Decreasing net global warming potential through partial substitution of urea with manure and slow-release fertilizer in a double-rice system

Objectives Straw returning is a common practice in rice production. In this case, we studied the effect and mechanism of partial substitution of urea with manure and slow-release fertilizeron decreasing the net global warming potential in a double-rice agro-ecosystem under long-term straw return condition to provides the basis for the realization of carbon sequestration and emission reduction.

Methods Field localized experiments were started since 2013 in a double-rice system in Jiangxi Province, China. Four nitrogen fertilizer treatments were setup on the base of whole straw of pre-crops returning to field, and total of N input of 165 kg/hm² in early rice and 195 kg/hm² in late rice. The treatments were: 1) 100% urea (CK); 2) 80% urea + 20% manure N (N1); 3) on the base of N1, applying extra Si, Zn and S (N2); 4) on the base of N2, 30% of total N replaced with coating urea (N3). Rice yield, above ground biomass and soil organic matter content was measured, the greenhouse gas emissions were measured during double-rice growth season in 2016.

Results Greenhouse gas emissions in late rice season were significantly higher than those in early rice season, e.g. the CH₄ emission was as high as 4 times (P < 0.05) and CO₂ emissions were 7.5%–9.3% higher (P > 0.05). Among the four treatments, the CO₂ emission was not significantly different in both early and late rice season; the N₂O emission was significantly decreased in the manure containing treatments (N1, N2 and N3), with decrement of 31.7%, 27.2% and 43.7% in early rice season, and 20.0%, 31.5% and 40.6% in late rice season in turn (P < 0.05). Manure application resulted in a significant increase of CH₄ emission compared with CK. In early rice season, the increment in CH₄ emission under N1, N2 and N3 were 13.1%, 13.9% and 21.4%, and 19.4%, 12.7% and 13.7% in late rice season, respectively (P < 0.05). Soil carbon sequestration potential in seasonal or yearly scale, which was estimated with above ground biomass, was significantly higher under manure treatments (N1, N2 and N3) comparing to CK (P < 0.05). In 2016, the net global warming potential (GWP) in early rice season was negative (carbon sink) due to 30% increment of the yield, while that was positive in late rice season (carbon source), and the whole year belonged to carbon source. Even so, the net GWP in N1, N2 and N3 treatments were significantly lower than that in CK, in order of N3 < N1 = N2 < CK.

Conclusions In the field with long-term straw returning, partial substitution of mineral N with manure would increase the absolute CH₄ emission, but the increased yield and carbon input would offset the adverse effect, and significantly decrease the N₂O emission and increase soil carbon storage, which would significantly decrease the net GWP in double rice system. The extra use of coating urea would strengthen the positive effect, but Si and micronutrient would not. Therefore, optimize fertilizer management to increase early rice yield is an efficient way to decrease greenhouse gas intensity in a double rice system due to much higher greenhouse gas emission in late rice season.