Multiscale Interdisciplinary Integration of Soil-Hydrology-Plant Interactions II: Rhizosphere Solute Movement
Presiding: L A Gaston, Louisiana State University; M A Celia, Princeton University
H42A-01 INVITED 10:35h
Representation of Soil Moisture, Evapotranspiration, and Solute Transport Across Different Length and Time Scales
In water-limited ecosystems, nonlinear interactions among soil, plants, and water, at the local scale, lead to complex functional relationships at larger spatial scales. The complexity includes non-uniqueness in upscaled relationships between total evaporation and average soil moisture, when upscaled variables are defined over spatial scales that are large relative to the scale of the original governing equations. This non-uniqueness can arise when water content is vertically averaged over the depth of the root zone, and when averaging is applied over larger horizontal spatial scales. Analytical solution methods can be used to predict and quantify some of the non-uniqueness, under appropriate simplifying assumptions required for the application of analytical methods. Numerical solutions allow for more general analyses and show more complex behavior patterns for systems with spatially and temporally variable rainfall, and spatially variable vegetation. When solute transport is added to the system, scaling effects in the moisture distribution and evapotranspiration function translate into complex functional relationships that apply to both contaminant transport and nutrient uptake. In this presentation, we will highlight the underlying causes of complexity and non-uniqueness, with a focus on estimates of water uptake functions, measures of plant health, and descriptions of nutrient transport in water-limited ecosystems.
H42A-02 INVITED 11:00h
Nitrate losses in subsurface tile drainage as affected by tillage, crop rotation, and fertilizer management
Nitrate emanating from artificial subsurface drains in the U.S. Midwest has been implicated as a strong contributing factor to water quality problems such as hypoxia in the Gulf of Mexico. Balancing the amount of N needed for optimum plant growth while minimizing nitrate transport to ground and surface waters, however, remains a challenge. Field studies are limited that investigate the water quality effects of multiple management practices such as nitrogen application timing and amount. Agricultural simulation models may be one method to cost-effectively investigate the effect of a variety of management practices under a variety of conditions. The Root Zone Water Quality Model (RZWQM) was calibrated to long-term (1990-2003) data near Nashua, Iowa, which includes 36 plots with a variety of crop rotations, tillage, and manure and fertilizer applications. The model adequately responds to year-to-year climate variation and to plot-to-plot management variation within a year. The calibrated model will be used to populate a database that quantifies the water quality and yield effect of multiple agricultural management practices under several climate and soil conditions. A database such as this may be a useful tool that simply and objectively quantifies the tradeoffs of management alternatives, which may help accelerate adoption of best management practices.
H42A-03 11:20h
Soil Moisture, Plant Nutrient Uptake, and Computer Simulation of Nutrient Export
Correct estimate of nutrient discharge from soil is important in nutrient management for protecting water quality of receiving water. Solute property, soil physical and chemical property, climatic factors, water supplies, microorganism activities, and plant uptake play important rolls in sub-ground nutrient cycle. Soil water movement provides forcing for transport of dissolved nutrients within the soil and their discharge from land to a receiving water. Solute export is proportional to water discharge and solute concentration in the soil pool. Plant uptake reduces the amount of nutrient in the soil and reduces export potential. Soil moisture level has a significant effect on the uptake of nutrient by plant, which involves both solute transport and complex physiological responses of plant. Plant nutrient uptake is not a monotonic function with soil moisture. Experiments show that plant uptake increases from a dry soil to moist soil, hence, reducing the potential of nutrient export. However, when moisture is over-sufficient in a wet soil, then more moisture would not cause higher nutrient uptake but increase export potential due to higher water discharge. This paper discusses the relationship among soil moisture, nutrient concentration, evapotranspiration, plant nutrient uptake and nutrient export from land, and presents a computer model on these processes, providing a method to simulate plant nutrient uptake and nutrient export under various moisture conditions.
H42A-04 11:35h
Role of model structure on the response of soil biogeochemistry to hydro-climatic fluctuations
Soil carbon and nutrient cycles are strongly affected by hydro-climatic variability, which interacts with the internal ecosystem structure. Here we test the implications of biogeochemical model structure on such dynamics by extending an existing model by the authors and coworkers. When forced by hydro-climatic fluctuations, the different model structures induce specific preferential nutrient paths among the soil pools, which in turn affect nutrient distribution and availability to microbes and plants. In particular, if it is assumed that microbes can directly assimilate organic nitrogen, plants tend to be inferior competitors for nutrients even in well-watered conditions, while if a certain amount of organic nitrogen is assumed to be mineralized without being first incorporated into microbial cells, vegetation can be advantaged over a wide range of soil moisture values. We also investigate the intensification of competition for nutrients (e.g., nitrogen) between plant and soil microbial communities under extreme hydrologic conditions, such as droughts and intense storms. Frequent rainfall events may determine ideal soil moisture conditions for plant uptake, enhancing nitrogen leaching while lowering oxygen concentration and inhibiting microbial activity. During droughts, the soil water potential often drops to the point of hampering the plant nutrient uptake while still remaining high enough for microbial decomposition and nitrogen immobilization. The interplay of microbe and vegetation water stress is investigated in depth as it controls the ability of one community (e.g., plants or soil microbes) to establish competitive advantage on the other. The long-term effects of these dynamics of competition and nutrient allocation are explored under steady-state and stochastic soil moisture conditions to analyze the feedbacks between soil organic matter and vegetation dynamics.
H42A-05 11:50h
The Vadose Zone's Dynamic Response to a Natural Precipitation Event: Water Migration Through the Unsaturated Subsurface Monitored by GPR
Water movement and its retentivity in the vadose zone is a fundamental link between precipitation, stormflow runoff and groundwater recharge. Moreover, the vertical distribution of water in the subsurface is fundamentally linked to vegetation since, following a precipitation event, the retentivity of near subsurface layers will determine the amount of water in the root zone. We report on a study of changes in the distribution of water in the subsurface during the hours and days following a precipitation event employing a sequence of high quality ground penetrating radar (GPR) measurements. The velocity of a radar signal in the subsurface is strongly modified by the soil water content (SWC), so that the technique provides an attractive means for non-invasively characterizing subsurface conditions using a Topp-like relation to infer SWC from GPR velocities. In this study, we focus on the vadose zone response to a natural precipitation event, by collecting a series of common mid-point (CMP) soundings at various times before, during, and after a 3 day period of rain at a geophysical test site in Southeastern New England. The shallow subsurface at the field site is unsaturated, and characterized to a depth of four meters by three layers: an organic rich soil layer (thickness = 0.9 m), a gravelly-sand layer (thickness = 2.9 m), underlain by a fine silty-sand, with bedrock at approximately 10 m deep. The CMPs were analyzed by carefully hand picking the first break of all observed phases: direct air, reflected, air refractions, and ground refracted phases. CMP soundings are ideal for this experiment allowing a combination of cross-checks. In particular, we have found it essential to complement the conventional analysis of reflected phases with the interpretation of ground refracted phases. Our study period involved a time of variable precipitation over 3 days, from July 23 to July 26, 2003, resulting in a total of 4.2 cm of water being added to the field site. The radar velocity of the organic rich soil layer decreased from 0.12 m/ns to 0.08 m/ns over the course of the 3 days, indicating a 0.14 m3/m3 increase in SWC in the upper meter of the subsurface. In addition, over the same period, the radar velocity of the deeper gravelly-sand decreased from 0.14 m/ns to 0.12 m/ns, corresponding to a 4% increase in SWC, from 0.065 m3/m3 to 0.1065 m3/m3. Of course, the systematically higher velocity of the gravelly-sand is consistent with the lower SWC expected for a low retentivity material. During the week following the precipitation event, a series of GPR measurements indicate that the SWC recovers slowly in the organic rich soil layer from its highest value of 0.26 m3/m3 to its pre-event value of 0.11 m3/m3 with a time constant of approximately 80 hr. Over the same period, the SWC of the top of the gravelly-sand layer (as inferred from the radar velocity of the refracted phase) recovers more quickly from 0.097 m3/m3 to its approximate pre-storm value of 0.066 m3/m3, with an approximate 20 hr time constant. However, the SWC of the entire gravelly-sand (as inferred from the reflected phase from its base), is systematically lower than its pre-storm value, suggesting the possibility that the SWC of the deeper layer is being replenished by the continuing drainage from the layer above. The overwhelming issue in our view is the fate of water in the organic soil layer. As the SWC diminishes by 0.14 m3/m3 over the post-storm period of 1 week, how much of this water drains vertically to the gravelly-sand, and how much is evapotranspirated to the atmosphere?