<p>Eutrophication due to phosphorus (P) enrichment continues to be a
primary water quality concern affecting freshwater and marine estuaries around
the world. Excessive anthropogenic P inputs, driven by the need to meet the
rising food and energy demands of a growing and increasingly urbanized
population, have resulted in the buildup of P creating legacy (historical) P
pools in agricultural landscapes. There is growing evidence that remobilization
of accumulated legacy P can interfere with conservation efforts aimed at
curbing eutrophication and improving water quality. Less is known about the
magnitude and effects of these legacy P pools on dissolved reactive P (DRP)
losses in tile-drained systems. This dissertation consists of three separate
inquiries into how legacy P may affect DRP losses in tile drains. In the first
inquiry, we examined the possibility of developing a suitable pedo-transfer
function (pedoTF) for estimating P sorption capacity (PSC). Subsequent
PSC-based indices (Phosphorus Saturation Ratio (PSR) and Soil Phosphorus
Storage Capacity (SPSC)) were evaluated using daily water quality data from an
in-field laboratory. The pedoTF derived from soil aluminum and organic matter
accurately predicted PSC (R<sup>2</sup> = 0.60). Segmented-line models fit
between PSR and soluble P (SP) concentrations in both desorption assays (R² =
0.69) and drainflows (R² = 0.66) revealed apparent PSR thresholds in close
agreement at 0.21 and 0.24, respectively. Linear relationships were observed
between negative SPSC values and increasing SP concentrations (R² = 0.52 and R<sup>2</sup>
=0.53 respectively), and positive SPSC values were associated with very low SP
concentrations in both desorption assays and drainflows. Zero SPSC was
suggested as a possible environmental threshold. Thus, PSC-based indices
determined using a pedoTF could estimate the potential for SP loss in tile
drains. Also, both index thresholds coincided with the critical soil test P
level for agronomic P sufficiency (22 mg kg<sup>-1</sup> Mehlich 3 P) suggesting
that the agronomic threshold could serve as an environmental P threshold. In
the second inquiry, PSC- based indices in addition to other site characteristics
present in a P index (PI), were used as inputs in the development of a
multi-layer feed-forward artificial neural network (MLF-ANN). The MLF-ANN was
trained, tested, and validated to evaluate its performance in predicting SP
loss in tile drains. Garson’s algorithm was used
to determine the weight of each site characteristic. To assess the performance
of ANN-generated weights, empirical data from an in-field laboratory was used
to evaluate the performance of an unweighted PI (PI<sub>NO</sub>), a PI
weighted using Lemunyon and Gilbert weights (PI<sub>LG</sub>), and an
ANN-weighted PI (PI<sub>ANN</sub>) in estimating SP losses in tile effluent.
The MLF-ANN provided reliable predictions of SP concentrations in tile effluent
(R<sup>2</sup> = 0.99; RMSE = 0.0024). Soil test P, inorganic fertilizer application
rate (FPR), SPSC, PSR, and organic P fertilizer application rate (OPR), with
weights of 0.279, 0.233, 0.231, 0.097, and 0.084, respectively, were identified
as the top five site characteristics with the highest weights explaining SP
loss in tile discharge. These results highlighted the great contribution of
both contemporary and legacy P sources to SP concentrations in tile discharge.
Also, PI<sub>ANN </sub>was the only PI with a significant exponential
relationship with measured annual SP concentrations (R<sup>2 </sup>= 0.60; p
< 0.001). These findings demonstrated that MLF-ANNs coupled with Garson’s
algorithm, can accurately quantify weights for individual site characteristics
and develop PIs with a strong correlation with measured SP in tile discharge.
Finally, in the third inquiry, we compared DRP loads and flow-weighted
mean DRP (FDRP) concentrations in P source and P sink soils and evaluated the
predominant DRP concentration – discharge (C-Q) behavior in these soils on a
daily and event scale. At the daily scale, C-Q patterns were linked to the soil
P status whereby a chemostatic and dilution behavior was observed for P source
and P sink soils, respectively. At the event scale, C-Q patterns were linked to
soil P status, flow path connectivity, and mixing of event water, matrix water,
and rising shallow groundwater. The predominant anti-clockwise rotational
pattern observed on P source soils suggested that, as the discharge event
progressed, contributions from P poor waters including matrix and shallow
groundwater resulted in lower DRP concentrations on the rising limb compared to
the falling limb. However, the variable flushing and dilution behavior observed
on the rising limb suggested that, in addition to discharge and soil P status,
rapid exchanges between P pools, the magnitude of discharge events (Q), and the
relative number of days to discharge peak (D<sub>rel</sub>) also regulated DRP
delivery. On the other hand, the predominant non-hysteretic C-Q behavior in P
sink soils suggest that DRP loss from these soils can be discounted. Our
collective results highlight the need for nutrient and conservation practices
focused on P drawdown, P sequestration, and P supply close to the crop needs,
which will likely be required to convert P sources to sinks and to avoid the
conversion of P sinks to sources. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/12253262 |
| Date | 07 May 2020 |
| Creators | Pauline Kageha Welikhe (8803301) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Evaluating_the_Effects_of_Legacy_Phosphorus_on_Dissolved_Reactive_Phosphorus_Losses_in_Tile-Drained_Systems/12253262 |
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