Nutrient uptake and growth of young trees in a P-deficient soil: Tree species and phosphorus effects

Phosphorus deficiency is widespread in the subhumid highlands of eastern Af rica but there are few data on the effect of P deficiency on the growth of agroforestry tree species, We studied the effect of P application on growth , nutrient uptake and dry matter partitioning in young trees of Calliandra calothyrsus, Cedrela serrulata, Eucalyptus grandis, Grevillea robusta, Mark hamia lutea, Senna spectabilis, and Sesbania sesban on a P-deficient soil ( Kandiudalfic Eutrudox, bicarbonate-EDTA extractable P = 1 mg kg(-1)) in wes tern Kenya. The trees were grown at two P levels (control and 500 kg added P ha(-1)) at 1 m(2) spacing in a randomized complete block design with thre e replications. Leaf K concentrations were in the low range for all species (5-9 mg g(-1)) and K deficiency may have limited responses to P. Averaged over species, P addition increased aboveground shoot dry matter by a factor of 2.6 at 62 and 124 days, but the response decreased to 1.3 at 325 days. The increases at 62 days were large in sesbania (5.4) and eucalyptus (3.2) but small in calliandra (1.4) and markhamia (1.1). Relative response to P w as more strongly correlated with shoot growth rate per unit root length amo ng species than with shoot growth rate alone. Calliandra, which had high ea rly growth rate but low response to added P, had an exceptionally high root length (6.0 km m(-2)) compared with the other species (0.3-2.1 km m(-2). P addition increased N and P content but decreased final shoot K content in sesbania and calliandra, and had little effect on K content in the other sp ecies. The high-yielding species (eucalyptus, sesbania and calliandra) accu mulated more than 30 g N and 2 g P m-2 in shoots in 325 days of growth. The proportion of total shoot N in wood (branch + stem) was in a higher range (67-75%) in the shrubby species (sesbania, calliandra, senna) than in the u pperstorey tree species (38-43%). Slow early shoot growth relative to total root length, and high specific root length (root length per unit root mass ) are proposed as criteria for the selection of species and provenances tha t are well adapted to P deficient soils. (C) 1999 Elsevier Science B.V. All rights reserved.