How fast any type of mulch might be digested will depend on how active a Soil Food Web you have as well as the type of material presented to them. The smaller the particles are the faster they will be digested, so a fine shredded bark mulch could be digested much faster then a course lumpy wood chip mulch. When used as mulch no high carbon material, such as bark fines or wood chips, should cause the soil Nitrogen levels to be tied up in the digestion of that mulch, that digestion should be a relatively slow process compared to what tilling the same material into the soil would be. I have not seen soil N depletion in any beds that I have mulched to the contrary I have seen plants that I have mulched with high carbon material turn a deeper green and grow much better after mulching because the soil is now evenly moist and the soil bacteria are bette able to feed the plants what they need for proper growth. Any vegetative material used for mulch will, eventually, supply organic matter to the soil as the Soil Food Webs works on and digests that material.

Pine bark is rich in lignin and the lipid suberin. Both resist the efforts of micro-organisms to cleave the hydrocarbon chains the bark is made of. Though heartwood and sapwood contain a fair amount of lignin, they are more cellulose than anything, and break down quickly, which causes N immobilization and a more rapid collapse of soil structure, not a desirable characteristic for bonsai. Other 'forest products' mixed into the bark can also be a problem. Hardwood bark can bring high soluble salts levels and a pH spike during the composting process, so using a product that is as close to 100% pine or fir bark as you can find is to be desired. I have ferreted out multiple sources for bark, so I have lots of options. I grow my short term plantings, like the garden display containers & veggies in a mix of 5 parts partially composted pine bark and one part each of sphagnum peat and perlite. I often find myself adding a little more perlite if the bark is very fine, but most have the opposite problem,. which is finding bark fine enough or bark that is partially composted. It's not necessary that the bark is partially composted; however, I just prefer it for it's water retention. For bonsai and all my long term plantings, I use equal parts by volume of Screened Turface Crushed granite - grower size, or #2 cherrystone and 1/8-1/4" fir or pine bark. Al The 5:1:1 mix I use is in the center here, fir bark at the top, and various other uncomposted pine products at 3,6,& 9: and this just lifted future bonsai is in the 1:1:1 gritty mix: Most people who look at the gritty mixture think you could never grow anything in it, but I guarantee that if you follow a few simple guidelines, that it will outperform (by FAR), peat/coir/compost-based soils. It's great for long term plantings like houseplants, cacti, succulents, trees, shrubs, because it retains it's structure and guarantees aeration indefinitely. The planting being root-bound is a factor long before the thought of soil collapse would even cross your mind. Al

There are pros and cons of each material. By the way, do you mean pine bark chips or the shredded wood that is a by-product of tree companies? (I've never heard of 'shredded' pine bark, is why I ask.) Anyway, there is no question that the wood product lasts much longer....pine bark can last for years without breaking down. Shredded wood decomposes faster than the bark, but will return more elements to the soil as it does so. Many of the tree companies are now using a different kind of chipper that creates very attractive little chips rather than that shredded stuff. Very nice looking. A real plus about the mixed wood chips is that they can often be had for free, as tree companies are very happy to find a place to donate their chips to, rather than pay to have them accepted into a landfill. This is my preferred mulching material, by far. Pine straw deteriorates quite rapidly in hot sun, first losing its color before it turns into dust. Pine straw needs to be obtained when it is fresh or the life span will be shortened even more. Pine straw adds organic compunds to the soil as it degrades, as do all of these materials, but they may be of limited value to the growing plants. Pine straw may need to be refreshed twice a year, which is a pain in the pocket book. Pine BARK lasts longest of all, since it is composed of lignins that even microorganisms can't break down. It maintains its color and consistency for a long time. My personal preferrance, after many years as a professional and now as a (fussy) homeower is the mixed wood chips from tree companies. One good application can last for up to three years before needing a top dressing, in my experience. Probably less in your climate. All three are great at buffering the soil from excessive heat, moisture loss, freezing temperatures, etc.

I think the shredded mulch may have retained too much water over the winter rains anyway - I had to remove some from around my lavenders which were getting moldy, the drainage is still poor in many places. Yes the kitties are lucky and happy to have a safe outdoor space now - they used to have a screened porch in our rental cottage for years, then we bought a condo which had no outdoor area. They definitely missed watching birds and squirrels. Here they are enjoying their "catio" planted with herbs sages and other cat-safe plants :) Some of these were taken before I added the mulch, when plants were pretty new last summer. Now they have grown but I'm battling whiteflies on most of it - sticky traps have not been very effective :(