Mycorrhizal assemblages and potential for enhancing growth of calliandra calothyrsus and sorghum bicolor in degraded lands: the case of Mabira Forest Reserve and semi-arid Soroti
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Forest degradation has been going on at an alarming rate, threatening the stability of many ecosystems in Uganda. In the last few decades, several restoration attempts have been made but the process has been very slow and on small scale, due to low resilience of the soil resource base and human interference. Successful consistent revegetation of drastically disturbed areas such as forests, mine sites and agricultural lands throughout the world has been achieved using the biological “tools”—mycorrhizal fungus inoculated tree seeds, seedlings, native shrubs, and grass species. However, the performance of these tools in different environments and plants has not yet been fully explored. The objective of this study therefore was to determine the distribution, abundance and diversity of mycorrhiza in selected restoration regimes of Mabira Forest and to compare the effect of selected arbuscular mycorrhiza (AMF) genera and their application rates for enhanced Calliandra calothyrsus and sorghum bicolor growth. The abundance and diversity of mycorrhiza was assessed through a survey conducted in six different forest regimes namely: 0-3 years old, 10-20 years, 20-30 years, 30-40 years, 40-50 years and over 55 years old compartments. In each of the forest regimes, an area measuring 150 x 50 m was demarcated and four composite soil samples collected at 30 cm soil depth. The collected soil sample was used in a sorghum bicolor mycorrhiza trap experiment with three replications for each forest regime. The performance of calliandra under different types and rates of AMF inoculation was assessed in the green-house using sterilized Mabira soils, while that of sorghum was conducted on-farm. AMF rich soils were collected from Serere, Soroti District for inoculum production. Four dominant genera (Glomus, Scutellospora, Gigaspora and Acaulospora) spores were isolated from the rhizosphere of sorghum in the laboratory. Each of them and their mixture were introduced onto calliandra seeds using the seed coating - VAM inoculum method at two concentrations of 30 spores and 50 spores. Three seeds per genera and rate, and a control (not inoculated seed) were grown in pots of 25 cm depth, and 15 cm diameter, each containing 3 kg of sterilized soil. Each treatment was replicated three times. On farm, the mixture of selected AMF isolates and the control (without AMF isolates) were replicated three times. AMF inoculated soil was then introduced in the sorghum planting holes on-farm. Results indicated that AMF spore counts were very low in Mabira Forest. However, there were seven arbuscular mycorrhiza genera constituting 18 species from the six forest regimes. The abundance of Glomus varied significantly within the forest regimes and was significantly higher than all the other genera, except Scutellospora. Gigaspora geosporum and Glomus clarisporum were the least abundant species and occurred in the 10-20 years and 20-30 years forest regimes, respectively. AMF diversity varied significantly among forest regimes with the 30-40 year old regime showing higher species diversity (9.48) than all the other regimes. All calliandra inoculated seedlings showed improved height and shoot dry matter weight) compared to the control (P<0.05) except for the AMF mixture treated calliandra at 50 spore rate. Glomus and Acaulospora had the highest increase in calliandra height, while AMF mixture showed the highest increase in shoot dry weight. On-farm the inoculated sorghum had a higher yield increase and was ten times taller than the control at 9 weeks after germination. Results of the study indicate a good scope for AMF inoculum selection and commercially utilizing the efficient AMF strains for forest restoration and agricultural productivity enhancement.