The NST 3.0 mechanistic nutrient uptake model was used to explore P uptake to a depth of 120 cm over a 126 d growing season in simulated buffer communities composed of mixtures of cottonwood (
The loss of P from agricultural lands has been a subject of growing interest in the environmental community for the past two decades. The shift in regulatory focus in the latter half of the 1990s from point sources to diffuse sources and the associated requirement that total maximum daily load (TMDL) estimates be developed has led to the extensive use of a variety of P transport models to describe both particulate and solution phase movement of P [
Paralleling the evolution of P transport models has been the development and implementation of various types of riparian buffers intended to retard the movement of P to surface waters [
In an earlier study, Kelly et al. [
As noted by Claassen and Steingrobe [
The first objective of this study was to use the NST 3.0 model to explore the uptake of P to a depth of 120 cm over a single growing season in simulated buffer communities composed of varying percentages of cottonwood, switchgrass, and smooth brome; three plant species commonly used in riparian buffers in the Midwestern U.S. The second objective was to use the model to explore the impacts of changes in key soil supply and plant parameters, as determined by sensitivity analysis, on estimates of P uptake by the three test species. And finally, the third objective was to use the model to explore potential P capture from each of three soil depth increments (0–30, 30–60, and 60–120 cm) over a 126d growing season. The buffer species tend to be deeprooted, so we hypothesized that P uptake from subsoil could be significant.
The NST 3.0 model was used to make all the calculations in this study. It is a transient model that requires numerical solution. The model along with necessary documentation is available for download from the website of the Department of Crop Sciences at Göttingen University (
Transport, sorption, and root parameters used in the NST 3.0 model to describe P uptake by cottonwood for the 0–120, 0–30, 30–60, and 60–120 cm soil depths.
Parameter  Units  0–120 cm  0–30 cm  30–60 cm  60–120 cm  


Diffusion coefficient in water^{†}  cm^{2} s^{−1} 




Θ  Volumetric soil water content  cm^{3} H_{2}O/cm^{3} soil  0.32  0.32  0.33  0.31 

Impedance factor  Unitless  0.33  0.33  0.35  0.32 

Water uptake at root  cm s^{−1} 





Initial solution concentration 






Buffer power  Unitless  314  45  477  421 

Maximum influx at high concentration 






Solution concentration when influx is 0.5 






Solution concentration when influx is zero 






Root radius  cm  0.023  0.0241  0.0217  0.0217 

Half distance  cm  1.05  0.213  0.378  2.578 

Initial root length  cm m^{−3}  3,041,322  2,094,524  663,672  283,126 

Root growth rate  cm d^{−1}  4,812  6,631  6,816  991 
^{
†}From Edwards and Huffman [
Transport, sorption, and root parameters used in the NST 3.0 model to describe P uptake by switchgrass for the 0–120, 0–30, 30–60, and 60–120 cm soil depths.
Parameter  Units  0–120 cm  0–30 cm  30–60 cm  60–120 cm  


Diffusion coefficient in water^{†}  cm^{2} s^{−1} 




Θ  Volumetric soil water content  cm^{3} H_{2}O/cm^{3} soil  0.37  0.39  0.36  0.36 

Impedance factor  Unitless  0.42  0.45  0.40  0.40 

Water uptake at root  cm s^{−1} 





Initial solution concentration 






Buffer power  Unitless  63  47  74  69 

Maximum influx at high concentration 






Solution concentration when influx is 0.5 






Solution concentration when influx is zero 






Root radius  cm  0.009  0.0121  0.0075  0.0085 

Half distance  cm  0.304  0.198  0.227  0.489 

Initial root length  cm m^{−3}  4,162,621  1,935,508  1,371,549  855,564 

Root growth rate  cm d^{−1}  8,484  10,506  10,306  4,641 
^{
†}From Edwards and Huffman [
Transport, sorption, and root parameters used in the NST 3.0 model to describe P uptake by smooth brome for the 0–120, 0–30, 30–60, and 60–120 cm soil depths.
Parameter  Units  0–120 cm  0–30 cm  30–60 cm  60–120 cm  


Diffusion coefficient in water^{†}  cm^{2} s^{−1} 




Θ  Volumetric soil water content  cm^{3} H_{2}O/cm^{3} soil  0.256  0.25  0.26  0.26 

Impedance factor  Unitless  0.243  0.23  0.25  0.25 

Water uptake at root  cm s^{−1} 





Initial solution concentration 






Buffer power  Unitless  73  28  98  92 

Maximum influx at high concentration 






Solution concentration when influx is 0.5 






Solution concentration when influx is zero 






Root radius  cm  0.0114  0.0135  0.009  0.0117 

Half distance  cm  0.505  0.255  0.347  0.914 

Initial root length  cm m^{−3}  2,486,379  1,464,477  793,155  228,747 

Root growth rate  cm d^{−1}  3,572  8,631  1,622  464 
^{
†}From Edwards and Huffman [
A solution depletion technique using intact plants and transient conditions, as described by Claassen and Barber [
For smooth brome, the depletion study was conducted at 49 d after germination and for switchgrass the plants were 54 d after germination [
The cottonwood depletion study followed procedures described in Kelly and Ericsson [
A field study was used to obtain estimates of the mean halfdistance between roots (
Soil samples collected at the start of the 2003 growing season were used to determine the solution and solid phase P concentrations in the soil. Soil solution was collected using the displacement column procedure described by Kovar and Barber [
Preliminary model runs were made using the 0–120 cm values listed in Tables
To explore model results further, a series of single factor sensitivity analyses were conducted with the 0–120 cm and 0–30 cm data using the approach described by Silberbush and Barber [
To explore the uptake of P in the 0–30, 30–60, and 60–120 cm soil depths, soil supply and root growth values were developed for each depth interval (Tables
Total estimated P uptake for the 126 d simulation period was 79.1 mmols m^{−3} for cottonwood, 69.5 mmols m^{−3} for switchgrass, and 61.0 mmols m^{−3} for smooth brome, all to a depth of 120 cm. Using these rates of uptake as a base, estimates of uptake on a per hectare basis were calculated for buffers composed of various percentages of the three cover types (Table
NST 3.0 model estimates of phosphorus uptake in kilograms per hectare to a depth of 120 cm over a 126 d growing season for buffers composed of varying percentages on an area basis of cottonwood, switchgrass, and smooth brome.
Plant community  Percentage contribution cottonwood/switchgrass/smooth brome  

66/17/17  33/33/33  0/50/50  100/0/0  0/100/0  0/0/100  
kg P ha^{−1}  
Cottonwood  16.3  8.1  0  24.5  0  0 
Switchgrass  3.6  7.1  10.8  0  21.5  0 
Smooth brome  3.2  6.2  9.4  0  0  18.9 
 

23.1  21.4  20.2  24.5  21.5  18.9 
Based on these simulations, the greatest amount of P is captured in a pure stand of cottonwood when compared to other buffer configurations. Assuming that this relationship holds for the longer term, the potential to capture and retain P on site is greater with the cottonwood given its longer lifespan and substantially larger level of standing biomass with the passage of time. The standing crop of the perennial grasses will come to relative equilibrium after a few years and the annual level of P intercepted will stabilize. If the grasses are removed annually, or even more frequently, the potential for P uptake will increase due to the fact that the level of P recycled annually by the standing vegetation will be reduced. The same would be true for the removal of the aboveground portion of the cottonwood, although it would be more practical to harvest the trees on a 7to10year cycle as a fiber or fuel byproduct. For example, Kelly et al. [
Each of the parameters used in the model was subjected to a single factor sensitivity analysis using the 0–120 cm and 0–30 cm values for each of the three cover type species. Results indicated that four factors,
Sensitivity analysis of predicted P uptake by cottonwood, smooth brome, and switchgrass using the NST 3.0 model showing the effect on predicted P uptake of varying individually the maximum nutrient influx rate at high concentration (
As noted in Figure
Values used to evaluate P uptake in each of the three depth intervals for each plant cover type are presented in Tables
Model exploration of the level of P uptake as a function of soil depth indicated that in all three cover types the level of uptake was highest in the 0–30 cm intervals (Table
NST 3.0 model estimates of phosphorus uptake in kilograms per hectare for four depth increments at the end of a 126 d simulated growing season for buffers composed of cottonwood, switchgrass, and smooth brome.
Plant community  Depth of soil increment (cm)  

0–120  0–30  30–60  60–120  
kg P ha^{−1}  
Cottonwood  24.5  20.4  2.7  0.8 
Switchgrass  21.5  15.3  7.3  4.7 
Smooth brome  21.4  18.8  5.4  2.0 
If the amount of P uptake for each of the three depth intervals within a cover type is summed, the estimate of total uptake obtained for switchgrass and smooth brome as compared to the value obtained for the same two cover types in the 0–120 cm simulation (Table
Figure
Effect of the distance from the root surface, as defined by the
Results of this study indicate that a simulation model such as NST 3.0 can provide both useful insights into the ability of various plant cover types to capture solution P and a means to explore which soil and plant factors are the most influential in predicting plant P uptake. A single factor sensitivity analysis for each cover type identified
The authors express their appreciation to Robin Sokolowsky for assistance with data summarization and to Dr. J. K. Kelly for assistance with figure preparation. Preparation of this paper was supported by funds from the Virginia Agricultural Experiment Station and the Virginia Tech College of Natural Resources and Environment.