Holocene vegetation signals in the Alashan Desert of northwest China revealed by lipid molecular proxies from calcareous root tubes

In the hinterland of deserts, it is difficult to reconstruct paleovegetation using fossil pollen because of the low pollen concentration. Therefore, an efficient method is needed to reconstruct the paleovegetation of desert regions. In this study, 34 Holocene calcareous root tube (CRT) samples were collected from the Alashan Desert in northwest China, and lipid molecular proxies from CRTs were selected to address this deficiency. The results show that n-alkanes mainly maximized at C27, C29, and C16, and that the carbon preference index is close to 1. Thus, the sources of n-alkanes from CRTs were the roots of higher plants and microorganisms, and thus changes in n-alkanes from CRTs could reveal variations in vegetation cover. The n-alkane Cmax of long-chain n-alkanes (C>25) in CRTs, maximizing at C27, indicated that vegetation in the Alashan Desert was characterized by shrub vegetation during the Holocene. Changes in the ratio of (C27+C29)/(C31+C33) indicated that the biomass of shrub vegetation increased during the period 7–2 cal ka BP. Moreover, the relative concentration of short-chain to long-chain n-alkanes decreased from 7 to 2 cal ka BP, suggesting that the effective moisture decreased during that period.

Roots as a site of hydrogen sulfide uptake in the hydrocarbon seep vestimentiferan Lamellibrachia sp

Vestimentiferan tubeworms have no mouth or gut, and the majority of their nutritional requirements are provided by endosymbiotic bacteria that utilize hydrogen sulfide oxidation to fix CO(2) into organic molecules. It has been assumed that all vestimentiferans obtain the sulfide, O(2) and CO(2) needed by the bacteria across the plume (gill) surface, but some live in locations where very little sulfide is available in the sea water surrounding the plume. We propose that at least some of these vestimentiferans can grow a posterior extension of their body and tube down into the sea-floor sediment, and that they can use this extension, which we call the ‘root’, to take up sulfide directly from the interstitial water. In this study of the vestimentiferan Lamellibrachia sp., found at hydrocarbon seeps in the Gulf of Mexico at depths of approximately 700 m, we measured seawater and interstitial sulfide concentrations in the hydrocarbon seep habitat, determined the structural characteristics of the root tube using transmission electron microscopy, characterized the biochemical composition of the tube wall, and measured the sulfide permeability of the root tube. We found that, while the sulfide concentration is less than 1 (μ)mol l(−)(1) in the sea water surrounding the gills, it can be over 1.5 mmol l(−)(1) at a depth of 10–25 cm in sediment beneath tubeworm bushes. The root tube is composed primarily of giant (β)-chitin crystallites (12–30 % of total mass) embedded in a protein matrix (50 % of total mass). Root tubes have a mean diameter of 1.4 mm, a mean wall thickness of 70 (μ)m and can be over 20 cm long. The tubeworm itself typically extends its body to the distal tip of the root tube. The root tube wall was quite permeable to sulfide, having a permeability coefficient at 20 degrees C of 0. 41×10(−)(3)cm s(−)(1), with root tube being 2.5 times more permeable to sulfide than trunk tube of the same diameter. The characteristics of the root suggest that it reaches down to the higher sulfide levels present in the deeper sediment and that it functions to increase the surface area available for sulfide uptake in a manner analogous to a respiratory organ.

A Root Tube - Pegging Pan Apparatus: Preliminary Observations and Effects of Soil Water in the Pegging Zone1

Soil conditions, especially water deficits in either the pegging or rooting zone or both, affect pod initiation and seed development of peanut (Arachis hypogaea L.). The objectives of this study were to 1) construct a root tube - pegging pan apparatus which would allow for physical separation of the rooting and pegging zones, 2) determine growth and development of peanut when grown in the apparatus, and 3) examine the effects of soil water in the pegging zone on the initiation and development of peanut fruits. An experimental apparatus was constructed to provide a mechanism for separation of the rooting and pegging zones, allowing for independent control of soil water in both zones. Root tubes (1.6 m long and 15 cm in dia.) were constructed of polyvinyl chloride tubing. Watering access tubes were inserted at 0, 0.5 and 1.0 m from the top of the root tube. The top of the root tube was closed with a convex cap having a 5-cm central hole through which a peanut plant was allowed to grow. A pegging pan (50 cm long × 35 cm wide × 20 cm deep) was fitted around the upper portion of the root tube. Preliminary studies demonstrated satisfactory shoot growth and pod development of peanut plants grown in the apparatus provided over-watering was avoided. It also appeared that pods and seeds formed in air-dry pegging zone soil. To examine this in more detail, an experiment was conducted comparing the effects of air-dry vs. moist (7 to 12% water by weight) pegging zone soil on pod and seed formation. The air-dry pegging zone reduced the percentage of tagged pegs which developed into full pods (those having reached full expansion) from 61 to 48% and reduced the growth rate of developing seeds and pods by 18 and 29%, respectively. The root tube - pegging pan apparatus provided a useful technique to gain a better understanding of peanut pod formation as influenced by soil water environments in the pegging zone.