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Background and aims: It is known that nitrogen (N) input modulates the rhizosphere priming effect (RPE); however, the magnitude and driving mechanisms of priming under increasing rates of fertilizer application remain unclear. Methods: 15N-urea (control, 75 (N75), 150 (N150), 225 (N225), and 300 (N300) kg N ha−1) was applied to a plant (maize)-soil (rice paddy) system and the RPE was monitored during the trumpet period (the most active stage) of plant growth. Results: Addition of N decreased soil-derived CO2 emission by 21.1–49.3% in the presence of plants. The RPE declined following N input in the control−N150 as a result of low microbial C:N imbalance, which decreased enzyme activities due to low microbial N mining and microbial activation, and high microbial metabolic efficiency (MME). In contrast, the RPE increased following N input in the N150 − N300, which was attributed to the high microbial C:N imbalance causing low MME, rather than the promotion of microbial N mining or microbial activation mechanisms. The microbial C:N imbalance was the result of N competition between microorganisms and plants. Conclusions: Thus, the combination of enzyme activities and MME mediates N-regulated RPE, while N competition drives the switching of RPE mechanisms. [ABSTRACT FROM AUTHOR]

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. Image 1 • Introduction of maize–rice (MR) into double rice (RR) decreased GHG emissions. • Crop yield of MR decreased slightly in 2017 and increased greatly in 2018 than RR. • Introducing MR reduced C footprint by lowering it in both season crops. • Higher increase of C input than C output led to larger net C sink in MR. • MR is an alternative system with low C footprint and high C sink and crop yield. [ABSTRACT FROM AUTHOR]

To assess the impacts of the incorporation of spring maize into paddy fields on the utilization of nitrogen (N) fertilizer by late rice, a two-year microplot experiment was performed using labeled 15N-urea to trace the fate of N fertilizer in spring maize-late rice (SM-LR) and early rice-late rice (ER-LR) systems. Compared with ER-LR system, SM-LR system showed higher grain yields and total N use of late rice. However, no difference in fertilizer-N use in late rice between the two cropping systems, and the utilization of soil-N rather than fertilizer-N was responsible for the increased total N uptake by late rice. Further microplot experiments on the fate of soil residual 15N showed a greater amount of soil residual 15N in the spring maize season than in the early rice season (122 vs. 72 kg N ha−1). However, soil residual 15N from the first season had a negligible contribution to the utilization of N in late rice. We conclude that the improvement in the soil N supply accounts for the increased total N use in late rice under the incorporation of spring maize into paddy fields. • Incorporation of spring maize in paddy fields increased total N uptake in late rice. • Fertilizer-N uptake in late rice was not improved by incorporating spring maize. • More soil residual 15N was found in the spring maize season than early rice season. • Soil residual 15N had a minor contribution to total N uptake in late rice. • Enhancement of soil-N supply accounted for the increased N uptake in late rice. [ABSTRACT FROM AUTHOR]