HUMAN IMPACT ON GLOBAL BIOGEOCHEMICAL CYCLES
Among the light elements of the periodic table, the human impact is greatest on the biogeochemical cycle for sulfur, where the annual human mobilization equals about 30% of what cycles naturally in the biosphere. By contrast, the human impact on the carbon cycle amounts to only about 7% of the underlying biotic cycle, while for nitrogen the human inputs contribute about 2% to the natural cycle. The comparisons are somewhat different if one compares the human mobilization to other processes that contribute new supplies of these elements to the biosphere. Here the human impact is 5% for C, 65% for N and 87% for S.
Human impact on the nitrogen cycle, largely by the production of fertilizer for agriculture, has stimulated rates of photosynthesis and plant growth in the natural and managed biosphere. Thus, changes in the nitrogen cycle yield concomitant changes in the global carbon cycle, leading to a sink for atmospheric CO2 that might otherwise contribute to global warming. Care must be taken to estimate the resulting mitigation of climate change, since microbial and pyro-denitrification return nitrogen to the pool of N2 in the atmosphere. Only about 1/3 of the nitrogen added to the land remains to stimulate net uptake and accumulation of carbon by the biosphere. Collectively the small sink for carbon from N and C fertilization of the biosphere, reforestation and afforestation demands that we take immediate action to curb fossil CO2 emissions if we are to have any hope at all to avoid significant climate change.