Greenhouse gas emissions during the rice season are reduced by a low soil C:N ratio using different upland-paddy rotation systems

Context

Upland-paddy rotation can improve multiple-cropping index and crop yields; however, the mechanisms underlying the effects of dry-season crop diversification on rice yields and greenhouse gas (GHG) emissions under multiple rotation systems remain unclear.

Objective

Here, we aimed to clarify the intrinsic mechanisms whereby rice yields and GHG emissions respond to the diversification of dry-season crops and lay a theoretical foundation for developing agronomic measures that can stabilize yields and reduce GHG emissions.

Methods

Using a positioning experimental site for upland-paddy rotation, we measured rice-season CH₄ and N₂O emissions, crop yields, GHG-emission intensity (GHGI) levels, soil physical and chemical properties in garlic–rice (GR), wheat–rice (WR) systems for 3 years (2019–2020, and 2022), and in a rapeseed–rice (RR) system for 1 year (2022). The soil microbial dynamics of the three systems were only tested in 2022.

Results

The WR system had the highest CO₂ emission equivalent (CO₂-eq), with a 3-year interval value of 1898.24–16794.30 kg·ha⁻¹, the lowest yield (8490.10–9773.46 kg·ha⁻¹), and the highest GHGI (0.22–1.83). The GR system had the highest rice yield (9718.91–10769.75 kg ha⁻¹), a lower CO₂-eq (1588.55–12567.51 kg·ha⁻¹), and therefore a lower GHGI (0.16–1.24). The RR system had the lowest GHGI in 2022 (benefiting from the lowest CO₂-eq) and a slightly higher yield than that of the WR system. CH₄ contributed to >88 % of the CO₂-eq under the three systems in 2020 and 2022. The higher soil C:N ratio of the WR system stimulated methanogenic microorganisms, coupled with higher microbial biomass C levels, and ultimately increased CH₄ emissions substantially. The soil C:N ratios of the GR and RR systems were significantly lower than that of the WR system because the soil total nitrogen (TN) of both systems was higher and increased CH₄ emissions were avoided. The higher levels of N nutrients (TN, NO₃^--N, and NH₄⁺-N) in the GR and RR systems also enhanced rice yields, with respective increases of 10.37 % and 1.22 %, compared with that of the WR system.

Conclusions

The diversified cultivation of dry-season crops in upland-paddy rotation systems affected rice yields and GHG emissions by changing the ratios of C and N.

Implications

Our findings highlight the importance of future research involving comprehensive agronomic measures to help reduce emissions, including fertilizer management, straw management, and tillage methods.