Green manure substitution for chemical nitrogen reduces greenhouse gas emissions and enhances yield and nitrogen uptake in rice rice cropping systems

Abstract

Although nitrogen (N) is important for rice growth, its excessive use can have negative environmental effects, such as greenhouse gas (GHG) emissions. Thus, sustainable and eco-friendly rice production demands precise N management strategies. This includes the use of milk-vetch (MV) as a green manure (GM) for substitution. However, how GM substitution for chemical N fertilizer (NF) affects yield, uptake, methane (CH 4 ) emissions, nitrous oxide (N 2 O) emissions and related microbial mechanisms in rice rice cropping systems remains poorly understood. The present study aimed to (i) investigate the effects of MV substitution for NF on grain yield, N uptake, and emissions of CH 4 , N 2 O, and GHG intensity; (ii) comparatively analyze the mechanistic effects of major microbial associated with CH 4 and N 2 O emissions under MV substitution for NF; and (iii) identify the optimal substitution level of NF by MV for mitigating GHG emission intensity while improving crop N uptake and yield in rice rice cropping systems. To address the aforementioned knowledge gap, we conducted a two-year field experiment based on a long-term study established in 2008. Six treatments, i.e., no fertilizer (N0), farmers’ N practice (N100), N100 and MV (N100MV), 80 % N100 and MV (N80MV), 60 % N100 and MV (N60MV) and only MV, were set up in a randomized complete block design in triplicate. Compared with the other treatments, N80MV significantly increased early and late rice yields, with its average N uptake exceeding that of N100, N100MV, N60MV, and MV by 126.3 %, 88.3 %, 54.2 %, and 31.5 %, respectively. The relative yield was strongly related to the N nutrition index (NNI), with the highest mean NNI values of 1.08 and 1.01 observed in N80MV during the early and late rice seasons, respectively. These findings identify N80MV as the optimal fertilization strategy for increasing both N nutrition and productivity. The balance between the mcr A and pmo A genes as well as between carbon (C) and N played a major role in explaining the variation in CH 4 emissions, whereas ammonia oxidation , the C:N ratio, available N, and the nir K gene played key roles in controlling N 2 O emissions. The moderate GWP and relatively high grain yield resulting from N80MV led to the mitigation of GHG emission intensity. The effectiveness of MV substitution for NF in mitigating GHG emissions while improving yield and N uptake in rice rice cropping systems can vary considerably on the basis of the NF levels substituted by MV. We suggest that substituting MV for 20 % N100 is a viable fertilization strategy not only for mitigating the GHG intensity but also for simultaneously improving yield and N uptake in rice rice cropping systems. Our findings have direct implications for extending our understanding of the dynamics of CH 4 and N 2 O emissions, along with their associated drivers, when GM substitutes for NF in rice rice cropping systems. • CH 4 emissions most driven by C/N and mcr A/ pmo A ratios. • N 2 O emissions were primarily controlled by Ammonia oxidation, available N, and nir K gene. • Full milk-vetch substitution raised CH 4 and lowered N 2 O; farmers’ N practice did opposite. • Milk-vetch substituted for 20 % chemical N-fertilizer decreased greenhouse gas intensity, while increasing yield and N-uptake.