Nitrogen fixation can also be accomplished artificially by various methods (see nitrogen). Humans annually fix vast amounts of nitrogen for industrial purposes and for use as fertilizer. Unfortunately, large-scale legume cultivation and artificial fixation may be upsetting the natural nitrogen cycle in the biosphere. There is some question whether natural denitrification can keep pace with fixation. For one thing, run-off of nitrate fertilizer can cause eutrophication of lakes and streams (see water pollution) and can foul drinking supplies. Another environmental problem is that inorganic fertilizers tend to depress legume fixation. As a consequence, root tissue remaining after harvest is poorer, and thus more fertilizer must be applied the following year.