Which human activity contributes excess nitrogen to ecosystems, leading to eutrophication and nitrous oxide release?

• A. Increased rainfall adds nitrogen
• B. Excess fertilizer use
• C. Natural nitrogen fixation by legumes
• D. Decreased soil moisture

Answer: (B)

Excess nitrogen entering ecosystems from human activity is most closely linked to fertilizer use. When nitrogen from fertilizers is applied beyond what plants can absorb, the surplus moves into water through runoff and leaching, fueling rapid algal growth and leading to eutrophication. In aquatic systems, this excessive growth can deplete oxygen as algae die and decompose, creating dead zones. At the same time, soil and manure management processes convert the extra nitrogen into nitrous oxide, a potent greenhouse gas that escapes to the atmosphere.

Other options don’t fit as the main driver: rainfall can bring nitrogen but is not primarily a human-caused input, natural nitrogen fixed by legumes adds nitrogen rather than causing excess, and reduced soil moisture affects transport and microbial activity but doesn’t provide an added nitrogen source.