Impact of Long-Term Application of Organic and Inorganic Nutrient Sources in a Maize-Bean Rotation to Soil Nitrogen Dynamics and Soil Microbial Populations and Activity

Abstract

Declining crop productivity is a big challenge to both the smallholder farming community and the researchers in crop production in Kenya. Nitrogen (N) is recognized as one of the most limiting nutrients to agricultural productivity in sub Saharan Africa. The high cost of mineral fertilizers and the adverse effects of continued fertilization have led to a search for alternative sources of nutrient supply. The effect of land management on soil nitrogen dynamics, microbial diversity and land sustainability in a long-term field experiment situated at the National Agricultural Research Laboratories, Kabete, near Nairobi, forms the subject of this thesis. The trial was established in 1976 to study the effect of continuous use of farmyard manure (FYM), crop residues and chemical fertilizers on crop yields and soil properties in a maize-bean rotation. A study was carried out to assess the effect of land management on soil physio-chemical properties and microbial populations. Soil chemical properties declined over time, while acidity and bulk density increased under continuous cropping with no inputs. The effect of organic and inorganic inputs on soil microbes, number and activities of various groups of microorganisms was monitored for one year in the 0 -15 and 15 -30 cm soil layers. Use of FYM alone or combined with chemical fertilizers gave significantly higher (p=0.05) numbers of microbes, microbial respiration and soil enzymatic activity than plots with no-inputs or with chemical fertilizers alone. The topsoil layer had significantly (p=0.05) higher microbial activity than the sub-soil regardless of treatment. Bacteria were more numerous (1 x 105 g dry soil -I) than fungi (Ix 103 g dry soil -I), which may lead to more SOM mineralization and less SOM retention in this cropping system. To study the dynamics of fertilizer nitrogen (N), labeled 15N fertilizer as calcium ammonium