Effects of soil organic matters status on inorganic nitrogen fertilizer use efficiency in Embu, Kabete and Maseno, Kenya
The management of nitrogen (N) nutrition is a vital aspect in maize production systems of Kenya. In Central, Western and parts od Eastern Kenya, high population density has eliminated the use of traditional fallow farming systems for replenishing soil fertility, while the high unevenly distributed rainfall has increased changes of N leaching. A study was conducted at Maseno, Embu and Kabete KARI field stations to investigate the fate of fertilizer N in the soil-plant system using treatments that had been receiving leaf prunings of Calliandra calothyrsus, Tithonia diversifolia and Leucaena leucocephala. The experimental design was a randomised complete block design with three replicates in all trials. In each treatment's main plot, two microplots were installed and in each one of them nitrogen-15(15N) fertilizer was applied at the rate of 80kg N ha-1 (split application). At the beginning and at the end of 2002 long rains, soil was sampled at different soil depths; 0-10, 10-30, 60-100, 100-150 and 150-200 cm for N and 15N enrichment analysis. Top soils (0-10cm) were analysed for pH, Ca, Mg, K, P and particle size. An incubation study was done on top soils (0-10cm) for nitrogen mineralization potential. At the end of the first season plnt samples were collected (maize grain, cob and stover) and analysed for N and 15N enrichment. At the end of the second season (short rains 2002), soil samples were collected for inorganic N analysis (to 200cm depth) and plant sample (maize grain) for yield data. Among the three trials, Maseno recorded the highest maize yield especially in calliandra treatment due to its ability to build soil organic matter in the soil over time. This was attributed to split application of fertilizer, evenly distributed rainfall and the influence of organic resource on soil organic matter status. These factors could have enhanced fertilizer nitrogen uptake and hence good crop yields in Maseno trial. Kabete trial recorded the poorest maize yields due to unevenly distributed rainfall. Limited soil moisture reduced both soil and fertilizer N uptake and this was reflected in very low crop yields. At Maseno trial, substantial nitrate concentration in the deeper soil layers was observed in the control plots followed closely by tithonia at the end of the 2002 long rains season. At Kabete trial most of the nitrate-N was left in the top soil as a result of low rainfall which could not move it into the lower soil depths. Calliandra at Embu trial recorded the highest mineral N concentration in the lower soil depths due to lack of synchrony between N release and plant uptake. At the end of 2002 short rains Kabete trial had the best overall performance of maize grain yields compared to Embu and Maseno trial due to mineralization of N that had been immobilized in the previous season. A large flush of mineralization and asynchrony in calliandra treatment at Embu and Kabete trials led to accumulation of inorganic N in the lower soil depths (150-200cm).