Stimulation of soil organic nitrogen pool: The effect of plant and soil organic matter degrading enzymes

- There is a link between forest trees and soil organic matter degrading processes.
- Organic matter degrading oxidative enzymes increase mobilization of soil organic N.
- The presence of plant have same effect on soil N pool than the enzymes alone.
- The sum of alkylamines and nitrate are in the same range in the soil.

The majority of nitrogen (N) in boreal forest soils is bound to soil organic matter (SOM) in forms not readily available to plants. Northern boreal forest ecosystems are often N limited, despite atmospheric N deposition, and the utilization of organic N from SOM is of crucial importance to the site productivity. The effect of microbial produced oxidative SOM degrading enzymes (laccase and manganese peroxidases) and proteases on soil N forms and availability was studied in a pot experiment with or without a Scots pine (Pinus sylvestris L.) seedling. The combination of SOM degrading enzymes and proteases decreased the total soil N content and increased the N losses significantly in the absence of the Scots pine seedlings. The total soil N content also decreased in the presence of the Scots pine seedlings, irrespective of the enzyme treatment. Most of the other N parameters studied were not sensitive to enzyme additions, and differed only between planted and non-planted treatments. Our results show that the alkylamine content of boreal forest soil are at the same levels as nitrate. We showed that SOM decomposition, stimulated by oxidative enzyme additions, is a key step in soil organic N utilization, while proteases alone do not increase N use from SOM. Plants stimulate N losses from SOM highlighting the importance of rhizosphere processes in soil C and N cycling.