Influence of indigenous trees on soil macrofauna and soil organic matter dynamics in tropical Miombo Woolands

Studying the interactions between the trees and the soil macrofauna would provide an insight into the activities and functions that take place below and above ground under the tree canopies. Objective of this study was to evaluate the effects of indigenous trees on soil macrofauna and soil organic matter dynamics in miombo woodland. Soil macrofauna study was conducted at the Kiberashi Sentinel Site, Tanzania, following the Land Degradation Survelance Framework nested sampling design across a total area of 100 square kilometers. A total of six-hundred and forty monoliths were excavated and collected, soil macrofauna separated by hand sorting, and soil samples prepared for spectroscopic analyses. Organisms were identified into main taxa. Focus group discussion and key informants interviews were conducted, where information on influence indigenous trees on the spatial and temporal distribution of soil macrofauna were collected. Furthermore, farmers identified trees that were beneficial to crop production, improve soil fertility and those that influenced soil macrofauna distribution. Five dominant indigenous tree species, Acacia seyal Delile, Albizia amara Boivin, Combretum molle R. Br. ex G. Don, Dombeya rotundifolia Planch and Vangueria infausta Burch were selected and abundance of soil macrofauna, soil aggrgegates fractions and carbon contents under these trees determined. The area around each tree was subdivided into four concentric zones: 0-2m from the trunk (the first monolith) (zone A), from 2 m to half diameter of the crown (zone B), from half diameter to the edge of the crown (zone C), and from the edge of the crown plus half diameter of the crown (zone D). Two trees were selected from these five tree species based on the quality of their residues for a microcosm study. V. infausta (high quality) and D. rotundifolia (low quality) residues were used in the microcosm study with the following treatments: Pontoscolex corethrurus were introduced into microcosms, each with two kilograms of luvisols soils mixed with the residues as follows and in the treatment and each replicated thrice: 100% V. infausta, 75% V. infausta+25% D. rotundifolia, 50% V. infausta+50% D. rotundifolia, 25% V. infausta+75% D. rotundifolia, 100% D. rotundifolia and control with no residues applied. Earthworm cast aggregate stability was assessed by wet-sieving and separated into three aggregates groups i.e. Total macroaggregates (>250 μm), microaggregates (53 – 250 μm) and silt+clay, and then further fractionated into microaggregate within macroaggregate and silt+clay within macroaggregate size fractions to assess aggregate composition and carbon distribution. The study revealed that, key agroecological variables (land use and soil moisture) and soil properties (total C, clay content, pH, available P and exchangeable bases) influenced significantly the abundances of soil macrofauna. , A. seyal increased significantly the abundance and biomass of millipedes and termites and influenced significant increase in quantities of total macroaaggregates and mean weight diameter of the soil aggregates, while C. molle and zone increased significantly microaggregates and microaggregates within macroaggregates quantities. On the other hand, A. seyal increased significantly the contents of carbon in whole soil and total macroaggregate, with total carbon contents decreasing with increase in distance from the tree trunk. Correlation analysis showed that there was significant correlation between millipedes and termites with total macroaggregates, mean weight diameter and total carbon contents in total macroaggregates, microaggregates and micoaggregates within macroaggregates. V. infausta residues fed to the P. corethrurus led to high quantities of cast and cocoon produced, while D. rotundifolia residues fed to the earthworms influenced quantities of microaggregates and microaggregates within macroaggregates as well as increased the contents of total carbon in microaggregates within macroaggregate. Therefore use of A. seyal and D. rotundifolia as agroforestry trees in farmers’ fields will play great roles in influencing millipedes and termites’ abundances, improve soil structure and carbon sequestration.