Which of the following is NOT a gain in the nitrogen cycle?

The loss of nitrogen by leaching is accurately identified as not a gain in the nitrogen cycle because it represents a removal process rather than an addition. In the nitrogen cycle, gains are typically associated with processes that increase nitrogen availability in the soil, enabling plants and microorganisms to utilize it.

In contrast, leaching involves the washing away of soluble nitrogen compounds, such as nitrates, from the soil into groundwater or surface water, thereby decreasing the amount of nitrogen available in the soil for plant uptake. This loss can lead to nutrient deficiencies for crops, which can negatively impact agricultural productivity.

On the other hand, the fixation of nitrogen refers to the conversion of atmospheric nitrogen into a form that plants can utilize, which is undeniably a gain. The addition of animal manure enriches soils with nitrogen and other essential nutrients, contributing to the fertility of agricultural land. Likewise, returning crop residues to the soil adds organic matter and nutrients back into the ecosystem, further enhancing soil health and nitrogen content. Thus, while fixation, manure addition, and the return of crop residues all contribute positively to the nitrogen cycle, leaching results in a detrimental loss.