Final answer:
Applying manure to alfalfa can lead to vegetative burn due to salts in the manure, disrupting soil nutrient content and potentially causing environmental issues such as nutrient leaching and air pollution.
Step-by-step explanation:
The concern when applying manure to alfalfa is primarily associated with the potential for vegetative burn from the salts present in the manure. Alfalfa (Medicago sativa) is a perennial herbaceous crop used in the production of hay for livestock feeding. It is a critical forage crop and also a photosynthetic autotroph which employs the C3 photosynthetic pathway and hosts Rhizobium bacteria for nitrogen fixation.
However, manure can contain high levels of salts which can cause 'burn' or damage to the alfalfa vegetation. This occurs because the salts can draw moisture out of plant tissues, leading to desiccation and damage. Furthermore, excessive salt can disrupt the soil's nutrient balance, making other essential nutrients, such as phosphorus, more prone to leaching, hence affecting the soil's overall nutrient content. These changes can impact the sustainability of alfalfa as a forage crop and the environment.
Additionally, using manure can also affect the nutrient cycles and can contribute to air pollution when biomass is burned, not to mention increasing CO₂ levels and depriving soil of nutrients from organic matter decomposition which are essential for plant growth.