Conservation agriculture increases phosphorus pools and stock in the top 30 cm of soil: A three-year study on a subtropical legume-rice rotation

The adoption of conservation agriculture (CA) practices is likely to alter soil phosphorus (P) pools and availability leading to changes in P fertilizer use and management. However, the impacts of CA practices on soil P fractions and their distribution in the soil profile under rice-based systems is not well-understood. This study examined the effect of two crop establishment practices (strip planting, SP and conventional tillage, CT) and two crop residue levels (high residue, HR- 40% of above ground biomass and low residue, LR- 15%) on P fractions in the soil profile under a lentil-mungbean-rice sequence. After three consecutive years of CA practice, covering nine crops, composite soil samples were collected at 0-5, 5-15, 15-30, 30-45 and 45-60 cm depths and analysed for solution P, NaHCO3-extracted inorganic (NaHCO3 Pi) and organic (NaHCO3Po) P, NaOH-extracted inorganic (NaOH Pi) and organic (NaOH Po) P, stable P, residue P, total P, soil organic carbon (SOC) and total nitrogen (N). The inorganic P (solution P, NaHCO3 Pi, NaOH Pi and stable P) comprised 53-59% while the organic P (NaHCO3 Po and NaOH Po) comprised only 6%-13% of total P. However, CA practices increased SOC with the corresponding increase in soil organic P fractions. HR increased solution P, NaOH Po, stable P, residue P, total P, as well as SOC contents along with the stocks of total carbon (C), N, and P at up to 15 cm while increasing NaHCO3 Pi, NaOH Pi and TN at up to 30 cm depth compared with the LR. Likewise, SP increased the inorganic P fractions (NaHCO3 Pi and NaOH Pi) at 0-30 cm depth over the CT while P fractions in the deeper soil layers were similar to each other. In conclusion, the increase in soil P pools at up to 15-30 cm depths especially because of increased crop residue retention should allow for a decrease in P fertilizer inputs in intensive rice-based systems.