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Rheinischen Friedrich-Wilhelms-Universität Bonn (2009)

Conservation tillage in Kenya : the biophysical processes affecting its effectiveness

Maguta Job Kihara

Titre : Conservation tillage in Kenya : the biophysical processes affecting its effectiveness

Auteur : Maguta Job Kihara

Université de soutenance : Rheinischen Friedrich-Wilhelms-Universität Bonn

Grade : Doctoral Thesis 2009

Résumé
Appropriate soil management is important for improved ecosystem functioning and high crop production. This study investigates how different tillage [reduced tillage (RT) and conventional tillage (CT)], crop residue (plus and minus crop residue) and cropping systems (soybean-maize intercropping, rotation and continuous maize) affected (i) soil aggregation, (ii) composition and diversity of microbial populations, (iii) crop residue (CR) disappearance and termite activity, (iv) nitrogen fixation and (v) crop productivity in Kenya. The main experiment in Nyabeda (western Kenya) had been established in 2003, while experiments in Matayos (western Kenya) and Machang’a (eastern Kenya) were established in 2005. Soybean-maize intercropping improved macroaggregation and reduced microaggregates and free silt and clay (P<0.05) compared with the other cropping systems. The proportion of soil large macroaggregates was 30% to 89% higher in RT than in CT, depending on depth. Addition of CR affected (P<0.05) soil aggregation mainly at the top 5 cm ; it increased the large macroaggregates (by up to 180%) in the soybean-maize intercropping systems. The composition of both bacteria and fungi communities was markedly different in the two tillage systems. With CR application, Simpson’s indices of fungi were in the order intercropping >rotation >continuous maize. In addition, intercropping had highest bacteria diversity indices in the Nyabeda site. CR affected bacteria composition (e.g, in Matayos) and lowered diversity of soil fungi (P<0.01) ; fungi Simpson’s index was 0.75 for plots without and 0.65 for plots with crop residue. Bacteria diversity was inversely related to silt and clay. Fungi diversity (Simpson’s index <0.7) was highly inversely related with aggregate mean weight diameter and with soil hot water-extractable carbon. CR disappearance was up to 85% of the initial residue in 3.5 months, and the relative contribution of macro- and mesofauna to residue disappearance was 70-95% for surface-placed and 30-70% for buried residues. Soil of termite galleries (mainly sheetings) was more enriched in carbon (1.6%) than bulk farm soil (1.4%) and mound soil (1.2% ; P<0.01) ; gallery soil and bulk farm soil had similar aggregates sizes but the values were lower (22-56% for >250<2000 µm aggregates ; P<0.05) than for mound soil. Soybean nitrogen derived from the atmosphere (%NDfA) ranged from 42-65% ; it was higher (P<0.05) in RT (55.6%) than in CT (48.2%). Nitrogen fixed seasonally in soybean aboveground plant parts was 26-48 kg N ha-1 with intercropping and 53-82 kg N ha-1 with rotation. Seasonal litter-fall contained about 15 kg N ha-1. Total fixed N under RT plus CR was at least 55% and 34% higher than in the other treatments (RT minus CR, CT plus CR, and CT minus CR) in intercropping and rotation systems, respectively. Seasonal average maize grain yields were 3.2-4.1 t ha-1 in continuous maize, 3.0-3.9 t ha-1 in soybean-maize rotation, and 1.8-2.8 t ha-1 in the soybean-maize intercropping system. Soybean grain yields were 0.92-0.99 t ha-1 in the soybean-maize rotation and 0.52-0.60 t ha-1 in the intercropping system. The net benefits were highest in the soybean-maize intercropping, followed by rotation > continuous maize. Soybean yields were similar between CT and RT ; maize yields were lower (P<0.05) in RT than CT. Overall net benefits for the 9 seasons were higher in CT than in RT. We conclude that (i) despite fast disappearance of CR, its application increases soil aggregation and influences microbial composition and diversity and nitrogen fixation ; (ii) for Ferralsols of western Kenya, combining RT and CR is important for improved soil structural stability and, intercropping maize and legume (soybean) leads to better soil structure and also gives higher net benefits than conventional rotation and continuous maize systems ; and (iii) RT is appropriate for soybean production ; maize yields are lower in RT than in CT due to surface crusing in the RT resulting from inadequate soil cover.

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