Jiafa Luo

Denitrification and N2O:N2 production in temperate grasslands: processes, measurements, modelling and mitigating negative impacts.

In this review we explore the biotic transformations of nitrogenous compounds that occur during denitrification, and the factors that influence denitrifier populations and enzyme activities, and hence, affect the production of nitrous oxide (N2O) and dinitrogen (N2) in soils. Characteristics of the genes related to denitrification are also presented. Denitrification is discussed with particular emphasis on nitrogen (N) inputs and dynamics within grasslands, and their impacts on the key soil variables and processes regulating denitrification and related gaseous N2O and N2 emissions. Factors affecting denitrification include soil N, carbon (C), pH, temperature, oxygen supply and water content. We understand that the N2O:N2 production ratio responds to the changes in these factors. Increased soil N supply, decreased soil pH, C availability and water content generally increase N2O:N2 ratio. The review also covers approaches to identify and quantify denitrification, including acetylene inhibition, (15)N tracer and direct N2 quantification techniques. We also outline the importance of emerging molecular techniques to assess gene diversity and reveal enzymes that consume N2O during denitrification and the factors affecting their activities and consider a process-based approach that can be used to quantify the N2O:N2 product ratio and N2O emissions with known levels of uncertainty in soils. Finally, we explore strategies to reduce the N2O:N2 product ratio during denitrification to mitigate N2O emissions. Future research needs to focus on evaluating the N2O-reducing ability of the denitrifiers to accelerate the conversion of N2O to N2 and the reduction of N2O:N2 ratio during denitrification.

A review of emissions of methane, ammonia, and nitrous oxide from animal excreta deposition and farm effluent application in grazed pastures

Abstract The agricultural sector in New Zealand is the major contributor to ammonia (NH3), nitrous oxide (N2O), and methane (CH4) emissions to the atmosphere. These gases cause environmental degradation through their effects on soil acidification, eutrophication, and stratospheric ozone depletion. With its strong agricultural base and relatively low level of heavy industrial activity, New Zealand is unique in having a greenhouse‐gas‐emissions inventory dominated by the agricultural trace gases, CH4 and N2O, instead of carbon dioxide which dominates in most other countries. About 96% of this anthropogenic CH4 is emitted by ruminant animals as a byproduct during the process of enteric fermentation. Methane is also produced by anaerobic fermentation of animal manure and many other organic substrates. In pastoral soils, NH3 and N2O gases are generated from N originating from dung, urine, biologically fixed N2, and fertiliser. The amount of these gaseous emissions depends on complex interactions between soil properties, climatic factors, and agricultural practices. In this review paper, the animal‐excretal inputs and farm‐effluent applications to New Zealand pastures are quantified. Data from overseas and New Zealand studies on CH4, NH3, and N2O emissions from excretal deposition and animal effluents, and the factors affecting these emissions, are synthesised with an aim to improve the New Zealand estimates of emissions from these sources. The practical implications of these emissions are described in relation to environmental impacts and management strategies for reducing these emissions.

Ammonia Volatilization and Nitrogen Utilization Efficiency in Response to Urea Application in Rice Fields of the Taihu Lake Region, China

Abstract Ammonia volatilization losses, nitrogen utilization Efficiency, and rice yields in response to urea application to a rice field were investigated in Wangzhuang Town, Changshu City, Jiangsu Province, China. The N fertilizer treatments, applied in triplicate, were 0 (control), 100, 200, 300, or 350 kg N ha −1 . After urea was applied to the surface water, a continuous airflow enclosure method was used to measure ammonia volatilization in the paddy field. Total N losses through ammonia volatilization generally increased with the N application rate, and the two higher N application rates (300 and 350 kg N ha −1 ) showed a higher ratio of N lost through ammonia volatilization to applied N. Total ammonia loss by ammonia volatilization during the entire rice growth stage ranged from 9.0% to 16.7% of the applied N. Increasing the application rate generally decreased the ratio of N in the seed to N in the plant. For all N treatments, the nitrogen fertilizer utilization Efficiency ranged from 30.9% to 45.9%. Surplus N with the highest N rate resulted in lodging of rice plants, a decreased rate of nitrogen fertilizer utilization, and reduced rice yields. Calculated from this experiment, the most economical N fertilizer application rate was 227 kg ha −1 for the type of paddy soil in the Taihu Lake region. However, recommending an appropriate N fertilizer application rate such that the plant growth is enhanced and ammonia loss is reduced could improve the N utilization Efficiency of rice.

Factors regulating denitrification in a soil under pasture

Abstract Experiments were conducted to obtain insights into factors regulating denitrification rate in a silt loam soil under permanent pasture in New Zealand, by removing possible limitations to denitrification during the incubation for the denitrification measurement. Soil temperature in the field was found to limit denitrification rate in all seasons relative to the denitrification rate measured at 25°C in the laboratory. This temperature effect was greatest in the cool–wet season. Additions of nitrate solution to soil cores stimulated denitrification rates in all seasons. This increase in denitrification rate suggests the availability of NO3− may have limited denitrification in this pasture soil. Denitrification rates also increased when soluble-carbon was added to the soil cores, but the magnitude of the effect depended on other edaphic factors. A large increase in denitrification rate was obtained by saturating the soil cores collected in most seasons, but particularly during the warm–dry period. However, little enhanced effect on denitrification rate by anaerobic incubation of soil cores was observed. These results suggest that the observed effect of water addition on denitrification rate may have been due to the easier diffusive movement of NO3−, or possibly soluble-C, to the microsites where denitrification was occurring in this pasture, and the creation of anaerobic sites in the soil may not have been as important to the increase of denitrification rate.

Grazing effects on denitrification in a soil under pasture during two contrasting seasons

The present study was designed to measure the effects of grazing events on denitrification in a pasture. A poorly-drained silt loam soil was used. The experiments were conducted both in the moist-cool winter, when a pasture was grazed with cows at a high stocking rate (about 300 cows ha−1) for 24 h, and in the dry-warm summer, when the pasture was grazed with cows at a low stocking rate (about 40 cows ha−1) for about 12 h. Denitrification rate was measured using the acetylene inhibition technique, by incubating soil cores (0–75 mm depth) in a closed system. In the moist-cool winter, the effect of grazing on denitrification was significant. An increase in nitrogen loss through denitrification generally occurred between 3 and 14 days after grazing, with the highest denitrification rate on d 10 following grazing. However, the measured total N loss through denitrification induced by grazing during that period was still very small, with less than 1% of the N returned in urine by the grazing animals being lost through denitrification in the 0–75 mm topsoil in the 2 weeks following grazing. Soil nitrate concentrations showed the same pattern as did denitrification rate. Grazing also had a temporary stimulating effect on denitrification enzyme activity (DEA). In the dry-warm summer, no systematic effect of grazing events at a low stocking rate on denitrification was observed, even though slightly higher concentrations of soil NO3− persisted for a long period after the grazing event. These results indicated that the effect of grazing on denitrification was influenced by soil conditions, particularly soil moisture content, in different seasons.

Nitrogen loss through denitrification in a soil under pasture in New Zealand.

Denitrification on several contrasting topographical sites in a New Zealand dairy-farm pasture was measured periodically over a year, using the acetylene inhibition technique, by incubating undisturbed soil cores in a closed system. The measured denitrification rates varied considerably both spatially and temporally. High coeAcients of variation (CV) and log-normal distributions of denitrification rate were often observed. The spatial variance in denitrification rate changed temporally and was apparently related to soil moisture content and the grazing pattern. Denitrification rates followed a marked seasonal pattern, with highest rates being measured during the wet winter and lowest rates during the dry summer and early autumn. DiAerences in denitrification rates among sites were not consistent. However, slightly higher denitrification rates were usually detected in the floor of a gully and in a gateway area than on other sites. Mean denitrification rates from individual dates were positively correlated to soil moisture content. However, there was a negative correlation between denitrification rate and soil nitrate concentration, respiration rate and temperature. An annual nitrogen loss of 4.5 kg N ha ˇ1 through denitrification was estimated in this legume-based dairy-farm pasture. Low soil moisture content was the primary factor limiting denitrification during the dry summer and early autumn. Low denitrification rates were also caused by lack of available soil NO3 -N. 7 2000 Elsevier Science

Quantification of reductions in ammonia emissions from fertiliser urea and animal urine in grazed pastures with urease inhibitors for agriculture inventory: New Zealand as a case study

Urea is the key nitrogen (N) fertiliser for grazed pastures, and is also present in excreted animal urine. In soil, urea hydrolyses rapidly to ammonium (NH4(+)) and may be lost as ammonia (NH3) gas. Unlike nitrous oxide (N2O), however, NH3 is not a greenhouse gas although it can act as a secondary source of N2O, and hence contribute indirectly to global warming and stratospheric ozone depletion. Various urease inhibitors (UIs) have been used over the last 30 years to reduce NH3 losses. Among these, N-(n-butyl) thiophosphoric triamide (nBTPT), sold under the trade name Agrotain®, is currently the most promising and effective when applied with urea or urine. Here we conduct a critical analysis of the published and non-published data on the effectiveness of nBTPT in reducing NH3 emission, from which adjusted values for FracGASF (fraction of total N fertiliser emitted as NH3) and FracGASM (fraction of total N from, animal manure and urine emitted as NH3) for the national agriculture greenhouse gas (GHG) inventory are recommended in order to provide accurate data for the inventory. We use New Zealand as a case study to assess and quantify the overall reduction in NH3 emission from urea and animal urine with the application of UI nBTPT. The available literature indicates that an application rate of 0.025% w/w (nBTPT per unit of N) is optimum for reducing NH3 emissions from temperate grasslands. UI-treated urine studies gave highly variable reductions (11-93%) with an average of 53% and a 95% confidence interval of 33-73%. New Zealand studies, using UI-treated urea, suggest that nBTPT (0.025% w/w) reduces NH3 emissions by 44.7%, on average, with a confidence interval of 39-50%. On this basis, a New Zealand specific value of 0.055 for FracGASF FNUI (fraction of urease inhibitor treated total fertiliser N emitted as NH3) is recommended for adoption where urea containing UI are applied as nBTPT at a rate of 0.025% w/w. Only a limited number of published data sets are available on the effectiveness of UI for reducing NH3 losses from animal urine-N deposited during grazing in a grazed pasture system. The same can be said about mixing UI with urine, rather than spraying UI before or after urine application. Since it was not possible to accurately measure the efficacy of UI in reducing NH3 emissions from animal urine-N deposited during grazing, we currently cannot recommend the adoption of a FracGASM value adjusted for the inclusion of UI.

Effect of long-term compost and inorganic fertilizer application on background N2O and fertilizer-induced N2O emissions from an intensively cultivated soil

The influence of inorganic fertilizer and compost on background nitrous oxide (N2O) and fertilizer-induced N2O emissions were examined over a maize-wheat rotation year from June 2008 to May 2009 in a fluvo-aquic soil in Henan Province of China where a field experiment had been established in 1989 to evaluate the long-term effects of manure and fertilizer on soil organic status. The study involved five treatments: compost (OM), fertilizer NPK (nitrogen-phosphorus-potassium, NPK), half compost N plus half fertilizer N (HOM), fertilizer NK (NK), and control without any fertilizer (CK). The natural logarithms of the background N2O fluxes were significantly (P<0.05) correlated with soil temperature, but not with soil moisture, during the maize or wheat growing season. The 18-year application of compost alone and inorganic fertilizer not only significantly (P<0.05) increased soil organic carbon (SOC) by 152% and 10-43% (respectively), but also increased background N2O emissions by 106% and 48-76% (respectively) compared with the control. Total N in soils was a better indicator for predicting annual background N2O emission than SOC. The estimated emission factor (EF) of mineralized N, calculated by dividing annual N2O emission by mineralized N was 0.13-0.19%, significantly (P<0.05) lower than the EF of added N (0.30-0.39%). The annual N2O emission in the NPK, HOM and OM soils amended with 300 kg ha(-1) organic or inorganic N was 1427, 1325 and 1178 g N ha(-1), respectively. There was a significant (P<0.05) difference between the NPK and OM. The results of this study indicate that soil indigenous N was less efficiently converted into N2O compared with exogenous N. Increasing SOC by compost application, then partially increasing N supply to crops instead of adding inorganic N fertilizer, may be an effective measure to mitigate N2O emissions from arable soils in the North China plain.

Nitrous oxide emissions from application of urea on New Zealand pasture

Abstract The use of nitrogen (N) fertiliser has been identified as a possible important source of nitrous oxide (N2O) from pastoral soils, and urea is the main form of N fertiliser used in New Zealand. The aim of this study was to examine the effects of urea application on N2O emissions from pastoral soil. A closed soil chamber technique was used to measure the N2O emissions from a pasture which received either 0 (control) or 50 kg N ha–1 (as urea) per application during different seasons between 2003 and 2005. Overall, urea fertiliser application generally increased N2O fluxes above control levels for up to 30 days, but the duration for which N2O levels were elevated depended on the season. These increases in the N2O fluxes were largely caused by a combination of changes in the soil mineral N level due to urea application, and moisture content of soil in different seasons. Nitrous oxide emissions were higher during the winter and spring measurement periods when the soil water‐filled pore space (WFPS) was mostly above field capacity, and the emissions were lower during the summer and autumn measurement periods when the soil WFPS was below field capacity. The estimated N2O emission factors for urea ranged between 0 and 1.56% of the urea‐N applied, with a calculated average emission factor of 0.56%. The findings of the seasonal measurements suggests that a reduction in the use of N fertilizers under wet winter or wet spring conditions in New Zealand could potentially reduce N2O emissions from pastoral soil.

Statistical analysis of nitrous oxide emission factors from pastoral agriculture field trials conducted in New Zealand

Between 11 May 2000 and 31 January 2013, 185 field trials were conducted across New Zealand to measure the direct nitrous oxide (N2O) emission factors (EF) from nitrogen (N) sources applied to pastoral soils. The log(EF) data were analysed statistically using a restricted maximum likelihood (REML) method. To estimate mean EF values for each N source, best linear unbiased predictors (BLUPs) were calculated. For lowland soils, mean EFs for dairy cattle urine and dung, sheep urine and dung and urea fertiliser were 1.16 ± 0.19% and 0.23 ± 0.05%, 0.55 ± 0.19% and 0.08 ± 0.02% and 0.48 ± 0.13%, respectively, each significantly different from one another (p < 0.05), except for sheep urine and urea fertiliser. For soils in terrain with slopes >12°, mean EFs were significantly lower. Thus, urine and dung EFs should be disaggregated for sheep and cattle as well as accounting for terrain.