Final answer:
Net primary productivity (NPP) increases initially with higher nitrogen input due to better availability of this crucial nutrient for plants, but it decreases at very high nitrogen levels because of negative impacts on soil and plant health.
Step-by-step explanation:
As nitrogen input increases, net primary productivity (NPP) increases at first and then decreases at high levels of nitrogen input. Net primary productivity refers to the rate at which plants and other photosynthetic organisms produce organic compounds in an ecosystem. Initially, NPP is likely to increase due to the enhanced availability of nitrogen, which is a critical nutrient for plant growth. However, at high levels of nitrogen input, negative effects such as nutrient imbalances, soil acidification, and potentially toxic levels of nitrate (NO3-) accumulation in soil can occur, leading to a decrease in NPP. These high levels of nitrates can also lead to increased leaching due to NO3-, which removes nutrients from the soil and can contribute to environmental issues such as eutrophication of water bodies.