2.5 Soil sampling and analysis
Soil samples were collected by laying random quadrats of 10m × 10m on the surface of the waste dump after 1 and 5-years of revegetation and natural forest (NF) as reference. Within each quadrat, composite soil samples were obtained by mixing five subsamples. A total of 110 composite soil samples were collected in a similar manner, 10 samples each at a consecutive depth of 0-10 and 10-20cm from the forest and both the revegetated dumps and 10 samples from 5-years old revegetated dumps under 0, 3, 6, 9 and 12 cm mulch thickness using a stainless-steel soil corer of 10 cm height and 8 cm internal diameter. The collected soil samples were air-dried at room temperature (25-290C) for a week and lightly crushed with a mortar and pestle. The coarse soil particles (>2 mm size) were separated from the crushed soil using standard mesh size gravimetrically. Bulk density (BD) was determined by the soil core method (Blake & Hartge, 1986). Soil pH was determined potentiometrically in soil: water (1:2.5, w/v) by the multi-parameter probe (HI2020, Hanna Instruments India). Soil organic carbon (SOC) was estimated by using the rapid dichromate oxidation technique (Nelson & Sommers, 1996). Soil organic matter (SOM) was determined as the weight differences after a loss on ignition (LOI) between 105oC and 375oC. Soil microbial biomass carbon (MBC) and nitrogen (MBN) were measured by chloroform fumigation /0.5 M K2SO4extraction method (Vance et al., 1987; Brookes et al., 1985). Soil urease activity (UA) was measured by the colorimetric method and the released NH4+ was quantified by UV-VIS Spectrophotometer (Guan, 1986). Dehydrogenase activity (DHA) in soil was estimated using 2, 3, 5-triphenyl tetrazolium chloride as a substrate (Casida, Klein, & Santoro, 1964). Plant available N (Av-N) and total N concentration were determined using a semiautomatic nitrogen estimation system (KJELODIST-EAS VA, Pelican Equipment, India) (Subbiah &Asija, 1956). Available phosphorus (Av-P) was determined by the Bray’s method (Bray & Kurtz, 1945) and Cation exchange capacity(CEC) of the soil was determined by Na-saturation method (soil was saturated with 1N sodium acetate solution, washed with ethanol and subsequently treated with 1N ammonium acetate) and measured by a flame photometer (ESICO-1388, Microprocessor flame photometer, India) (Jackson, 1973). Soil organic carbon and total N stock was calculated by using respective elemental concentration in soils, specific soil depth and bulk density as follows:
\(\text{SOC\ }\text{or\ N\ }\text{stock\ }\left(\text{Mg\ }\text{ha}^{-1}\right)=[SOC\ or\ N\ conc\times\text{BD}\times T\ \times 100]\)(1)
Where SOC is soil organic carbon concentration (%), N is total nitrogen concentration (%), BD is bulk density (Mg m-3), T is soil thickness (m).
2.6 Soil CO2flux measurement
Rate of soil CO2 flux was measured at the revegetated dump surface during two developmental ages and natural Sal forest for a week (24 hours every 20 min interval for 90 seconds) by LICOR LI-8100 Infrared gas analyzer (LICOR Inc. Lincoln, NE, USA). Temperature probe (type E-thermocouple, p/n 8100-201; 6.4 mm diameter, 25 cm length) was inserted up to a depth of 10cm to record soil temperature. Similarly, soil moisture was measured by a moisture probe (ECHO Model EC-5, p/n 8100-202; 5 cm length). The Probe output is given as mV (700–1300 mV), and volume of water content is expressed as m3/m3 by using the following regression equation as given in the LI-8100 instruction manual:
\(\text{VWC\ }\left(\frac{m^{3}}{m^{3}}\right)=\ -\ 3.14\ \times 107\ x\ mV\ +\ 1.16\ \times\ 103\ x\ mV\ -\ 0.612\ \)(2)
In the laboratory, the calibration curve was prepared between millivolts (mV) and moisture content (percent, w/w basis) ranging from 525 to 1190mV and corresponding moisture percentage ranging from 0.5 % (dry soil) to 20.4 % (wet soil) was calculated as follows:
\(Moisture\ (\%,\ w/w\ basis)\ =\ 0.03\ \times\text{\ mV}-\ 14.41\ \ \)[R2= 0.909] (3)