The influence of carcass decomposition on soil nutrient composition and its effects on growth of Gazania rigens (L.).

Abstract

This research project forms part of the Department of Nature Conservation, Unisa’s: Death in the long grass: The ecological implications of carcass decomposition in a southern African grassland project. Decomposing carcasses leach nutrients into soils, forming dense nutrient rich islands in grasslands, which contribute to the recycling of nutrients and energy through the ecosystem. Large decomposing herbivore species, such as blue wildebeest (Connochaetes taurinus), create much larger volumes of nutrient influx into soils of cadaver decomposition islands (CDI) compared to smaller decomposing animals, such as rodents (Rattus norvegicus). Plant growth is negatively affected immediately after the influx of carcass decomposing liquids in CDI soils, resulting in plant death within the CDI; however, field observations have indicated plant growth returns after several months. The aim of the project was to develop an understanding of the ecological dynamics associated with the decomposition of wild animal carcasses in a grassland ecosystem as nutrient islands. This project investigated the effect of liquid influx from decomposing blue wildebeest (Connochaetes taurinus) carcasses on soil nutrient content at various stages of decomposition, and the effect thereof on the growth of the indigenous plant Gazania rigens (L.). The findings of this research project could be applied positively in the development of organic growth media by making use of animal waste products incorporated with sustainable plant products, such as coir and bagasse, to enhance their performance, as the current source of organic growth media, peat moss, is seen as unsustainable. Ten blue wildebeest carcasses, five caged and five pegged, were placed at suitable localities in grasslands at Telperion Nature Reserve, Mpumalanga, South Africa. Soil samples were collected at each site before the placement of carcasses and at a three-week interval after placement of carcasses. Soil samples collected from each carcass site were homogenised, air-dried, some frozen (for nitrogen analyses) and analysed at Unisa’s CAES laboratories for soil texture, colour, electrical conductivity (EC), and potential hydrogen (pH), organic matter (OM), exchangeable bases; sodium (Na), magnesium (Mg), calcium (Ca) and potassium (K) and cation exchange capacity (CEC), total nitrogen (TN), total carbon (TC) and C:N ratios, available phosphorus (P), and micronutrients; copper (Cu), iron (Fe) and zinc (Zn). To assess the influence of soil nutrient inputs on plant growth, 120 five-week-old G. rigens seedlings were planted in the soils collected from around each carcass before placement of carcasses, and at three-week intervals during the decomposition process. After 12 weeks of growth under controlled environmental conditions, plants were harvested and individual organs measured, counted, weighed and air-dried to obtain dried organ weights. Soil properties showed significant temporal changes after placement of caged and pegged carcasses, especially EC, pH, OM, exchangeable bases; Na, Mg, Ca and K, nitrogen (N), C:N ratios, and Cu, compared to before placement of carcasses. Soil colours and textures remained unchanged throughout carcass decomposition periods compared to before the placement of carcasses. Noticeable differences were observed between plants grown in soils collected from decomposing caged and pegged carcasses, compared to those grown in soils collected before placement of carcasses, especially in the number of leaves, leaf lengths, fresh and dry leaf weights and number of dead leaves, as well as fresh buds, flowers and seed heads. Soil aspects reported to have the most significant temporal influence on G. rigens growth were EC, pH, OM, Mg, CEC, N, carbon (C), C:N ratios, P, Cu, Fe and Zn. Each of the growth parameters was affected by one or more of these soil aspects, especially plant leaves and flowering organs. This study reported significant changes in soils and plant growth parameters; however, no significant changes were reported in the two treatments, caged carcasses compared to pegged carcasses. The implication of this study is that the use of soils from around decomposing carcasses as a medium for growth of ornamental crops is not detrimental to plant growth. Recommendations are to investigate methods to use carcass waste in composting techniques for growth medium in which to grow ornamental and edible plants