Impact of trees on water and nutrients dynamics in smallholder maize-based farming systems in Trans-Nzoia, Rift valley, Kenya

Abstract

With decreasing land size, caused by fragmentation as family size increases, competition for this scarce resource has led to prior consideration on what tree species to plant as complementarities to household economy or as an encouraged agroforestry intervention measure. Even though farmers can instinctively anticipate crop yield losses as trees grow, they would likely be unable to accurately predict the period of viable intercropping and their effects on soil resources. The current study hypothesizes that, the adoption and management of trees in agricultural fields is largely influenced by household resource endowment, land tenure and the period of occupation by current households. Farmers are also hypothesized to be able to detect differences in soil quality within their farms by using local soil quality indicators which can be confirmed by chemical soil analyses and they understand changes in soil resulting from the presence of trees on farm. In addition, different tree species at varying age/ stage of growth are hypothesized to have different contribution to an agroecosystem in terms of competition, complementarity or balanced off effects on below and above ground resources necessary for crop growth and productivity. The study evaluated agroforestry adoption and practices within smallholder farms in a former large-scale maize growing area of Trans-Nzoia County, Kenya to understand the structure, densities and utilization of tree populations in agricultural landscapes is useful in determining the species influencing agroecosystem functions. This was followed by investigating how household resource endowment, land tenure and time under current management affect the adoption and resulting agroforestry practices. Five settlement schemes which were formerly large estates dominated by maize mono2 cropping were selected for the current study. Tree inventories of the farms was obtained through transect walks across each settlement. A total of 123 farms were assessed representing households of different resource endowment levels, tenure and number of years under current management. Different analyses were carried out including farm size and tree number, tree density, species richness, tree diversity and utilization of the dominant tree species. This was followed by an assessment of water and nutrient dynamics in maize (Zea mays) and dominant tree species; (Calliandra calothyrsus, Sesbania sesban, Grevillea robusta, Eucalyptus spp, Croton macrostachyus and Markhamia lutea) intercrop in the smallholders’ farms. The nature and extent of interaction was evaluated to establish the existing tree-crop relationships and effect on crop productivity in the farms. To understand the short and long-term effect of tree integration in the farms on soil crop productivity, water and nutrient availability, Water Nutrient Light Capture in Agroforestry Systems (WaNuLCAS) model was parameterized and simulations run of three selected tree species (Grevillea robusta, Croton macrostachyus and Markhamia lutea). Finally, to better understand the soil fertility problem in the study area we assessed farmers knowledge on soil qualities in their farms, indicators used to detect changes in soil quality which was evaluated by comparing with scientific knowledge. Lastly, their perception of contribution of dominant tree species on soil quality was investigated. In total, the study identified 44 tree/shrub species, 24 of which were indigenous and the rest exotic. However, the exotic tree species dominated strongly in abundance with Eucalyptus spp being the most frequent taxon and constituting 34.6% of all trees. Species richness was found to be low compared to other agricultural landscapes in the region. Resource 3 constrained households were found to prefer fruit tree species and maintained high tree diversity in the farms. Households with secure tenure had higher tree diversity than those without who had higher species richness and opted for fast growing fodder and fertilizer/firewood trees. Younger farms had fewer trees but highest species richness than older farms. Results showed that farmers possess the knowledge of the crops that best perform in each soil type they identified. Farmers also use plants as indicators of differences in soil quality in the farm. The smallholders’ farmers have a wealth of experience on local indicators of soil quality and contribution of agroforestry trees in maize production systems. They know how to distinguish between fertile and infertile soils using visual and morphological soil characteristics. Farmers’ perceptions of soil quality were substantiated through soil chemical analyses and pH, boron, ECd, ExAc, potassium and magnesium provided precise information on these differences. The ability of the trees to increase soil fertility through leaves decomposition, nitrogen fixing, erosion control and provision of minimal shade during dry seasons were the preferred attributes of agroforestry tree species. Great tree management diversity was exhibited among the farmers whereby they selectively prune trees perceived as competitive to crops but still want to maintain them in the farm such as Eucalyptus spp and G. robusta. The smallholder farmers in the study area remain important maize producers in Kenya with an average maize yield of 6.5 tons ha-1 recorded. Maize yield under the dominant tree species showed significant differences (P < 0.001) with leguminous species (C. calothyrsus and S. sesban) recording the highest amount of grain weight. Dominant tree species within the farms were also shown to significantly (P < 0.001) influence the spatial distribution of soil water. The study found that under Eucalyptus spp (exotic) and S. sesban (native) the amount of water was reduced compared to soil away from the tree influence while under G. robusta (exotic) and C. macrostachyus (native) soil moisture was increased under the trees. S. sesban (leguminous) and C. macrostachyus (non-leguminous) were shown to have highest turnover rate of plant residues compared to the rest of dominant tree species while Eucalyptus spp (non-leguminous) and G. robusta (non-leguminous) recorded lowest turnover rate. Agroforestry zones recorded significant difference in amount of soil pH (P = 0.074) and Mg (P = 0.034) under S. sesban which was not reported from other tree species. The nutrient concentrations decreased with distance from tree stems and with soil depth, a pattern caused by nutrient accumulation from litter fall below and high rate of residue decomposition around S. sesban canopy. Modelling using WaNuLCAS suggested that individual traits of tree species, management practices such as crop choices, tree selection, intercrop spacing and age are important factors affecting water use, nutrient availability and biomass production in smallholders’ maize-based farms in Trans-Nzoia County. M. lutea was also shown to assist in P recycling in the farms. In contrast, there was increased water competition exhibited under G. robusta which in turn translated to low crop productivity under the tree. In conclusion, the study explains that; (1) the establishment of a diverse tree cover in the study area involves the simultaneous action of three main drivers, namely, household resource endowment, land tenure and time under current management, (2) farmers hold complex ecological knowledge or local knowledge on indicators of soil quality and contribution of agroforestry tree in their farms and they recognize the tradeoffs underlying a biodiverse agroforestry system. Their creativ capability in the utilisation of local knowledge was also demonstrated, (3) differences in water availability and maize productivity under dominant tree species in smallholder farms was shown to be much more complex than in monocultural systems; and (4) different tree species contribute differently to soil nutrients in agroforestry systems therefore it is not always beneficial to grow intimate mixtures of trees and crops. Farmers should be encouraged to incorporate tree species that exhibited less competition with crop into their farms such as S. sesban while avoiding those exhibiting increased competition such as Eucalyptus spp. However, management of such tree species proved more important to the farmer than total elimination and the integration of local knowledge with scientific can be a good tool to enhance productivity of agroforestry systems.