| dc.description.abstract | A study was conducted to estimate and characterize the structure and function of
a rangeland ecosystem on the Njemps flats of Baringo District, Kenya. Herbaceous primary
production and productivity were estimated, and energy flow and nitrogen cycling
characterized over a two-year period.
Total aboveground standing crop ranged from 84.6 g m-2 to 295.4 g m-2, with a
mean of 162.3±60.6 g m-2. Mean monthly aboveground standing crop for 1992 and 1993
was 142.8 ±53.8 and 178.5±63.3 g m-2 respectively. The two values were significantly
different (P<0.10). Aboveground biomass yield ranged from 17.7 g m-2 to 242.7 g m-2,
with a mean of 104.3±58 g m-2 and a coefficient of variation of 58%. Mean aboveground
standing crop was 59±24 g m-2. Monthly values ranged from 28.8 g m-2 to 120 g m-2, with
a 38% coefficient of variation.
The range for total belowground standing crop was from 83.3 g m-2 to 232.7 g m-2,
and a mean of 155.2±46 g m-2. The values had a coefficient of variation of30%. Mean
total monthly belowground plant material yield for 1992 and 1993 was 137.6±41 g m-2 and
169.9 ±46 g m-2. The coefficients of variation were 59% and 28% respectively. The mean
monthly belowground biom..ass yield was 51.6±33 g m-2 with a coefficient of variation of
64%. Mean monthly yield 'for belowground dead material was 103.7±32 g m-2, with a
coefficient of variation of 31%. There was no significant difference (P>O.OI) in the mean
belowground dead material yield between 1992 and 1993.
Aboveground:Belowground plant material ratios ranged from 0.55 to 2.31. Ratios
greater than 1.00, thus indicating higher quantity of plant material aboveground than
belowgroundr were observed in March and October of 1992 and July and October of 1993.
Monthly litter production spread from '31.4 g m-2 to 130 g m-2. Mean monthly
yield was 92.5±26 g m-2, with a 28% coefficient of variation. There was no significant
difference (P>O.OI) in lit!er yield between 1992 and 1993.
Rate of decomposition for aboveground material ranged from 0.005 g g-1 day-l to
0.084 g g-1 dayI. The m,e.an annual rate of decomposition was 0.026 g g-1 dayJ.
Belowground plant material rates of decomposition spread from 0.009 g g-1 day-l to 0.062
g g-1 dayl , with a mean annual rate of 0.041 g g-1 day-I. Belowground material
consistently decomposed faster than aboveground material. Peaks in both aboveground and
belowground material decomposition rates coincided with rainfall peaks.
In 1992, annual NPP was 439.2 g m-2, giving a net primary productivity of 1.22 g
m-2day-l. Monthly NPP ranged from 17.2 g m-2 to 90.1 g m-2. In 1993, annual NPP was
944.5 g m-2, equivalent to a net primary productivity of 2.62 g m-2 day-I. Monthly NPP
was between 27.4 g m-2 and 548.6 g m-2. Over the 1992-1993 period, NPP was 1383.7 g
m-2, equivalent to a productivity of 1.92 g m-2day-l. Trends in monthly NPP closely
followed the trend in rainfall.
Turnover rates and times varied between years and compartments. Highest
turnover rate was for aboveground biomass (0.40), which was closely followed by the grass
litter and belowground biomass compartments with 0.32 and 0.30 respectively. Lowest
turnover rate (0.13) was recorded from the aboveground dead compartment. The turnover
times for the aboveground biomass, grass litter, and belowground biomass compartments
were 2.5 years, 3.1 years and 3.3 years respectively. The dead aboveground biomass had
a turnover time of 4.5 -7.7 years. High turnover rates were associated with higher rainfall.
Energy contents in the aboveground and belowground plant materials during both
seasons were 17.9 KJ g-1 and 21.6 KJ g-1 respectively. Aboveground live and dead plant
material compartments had one and a half times more standing crop of energy during the
dry season compared to the wet season. During the dry season there was net loss of energy
from the aboveground dead compartment. The litter compartment had a net loss of energy
during both seasons.
Energy content in the belowground live compartment was 1135.0 and 1118.9 KJ m-
2 for the dry and wet- season respectively. There was net loss of energy from this
compartment during both seasons. During the dry season, average energy accumulation in
aboveground and belowground compartment occurred at a rate of 25.4 KJ m-2 dayl and
16.5 KJ m-2 day-l respectively. In the wet "season, average energy accumulation in
aboveground and belowground compartments was 27.8 KJ m-2 day! and 22.0 KJ m-2 day- 1 respectively.
The total standing stock of nitrogen in plant compartments during the dry and wet
season was 1'700 mg m-2 and 9650 mg m-2 respectively. The two values were significantly
different (P<0.05). Among the aboveground compartments, in the dry season, live dicots
had the highest nitrogen content (2.4%). The dead dicots and litter compartments had 2%
nitrogen. Live plant material had highest nitrogen content (1.8%) among the below ground
compartments. The lowest nitrogen content was recorded in the aboveground dead grass
compartment (0.7%). During the wet season, litter compartment had highest nitrogen
content (2.9%) whereas the lowest value was for the aboveground dead grass compartment
(1.3%). The live belowground material had 3% nitrogen, which was the highest among the
belowground compartments.
Incubation of dry season soil revealed that over a period of twelve weeks, and with
7% soil moisture content, the mineralization potential was about 26.0 and 23.0 ug g-l soil
for NH4+-N and N03--N respectively. The pH of the incubated soil remained within
narrow limits, ranging from 6.4 and 6.8. The mineralization potential for the wet season soil,
having 27% moisture content, was estimated to be 35.0 and 28.0 ug g-l soil forNH4+-N
and N03--N respectively. Soil pH during wet season soil incubation ranged between 6.9 and
8.8.
Total soil nitrogen pool during the dry season was estimated to be 2847.6 g m-2 in
the top 30 em soil horizon. Mineralised nitrogen constituted 0.05%. Standing crop of
nitrogen in the aboveground plant material compartments was 2320 mg m-2, 920 mg m-2
and 1950 mg m-2 for the live material, dead material and litter compartments respectively.
An accumulation of nitrogen was evident in the aboveground live compartment.
Standing stock of n..itrogen in the belowground plant material compartments was
930 mg g-l and 1580 mg g-l for the live and dead compartments respectively. There was
relative accumulation of nitrogen in the mineralized nitrogen pool, whereas the
belowground live compartment had a net loss of nitrogen.
Total standing stock of nitrogen in the soil pool during the wet season was 5152.8
g m-2 in the top 30 ern soil horizon, which was higher than that recorded during the dry
season (P<0.05). Mineralized nitrogen constituted Q;07%. Standing stocks of nitrogen in
the belowground live and dead compartments were 1550 and 2130 mg m-2 respectively.
Both values were higher than those recorded during the dry season. There was a net loss
of nitrogen from the belowground dead compartment, Standing stocks of nitrogen in
aboveground compartments were 2420 mg m-2, 2300 mg m-2 and 1050 mg m-2 for the
aboveground live, dead, and litter. compartments respectively. There was a significant
difference (P<0.05) between standing stock of nitrogen in the aboveground live and dead
compartments, and between aboveground dead and litter compartments.
On the whole, structure and function of the Njemps flats rangeland ecosystem was
episodic in nature and closely correlated with the rainfall trend. This was shown by the high
variability of the data on aboveground and belowground biomass compartments, NPP,
decomposition and turnover rates. The ecosystem was in a non-equilibrium state, having
STF values between most compartment being either greater or less than 1.
The high primary productivity reported from this arid rangeland ecosystems puts
these areas under focus as C02 sinks whose role in the amelioration of the imminent
warming of global climate could be more important than is currently thought. In terms of
management and utilization by man, the ecosystem characteristics imply a high level of
flexibility, the sole objective being to seize opportunities while spreading and reducing
risks. | en |
