True or False: Immobilization can be caused by mature grass cover crops.

The statement is true because immobilization refers to the process by which nutrients—particularly nitrogen—become unavailable for plant uptake as they are absorbed by microbial biomass. Mature grass cover crops can contribute to immobilization as they provide a rich carbon source for soil microorganisms. When these cover crops decompose, the microbial population increases and utilizes available nitrogen for their growth, thus temporarily reducing nitrogen levels in the soil for other plants.

Mature grass cover crops contain a larger amount of plant material and can lead to a significant demand for nitrogen due to the decomposition process. As the grass breaks down, soil microbes may use the nitrogen present in the soil, leading to a situation where there is a lag in nitrogen availability for subsequent crops. This effect can be less pronounced with younger cover crops, as they may not have as extensive a root system or as much biomass for the microbes to use, thereby reducing the likelihood of significant immobilization.

While legumes can also contribute to nitrogen dynamics in the soil, their primary role often focuses on nitrogen fixation rather than the immobilization effect. Hence, the statement accurately reflects the role of mature grass cover crops in nutrient dynamics within the soil.