The long-term fate of deposited nitrogen in temperate forest soils

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 15 N. 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 15 N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant 15 N sink after 2 decades, (2) long-term increased N deposition would lead to lower 15 N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on 15 N recovery in the soil. The results show that large amounts of the added 15 N remain in the soil after 2 decades and at 2 out of 4 sites the 15 N 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

Dynamics and multi‐annual fate of atmospherically deposited nitrogen in montane tropical forests

“…In addition, this pattern of initial losses in the first few months is similar to that observed in several temperate forest experiments (Figure 4), suggesting that total long‐term ecosystem retention might be determined by the initial loss. A recent long‐term study also suggested that 15 N initially retained would remain in temperate forests while newly deposited N would be lost from the system (Veerman et al, 2020). We speculate that 15 N initially retained after the first few months might be converted to organic forms and then enters the ecosystem's internal N cycling.…”

“…We attribute this to fast litter turnover in tropical forests, resulting in the small capacity of organic soil to retain the added 15 N (Gurmesa et al, 2016; Liu, Yu, et al, 2017; Wang et al, 2018). The 15 N initially retained in the organic soil could be transferred to the mineral soil, or released and assimilated by plants (Veerman et al, 2020; Wessel et al, 2013). In numerous studies in temperate forests, 15 N recovery decreased over time in the organic soil, which was attributed to litter decomposition, physical leaching, or downward transport by soil fauna (Goodale, 2017; Li et al, 2019; Nadelhoffer et al, 2004).…”

“…The mineral soil was still the largest sink for deposited N after 3 years. The long‐term persistence of 15 N in mineral soil has been attributed to the incorporation of 15 N into stable soil organic matter (SOM) pools (Goodale et al, 2015; Perakis & Hedin, 2001; Veerman et al, 2020). Previous studies have demonstrated that inorganic N could be incorporated into the organic N pool through microbial accumulation, condensation of N in microbial enzymes with phenolic compounds, or abiotic reactions of inorganic N with SOM (Fuss et al, 2019; Goodale et al, 2015; Johnson, 1992; Johnson et al, 2000; Lewis & Kaye, 2012; Liu, Peng, et al, 2017), while inputs of high C/N woody debris would also promote the immobilization of 15 N by microbes (Lajtha, 2020).…”

Experimental Design and Interpretation of Terrestrial Ecosystem Studies Using 15N Tracers: Practical and Statistical Considerations

“…This time-course contrasts very much with the rapid disappearance of 15 N in nitrate leached from this forest (Providoli et al, 2005;Schleppi et al, 2017). On the other hand, 15 N available to trees decreases markedly from year to year while tracer in the bulk soil remains quite stable for decades, as observed in this experiment as well as in three other experimental forests across Europe (Veerman et al, 2020). This clearly shows that different N pools with different turnover times are involved.…”

“…Compared with short-term studies with a time scale of days, long-term studies pose some specific challenges and opportunities. The long-term fate of 15 N tracers is especially useful to assess the effect of slow changes, as brought about by atmospheric deposition (Veerman et al, 2020;Wessel et al, 2021) or climate change (Cheng et al, 2019). Interpretation of long-term tracer experiments can be a challenge, but can be improved by the use of models (Currie, 2007).…”