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.