Data from: Soil acidification exerts a greater control on soil respiration than soil nitrogen availability in grasslands subjected to long-term nitrogen enrichment

Terrestrial ecosystems worldwide are receiving increasing amounts of biologically reactive nitrogen (N) as a consequence of anthropogenic activities. This intended or unintended fertilization can have a wide-range of impacts on biotic communities and hence on soil respiration. Reduction in below-ground carbon (C) allocation induced by high N availability has been assumed to be a major mechanism determining the effects of N enrichment on soil respiration. In addition to increasing available N, however, N enrichment causes soil acidification, which may also affect root and microbial activities. The relative importance of increased N availability vs. soil acidification on soil respiration in natural ecosystems experiencing N enrichment is unclear. We conducted a 12-year N enrichment experiment and a 4-year complementary acid addition experiment in a semi-arid Inner Mongolian grassland. We found that N enrichment had contrasting effects on root and microbial respiration. N enrichment significantly increased root biomass, root N content and specific root respiration, thereby promoting root respiration. In contrast, N enrichment significantly suppressed microbial respiration likely by reducing total microbial biomass and changing the microbial community composition. The effect on root activities was due to both soil acidity and increased available N, while the effect on microbes primarily stemmed from soil acidity, which was further confirmed by results from the acid addition experiment. Our results indicate that soil acidification exerts a greater control than soil N availability on soil respiration in grasslands experiencing long-term N enrichment. These findings suggest that N-induced soil acidification should be included in predicting terrestrial ecosystem C balance under future N deposition scenarios.

受人类活动影响，全球陆地生态系统接收的生物活性氮（biologically reactive nitrogen）总量持续攀升。这类有意或无意的氮素施肥行为，会对生物群落乃至土壤呼吸（soil respiration）过程产生广泛影响。以往研究普遍认为，高氮有效性诱导的地下碳分配（below-ground carbon allocation）减少，是氮富集影响土壤呼吸的核心机制。然而，氮富集不仅提升了土壤有效氮含量，还会引发土壤酸化，进而可能对根系与微生物活动产生影响。但在发生氮富集的自然生态系统中，提升的氮有效性与土壤酸化二者对土壤呼吸的相对重要性，目前尚未明确。本研究于内蒙古半干旱草原开展了为期12年的氮富集实验，以及配套的为期4年的酸添加对照实验。研究结果表明，氮富集对根系呼吸与微生物呼吸产生了截然相反的调控效应。氮富集显著提升了根系生物量、根系氮含量与比根呼吸速率（specific root respiration），进而促进了根系呼吸。与之相反，氮富集显著抑制了微生物呼吸，其潜在机制可能为降低微生物总生物量（microbial biomass）并改变微生物群落组成（microbial community composition）。氮富集对根系活动的影响同时受土壤酸化与有效氮含量提升的共同调控，而对微生物活动的影响则主要源自土壤酸化，这一结论通过酸添加实验的结果得到了进一步验证。本研究结果显示，在长期经受氮富集的草原生态系统中，土壤酸化对土壤呼吸的调控作用要强于土壤氮有效性。上述研究结果提示，在未来氮沉降（N deposition）情景下预测陆地生态系统碳平衡时，应当将氮诱导的土壤酸化因素纳入考量范畴。