A greenhouse reuses the same nutrient solution to grow several lettuce crops. Between crops, the water volume and the concentrations of all mineral ions except nitrate ions are restored. The latest crop has a lower dry mass than the first crop.
Which explanation accounts for the decrease in crop dry mass after repeated use of the solution?
Earlier crops absorbed nitrate ions, so less nitrogen was available to make carbohydrates and lipids.
Earlier crops released extra nitrate ions into the solution, so the latest crop received too much nitrogen.
Earlier crops converted nitrate ions into nitrogen gas, so less nitrogen remained for the latest crop.
Earlier crops absorbed nitrate ions, so less nitrogen was available to make amino acids and proteins.
101 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.