Managed loblolly pine (P. taeda L.) forests comprise a major land-use across the “wood basket” of the southeastern US. Lower coastal plain loblolly pine forests can enhance carbon (C) storage in fast-growing vegetation and soils, representing a significant opportunity to manage these forests for improved soil health through an understanding of soil C and nitrogen (N) storage. Furthermore, these forests have unrealized potential to produce biomass for emerging bioenergy markets. Despite this, very little is known about how intensified management (herbaceous bioenergy production) of these forests influences short-term soil organic C (SOC) and soil organic N (SON) pools in surface and subsurface soil horizons. The field site was located in the Lower Coastal Plain of North Carolina, USA in which trees were planted in bedded (e.g., raised) rows (bed) and intercropped with or without switchgrass (P. virgatum L.) between the bedded rows of trees (interbed). Soils were collected from four depths (0–5, 5–15, 15–30, and 30–45 cm) and two locations (bed and interbed regions) and fractionated into light and heavy mineral-associated SOC and SON pools using a sequential density fractionation technique. Bulk soil SOC and SON concentration as well as C:N ratio decreased with depth for both treatments, while ¹⁵N became enriched. Free and occluded lighter fractions (< 1.65 g cm⁻³) had higher SOC and SON concentration (~45% C and ~ 1.15% N) compared to medium (1.65–2.00 g cm⁻³) and heavy (> 2.00 g cm⁻³) fractions. At the 0–5 cm depth in interbeds, SOC and SON were concentrated in the free light fraction (~45%), but transitioned to being concentrated in the heaviest mineral-associated fraction at the 15–30 (~55%) and 30–45 (~70%) cm depth. The ¹⁵N analysis of density fractions indicated progressive enrichment with increasing density, suggesting increasing incorporation of microbially-derived products into stable mineral-associated soil organic matter (SOM) pools. In the pine-switchgrass treatment, SOC and SON concentrations were higher in the light and medium fractions in beds and interbeds compared to the pine only treatment, particularly at the 30–45 cm depth. The C:N ratio in the pine-switchgrass treatment was higher compared to the pine only treatment indicating inputs of new plant-derived organic matter. Furthermore, the δ¹³C stable isotope signature was enriched in both the occluded light fraction (1.65 g cm⁻³) and the mineral-associated fraction (1.65–1.85 g cm⁻³). Our results suggest that switchgrass intercropping is contributing to the early buildup of SOM in particulate and mineral-associated SOM pools possibly through additions of new root-derived organic matter and the positive effect on soil microbial activity through priming of microbes.

管理火炬松 ( P. taedaL.) 森林是美国东南部“木篮子”的主要土地用途。较低的沿海平原火炬松林可以增强快速生长的植被和土壤中的碳 (C) 储存，这是通过了解土壤碳和氮 (N) 储存来管理这些森林以改善土壤健康的重要机会。此外，这些森林尚未实现为新兴生物能源市场生产生物质的潜力。尽管如此，关于这些森林的集约化管理（草本生物能源生产）如何影响表层和地下土壤层中的短期土壤有机碳 (SOC) 和土壤有机氮 (SON) 库，我们知之甚少。该田地位于美国北卡罗来纳州的下沿海平原，在那里种植树木（例如，P. virgatum L.) 之间的树木（间床）。从四个深度（0-5、5-15、15-30 和 30-45 厘米）和两个位置（床和夹层区域）收集土壤，并使用连续的方法将土壤分为轻重矿物相关 SOC 和 SON 池密度分馏技术。两种处理的大块土壤 SOC 和 SON 浓度以及 C:N 比均随深度降低，而¹⁵ N 变得富集。自由和闭塞较轻馏分（<1.65克厘米^-3）具有更高的SOC和SON浓度（〜45％C和〜1.15％N）相比，培养基（1.65-2.00克厘米^-3）和重（>2.00克厘米^{- 3}) 分数。在夹层 0-5 cm 深度处，SOC 和 SON 集中在游离光部分 (~45%)，但在 15-30 (~55%) 和 30 处转变为集中在最重的矿物伴生部分–45 (~70%) 厘米深度。在¹⁵密度分数的 N 分析表明随着密度的增加逐渐富集，表明微生物衍生产品越来越多地融入稳定的矿物相关土壤有机质 (SOM) 池中。在松树柳枝稷处理中，与仅使用松树处理相比，床层和夹层中轻质和中等部分的 SOC 和 SON 浓度更高，尤其是在 30-45 厘米深度处。松树柳枝稷处理中的 C:N 比高于仅使用松树的处理，表明输入了新的植物衍生有机物质。此外，δ ¹³ C 稳定同位素特征在吸留轻组分 (1.65 g cm ^-3 ) 和矿物相关组分 (1.65–1.85 g cm ^-3）。我们的研究结果表明，柳枝稷间作有助于颗粒物和矿物相关 SOM 池中 SOM 的早期积累，这可能是通过添加新的根源性有机物质以及通过引发微生物对土壤微生物活动的积极影响来实现的。