Potential nutrient inputs and soil property changes in woodlots of Leucaena leucocephala and Senna siamea fifteen years after establishment

Quantitative estimates of litter and potential nutrient inputs and release and changes in selected soil chemical properties of Leucaena leucocephala and Senna siamea woodlots fifteen years after establishment were studied in three experiments as follows; i. Litter and potential nutrients inputs in Leucaena leucocephala and Senna siamea woodlots. ii ‘In situ’ decomposition and mineralization of Leucaena leucocephala and Senna siamea foliage. iii Soil chemical properties in woodlots of Leucaena leucocephala and Senna siamea fifteen years after establishment. Experiment I compared litter and potential nutrient inputs in Leucaena leucocephala and Senna siamea woodlots. Litter inputs were estimated by installing 12 trap nets in each woodlot randomly. Total litter collected from June to December, 2003 in Leucaena leucocephala woodlot was 400 kg/ha with average monthly collection of 67 kg/ha. In the Senna siamea woodlot, a total of 455 kg/ha of litter was collected with an average monthly collection of 76 kg/ha. Total litter inputs did not differ significantly between the woodlots hence, Leucaena leucocephala or Senna siamea species could be used in Agroforestry practice where litter input is a management objective. Potential nutrient inputs in leaves, twigs and branches for the six month period were computed by multiplying the values of component oven dry weights by the respective nutrient element concentration. Senna siamea could potentially contribute in its litter components (leaves, twigs and branches) N, 1024 kg/ha; P, 290 kg/ha; K, 824 kg/ha; Ca, 719 kg/ha and Mg, 441 kg/ha whereas Leucaena leucocephala could contribute N, 843 kg/ha; P, 164 kg/ha; K, 657 kg! ha, Ca, 347 kg/ha and Mg, 233 kg/ha. Potential N and K inputs did not differ significantly between the woodlots (P > 0.05). However, potential P, Ca and Mg inputs were significantly (P < 0.05) higher in Senna siamea litter compared to Leucaena leucocephala litter. Senna siamea could therefore, be a preferred species to improve soil fertility if nutrient inputs are part of management objective. In experiment II, rates of decomposition and mineralization of N, P, K, Ca and Mg in Leucaena leucocephala and Senna siamea foliage were compared using nylon bags from August to December, 2003. Decomposition rate of Leucaena leucocephala foliage was significantly (P > 0.05) faster than that of Senna siamea foliage. The decomposition rate constants (Kj) of Leucaena leucocephala foliage varied between 0.013 and 0.069. That of Senna siamea foliage varied between 0.00 1 and 0.076. For long term effect on soil, Senna siamea had a better potential as soil cover due to the slower decomposition of its foliage. Between species, mineralization of Mg was significantly faster in decomposing foliage of Leucaena leucocephala whereas mineralization of N, P, K and Ca were similar. Within species N, P, K and Ca release were faster in decomposing foliage of Senna siamea whereas Mg release was faster in decomposing foliage of Leucaena leucocephala. Nutrient release constant (Ku) of decomposing foliage of Leucaena leucocephala varied between 0.004 and 0.011 and in decomposing foliage of Senna siamea K varied between 0.003 and 0.007. Based on the Jçvalues the pattern of release in Leucaena leucocephala foliage was in the order N> Ca> Mg> K= P. In Senna siamea foliage the pattern of release was in the order K> Mg > N P Ca. Experiment III compared selected soil chemical properties between and within the woodlots and changes in selected soil chemical properties between the woodlots fifteen years after establishment. Using soil auger, soil samples were collected from two depths; 0-15 cm and 15-30 cm within three quadrats of size 15 cm x 25 cm randomly laid in the woodlots. These were analyzed for soil pH, % OM, % OC, total N, available P, exchangeable K, Ca and Mg, exchangeable acidity (Al + H), effective CEC and % base saturation. Within 0-15 cm depth, available P and exchangeable K were significantly higher within Senna siamea woodlot compared to Leucaena leucocephala woodlot. All other soil properties did not differ significantly (P > 0.05) in both woodlots. Within 15 - 30 cm depth, only available P was significantly higher in the Senna siamea woodlot compared to Leucaena leucocephala woodlot. Within 0-15 cm depth, there were appreciable increases in soil pH, % OC, exchangeable Ca and effective CEC in both Woodlots fifteen years after establishment. However, levels of total N, available P and exchangeable K decreased in both woodlots. Within 15-30 cm, soil pH decreased within Senna siamea woodlot whereas % OC, total N and available P decreased within Leucaena leucocephala woodlot. Harvests that remove greater portions of Senna siamea and Leucaena leucocephala woody biomass could potentially deplete N, P, and K fertility.

Source: KNUST