Human activities have doubled the loading of ecosystems with reactive nitrogen (N) contributing to many serious problems such as eutrophication, climate change and pollution. As most anthropogenic reactive N is created to supply plant-available N in agriculture, one of the ways to mitigate the situation is to improve N use efficiency in crop production. It is also potentially more sustainable to supply N to crops using biological N fixation (BNF) rather than synthetic N fertilisers, because BNF does not rely on energy from fossil fuels to create plant-available N. Soil pH affects N transformations (e.g. nitrification is generally slower in low pH), but pH effects during decomposition of green manure and pH interactions with physico-chemical properties of the plant material are not well known. Here, effects of soil pH on N release, mineral-N dynamics and N2O emissions during plant material decomposition were studied. One of the objectives was to establish if regulation of soil pH could be used to manipulate N supply from green manures to crops. It was the first time such study was conducted using a long-term pH gradient (Woodlands Field, Craibstone, Aberdeen, UK). This enabled to avoid short-term effects of pH change on soil biochemical processes and confounding effects of other variables, such as soil texture and organic matter content, which vary when soils from different locations are used. Field and laboratory experiments showed that soil acidification is unlikely to be useful as a means regulate N supply from green manure (e.g. to reduce risk of nitrate leaching) as it did not significantly affect N release and nitrate dynamics. Comparison of different green manure species showed that tannin-rich plant material and purified tannins can interact with soil pH affecting soil microbial community composition and N2O emissions, but the effects were not related causally, which warrants further investigation.