Ecosystem flip-flops in response to anthropogenic N deposition: The

importance of long-term experiments George L Vourlitis, Biological Sciences

Department, California State University San Marcos

2003

2017

• Chaparral can receive up to 35 kgN ha-1 y-1 and coastal sage scrub (CSS) can receive up to 20 kgN ha-1 y-1; however, values can vary with elevation and degree of exposure.

• 90% is dry deposition.

• N accumulates in the summer and fall and becomes available as a large pulse following the first rainfall event. Fenn et al. (2010)

Atmospheric N deposition

• Chaparral and CSS productivity are thought to be N limited. N enrichment has the potential to alter chaparral and CSS C and N storage.

Research objectives and approach

We used a long-term field experiment to assess how dry-season N inputs altered chaparral and CSS C and N storage and cycling. We hypothesized that N addition would significantly

(1) increase rates of C and N accumulation.

(2) increase the cover and dominance of exotic annuals

Research objectives and approach

We used a long-term field experiment to assess how dry-season N inputs altered chaparral and CSS C and N storage and cycling. We hypothesized that N addition would significantly

(1) increase rates of C and N accumulation.

(2) increase the cover and dominance of exotic annuals

• N-fertilizer (50 kgN/ha) is added in the fall of each year since September 2003.

• Samples were collected seasonally (March, June, September, and December).

• Soil (0-10 cm), surface litter pool, litter production, root biomass (0-10 cm), plant tissue, aboveground biomass and plant cover, leaching (1 m).

• C and N content of tissue, litter, soil, roots, extractable N, pH.

Experimental design

10m

10m

• N-fertilizer (50 kgN/ha) is added in the fall of each year since September 2003.

• Samples were collected seasonally (March, June, September, and December).

• Soil (0-10 cm), surface litter pool, litter production, root biomass (0-10 cm), plant tissue, aboveground biomass and plant cover, leaching (1 m).

• C and N content of tissue, litter, soil, roots, extractable N, pH.

Experimental design

10m

10m

• N-fertilizer (50 kgN/ha) is added in the fall of each year since September 2003.

• Samples were collected seasonally (March, June, September, and December).

• Soil (0-10 cm), surface litter pool, litter production, root biomass (0-10 cm), plant tissue, aboveground biomass and plant cover, leaching (1 m).

• C and N content of tissue, litter, soil, roots, extractable N, pH.

Experimental design

10m

10m

• N-fertilizer (50 kgN/ha) is added in the fall of each year since September 2003.

• Samples were collected seasonally (March, June, September, and December).

• Soil (0-10 cm), surface litter pool, litter production, root biomass (0-10 cm), plant tissue, aboveground biomass and plant cover, leaching (1 m).

• C and N content of tissue, litter, soil, roots, extractable N, pH.

Experimental design

10m

10m

Chaparral site description

•Evergreen chaparral: Adenostoma fasciculatum, Ceanothus greggii. •Ambient atmospheric N deposition 2-3 kgN ha-1 y-1. •53 cm of precipitation (rain and snow) annually. •Burned in July 2003

May 2002

March 2004

Fenn et al. (2010) JEMA 91: 2404-2423

Sky Oaks Field Station

• Added N rapidly acidified surface (0-10 cm) soil

• Soil extractable soil N increased significantly after 1 year of addition.

• N addition significantly increased NO3

- leaching during the first 3 years of N addition (Vourlitis et al. 2009).

Chaparral extractable N and pH

Mean + se; n = 4 plots/treatment Vourlitis and Hentz (2016) JGR-B

2003 2005 2007 2009 2011 2013 2015

Precipitation (m

m)

0

200

400

600

800

1000

pH

5.0

5.5

6.0

6.5

7.0

2003 2005 2007 2009 2011 2013 20150

200

400

600

800

1000

Ext

ract

able

N (g

N/m

2 )

0

2

4

6

8

10

N1,6 = 96.9***T12,72 = 35.4***NxT12,72 = 20.8***

N1,6 = 22.2**T12,72 = 3.2***NxT12,72 = 3.0**

Control+N

Aboveground biomass production During the first 6 years, N plots had significantly lower aboveground biomass than control plots.

2003 2005 2007 2009 2011 2013 2015

Abo

vegr

ound

bio

mas

s (g

dw/m

2 )

0

200

400

600

800

1000Control+N

Mean + se; n = 4 plots/treatment Vourlitis and Hentz (2016) JGR-B

pH

2003-09

pH

-0.5

-0.4

-0.3

-0.2

-0.1

0.0Extractable N

2003-09

Ext

ract

able

N (g

N/m

2 )

0.00.20.40.60.81.01.21.41.6

ANPP

2003-09

AN

PP

(gC

m-2

y-1

)

-40

-30

-20

-10

Resulting in significantly lower ANPP for N plots

N-Control: Mean + 95% CI; n = 4 plots/treatment

N-Control: Mean + 95% CI; n = 4 plots/treatment N-Control: Mean + 95% CI; n = 4 plots/treatment

Dry season N addition significantly increased N availability and soil acidity

2003 2005 2007 2009 2011 2013 2015

Abo

vegr

ound

bio

mas

s (g

dw/m

2 )

0

200

400

600

800

1000

1200Control+N

N1,6 = 0.2T11,66 = 51.2***NxT11,66 = 5.3***

Aboveground biomass increment Mean + se; n = 4 plots/treatment Vourlitis and Hentz (2016) JGR-B During the last 6 years, aboveground

biomass of +N plots either equaled or exceeded control plots.

pH

2003-09 2009-15

pH

-1.0

-0.8

-0.6

-0.4

-0.2

0.0Extractable N

2003-09 2009-15

Ext

ract

able

N (g

N/m

2 )

0.0

0.5

1.0

1.5

2.0

2.5

3.0

ANPP

2003-09 2009-15

AN

PP

(gC

m-2

y-1

)

-60

-40

-20

0

20

40

60N-Control: Mean + 95% CI; n = 4 plots/treatment

Resulting in significantly higher ANPP for N plots

N-Control: Mean + 95% CI; n = 4 plots/treatment N-Control: Mean + 95% CI; n = 4 plots/treatment

Extractable N and soil acidification increased by > 2x in N plots

CSS Site description

Coastal sage scrub (CSS) •Santa Margarita Ecological Reserve •Artemisia californica, Salvia mellifera. •Ambient atmospheric N deposition 6-8 kgN ha-1 y-1. •36 cm of precipitation annually. •Has not experienced fire for about 35-40 years.

Wet season

Dry season Fenn et al. (2010) JEMA 91: 2404-2423

• Added N acidified surface (0-10 cm) soil after 4 years

• Soil extractable soil N increased significantly after 2 years of addition.

CSS extractable N and pH

Mean + se; n = 4 plots/treatment Vourlitis (2017) Oecologia

Year2003 2005 2007 2009 2011 2013 2015

0

100

200

300

400

500

600

700

800

900

Soi

l ext

ract

able

N (g

N/m

2 )

0

2

4

6

8

10

12N1,6 = 82.3***T13,78 = 34.9***NxT13,78 = 21.6***

2003 2005 2007 2009 2011 2013 2015

Precipitation (m

m)

0

100

200

300

400

500

600

700

800

900

Soi

l pH

5.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0N1,6 = 122.1***T13,78 = 19.6***NxT13,78 = 16.5***

Control+N

CSS cover and species composition

After 4 years of N addition: Artemisia californica increased significantly in N plots. Salvia mellifera decreased significantly in N plots. Brassica nigra cover was < 1 % in both control and N plots. Drier period (except 2005).

h)

2004 2006 2008 2010 2012 2014 2016-5

0

5

10

15

20

25

A. californica

0

20

40

60

80NControl

S. mellifera

Abso

lute

cov

er (%

)

0

10

20

30

40

50

B. nigra

Year

2004 2006 2008 2010 2012 2014 20160

5

10

15

20

25

30

Diff

eren

ce in

cov

er (%

)

-20

-10

0

10

20

30

f)

-30

-20

-10

0

10

20

*

*

Mean + se; n = 4 plots/treatment Vourlitis (2017) Oecologia

CSS cover and species composition

After 8 years of N addition: Artemisia californica cover begins to decline in N plots. Salvia mellifera cover begins to increase in N plots. Brassica nigra cover begins to increase in N plots. Wetter period.

h)

2004 2006 2008 2010 2012 2014 2016-5

0

5

10

15

20

25

A. californica

0

20

40

60

80NControl

S. mellifera

Abso

lute

cov

er (%

)

0

10

20

30

40

50

B. nigra

Year

2004 2006 2008 2010 2012 2014 20160

5

10

15

20

25

30

Diff

eren

ce in

cov

er (%

)

-20

-10

0

10

20

30

f)

-30

-20

-10

0

10

20

** *

*

*

Mean + se; n = 4 plots/treatment Vourlitis (2017) Oecologia

CSS cover and species composition

After 13 years of N addition: Artemisia californica cover is the same in N and control plots. Salvia mellifera cover is significantly lower in N plots. Brassica nigra cover is significantly higher in N plots. Drier period.

Mean + se; n = 4 plots/treatment Vourlitis (2017) Oecologia

h)

2004 2006 2008 2010 2012 2014 2016-5

0

5

10

15

20

25

A. californica

0

20

40

60

80NControl

S. mellifera

Abso

lute

cov

er (%

)

0

10

20

30

40

50

B. nigra

Year

2004 2006 2008 2010 2012 2014 20160

5

10

15

20

25

30

N1,6=1.1T51,306=5.8***NxT51,306=1.0

N1,6=2.1T51,306=3.9***NxT51,306=1.4*

N1,6=2.2T51,306=5.9***NxT51,306=1.9***

Diff

eren

ce in

cov

er (%

)

-20

-10

0

10

20

30

f)

-30

-20

-10

0

10

20

** *

* *

* * **

**

*

CSS cover and species composition

Total cover = 55% B. nigra cover = 20%

SMER: Plot 3 (Control) SMER: Plot 4 (+N)

Total cover = 66% B. nigra cover = 52%

Discussion/conclusions We hypothesized that N addition would significantly

(1) increase rates of C and N accumulation.

Chaparral: Yes, but only after 7 years of N addition

CSS: Yes, but only when rainfall exceeded ca. 400 mm/y (Vourlitis 2012)

(2) Increase the cover and dominance of exotic annuals

Discussion/conclusions We hypothesized that N addition would significantly

(1) increase rates of C and N accumulation.

Chaparral: Yes, but only after 7 years of N addition

CSS: Yes, but only when rainfall exceeded ca. 400 mm/y (Vourlitis 2012)

(2) Increase the cover and dominance of exotic annuals

Chaparral: No, no change in species composition or richness after 14 years of N fertilization.

CSS: Yes, but only after 10 years of fertilization and prolonged drought.

Discussion/conclusions We hypothesized that N addition would significantly

(1) increase rates of C and N accumulation.

Chaparral: Yes, but only after 7 years of N addition

CSS: Yes, but only when rainfall exceeded ca. 400 mm/y (Vourlitis 2012)

(2) Increase the cover and dominance of exotic annuals

Chaparral: No, no change in species composition or richness after 14 years of N fertilization.

CSS: Yes, but only after 10 years of fertilization and prolonged drought.

These data indicate that ecosystem responses to N deposition are complex, may change over time, and interact with other environmental variables.

Long-term studies are needed to understand how N deposition will alter ecosystem structure and function.

Thank you!

Field Stations Program College of Arts and Sciences at San Diego State University