Effects of land use on regional nitrous oxide emissions in the humid tropics of Costa Rica : extrapolating fluxes from field to regional scales

R.A.J. Plant

Research output: Thesisinternal PhD, WU


<p>Atmospheric concentrations of the greenhouse gas nitrous oxide (N <sub>2</sub> O) have increased significantly since pre-industrial days. Greenhouse gases absorb infrared radiation reflected by earth's surface, thereby causing global warming. The increase in atmospheric N <sub>2</sub> O concentrations is attributed to human activities. The relative contribution of N <sub>2</sub> O to the anthropogenic greenhouse effect is about 5%. The two major natural sources of N <sub>2</sub> O are soils and oceans, while agricultural soils comprise the main anthropogenic source.</p><p>Nitrous oxide if formed in soil as an intermediate product from nitrification and denitrification, soil processes that operate at the microsite scale. Land use changes strongly affect soil nitrogen (N) cycling; especially conversion of natural forest to agricultural land generally increases N <sub>2</sub> O emissions. Therefore, land use is an important distal process control on N <sub>2</sub> O emissions from soil. Effects of land use change on N <sub>2</sub> O must be studied at scales relevant to agricultural land use planning and policy making.</p><p>This calls for methods to extrapolate plot-scale measurements that are highly variable in space and time. Neglecting spatial heterogeneity of fluxes and process controls can lead to serious errors in areal and regional flux estimates. The objective of the work summarized in this thesis was to study effects of land use on regional N <sub>2</sub> O emissions by extrapolating plot-scale N <sub>2</sub> O measurements in the Northern Atlantic Zone of Costa Rica (2817 ha). A body of earlier work has been carried out in this humid tropical region, and the concurrent availability of data on soils, land use, climate and N <sub>2</sub> O emissions for a sizeable area provided a unique opportunity for an in-depth methodological study on extrapolation. Moreover, the land use history of the area is representative for humid tropical regions in Latin America.</p><p>A well-tested process-based ("mechanistic") simulation model driven by rainfall events (dndc) was used to estimate fluxes from unsampled fields and land units. Land units are defined by distal process controls such as soil type, management and climate. The model, originally designed to simulate nitrogen oxide emissions under temperate climatic conditions, was adapted to justify application to humid tropical pastures and banana plantations. First, the original formulations of evapotranspiration and soil water flow were replaced by routines relevant to tropical soils. Second, functions simulating <em>i)</em> cattle grazing and <em>ii)</em> steady input of organic matter through root turnover and the return of excrements to the pasture were added. Third, an explicit treatment for the immobilization of N was added.</p><p>The adapted simulation model was tested against field measurements of <em>i)</em> N <sub>2</sub> O and nitric oxide (NO) fluxes from a chronosequence of pastures on Inceptisols and <em>ii)</em> N <sub>2</sub> O fluxes from a banana plantation on Andisols and Inceptisols. For the pasture chronosequence, the model formulation was consistent for annual N dynamics and annual nitrogen oxide emissions. In contrast, simulated daily dynamics of nitrogen oxide emissions did not match field observations. The differences in local weather on the seven sampled pasture sites comprising the chronosequence may have caused a significant part of the mismatch. Annual emissions calculated by the model are essentially cumulative daily fluxes, so daily comparisons provide a more conclusive insight in the model's performance than annual comparisons.</p><p>Simulated daily N <sub>2</sub> O fluxes from soils below a banana plantation were compared with data from monthly and frequent field sampling. Different model parameterizations were used to represent fertilizer inputs below banana plants and crop residue additions between plants. For both the Andisol and the Inceptisol, simulated below-plant fluxes matched frequently measured fluxes better than monthly measured fluxes. Simulated between-plant fluxes matched monthly measured fluxes better than frequently measured fluxes. The simulated annual N <sub>2</sub> O-N losses for the Inceptisol and Andisol were 6 and 15 kg ha <sup>-1</sup> , respectively. Field-measured annual losses were 6 and 13 kg ha <sup>-1</sup> . In addition, three banana fertilization scenarios on an Andisol were studied. With fewer equal splits of fertilizer-N, the simulated N <sub>2</sub> O-N loss declined. With more equal splits losses increasingly depended on the amount of fertilizer-N.</p><p>An expert system for quantifying inputs and outputs of pastures (pastor) was linked with the simulation model to produce frequency distributions of N <sub>2</sub> O and NO emissions for one current pasture management system ("Natural") and two alternative systems ("Grass-Legume" and "Fertilized Improved"). Current forest-derived natural pastures deplete soil nitrogen stocks and therefore are unsustainable. Alternative management aims to utilize soil-N in a sustainable manner. The expert system was set up to generate parameter sets representing different land use options for the three management systems. The simulation model was rerun for each parameter set.</p><p>Simulated annual N <sub>2</sub> O-N losses twenty-five years after pasture establishment were 3-5 kg ha <sup>-1</sup> for natural pastures, 12-15 for grass-legume mixtures, and 7-28 for fertilized grasses. Simulated annual losses of NO-N were 1-2 kg ha <sup>-1</sup> for natural pastures, 7-8 for grass-legume mixtures, and 3-16 for fertilized grasses. Regression analysis showed that annual C input to the soil explained N <sub>2</sub> O losses, and that NO losses were explained by biomass production. Nitrous oxide and NO emissions from pastures may increase by a factor 3-5 when natural pastures are converted to improved pastures. Such conversion may increase the sustainability of the pasture by stopping the decline of soil N, but the change is not necessarily sustainable from a global perspective because it increases the emission of N oxides.</p><p>The regional N <sub>2</sub> O flux from soils below primary and secondary forest, pastures, and banana plantations was explored by linking the simulation model with an extant Geographic Information System (gis) on soils and land use. Land units on the overlaid soil and land use coverage were linked with the nearest of seven available meteorological stations. Monte Carlo-based sensitivity analysis was used to identify clay content, initial soil organic C, bulk density and pH as required map attributes and key driving model variables. For 217 different land units, model simulations were repeatedly carried out using climate data for seven different years.</p><p>The estimated regional N <sub>2</sub> O-N flux was 1.8-2.1 Gg yr <sup>-1</sup> . A full-fledged regional analysis of N <sub>2</sub> O emissions was performed using both deterministic and stochastic descriptions of key model inputs. The stochastic descriptions accounted for soil and land use heterogeneity across (non-georeferenced) fields within eleven different land units. Using Monte-Carlo integration, frequency distributions of fluxes were obtained per land unit class. Regional fluxes were calculated by summing expected values of the distributions weighted by area. Stochastic incorporation of both soil and land use variability resulted in areal flux estimates that were 14-22% lower than those estimated with deterministic model runs, suggesting concavity in the relationship between key model parameters and N <sub>2</sub> O fluxes.</p><p>Spatial flux patterns for 1992 land use and two alternative land use scenarios were evaluated using stochastic inputs. With contemporary management of banana plantations and natural grasses, the regional N <sub>2</sub> O-N flux (standard deviation in parenthesis) was 1.0 (0.4) Gg yr <sup>-1</sup> . Replacing natural grasses by sustainable grass-legume mixtures on relevant soil groups and allowing different fertilization levels on banana plantations increased the regional flux to 1.6 (0.5) Gg yr <sup>-1</sup> . When all natural grasses were replaced by fertilized improved species and different fertilization levels were allowed on banana plantations, the regional flux increased to 1.9 (1.2) Gg yr <sup>-1</sup> .</p><p>Land use activities that are sustainable in terms of economic profit and soil fertility may be unsustainable when including N <sub>2</sub> O emission as an extra indicator. Soil variations, dominating regional patterns, must be incorporated when inventorying N <sub>2</sub> O emissions. Spatial heterogeneity of soil properties regulates emissions at finer scales than typically employed in regional soil surveys. A stochastic description of key variables may therefore be an efficient way to reduce aggregation errors in regional flux estimates.</p><p>Future challenges include studies on effects of land use conversions, resulting in new spatial layouts of land units, on regional N <sub>2</sub> O fluxes. Also, the simulation model's implicit upscaling of emissions from soil microsite to field scales may be a potential research area.</p>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Bouma, J., Promotor
  • van Breemen, N., Promotor
Award date22 Feb 1999
Place of PublicationS.l.
Print ISBNs9789058080011
Publication statusPublished - 1999


  • nitrous oxide
  • emission
  • land use
  • silvicultural conversion
  • scaling
  • aggregation
  • costa rica

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