Cycling and retention of nitrogen in European beech (<i>Fagus sylvatica</i> L.) ecosystems under elevated fructification frequency

• Main Authors: R. Brumme, B. Ahrends, J. Block, C. Schulz, H. Meesenburg, U. Klinck, M. Wagner, P. K. Khanna
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-06-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/18/3763/2021/bg-18-3763-2021.pdf

<p>Atmospheric deposition of nitrogen (N) has exceeded its demand for plant increment in forest ecosystems in Germany. High N inputs increased plant growth, the internal N cycling within the ecosystem, the retention of N in soil and plant compartments, and the N output by seepage water. But the processes involved are not fully understood, notably the effect of fructification in European beech (<i>Fagus sylvatica</i> L.) on N fluxes. The frequency of fructification has increased together with air temperature and N deposition, but its impact on N fluxes and the sequestration of carbon (C) and N in soils have been hardly studied. A field experiment using <span class="inline-formula">¹⁵</span>N-labeled leaf litter exchange was carried out over a 5.5-year period at seven long-term European beech (<i>Fagus sylvatica</i> L.) monitoring sites to study the impact of current mast frequency on N cycling. Mean annual leaf litterfall contained 35 kg N ha<span class="inline-formula">⁻¹</span>, but about one-half of that was recovered in the soil 5.5 years after the establishment of the leaf litter <span class="inline-formula">¹⁵</span>N exchange experiment. In these forests, fructification occurred commonly at intervals of 5 to 10 years, which has now changed to every 2 years as observed during this study period. Seed cupules contributed 51 % to the additional litterfall in mast years, which creates a high nutrient demand during their decomposition due to the very high ratios of C to N and C to phosphorus (P). Retention of leaf litter <span class="inline-formula">¹⁵</span>N in the soil was more closely related to the production of total litterfall than to the leaf litterfall, indicating the role of seed cupules in the amount of leaf N retained in the soil. Higher mast frequency increased the mass of mean annual litterfall by about 0.5 Mg ha<span class="inline-formula">⁻¹</span> and of litterfall N by 8.7 kg ha<span class="inline-formula">⁻¹</span>. Mean net primary production (NPP) increased by about 4 %. Mean total N retention in soils calculated by input and output fluxes was unrelated to total litterfall, indicating that mast events were not the primary factor controlling total N retention in soils. Despite reduced N deposition since the 1990s, about 5.7 out of 20.7 kg N ha<span class="inline-formula">⁻¹</span> deposited annually between 1994 and 2008 was retained in soils, notably at acid sites with high <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">N</mi><mo>/</mo><mi mathvariant="normal">P</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="beae1693920d21781f0a96f1c036b266"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3763-2021-ie00001.svg" width="23pt" height="14pt" src="bg-18-3763-2021-ie00001.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">C</mi><mo>/</mo><mi mathvariant="normal">P</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fdaeee00bef362ec397222599b701784"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3763-2021-ie00002.svg" width="23pt" height="14pt" src="bg-18-3763-2021-ie00002.png"/></svg:svg></span></span> ratios in the organic layers and mineral soils, indicating P limitation for litter decomposition. Trees retained twice as much N compared to soils by biomass increment, particularly in less acidic stands where the mineral soils had low <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">C</mi><mo>/</mo><mi mathvariant="normal">N</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="497f528d2af018b165f67656d390570d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3763-2021-ie00003.svg" width="24pt" height="14pt" src="bg-18-3763-2021-ie00003.png"/></svg:svg></span></span> ratios. These results have major implications for our understanding of the C and N cycling and N retention in forest ecosystems. In particular the role of mast products in N retention needs more research in the future.</p>