Date of data collection: 2016-2018

Geographic location of data collection: Guyuan, Ningxia, China (106°23′E, 36°15′N)

These data were generated to (i) investigate the responses of soil properties, biological communities and multifunctionality to decreased soil pH; (ii) determine the potential biotic and abiotic pathways that soil pH may drive multifunctionality. In 2016, a 17 m × 40 m semi-arid grassland plot with an initial pH value of 8.05 and uniform vegetation was selected. The experiment was granted by the administration of Yunwu Mountain National Nature Reserve. A randomized block design was used with five treatments and six replicates per treatment. A total of 30 plots were established. All plots were 2 m × 2 m and separated by 1 m buffer zones. The treatments included five levels of acid addition rate (0, 0.23, 0.56, 3.60, and 9.01 mol H⁺ m^-2) in the form of sulphuric acid solution. In late August 2017, the plant communities achieved their peak biomass, and were surveyed and harvested in a 0.5 m × 1 m quadrat in each plot to determine the plant community diversity and estimate above-ground biomass. After harvesting the plants, six soil cores (0-15 cm deep, 2.5 cm diameter) per plot in each of the six blocks were collected and pooled by plot as a replicate for further chemical analyses.

1. Plant community and primary productivity 

Total numbers of species and individuals observed in each quadrat were measured as species richness and abundance, respectively. The species diversity of each plot was also characterized by Shannon Wiener's diversity index and Simpson’s diversity index. The species evenness was characterized by Pielou's evenness index. We measured soil respiration, root respiration, ecosystem respiration (ER) and net ecosystem C exchange (NEE) one time in mid-August 2017. Gross primary productivity (GPP) is the sum of NEE and ER.

2. Soil properties

Soil pH was measured in a 1/2.5 ratio (soil/water) suspension. A rapid method of soil carbonate analysis using gas chromatography (GC-7890B, Agilent, Santa Clara, CA, USA) was used to measure soil carbonate. A 20 g subsample of the fresh soil was oven-dried at 105 °C for 24 h to determine soil moisture. Ammonium and nitrate concentrations in soil extracts were measured using a segmented flow analyzer (Skalar SAN Plus; Skalar Inc., Breda, Netherlands). SOC was measured using the Walkley-Black method. Dissolved organic carbon (DOC) in extracts was determined with a TOC analyzer (Elementar Vario Micro Cube, Germany). Microbial biomass carbon (MBC) and nitrogen (MBN) were determined using the chloroform extraction methods, as conversion factors KC (0.38) and KN (0.45) for MBC and MBN, respectively. Available phosphorus concentrations in the extracts were measured by using the molybdate blue colorimetric method. The concentrations of the extractable cations were determined by an inductive coupled plasma emission spectrophotometer (Thermo 6300, Thermo Electron, Milford, MA, USA). 

3. Potential extracellular enzymes activities

Potential extracellular enzyme activities relating to C-cycling (α-1,4-glucosidase, β-1,4-glucosidase, β-cellobiose, β-1,4-xylosidase, phenol oxidase and peroxidase), N-cycling (leucine amino peptidase, β-1,4-N-acetyl-glucosaminidase) and P-cycling (alkaline phosphatase) were measured as decomposition and nutrient release functions of ecosystems. The activities of all extracellular enzymes, except for phenol oxidase and peroxidase, were quantified by high throughput fluorometric assay in 96-well microtiter plates. The phenol oxidase and peroxidase activities were measured spectrophotometrically using l-3,4-dihydroxy phenylalanine (DOPA) as the substrate in clear 96-well microplates. The soils were assayed at a pH of 8.0 by suspending approximately 2.5 g of soil in 50 mL of 50 mM sodium acetate buffer. 

4. Potential net N mineralization rates and potential nitrification rates 

Potential net N mineralization rates were determined using the aerobic incubation procedure (Evans et al., 2001). Briefly, 12.5 g fresh soil was placed in a 150 mL specimen bottle, which was then closed with a perforated plastic cap to allow gas exchange while minimizing evaporation. These samples were incubated for 30 days at 25 °C. Potential net N mineralization rates were calculated by the change in the concentrations of total inorganic N before and after the incubation. 

Potential nitrification rates were measured by the chlorate inhibition method (Hoffmann et al., 2007). Briefly, for each sample, two subsamples (2.5 g of fresh soil) were incubated in 50 mL centrifuge tubes containing 10 mL of ammonium sulfate solution (10 mM) and 50 μL of sodium chloride solution (1.5 M). One subsample was added with 2.5 mL of potassium chlorate (2 M) to inhibit nitrite oxidation before incubation, while the other one was not added until an incubation of 5 h in the dark in a shaker (175 rpm; 25 °C). Potential nitrification rates were calculated as the linear accumulation of nitrite concentration between time 0 and 5 h during the incubation. 

Refences:

Evans, R. D., Rimer, R., Sperry, L., & Belnap, J. (2001). Exotic plant invasion alters nitrogen dynamics in an arid grassland. Ecological Applications, 11(5), 1301–1310. https://doi.org/10.1890/1051-0761(2001)011[1301:EPIAND]2.0.CO;2

Hoffmann, H., Schloter, M., & Wilke, B. M. (2007). Microscale-scale measurement of potential nitrification rates of soil aggregates. Biology and Fertility of Soils, 44(2), 411–413. https://doi.org/10.1007/s00374-007-0227-5