Contrasting potential for biological N₂ fixation at three polluted central European <i>Sphagnum</i> peat bogs: combining the ¹⁵N₂-tracer and natural-abundance isotope approaches

<p>Availability of reactive nitrogen (N<span class="inline-formula">_r</span>) is a key control on carbon (C) sequestration in wetlands. To complement the metabolic demands of <i>Sphagnum</i> in pristine rain-fed bogs, diazotrophs supply additional N<span class="inline-formula">_r</span> via biological nitrogen fixation (BNF). As breaking the triple bond of atmospheric N<span class="inline-formula">₂</span> is energy-intensive, it is reasonable to assume that increasing inputs of pollutant N<span class="inline-formula">_r</span> will lead to BNF downregulation. However, recent studies have also documented measurable BNF rates in <i>Sphagnum</i>-dominated bogs in polluted regions, indicating the adaptation of N<span class="inline-formula">₂</span> fixers to changing N deposition. Our aim was to quantify BNF in high-elevation peatlands located in industrialized central Europe. A <span class="inline-formula">¹⁵</span>N<span class="inline-formula">₂</span>-tracer experiment was combined with a natural-abundance N-isotope study at three <i>Sphagnum</i>-dominated peat bogs in the northern Czech Republic in an attempt to assess the roles of individual BNF drivers. High short-term BNF rates (8.2 <span class="inline-formula">±</span> 4.6 g N m<span class="inline-formula">²</span> d<span class="inline-formula">⁻¹</span>) were observed at Malé mechové jezírko, which receives <span class="inline-formula">∼</span> 17 kg N<span class="inline-formula">_r</span> ha<span class="inline-formula">⁻¹</span> yr<span class="inline-formula">⁻¹</span>. The remaining two peat bogs, whose recent atmospheric N<span class="inline-formula">_r</span> inputs differed from Malé mechové jezírko by only 1–2 kg ha<span class="inline-formula">⁻¹</span> yr<span class="inline-formula">⁻¹</span> (Uhlír̆ská and Brumiště), showed zero BNF. The following parameters were investigated to elucidate the BNF difference: the NH<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="51ca01690260423140b5b0de9583232a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="soil-9-623-2023-ie00001.svg" width="8pt" height="15pt" src="soil-9-623-2023-ie00001.png"/></svg:svg></span></span>-N <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="265e2a7d42d09da6c1e252e5649f9787"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="soil-9-623-2023-ie00002.svg" width="8pt" height="14pt" src="soil-9-623-2023-ie00002.png"/></svg:svg></span></span> NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="d615913ec88b34ee0c05b0f0374db64d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="soil-9-623-2023-ie00003.svg" width="9pt" height="16pt" src="soil-9-623-2023-ie00003.png"/></svg:svg></span></span>-N ratio, temperature, wetness, <i>Sphagnum</i> species, organic-N availability, possible P limitation, possible molybdenum (Mo) limitation, SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="820c2a883eb6831c3f3dfbf9fe5018ac"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="soil-9-623-2023-ie00004.svg" width="13pt" height="17pt" src="soil-9-623-2023-ie00004.png"/></svg:svg></span></span> deposition, and pH. At Malé mechové jezírko and Uhlír̆ská, the same moss species (<i>S. girgensohnii</i>) was used for the <span class="inline-formula">¹⁵</span>N<span class="inline-formula">₂</span> experiment; therefore, the host identity could not explain the difference in BNF at these sites. Temperature and moisture were also identical in all incubations and could not explain the between-site differences in BNF. The N : P stoichiometry in peat and bog water indicated that Brumiště may have lacked BNF due to P limitation, whereas non-detectable BNF at Uhlír̆ská may have been related to the 70-fold higher SO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="af7843922aebc745671236cec92f9569"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="soil-9-623-2023-ie00005.svg" width="13pt" height="17pt" src="soil-9-623-2023-ie00005.png"/></svg:svg></span></span> concentration in bog water. Across the sites, the mean natural-abundance <span class="inline-formula"><i>δ</i>¹⁵</span>N values increased in the following order: atmospheric deposition (<span class="inline-formula">−</span>5.3 <span class="inline-formula">±</span> 0.3 ‰) <span class="inline-formula"><i>&lt;</i></span> <i>Sphagnum</i> (<span class="inline-formula">−</span>4.3 <span class="inline-formula">±</span> 0.1 ‰) <span class="inline-formula"><i>&lt;</i></span> bog water (<span class="inline-formula">−</span>3.9 <span class="inline-formula">±</span> 0.4 ‰) <span class="inline-formula"><i>&lt;</i></span> atmospheric N<span class="inline-formula">₂</span> (0.0 ‰). Only at Brumiště was N in <i>Sphagnum</i> significantly isotopically heavier than in atmospheric deposition, possibly indicating a longer-term BNF effect. Collectively, our data highlight spatial heterogeneity in BNF rates under high N<span class="inline-formula">_r</span> inputs as well as the importance of environmental parameters other than atmospheric N<span class="inline-formula">_r</span> pollution in regulating BNF.</p>