When it Comes to Nitrogen Leaching, Not All Cover Crop Practices Are the Same

Abstract

Cover crops are subsidized by taxpayers for use on more than 600,000 acres of agricultural fields in Maryland as part of an initiative to protect water quality and the Chesapeake Bay. As cover crops grow and take up nutrients, the water leaching from fields is cleaned up, especially with regard to nitrogen. However, the way that cover crops are typically managed may not be optimal for improving water quality. The Weil lab's previous work has shown that the effectiveness of cover crops in reducing N leaching during the winter is dramatically affected by how early the cover crops are established, with cover crops planted in mid-October having little impact on N leaching compared to those planted a month earlier. The challenge is to find ways of getting cover crops established in early September, a time frame usually not possible with the typical practice of drilling cover crop seed after harvesting the corn or soybean cash crop. For this reason we studied a mixed species cover crop (radish, rye, and crimson clover) that was interseeded into standing soybeans canopies as compared to the standard practice of post-harvest drilling, and a no-cover crop control. We conducted the replicated experiment on two coastal plain fields with soils of contrasting textures formed in silty/clayey sediments, and in sandy sediments. This experiment was established at the Beltsville Facility of the Central Maryland Research and Education Center, with funding from Shore Rivers, LLC and the Maryland Soybean Board. The early planted cover was planted by broadcasting seed into a standing soybean canopy at leaf yellowing using a hiboy air-seeder on September 11, 2017. In each field, suction lysimeters were installed (Figure 1) to one-meter depth and samples were collected using a 85 kPa vacuum approximately every two weeks between December 17, 2017 and May 7, 2018. Soil pore water samples were filtered to remove particulate matter and frozen until they were analyzed for NO3-N and NH3-N on a LaChat® Flow Injection Analyzer. Figure 1. Nitrate-N concentrations in porewater from 1 m depth in fields of contrasting soil texture. Average of all sample dates during the 2017-18 winter-spring leaching season (N=33). Error bars are one standard error.

Full text

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VOLUME 10, ISSUE 1 AGRONOM Y NE WS: A P R I L 2 0 1 9
When it Comes to Nitrogen Leaching, Not All Cover Crop Pracces Are
the Same
Ian Goralczyk, Nathan Sedghi, and Ray Weil
University of Maryland, Department of Environmental Science & Technology
Cover crops are subsidized by taxpayers for use on
more than 600,000 acres of agricultural elds in
Maryland as part of an iniave to protect water
quality and the Chesapeake Bay. As cover crops grow
and take up nutrients, the water leaching from elds is
cleaned up, especially with regard to nitrogen.
However, the way that cover crops are typically
managed may not be opmal for improving water
quality. The Weil lab’s previous work has shown that
the eecveness of cover crops in reducing N leaching
during the winter is dramacally aected by how early
the cover crops are established, with cover crops
planted in mid-October having lile impact on N
leaching compared to those planted a month earlier.
The challenge is to nd ways of geng cover crops
established in early September, a me frame usually
not possible with the typical pracce of drilling cover
crop seed aer harvesng the corn or soybean cash
crop. For this reason we studied a mixed species cover
crop (radish, rye, and crimson clover) that was
interseeded into standing soybeans canopies as
compared to the standard pracce of post-harvest
drilling, and a no-cover crop control. We conducted the
replicated experiment on two coastal plain elds with
soils of contrasng textures formed in silty/clayey
sediments, and in sandy sediments.
This experiment was established at the Beltsville
Facility of the Central Maryland Research and
Educaon Center, with funding from Shore Rivers, LLC
and the Maryland Soybean Board. The early planted
cover was planted by broadcasng seed into a standing
soybean canopy at leaf yellowing using a hiboy air-
seeder on September 11, 2017. In each eld, sucon
lysimeters were installed (Figure 1) to one-meter depth
and samples were collected using a 85 kPa vacuum
approximately every two weeks between December 17,
2017 and May 7, 2018. Soil pore water samples were
ltered to remove parculate maer and frozen unl
they were analyzed for NO3-N and NH3-N on a LaChat®
Flow Injecon Analyzer.
Figure 1. Nitrate-N concentraons in porewater from 1 m depth in elds of contrasng soil texture. Average of all sample
dates during the 2017-18 winter-spring leaching season (N=33). Error bars are one standard error.

4
VOLUME 10, ISSUE 1 AGRONOM Y NE WS: A P R I L 2 0 1 9
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One eld had a
silt loam surface
texture and a clay
loam subsoil (Russet-
Chrisana Complex).
The other eld had a
loamy sand surface
texture and sandy
loam subsoil
(Evesboro-Downer
Complex). By ulizing
elds of contrasng
soil textural classes
we can determine the
eecveness of
these cover cropping
methods with a
range of soil
condions in order
to broaden the scope of this study.
Cover crop use made a major dierence in nitrate
concentraons measured in the porewater collected at
1 m depth (Figure 2). Nitrate concentraons were
reduced most where cover crops were established the
earliest. As expected, the nitrate concentraons in the
leaching water, as well as the impact of early cover crop
establishment, were greatest on the sandy soil site.
While there were some individual samples that
exceeded the EPA safe drinking water standard for
nitrate-N (10.0 ppm), the average of all individual
treatments was below this standard, and nitrate
concentraons were consistently lower for the early
interseeded cover crop treatment. A major reason why
lower nitrate concentraons at one meter depth were
observed for cover cropped plots is that the nitrate was
taken up by cover crops roots and largely translocated
to the aboveground plant ssue. This process captures
the N before it leaves the potenal roong zone and
recycles it to the surface soil where it may be released
for use by future crops. This release could lead to
decreased need for ferlizer nitrogen applicaon to the
following corn crop. Our data suggest that if similar
cover crop interseeding pracces (using aerial or ground
-based methods) were applied on a large scale on
commercial farms, the reducon in nitrogen loading to
the Chesapeake Bay could be substanal. We can also
conclude that early-planted cover crops are eecve for
reducing nitrate leaching on soils with a range of
textural classes.
While these results are promising, it is important to
note that they represent only one year out of a three
year project, and that more data will be collected on
dierent elds and with dierent cover cropping
methods. We hope to provide farmers with guidance on
opmizing cover crop species mixtures, planng dates
and methods in order to enhance the impact of cover
crops on nitrogen polluon while also improving soil
health and farm protability.
Figure 2. Undergraduate researcher
in the Weil Lab, Ian Goralczyk,
installing a sucon lysimeter for
collecng soil porewater samples.