Enhanced growth of trees under elevated atmospheric CO concentration (‘CO fertilisation’) can potentially reduce a fraction of anthropogenic CO emissions but is anticipated to become progressively constrained by nitrogen (N) limitation in temperate ecosystems. However, it is believed that this constraint may be mitigated if trees under elevated CO (eCO) prime microbial activity in their rhizosphere to release available N. We assessed whether mature trees under eCO regulate N availability in their rhizosphere to meet increased N demand. We hypothesized that eCO primes N mineralization in the rhizosphere while reducing N losses through nitrification and denitrification. This study was conducted in a mature English-Oak-dominated temperate forest in central England, in the sixth year of Free Air CO Enrichment (FACE). In the summer of 2022, we measured N transformations, enzyme activities, and nutrient pools in the rhizosphere and bulk soil of the organic layer (0–7 cm) under laboratory conditions. While the rhizosphere was found to be inherently more active (i.e. positive N priming) than the bulk soil, the effect of eCO were not consistently stronger in the rhizosphere. Available soil N, dissolved organic carbon and microbial biomass were enhanced under eCO in bulk and rhizosphere soils. Net N mineralization was enhanced under eCO in the bulk and rhizosphere soils while leucine aminopeptidase activity, associated with organic N depolymerization, was enhanced solely in the rhizosphere. Despite higher C and N availability creating potential hot spots, nitrification was reduced under eCO and denitrification remained unaffected in the rhizosphere, demonstrating a more efficient conservation of N under eCO. Our findings demonstrate that eCO stimulates N-mining and reduce N losses in the rhizosphere. Furthermore, the tenfold difference in N turnover rates between rhizosphere and bulk soils suggests that expanding rhizosphere mass from increased root biomass may help trees under eCO to meet higher N demand.

中文翻译：

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在二氧化碳浓度升高的情况下，根际在提高成熟温带森林氮利用率方面的作用


在大气二氧化碳浓度升高的情况下促进树木生长（“二氧化碳施肥”）可能会减少一部分人为二氧化碳排放，但预计将逐渐受到温带生态系统中氮（N）限制的限制。然而，人们相信，如果树木根际的二氧化碳（eCO）主要微生物活性升高以释放可用的氮，则这种限制可能会得到缓解。我们评估了eCO下的成熟树木是否调节其根际的氮可用性以满足增加的氮需求。我们假设 eCO 促进根际氮矿化，同时通过硝化和反硝化减少氮损失。这项研究是在英格兰中部一个以英国橡树为主的成熟温带森林中进行的，当时正值自由空气二氧化碳富集 (FACE) 的第六年。 2022 年夏季，我们在实验室条件下测量了根际和有机层（0-7 厘米）的土壤中的氮转化、酶活性和养分库。虽然发现根际本质上比大块土壤更活跃（即正氮引发），但 eCO 的影响在根际并不总是更强。在 eCO 作用下，大块土壤和根际土壤的有效土壤氮、溶解有机碳和微生物量均得到提高。在 eCO 作用下，大块土壤和根际土壤中的净氮矿化得到增强，而与有机氮解聚相关的亮氨酸氨肽酶活性仅在根际土壤中得到增强。尽管较高的碳和氮可用性产生了潜在的热点，但在 eCO 下硝化作用有所减少，而根际反硝化作用并未受到影响，这表明 eCO 下可以更有效地保存氮。 我们的研究结果表明，eCO 可以刺激氮的开采并减少根际的氮损失。此外，根际和土体土壤之间氮周转率的十倍差异表明，通过增加根生物量来扩大根际质量可能有助于 eCO 下的树木满足更高的氮需求。