This document summarizes research on nutrient delivery to tile drains from various water sources. The objectives were to determine dominant water flow paths to tile drains, quantify dissolved reactive phosphorus (DRP) fluxes, and assess linkages between water flow paths and nutrient delivery. Field data from 2017-2019 showed tile water was mostly groundwater, with some soil water and precipitation. DRP concentration was higher when more water was from near the soil surface. Fertilizer application timing and placement affected DRP loss. Understanding both preferential flows and groundwater dynamics is important for effective phosphorus management.

1. Source and
transport
controls on
nutrient delivery
to tile drains
Williams, Penn, and McAfee

2. Terminology
Fertilizer P
fertilizer that hasn’t equilibrated with the
soil; “new P” or “incidental P”
Soil P P that is loosely or strongly bound to the
soil; “old P” or “legacy P”
water stored in the soil profile; “old water”
or “pre-event water"
Soil water
Groundwater
Discharge &
concentration
“new water” or “event water"
Precipitation

3. Objectives
Determine dominant water flow paths to tile drains and
factors that may influence this partitioning
Quantify DRP fluxes and the relative contribution of recent
nutrient management practices to overall losses
Assess linkages between water flow paths and nutrient
delivery to tile drains

4. Field data
Daily tile discharge and water quality
Weekly precipitation, groundwater (2 m), and
soil water sampling (10-80 cm)
June 2017 – August 2019

5. Tile water is…mostly groundwater, some
soil water, and a little precipitation
Percent
of
Tile
Q
0
20
40
60
80
100
Check out
our poster!

6. 0
10
20
30
40
50
0 0.5 1 1.5 2 2.5
Tile
Q
(mm)
Water table depth (m)
0.0
0.2
0.4
0.6
0.8
1.0
WT<1m
91% of total Q
Fraction
of
tile
Q
WT>1m
9% of total Q
0.0
0.2
0.4
0.6
0.8
1.0
Discharge and
water sources
vary with
antecedent
conditions

7. Dissolved reactive P concentration
Fertilizer application
Nutrient management matters!
But…let’s first consider soil P loss
and periods without fertilizer
application

8. R² = 0.1146
0.0
0.1
0.2
0.3
0.0 0.3 0.5 0.8 1.0
R² = 0.4777
0.0
0.1
0.2
0.3
0.0 0.3 0.5 0.8 1.0
R² = 0.5342
0.0
0.1
0.2
0.3
0.0 0.3 0.5 0.8 1.0
DRP
concentration
(mg/L)
Qprecip/Q
Qsoil/Q
Qgw/Q
Water sources from
near the soil surface
= greater DRP
concentration

9. Discharge and water sources vary with
antecedent conditions impacting DRP
0
10
20
30
40
50
0 0.5 1 1.5 2 2.5
Tile
Q
(mm)
Water table depth (m)
0.0
0.2
0.4
0.6
0.8
1.0
WT<1m
91% of total Q
Fraction
of
tile
Q
WT>1m
9% of total Q
0.0
0.2
0.4
0.6
0.8
1.0
<1m >1m
DRP
(mg/L)
WT<1m
Lower, less variable DRP
WT>1m
Greater, more variable DRP

10. Groundwater
Soil water
Precipitation
Soil water
Groundwater
Precipitation
DRP
(mg/L)
DRP
(mg/L)
WET conditions
DRY conditions
• Water table at/above tile depth
• Large flows ∝ precipitation
• Groundwater dominated
• Lower, less variable DRP
• Water table at/below tile depth
• Small or negligible flows
• Soil water/precip. dominated
• Greater, more variable DRP

11. The tale of two fertilizer applications
Fertilizer application
Fall 2017 Spring 2019
35.6 kg P/ha
Broadcast
Dry conditions…
followed by a few
large (relative) flow
events
Max DRP=5.62 mg/L
19.4 kg P/ha
Subsurface with planter
Wet conditions…
followed by a few small
(relative) flow events
Max DRP=0.36 mg/L

12. Fall 2017 Spring 2019
Fertilizer P = 44%
Soil P = 56%
Fertilizer timing and placement could
have decreased DRP loss by ~40%

13. Why you
should care
In our data collection and
modeling, we tend to focus on
the preferential flows
But…groundwater dynamics have
a strong influence on discharge
and DRP transport
1. Largest component of flow
2. Hydrologic buffer (dilution)
3. Chemical buffer (contact
time)
So, we need to study both!

14. Quantifying the
interaction between
P source and
transport is critical
for effective P
management

15. Want
more?
mark.williams2@usda.gov