Nitrous Oxide Emissions from an Alpine Grassland as Affected by Nitrogen Addition

Nitrogen (N) addition is an important nutrient strategy for alpine grassland in northwestern China to improve productivity for livestock needs. A field experiment was conducted in a semi-arid alpine grassland in northwestern China to investigate the effect of N addition rates on soil N2O emissions over the growing seasons of 2017 and 2018. Treatments included six N addition rates (0, 10, 30, 60, 120, 240 kg N ha−1 y−1), which were applied before each growing season. The N2O fluxes increased with N addition rates and showed different episodic changes between the two growing seasons. In 2017, the maximum N2O flux rate occurred within 2 weeks following N addition. In 2018, however, the maximum N2O flux rate occurred later in the growing season due to a heavy rainfall event. Growing season cumulative N2O emissions ranged between 0.32 and 1.11 kg N ha−1, and increased linearly with N addition rates. Increasing N addition rates over 60 kg N ha−1 yr−1 did not further increase plant above-ground biomass. The inter-annual variability of N2O flux suggests the importance of soil moisture in affecting N2O emissions. It is particularly important to avoid over-applying N nutrients beyond plant needs to reduce its negative effect on the environment while maintaining livestock productivity. The N2O flux rate increased with soil dissolved organic carbon (DOC) and soil pH. These results suggest the optimal N addition rate to the livestock grassland in this region should be 60 kg N ha−1 yr−1.

Diurnal Tree Stem CH4 and N2O Flux Dynamics from a Riparian Alder Forest

Tree stems play an important role in forest methane (CH4) and nitrous oxide (N2O) flux dynamics. Our paper aimed to determine the unknown diurnal variability of CH4 and N2O exchange in grey alder tree stems. The gas fluxes in tree stems and adjacent soil were measured using manual static and dynamic chamber systems with gas chromatographic and laser-spectroscopic analysis, respectively. The alder trees were predominant emitters of CH4 and N2O; however, N2O emission from stems was negligible. The soil mainly emitted N2O into the atmosphere and was both a source and sink of CH4, depending on environmental conditions. Neither the tree stems nor the riparian forest soil showed significant differences in their CH4 and N2O fluxes between the daytime and nighttime, independently of the exchange rates. In contrast to several previous studies revealing a diurnal variability of greenhouse gas fluxes from tree stems, our investigation did not show any clear daytime–nighttime differences. On the other hand, we found quite clear seasonal dynamics initiated by changing environmental conditions, such as temperature and soil water conditions and tree physiological activity. Our results imply a transport role of tree stems for soil-produced CH4 and N2O rather than the production of these gases in tree tissues, even though this cannot be excluded.

Eco-Friendly Yield and Greenhouse Gas Emissions as Affected by Fertilization Type in a Tropical Smallholder Rice System, Ghana

Data on greenhouse gas emission levels associated with fertilization applied in smallholder paddy rice farms in Ghana are scanty. The current study investigated fertilization types to determine their eco-friendliness on yield, Global Warming Potential (GWP) and Greenhouse Gas Intensity (GHGI) in a major rice season in the forest zone of Ghana. In total, five treatments were studied viz Farmer Practice (BAU); Biochar + Farmer Practice (BAU + BIO); Poultry Manure + Farmer Practice (BAU + M); Biochar + Poultry Manure + Farmer Practice (BAU + BIO + M); and Control (CT). Fluxes of methane (CH4) and nitrous oxide (N2O) were measured using a static chamber-gas chromatography method. N2O emissions at the end of the growing season were significantly different across treatments. BAU + BIO + M had highest N2O flux mean of 0.38 kgNha−1day−1 (±0.18). BAU + M had the second highest N2O flux of 0.27 kgNha−1day−1 (±0.08), but was not significantly different from BAU at p > 0.05. BAU+BIO recorded 0.20 kgNha−1day−1 (±0.12), lower and significantly different from BAU, BAU + M and BAU + BIO + M. CH4 emissions across treatments were not significantly different. However, highest CH4 flux was recorded in BAU+BIO at 4.76 kgCH4ha−1day−1 (±4.87). GWP based on seasonal cumulative GHG emissions among treatments ranged from 5099.16 (±6878.43) to 20894.58 (±19645.04) for CH4 and 756.28 (±763.44) to 27201.54 (±9223.51) kgCO2eqha−1Season−1 for N2O. The treatment with significantly higher yield and low emissions was BAU + M with a GHGI of 4.38 (±1.90) kgCO2eqkg−1.

How Tillage and Fertilization Influence Soil N2O Emissions after Forestland Conversion to Cropland

Soil nitrous oxide (N2O) emissions are influenced by land use adjustment and management practices. To meet the increasing socioeconomic development and sustainable demands for food supply, forestland conversion to cropland occurs around the world. However, the effects of forestland conversion to cropland as well as of tillage and fertilization practices on soil N2O emissions are still not well understood, especially in subtropical regions. Therefore, field experiments were carried out to continuously monitor soil N2O emissions after the conversion of forestland to cropland in a subtropical region in Southwest China. One forestland site and four cropland sites were selected: forestland (CK), short-term croplands (tillage with and without fertilization, NC-TF and NC-T), and long-term croplands (tillage with and without fertilization, LC-TF and LC-T). The annual cumulative N2O flux was 0.21 kg N ha−1 yr−1 in forestland. After forestland conversion to cropland, the annual cumulative N2O flux significantly increased by 76‒491%. In the short-term and long-term croplands, tillage with fertilization induced cumulative soil N2O emissions that were 94% and 235% higher than those from tillage without fertilization. Fertilization contributed 63% and 84% to increased N2O emissions in the short-term and long-term croplands, respectively. A stepwise regression analysis showed that soil N2O emissions from croplands were mainly influenced by soil NO3− and NH4+ availability and WFPS (water-filled pore space). Fertilization led to higher soil NH4+ and NO3− concentrations, which thus resulted in larger N2O fluxes. Thus, to reduce soil N2O emissions and promote the sustainable development of the eco-environment, we recommend limiting the conversion of forestland to cropland, and meanwhile intensifying the shift from grain to green or applying advanced agricultural management practices as much as possible.

C3 and C4 Grass Species: Who Can Reduce Soil Nitrous Oxide Emissions in a Continental Arid Region?

In order to relieve grazing pressure, drought-tolerant grass species are widely cultivated in arid regions. However, soil N emission is largely neglected while pursuing forage yield. We carried out a randomized block study to investigate whether and how the C3 and C4 grass species differ in soil N emission in a typical salinized field with temperate continental arid climate in the northwest inland regions, China. We quantified soil N2O flux from two C3 (barley and rye) and two C4 grass species [corngrass and sorghum hybrid sudangrass (SHS)] in fields during the growing season (from May to September) by using the static box method, and then determined the relationships between soil N2O fluxes and forage yield and soil properties. Results show that soil available nitrogen, soil temperature, pH, soil organic carbon, and total nitrogen were correlated, but soil water was anti-correlated with soil N2O fluxes. In addition, N2O flux increased significantly faster with soil temperature in C4 than in C3 grass fields. Although the lower total N2O emission fluxes were detected for C3 species, the lower yield-scaled N2O was detected for C4 species. Our study provided insights into the determination of grass species and the understanding of mechanisms regulating N2O fluxes in C3 and C4 species in the continental arid regions.

Warming Increases Nitrous Oxide Emission from the Littoral Zone of Lake Poyang, China

Littoral wetlands are globally important for sustainable development; however, they have recently been identified as critical hotspots of nitrous oxide (N2O) emissions. N2O flux from subtropical littoral wetlands remains unclear, especially under the current global warming environment. In the littoral zone of Lake Poyang, a simulated warming experiment was conducted to investigate N2O flux. Open-top chambers were used to raise temperature, and the static chamber-gas chromatograph method was used to measure N2O flux. Results showed that the littoral zone of Lake Poyang was an N2O source, with an average flux rate of 8.9 μg N2O m−2 h−1. Warming significantly increased N2O emission (13.8 μg N2O m−2 h−1 under warming treatment) by 54% compared to the control treatment. N2O flux in the spring growing season was also significantly higher than that of the autumn growing season. In addition, temperature was not significantly related to N2O flux, while soil moisture only explained about 7% of N2O variation. These results imply that N2O emission experiences positive feedback effect on the ongoing warming of the climate, and abiotic factors (e.g., soil temperature and soil moisture) were not main controls on N2O variation in this littoral wetland.

A mass spectrometric multiple soil-gas flux measurement system with portable high-resolution mass spectrometer MULTUM coupled to automatic chamber for continuous field observation

We developed a mass spectrometric soil-gas flux measurement system using a portable high-resolution multi-turn time-of-flight mass spectrometer, called MULTUM, combined with an automated soil-gas flux chamber for continuous field measurement of multiple gas concentrations. The developed system continuously measures concentrations of four different atmospheric gases (i.e., N2O, CH4, CO2, and O2), of which the concentrations range over six orders of magnitude at a time within a single gas sample. The measurements were performed every 2.5 min with analytical precisions (two standard deviations) of ±34 ppbv for N2O, ±170 ppbv for CH4, ±16 ppmv for CO2, and ±0.60 vol% for O2 at their atmospheric concentrations. The developed system was used for continuous field soil–atmosphere flux measurements of greenhouse gases (GHGs: N2O, CH4, and CO2) and O2 with 1 h resolution. The minimum quantitative fluxes (two standard deviations) were estimated through simulation as 70.2 µg N m−2 h−1 for N2O, 139 µg C m−2 h−1 for CH4, 11.7 mg C m−2 h−1 for CO2, and 9.8 g O2 m−2 h−1 (negative) for O2, whereas the estimated minimum detectable fluxes (two standard deviations) were 17.2 μg N m−2 h −1 for N2O, 35.4 μg C m−2 h−1 for CH4, 2.6 mg C m−2 h−1 for CO2, and 2.9 g O2 m−2 h−1 for O2. The developed system was deployed in the University Farm of the Ehime University (Matsuyama, Ehime, Japan) for a field observation over five days. Interestingly, an abrupt increase in N2O flux from 70 to 682 µg N m−2 h−1 was observed a few hours after the first rainfall, whereas no obvious increase in the CO2 flux was observed, although the temporal responses were different from those observed in a laboratory experiment. No abrupt N2O flux change was observed in succeeding rainfalls. Continuous multiple-gas flux and concentration measurements can be a powerful tool for tracking and understanding of underlying biological and physicochemical processes in the soil through measuring more tracer gases, such as volatile organic carbon gases, reactive-nitrogen gases, and noble gases by taking advantage of the broad versatility of mass spectrometry in detecting broad range of gas species.