Plant available water and tree seedling survival in hydrogel amended soils under induced drought conditions
In order to increase plantation forests and promote on-farm tree growing in the drier parts of the tropics for provision of tree products and services, there is a need to find solutions to the low soil moisture resulting from drought that limit tree growing. Superabsorbent Polyacrylate (SAP) hydrogels that absorb and retain water many times their own weight and release to plant roots are potential solutions for tree establishment in drought prone areas. The objectives of this study were to i) Determine the effect of hydrogel amendment to different soils on plant available water and survival of trees under drought conditions ii) Assess the effect of hydrogel amendment on irrigation frequency and plant biomass production in sandy soil iii) Examine the effect of hydrogels on the development of leaf water potential and osmolarity of the cell sap in selected tree species grown in temperate soil types and iv) Assess the effect of hydrogels on water retention, plant available water (PAW) and tree survival in selected temperate tree species and soil types. Experiments were conducted under greenhouse conditions in both Uganda (tropical condition) and Germany (temperate condition). Five soil types (sand, loam, silt loam, sandy loam and clay) were amended at 0, 0.2 and 0.4 % hydrogel and grown with seedlings of nine tree species (Eucalyptus grandis, Eucalyptus citriodora, Pinus caribaea, Araucaria cunninghamii, Melia volkensii, Grevillea robusta, Azadirachta indica, Maesopsis eminii and Terminalia superba) for eight weeks before subjection to water stress. PAW was determined by obtaining the difference between weight of the pots at field capacity and at death of the seedlings, while survival time was computed in terms of days survived by seedlings after initiation of drought conditions. Evapotranspiration was estimated from the ratio of PAW to number of days survived. Seeds of Agrostis stolonifera were sown in three identical buckets whose upper 25 cm layers were amended at 0, 0.2 and 0.4 % hydrogel and the lower 25 un-amended to examine the effect of hydrogel amendment on irrigation frequency and plant biomass production. Irrigation in each bucket was achieved when potential meter was 600 cm bars. Leaf water potential and osmolarity for temperate tree seedlings planted in three soil types (sand, loam and clay) were measured at field capacity, wilting point and at death of the seedlings using a pressure chamber and Osmometer (freezing point depression method) respectively. Water retention by the soils amended with hydrogel of various grain sizes was determined gravimetrically. All data were subjected to Analysis of Variance (ANOVA). Hydrogel amendment to both tropical and temperate soils significantly increased PAW in the soils, prolonged tree seedling survival time and reduced the evapotranspiration in over 90% of the seedlings compared with the control under green house conditions. Araucaria cunninghammi and Pinus sylivestris survived 69 and 71 extra days respectively in sand amended with 0.4 % hydrogel after the death of the tree seedlings in un-amended sandy soil. The hydrogel increased the shoot and root biomass of Agrostis stolonifera by 2.2 and 4 times respectively compared to the control. There was reduced irrigation frequency, but more biomass production of plants in the 0.4 % hydrogel amended sandy soil. Amending sandy soils with 0.4 % hydrogel enhances seedling growth under drought conditions. At wilting point, Leaf Water Potential (Ψleaf) was higher in the 0.4 % hydrogel amendment compared to the control in over 70% of the treatments. The hydrogel induced higher drought tolerance to trees grown in the sandy than loamy and clayey soils. Drought tolerance by temperate tree seedlings grown in sandy soil was in the order P. sylvestris > P. abies > F. sylvatica. The small-grained hydrogel absorbed and retained more water than medium and large-grained hydrogels. Water absorption and retention by hydrogel was higher in sandy than clayey and loamy soil; implying that hydrogels can be confidently applied to enhance growth and survival of tree seedlings planted in sandy soils in dry areas of Uganda.