DOI: 10.5061/dryad.931zcrjg7
title: Data from: Changes in levels of enzymes and osmotic adjustment compounds in key species and their relevance to vegetation succession in abandoned croplands of a semiarid sandy region

description: Reclamation of cropland from grassland is regarded as a main
 reason for grassland degradation; understanding succession from abandoned
 cropland to grassland is thus crucial for vegetation restoration in arid
 and semiarid areas. Soil becomes dry when cropland is reverted to
 grassland, and enzyme and osmotic adjustment compounds may help
 plants to adapt to a drying environment. Croplands that were abandoned in
 various years on the Ordos Plateau in China, were selected for
 the analysis of the dynamics of enzymes and osmotic adjustment compounds
 in plant species during vegetation succession. With increasing number of
 years since abandonment, levels of superoxide
 dismutase increased in Stipa bungeana, first
 decreased and then increased in Lespedeza davurica and
 Artemisia frigida, and fluctuated in Heteropappus altaicus.
 Levels of peroxidase and catalase in the four species
 fluctuated; levels of proline, soluble sugar and soluble
 protein either decreased or first
 increased and then generally
 decreased. According to a drought
 resistance index, the drought resistance of the four
 species was ranked in descending order as follows: S.
 bungeana &gt; A. frigida &gt; H. altaicus &gt; L.
 davurica. The drought resistance ability of the different species
 was closely linked with vegetation succession from communities
 dominated by annual and biennial species (with main accompanying species
 of L. davurica and H. altaicus) to communities
 dominated by perennial species (S. bungeana and A.
 frigida) when soil became dry owing to increasing evapotranspiration after
 cropland abandonment. The restoration of S. bungeana
 steppe after cropland abandonment on the Ordos
 Plateau is recommended both as high-quality forage and for
 environmental sustainability.
description: Experimental design Field experiments were
 conducted in July 2017. We selected typical abandoned corn cropland
 on hard
 ridge (3, 6, 10, 15 and 20 years since abandonment, based on the
 information provided by the landowners), and the control was
 uncultivated natural vegetation
 near abandoned corn croplands. To ensure similar environments for
 all sampling plots, all plots were set within 2 km in similar terrain and
 landform. The species composition of the natural vegetation
 was similar
 to the vegetation before the corn croplands were established. Three
 separate replicates were set up for each
 age of
 abandoned cropland and natural
 vegetation with the same cropping history
 as well as the same time since
 abandonment. In each replicate
 of each
 age of
 abandoned cropland and natural vegetation, three
 leaf-sampling plots (20 m × 20 m) were established, and the
 topmost fully developed
 functional leaves (upper portion of stems) were collected from over ten
 plants chosen randomly in the morning of one day. Leaf samples were collected from
 the four species that were found in all sample plots:
 <i>S.</i><i> bungeana</i>, <i>L.</i><i> davurica</i>, <i>A. f</i><i>rigida</i> and <i>H. altaicus</i>. All species were deciduous. Leaf samples were immediately placed in a foam box with dry ice, brought to the laboratory, and kept in a -80°C ultralow temperature freezer. SOD, POD, CAT, MDA, proline (Pro), soluble sugar (SS), and soluble protein (SP) concentrations were analyzed separately with SOD, POD, CAT, MDA, Pro, SS and SP assay kits (Comin Biotechnology Co., Ltd., Suzhou, China) (Pan et al., 2016). Leaf samples that had been maintained at -80°C were ground into a powder with liquid nitrogen. Using a sodium phosphate (Na<sub>2</sub>HPO<sub>4</sub>/NaH<sub>2</sub>PO<sub>4</sub>) buffer, SOD, POD, CAT, MDA, Pro and SP were extracted by homogenizing on ice (0.1 g leaf tissue for each SOD, POD, CAT, MDA, Pro and SP assay with 1 mL buffer). To isolate the supernatants for the SOD, POD, CAT and MDA assays, the homogenates were centrifuged at 8,000 ×g at 4°C for 10 min. For the Pro assay, the homogenates were shaken in a boiling water bath (90°C) for 10 min, cooled and then centrifuged at 1,000 ×g at 25°C for 10 min. For the SP assay, the homogenates were centrifuged at 10,000 ×g at 4°C for 10 min. For the SS assay, the leaf samples that had been maintained at -80°C were ground into a powder in liquid nitrogen, then 0.1 g leaf tissue was homogenized with 1 mL distilled water and maintained in a boiling water bath for 10 min. After cooling, the mixture was centrifuged at 8,000 ×g at 25°C for 10 min to produce the supernatant. In each 20 m × 20 m leaf-sampling plot, quadrats covering 5 m × 5 m for semishrub species and 1 m × 1 m for grass species were established. To ensure accurate measurements of biomass, leaves were not collected in these quadrats for analyses of enzyme activity, MDA, or osmotic adjustment compounds. To obtain the aboveground biomass, the aboveground parts of every species were harvested separately, taken to the laboratory and dried to a constant weight at a temperature of 80°C (Cai<i> </i>et al., 2018). In each 20 m × 20 m leaf sample plot, intact soil cores were collected randomly using a cutting ring (volume of 100 cm<sup>3</sup>) from five soil depths (0–5, 5–10, 10–20, 20–30 and 30–40 cm) after removing any rocks and litter. After collecting the soil samples, we immediately measured the fresh weight (FW), and then the samples were taken to the laboratory and oven-dried at 105°C to a constant weight to measure the dry weight (DW). Soil water content (SWC) was calculated as SWC (%) = (FW - DW)/FW.   Statistical analysis To evaluate the drought resistance ability of a species, principal component analysis (PCA) was used to develop an index (Wold et al., 1987). Averaged data from three replicates for each of the seven parameters (SOD, POD, CAT, MDA, SS, Pro, and SP) related to the drought resistance of <i>S.</i><i> bungeana</i>, <i>L.</i><i> davurica</i>, <i>A. f</i><i>rigida</i> and <i>H. altaicus</i> in different abandoned cropland and natural vegetation were converted into 7 principal components for analysis, and then the results of the PCA were used to build a drought resistance index. The weight (<i>Wi</i>) and the comprehensive evaluation index (<i>D</i>) of each plant were estimated using the following equations (Wang et al., 2015):       (1)      <i>Wi</i> =Pi/(P1+P2+...+Pn)  i = 1, 2, 3, …, n                          (2)      <i>D</i> =U1*W1+U2*W2+...+Un*Wn i = 1, 2, 3, …, n                     where <i>P</i><i><sub>i</sub></i> is the contribution rate of principal component <i>i</i>, <i>U</i><i><sub>i</sub></i> is the subordinative function of principal component <i>i</i>, and <i>D</i> is the drought resistance index of the species. The higher the <i>D</i> value, the higher the drought resistance. Within each of three replicates, samples were collected from three subplots; data from three subplots were averaged as one datum point for each replicate. A statistical analysis was performed by two-way ANOVA. If significant differences were found, Duncan’s test was used to determine mean differences between treatments (<i>P </i>&lt; 0.05) (Kabacoff, 2015). The relationships among SOD, POD, CAT, MDA, Pro, SS and SP were examined using Pearson’s correlation analysis. All statistical analyses, including the test for homogeneity of variance, were performed using SPSS Statistics 17.0 (SPSS Inc., Chicago, IL, USA).
description: Microsoft Excel was used to create this data file, which contains
 data for Leaf enzyme activities, MDA, osmotic adjustment compounds and
 aboveground biomass of dominant species on the Ordos Plateau in
 China.