Decomposition of Cynodon dactylon (L.) Pers. in the seasonally flooded areas of the Pongolo river floodplain: pattern and significance of dry matter and nutrient loss

Cynodon dactylon meadows occupying seasonally inundated areas of the Pongolo river floodplain, South Africa, decompose during periods of submergence. The loss of dry matter and nutrients from both fresh and dried material enclosed in litter bags was studied. The pattern of loss of dry matter was diphasic. Loss was more rapid from dried than from fresh material, half the mass being lost in 18 and 28 days respectively. Nutrient loss followed an exponential pattern. Nitrogen, phosphorus and magnesium were lost more rapidly from dried than from fresh material whilst rates of loss of sodium, potassium and calcium were similar. The significance of Cynodon decomposition in the functioning of the floodplain is assessed from estimates of the above- and below-ground standing crop around one small lake before and after submergence. It is concluded that decomposition of plants growing in seasonally inundated areas of the floodplain may contribute significantly to the productivity of the system, but that the extent of the contribution can be greatly influenced by the flooding regime. The completion of the Pongolapoort dam upstream of the floodplain will alter the natural flooding pattern, effecting a reduction in floodplain productivity, unless a programme for flood release can be initiated.