High Carbon materials do not "suck" Nitrogen from your soil but if that high Carbon material is worked into the soil the soil bacteria will get busy digesting it and they will utilize all the available Nitrogen to do that and concentrate on digesting that material and not on feeding your plants. But once they die off that Nitrogen they used will be put back into your soil for future use. Since my sandy soil, in the higher areas, drains so well I use shredded oak leaves as a mulch on the beds the Iris are planted in, but not in the lower areas where the high water table has soil moisture nearer the surface, until in mid July (usually) when the water table drops way below the roots zone.

Suzie- I don't know if Comet will work on all kinds of rot. But the kind I get is as described by klavier above that begins at the base of the crown. Probably any bleach treatment would work, but the grittiness of the Comet may help it stick to the rhizome and help dry it out. I scrape all the soft parts of the rhizome off with my fingernail and even some of the firm parts ( you can use a knife or spoon or whatever, but I like to feel with my fingernail when I'm hitting firm rhizome). I then cover the whole rhizome top and bottom with Comet (using my fingers to make sure all the scraped parts get covered). I cover the ground around the rhizome with it also out to a couple of inches. I check it every few days and may have to scrape additional rot and reapply the Comet. Or if it rains, I reapply the Comet. I also dig a little ditch around the rhizome so if it does rain, no run-off will run over the rhizome (mine are on a slope). I'm trying Laetitia's method right now with a new one that's rotting. It's been out of the ground drying for a few days, coated with Comet. When there's no chance of rain for a few days, I'll put it back in the ground. Neil

Based on what I've read here, mulching in the winter in an invitation to rot. This link has information. There were a few more hits when I searched this forum for "mulch'. This information on borers came from another post. Life History Adults (moths) emerge in September and October from the soil, mate, and deposit eggs on the dead leaves, particularly those that are dried and crinkled. Eggs usually are laid in crevices or rolled areas of the leaves, in groups of three to five or more. A single female moth is capable of depositing up to 1,000 eggs in her lifetime. The egg overwinters and begins to develop in early spring. Eggs hatch during April and May and the young larvae begin to feed on young, rapidly growing iris leaves. After feeding on leaf surfaces, causing scars from which sap runs down the leaf, the larvae assume a boring habit, entering the leaves and gradually mining downward. Leaf mining injury is characterized by dark, water-soaked areas or slits in the leaves. Some chewing also occurs on leaf margins. When about half-grown (one-half to three-fourths inch long), iris borers reach the rhizomes, where they feed until late July or early August. As the borers feed, bacteria infect the rhizome, initiating the soft rot. When fully grown, iris borer larvae are about 2 inches long, cylindrical, smooth, and plump with whitish to pinkish bodies and dark brown heads. Shortly after this stage is reached the larva leaves the iris rhizome and constructs a "cell" in the soil where the pupa stage is passed. The pupa is a non-feeding transition stage between the larva (or worm) and the adult moth. Pupae are dark brown to black and are usually found in the top 2 inches of soil. After two to three weeks in the pupa stage, the iris borer moth emerges and the cycle is completed when the eggs are deposited. Moths are typical millers--dark grey to brown, with a wingspan of about 2 inches. There is only one generation per year. Prevention One key to iris borer prevention is removal and destruction of the previous year's dead foliage before April 1. This sanitation practice will reduce the problem by destroying overwintering eggs in the residues. Also on prevention.................... You can significantly reduce the likelihood of a large borer infestation by keeping all dead and diseased iris leaves and other vegetative matter cleaned out of your iris beds, esp in late summer/early fall when borer moths are looking for such materials on which to lay their eggs. Another thorough garden clean-up in early spring with help to further reduce the number of surviving borer eggs before they can hatch. Burn or discard all garden cleanings; DO NOT COMPOST! Here is a link that might be useful: mulch

During all my horticultural years I have never dealt with bacterial soft rot (thank goodness!) I had to search the subject which you should also do. You will note that it develops when excess watering takes place in temperatures between 30-90 degrees. In your reading you will find suggested remedies. If this were my problem I would load my hose-end sprayer with full strength hydrogen peroxide (located in any drug store), set the dial for 5 ounces and carefully saturate the soil in that area of the garden while we are experiencing these present high temperatures. Or fill the hose-end sprayer with full strength Clorox and set the dial for 3 tablespoons. (The former method would be my choice.) Leave infected bed unplanted until next spring. In your reading you will note certain plants resistant to the rot. It would make sense to plant some of those in that location for a few years. And remember not to over water!