Irrigation and phytolith formation: an experimental study

Abstract

It has been proposed that phytoliths from archaeological sites can be indicators of water availability and hence inform about past agricultural practices (Rosen and Weiner, 1994;Madella et al., 2009). Rosen and Weiner (1994) found that the number of conjoined phytoliths from cereal husks increased with irrigation while Madella et al. (2009) demonstrated that the ratio of long-celled phytoliths to short-celled phytoliths increased with irrigation. In order to further explore these hypotheses, wheat and barley were experimentally grown from 2005 to 2008 in three different crop growing stations in Jordan. Four different irrigation regimes were initially employed: 0% (rainfall only), 80%, 100% and 120% of the optimum crop water requirements, with a 40% plot being added in the second and third growing seasons. Each plot measured 5 m  5 m and a drip irrigation system was used. Environmental variables were measured on a daily basis, and soil and water samples were taken and analysed at the University of Reading. Phytoliths from the husks of these experimentally grown plants were extracted using the dry ashing method. Results demonstrate that although the number of conjoined cells increases with irrigation, there were considerable intersite and inter-year differences suggesting that environmental variables other than water availability affect phytolith uptake and deposition. Furthermore, analytical experiments demonstrated that conjoined phytoliths are subject to change or breakage by external factors, making this methodology problematic to apply to archaeological phytolith assemblages that have an unknown taphonomic history. The ratio of long cells to short cells also responded to increased irrigation, and these forms are not subject to break up as are conjoined forms. Our results from the modern samples of durum wheat and six-row barley show that if an assemblage of single-celled phytoliths consists of over 60% dendritic long cells then this strongly suggests that the crop received optimum levels of water. Further research is needed to determine if this finding is consistent in phytolith samples from the leaves and stems, as suggested by Madella et al. (2009), and in other species of cereals. If this is the case then phytoliths are a valuble tool for assessing the level of past water availability and, potentially, past irrigation.

Phytoliths and rice: from wet to dry and back again in the Neolithic Lower Yangtze

“…After the development of paddy fields with irrigation systems, we would expect to see a return to higher ratios of phytoliths from environmentally controlled silicification. There are several potential issues however; one is that rice generally grows in much more humid conditions than the south-west Asian winter cereals previously considered (Madella et al 2009; Jenkins et al 2010). More water and greater evapotranspiration are likely to cause higher phytolith production overall.…”

Dry, rainfed or irrigated? Reevaluating the role and development of rice agriculture in Iron Age-Early Historic South India using archaeobotanical approaches

“…‘Fixed’ morphotypes (grass phytoliths with a rate of production that is not related to water availability), ‘sensitive’ morphotypes (grass phytoliths that have a rate of production directly correlated with water availability) and ‘other grass multi-cells’ (grass multi-celled phytolith panels) were separately classified (Table 5). In grasses, the long cells and stomatal cells (sensitive morphotypes) which show a quantifiable difference in production in Triticum aestivum , Triticum dicoccum , Hordeum vulgare and Hordeum aegiceras grown in controlled dry and wet conditions (Madella et al 2009) and Triticum durum and Hordeum vulgare grown in agricultural fields (Jenkins et al 2011, 2016). This pattern has been demonstrated to apply to rice using modern field samples from India (Weisskopf et al 2015) and China (Huan et al 2018), as well as archaeological samples from both field and cultural contexts in Neolithic China by Weisskopf et al 2015).…”

“…A sensitive to fixed phytolith index (where a higher value = wetter growing conditions) correlated with the archaeological, geoarchaeological and macrobotanical evidence for field cultivation systems, with Tianloushan showing 2.5, Caoxieshan showing 0.7 and Maoshan showing 2.0. Additionally, the production of multi-cell panels (conjoined cells) appears to occur more frequently in Triticum dicoccum , Triticum aestivum , Hordeum vulgare and Hordeum spontaneum grown in wet conditions (Jenkins et al 2011; Rosen and Weiner 1994). However, the processes of disarticulation in archaeological contexts are not clear (Jenkins et al 2011 p. 370), and my personal observations suggest that there may be an increased rate of post-depositional disarticulation in more water-rich environments.…”

“…Additionally, the production of multi-cell panels (conjoined cells) appears to occur more frequently in Triticum dicoccum , Triticum aestivum , Hordeum vulgare and Hordeum spontaneum grown in wet conditions (Jenkins et al 2011; Rosen and Weiner 1994). However, the processes of disarticulation in archaeological contexts are not clear (Jenkins et al 2011 p. 370), and my personal observations suggest that there may be an increased rate of post-depositional disarticulation in more water-rich environments. Nevertheless, ‘other grass multi cells’ were categorised separately, as potential indicators of wet land environments (based on Jenkins et al 2011 and Rosen and Weiner 1994), but also as potential indicators of dryland cultivation systems (as per Fig.…”

Early agriculture in Sri Lanka: New Archaeobotanical analyses and radiocarbon dates from the early historic sites of Kirinda and Kantharodai (Kandarodai)

“…Using the sensitive vs. fixed model (Madella et al 2009, Jenkins et al 2010 where sensitive represents wet rice agriculture and fixed dry or rainfed arable systems, Kanthorodai and Kirinda were compared to the phytoliths from sites in Uttar Pradesh and Odisha, India analysed by Harvey et al (2006); Harvey and Fuller (2005). the Iron Age, 1300 1000BC at Golbai Sassan and 1400-1000BC at Gopalpur (Harvey et al 2006).…”