Designing Technosols to reduce salinity and water stress of crops growing under arid conditions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy of Applied Science in Soil Science at Massey University, Palmerston North, New Zealand

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Salt- and plant-water stress are widely considered to be major abiotic stresses threatening crop production in dry areas. Innovative methods to alleviate salt and plant-water stress that are both practical and economically efficient are in great demand. While the most common reclamation strategy for salt-affected soils is to flush the salts out of the root zone with low salinity/sodicity water, this is challenged by the fact that water is commonly scarce in areas affected by salt stress. The use of specific soil amendments or a combination of them in such areas may well solve some of these problems. Biochar has, in fact, been shown that, in some instances, it is able to effectively reduce salt stress to plants. Other porous materials, such as pumice, have not yet been considered although pumice has been reported to contribute to water retention under arid conditions. Further potential amendments include organic residues, as they can produce beneficial impacts on plant growth by improving soil functions. To date, however, limited research has attempted to unravel (and compare) the effects of either pumice and/or biochar in alleviating salt and plant-water stress. There is also scant information on (i) how to minimise the impact of biochar on the salinity of soils in dry regions; (ii) the underlying mechanisms explaining how the use of either pumice or biochar amendments can decrease soil salinity under arid conditions; and (iii) whether individual or combined additions of either pumice and/or an organic amendment, algae, to a sandy soil alleviates salt and water-stress on plant growth. Therefore, my objective in this study is to investigate whether these amendments can be used in the formulation of Technosols specifically designed to reduce salinity and water-stress of crops growing under arid environments. A quantitative review of the literature was carried out to evaluate what type of biochar and under what conditions its use is suitable in dryland soils. For this, a meta-analysis of 40 studies published between 2013 and 2020 using pairwise comparisons was carried out to evaluate the short-term effect of biochar on the salinity using electrical conductivity (EC) as the proxy for soils under dry environments (Mediterranean, arid, semi-arid climates, or under simulated dry and saline conditions). The results indicated that in terms of the risk of biochar increasing soil salinity, (i) biochars made from high-ash material should not be applied to soils in dry regions; and (ii) the addition of biochars made from relatively low-ash ligneous material at application rates ≤ 20 t ha-1 is suitable as an amendment to soils under dry environments. The use of a leaching fraction is recommended. Water-borne salt transport in soils under arid conditions is strongly related to the influence of amendments on the soil’s mobile-immobile water fractions. For this, the influence of the porosity and pore-size distribution of pumice and biochar (produced from willow wood chips at a highest heating temperature of 350 °C) on the mobile-water content when added to a sandy soil were investigated. Pumice and biochar (of 1.5-, 3-, and 6-cm Ø) were characterised using Scanning Electron Microscope (SEM) technology. The fraction of mobile-water present in these amendments, previously added to a sandy soil at different application rates and particle sizes, was determined using a tracer (Na+) technique. The results showed that the overall larger contribution of pumice to the water mobility than that of biochar under near-saturated conditions could be related to its relatively higher levels of macro-scale plus meso-scale porosity, and this increased as the pumice particle size increased. Both pumice and biochar had a predominance of pores with a Ø < 30 μm and relatively high total porosity, which are expected to contribute to water retention dilution of salinity when these amendments are added to salt-affected sandy soils. In order to evaluate the effects of pumice and biochar amendments on water retention and salinity of a sandy soil under simulated arid conditions, pumice and biochar of different particle sizes (1.5-, 3-, and 6-cm Ø) were separately added at different rates (3, 6, and 12%, v/v basis) to the soil. This was drip irrigated with an artificial saline water under non-draining conditions. Pebbles applied at identical rates and sizes as pumice and biochar, were used as positive controls, whereas no amendment was the negative control. Treatments underwent 10 wetting and drying cycles at 35 ℃ at the end of which, the residual soil was separated from the amendments. We found that (i) the EC of the residual soil followed the order pumice < biochar < positive control = negative control, with differences where existing, being significant at p < 0.05; (ii) the smallest EC and sodium adsorption ratio (SAR) values of the residual soil were achieved when applying 12% pumice, regardless of the particle size; the opposite pattern (12% > 6% > 3%) was observed in the pumice when analysed separately from residual soil; (iii) pumice and biochar treatments retained an increasing amount of water in the soil after each drying cycle (significant at p < 0.05); and (iv) at the end of the experiment, the EC values of the leachates indicated that salts retained in pumice were more slowly mobilised than those in the biochar. The application of either pumice or biochar can contribute to a decrease soil salinity, but pumice could additionally serve as a tool to remove salts from salt-affected soils. In order to investigate whether individual or combined additions of either pumice (PU) and/or algae (AL) to a sandy soil could alleviate the impact of irrigation with saline water on the growth of lucerne (Medicago sativa L.) under simulated semi-arid conditions. A plant growth chamber study was conducted that included six treatments that received saline water (6.4 dS m-1): T1 (sand – positive control), T2 (sand + 3% (v/v basis) PU), T3 (sand + 12% PU), T4 (sand + 3% PU + 2% AL), T5 (sand + 12% PU + 2% AL), T6 (sand + 2% AL). A seventh treatment was T7 (sand – negative control), to which deionised water was added. All treatments underwent 14 cycles of irrigation wetting and drying events (at 27 ± 1 ℃/ 16 ± 1 ℃ day/night). Results showed that, at the end of the experiment and compared with the positive control (T1), the two treatments with the largest application rate of PU (T5 and T3) showed the largest (significant at p < 0.05) reduction in soil EC, SAR, and water-extractable ions among those treatments receiving saline water (T1-T6). Lucerne in treatments T1-T6 always had a smaller (p < 0.05) biomass, leaf dry weight (DW), and relative growth rate than the treatment receiving deionised water (T7) (DW: 2.3 g m-2), but values for treatment T5 (DW: 1.7 g m-2) were significantly larger (p < 0.05) than for treatments T1-T4 and T6 (DW < 1.1 g m-2). Overall, the results obtained suggest that, if proven feasible at a field scale, the combined addition of PU (12%), by reducing salinity and contributing to water retention, and AL (2%), by adding nutrients and/or bioactive compounds, could be used to mitigate salt stress and improve plant growth in sandy soils under arid conditions. The information obtained in this thesis supports the use of pumice and algal amendments as ingredients of Technosols designed to reduce salinity and water-stress of crops growing under arid conditions. Both materials are easily available (if to be used in areas close to a volcanic region and at the seaside), low cost, and their use in agriculture may open new doors to deal with the current problems faced in dry regions.
The following Figures have been removed for copyright reasons: Figs 2-2 & 2-3 (=Menzies et al, 2015 Figs 1b & 3); 2-4 (=Rengasamy, 2010 Fig 1); 2-8 (=Troll et al., 2017 Figs 7a-7d).
Biochar, Pumice, Soil amendments, Soils, Salts in, Arid regions agriculture