I have so many footprints these days I'd need an expert tracker to sort them out.

There's my carbon footprint, the one most people are familiar with, with dozens of calculators that happily inform me just how excessive is the carbon gushing from my First World life choices.

Water is an issue too, and I can measure that.

Now, it seems, I have to think about my nitrogen footprint as well.

In some ways, nitrogen is great stuff. It's in fertilizer, and makes plants grow. Great for the veggie garden! Hungry nations need it.

But get too much of it, let nitrogen run amok, and it turns evil.

Nitrogen in various forms can cause urban smog. It's a greenhouse gas.

The world's nitrogen scientists have become so concerned about out-of-control nitrogen that they've formed the International Nitrogen Initiative (see www.initrogen.org), aimed at optimizing nitrogen's beneficial impacts and reining in the bad.

In the Mid-Atlantic, one of the major problems with nitrogen is its effect on coastal bays, which are - or should be - carpeted with sea grass beds that support a teeming food web, which tops out in delicacies like crabs, flounder, and shellfish.

Perversely, nitrogen doesn't make the sea grass grow. Instead, it fuels blooms of algae, which kill the grasses - first by shading them, and then, when the algae finally finish gorging and die, by sinking to the bottom and smothering them. After that, the shellfish go into a tailspin.

In 2007, the National Oceanic and Atmospheric Administration looked at 99 coastal estuaries and found that 65 percent were moderately to highly "eutrophic" - specifically, overnourished, but in effect, dying.

It's not just the fertilizer we slather on our lawns and gardens. Nitrogen also literally falls from the sky. It's produced when we burn fossil fuels - in electric power plants and in cars, for instance.

The problem is endemic along the Jersey Shore - something to think about this summer as people cross the bay causeways, headed for the beaches.

It's especially bad in Barnegat Bay, ranked among the worst 15 in the national U.S. Geological Survey study.

Part of the reason is that the bay doesn't flush. It has only two narrow openings to the ocean, and scientists have estimated a bucket of water stays in the bay for 70 days.

Another reason is that the soil in the bay's watershed isn't good for growing turf, so people who want pretty lawns have to fertilize the heck out of them. And then all the excess washes into the bay.

Things have become so bad that an advocacy group, Save Barnegat Bay, is pushing Ocean County to pass a fertilizer ordinance, limiting the kinds of fertilizer that can be used, when it can be applied and where - as in not closer than 10 feet to water bodies.

So enough with the fertilizer, please.

As for the rest, there are many ways individuals can cut their contribution to the planet's nitrogen overload.

And yes, there's a calculator. Produced by the Chesapeake Bay Foundation, it's aimed at those who live near the bay, another nitrogen hotspot. But it translates well to other watersheds, scientists tell me, especially coastal estuaries.

As usual, I began the test (at www.cbf.org) with hubris. Surely I wouldn't be a a problem.

Ha! After factoring in my driving habits, my garden's fertilizer regime (I don't fertilize the lawn), even my electricity use and sewer type (I have septic), the dastardly thing concluded that my yearly nitrogen footprint is 20.2 pounds.

Ideally - if I were in the Chesapeake watershed - it should be closer to 8.

The big culprit is my septic system, which the calculator says contributes 13.6 pounds of nitrogen a year. A top-of-the-line municipal sewage treatment plant would lower my input to 3.6 pounds a year.

That's not realistic for me, but the calculator lists plenty of other things I can do - including planting more trees, which soak up nitrogen - factoring in various "what-if" scenarios.

Oof. Guess I'll have to go on a nitrogen diet.