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 | | Anand, Sanjay
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|  | Developing a taro (Colocasia esculenta) production system based on genotype and fallow system for economic and environmental sustainability under local conditions in Samoa
Author:Anand, Sanjay
Institution: University of the South Pacific.
Award: Ph.D.
Subject: Fallowing -- Samoa, Cropping systems -- Samoa, Taro -- Samoa
Date: 2016
Call No.: pac S 602 .8 .A53 2016
BRN: 1206839
Copyright:20-40% of this thesis may be copied without the authors written permission
Abstract: Developing agricultural production systems able to conserve soil quality is essential to guarantee the current and future capacity of soil to provide ecosystem services. This study investigated the efficacy of six month fallow effects of four contrasting cover crops namely, grass, mucuna, erythrina and biochar over four agro-ecological taro growing zones in Samoa. Understanding mineralisation patterns of green manure residues is crucial in the synchronisation of nutrient release from plant residue and uptake by plants. This research work focused on tailoring the release of nutrients from green manure that has been applied as mulch, with the aim of improving the efficiency with which green manure nutrients are used in taro cropping systems. All the organic soil amendments were evaluated in the screen house and multi location field conditions in an attempt to estimate the rates of decomposition of green manure residues and their effects on the dynamics of soil labile C, fluorescein diacetate hydrolysis activity (broad spectrum soil biological activity) and the potentially mineralisable N pools. The mineral N fluxes (NH4 +-N and NO3 --N) from the embedded covered core in-situ aerobic incubations as well as the net mineralisation potentials from subsequent mineralisation of green manure mulch residues were also studied. The influence of the green manure supply on the yield and nutrient content of two taro (Colocasia esculenta) cultivars was also determined. The effects of the decomposition of cover crop mulches on nematode population were also evaluated. In addition to the biochemical indicators mentioned above, soil phosphatase and urease assays were investigated during 90 days incubation period of the mulch residues at different rates in pots under the screen house conditions. The nutrient uptake and nutrient use efficiency of the two taro cultivars: Samoa 1 and Samoa 2; used for this research was determined in a separate pot experiment. Results from this study indicated that all the fallow treatments significantly improved the soil active carbon stocks upon decomposition; which however, were largely dependent on the biomass production. Mucuna fallow contributed to the largest additions of biomass across all the agro-ecological sites and as such proved to be the superior cover crop with regards to improving soil active carbon, soil biological activity vi as well as the potentially mineralisable N pools. Mucuna fallows also resulted in significantly greater inputs of mineral N to the soil system and showed significant net mineralisation potentials over two of the sites. It significantly contributed to the suppression of plant parasitic nematodes while enhancing the activity of free living genera. Nutrient uptake and the corresponding yields of taro were comparatively higher under the mucuna fallow, both with and without supplementation with complete mineral fertilisers. Comparable yields under biochar treated plots were observed, owing to appreciable amounts of K uptake. The comparative economic analysis of the mucuna fallow technology against the traditional grass fallow revealed 98% and 48% higher gross margin for Salani and Safaatoa sites, respectively; while 21 fold increases was observed for the Siufaga site. The potential practical benefits of the mucuna legume technology for South Pacific taro farmer’s looks promising in terms of increased yields reduced labour requirements, reduced fertiliser inputs, and suppression of weeds and plant parasitic nematode population. Significant positive associations were found to exist between the yield of cultivar Samoa 2 and mean levels of FDA, PMN, NH4 +-N and NO3 - -N. However, yield of cultivar Samoa 1 showed no significant correlation with these soil parameters. Mean levels of labile C did not correlate with the yields of any of the cultivars. The rate of decomposition and subsequent release of mineral N was also favourable for the decomposing mucuna litter. Although erythrina had higher N content than mucuna, most of it was released so rapidly during the initial stages of decomposition that it could not be resourcefully made use of by the taro crop. Conversely, the rate of decomposition and the N release pattern from the mucuna residues was more gradual and synchronised well with the vegetative growth phase of the taro crop. vii The dry matter accumulation and nutrient uptake of the two taro cultivars revealed that the cultivars exhibited significant differences for the various nutrients with regards to their efficiency of utilisation towards production of a unit of edible dry matter. Cultivar Samoa 1 had a higher nutrient use efficiency for N, P, K, Mg, Mn and Cu over cultivar Samoa 2. However, for Ca, Fe and Zn, cultivar Samoa 2 had a higher nutrient use efficiency over cultivar Samoa.
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