A model ecosystem initially contains 60 mg of available mineral ions in its soil. Each week, plants take up 15 mg and decomposition returns 10 mg. No mineral ions enter or leave by other routes.
Decomposition is then prevented, but plant uptake continues at the same rate for two weeks.
Compared with continued decomposition, how much lower is the mass of available mineral ions after these two weeks?
10 mg10\,\text{mg}10mg lower, because uptake removes mineral ions twice.
30 mg30\,\text{mg}30mg lower, because uptake removes mineral ions twice.
50 mg50\,\text{mg}50mg lower, because recycling no longer replaces mineral ions.
20 mg20\,\text{mg}20mg lower, because recycling no longer replaces mineral ions.
44 exam-style questions on Edexcel GCSE Biology 9.3 Material cycles and decomposition, covering 9.3.1 Cycling of materials through ecosystems, 9.3.2 The carbon cycle, 9.3.3 The water cycle, 9.3.4 The nitrogen cycle, 9.3.5 Indicator species and pollution, 9.3.6 Decomposition in preservation and composting, and 9.3.7 Rate changes in decay of biological material. Each one has a worked solution and a mark scheme showing where the marks go.