Carbon budget and sustainable harvesting of cyperus papyrus L. (Papyrus) in a papyrus swamp in lake Naivasha, Kenya
Kariuki, Francis Wachira
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Carbon budget and sustainable harvesting of Cyperus papyrus (papyrus) in a papyrus swamp in Lake Naivasha were investigated. Carbon dioxide fluxes and photosynthesis were measured using eddy covariance and IRGA techniques respectively. Sustainable harvesting was assessed using five treatments, control, harvesting at intervals of 6, 12, 24 and 36 months each replicated five times in a Latin Square Design Carbon sequestration in the swamp was investigated through decomposition using litterbag and analysis of swamp components for carbon and nitrogen. Light compensation point was reached at PAR value of 1200 m-2s-1 between 08.00 and 09-00 hours. Mean maximum midday CO2 uptake was -24.40 ± 1.36 mol m-2s-1 corresponding peak PAR (1882 mol m-2s-1) with -31.19 and -18.92 mol m-2s-1 as the highest and the lowest uptake values respectively for days with monomodal peaks. On those days with bimodal peaks, the initial peak was -24.87 ± 9.29 mol m-2s-1 between 11.00 and 12.00 hours and a second peak at about 14.00 hours of -29.29 ± 1.27 mmol m-2s-1. Mean CO2 efflux was 10.28 ± 1.38 mol m-2s-1. Mean water vapour flux were 0.00 and 8.25 mmol ± 1.27 mmol m2s-1 for night and day respectively. The day peak corresponded to peak PAR, temperature and sensible heat. Papyrus had a maximum assimilation rate of 68.7 umol m-2s-1 and Carboxylation Efficiency (CE) of 0.06 compared to assimilation rate of 36.7 umol m-2s-1 and CE of 0.02 in Cyperus immensus. However, C. papyrus was CO2 saturated at a lower internal carbon dioxide concentration that C. immensus (C4), an indication of intermediate characteristics. Papyrus above ground biomass and culm density at 6 months harvesting interval showed significant (P<0.05) difference in terms of harvesting intervals. The overall culm density for six months intervals was 7.78±1.33 initial, 11.66 ± 2.55 and 11.62 ± 5.94 during the second and third harvest intervals respectively. This insignificantly declined (P>0.05) TO 10.66 ± 2.89 when the fourth harvest was delayed by three months and declined significantly (P<0.05) further to 4.98 ± 1.65 on the fifth harvest. The 12 months intervals showed the overall density increasing insignificantly (P>0.05) from initial 6.04 ± 2.35 to 9.36 ±2.46 at the second harvest. This declined significantly (P<0.05) to 7.42± 2.04 at the time of the third harvest. The above ground biomass showed an initial significant (P<0.05) decreased to 1087.64 ± 695.10 from 3290.52 ± 637.04 and thereafter significantly (P<0.05) increased to 1773.18 ± 496.27 gm-2 upon the third harvest. This was an indication that papyrus biomass was not density dependent. With the 24 months interval, the overall density showed a fairly stable state with density of 10.07 ± 1.99 and 11.18 ±1.73 culms m-2 initial and second harvests respectively. The results show that the rate of recruitment was approximately equal to that of mortality indicating a stable community. The above ground biomass significantly (P<0.05) increased from initial 2697.61±597.60 ±818.89 gm-2 a 25% increase. Decomposition was found to vary with the position (habitat) of litter at the site. The percentage material remaining after 300 days of decomposition was 74.05, 73.76 and 86.54 under the detritus, in water and on the detritus respectively. The decomposition rates after 90 days were 0.36, 0.81 and 1.02 and, the half-life were 3.94, 1.87 and 1.94 on the detritus, in water and under the detritus respectively. Analysis of papyrus plant organs and swamp components showed that percentage carbon was highest 47.92 ± 0.25 in the umbels and lowest 43.86 ± 0.63 in roots. Percentage carbon was 44.29± 0.18 in culms, and 47.08 ± 0.15 in rhizomes. Detritus and peat had percentage carbon of 37.95 ± 2.30 and 18.23 ± 0.23 respectively. Percentage nitrogen content was least 0.329 ± 0.02 and highest 1.297 ± 0.04 in the roots. Culms had 0.934 ± 0.04 while rhizomes had 1.219 ± 0.06% nitrogen content. The whole plant had 45.85± 1.92 and 1.14 ± 0.15% carbon and nitrogen contents respectively and C: N ration of 40:1. In detritus and peat, percentage nitrogen contents were 1.84 ± 0.14 and 0.977 ± 0.08 respectively. In terms of C:N ratio, the highest among the plant organs was 134:1 in senescent culms and least 34:1 in roots and 39:1 and 43:1 in rhizomes and umblels respectively and, 47:1 in culms. It was 19:1 and 21:1 in peat and detritus. This shows that the C:N ratio for papyrus organs was significantly (P<0.05) higher than the 17:1 dietary requirement of proteins for most animals. Carbon exchange indicates that papyrus swamps are presently sources of carbon. This status can be reversed through sustainable harvesting, which in the short term will be a source of carbon but will reverse once the amount of detritus is substantially reduced. Sustainable harvesting was found to be possible at 24 months intervals.