McCulley R L, Jobbágy E G, Pockman W T, Jackson R B
Department of Biology, Duke University, Durham, NC 27708-0340, USA.
Oecologia. 2004 Dec;141(4):620-8. doi: 10.1007/s00442-004-1687-z. Epub 2004 Aug 19.
Explanations for the occurrence of deep-rooted plants in arid and semi-arid ecosystems have traditionally emphasized the uptake of relatively deep soil water. However, recent hydrologic data from arid systems show that soil water potentials at depth fluctuate little over long time periods, suggesting this water may be rarely utilized or replenished. In this study, we examine the distributions of root biomass, soil moisture and nutrient contents to 10-m depths at five semi-arid and arid sites across southwestern USA. We couple these depth distributions with strontium (Sr) isotope data that show deep (>1 m) nutrient uptake is prevalent at four of the five sites. At all of the sites, the highest abundance of one or more of the measured nutrients occurred deep within the soil profile, particularly for P, Ca2+ and Mg2+. Phosphate contents were greater at depth than in the top meter of soil at three of five sites. At Jornada, for example, the 2-3 m depth increment had twice the extractable P as the top meter of soil, despite the highest concentrations of P occurring at the surface. The prevalence of such deep resource pools, and our evidence for cation uptake from them, suggest nutrient uptake as a complementary explanation for the occurrence of deep-rooted plants in arid and semi-arid systems. We propose that hydraulic redistribution of shallow surface water to deep soil layers by roots may be the mechanism through which deep soil nutrients are mobilized and taken up by plants.
传统上，对于干旱和半干旱生态系统中深根植物出现的解释一直强调对相对深层土壤水分的吸收。然而，近期来自干旱系统的水文数据表明，深层土壤水势在很长一段时间内波动很小，这表明这种水分可能很少被利用或补充。在本研究中，我们在美国西南部五个半干旱和干旱地点，研究了根生物量、土壤水分和养分含量在10米深度内的分布情况。我们将这些深度分布与锶（Sr）同位素数据相结合，这些数据表明在五个地点中的四个，深层（>1米）养分吸收很普遍。在所有地点，一种或多种被测养分的最高丰度出现在土壤剖面深处，特别是磷、钙离子和镁。在五个地点中的三个，土壤深层的磷酸盐含量高于表层一米的土壤。例如，在乔纳达，尽管土壤表层的磷浓度最高，但2 - 3米深度增量处的可提取磷是表层一米土壤的两倍。这种深层资源库的普遍存在，以及我们关于植物从其中吸收阳离子的证据，表明养分吸收是干旱和半干旱系统中深根植物出现的一个补充解释。我们提出，根系将浅层地表水水力再分配到深层土壤层可能是深层土壤养分被植物调动和吸收的机制。
