Mulch and soil carbon

Many growing strategies in permaculture involve the use of high-carbon mulch, such as wood chip and straw. Among other things, this adds to the levels of soil carbon.

Just how sustainable is this? If we are using wood chip surely we're just moving carbon from one place to another, which is going to skew data on soil carbon (and other nutrients) towards our habitats, while ignoring the consequences elsewhere?

In the case of straw, adding artificial or even organic fertilisers to a field, then moving the straw from their field to our forest doesn't really strike me as wildly sustainable.

Does anyone have any thoughts (better yet numbers) on this? Do we need to rethink mulching and, if so, how?

The way I see it is that while you may need to import some mulch in the beginning it eventually becomes a self regeneration system and can even be used to export this material to extensions of the system. So in answer I see it as non-sustainable if done over a long period BUT as a permaculture forest is a long term project and will generate much more mulch it is sustainable, and justifiable in the short term.

This is a great question for me as I am trying to plant guilds on almost pure sand. It seems to take 4-5 years before trees and shrubs respond to their wood chip mulch and start to flourish. And of course one has to consider the carbon costs of transporting wood chips.

We have to look at the full lifecycle analysis of a practice, which includes in this case the energy required to chip the wood and transport the material. With that said, mulching is a great practice in the early years of rehabilitating a degraded sites. In the long term however we want to produce our own mulch materials right on-site. The best place I've ever seen doing this is Las Canadas in Mexico, where they grow giant biomass grasses and woody legumes to produce compost in their bio intensive areas, and for chop and drop in their food forest. Last fall I planted out a bunch of giant miscanthus grass in my garden to provide mulch for my annual beds. My perennial beds most themselves now that they are more mature, and if you annual crops like grains produce enough of their own mulch from their crop residues, the most annual crops require some mulch from somewhere else.

In the city, it seems like the supply of free wood chip mulch is unlimited. I never have any trouble flagging down a truck and getting them to dump it on my driveway. I live in suburban Los Angeles and there are so many tree trimming crews working on public streets and in the parks, as well as private tree crews working in our neighborhood. I just go out to the backyard and listen for the whine of a chipper somewhere within a half-mile distance, then get in the car and drive till I find them. More often than not, they are thrilled to dump it for free.

When I see how much carbon is loaded into the green colored garbage cans on my street every week (lawn clippings, branches, etc.), it just amazes me. Every bit of that could be piled up somewhere in the back yard of these homes and left to break down naturally. You wouldn't even have to chip it or hugelculture it -- just pile it up somewhere and let the bacteria and fungi do their thing.

If you live in the country, however, getting and spreading that much mulch might be a huge challenge. I've got a big yard and can only handle 4 or 5 truckloads (or so) a year. If you wanted to put down a 6 inch layer of wood chips over a couple of acres, you'd need to invest in a front-end loader and some sort of manure spreader (ala Joel Salatin), but would you have the supply to do so? There are 4840 square yards in an acre. If you put 6 inches of chips down, that would be 1/6th of a yard of chips per square yard. 4840 divided by 6 = 806 yards of mulch. A standard dump truck used to hold 9 yards (hence, the old saying, "The whole 9 yards"). A BIG chip truck might hold 20 yards or so. So you would need about 40 BIG truck loads of chips to cover an acre with 6 inches of chips. That's a LOT of truck loads, and 6 inches of chips break down (at least in my climate) within 6 months. You'd need to do that at least once or twice a year to keep a decent mulch layer.

I'd focus on cover crops.

I am with Eric here. If I can get mulch free of cheap, I'll take it to jump start a system. But long run, unless you have some municipal source, you eventually need to grow your own, even if it is just chop and drop. The only place I differ with Eric is that I have figured out how to grow my own with annuals in a permaculture type system. First by making sure I have loads of grass growing between my crop rows as a living mulch with perennials, and second with off season cover crops.

But don't fool yourself, most mulches don't really add a whole lot of carbon long term in my opinion. Some does get pulled down by worms and other things. But most will decompose away eventually. You'll be surprised how quickly after you stop adding mulch the O-horizon reduces. Few years after stopping you are back pretty close to where you started. So one way or another you need to keep adding that mulch.