The positive effects of afforestation in karst rocky desertification areas are often limited by soil nitrogen (N) availability. However, how plant N limitation evolves in response to prolonged afforestation as well as the mechanisms underlying plant N limitation and soil N availability are poorly understood. In this study, plant leaves and soil samples were collected from cropland, plantations 10, 20 and 40 years after their establishment on abandoned cropland in a karst rocky desertification area. The natural succession of shrubland served as the control. Foliar N/phosphorus (P) stoichiometry was used to evaluate plant N limitation, and the N concentration and δN value of foliar and soil were measured to evaluate the changes in soil N availability. Soil N transformation rates were also determined, using N tracing technique to elucidate the mechanism underlying soil inorganic N availability. The plant community-level foliar N/P ratio (13.5) was lower than 14 following 10-y afforestation, indicating that plant growth is predominantly N limited. With the prolonged afforestation, the plant community-level foliar N/P ratio increased to 14.0–15.5, suggesting decreased plant N limitation. This could be attributed to increased soil N availability confirmed by the raises in the N concentration and δN of plant community-level foliar and soil δN with prolonged afforestation. Compared to cropland, afforestation increased the soil inorganic N supply capacity, by stimulating organic N mineralization, heterotrophic nitrification and the release of adsorbed ammonium, more obviously with prolonged afforestation. The rates of mineralization, heterotrophic nitrification and the release of adsorbed ammonium correlated significantly and positively with the plant community-level foliar N concentration, N/P ratio, and δN, indicating that a larger soil inorganic N supply alleviates plant N limitation. This effect can be attributed to the increases in organic matter content, bacterial and fungal abundances and soil structure improvement following afforestation, all of which facilitate mineralization and heterotrophic nitrification. However, plant N limitation after 40 years of afforestation still exists, suggesting that monoculture afforestation may need to be adapted to maximize soil N availability and to alleviate plant N limitation in karst rocky desertification areas.

中文翻译：

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亚热带喀斯特地区造林增加土壤无机氮供应能力并降低植物氮限制

喀斯特石漠化地区造林的积极作用往往受到土壤氮（N）可用性的限制。然而，植物氮限制如何随着长期植树造林而演变，以及植物氮限制和土壤氮有效性的机制知之甚少。本研究在喀斯特石漠化地区废弃农田上采集了农田、人工林10年、20年和40年后的植物叶子和土壤样本。灌木丛的自然演替作为对照。采用叶面氮/磷（P）化学计量评价植物氮素限制，并通过测量叶面和土壤的氮浓度和δN值来评价土壤氮素有效性的变化。还使用氮示踪技术测定了土壤氮转化率，以阐明土壤无机氮有效性的机制。经过 10 年造林后，植物群落水平叶面 N/P 比值 (13.5) 低于 14，表明植物生长主要受氮限制。随着造林时间的延长，植物群落水平叶面N/P比值增加至14.0-15.5，表明植物氮限制减少。这可能是由于随着造林时间的延长，植物群落叶面的氮浓度和 δN 以及土壤 δN 的增加证实了土壤氮可用性的增加。与耕地相比，造林通过刺激有机氮矿化、异养硝化和吸附铵的释放，提高了土壤无机氮供应能力，且随着造林时间的延长，效果更加明显。矿化、异养硝化和吸附铵释放速率与植物群落水平叶面氮浓度、N/P比和δN显着正相关，表明较大的土壤无机氮供应缓解了植物氮限制。这种效应可归因于造林后有机质含量、细菌和真菌丰度的增加以及土壤结构的改善，所有这些都促进了矿化和异养硝化作用。然而，造林40年后，植物氮的限制仍然存在，这表明可能需要调整单作造林，以最大限度地提高土壤氮的利用率，并缓解喀斯特石漠化地区植物氮的限制。