Managing Soil Organic Carbon for Multiple Benefits: The Case of Africa - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 310

system estimated that there would be a net
loss of soil C between 2000 and 2030 in Kenya.

Grass and shrubs

Whole grass and shrub cover in 1 × 1 m
plots are cut and weighed in the field and
subsamples dried in the oven. The average
dry weights per 1 × 1 m space are then calcu-
lated and used to determine the aboveground
biomass per hectare.

Monitoring of Soil Carbon

Soil databases that only hold data on total
organic carbon content and limited infor-
mation on land use (history) can provide
only limited information on the dynamics
of carbon during land use or climate-
induced changes in different agroecosys-
tems (Batjes and Sombroek, 1997). None the
less, they remain critical in estimating the
size of the global soil C and pools. Direct
field sampling and laboratory measurements
of soil carbon in Kenya have been going on
for over half a century and the data exist in
the form of numerous technical and research
reports, theses, journal papers and work-
shop proceedings, annual reports and geo-
graphic information system (GIS) databases.
A combination of biomass measurements
and empirical equations has also been em-
ployed in Kenya (Woomer, 2003; Kamoni and
Macharia, 2011). For trees, this normally
involves measurements of the diameter at
breast height (1.3 m from the ground) of a
number of trees within each farm, and the
total number of trees in the farm estimated
visually. The aboveground biomass (AGB) is
then estimated using allometric equations
(Woomer, 2003), originally from FAO (1997):

Root biomass

The total root biomass for trees, fruit trees,
bananas, coffee and tea are calculated using
the estimates below (Woomer, 2003).
Total root biomass = root biomass (0.35
AGB) + leaf drop (0.15 AGB) + fine root
turnover (0.15 AGB) but for maize, beans,
cowpeas, watermelon, grass and shrubs,
only the 0.35 AGB factor was used.

Maize and legumes

The relationship between yield, total bio-
mass and harvest index (HI), that is total
biomass = yield × HI, was used to esti-
mate the aboveground biomass for maize,
beans and cowpeas (Kamoni and Machar-
ia, 2011). Information on crop yields and
area planted per crop was provided by the
farmers. Area measurements for small,
planted plots were taken during inter-
views.

in dry zones (<1500 mm year -1 )

Y = exp (-1.996 + 2.32 ln D )
and in moist zones (1500-4000 mm year -1 )

Converting Biomass
to Carbon Stocks

Y = exp (- 2 .134 + 2 .53 ln D )

where Y is the aboveground tree biomass in
kg tree − 1 , exp = 2.71828 … or 22/ 7 and D is
the measured tree diameter at breast height
(DBH) in cm (calculated from the circumfer-
ence−diameter relationship, i.e. C = π D ).
The equations below (Hairiah et al .,
2001) were used to calculate the aboveground
biomass for bananas and coffee:

Total biomass (kg ha -1 ) is multiplied with a
factor of 0.47 to convert to C (kg ha -1 ) (Woom-
er, 2003).
Kamau et al . (2008) used destructive
methods (uprooting) to measure tea bio-
mass in Kericho, Kenya. Batjes (2004,
2011) used information contained in the
International Soil Reference and Informa-
tion Centre (ISRIC) soil profile pits data-
base for Kenya (bulk density, per cent of

Y = 0.303 D 2 .1345

Y = 0.281 D 2 .0635