2.3.3. The Soil Samples Collection

Three soil cores (0 to 15 cm depth) were collected from each plot following rice harvest in November 2016, 2017 and 2018. Plant debris was removed and the soil separated into two parts: (1) air-dried at room temperature and then ground to pass a 2-mm sieve; (2) stored at 4 ◦C for subsequent microbial activity measurements. Soil pH, soil water and soil organic matter (SOM) were analyzed using methods described by Lu [25]. Briefly, soil pH was measured using a glass electrode with a soil-to-water ratio of 1:2.5. Soil water content was determined gravimetrically by weight difference before and after at least six hours of oven drying at 105 ◦C. The SOM was determined using the dichromate oxidation method. A subsample of the air-dried, ground soil was further ground to pass a 0.15-mm sieve and used to determine total Cd and Pb concentrations and sequentially extractable heavy metals fractions as described below [1,7].
