Trouble with nitrogen loss. To mulch or not mulch?

I tested my soil. Both the original soil and beds have plenty of potassium, phosphorus, good pH, but nitrogen depleted.
Reason: there is continuous rainfall nearly every day. In winter, soil is mostly bare and exposed to thaws, rain and winds.

Soil tests have determined that only if a bed has a thick grass mulch, then nitrogen levels are kept good. Nitrogen fixing species help but the increase is not as large as for a mulch.

However, I do like to practice Fukuoka´s style, with vegetables self-sowing themselves for the next year. And these volunteers cannot germinate properly through a thick mulch, or when they fall over it. Also late summer green manure does not germinate in such a thick mulch, unless I lay new soil over the mulch.

And also the mulch cause a big problem with slugs (climate is too wet). Nevertheless bare soil is not good, due to erosion and heavy nitrogen loss.

It is mid summer now in Iceland, crops are lush and flowering, yet everything will die down within 2 months and I need to find a permanent solution to avoid nitrogen loss during the long winter season (about 9 months, from September to May). Some beds are raised beds, some are huegelkultur, some are flat beds, all share the same problem.

What should I do?

What is the organic content of your soil like? I'd be thinking longer term about doing everything you can to build your soil - increased organic matter should help retain nitrogen and other nutrients.

It maybe that in your situation fukuoka style selfseeding of annuals just isn't viable, or at least not until your soil issues are resolved. Can you establish a good mix of perennials in your climate, or are you limited to annuals? What about establishing some dedicated mulch crops adjacent to your planting areas ie comfrey patches to mine for that leeched nitrogen? Alfalfa for nitrogen fixing and mulch materials?

The organic matter content of my soil is rather high, I have been laying cut lupins, grass, compost, banana peels, over the last 3 years, since I start. However soil always tends to be clayish and suffer from compaction (it was a lawn). Liming helped quite a bit as it made the clay more friable. I was afraid of raising the pH (which was already 7.0 in the original soil) but it seems that the pH is now 6.5 because not only of the organic matter/humus content but some peat moss on it, even after liming it!

I am unsure of deep digging because I am afraid of reducing the nitrogen levels by doing so. Or perhaps it could be just the thing to do.

Independently of deep digging my beds, I will ensure that most of beds will be heavily mulched with organic matter this year, perhaps with a tiny cover of both soil and mulch, so that seedlings could still germinate there. In some spots I will not mulch and I will let green manure do its job. In the spots where nitrogen was low this year, I will plant mostly nitrogen fixing species (broad beans and peas) next year. But I will plant always a few of them in the other beds as well.

And I will try the comfrey/ alfalfa strategy, though i am not sure whether they survive the winter here. At least i will plant a native clover, though it will become a sort of a weed with time. Hopefully it will form a balance with the self-sowing vegetables over the years.

There is a perennial lupin that grows everywhere, but it is an invasive and quite agressively spreading species, and so I am a little concerned in planting it right in the middle of my beds, but it grows everywhere just a few meters away. But I do use it as to mulch, and make compost.

I have been introducing perennials to some beds too. So in those spots I do not want to change much the soil, at least next to the plants.

I'm not sure of the legal implications of gathering sea bird guano in Iceland, but that along with seaweed might help. Since your losses happen outside of the growing season through leaching, you could gather rich pioneer species, compost before the cold weather and then rake it all into a nice high pile where it would be covered in a tarp to prevent soaking in the winter rain. In the spring you'll have a nice rich layer of dark colored material to help with warm up.

You mentioned the soil being clayish. Is it clay or is it very fine volcanic ash ? Clay is good at retaining nutrients while ash is known to leach badly.