permaculture

[Toby Hemenway <toby@patternliteracy.com>]

Robyn Francis wrote:
> Mulches with a hard cellulose structure such as sawdust, wood chips,
> eucalyptus leaves and pine needles have high N demands to break down. Some N 
> will be drawn from air but can also use up N from the top soil layer.
I'm having a hard time figuring out a mechanism for nitrogen to be drawn 
up more than a couple of millimeters, then across an interface, and into 
a fresh mulch layer. Rain and gravity will carry N downward. 
Volatilization might cause nitrogenous gases to rise, but mulch won't 
have any effect on that. If anything, a mulch layer would slow that down 
by keeping the soil cool. Movement of N upward seems to require an 
organism that gets its N in the soil, takes it upward into the mulch, 
and then eats the mulch carbon to balance the nitrogen, or vice versa. 
What would move more than a couple of mm? Bacteria won't move that far. 
Fungal hyphae can transport nutrients, but it would be unusual to have a 
fungus with intact mycelial strands that cross the interface, from 
nicely decomposed earth into rough woody mulch. Insects or worms? Maybe, 
but it seems weird. If someone knows of a mechanism, I stand eager to be 
enlightened.

I've always understood that the only place there is N lockup with mulch 
is right at the mulch/soil interface. The mulch doesn't have much 
nitrogen in it to start with (400 to 1 C/N or so), so what happens in 
the mulch itself can't be called locking up, just absence. The interface 
is where the most active breakdown of woody mulch initially happens, 
until the wood's lignin is broken down enough for large-scale 
decomposition to start within the body of the mulch. I've never noticed 
N-starvation in wood-mulched plants (and I use woody mulch almost 
exclusively. Quite the contrary--plants love it. But like I say, I'll 
listen to an alternative explanation.

Toby
http://patternliteracy.com



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