Which nutrient is commonly considered the most limiting to net primary productivity (NPP)?

Nitrogen is often regarded as the most limiting nutrient to net primary productivity (NPP) in many ecosystems, particularly in terrestrial environments. This is because nitrogen is a crucial component of amino acids, proteins, and nucleic acids that are necessary for plant growth and development. Since plants rely on nitrogen to synthesize these essential compounds, a deficiency in nitrogen can directly restrict their ability to photosynthesize and, thus, their productivity.

In many ecological contexts, nitrogen availability influences plant growth more dramatically than other nutrients because while phosphorus, potassium, iron, and other minerals are important, they typically do not limit productivity to the same extent as nitrogen. This is especially true in ecosystems where nitrogen inputs are low, such as those with old, weathered soils.

The role of nitrogen in limiting primary productivity is further illustrated by the phenomenon of eutrophication, where nutrient over-enrichment often results in increased algal blooms, which can subsequently deplete oxygen levels and harm aquatic ecosystems. This indicates that, while organisms need various nutrients to thrive, nitrogen often surfaces as a bottleneck in ecosystem productivity.

Thus, nitrogen's critical role in the growth processes of primary producers makes it the nutrient most commonly linked with limitations on net primary productivity.