Tropical rainforests have much higher decomposition rates than TG, yet the rainforest soil is low in nutrients. Clearly, rates of decomposition alone do not determine nutrient levels. Compare the adaptations for desert living in a camel and a desert cicada. Where applicable, indicate to which component of the water budget each corresponds. Further recall that the fat is not an adaptation to water shortage.
All aerobic organisms produce metabolic water all the time - from cellular respiration.
The stored fat is metabolized in response to energy needs, not in response to water needs. In fact, increasing metabolism both produces water and costs water - via the increased evaporative loss from increased breathing to supply the needed oxygen for respiration. It further reduces the need for cooling by facing into the sun and having fur that insulates it from the sun. The cicada remains under the shade of leaves, but nevertheless is subjected to extremely high temperatures in mid-day.