Billings S A, Richter D D
Department of Ecology and Evolutionary Biology, Kansas Biological Survey, University of Kansas, Lawrence, KS 66047, USA.
Oecologia. 2006 Jun;148(2):325-33. doi: 10.1007/s00442-006-0366-7. Epub 2006 Feb 8.
Understanding what governs patterns of soil delta15N and delta13C is limited by the absence of these data assembled throughout the development of individual ecosystems. These patterns are important because stable isotopes of soil organic N and C are integrative indicators of biogeochemical processing of soil organic matter. We examined delta15N of soil organic matter (delta15NSOM) and delta13CSOM of archived soil samples across four decades from four depths of an aggrading forest in southeastern USA. The site supports an old-field pine forest in which the N cycle is affected by former agricultural fertilization, massive accumulation of soil N by aggrading trees over four decades, and small to insignificant fluxes of N via NH3 volatilization, nitrification, and denitrification. We examine isotopic data and the N and C dynamics of this ecosystem to evaluate mechanisms driving isotopic shifts over time. With forest development, delta13CSOM became depth-dependent. This trend resulted from a decline of approximately 2 per thousand in the surficial 15 cm of mineral soil to -26.0 per thousand, due to organic matter inputs from forest vegetation. Deeper layers exhibited relatively little trend in delta13CSOM with time. In contrast, delta15NSOM was most dynamic in deeper layers. During the four decades of forest development, the deepest layer (35-60 cm) reached a maximum delta15N value of 9.1 per thousand, increasing by 7.6 per thousand. The transfer of > 800 kg ha(-1) of soil organic N into aggrading vegetation and the forest floor and the apparent large proportion of ectomycorrhizal (ECM) fungi in these soils suggest that fractionation via microbial transformations must be the major process changing delta15N in these soils. Accretion of isotopically enriched compounds derived from microbial cells (i.e., ECM fungi) likely promote isotopic enrichment of soils over time. The work indicates the rapid rate at which ecosystem development can impart delta15NSOM and delta13CSOM signatures associated with undisturbed soil profiles.
由于缺乏在各个生态系统发展过程中收集的这些数据，对控制土壤δ15N和δ13C模式的因素的理解受到限制。这些模式很重要，因为土壤有机氮和碳的稳定同位素是土壤有机质生物地球化学过程的综合指标。我们研究了美国东南部一个正在演替的森林四个深度的存档土壤样本在四十多年间的土壤有机质δ15N（δ15NSOM）和δ13CSOM。该地点是一片旧耕地松林，其中氮循环受到以前农业施肥、树木在四十多年间大量积累土壤氮以及通过氨挥发、硝化作用和反硝化作用的氮通量小到可忽略不计的影响。我们研究了该生态系统的同位素数据以及氮和碳动态，以评估驱动同位素随时间变化的机制。随着森林的发展，δ13CSOM变得与深度相关。这一趋势是由于森林植被输入的有机质导致矿质土壤表层15厘米处下降了约2‰，降至-26.0‰。较深层的δ13CSOM随时间变化相对较小。相比之下，δ15NSOM在较深层最为活跃。在森林发展的四十多年间，最深层（35 - 60厘米）的δ15N最大值达到9.1‰，增加了7.6‰。超过800 kg·ha-1的土壤有机氮转移到正在演替的植被和林地中，并且这些土壤中外生菌根（ECM）真菌的比例明显很大，这表明通过微生物转化的分馏作用必定是这些土壤中δ15N变化的主要过程。源自微生物细胞（即ECM真菌）的同位素富集化合物的积累可能会随着时间的推移促进土壤的同位素富集。这项工作表明生态系统发展能够快速赋予与未受干扰土壤剖面相关的δ15NSOM和δ13CSOM特征。
