Fate of simulated atmospherically deposited nitrogen in a plant-soil mesocosm

Soil is considered as a major retention pool for atmospherically deposited nitrogen (N) in forest ecosystems. A 16-week two-way factorial greenhouse experiment was conducted to test the hypothesis that the variation of soil organic matter (SOM) and the presence of a functional plant (Salix dasyclados seedling) will affect the retention of deposited N (simulated with weekly input of ¹⁵NH4⁺). Most ¹⁵N added was recovered in the soil at the end of the experiment. Plant significantly increased soil retention of ¹⁵N, especially in SOM-rich organic soils (P<0.001). A large proportion of soil-retained ¹⁵N, however, existed as extractable inorganic N (P<0.001). More ¹⁵N was recovered in plant biomass in mineral soil treatment (15.3±3.8%) than in organic soil treatment (9.2±1.4%) but the difference was not significant (P=0.154). While plants reduced the overall N concentrations in leaching, we found they increased ¹⁵N loss from the organic soil pots (19.4±3.1 vs. 9.8±4.7% in plant vs. no-plant treatments) but decreased ¹⁵N loss from the mineral soil pots (10.4±3.1 vs. 15.0±5.1%). Overall ¹⁵N recovery in measured pools (soil, plant biomass, and leaching) averaged 85.5% in the organic soil-plant treatment, but averaged only 33% in the no-plant pots. Our results suggest that the interaction between soil characteristics and functional plants has strong effects on the retention of newly deposited N. Plants increase N retention in soil, in addition to the direct uptake of newly-added N. The contribution of deposited N to plant could be more important in low-SOM soils than in high-SOM soils.