Termites are major ecosystem engineers in African savanna creating soil heterogeneity and nutrient hotspots in the form of mounds.

1
INFLUENCE OF TERMITES ON STABILITY AND REACTIVITY
OF PHOSPHORUS IN EAST AFRICAN SAVANNA
Lucy Ngatia1, A. E. Normand1, B.L. Turner2, P.W.
Inglett1 and K. R. Reddy1 1 Wetland
Biogeochemistry Laboratory, Soil and Water
Science Department, University of Florida 2
Smithsonian Tropical Research Institute

• INTRODUCTION
• Termites are major ecosystem engineers in African
  savanna creating soil heterogeneity and nutrient
  hotspots in the form of mounds.
• Plant productivity in African savanna is limited
  by cycling of phosphorus (P) and nitrogen (N).
• However, little is known about the influence of
  termite mounds on P reactivity and stability.
• Objective To determine the influence of termite
  mounds on soil P distribution and reactivity.
• Hypothesis The termites increase the phosphorus
  availability on the termite mounds.

RESULTS
(b)
Termite mound
A (Center)
B (Edge)
Termite

• MATERIALS AND METHODS
• Study site
• Mpala Research Centre (MRC) and associated Mpala
  Ranch which encompasses 190 km2 of semi arid
  savanna within the Laikipia District of Rift
  Valley Province, Kenya.
• Laikipia District is a semi arid Acacia savanna
  that boasts the largest population of wildlife in
  Kenyas non protected areas.
• Livestock and wildlife co-exist and depend on
  natural vegetation including Acacia drepanolobium
  (overstory) and grass (understory)for feed.
• Methods
• 42 soil composite samples from the centre, edge
  and 5m away from the termite mound were
  collected up to 10 cm depth (Fig 2), air dried
  for 12 days and analyzed for select chemical
  parameters .
• Soil phosphorus forms were determined on alkali
  extracts using 31P nuclear magnetic resonance
  (NMR) spectroscopy.
• Isotope signature d13C and d15N were determined.
  
• Total P was determined using ashing method.
• Available P determined using Mehlich-1 method.
• Statistical analysis was done using JMP 8.0
  software program and results were considered
  significantly different at plt0.05 level.
• Study site

5m
Fig 5. Relationship between (a) d15N vs TP and
(b) d15N vs NP ratio within and around the
termite mound
C (5 m away)
Table 2. Soil nutrient concentration as
influenced by termite mound
C N N P LOI CN CP
g kg-1 g kg-1 g kg-1 g kg-1 Ratio Ratio
Centre 20.01.8a 1.90.2a 0.370.06a 0.370.06a 14.80.4ab 10.40.2b 648b
Edge 24.61.3a 2.20.1a 0.290.03ab 0.290.03ab 14.00.4bc 11.30.2a 11120ab
5 m away 20.61.0a 1.80.1a 0.210.02bc 0.210.02bc 12.90.4c 11.20.1a 12219a
Off mound 21.21.2a 2.00.1a 0.140.01c 0.140.01c 15.60.3a 10.60.1b 1514a
P value 0.1 0.2 0.0006 0.0006 lt.0001 0.0002 0.0014
Fig 2. Termite mound, termite on a hand help
chunk of soil and sampling points A, B, C.
Fig 3. Source of soil organic matter as indicated
by isotope signatures
Available P K Ca Mg Fe Al
mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1
Centre 111a 67944ab 6690226a 69841b 14.61.5c 57017a
Edge 81b 81266ab 4504402b 72153b 22.12.9bc 46835c
5 m away 71b 64914b 4124236b 77715ab 38.93.5a 54019ab
Off mound 61b 84750a 4139200b 88325a 30.32.7ab 47821bc
P value 0.0001 0.0109 lt.0001 0.0067 lt.0001 0.0105

• The ?13C (Fig. 3 -15.2 to -17.3) indicated that
  grass was the main source of soil organic matter.
  
• Total P and available P were three and two times
  higher, respectively, at the center of the
  termite mound (0.370.06 g kg-1 and 111 mg kg-1)
  compared to off termite mound soils (0.140.01 g
  kg-1 and 61 mg kg-1) (Table 2) indicating that
  termite activities promote P accumulation.
• NMR results indicated that at all sites near the
  termite mound soil P was dominated by
  orthophosphate, phosphomonoesters, phosphodiester
  (DNA) and pyrophosphate while in contrast, off
  mound soil P was dominated by orthophosphate and
  phosphodiesters. This indicate that termite
  promote both P availability and stability (Fig.
  4Table 1).
• ?15N was negatively correlated with soil P and
  positively with NP ratio potentially suggesting
  N2 fixation by termites (Fig 5a,b).
• Total P, Ca, and Al were significantly higher at
  the center of the termite mound while K and Mg
  were higher off the termite mound (Table 2),
  suggesting that termites could be pumping P, Ca
  and Al from the deeper soil horizons to upper
  horizon.
• The presence of termite mounds in the savanna is
  important because they promote short term
  availability of P to plants through
  orthophosphate as well as long term P
  availability through pyrophosphate.
• P heterogeneity created by termite mound
  potentially influences plant productivity and
  feed quality to livestock and wild animals.
• Nitrogen fixation by the termites in the savanna
  is likely to reduce N limitation to plant
  productivity.

DISCUSSION AND CONCLUSIONS
Fig 4. 31P spectra with functional groups
referenced using 85 phosphoric acid
Table 1. Recovered P fractions in alkali extracts
using 31P nuclear magnetic resonance (NMR)
spectroscopy.
  TP NaOH -P Ortho-P Phosphomonoester DNA Pyrophosphate
mg kg-1 mg kg-1
Acacia 1812 1606.8 77 20 3 -
Grass 1011 1010 75 20 3.5 1.5
Center 370 121.68 61 31 5 3
Edge 290 65.38 35 59 3 3
5 m away 210 53.88 39 55 2 4
Black cotton soil 140 30.58 55 35 10 -

Fig 1. Study site and typical/Model P functional
groups in 31P NMR spectrum (Cheesman et al.)
STRI and NCCR is hereby acknowledged for
providing the necessary funding for carrying out
this research.
Corresponding author Lucy Ngatia, Soil and Water
Sciences Dept., 2181 McCarty Hall A. PO Box
110290, Gainesville, FL 32611-0290.
lucyngatia_at_ufl.edu