2.1 Experimental site
A field experiment was conducted from September 2017 to December 2019 (27 months total) in a farmer’s field in Madurai, Tamil Nadu state, India (9°43’22.37” N 77°46’51.61” E; 175 m asl) (Seki et al., 2019). The mean annual air temperature was 24.7 °C and the annual rainfall was 820 mm (692–857 mm; 2017–2019). This area has 40–75% of annual rainfall during the rainy season (South-West monsoon: June–September, and North-East monsoon: October–December). Due to the low SOC content (Seki et al., 2019), the experimental field should be representative of the degraded cropland soils in this area (Lal, 2004b). Soil was classified into Typic Haplustepts (Soil Survey Staff, 2014). The value for the selective physicochemical properties of the surface layer (0–15 cm depth) in this site were: soil pH (1:5 water) of 8.5, SOC of 3.2 g kg−1, inorganic carbon (IC) of 0.1 g kg−1, clay content of 27.2%, cation exchange capacity of 25.1 cmolc kg−1, and soil bulk density of 1.57 g cm−3. Surface SOC stock (0–15 cm depth) was 8.3 Mg C ha−1. TC was measured by a dry combustion method with a NC analyzer SUMIGRAPH NC TR-22 (Sumika Chemical Analysis Service, Japan). IC was measured following the method provided by Bundy and Bremner (1972). Briefly, the soil sample was treated with 1M HCl at room temperature for 24 h, and then unreacted HCl that was not released as CO2 from carbonates was determined by titration with 1M NaOH to calculate the IC content. SOC was calculated as follows: SOC = total carbon (TC) – IC.