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 strategy to combat soil degradation by storing soil organic carbon (SOC) and total nitrogen (TN). We quantified the impacts of NT cropping systems on the changes in SOC and TN stocks and in particulate and mineral-associated organic matter fractions (POM and MAOM), to 100 cm depth, from three 13-year-old experiments in a tropical red Oxisol in Cambodia using diachronic and equivalent soil mass approaches. Established in 2009 and arranged in a randomized complete-block design with triplicates, the experiments included maize (MaiEx)-, soybean (SoyEx)-, and cassava (CasEx)-based cropping systems. Each experiment comprised three treatments: (1) mono-cropping of main crops (maize, soybean, and cassava) under conventional tillage (CTM); (2) mono-cropping of main crops under NT systems with the use of cover crops (NTM); and (3) bi-annual rotation of main crops under NT systems with the use of cover crops (NTR), with both crops being presented every year and represented by NTR1 and NTR2. Soil samples were collected in 2021, 10 years after the last sampling. All the NT systems significantly (p<0.05) increased SOC stock in the topsoil in SoyEx and MaiEx and down to 40 cm in CasEx. Considering the whole profile (0–100 cm), the SOC accumulation rates ranged from 0.86 to 1.47 and from 0.70 to 1.07 Mg C ha−1 yr−1 in MaiEx and CasEx, respectively. Although SOC stock significantly increased in CTM at 0–20 cm in MaiEx and CasEx, it remained stable at 0–100 cm in all the experiments. At 0–5 cm, NTR systems significantly increased TN stock in all the experiments, while, in NTM systems, it was only significant in MaiEx and SoyEx. At 0–100 cm, TN stock in all the experiments remained stable under NTR systems, whereas a significant decrease was observed under NTM systems in SoyEx and CasEx. Although C-POM stock significantly increased under all NT systems limited to 0–10 cm in MaiEx and SoyEx, all the NT systems significantly increased C-MAOM stock in the 0–10 cm layer in MaiEx and SoyEx and down to 40 cm in CasEx. All the NT systems significantly increased N-POM stock at 0–10 cm in MaiEx and SoyEx, while a significant decreased in N-MAOM stock was observed below 5 cm in CasEx and below 40 cm in MaiEx and SoyEx. Our findings showed that long-term NT systems with crop species diversification accumulated SOC not only on the surface but also in the whole profile by increasing SOC in both the POM and MAOM, even in the cassava-based system. This study highlights the potential of NT systems for storing SOC over time but raises questions about soil N dynamics.