Taking Cover Crops to the Next Level with Mixtures – Focus on C:N Ratios

Charlie WhiteCrop Management Team

What type of cover crop would lead to each result for corn yields and N leaching?

Annual Ryegrass

No Cover Red Clover + Ryegrass

Red Clover100110120130140150160170180190200

0 lbs N/ac160 lbs N/ac

0

25

50

75

100

125

150 Chart TitleCC #1 CC #3

Corn

Yie

ldBu

/ac

Nitr

ate

Leac

hing

lbs N

/ac

CC #2

Corn Fertilizer

Ryegrass

Ryegrass + Red Clover

Red Clover

Themes for today• N supply vs. N retention functions• Cover crop types

- grasses, brassicas, legumes

Mechanisms to learn about• N acquisition strategies• Decomposition and N release• Competition & synergies in mixes

How do cover crop species and mixes affect nitrogen management?

Cover crop functions and species should be tailored to meet the needs and constraints of each farming system

Categories of nitrogen management objectives• Retain N against leaching• Supply N to the next crop• Both retain & supply N

Cover crop planting window affects species options• Late summer after small grain

– many species options• Early fall after corn silage

– still a few good options• Late fall after shell corn or beans

– only a couple species left

• Grasses and brassicas only acquire N from the soil• Legumes can acquire N from the atmosphere so there is less

demand from the soil

Nitrogen retention: N acquisition strategy of cover crops affects nitrate leaching

0

20

40

60

80

100

120

140

160

180

over

-win

ter N

leac

hing

(kg/

ha)

a

ab

bcc c

cd

de de de e e e

Red Clover vs.Austrian

Winter Pea

Nitrogen retention: N acquisition strategy of cover crops affects nitrate leaching

Anion Exchange Resin Capsules

Nitrogen supply: N is released from cover crop residues by microbial decomposition

Fava Bean Residues Sorghum – SudangrassResidues

Ryegrass Residues

Red Clover Residues

Decomposition

With high N residues, microbes use some of the N , the rest goes to the soil as NH4

+ “Mineralization”

Carbon

Nitrogen

~ 50% respired as CO2

(Energy gain for the microbes)

~50% assimilated (Building blocks)

Carbon

Atmosphere

Available Soil NNH4

+

Cove

r Cro

p Re

sidue

s Microbial Biom

ass

Nitrogen

C:N = 10:1

C:N = 10:1

Decomposition

Carbon

Nitrogen

~ 50% respired as CO2

(Energy gain for the microbes)

~50% assimilated (Building blocks)

Carbon

Atmosphere

Available Soil NNH4

+

Cove

r Cro

p Re

sidue

s Microbial Biom

ass

Nitrogen

C:N = 20:1

C:N = 10:1

With moderate N residues, microbes use all of the N

Decomposition

With low N residues, microbes use all of the cover crop N plus some soil N “Immobilization”

Carbon

Nitrogen

~ 50% respired as CO2

(Energy gain for the microbes)

~50% assimilated (Building blocks)

Carbon

Atmosphere

Available Soil NNH4

+

Cove

r Cro

p Re

sidue

s Microbial Biom

ass

Nitrogen

C:N = 40:1

C:N = 10:1

C:N ratio regulates N availability from cover crop decomposition

• C:N below 20 – Nitrogen Mineralization – NH4+ produced

• C:N between 20 and 30 – Neutral Mineralization/Immobilization• C:N above 30 – Nitrogen Immobilization – NH4

+ and NO3- consumed

Cover Crop Species Typical Range of C:N

Clovers 9 – 12

Austrian Winter Pea 9 – 12

Soybean 12 – 15

Fava Bean 13 – 19

Canola 11 – 20

Radish 12 – 14

Cereal rye, triticale 9 – 40

Annual ryegrass 10 – 30

Oats 17 – 30

Sorghum Sudangrass 30 – 40

C:N ratio regulates N availability from cover crop decomposition

Oct Nov Dec Mar Apr May Jun0

5

10

15

20

25

30

Red CloverNo CoverRed Clover + RyegrassRyegrass

Soil

Nitr

ate

(ppm

N)

Annual Ryegrass

No Cover Red Clover + Ryegrass

Red Clover100110120130140150160170180190200

0 lbs N/ac160 lbs N/ac

Corn

Yie

ld (b

u/ac

)

40 lbs N/ac 70 lbs N/ac 140 lbs N/acCover Crop N contentCover Crop C:N 24 16 10

Burn

dow

n

C:N ratio of a cover crop can be managed through species selection, management timing, and species mixtures

Triticale

Annual Ryegrass

Early boot stage

C:N ratio of a cover crop can be managed through species selection, management timing, and species mixtures

Annual Ryegrass

Triticale

Cereal RyeC:N = 16 C:N = 17 C:N = 23

April 16Penn England FarmCentral PA

Managing C:N ratio in legume-grass mixes: Red clover is a poor competitor

Ryegrass10 lbs/ac

Ryegrass + Red Clover10 lbs/ac + 12 lbs/ac

Red Clover12 lbs/ac

40 lbs N/ac 70 lbs N/ac 140 lbs N/acCover Crop N Cover Crop C:N 24 16 10

Red clover plus increasing seeding rates of triticale

Managing C:N ratio in legume-grass mixes: Red clover is a poor competitor

0 10 20 30 40 50 60 700

20406080

100120140160180

051015202530354045

Rock SpringsBig FlatsC:N Rock SpringsC:N Big Flats

Triticale Seeding Rate (lbs/ac)

Tota

l Cov

er C

rop

Biom

ass N

(lbs N

/ac)

C:N

Vining legumes make a better companion with grasses

Rye Hairy VetchRye + Hairy Vetch

+ Canola

Study by Denise Finney, PSU Graduate Student

Vining legumes make a better companion with grasses

Study by Denise Finney, PSU Graduate Student

Rye Rye + HV + Canola

Hairy Vetch0

20406080

100120140160180

0 lbs N/ac135 lbs N/ac

Corn

Yie

ld (b

u/ac

)Rye Rye + HV + Canola Hairy Vetch

Biomass N (lbs N/ac) 60 141 165C:N 42 16 9Nitrate Leaching (lbs N/ac) 1 3 23

•Cover crop mixtures need to be tailored to specific farm goals and constraints•Grasses work well at retaining N against leaching• Legumes work best at supplying N to the next crop

- Low C:N ratio of legumes causes mineralization upon decomposition

•Cover crop mixes can balance N retention and N supply functions•Vining species of legumes work best in a mix with

grasses

Conclusions

Thank You!

Feel free to contact me for more information:Charlie White

cmw29@psu.edu814-863-9922

Funding for this work was

provided by:

USDA NIFAOREI Program

USDA NRCS Conservation

Innovation Grant