Combined effects of nitrogen fertilizer and biochar on greenhouse gas emissions and net ecosystem economic budget from a coastal saline rice field in southeastern China

Effects of biochar application methods on greenhouse gas emission and nitrogen use efficiency in paddy fields

Biochar application in rice production reduces nitrogen loss and greenhouse gases. We conducted in situ experiments for 3 years, with N210B0 (210 kg N ha-1) as the control. Two biochar application methods (B1:15 t ha-1 biochar applied once and B2: biochar applied three times at 5 t ha-1 yr-1) combined with two nitrogen levels (N210: 210 kg N ha-1 and N168: 168 kg N ha-1) were used. Soil physicochemical properties, CH4 and N2O emissions, functional gene abundance, rice yield, and nitrogen use efficiency were analyzed. Both methods improved the physicochemical properties of the soil, however, B1 was less effective than B2 in increasing soil pH, bulk density, organic carbon, total nitrogen, and microbial biomass nitrogen in year 3. B1 had a higher CH4 emission mitigation effect than B2 in 3 consecutive years, mainly due to the higher pmoA gene abundance. B1 showed a higher reduction effect of N2O emissions compared to B2 in year 1, but the opposite was observed in years 2 and 3. B2 had a higher abundance of AOB, nirK, and nosZ genes compared to B1 in year 3. Compared with N210B0, rice yields were increased by 9.1 %, 9.6 %, and 3.6 % with N210B1, N210B2, and N168B2, respectively, over 3 years, while N168B1 improved yields in the previous 2 years. Biochar improved nitrogen use efficiency over 3 consecutive years directly due to increased use efficiency of panicle fertilizer; the effect of B1 was greater than that of B2 during years 1 and 2, while the opposite was observed in year 3. Both Biochar applied once and three times appeared to be promising practices to increase yield and mitigate GHGs. From the GHGI perspective, the biochar applied once combined with 168 kg N ha-1 can further improve nitrogen use efficiency, and reduce GHGs without hindering improvements in rice yield.

A global meta-analysis of yield-scaled N2 O emissions and its mitigation efforts for maize, wheat, and rice

Maintaining or even increasing crop yields while reducing nitrous oxide (N2 O) emissions is necessary to reconcile food security and climate change, while the metric of yield-scaled N2 O emission (i.e., N2 O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta-analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield-scaled N2 O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield-scaled N2 O emissions across all available data followed the order wheat (322 g N Mg-1 , with the 95% confidence interval [CI]: 301-346) > maize (211 g N Mg-1 , CI: 198-225) > rice (153 g N Mg-1 , CI: 144-163). Yield-scaled N2 O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield-scaled N2 O emissions with increasing N use efficiency or in N2 O emissions for production systems with cereal yields >10 Mg ha-1 (maize), 6.6 Mg ha-1 (wheat) or 6.8 Mg ha-1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade-offs between crop production and N2 O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced-efficiency N fertilizers significantly reduced yield-scaled N2 O emissions at similar or even higher cereal yields. Our data-driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production.

Paddy rice yield and greenhouse gas emissions: Any trade-off due to co-application of biochar and nitrogen fertilizer? A systematic review

Combined application of biochar and nitrogen (N) fertilizer could offer opportunities to increase rice yield and reduce methane emissions from paddy fields. However, this strategy may increase nitrous oxide (N2O) emissions, hence its interactive effects on GHG emissions, global warming potential (GWP) and GHG intensity (GHGI) remained poorly understood. We conducted a systematic review to i) evaluate the overall effects of combined application of biochar and N fertilizer rates on GHGs emissions, GWP, rice yield, and GHGI, ii) determine the quantities of biochar and N-fertilizer application that increase rice yield and reduce GHGs emissions and GHGI, and iii) examine the effects of biochar and different types of nitrogen fertilizers on rice yield, GHGs, GWP, and GHGI using data from 45 research articles and 183 paired observations. The extracted data were grouped based on biochar and N rates used by researchers as well as N fertiliser types. Accordingly, biochar rates were grouped into low (≤9 tons/ha), medium (>9 and ≤ 20 ton/ha) and high (>20 tons/ha), while N rates were grouped into three categories: low (≤140 kg N/ha), medium (>140 and ≤ 240 kg N/ha), and high (>240 kg N/ha). For fertiliser types, N rates were grouped as: low (≤150 kg N/ha), medium (>150 and ≤250 kg N/ha), and high (>250 kg N/ha) and N types into: urea, NPK, NPK plus urea (NPK_urea) and NPK plus (NH4)2SO4 (NPK_(NH4)2SO4). Results showed that biochar and N fertiliser significantly affected GHGs emissions, GWP, GHGI and rice yield. Compared to control (i.e., sole N application), co-application of high biochar and medium N rates significantly decreased CH4 emission (82 %) while low biochar with low N rates enhanced CH4 emission (114 %). In contrast, high biochar combined with low N decreased N2O emission by 91 % whereas medium biochar and high N rates resulted in 82 % increase in N2O emission relative to control. The highest GWP and GHGI were observed under co-application of medium biochar and low N rates. Highest rice yield was observed under low biochar rate and high N rate. Regardless of N fertiliser type and biochar rates, increasing N rates increased rice yield and N2O emissions. The highest GWP and GHGI were recorded under sole NPK application. Combination of low biochar and medium N produced low GHGs emissions, high grain yield, and the lowest GHGI, and could be recommended to smallholder farmers to increase rice yield and reduce greenhouse gas emissions from paddy rice field. Further studies should be conducted to evaluate the effects of biochar properties on soil characteristics and greenhouse gas emissions.

Agricultural diversification promotes sustainable and resilient global rice production

Rice is a staple food for half of the human population, but the effects of diversification on yields, economy, biodiversity and ecosystem services have not been synthesized. Here we quantify diversification effects on environmental and socio-economic aspects of global rice production. We performed a second-order meta-analysis based on 25 first-order meta-analyses covering four decades of research, showing that diversification can maintain soil fertility, nutrient cycling, carbon sequestration and yield. We used three individual first-order meta-analyses based on 39 articles to close major research gaps on the effects of diversification on economy, biodiversity and pest control, showing that agricultural diversification can increase biodiversity by 40%, improve economy by 26% and reduce crop damage by 31%. Trade-off analysis showed that agricultural diversification in rice production promotes win-win scenarios between yield and other ecosystem services in 81% of all cases. Knowledge gaps remain in understanding the spatial and temporal effects of specific diversification practices and trade-offs.

Contrasting effects of different field-aged biochars on potential methane oxidation between acidic and saline paddy soils

There is recognition that biochar addition is an appropriate measure to mitigate methane (CH₄) emissions by promoting potential methane oxidation (PMO) in the field. However, the mechanism for different field-aged biochars and effective duration after field application are not well documented. Based on a long-term field experiment, biochar was field aged and separated from two contrasting acidic (Ba) and saline (Bs) paddy fields. Then, the effects of different aged biochars on PMO in acidic and saline paddy soils were explored by incubation experiment. There were five treatments for each soil group: soil without biochar (CK), biochar-enriched paddy soil (2 or 6 years) (NB), fresh biochar amendment (Bf), aged biochar separated from acidic paddy soil amendment (Ba), and aged biochar separated from saline paddy soil amendment (Bs). Results showed that saline paddy soils had a significantly higher PMO than acidic paddy soils under treatment without biochar, and that PMO in acidic paddy soil was enhanced by various biochar amendments, whereas those biochar amendments had no significant effects on PMO in saline paddy soil. PMO was positively correlated with pmoA abundance, N consumption rate and pH of soil-biochar mixture. Aged biochar separated from different fields had conflicting influences on soil pH, N consumption rate and PMO. Ba lost its initial effect on changing PMO as compared to Bf treatment when added back into acidic paddy soil. To the contrary, the acidic paddy soil NB treatment containing biochar added six years before possessed the highest value of PMO among all ten treatments. This study suggested that acidic paddy soil with biochar amendment could mitigate CH₄ emissions by promoting PMO for a prolonged period, though aged biochar separated from the same field had a limited impact on reducing CH₄ emissions.

A quantitative review of the effects of biochar application on rice yield and nitrogen use efficiency in paddy fields: A meta-analysis

Applying biochar to paddy fields is a helpful approach that potentially increases rice production and nitrogen use efficiency (NUE) to ensure food security and protect the ecological environment. Notwithstanding, reviewing most of the previous experimental studies on the impacts of biochar reveals a considerable inconsistency in the proposed results. The present study conducts a comprehensive meta-analysis on the literature published before February 2021 to investigate the impacts of biochar properties, experimental conditions, and soil properties on rice yield and NUE. The meta-analysis results show that biochar application increases rice yield and NUE by 10.73% and 12.04%, respectively. The most significant improvements in the soil properties are seen in alkaline soils and paddy soils with a fine-textured. In addition, the benefits of biochar are significantly enhanced when produced at 500-600 °C with livestock manure due to the existence of more nutrients compared to other feedstocks. Analysis of water management reveals that biochar application under water-saving irrigation is more effective in increasing rice productivity. In terms of application rates, the >20 t/ha biochar and 150-250 kg/ha nitrogen fertilizer are recommended for improving rice yield and NUE. Regardless of existing uncertainty due to the lack of long-term experimental data, those investigated factors have significant implications for biochar management strategies in rice growth systems.