Diachronic assessment of soil organic C and N dynamics under long-term no-till cropping systems in the tropical upland of Cambodia

Abstract. No-till (NT) cropping systems have been proposed as a potential strategy to combat soil degradation and global warming by storing soil organic carbon (SOC) and nitrogen (N). Yet, there are ongoing debates about the real benefits of NT systems and factors influencing SOC and N accumulation. Assessing the dynamics of SOC and N on the long-term is needed to fill knowledge gaps and provide robust scientific evidence for potential additional SOC storage. We quantified the changes in SOC and N stocks and fractions down to 100 cm depth from three 13-year-old experiments in a tropical red Oxisol in Cambodia, comparing conventional tillage (CT) to NT monocropping and NT crop rotation systems using a diachronic and equivalent soil mass (ESM) approach. The three experiments comprised maize-, soybean-, and cassava-based cropping system trials, hereafter called MaiEx, SoyEx, and CasEx, respectively. Soil samples were collected in 2021, 10 years after the first sampling in 2011, at 7 depths: 0–5, 5–10, 10–20, 20–40, 40–60, 60–80, and 80–100 cm. Over the 10-year period (2011–2021), significant impacts on SOC stock and its vertical distribution differed among the NT systems and in the three experiments. In MaiEx and CasEx, the soils under all the NT systems significantly (P > 0.05) accumulated SOC stock across the soil depths, with the accumulation ranging from 6.97 to 14.71 Mg C ha^-1 in the whole profile (0–100 cm). In SoyEx, significant increase in SOC stock was limited to the top 0–20 cm under NT monocropping, whereas NT crop rotation systems had significantly accumulating SOC stock from 0 to 80 cm depths. When considering 0–100 cm as a single stratum, the annual SOC cumulative rate in NT systems ranged from 0.86–1.47, 0.65–1.00, and 0.70–1.07 Mg C ha^-1 yr^-1 in MaiEx, SoyEx, and CasEx, respectively. In the top 0–10 cm, NT systems significantly increased C concentration in particulate organic matter (POM) by 115 %, 118 %, in MaiEx and SoyEx, respectively, and by 37 % in CasEx although not significantly. Similarly, at 0–10 cm depth, NT systems significantly enhanced C concentration in the mineral-associated organic matter (MAOM) by 33 %, 21 %, in MaiEx and SoyEx, respectively. Significant increase of C in MAOM was also observed from 0 to 40 cm in CasEx. In contrast, total N stock in NT systems increased in the surface 0–5 cm depth but decreased below 10 cm and in the whole profile (0–100 cm), particularly under NT monocropping with an annual loss rate of -0.10 and -0.17 Mg N ha^-1 yr^-1 in SoyEx and CasEx, respectively. Although NT systems increased N concentration in POM in the top 0–10 cm of MaiEx and SoyEx, a decreasing trend was observed below 10 cm depth. The N concentration in POM under NT systems in CasEx also decreased with soil depth. From 2011 to 2021, N concentration in MAOM under NT systems remained stable in MaiEx and SoyEx in the top 0–5 cm, but significant decreases in MaiEx and CasEx below 5 cm. Our findings suggest that adopting NT cropping systems with diverse crop and cover crop species and high biomass C inputs in the long-term leads to SOC accumulation not only in the surface but also in deeper layers, by increasing both the C pools in the POM and MAOM size fractions, even on the cassava-based system, which is believed to be an annual crop that could cause serious soil fertility depletion. This study highlights the potential of NT cropping systems to store SOC over time, but raises questions about soil N dynamics.