water Communication Isotopic Signatures as an Indicator of Long-Term Water-Use Efficiency of Haloxylon Plantations on the Dried Aral Sea Bed Asia Khamzina 1,* , Jiae An 2, Hanna Chang 1 and Yowhan Son 1 1 Department of Environmental Science and Ecological Engineering, Korea University, 145 Anam-Ro, Seongbuk-Gu, Seoul 02841, Korea; [email protected] (H.C.); [email protected] (Y.S.) 2 Division of Restoration Research, National Institute of Ecology, Yeongyang 36531, Korea; [email protected] * Correspondence: [email protected] Received: 23 November 2019; Accepted: 25 December 2019; Published: 27 December 2019 Abstract: The desiccation of the Aral Sea due to water withdrawal from contributing rivers has resulted in an unprecedented change in the region’s climate, from maritime to hot dry desert. Afforestation has been implemented on the desiccated seafloor—the Aralkum Desert—for stabilizing the exposed substrate. However, studies on the long-term status of the afforested sites are limited. Here, we examined C and N isotopic signatures in Haloxylon aphyllum plantations, as indicators of time-integrated plant response to the prevalent water and salinity constraints, in northern Aralkum, Kazakhstan. Foliar 13C composition analysis in a chronosequence of H. aphyllum plantation sites (aged 1–27 years) on the sandy substrate revealed a significant trend towards higher water-use efficiency in older plantations, possibly in response to declining water availability. A lack of correlation between plant 13C signature and soil electrical conductivity suggests no history of salt stress despite the saline environment. Furthermore, 15N enrichment in plant tissue in the water-limited Aralkum ecosystem indicates the relative openness of N cycling. There was an increase in species richness and self-propagation at the plot scale, indicating successful afforestation effort. Coupled with other approaches, isotope discrimination might elucidate mechanisms underlying stress tolerance in H. aphyllum, which could support the afforestation efforts. Keywords: afforestation; Aralkum; Central Asia; saksaul; 13C; 15N 1. Introduction The desiccation of the Aral Sea due to withdrawal of water from the contributing rivers has resulted in a significant change in the region’s climate, from maritime to dry hot desert, with irreversible consequences for the regional ecosystems and local livelihoods [1–3]. Afforestation has been implemented on the dried seafloor, the man-made Aralkum Desert, for stabilization of the exposed substrate, in order to prevent soil erosion, salty dust storms, and adverse effects on human health. The newly vegetated areas also hold potential for carbon sequestration, foraging, and wildlife conservation [1,4]. Black and white saksaul species (Haloxylon spp.), native to the central Asian deserts, have been the pioneering species in the afforestation efforts dating to the late 1980s and more recent decades. Several studies have evaluated the phytoremediation potential of Haloxylon spp. in test sites at the crucial stage of plantation establishment [5–7]. However, studies addressing the long-term development of vegetation are limited, and they have mostly focused on natural succession in various exposed substrates [1,7] and the spatial dynamics of vegetation cover [8]. Long-term development and sustainability of afforested plots have eluded attention, often due to limited feasibility for conducting field studies in remote locations with lacking research infrastructure for continuous measurement of soil-plant-atmosphere dynamics. Water 2020, 12, 99; doi:10.3390/w12010099 www.mdpi.com/journal/water WaterWater2020 2020, 12,, 12 99, x FOR PEER REVIEW 2 of2 7 of 7 Stable carbon (C) and nitrogen (N) isotopic signatures of plants and soils can serve as tracers thatStable indicate carbon time-integrated (C) and nitrogen responses (N) isotopic of plants signatures to the ofenvironmental plants and soils constraints can serve [9,10]. as tracers These that indicateindicators time-integrated are thus suitable responses for a of rapid plants assessment to the environmental of environmental constraints influences, [9,10]. that These is, indicators water and are thussalinity suitable constraints for a rapid prevalent assessment in Aralkum, of environmental on plant influences,functions that that is,cannot water be and easily salinity measured constraints or prevalentsurmised in from Aralkum, yield data on plant alone. functions Rainfall variability that cannot is beconsidered easily measured the most or significant surmised factor from affecting yield data alone.the CRainfall and N isotopic variability composition is considered in dryland the plants most [11,12]). significant A low factor soil water affecting potential the C due and to N dryness isotopic compositiondecreases intercellular in dryland plants CO2 concentration [11,12]). A low through soil water stomatal potential closure, due resulting to dryness in 13 decreasesC enrichment intercellular in the foliar tissue. The plant C isotopic composition (δ13C) is also13 positively related to the intrinsic water- CO2 concentration through stomatal closure, resulting in C enrichment in the foliar tissue. The plant C isotopicuse efficiency composition (WUE, the (δ 13ratioC) isof alsowater positively loss to biomass related gain), to the because intrinsic a decrease water-use in water efficiency availability (WUE, theinduces ratio of stomatal water loss closure to biomass thereby gain), enhancing because the aWUE decrease [9]. Similarly, in water availabilitysoil salinity inducesis found stomatal to be 13 closurecorrelated thereby with enhancing δ C because the salinity-induced WUE [9]. Similarly, water soil stress, salinity due to is a found lower towater be correlated potential of with salineδ13 C soils and thus reduced water uptake by the roots [13], reduces stomatal conductance and because salinity-induced water stress, due to a lower water potential of saline soils and thus reduced consequently increases intrinsic WUE. Natural abundance of 15N in plants and soils can be used as water uptake by the roots [13], reduces stomatal conductance and consequently increases intrinsic an indicator of N cycling in an ecosystem [14], but the interpretation of 15N signatures is more complex WUE. Natural abundance of 15N in plants and soils can be used as an indicator of N cycling in an as they reflect the net result of a range of processes [10]. ecosystem [14], but the interpretation of 15N signatures is more complex as they reflect the net result of In this study, we conducted a preliminary evaluation of the long-term status of afforested sites a rangeusing ofstable processes C and [ 10N ].isotopic signatures as indicators of plant vigor in response to the prevalent waterIn this and study, salinity we constraints conducted in a Aralkum. preliminary The evaluation specific objectives of the long-term of our study status were of as aff follows:orested sites(i) usingexamining stable Cthe and range N isotopicof 13C and signatures 15N signatures as indicators in plant communities of plant vigor of Haloxylon in response-based to afforestation the prevalent waterseries and in salinitynorthern constraints Aralkum, (ii) in Aralkum.assessing long-term The specific variations objectives in δ13 ofC ourand studyδ15N in were the afforestation as follows: (i) 13 15 examiningchronosequence the range sites, of andC and (iii) interpretingN signatures the in observed plant communities variations ofin Haloxylonrelation to-based the plant afforestation water seriesstatus. in northern Aralkum, (ii) assessing long-term variations in δ13C and δ15N in the afforestation chronosequence sites, and (iii) interpreting the observed variations in relation to the plant water status. 2. Materials and Methods 2. Materials and Methods The study area in northern Aralkum covers about 9600 km2 of 44.822° N and 45.741° N latitude 2 andThe 60.014° study E area and in61.332° northern E longitude Aralkum (Figure covers 1). about The area 9600 has km beenof 44.822 entirely◦ N desiccated and 45.741 since◦ N about latitude and1977. 60.014 ◦ E and 61.332◦ E longitude (Figure1). The area has been entirely desiccated since about 1977. (a) (b) FigureFigure 1. 1.Study Study area area in in northern northern Aralkum, Aralkum, KazakhstanKazakhstan ( (aa)) and and locations locations of of the the surveyed surveyed afforestation afforestation sitessites (b ).(b Source:). Source: Landsat-8 Landsat-8 OLI OLI acquired acquired onon 17–3117–31 August 2018. 2018. WGS WGS 1987 1987 UTM UTM Zone Zone 40N. 40N. AffAfforestationorestation sites sites of of di differentfferent establishment establishment agesages were selected selected for for reconstructing reconstructing the the pattern pattern of of plantplant development development and and responses responses to to environmental environmental constraints.constraints. Seven Seven afforest afforestationation sites sites established established duringduring 1991–2017 1991–2017 (Table (Table1 )1) byby plantingplanting seedlings seedlings or or seeds seeds (only (only in 1991) in 1991) of Haloxylon of Haloxylon aphyllum aphyllum were weresurveyed surveyed on on20–21 20–21 August August 2018. 2018. These These plantations plantations were were set setup upon ontypical typical barren barren plain plain areas areas characterizedcharacterized by by a sandya sandy substrate substrate (sand (sand fraction fraction == 90%) with with remnant remnant seashells. seashells. In In most most of ofthe the sites sites thatthat we we visited, visited, the the original