『Abstract
Precipitation variability and magnitude are expected to change
in many parts of the world over the 21st century. We examined
the potential effects of intra-annual rainfall patterns on soil
nitrogen (N) transport and transformation in the unsaturated soil
zone using a deterministric dynamic modeling approach. The model
(TOUGHREACT-N), which has been tested and applied in several experimental
and observational systems, mechanistically accounts for microbial
activity, soil moisture dynamics that respond to precipitation
variability, and gaseous and aqueous tracer transport in the soil.
Here, we further tested and calibrated the model against data
from a precipitation variability experiment in a tropical system
in Costa Rica. The model was then used to simulate responses of
soil moisture, microbial dynamics, N leaching, and N trace-gas
emissions to changes in rainfall patterns; the effect of soil
texture was also examined. The temporal variability of nitrate
leaching and NO, NH3, and N2O
effluxes were significantly influenced by rainfall dynamics. Soil
texture combined with rainfall dynamics altered soil moisture
dynamics, and consequently regulated soil N responses to precipitation
changes. The clay loam soil more effectively buffered water stress
during relatively long intervals between precipitation events,
particularly after a large rainfall event. Subsequent soil N aqueous
and gaseous losses showed either increases or decreases in response
to increasing precipitation variability due to complex soil moisture
dynamics. For a high rainfall scenario, high precipitation variability
resulted in as high as 2.4-, 2.4-, 1.2-, and 13-fold increases
in NH3, NO, N2O and
NO3- fluxes, respectively, in
clay loam soil. In sandy loam soil, however, NO and N2O
fluxes decreased by 15% and 28%, respectively, in response to
high precipitation variability. Our results demonstrate that soil
N cycling responses to increasing precipitation variability depends
on precipitation amount and soil texture, and that accurate prediction
of future N cycling and gas effluxes requires models with relatively
sophisticated representation of the relevant processes.
Keywords: GHG emission; Vadoze zone; Biogeochemistry; Climate
change; N cycle; Nitrate leaching』
1. Introduction
2. Methods
2.1. TOUGHREACT-N model
2.2. Model calibration and testing
2.3. Scenario analysis
3. Results
3.1. Experimental data vs. model predictions comparison
3.2. Soil moisture response in the model experiments
3.3. Soil nitrogen gas efflux response
3.4. Soil nitrate leaching response
3.5. Soil depth integrated N turnover rates
4. Discussion
4.1. Soil moisture response to precipitation variability
4.2. Effect of precipitation variability on N losses
4.3. Implications for ecosystem responses
4.4. Implication for response sensitivity
5. Summary
Acknowledgements
Appendix 1. Model description
1.1. Soil moisture dynamics
1.2. Multiphase transport
1.3. Chemical and biological reactions
1.4. The nitrogen cycle
1.5. Nitrification, denitrification and aerobic respiration
1.6. Microbial dynamics
1.7. The carbon cycle
References