I suspect that grass clippings do take some nitrogen from the soil, as they decompose in the lawn. But the nitrogen doesn't disappear, it ends up in the bacteria and fungi that are decomposing the grass clippings. Eventually, the micro-organisms end up back in the soil, they die, and the nitrogen is released into the soil. If the decomposition of grass clippings feeds microbes that are capable of metabolizing atmospheric nitrogen, you could end up with a net gain of nitrogen in the soil.

You DO end up with a net gain. Decomposing organic matter only results in a temporary tie-up of available nitrogen - it doesn't actually "rob" the soil of nitrogen. Once decomposed, the nitrogen will be released by the soil organisms. Since grasscycling with a mulching mower minces the clippings into very small particles, the decomposition process is rapid and the nitrogen is released quickly. Tests have proven that grasscycling (mulching mowers) results in increased nutrient availability to the lawn, reducing the need to apply other fertiilizers.

Agree gardengal - wasn't there a big study in Michigan university on this - I have to look it up. All I know from experience is the lushest lawns in our neighborhood are the ones that don't have grass clipping bags at the curb!

Taking away the clippings results in a steady loss of matter, nitrogen, carbon and the other micro nutrients in those grass clippings. Leaving them behind recycles them through the system. They break down and the matter is re-used for future growth. Leaving the clippings cannot logically increase the nutrients that the grass uses to grow, it just returns them.

ISTM that my compost pile loves grass clippings. With this in mind I suspect grass clippings are very well liked by the micro-organisms that do the actual breaking down. If it's so good for the organisms in the CP, it's probably equally good for the organisms in the lawn/soil.

I'd only advocate the removal of clippings if the grass was so tall that the clippings caused other harm by "smothering".

Lloyd

Those green grass clippings you mulch mow back into your turf will have lots of Nitrogen in them which will help feed your soil bacteria which will then feed the grass that is growing.

If you want your compost pile to heat up real nice, mix in those high-nitrogen grass clippings.