Impact of soil fertility management strategies on diversity and

Abstract

The role of soil invertebrate fauna in decomposition of organic residues and thus nutrient release, soil structure and soil-water relations is well appreciated. The scope, however, of their manipulation to derive the potential benefits is little understood. A study was undertaken to assess how soil fertility management strategies within an agroforestry system affect soil biodiversity (macrofauna diversity and populations). The study was conducted on-farm during the 1997 Short Rains (Oct 1997-Feb 1998) in western Kenya with the following treatments: (1) control without any input, (2) fertilizer at 120 kg N, 150 kg P and 100 kg K/ha, (3) Tithonia diversiflia biomass at 5 t ha dry weight and (4) Senna spectabilis at 5 t/ha dry weight. Riparian natural forest and a 6-year grassland fallow were assessed and used as reference standards. Macrofauna diversity and populations were monitored in soil monoliths (25 x 25 x 30 cm) at the beginning of the season, 6 weeks after sowing maize and at maize harvest. Faunal diversity was assessed using the Shannon-Wiener index of diversity (H'). Data were subjected to analyses of variance (ANOVA) in a split-plot design and treatment differences evaluated using least significance difference (LSD) at P = 0.05. Macrofauna diversity and populations of most of the macrofauna groups were significantly higher under natural forest (H' = 2.31) than in all the other treatments (each H' = <1). Although the 2 organic residues did not affect faunal diversity, addition of senna significantly increased earthworm population by 400% and tithonia by 240% over the control. Even though termite population increased by 145% in senna and 118% in tithonia over the control, the increase was not significant because of high variability between replications of the same treatment. Fertiliser did not change either the diversity or population of termites and earthworms. The study indicates that: (1) Short-term addition of organic residues do not significantly increase faunal diversity, (2) Soil invertebrate fauna populations can be manipulated by external additions of organic residues such as senna and (3) Changes in land use systems, especially those that reduce the amount, range and diversity of food resources for macrofauna bring about instability in below ground biodiversity. There is therefore a need to clearly define the conditions in which such impairment is critically important to agricultural production and sustainability and to determine what management interventions may be made to alleviate or ameliorate problems resulting from loss of diversity.