Search

The application of mineral fertilizers can effectively enhance crop yields. However, this potential benefit may be diminished if the use of mineral fertilizers leads to a substantial decline in soil organic carbon (SOC) and an increase in soil greenhouse gas (GHG) emissions. This study aimed to determine the optimal fertilizer combinations and rates for improving SOC and maize yield while reducing GHG emissions in the semi-arid uplands of Kenya. Data were collected from five different fertilizer treatments (N50, N100, N150, N100+manure, and N100+straw) compared to a control (N0) in a long-term experimental field, which was used to run and validate the DNDC model before using it for long-term predictions. The results showed that the combination of mineral fertilizer and straw resulted in the highest SOC balance, followed by that of fertilizer and manure. All fertilized treatments had higher maize grain yields compared to low-fertilizer treatment (N50) and control (N0). Daily CO2 fluxes were highest in the treatment combining mineral fertilizer and manure, whereas there were no significant differences in N2O fluxes among the three tested treatments. The findings of this study indicate that the judicious application of mineral fertilizer, animal manure, and straw has great potential in enhancing SOC and maize yields while reducing GHG emissions, thereby providing practical farming management strategies in semi-arid Kenya. [ABSTRACT FROM AUTHOR]

Incorporating crop residues into the soil is an effective method for improving soil carbon sequestration, fertility, and crop productivity. Such potential benefits, however, may be offset if residue addition leads to a substantial increase in soil greenhouse gas (GHG) emissions. This study aimed to quantify the effect of different crop residues with varying C/N ratios and different nitrogen (N) fertilizers on GHG emissions, yield, and yield-scaled emissions (GHGI) in winter wheat. The field experiment was conducted during the 2018–2019 winter wheat season, comprising of four residue treatments (no residue, maize residue, soybean residue, and maize-soybean mixed residue) and four fertilizer treatments (control, urea, manure, and manure + urea). The experiment followed a randomized split-plot design, with N treatments as the main plot factor and crop residue treatments as the sub-plot factor. Except for the control, all N treatments received 150 kg N ha−1 season−1. The results showed that soils from all treatments acted as a net source of N2O and CO2 fluxes but as a net sink of CH4 fluxes. Soybean residue significantly increased soil N2O emissions, while mixed residue had the lowest N2O emissions among the three residues. However, all residue amendments significantly increased soil CO2 emissions. Furthermore, soybean and mixed residues significantly increased grain yield by 24% and 21%, respectively, compared to no residue amendment. Both soybean and mixed residues reduced GHGI by 25% compared to maize residue. Additionally, the urea and manure + urea treatments exhibited higher N2O emissions among the N treatments, but they contributed to significantly higher grain yields and resulted in lower GHGI. Moreover, crop residue incorporation significantly altered soil N dynamics. In soybean residue-amended soil, both NH4+ and NO3− concentrations were significantly higher (p < 0.05). Conversely, soil NO3− content was notably lower in the maize-soybean mixed residue amendment. Overall, our findings contribute to a comprehensive understanding of how different residue additions from different cropping systems influence soil N dynamics and GHG emissions, offering valuable insights into effective agroecosystems management for long-term food security and soil sustainability while mitigating GHG emissions. [ABSTRACT FROM AUTHOR]

Nitrogen (N) fertilizer and crop residue amendments are important agricultural practices that could increase soil health, fertility, and crop yield. Such practices may also change soil denitrification processes where contradictory observations have been reported on soil N2O emissions with fewer studies on N2 emissions due to its large atmospheric background concentrations limiting its soil-borne measurement. This study aims to investigate N2O production and reduction of N2 emissions under a conducive denitrifying environment (like anaerobic microsites, 80% WFPS, available N and C) after rice straw amendment and KNO3 application to three different soil types (fluvo-aquic, black, and paddy soils). In this regard, three treatments for three different soil types were set consisting of (a) a non-amended treatment (control), (b) a KNO3 treatment (KNO3, 20 mM KNO3), and (c) a straw plus KNO3 treatment (2.5 g rice straw kg−1 dry soil and 20 mM KNO3), which were incubated under 80% WFPS. Moreover, direct N2O and N2 fluxes were measured over 17 days in the current incubation experiment with a robotized incubation system using a helium atmosphere. Results showed that rice straw amendment combined with N fertilizer increased both N2O and N2 fluxes compared with control or KNO3 treatments in all three soil types. Overall, compared with the black and paddy soils, the N2O and N2 fluxes were higher in the fluvo-aquic soil, with a maximum of 234.2 ± 6.3 and 590.1 ± 27.3 g N ha−1 from F_SK treatment, respectively, during the incubation period. The general trends in three soil types of both N2O and N2 emissions were control < KNO3 < rice straw plus KNO3 treatments. Straw amendment in combination with KNO3 can stimulate a high denitrification rate (less N2O and higher N2), whereas their effect on stoichiometric ratios of N2O/(N2O + N2) highly depends on soil nitrate concentration, oxygen level, soil moisture content, and labile C. The current study underscores that the rice straw amendment in combination with N fertilizer can trigger denitrification with less increment on soil N2O but higher N2 emissions under conditions favoring denitrification. [ABSTRACT FROM AUTHOR]

Purpose: The key environmentally beneficial process that substantially removes reactive nitrogen from biosphere is a complete denitrification. The science of measuring and constraining nitrous oxide (N2O) emissions has advanced significantly; however, despite several attempts, in situ dinitrogen (N2) measurement is still a great challenge and is poorly understood due to the high atmospheric N2 background. This study aimed at estimating field-scale inferred N2 emissions using data of field N2O emissions and laboratory-measured N2O/(N2O + N2) ratios and correlating those emissions with the soil-environmental factors. Materials and methods: Closed static chamber and He/O2 direct measurement methods were used at field and laboratory scale, respectively. For each treatment (varying N fertilizer rates), on each sampling date, N2O and N2 emissions were measured at laboratory and N2O emissions at field-scale, allowing the calculation of field-scale inferred N2 emissions. Results: The results demonstrate that field-scale inferred cumulative N2 emissions were 1.35, 1.48 and 1.60 times greater than laboratory-measured cumulative N2 emissions in low nitrogen level (LNL), medium nitrogen level (MNL) and high nitrogen level (HNL) treatments, respectively. This suggests that estimating N2 emissions at the field-scale in agricultural soil could give more insight on N cycling processes. Moreover, N fertilizer application rates increased linearly both field and laboratory cumulative N2O and N2 emissions. Both positive and negative relationships between soil-environmental parameters and N2O, N2 and their N2O/(N2O + N2) ratios at field and laboratory-based indicate their heterogeneous roles in N2O formation and reduction processes. Conclusion: The results provide complementary insights into field-scale N2 emissions in agricultural soil and help in closing the knowledge gap in the N balance. Linear relationships between the emissions (N2O and N2) and N fertilizer rates observed suggest that climate change mitigation options could be achieved by optimizing the N fertilization rates since N2O and N2 emissions are enhanced by increasing N inputs. As our results present the field-scale inferred N2 emissions, there is still a need to design a robust methodological approach that will enable researchers to directly quantify field N2 emissions. [ABSTRACT FROM AUTHOR]

Micro sensors have large potential to measure soil nitrous oxide (N2O) emissions. However, because of its low sensitivity to get the electrical signals on soil with low moisture content, little research has been conducted in semi-arid agro-ecosystem. Moreover, the use of N2O microelectrode studies were largely focused on the marshy fields, paddy fields and waste waters with high soil moisture content. In order to characterize the boundary of the sensor signal (upper and lower), purified water without N2O is usually used as the calibration medium to calibrate the electrode. However, the physical and chemical properties of purified water and soil leaching solutions are very different. In this study, we improved and verified a microelectrode calibration method based on selecting appropriate media and optimal electrolyte concentration. In addition, we applied the improved microelectrode calibration method to continuously observe N2O flux from the soil column in a 30-day-long incubation experiment. The results pointed to noteworthy improvement of the microelectrode calibration compared to the current method, with soil solution validated as a calibration medium at optimal electrolytic concentration of 7:1-8:1, expressed as water/soil ratio. The emission of N2O from soil with different soil fertility were also significantly different. Our research is beneficial to establish the microelectrode calibration method of N2O emission under low moisture conditions from soil microenvironment, and also points on management practices of agro-ecosystem for N2O mitigation strategies. [ABSTRACT FROM AUTHOR]