h⁻¹), slightly lower at the SF site (−25.6 ± 1.3 SE μg C m⁻² h⁻¹) and lowest for the RP site (−5.7 ± 0.5 SE μg C m⁻² h⁻¹). At all sites, CH₄ uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N₂O emissions, CH₄ uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2 month measurement period, CO₂ emissions at the PF site increased significantly from 50 mg C m⁻² h⁻¹ up to 100 mg C m⁻² h⁻¹ (mean value 68.8 ± 0.8 SE mg C m⁻² h⁻¹), whereas CO₂ emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0 ± 1.8 SE mg C m⁻² h⁻¹ (SF) and 34.9 ± 1.1 SE mg C m⁻² h⁻¹ (RP). A dependency of soil CO₂ emissions on changes in soil water content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO₂ emissions as compared to control chambers. Correlation of CO₂ emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.     

Nitrous Oxide Emission from Grazed Grassland Under Different Management Systems

Nitrous oxide (N₂O) emissions from grazed grasslands are estimated to be approximately 28% of global anthropogenic N₂O emissions. Estimating the N₂O flux from grassland soils is difficult because of its episodic nature. This study aimed to quantify the N₂O emissions, the annual N₂O flux and the emission factor (EF), and also to investigate the influence of environmental and soil variables controlling N₂O emissions from grazed grassland. Nitrous oxide emissions were measured using static chambers at eight different grasslands in the South of Ireland from September 2007 to August 2009. The instantaneous N₂O flux values ranged from -186 to 885.6 μg N₂O-N m⁻² h⁻¹ and the annual sum ranged from 2 ± 3.51 to 12.55 ± 2.83 kg N₂O-N ha⁻¹ y⁻¹ for managed sites. The emission factor ranged from 1.3 to 3.4%. The overall EF of 1.81% is about 69% higher than the Intergovernmental Panel on Climate Change (IPCC) default EF value of 1.25% which is currently used by the Irish Environmental Protection Agency (EPA) to estimate N₂O emission in Ireland. At an N applied of approximately 300 kg ha⁻¹ y⁻¹, the N₂O emissions are approximately 5.0 kg N₂O-N ha⁻¹ y⁻¹, whereas the N₂O emissions double to approximately 10 kg N ha⁻¹ for an N applied of 400 kg N ha⁻¹ y⁻¹. The sites with higher fluxes were associated with intensive N-input and frequent cattle grazing. The N₂O flux at 17°C was five times greater than that at 5°C. Similarly, the N₂O emissions increased with increasing water filled pore space (WFPS) with maximum N₂O emissions occurring at 60–80% WFPS. We conclude that N application below 300 kg ha⁻¹ y⁻¹ and restricted grazing on seasonally wet soils will reduce N₂O emissions.     

Large Greenhouse Gas Emissions from a Temperate Peatland Pasture

Agricultural drainage is thought to alter greenhouse gas emissions from temperate peatlands, with CH₄ emissions reduced in favor of greater CO₂ losses. Attention has largely focussed on C trace gases, and less is known about the impacts of agricultural conversion on N₂O or global warming potential. We report greenhouse gas fluxes (CH₄, CO₂, N₂O) from a drained peatland in the Sacramento-San Joaquin River Delta, California, USA currently managed as a rangeland (that is, pasture). This ecosystem was a net source of CH₄ (25.8 ± 1.4 mg CH₄-C m⁻² d⁻¹) and N₂O (6.4 ± 0.4 mg N₂O-N m⁻² d⁻¹). Methane fluxes were comparable to those of other managed temperate peatlands, whereas N₂O fluxes were very high; equivalent to fluxes from heavily fertilized agroecosystems and tropical forests. Ecosystem scale CH₄ fluxes were driven by “hotspots” (drainage ditches) that accounted for less than 5% of the land area but more than 84% of emissions. Methane fluxes were unresponsive to seasonal fluctuations in climate and showed minimal temporal variability. Nitrous oxide fluxes were more homogeneously distributed throughout the landscape and responded to fluctuations in environmental variables, especially soil moisture. Elevated CH₄ and N₂O fluxes contributed to a high overall ecosystem global warming potential (531 g CO₂-C equivalents m⁻² y⁻¹), with non-CO₂ trace gas fluxes offsetting the atmospheric “cooling” effects of photoassimilation. These data suggest that managed Delta peatlands are potentially large regional sources of greenhouse gases, with spatial heterogeneity in soil moisture modulating the relative importance of each gas for ecosystem global warming potential.     

Seasonal accumulation and partitioning of nitrogen by lentil

Although wheat (Triticum aestivum L.) is the dominant crop of the semi-arid plains of Canada and the western United States, lentil (Lens culinaris Medik.) has become an important alternative crop. Sources and seasonal accumulation of N must be understood in order to identify parameters that can lead to increased N₂-fixing activity and yield. Inoculated lentil was grown in a sandy-loam soil at an irrigated site in Saskatchewan, Canada. Wheat was used as the reference crop to estimate N₂ fixation by the A-value approach. Lentil and wheat received 10 and 100 kg N ha⁻¹ of ammonium nitrate, respectively. Crops were harvested six times during the growing season and plant components analyzed. During the first 71 days after planting the wheat had a higher daily dry matter and N accumulation compared to lentil. However, during the latter part of the growing season, daily dry matter and N accumulation were greater for lentil. The maximum total N accumulation for lentil at maturity was 149 kg ha⁻¹. In contrast, wheat had a maximum N accumulation of 98 kg ha⁻¹ in the Feekes 11.1 stage, or 86 days after planting. The maximum daily rates of N accumulation were 3.82 kg N ha⁻¹ day⁻¹ for lentil and 2.21 kg N ha⁻¹ day⁻¹ for wheat. The percentage of N derived from N₂ fixation (% Ndfa) ranged from 0 at the first harvest to 92 % at final harvest. Generative plant components had higher values for % Ndfa than the vegetative components which indicates that N in the reproductive plant parts was derived largely from current N₂ fixation and lentil continued to fix N until the end of the pod fill stage. At final harvest, lentil had derived 129 kg N ha⁻¹ from N₂ fixation with maximum N₂-fixing activity (4.4 kg N ha⁻¹ day⁻¹) occurring during the early stages of pod fill. Higher maximum rates of N₂-fixing activity than net N accumulation (3.82 kg N ha⁻¹ day⁻¹) may have been caused by N losses like volatilization. In addition, lentil provided a net N contribution to the soil of 59 kg ha⁻¹ following the removal of the grain.     

Gaps and Soil C Dynamics in Old Growth Northern Hardwood–Hemlock Forests

Old growth forest soils are large C reservoirs, but the impacts of tree-fall gaps on soil C in these forests are not well understood. The effects of forest gaps on soil C dynamics in old growth northern hardwood–hemlock forests in the upper Great Lakes region, USA, were assessed from measurements of litter and soil C stocks, surface C efflux, and soil microbial indices over two consecutive growing seasons. Forest floor C was significantly less in gaps (19.0 Mg C ha⁻¹) compared to gap-edges (39.5 Mg C ha⁻¹) and the closed forest (38.0 Mg C ha⁻¹). Labile soil C (coarse particulate organic matter, cPOM) was significantly less in gaps and edges (11.1 and 11.2 Mg C ha⁻¹) compared to forest plots (15.3 Mg C ha⁻¹). In situ surface C efflux was significantly greater in gaps (12.0 Mg C ha⁻¹ y⁻¹) compared to edges and the closed forest (9.2 and 8.9 Mg C ha⁻¹ y⁻¹). Microbial biomass N (MBN) was significantly greater in edges (0.14 Mg N ha⁻¹) than in the contiguous forest (0.09 Mg N ha⁻¹). The metabolic quotient (qCO₂) was significantly greater in the forest (0.0031 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹) relative to gaps or edges (0.0014 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹). A case is made for gaps as alleviators of old growth forest soil C saturation. Relative to the undisturbed closed forest, gaps have significantly less labile C, significantly greater in situ surface C efflux, and significantly lower decreased qCO₂ values.     

Effects of Carbon Dioxide Enrichment and Nitrogen Addition on Inorganic Carbon Leaching in Subtropical Model Forest Ecosystems

Soil mineral weathering may serve as a sink for atmospheric carbon dioxide (CO₂). Increased weathering of soil minerals induced by elevated CO₂ concentration has been reported previously in temperate areas. However, this has not been well documented for the tropics and subtropics. We used model forest ecosystems in open-top chambers to study the effects of CO₂ enrichment alone and together with nitrogen (N) addition on inorganic carbon (C) losses in the leachates. Three years of exposure to an atmospheric CO₂ concentration of 700 ppm resulted in increased annual inorganic C export through leaching below the 70 cm soil profile. Compared to the control without any CO₂ and N treatments, net biocarbonate C (HCO₃ ⁻-C) loss increased by 42%, 74%, and 81% in the high CO₂ concentration treatment in 2006, 2007, and 2008, respectively. Increased inorganic C export following the exposure to the elevated CO₂ was related to both increased inorganic C concentrations in the leaching water and the greater amount of leaching water. Net annual inorganic C (HCO₃ ⁻-C and carbonate C: CO₃ ²⁻-C) loss via the leaching water in the high CO₂ concentration chambers reached 48.0, 49.5, and 114.0 kg ha⁻¹ y⁻¹ in 2006, 2007, and 2008, respectively, compared with 33.8, 28.4, and 62.8 kg ha⁻¹ y⁻¹ in the control chambers in the corresponding years. The N addition showed a negative effect on the mineral weathering. The decreased inorganic C concentration in the leaching water and the decreased leaching water amount induced by the high N treatment were the results of the adverse effect. Our results suggest that tropical forest soil systems may be able to compensate for a small part of the atmospheric CO₂ increase through the accelerated processing of CO₂ into HCO₃ ⁻-C during soil mineral weathering, which might be transported in part into ground water or oceans on geological timescales.     

Variations in soil N cycling and trace gas emissions in wet tropical forests

We used a previously described precipitation gradient in a tropical montane ecosystem of Hawai’i to evaluate how changes in mean annual precipitation (MAP) affect the processes resulting in the loss of N via trace gases. We evaluated three Hawaiian forests ranging from 2200 to 4050 mm year⁻¹ MAP with constant temperature, parent material, ecosystem age, and vegetation. In situ fluxes of N₂O and NO, soil inorganic nitrogen pools (NH₄⁺ and NO₃⁻), net nitrification, and net mineralization were quantified four times over 2 years. In addition, we performed ¹⁵N-labeling experiments to partition sources of N₂O between nitrification and denitrification, along with assays of nitrification potential and denitrification enzyme activity (DEA). Mean NO and N₂O emissions were highest at the mesic end of the gradient (8.7±4.6 and 1.1±0.3 ng N cm⁻² h⁻¹, respectively) and total oxidized N emitted decreased with increased MAP. At the wettest site, mean trace gas fluxes were at or below detection limit (≤0.2 ng N cm⁻² h⁻¹). Isotopic labeling showed that with increasing MAP, the source of N₂O changed from predominately nitrification to predominately denitrification. There was an increase in extractible NH₄⁺ and decline in NO₃⁻, while mean net mineralization and nitrification did not change from the mesic to intermediate sites but decreased dramatically at the wettest site. Nitrification potential and DEA were highest at the mesic site and lowest at the wet site. MAP exerts strong control N cycling processes and the magnitude and source of N trace gas flux from soil through soil redox conditions and the supply of electron donors and acceptors.     

Nitrogen dynamics and soil nitrate retention in a <Emphasis Type="Italic">Coffea arabica</Emphasis>—<Emphasis Type="Italic">Eucalyptus deglupta</Emphasis> agroforestry system in Southern Costa Rica

Nitrogen fertilization is a key factor for coffee production but creates a risk of water contamination through nitrate (NO₃⁻) leaching in heavily fertilized plantations under high rainfall. The inclusion of fast growing timber trees in these coffee plantations may increase total biomass and reduce nutrient leaching. Potential controls of N loss were measured in an unshaded coffee (Coffea arabica L.) plot and in an adjacent coffee plot shaded with the timber species Eucalyptus deglupta Blume (110 trees ha⁻¹), established on an Acrisol that received 180 kg N ha⁻¹ as ammonium-nitrate and 2,700 mm yr⁻¹ rainfall. Results of the one year study showed that these trees had little effect on the N budget although some N fluxes were modified. Soil N mineralization and nitrification rates in the 0–20 cm soil layer were similar in both systems (≈280 kg N ha⁻¹ yr⁻¹). N export in coffee harvest (2002) was 34 and 25 kg N ha⁻¹ yr⁻¹ in unshaded and shaded coffee, and N accumulation in permanent biomass and litter was 25 and 45 kg N ha⁻¹ yr⁻¹, respectively. The losses in surface runoff (≈0.8 kg mineral N ha⁻¹ yr⁻¹) and N₂O emissions (1.9 kg N ha⁻¹ yr⁻¹) were low in both cases. Lysimeters located at 60, 120, and 200 cm depths in shaded coffee, detected average concentrations of 12.9, 6.1 and 1.2 mg NO₃⁻-N l⁻¹, respectively. Drainage was slightly reduced in the coffee-timber plantation. NO₃⁻ leaching at 200 cm depth was about 27 ± 10 and 16 ± 7 kg N ha⁻¹ yr⁻¹ in unshaded and shaded coffee, respectively. In both plots, very low NO₃⁻ concentrations in soil solution at 200 cm depth (and in groundwater) were apparently due to NO₃⁻ adsorption in the subsoil but the duration of this process is not presently known. In these conventional coffee plantations, fertilization and agroforestry practices must be refined to match plant needs and limit potential NO₃⁻ contamination of subsoil and shallow soil water.     

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.