I had what I thought was tied up nitrogen issues in my garden (I think due to new raised bed on cardboarded lawn in one area, and new bed made of turned over sod, just enough compost to plant seeds and then mulched heavily with straw.)
In both areas, plants were starting to get yellow green. I side-dressed with coffee grounds from Starbucks, and when that helped but not enough, I got out the heavy guns and mist-sprayed with fish-emulsion (eww stinky -- I think it works better to do this in the evening after sun no longer touches the plants. It also keeps from attracting flies. The morning dew helps to dissipate the concentration on the plants and it's not so "strong" by next day) as well as kelp solution, and finally with compost tea. The fish emulsion and compost tea were also used in watering can to water the roots. (Not all on the same day -- this was a week-long regimen) All the plants looked MUCH better by 3rd day. I've since side-dressed the raised bed veggies with compost mixed with alfalfa pellets, although I still have to do that for the other bed, and I plant to go get some more coffee grounds.
Now, I was RE-READING your post, and noticed you said you mulched heavily with the paper *LAST YEAR* and *TILLED IN THE PAPER* with horse manure *WHEN YOU PLANTED* Does that mean that the shredded office paper was still INTACT this spring? This begs the question -- what kind of papers? and what kind of ink were the papers printed with? If water-soluble ink-jet on regular white copy/printer paper they should have broken down shouldn't they? However, many ink-jets use non water-soluble ink - can't have legal documents washing away with a little rain.... If they are being printed with laser printers that use fine plastic powder or thermal transfer printer that use micro-thin plastic film melted onto the paper, and the papers were heavily printed, then you essentially have coated paper and that would explain them not decomposing.... There IS also the dioxin issue with bleached white paper.