An ecologist investigated the rate of decomposition of leaf litter from two different species of tree: Silver Birch (Betula pendula) and English Oak (Quercus robur).
The ecologist placed 10.0 g10.0\text{ g}10.0 g of dried birch leaves into a mesh litter bag. An identical mesh litter bag was filled with 10.0 g10.0\text{ g}10.0 g of dried oak leaves.
Both litter bags were buried at a depth of 10 cm10\text{ cm}10 cm in soil containers kept inside a laboratory incubator.
The remaining dry mass of the leaf litter in each bag was determined every 7 days for a period of 56 days.
State two variables, other than the initial dry mass of the leaf litter and the depth of burial, that must be controlled in this investigation.
