We often take the soil beneath our feet for granted, but in more than three decades of studying environmental chemistry, William H. Schlesinger has played an important role in establishing how soil can affect forests, deserts, and even climate. Humans can, with better management, potentially increase the carbon content of soils and thereby store some of the excess carbon dioxide changing our planet's atmosphere, says Schlesinger, who is president of the Cary Institute of Ecosystem Studies in Millbrook, N.Y., and the former dean of the Nicholas School of the Environment and Earth Sciences at Duke University. On Monday, Schlesinger, who is also a trustee of the Natural Resources Defense Council, will give a lecture in New York City as part of NRDC's Visionary Speaker Series. He will focus on nitrogen and how it affects human health, climate, and the ozone layer. He discussed the subject in advance with journalist Michael O. Allen.

How would you define the nitrogen problem?

About 78 percent of the mass of the Earth's atmosphere is nitrogen gas. But the nitrogen in the atmosphere can't be used directly by plants and crops until it is converted to one of the forms we call active or available nitrogen. Nature can do that transformation. There are some bacteria that can do that, but it is what we call metabolically expensive. It's a difficult reaction. So humans, of course, being the ingenious creatures that we are, discovered a way to do that synthetically. And that is the genesis of the entire fertilizer industry of today. That started about a hundred years ago, but it's really since WWII that it's come on strong. Statistics show that half of all fertilizers ever used have been used since the mid-1980s.

So we're now supplying the natural world with about twice the nitrogen it had a hundred years ago. It gets in the atmosphere. It gets in streams and rivers that are draining from farm fields. It gets into ground waters. When the ground waters are subsequently used as drinking water for humans, the concentration of nitrates can exceed a recognized public health standard. High nitrates in water cause a syndrome in young children and infants called blue baby syndrome. A significant faction of nitrogen also gets converted into nitrous oxide, which causes a number of difficulties. One is that it's a greenhouse gas in the atmosphere. Another is that it is now thought to be the major source of ozone destruction in the stratosphere.

So it's a complicated problem. How do you explain it to the public?

The problem with nitrogen is these issues are not simple. You can't just say, humans added nitrogen to the environment and that's bad because....  All of my descriptions have several steps. You add it to the farm field and it runs away from the farm field, runs all the way down to the coastal ocean, causes an algal bloom. The algal bloom by itself is probably not a problem, but when it dies it causes a problem. All the impacts have multiple steps. The public hasn't heard much about them, so it's going to take very careful and thoughtful descriptions to get people to understand that this is not a good thing.

What are the major sources of the nitrogen buildup and how can we begin to cut back?

The first thing to realize is that the vast majority of the problem is derived from large-scale agriculture. That's really where the education needs to be aimed. Those operations need to use nitrogen fertilizer sparingly, judiciously, at the highest efficiency that can be achieved. In other words, not just spread it on the landscape willy-nilly but to put it on the landscape when the crop most needs it and is likely to accumulate it. That would mean that less nitrogen is lost to runoff.

Another big thing that would be helpful would be for agriculture and actually our society in general to recycle its waste more effectively. Right now we add a lot of nitrogen to corn crops in Iowa and we harvest that crop and ship it North Carolina and feed it to pigs. And, of course, the pigs excrete it. Instead of gathering much of it and taking it back to Iowa, where it might be reused, we store it in large lagoons. Sometimes those lagoons break and dump vast quantities of nitrogen into the local river. We don't have a scheme, an economic system put together that is very effective at recapturing and reusing nitrogen.

If you bring it back to the individual citizen, a lot of the things that I said could apply, but they'll apply on a much smaller scale. The rate at which home gardeners and golf courses use nitrogen as a fertilizer is vastly in excess of what's needed. Because it's not terribly expensive, people decide if a little bit is okay then a lot must be better. A lot of excess washes off and contaminates the ground water. We do need to get the education of this down to the individual level, as well.

There are some lawmakers who still call global warming a "hoax." How can you get government to be part of the solution when some of its most prominent members take that approach to environmental issues?

I believe we need to have a continued presence at congressional briefings, educational sessions, and in the newspapers and magazines and on television. Scientists need to explain as clearly as possible what the problem is, what the basis of it is, and that there are solutions. I guess I'm old-fashioned in the belief that this kind of grassroots education would be productive and bring people around. I don't see much other hope. Every now and again, when you get an administration like the last administration with eight years in which they actively discouraged scientists from being at the table, that can really set things back.

Is there any good news on this front?

We've got a lot of success stories. When I was in college in the 1970s, some of the problems were DDTs and leaded gasoline. Both of those have since been outlawed. About ten years later, one of the big issues was the chlorofluorocarbons and ozone destruction. Those have been substantially scaled down. In every case the progress wasn't easy or swift. And it took a lot of convincing. It had setbacks along the way, but eventually wiser heads prevailed.

I noticed this line on your curriculum vitae, and I just have to ask about it: "Web: World Wrestling Entertainment." What's that about?

You're talking about my buddy, John "Bradshaw" Layfield, the WWE Smackdown champion wrestler. He came with us on a trip to Antarctica. I have to admit he started off the trip being rather skeptical, but by the end I'd made him into an environmentalist.

Read interviews with more NRDC visionary speakers.