Abstract
Increased anthropogenic nitrogen (N) inputs can alter the N cycle and affect forest ecosystem functions. The impact of increased N deposition depends among others on the ultimate fate of N in plant and soil N pools. Short-term studies (3–18 months) have shown that the organic soil layer was the dominant sink for N. However, longer time scales are needed to investigate the long-term fate of N. Therefore, the soils of four experimental forest sites across Europe were re-sampled ~ 2 decades after labelling with 15N. The sites covered a wide range of ambient N deposition varying from 13 to 58 kg N ha−1 year−1. To investigate the effects of different N loads on 15N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant 15N sink after 2 decades, (2) long-term increased N deposition would lead to lower 15N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on 15N recovery in the soil. The results show that large amounts of the added 15N remain in the soil after 2 decades and at 2 out of 4 sites the 15N recovery levels are higher in the mineral soil than in the organic soil. The results show no clear responses of the isotopic signature to the changes in N deposition. Several environmental drivers are identified as controlling factors for long-term 15N recovery. Most drivers that significantly contribute to 15N recovery are strongly related to the soil organic matter (SOM) content. These findings are consistent with the idea that much of the added 15N is immobilized in the SOM. In the organic soil layer, we identify C stock, thickness of the organic layer, N-status and mean annual temperature of the forest sites as most important controlling factors. In the mineral soil we identify C stock, C content, pH, moisture content, bulk density, temperature, precipitation and forest stand age as most important controlling factors. Overall, our results show that these temperate forests are capable of retaining long-term increased N inputs preferably when SOM availability is high and SOM turnover and N availability are low.
Highlights
In the last six decades, intensification in agricultural practices and fossil fuel combustion have strongly increased both atmospheric nitrogen (N) emission and its deposition to the terrestrial environment on a global scale (Erisman et al 2011; Wessel et al 2013)
For the other two sites the 15N recovery had become higher in the mineral soil than in the organic soil, a significant difference was found only for the high N treatments at al 1998 (AL) (p = 0.006) and at KH (p = 0.01)
The drivers that contributed to the prediction of 15N recovery were mostly related to soil organic matter (SOM) accumulation and turnover processes
Summary
In the last six decades, intensification in agricultural practices and fossil fuel combustion have strongly increased both atmospheric nitrogen (N) emission and its deposition to the terrestrial environment on a global scale (Erisman et al 2011; Wessel et al 2013). Increased anthropogenic N inputs can alter the N cycle and affect ecosystem functions. The impact of increased N input to the environment depends on the fate of N in a specific ecosystem (Niu et al 2016). In N-limited forest ecosystems, increased N deposition. Long-term chronic N deposition may exceed the capacity for N uptake by vegetation and microbes and cause N saturation and N loss from the soil (Aber et al 1989; Lovett and Goodale 2011; Lu et al 2011). Nitrogen saturation may lead to soil acidifications and forest decline and increased N leaching losses can cause eutrophication of freshwaters (Aber et al 1989; Jonard et al 2015). Increased N deposition can contribute to the production of the greenhouse gas N2O
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