Double paddy rice conversion to maize–paddy rice reduces carbon footprint and enhances net carbon sink

Increasing water scarcity and demand for biofuels and fodder has resulted in a change to traditional farming cropping system from double paddy rice (early rice–late rice, sign as ERLR) to maize–paddy rice (spring maize–late rice, sign as SMLR) in Asia. Whether the introduction of SMLR would induce lower greenhouse gas (GHG) emissions and higher net carbon (C) sink remains unclear. The objective of this study is to conduct a comprehensive assessment of the C footprint and net ecosystem carbon budget (NECB) for ERLR and SMLR systems, based on a two–year (2017–2018) paddy field experiment in southeastern China. Results showed that introducing SMLR resulted in a slight decrease of grain yield by 3.78% in 2017 and a pronounced increase by 17.6% in 2018. The introduction of SMLR into ERLR significantly reduced the C footprint by 35.1–41.7%. This was attributed to spring maize having a 60.1–64.5% lower C footprint relative to early rice and the C footprint of later rice in SMLR being 17.7–19.0% lower than in ERLR. Methane emissions were the largest contributor to the C footprint in both cropping systems, accounting for 52.7–54.4% and 37.2–39.6% in ERLR and SMLR, respectively. This composition of C footprint was similar to that of early rice in ERLR and late rice in ERLR and SMLR. However, GHG emissions from the manufacture of nitrogen fertilizers were the largest fraction of the C footprint in spring maize production, accounting for 40.0–49.3%. Although SMLR resulted in a greater C output due to increased direct carbon dioxide emissions from soils (by 60.1–142%), the introduction of SMLR produced a higher C input. Furthermore, the increase of C input outweighed the increase of C output, leading to a significant increase of NECB by 80.1–147%. Results from this study demonstrate that the introduction of SMLR into ERLR is highly effective strategy for reducing C footprint and enhancing the net C sink as well as maintaining high grain yield.