10.42 The retention of nitrogen in the soil is an important consideration in cultivation practices, including the cultivation of forests. Two methods for preparing plots for planting pine trees after clear-cutting were compared on the basis of the percentage of labeled nitrogen recovered.

Method A leaves much of the forest floor intact, while method B removes most of the organic material. It is clear that method B will produce a much lower recovery of nitrogen from the forest floor. The question of interest is whether or not method B will cause more nitrogen to be retained in the microbial biomass, as a compensation for having less organic material available.

Percentage of nitrogen recovered in the microbial biomass was measured on six text plots for each method. Method A plots showed a mean of 12 and a standard deviation of 1. Method B plots showed a mean of 15 and a standard deviation of 2. At the 10% significance level, should we say that the mean percentage of recovered nitrogen is larger for method B? (See Vitousek and Matson, Science, July 6, 1984, for more details.)