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Crop straw retention in the rice-based rotation cropland has been widely accepted as an effective method to improve soil quality in China. Rice–wheat rotation cropland is one the most prevalent rice-based rotation patterns, where it only exploits a small proportion of the total agricultural land yet feeds the majority of the Chinese population. Previous studies indicated that the incorporation of fore-rotating crop straw can effectively facilitate soil carbon sequestration in rice paddy fields. However, the application of crop straw may increase methane (CH4) emissions from rice paddies due to the anaerobic soil condition. To mitigate CH4 emissions from rice paddies while still preserving their soil carbon sequestration ability, a field experiment was conducted in the 2012–2016 rice growing seasons to determine the optimal low-carbon crop straw retention strategy. Five treatments with different wheat straw retention strategies were employed in this study, including non-fertilization and non-straw (Control), conventional fertilization without straw incorporation (CF), conventional fertilization with wheat straw incorporation (FS), slow-release fertilizer combined with wheat straw (SFS), and conventional fertilization with wheat-straw-derived biochar (FB). The results indicated that FS, SFS, and FB treatments significantly increased soil carbon sequestration in comparison with CF treatment. However, the increment of soil carbon sequestration was offset by raw wheat straw induced excess CH4 emissions under FS and SFS treatments. In contrast, the application of wheat-straw-derived biochar significantly promoted soil carbon sequestration, but showed no significant effect on CH4 emissions. Collectively, to the farmers, who aim to achieve agricultural carbon neutrality, the application of straw-derived biochar is worthy of consideration in rice cultivation processes. [ABSTRACT FROM AUTHOR]

Returning straw directly to the field soil is an important strategy that is widely advocated for the disposal of cereal straw. Straw carbonization to form biochar is an innovative approach for treating straw, and its incorporation into the soil is beneficial for improving soil quality. To assess the long-term impacts of these two technologies on greenhouse gas (GHG) emissions and crop yields, a decade-long field study, comparing three treatments, namely direct incorporation of straw from the crop into the field (SCF), incorporation of biochar from straw into the field (BCF), both combined with chemical fertilizer, and the incorporation of chemical fertilizer alone into the field (CF), was conducted on a rice-wheat rotation system in eastern China. Results showed that SCF and BCF achieved the same rice yield as CF treatment alone, while BCF caused a slight decrease in wheat yield. However, there was no significant difference in wheat yield between SCF and BCF, with no significant 'season × treatment' interactions observed. Over the ten-year observation period, rice yields were found to be greatly influenced by air temperature and precipitation, whereas wheat yields were more dependent on precipitation. In terms of GHG emissions, SCF increased annual methane (CH 4) emissions by 144 % but reduced annual nitrous oxide (N 2 O) emissions by 40 %, resulting in an 80 % increase in annual global warming potential (GWP) compared with CF. On the other hand, BCF reduced CH 4 and N 2 O emissions by 58 % and 25 %, respectively, relative to SCF, in the rice season. Despite slightly higher N 2 O emissions in the wheat season, BCF reduced the annual GWP by 51 %, relative to SCF. Additionally, the relative CH 4 emissions of SCF/CF and BCF/CF decreased over time, with BCF showing lower CH 4 emissions than CF after five years. Overall, the findings suggest that straw carbonization into biochar before soil incorporation could be a promising approach to mitigating GHG emissions and maintaining crop yields compared with incorporating straw directly into the soil in the rice-based rice-wheat rotation systems. • SCF and BCF achieved the same rice yield as CF treatment alone. • There was no significant difference in wheat yield between SCF and BCF. • SCF increased annual GWP by 80 % compared with CF. • BCF decreased annual GWP by 12 % relative to CF and by 51 % compared with SCF. • SCF/CF and BCF/CF ratio in CH 4 emissions decreased over the ten years. [ABSTRACT FROM AUTHOR]

A field experiment in Yangtze River delta of China was conducted to investigate the effects of four treatments (conventional chemical fertilizers (CF), chemical fertilizers incorporated with wheat straw (CF-WS), slow-release urea fertilizer incorporated with wheat straw (SCF-WS), and no fertilizers (Non-F)) on methane emission, rice grain yield, and straw biomass in three (2013, 2014, and 2015) consecutive growing seasons. Similar air temperature and precipitation were recorded in 2014 and 2015 normal seasons; however, the 2013 season with a 2.1–2.3 °C higher mean air temperature and a 61–64% lower precipitation than the normal seasons was considered as a warm-and-dry season. No significant differences in seasonal total methane emissions across all treatments were observed in 2014 and 2015 seasons. Seasonal total methane emissions were 58–294% higher in 2013 season compared to 2014 and 2015 seasons, which suggests that higher ambient temperatures increase methane emissions from paddy fields. The enhanced methane emission was seen mainly during the mid-to-late growth stages after mid-season drainage. CF-WS and SCF-WS significantly increased methane emissions before mid-season drainage relative to CF, and thereby enhanced methane emissions in both normal and warm-and-dry seasons in a seasonal scale. While rice grain yield in each treatment in 2013 season was close to or lower than that in 2014 and 2015 seasons, straw biomass in CF, CF-WS, and SCF-WS in 2013 season were significantly higher than those in 2014 and 2015 seasons, by both 29%, 33–41%, and 15–16%, respectively. Enhancements in methane emissions at higher air temperatures were significantly correlated to its greater straw biomass. Furthermore, combination of slow-release urea fertilizer and straw by ploughing can significantly increase above-ground biomass yields and nitrogen use efficiency in both normal and warm-and-dry seasons. [ABSTRACT FROM AUTHOR]

The effectiveness of plant residue addition to increase crop nitrogen (N) use efficiency while reducing N losses is dependent on whether residue management can improve the match between N immobilization-mineralization turnover and plant demand. To understand the effects of residue management on N cycle processes, we conducted a field study that included three treatments. These were a control without N fertilizer (CK), 15 N-labelled urea (UR), and 15 N-labelled urea + wheat straw (URS). Nitrogen immobilized in the top soil layer (0–20 cm) in the urea only treatment tended to decrease after the basal application and the second top dressing, but increased after the first top dressing, whereas the urea + straw treatment increased immobilization up to the second top dressing, before decreasing towards the end of the study. The addition of wheat straw led to a 24% reduction in ammonia loss after the basal application, corresponding well to lower soil extractable ammonium contents. The N uptake by the plant in the urea + straw treatment corresponded to 25% of applied N, which was higher than the 20% recorded in the urea only treatment. At harvest, 31% of the applied N was found in the 0–60 cm soil layer after the addition of wheat straw, whereas there was only 25% without straw addition. The results showed that 45% applied N was unaccounted for when wheat straw was added, compared to 55% unaccounted for without wheat straw. These indicate that the addition of wheat straw may increase plant N uptake by contributing to N immobilization early in the season, but then improving its subsequent mineralization later in the season. [ABSTRACT FROM AUTHOR]

[Display omitted] A field experiment was conducted in Shanghai, China, to examine whether different organic amendment methods can influence the emissions of methane (CH 4) from a rice paddy by regulating the availability of photosynthate carbon (C), which is the methanogenic substrate for the production of CH 4. Three treatments involving different organic amendment methods, rice–wheat without organic amendment (RW), rice–wheat with straw incorporation (RW-S) and Chinese milk vetch-rice with green manure incorporation (RM), were employed during the 2018 rice growing season. The results showed that, compared with the RW treatment, the RW-S and RM treatments significantly increased photosynthate C-induced CH 4 emissions by factors of 5.55 and 1.74 in the tillering stage and 6.03 and 1.62 in the booting stage, respectively. However, the contribution ratios of the photosynthate C-induced CH 4 fluxes to the gross fluxes of CH 4 in descending order were RM > RW > RW-S, which can be attributed to background differences in CH 4 fluxes among the different organic amendment methods. Compared with the tillering stage, the average contribution ratios of photosynthate C-induced CH 4 emissions in the booting stage were 1.59, 4.43 and 2.46 times higher for the RW, RW-S and RM treatments, respectively. These results indicated that the different organic amendment methods affected CH 4 emissions from the rice paddies not only via direct C inputs but also by influencing the availability of rice photosynthate C in rhizosphere. [ABSTRACT FROM AUTHOR]

This study investigated the straw harvested after nitrogen (N) fertilizer application levels (0, 75, 150 and 225 kg N hm−2). The N fertilizer increased straw yield by 115.4–190.6%. In addition to N fertility, the pyrolysis temperatures (300, 500 and 700 °C) induced significant changes of the straw-derived biochar samples. The yield reduced from 41.4 wt% to 23.2 wt%, the residence time increased from 272 yr to 2194 yr, and the residual coefficient of organic C (F p e r m ) increased from 0.65 to 0.93 for the biochar samples as the temperature rising. The parameters of C sequestration were mainly affected by pyrolysis temperature. The N + P 2 O 5 +K 2 O content (5.6–8.8%) of the biochar samples was more in the 500 °C treatment. The characteristics of nutrient supply were affected by both the N fertility and pyrolysis temperature. The N fertilizer rate of 150 kg N hm−2 in the soil for wheat straw, together with the 500 °C treatment induces the best nutrient donor and C sequestration after biochar incorporation into the soil. [Display omitted] • Nitrogen fertilizer rate at 150 kg N hm−2 produces nutrient-rich straw. • Nitrogen fertilizer and pyrolysis temperatures produce desired straw biochars. • Highest N + P 2 O 5 +K 2 O content occurred in the 500 °C biochar treatment. • The MRT of 1151–1294 yr calculated in the 500 °C biochar samples. [ABSTRACT FROM AUTHOR]