How have humans changed the global nitrogen cycle?

Humans have significantly impacted the global nitrogen cycle through various activities. One of the ways humans have changed the nitrogen cycle is by greatly increasing the amount of fixed nitrogen. Fixed nitrogen refers to nitrogen that has been converted from its atmospheric form (N2) into forms that can be readily used by plants and other organisms. This increase in fixed nitrogen is primarily due to the widespread use of synthetic fertilizers in agriculture. These fertilizers contain nitrogen compounds such as ammonia and nitrates, which are easily taken up by plants. As a result, the amount of fixed nitrogen available in the environment has increased, leading to changes in ecosystem dynamics.

Additionally, human activities have also raised the speed at which nitrogen travels through the cycle. This is mainly attributed to the combustion of fossil fuels, which releases nitrogen oxides into the atmosphere. These nitrogen oxides can then undergo various chemical reactions and eventually return to the Earth's surface through deposition, such as acid rain. This accelerated cycling of nitrogen can have detrimental effects on ecosystems, including increased nutrient runoff into water bodies and the formation of harmful algal blooms.

Therefore, the correct answer is D. Humans have greatly increased the amount of fixed nitrogen and raised the speed at which nitrogen travels through the cycle.