2.2 Experimental set-up
The experiment included the following four treatments with three replicates:
(1) Control plot (nothing applied to the soil); hereafter referred to as ‘C plot’
(2) Biochar plot (8.2 Mg C ha−1) (applied only one time at the beginning of the experiment); hereafter referred to as ‘B plot’
(3) FYM plot (1.1 Mg C ha−1 yr−1) (applied every year i.e., three times during the whole experiment); hereafter referred to as ‘M plot’
(4) Biochar (8.2 Mg C ha−1) and FYM (1.1 Mg C ha−1 yr−1) plot (each applied in the same way as the B and M plots above); hereafter referred to as ‘BM plot’,
Each experimental plot (8 m × 5 m) was arranged in a randomized block design with a 1 m buffer zone.
Table S1 indicates the summary of three years of crop cultivation and land management. Sorghum (Sorghum bicolor (L.) Moench) was cultivated three times during the experimental period. Every year before cultivation, plowing (0–15 cm) was done using hand hoes. In the B and BM treatments, biochar was applied only in Sep 2017, while FYM was applied three times in Sep 2017, Aug 2018, and Aug 2019 (every year before sorghum cultivation) in the M and BM treatments. Both biochar and FYM were incorporated into the soil (0–15 cm depth) using hand hoes. Biochar applied in this experiment was produced from mesquite wood (Prosopis juliflora ) and pyrolyzed with the heap method that local people traditionally use for making charcoal (Srinivasarao et al., 2013). Prosopis juliflora has recently been utilized and/or eliminated in India to control its invasion because it is recognized as an invasive species that can cause reductions in water resources and farmlands (Wakie et al., 2016). The amount of FYM added was representative of the traditional amount applied in the experimental area, and FYM has been incorporated by local farmers every 1–3 years. The application amount of biochar and FYM C was determined by measuring the dry weight, as well as the C content of biochar and FYM by a dry combustion method as mentioned above. Table 1 shows the chemical properties of biochar and FYM.
In all treatment plots, sorghum was planted according to rainfall in each season: in the first year, sorghum was planted in Oct 2017 and harvested in Jan 2018, while in the second and third years, sorghum was planted in August and harvested in December. Every year, sorghum was planted at the rate of 1.75 g m−2 (plant-to-plant distance was 30 cm). During each cultivation period, weeding was carried out with hand hoes every month after planting. After harvesting, aboveground biomass (leaf and stem) was removed outside the field, according to local farmers’ traditional way for animal feed, while belowground biomass (root) was retained. To evaluate belowground C input, i.e., sorghum roots, root biomass were collected from a soil volume of 30 cm (plant spacing) × 30 cm (plant spacing) × 15 cm (depth) for each plot by completely digging out the root system manually at the end of each cultivation period (in Jan 2018, Dec 2018, and Dec 2019). The root samples were washed and dried for more than two days at 70 °C, and the C content and its weight were measured as mentioned above.
During the non-cultivation period, i.e., from after harvesting to the next cultivation period (Feb–Jul in 2018, and Jan–Jul in 2019), weeding was conducted by hand every 2–3 months to maintain bare land in all treatment plots.