Tree roots penetrate the soil to several meters depth, but the role of subsoils for the supply of nutrient elements such as phosphorus (P) to the trees is poorly understood. Here, we tested the hypothesis that increased P deficiency in the topsoil results in an increased microbial recycling of P from the forest subsoil. We sampled soils from four German temperate forest sites representing a gradient in total P stocks. We analyzed the oxygen isotopic composition of HCl-extractable phosphate (δ18OP) and identified differences in P speciation with increasing soil depth using X-ray absorption near-edge structure (XANES) spectroscopy. We further determined microbial oxygen demand with and without nutrient supply at different soil depths to analyse nutrient limitation of microbial growth and used nanoscale secondary ion mass spectrometry (NanoSIMS) to visualize spatial P gradients in the rhizosphere. We found that δ18OP values in the topsoil of all sites were close to the isotopic signal imparted by biological cycling when oxygen isotopes in phosphate are exchanged by enzymatic activity. However, with increasing soil depth and increasing HCl-P concentrations, δ18Ο values continuously decreased towards values expected for primary minerals in parent material at depths below 60 cm at sites with high subsoil P stocks and below more than 2 m at sites with low subsoil P stocks, respectively. For these depths, XANES spectra also indicated the presence of apatite. NanoSIMS images showed an enrichment of P in the rhizosphere in the topsoil of a site with high P stocks, while this P enrichment was absent at a site with low P stocks and in both subsoils. Addition of C, N and P alone or in combination revealed that microbial activity in subsoils of sites with low P stocks was mostly P limited, whereas sites with high P stocks indicated N limitation or N and P co-limitation. We conclude that subsoil P resources are recycled by trees and soil microorganisms. With continued weathering of the bedrock and mobilisation of P from the weathered rocks, P cycling will proceed to greater depths, especially at sites characterised by P limitation.

中文翻译：

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温带森林生态系统底土中磷的生物地球化学循环

树根可穿透土壤达数米深，但人们对底土为树木提供磷 (P) 等营养元素的作用知之甚少。在这里，我们检验了表土中磷缺乏增加导致森林底土中磷的微生物循环增加的假设。我们从代表总磷储量梯度的四个德国温带森林地点取样土壤。我们分析了 HCl 可萃取磷酸盐 (δ18OP) 的氧同位素组成，并使用 X 射线吸收近边结构 (XANES) 光谱确定了随土壤深度增加而 P 形态的差异。我们进一步确定了在不同土壤深度有和没有养分供应的微生物需氧量，以分析微生物生长的养分限制，并使用纳米级二次离子质谱法 (NanoSIMS) 来可视化根际中的空间 P 梯度。我们发现，当磷酸盐中的氧同位素通过酶活性交换时，所有地点表土中的 δ18OP 值接近生物循环传递的同位素信号。然而，随着土壤深度的增加和 HCl-P 浓度的增加，δ180 值不断下降，在深度低于 60 cm 的地方，在高底土 P 库和低于 2 m 的地方，在低底土 P 的地方，母体材料中的原生矿物质的预期值股票，分别。对于这些深度，XANES 光谱也表明存在磷灰石。NanoSIMS 图像显示，在 P 储量高的地点的表土中，根际 P 富集，而在 P 储量低的地点和两种底土中都不存在这种 P 富集。单独或组合添加 C、N 和 P 表明，低 P 储量地点的底土中的微生物活动主要受 P 限制，而高 P 储量地点表明 N 限制或 N 和 P 共同限制。我们得出结论，底土磷资源被树木和土壤微生物循环利用。随着基岩的持续风化和风化岩中磷的迁移，磷循环将进行到更大的深度，特别是在以磷限制为特征的地点。单独或组合添加 C、N 和 P 表明，低 P 库位点的底土中的微生物活动主要受 P 限制，而高 P 库的站点表明 N 限制或 N 和 P 共同限制。我们得出结论，底土磷资源被树木和土壤微生物循环利用。随着基岩的持续风化和风化岩中磷的迁移，磷循环将进行到更大的深度，特别是在以磷限制为特征的地点。单独或组合添加 C、N 和 P 表明，低 P 库位点的底土中的微生物活动主要受 P 限制，而高 P 库的站点表明 N 限制或 N 和 P 共同限制。我们得出结论，底土磷资源被树木和土壤微生物循环利用。随着基岩的持续风化和风化岩中磷的迁移，磷循环将进行到更大的深度，特别是在以磷限制为特征的地点。