Limitations on Grass Productivity in a Southern African Savanna Lydia P. Ries1*, Herman H. Shugart1 , Barney S. Kgope2

1Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903 USA2South African National Biodiversity Institute, Cape Town, South Africa

*Corresponding Author: lydiaries@virginia.edu, +1-434-924-4303

Introduction

•Pleistocene savanna soils such as those in the Kalahari Basin of southern Africa are subject to nutrient losses through long-term leaching.

•Unlike Nitrogen (N), Phosphorus (P) has limited inputs relative to losses in African savanna ecosystems and would seemingly be limiting to plants.

•Light may also have interactive effects with nutrients in savannas due to its heterogeneous availability.

•Forcings such as human-induced fires and increased grazing pressures alter light and nutrient cycles in savannas (Skarpe 1990).

•Understanding the role of light and nutrients in savanna productivity is imperative to understanding the future of savannas as bush encroachment, desertification become more widespread in this ecosystem type.

Materials & Methods

•Research conducted in an open woodland savanna (Kirkia acumunata, Baikaea plurijuga, Diplorhynchus condylocarpon) in northeastern Botswana (18.66 S, 25.50 E), 698 mm rain/year.

•Additional research conducted along IGBP-designated Kalahari Transect (data not included here) pictured above.

•Added aqueous N (20 g/m2) and P (10g/m2) to 1-m2 plots of randomly selected grasses, Schmidtia pappophoroides, during 2005 dry season.

• Measured net carbon assimilation under a range of incident photosynthetic active radiation (PARi) levels using a LI-6400 (Li-Cor, Lincoln, NE, USA).

•Harvested total AG biomass during 2006 wet season. Dried leaf tissue was analyzed for Kjeldahl total P and total percent C and N.

Abstract

Despite the ubiquity of sub-tropical savannas throughout the Earth, limitations of savanna productivity are understudied relative to other terrestrial systems. In particular, there has been little attention on the role of phosphorus (P) in savanna productivity and structure. This study examined the role of increased nitrogen and phosphorus in grass productivity in a woodland savanna in Botswana. We added aqueous forms of nitrogen (N) and P to selected individual grasses. During the following growing season we measured foliar nutrient concentrations and photosynthetic response at various light levels to estimate the productivity response. We observed an increase in foliar P concentrations but for not % C and % N in leaves. We also observed a significant increase in net carbon assimilation and Amax for all amended grasses. These results suggest that the aboveground productivity of these woodland savanna grasses is limited by both N and P. Additionally, under constant C02 availability, photosynthesis is limited by nutrients for light levels greater than 1000 μmol m-2 s-1 This research will help broaden our understanding of the biogeochemical processes that govern savanna productivity. Ultimately, these data can be used to model ecological succession of savannas under scenarios in which bush encroachment and desertification may alter availability of light and nutrients.

Discussion

•N, P and N+P addition increased AG biomass production for grasses suggesting a nutrient limitation on productivity. The largest increase was seen when both N and P were added, possibly due to overcoming nutrient uptake by the soil microbial community.

•The coinciding increase in foliar P (as in Ludwig et al. 2001) and photosynthesis under increased levels of soil P suggests that P was possibly one of the rate limiting factors, Wp (Triose phosphate), during photosynthesis as suggested by Farquar (1980).

•This trend was not as evident under increased levels of soil N. Higher levels of photosynthesis at were seen for higher levels of light but an increase in foliar N was not detected.

•The control grasses reached their light saturation point at 200 (m -2 s-1). After this point, an increase in nitrogen had the largest effect on photosynthesis, suggesting N is most limiting for light levels greater than 200 (m-2 s-1). P was also limiting at these levels as seen in higher Amax(2000) but not as significant those seen under increased N.

Future Research

Further results will include data from the IGBP Kalahari Transect (depicted above) to be used in examining the interactive effects of nutrients, light and soil moisture along a precipitation gradient. Response variable for data collected 2005-2006 include:

•AG biomass productivity (g/m2)

•Foliar nutrient concentrations (g/mg)

•Net carbon assimilation, stomatal conductance (μm-2 s-1 )

•Water use efficiencyReferences

Farquhar, G. D., von Caemmerer, S., Berry, J.A. (1980). "A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species." Planta 149: 977-988.

Ludwig, F., de Kroon, H., Berendse, F., Prins, H.H.T., H. de Kroon, et al. (2001). "Effects of nutrients and shade on tree-grass interactions in an East African savanna." Journal of Vegetation Science 12: 579-588.

Skarpe, C. (1990). "Structure of the woody vegetation in disturbed and undisturbed arid savanna, Botswana." Vegetatio 87: 11-18.

Acknowledgements

University of Virginia Department of Environmental Sciences, NASA Award NNG-04-GM71G and Oberlin College Alumni Fund, University of Botswana, HOORC, Maun, Botswana, Pandamatenga Agricultural Research Station, Rangeland Ecology Unit, Botswana.

1. Aboveground (AG) Biomass

Total AG biomass varied significantly between control and nutrient treatments (Figure 1). The combined N+P treatment (p=0.015) produced more than three times than the control treatment.

2. Nutrient Concentrations

P foliar levels (p=0.007) were significantly higher than the control in the P treatment but not in the N (p=0.061) or N+P (p=0.081) treatments (Figure 2). There was no significant effect of nutrients on foliar C:N (F= 0.56, p=0.65) (means not presented here).

3. Photosynthetic Response

Light response curves (Figure 3) show increases in maximum grass net carbon assimilation for all treatments (N, P and N+P) relative to control grasses. These curves suggest a light limitation on photosynthesis at < 500 μm-2 s-1 and nutrient limitations, particularly N, for light levels > 500 μm2 s-1 . Ultimately, data will be to asses limitation of P in carbon assimilation model (Figure 4)

Results

Light Response Curves for 4 Nutrient Treatments

-10

0

10

20

30

40

50

60

0 500 1000 1500 2000 2500

PARi (μmol m-2 s-1)

Net

Car

bo

n A

ssim

ilat

ion

(μ

mo

l m

-2 s

-1)

N

C

P

N+P

Figure 3. Light response curves under fixed CO2 for all nutrient treatments

Aboveground Dry Biomass Means for All Treatments

0

50

100

150

200

250

C N N+P PTreatment

Dry

Bio

mas

s (g

/m2 )

Figure 1. Total aboveground dry grass biomass means for all nutrients treatments (Carbon, Nitrogen, Nitrogen + Phosphorus and Phosphorus). Letters indicate significant differences between treatments.

a

ab

b

a

Foliar [P] for All Treatments

0

1

2

3

4

C N N+P P

Treatment

Pho

spho

rus

(mg/

g)

Figure 2. Foliar P concentrations were higher in P only treatment.

bcc

aba

IGBP Kalahari Transect Field Sites

c

ocn P

PVA

5.0

1

1. Rubisco limiting rate (Wc):

o

occ

ccc

K

PKP

PVW

1

max

2. RuBP limiting rate (Wj):

o

c

cj P

P

JPW

4

3. Triose phosphate limiting rate (Wp):

c

op P

PWUW

*5.0

3 min

Net carbon assimilation

Figure 4. Ries and Kgope collecting physiology data to be used in modeling limitations on net carbon assimilation (Farquhar, 1980).

Open Acacia savanna, central Botswana