Does anyone know if in-situ composting is as effective as top dressing for garden production? For the past six months I have been adding ripped cardboard and cut brown paper to my kitchen scraps then I bury the mix in trenches around the garden and yard. It is amazing how quickly the waste breaks down. The number of worms in the garden has increased significantly. I just can't find any articles indicating if top dressing is still needed/recommended in addition. I think I am missing the 'mulch' factor with this method but it sure is a lot cleaner than previous attempts at the free standing method.
What you are doing is sometimes called trench composting. In some ways it is more effective than composting in a compost bin, because the soil organisms are there to break it down and there is likely to be much less nitrogen loss. Plus you don't have to do the work of turning it! The downside is that the nutrients and carbon are in the trenches, not in the soil in general. However, worms and other soil organisms will move them around, and roots will grow into them.
Top dressing for soil improvement depends on nutrients leaching down and soil organisms pulling bits of material into the ground. If the top dressing is evenly spread the resulting benefits will also be evenly spread. If one area needs more help it can be given more attention. Top dressing is often not really effective as a mulch, it depends on what the top dressing is and how thickly it is applied and how fast it breaks down. Straw, for example, is a good mulch because it doesn't break down fast and basically lasts the season (at least it lasts for my short season), but it isn't doing much for the soil while it is being a mulch.
Every garden is its own little unique ecosystem. Look at your garden and see how the trench compositing is benefiting it overall. If it is working, great. If some areas need more, then top dress there or do more trench composting there. For mulch why does your garden need mulch? What is the function of the mulch? Can you get the same result with a living mulch? Or planting closely enough that your plants form a closed canopy?