Edzo Veldkamp

Testing a Conceptual Model of Soil Emissions of Nitrous and Nitric Oxides

n ve s ti ga tors from many diverse disciplines—agron om i s t s , a tm o s ph e ric ch em i s t s , eco l ogi s t s , geoch em i s t s , m eteoro l ogi s t s , and microbi o l ogi s t s — a ll stu dy em i s s i ons of n i trous ox i de (N 2 O) and nitric ox i de (NO) from soi l s . Th ei r com m on interest in soil em i s s i ons of n i trogen ox i des stem s f rom the attem pt to answer the fo ll owing qu e s ti on s : • Are em i s s i ons of N 2 O and NO from soils of su f f i c i en t m a gn i tu de to sign i f i c a n t ly affect the regi onal and gl ob a l bu d gets of these gases in the atm o s ph ere ,a n d , i f s o , do these em i s s i ons also sign i f i c a n t ly affect gl obal warm i n g and the ch emical processes of ozone (O 3 ) pr odu cti on in the tropo s ph ere and ozone de s tru cti on in the stra to sph ere ?

Spatial and temporal variation in soil CO2 efflux in an old-growth neotropical rain forest, La Selva, Costa Rica

Our objectives were to quantify and compare soil CO2 efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO2 release. We measured soil CO2 efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river terraces (‘old alluvium’) and the other three were developed on old lava flows (‘residual’). At the same time we measured soil CO2 concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep). Between ‘old alluvium’ sites, the two-year average CO2 flux rates ranged from 117.3 to 128.9 mg C m−2 h−1. Significantly higher soil CO2 flux occurred on the ‘residual’ sites (141.1 to 184.2 mg C m−2 h−1). Spatial differences in CO2 efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content. Spatial variability in CO2 storage was high and the amount of CO2 stored in the upper and lower soil profile was different between ‘old alluvial’ and ‘residual’ sites. The major factor identified for explaining temporal variations in soil CO2 efflux was soil water content. During periods of high soil water content CO2 emission decreased, probably due to lower diffusion and CO2 production rates. During the 2-year study period inter-annual variation in soil CO2 efflux was not detected.

Effects of pasture management on N2O and NO emissions from soils in the humid tropics of Costa Rica

Emissions of nitrous oxide (N2O) and nitric oxide (NO) from agricultural soils in the tropics are important in the global budgets of these trace gases. We made monthly measurements of N2O and NO emissions from pastures with three different management systems on volcanic soils in northwestern Costa Rica: traditional (no N input from fertilizer or legumes), pastures with a grass-legume combination, and pastures fertilized with 300 kg N ha−11 yr−1. Average annual N2O emissions were 2.7 ng N cm−2 h−1 from the traditional pastures, 4.9 ng N cm−2 h−1 from the grass-legume pastures, and 25.8 ng N cm−2 h−1 from the fertilized pastures. Average annual NO emissions were 0.9, 1.3, and 5.3 ng N cm−2 h−1 from traditional, grass-legume and fertilized pastures, respectively. In a separate experiment the effects of ammonium, nitrate, and urea-based fertilizer mixtures on nitrogen oxide fluxes were compared. We measured nitrogen oxide fluxes following four different fertilization events. Nitrogen oxide fluxes were among the highest ever measured. The difference in soil water content between the fertilization events had a far greater effect on N2O and NO emissions than the effect of fertilizer composition. We conclude that the concept of “emission factors” for calculating N2O and NO emissions from different types of N fertilizer is flawed because environmental factors are more important than the type of N fertilizer. To estimate fertilizer-induced N2O emission in tropical agriculture, stratification according to soil moisture regime is more useful than stratification according to fertilizer composition.

Geographic bias of field observations of soil carbon stocks with tropical land-use changes precludes spatial extrapolation

Accurately quantifying changes in soil carbon (C) stocks with land-use change is important for estimating the anthropogenic fluxes of greenhouse gases to the atmosphere and for implementing policies such as REDD (Reducing Emissions from Deforestation and Degradation) that provide financial incentives to reduce carbon dioxide fluxes from deforestation and land degradation. Despite hundreds of field studies and at least a dozen literature reviews, there is still considerable disagreement on the direction and magnitude of changes in soil C stocks with land-use change. We conducted a meta-analysis of studies that quantified changes in soil C stocks with land use in the tropics. Conversion from one land use to another caused significant increases or decreases in soil C stocks for 8 of the 14 transitions examined. For the three land-use transitions with sufficient observations, both the direction and magnitude of the change in soil C pools depended strongly on biophysical factors of mean annual precipitation and dominant soil clay mineralogy. When we compared the distribution of biophysical conditions of the field observations to the area-weighted distribution of those factors in the tropics as a whole or the tropical lands that have undergone conversion, we found that field observations are highly unrepresentative of most tropical landscapes. Because of this geographic bias we strongly caution against extrapolating average values of land-cover change effects on soil C stocks, such as those generated through meta-analysis and literature reviews, to regions that differ in biophysical conditions.

Nitrogen oxide emissions from a banana plantation in the humid tropics

Use of nitrogen fertilizer is thought to contribute significantly to the increase of atmospheric nitrous oxide (N2O) and nitric oxide (NO). While the current increase of fertilizer use is concentrated in tropical areas, nearly all studies of nitrogen oxide emissions have been conducted in agricultural systems in temperate areas. We measured N2O and NO fluxes from a fertilized banana plantation in the humid tropics of Costa Rica, where 360 kg N ha−1 yr−1 is applied. Using chamber techniques, we sampled an Andisol and an Inceptisol on a monthly basis. Twice on each soil type, we sampled intensively in time following fertilizer applications. There is a strong spatial and temporal dependence of nitrogen oxide emissions on place and time of fertilizer application. We find greater mean N2O and NO emissions from the Andisol (31.4 ng N2O-N cm−2 h−1 and 55.6 ng NO-N cm−2 h−1) than from the Inceptisol (9.3 ng N2O-N cm−2 h−1 and 41.1 ng NO-N cm−2 h−1) under the plants where fertilizer is typically applied. The percentages of applied fertilizer-N that are converted into nitrogen oxide (“yield”) are between 1.26 and 2.91% for N2O and between 5.09 and 5.66% for NO depending on soil type. We consistently calculate higher nitrogen oxide yields based on intensive sampling versus monthly sampling. Temporal variation in nitrogen oxide emissions probably causes monthly sampling to underestimate mean annual fluxes. Our results suggest that in some tropical systems a higher percentage of applied nitrogen may be lost in gaseous form than in temperate agriculture. Current global estimates of N2O and NO sources from tropical agriculture are based on information from temperate areas and may cause an underestimate of the contribution of tropical agriculture to the budgets of these trace gases.

Fertilizer-induced nitric oxide emissions from agricultural soils

We summarize and evaluate 23 studies of the effect of fertilizer use on nitric oxide (NO) emission from agricultural soils. To quantify this effect we selected only field-scale studies with duration of at least one complete growing season and excluded studies with a legume as the principle crop. Only 6 studies met the established criteria, resulting in a total of 12 observations of soil/crop/fertilizer combinations, all in temperate areas. For these studies, the amount of NO emitted was linearly related to the amount of fertilizer applied (R2 = 0.64) and about 0.5% of applied nitrogen was emitted as NO during the crop growing season. The available data are too limited to separate the effects of fertilizer type, soil type, or crop management.

Impact of elevated N input on soil N cycling and losses in old‐growth lowland and montane forests in Panama

Nitrogen deposition is projected to increase rapidly in tropical ecosystems, but changes in soil-N-cycling processes in tropical ecosystems under elevated N input are less well understood. We used N-addition experiments to achieve N-enriched conditions in mixed-species, lowland and montane forests in Panama. Our objectives were to (1) assess changes in soil mineral N production (gross rates of N mineralization and nitrification) and retention (microbial immobilization and rapid reactions to organic N) during 1- and 9-yr N additions in the lowland forest and during 1-yr N addition in the montane forest and (2) relate these changes to N leaching and N-oxide emissions. In the old-growth lowland forest located on an Inceptisol, with high base saturation and net primary production not limited by N, there was no immediate effect of first-year N addition on gross rates of mineral-N production and N-oxide emissions. Changes in soil-N processes were only apparent in chronic (9 yr) N-addition plots: gross N mineralization and nitrification rates, NO3- leaching, and N-oxide emissions increased, while microbial biomass and NH4+ immobilization rates decreased compared to the control. Increased mineral-N production under chronic N addition was paralleled by increased substrate quality (e.g., reduced C:N ratios of litterfall), while the decrease in microbial biomass was possibly due to an increase in soil acidity. An increase in N losses was reflected in the increase in 15N signatures of litterfall under chronic N addition. In contrast, the old-growth montane forest located on an Andisol, with low base saturation and aboveground net primary production limited by N, reacted to first-year N addition with increases in gross rates of mineral-N production, microbial biomass, NO3- leaching, and N-oxide emissions compared to the control. The increased N-oxide emissions were attributed to increased nitrification activity in the organic layer, and the high NO3- availability combined with the high rainfall on this sandy loam soil facilitated the instantaneous increase in NO3-leaching. These results suggest that soil type, presence of an organic layer, changes in soil-N cycling, and hydrological properties are more important indicators than vegetation as an N sink on how tropical forests respond to elevated N input.

Soil organic carbon dynamics: variability with depth in forested and deforested soils under pasture in Costa Rica.

Dynamics of soil organic carbon (SOC) inchronosequences of soils below forests that had beenreplaced by grazed pastures 3–25 years ago, wereinvestigated for two contrasting soil types (AndicHumitropept and Eutric Hapludand) in the Atlantic Zoneof Costa Rica. By forest clearing and subsequentestablishment of pastures, photosynthesis changes froma C-3 to a C-4 pathway. The accompanying changes inC-input and its δ13C and 14Csignals, were used to quantify SOC dynamics. C-input from rootturnover at a pasture site was measured by sequentialharvesting and 14C-pulse labelling. With aspatial resolution of 5 cm, data on total SOC,δ13C and δ14C of soil profileswere interpreted with a model that distinguishes threepools of SOC: ‘active’ C, ‘slow’ C and ‘passive’ C,each with a 1-st order decomposition rate(ka, ks and kp). The modelincludes carbon isotope fractionation and depth-dependentdecomposition rates. Transport of C between soillayers was described as a diffusion process, whichaccounts for physical and biotic mixing processes.Calibrated diffusion coefficients were 0.42 cm2yr-1 for the Humitropept and 3.97 cm2yr-1 for the Hapludand chronosequence.Diffusional transport alone was insufficient foroptimal simulation; it had to be augmented bydepth-dependent decomposition rates to explain thedynamics of SOC, δ13C andδ14C. Decomposition rates decreasedstrongly with depth. Upon increased diffusion,differences between calibrated decomposition rates ofSOC fractions between surface soils and subsoilsdiminished, but the concept of depth-dependentdecomposition had to be retained, to obtain smallresiduals between observed and simulated data. At areference depth of 15–20 cm ks was 90 yr-1in the Humitropept and 146 yr-1 in the Hapludand.Slow C contributed most to total organic C in surfacesoils, whereas passive C contributed most below 40 cmdepth. After 18–25 years of pasture, net loss of C was2180 g C m-2 for the Hapludand and 150 g m-2for the Humitropept soil.

Calibration of time domain reflectometry technique using undisturbed soil samples from humid tropical soils of volcanic origin

Time domain reflectrometry (TDR) is used to measure the apparent dielectric number (Ka) in soils. We studied two soil types (Humitropept and Hapludand) of low bulk density (about 0.7 Mg m−3 at 0.05 m to 0.8 Mg m−3 at 0.3 m depth) and high organic matter content (about 7% at 0.05 m to 4% at 0.3 m depth). Soils are located in a humid tropical environment (average annual soil water content is 0.51 to 0.58 m3 m−3). For calibration, undisturbed soil blocks, with a TDR probe installed in the center, were saturated and then allowed to dry by evaporation. Volumetric water content was calculated from measured Ka values and from gravimetric measurements. Because we used undisturbed soil samples, our calibration accounts for the natural heterogeneity in soils. We tested the suitability of various calibration functions relating Ka to soil water content for our soils. TDR technique underestimated the actual soil water content by 0.05–0.15 m3m−3, when using the widely applied Topp calibration function. A three-phase mixing model with a geometry parameter, α=0.47, fit our data best. We consider mixing models to be a robust approach for calibration of TDR technique on various soils.

Tropical Andean Forests Are Highly Susceptible to Nutrient Inputs—Rapid Effects of Experimental N and P Addition to an Ecuadorian Montane Forest

Tropical regions are facing increasing atmospheric inputs of nutrients, which will have unknown consequences for the structure and functioning of these systems. Here, we show that Neotropical montane rainforests respond rapidly to moderate additions of N (50 kg ha−1 yr−1) and P (10 kg ha−1 yr−1). Monitoring of nutrient fluxes demonstrated that the majority of added nutrients remained in the system, in either soil or vegetation. N and P additions led to not only an increase in foliar N and P concentrations, but also altered soil microbial biomass, standing fine root biomass, stem growth, and litterfall. The different effects suggest that trees are primarily limited by P, whereas some processes—notably aboveground productivity—are limited by both N and P. Highly variable and partly contrasting responses of different tree species suggest marked changes in species composition and diversity of these forests by nutrient inputs in the long term. The unexpectedly fast response of the ecosystem to moderate nutrient additions suggests high vulnerability of tropical montane forests to the expected increase in nutrient inputs.