| Anthropogenic Nitrogen (N) fixation has elevated N inputs through atmospheric N deposition to global terrestrial ecosystems, where soil is considered as a major retention pool for atmospherically deposited N in forest ecosystems. However, the effects of plant and soil characteristics on the capacity of soil to retain N are not well understood. A 16-week two-way factorial greenhouse experiment was conducted to test the hypothesis that variation in soil organic matter (SOM) and the presence of a functional plant (Salix dasyclados seedling) will affect (1) N cycling rates in the soil and N leaching losses, and (2) the retention of atmospheric-deposited N (simulated with 15N label) in the integrated plant-soil system. All treatment pots were supplied with 2 mg•week−1 15N in the form of (15NH4)2SO4 from Week 5 to Week 16. The leaching volume and N leaching losses from all the pots were monitored throughout the experiment. At the end of the experiment, the soil total N, soil extractable N, net N mineralization rates, and net nitrification rates were determined. I also measured 15N enrichment (Atomic percentage enrichment, or APE%) in each N pool, including the plant, soil at the end of the experiment, and soil leachate throughout the experiment. Soils with higher percentage of SOM tended to have higher soil N cycling rates and greater N leaching losses, while the presence of Salix dasyclados significantly reduced soil N mineralization rates, nitrification rates, leachate volume, and NH4+-N concentrations in soil solution and soil leachate. However, Salix dasyclados had little effect on the reduction of NO3−-N in soil solution and soil leachate. After Week 16, most 15N added was recovered in the soil pool. Plants significantly increased the retention of 15N in soil, especially in the High-SOM treatments than in the Low-SOM treatments (P<0.001). A large proportion of soil-retained 15N, however, existed as extractable inorganic N, which was significantly higher in the High-SOM/plant treatment than in other treatments (P<0.001). More 15N was recovered in plant biomass in the Low-SOM soil treatment (15.3±3.8%) than in the High-SOM soil treatment (9.2±1.4%) (P=0.154). Significantly higher 15N APE% was also found in every component of the former than the later. While plants reduced the overall N concentrations in the leachate, they increased 15N leaching loss from the High-SOM pots (19.4 vs. 9.8% in plant and no-plant treatments), respectively, but decreased 15N loss from the Low-SOM pots (10.4 vs. 15.0%). Overall 15N recovery in measured pools (soil, plant biomass, and leaching) averaged 85.5% in High-SOM/plant treatment, but only 33% in no-plant pots. The 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 added N. The plant biomass could be more important to retain atmospheric deposited N in the Low-SOM treatment than in the High-SOM treatment. |
