We are planting three 4x8 raised beds at the local library where I work. I can get pine straw for free, but my husband feels like the wheat straw is superior. He thinks it is possible that over time the pine straw could create too much acidity in the beds.
The amount of acidity that pine straw adds to the soil is just about unmeasurable --- it's barely anything. If you were composting hundreds of yards of pine straw and then adding that into your soil, perhaps you'd see the PH move, but just putting 3 inches on top of a raised bed will not move the needle much (if any). Do you know what your soil PH is? I've got heavy clay soil, so it's already a bit on the alkaline side of things. Pine straw, if it does raise the acidity, would only be a friend in my case.
All things considered, I like working with pine straw.
1. It's short leaf length makes it easier to move around than longer wheat/oats/rice/barley straw.
2. I have no hard data to confirm this, but it would seem to me to be more nutritionally dense for your soil than regular straw. Trees are the ultimate dynamic accumulators, mining the deep soil for nutrients. It only stands to reason that deep-rooted plants would accumulate more minerals than a shallow-rooted, short lived plant like wheat, barley, oats, etc. Those nutrients would be found in the bio mass that they produce: in this case, the pine needles or the wheat stalk.
3. Its free. You can rake up the pine straw for free, whereas around here, a bale of straw is 8 to 10 bucks (I live in Los Angeles county, so everything like that would have to be trucked-in from afar). In terms of your carbon footprint, if you've got a local supply of pine straw, that's better than hauling in bales of wheat straw from some wheat producing location (Kansas?).
4. I like the look of pine straw: a nutty brown color. It lays down so nicely and doesn't blow around or move much once you've set it in place.
5. The worms go crazy for pine straw in my garden, whereas where I grow oats as a winter cover crop and then chop and drop it in spring, it doesn't get eaten by the worms nearly as quickly. So you can take that as a positive or a negative. If you want your mulch to feed the soil and the soil food web, in my experience, pine straw is superior. If you want it to lay there on the surface longer and knock down weeds as well as retain moisture, grain straw appears to have a longer shelf life. However, I'd argue that as the worms incorporate that pine straw humus into the soil, water retention goes up. Carbon in the soil (once it's passed through the digestive tract of a worm) becomes an amazing sponge to infiltrate and hold moisture. Eventually, even grain straw will break down and hopefully the worms will carry it down into the soil profile.
I've used both, and of course ANY mulch is better than no mulch at all. Perhaps try a bit of each and see what you like better. If you've got more than one bed, split it 50/50 and see what you like better.
"The rule of no realm is mine. But all worthy things that are in peril as the world now stands, these are my care. And for my part, I shall not wholly fail in my task if anything that passes through this night can still grow fairer or bear fruit and flower again in days to come. For I too am a steward. Did you not know?" Gandolf
I would hypothesize the main benefit from grass/grain straw lies in how the hollow stems would aerate your soil as they incorporate. I would not worry about acidity with pine needles or conifer wood in mulch or hugel beds, they are made more ph neutral by endemic fungal inoculants in the rotting wood.
This is all just my opinion based on a flawed memory