T.J. van der Weerden

Ammonia emission factors for N fertilizers applied to two contrasting grassland soils

Ammonia volatilization from nitrogen (N) fertilizer applied throughout the year to two soil types was measured using a system of small wind tunnels. Losses from urea ranged from 12 to 46% of the applied N. Small losses, averaging <1%, were measured from ammonium nitrate (AN) and calcium nitrate applications. Factors influencing these losses are discussed. Using these results and those from other workers, emission factors for urea and AN applications to grassland in the UK were determined as 23.0 and 1.6% of the applied N, respectively. Emission factors for these fertilizers when applied to arable land were estimated as 11.8 and 0.8%, respectively. The emission factor for all other applied N (as straight and compound fertilizers) was assumed to be similar to that for AN. Calculations showed that fertilizer applications to agricultural land in the UK contributes 34 kt NH3-N per year, equivalent to 17% of the total annual NH3 emission.

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.

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.

Using near-continuous measurements of N2O emission from urine-affected soil to guide manual gas sampling regimes

Abstract Nitrous oxide (N2O) fluxes are influenced by fluctuating soil temperature and external factors such as rainfall events. Therefore, sampling time, frequency of sampling after nitrogen (N) application and weather interactions are critical when determining cumulative N2O losses and associated emission factors. Using automated chambers, three short-term field trials were conducted to measure N2O fluxes from urine-affected pasture in southern New Zealand to determine the influence of soil temperature and light on diurnal N2O variation and the effect of sampling frequency on estimated cumulative N2O losses. Diurnal N2O flux patterns were often interrupted by rainfall events, but when present they were mainly driven by fluctuations in soil temperature at 0–2 cm depth. Mean daily N2O fluxes occurred between 10:00–12:00 h and 18:00–21:00 h. Analysis of all data, including rainfall-affected periods, showed that gas sample collection three times a week between 10:00–12:00 h provided zero bias in calculated cumulative emissions when compared with those based on frequent, 2-hourly, flux measurements. To account for resource limitations in field campaigns for estimating cumulative N2O emissions, an alternative approach is to sample two times a week, when fluxes can be expected to be large (e.g. first 4–6 weeks following urine deposition on to pasture), with additional sampling following significant rainfall. This latter approach produced an average bias of +4%, but ranged from −3 to +18%.

Review of greenhouse gas emissions from the storage and land application of farm dairy effluent

The amounts of farm dairy effluent stored in ponds and irrigated to land have steadily increased with the steady growth of New Zealands dairy industry. About 80% of dairy farms now operate with effluent storage ponds allowing deferred irrigation. These storage and irrigation practices cause emissions of greenhouse gases (GHG) and ammonia. The current knowledge of the processes causing these emissions and the amounts emitted is reviewed here. Methane emissions from ponds are the largest contributor to the total GHG emissions from effluent in managed manure systems in New Zealand. Nitrous oxide emissions from anaerobic ponds are negligible, while ammonia emissions vary widely between different studies, probably because they depend strongly on pH and manure composition. The second-largest contribution to GHG emissions from farm dairy effluent comes from nitrous oxide emissions from land application. Ammonia emissions from land application of effluent in New Zealand were found to be less than those reported elsewhere from the application of slurries. Recent studies have suggested that New Zealands current GHG inventory method to estimate methane emissions from effluent ponds should be revised. The increasing importance of emissions from ponds, while being a challenge for the inventory, also provides an opportunity to achieve mitigation of emissions due to the confined location of where these emissions occur.

Comparison between APSIM and NZ-DNDC models when describing N-dynamics under urine patches

Nitrous oxide (N2O) emissions from soil are the result of complex interactions between physical, chemical and biological processes. We compared two process-based models (APSIM and NZ-DNDC) with measurements of N2O emissions, soil and content (0–75 mm) and water-filled pore space from a series of field campaigns where known amounts of animal urine-N were applied to four soil types under permanent pastures, in two regions within New Zealand, at different times of the year. We also compared cumulative N2O emissions with an N2O inventory emission factor approach (EF3 method). Overall, the two process-based models performed less well than the EF3 method for simulating cumulative N2O emissions over the complete data set. However, in winter, the APSIM model correlated well with measurements (r = 0.97), while NZ-DNDC performed well on the Otago soils (r = 0.83 and 0.92 for Wingatui and Otokia, respectively). The process-based models have the potential to account for the effect of weather conditions and soil type on N2O emissions that are not accounted for by the EF3 method. However, further improvements are currently needed. The fractions of N lost to different processes within the complex soil–plant atmosphere system differed between the two models. The size of the predicted plant uptake, leaching and NH3 volatilisation fluxes are large compared with N2O emissions and could affect the simulated soil N pools and thus the predicted N2O fluxes. To simulate N2O fluxes accurately, it is therefore important to ensure these processes are well modelled and further validation studies are needed.

Nitrous oxide, ammonia and methane emissions from dairy cow manure during storage and after application to pasture

Housing dairy cattle off-paddock in animal confinement facilities provides an alternative to grazing winter crops in southern New Zealand, but the associated greenhouse gas (GHG) emissions from such systems are poorly understood. Nitrous oxide (N2O), methane (CH4) and ammonia (NH3) emissions were measured from stored and land-applied dairy manures collected from winter-housed cows. Emissions increased with storage period, suggesting that losses could be minimised by reducing the storage period, provided ground conditions are suitable for manure spreading and there is minimal risk of nutrient run-off/leaching. After land application, NH3 emissions varied according to manure type, whereas N2O and CH4 emissions were negligible. The rate of manure application did not influence emission factors for any of the three GHGs. When manure emissions were converted to carbon dioxide equivalents (CO2e) per wintered cow, it was found that relative GHG emissions from manure were greatest during storage compared with land application, although these results require verification on a farm scale.

Evaluating the economic and production benefit of removing dairy cows from pastures in response to wet soil conditions

ABSTRACT This study used the DairyNZ Whole Farm Model to assess the cost–benefit of duration-controlled grazing (DCG) to reduce the time dairy cows spend on pastures with high soil water content. Within the model, grazing duration was reduced from 21 hours down to 17, 13 or 0 hours when soil moisture was above a critical water content. Scenario farms encompassed four climatic regions of New Zealand and two soil types. Regardless of region, soil type or grazing duration, implementation of DCG was not profitable except in one single instance. Furthermore, increased pasture production in response to DCG was minimal due to poor pasture management. Farms located on poorly drained soils experienced more wet days per year, thereby increasing the frequency of standoff pad use and higher operational costs. Results indicate that any financial benefit that was gained from protecting imperfectly drained and poorly drained soils through DCG was diminished by a high capital repayment and operational cost associated with the off-paddock facility.

Nitrous oxide emissions from cattle urine deposited onto soil supporting a winter forage kale crop

ABSTRACT Wintering cows on forage crops leads to urine being excreted onto wet, compacted soils, which can result in significant emissions of nitrous oxide (N2O). A field trial was conducted to determine the N2O emission factor (EF3; proportion of urine-N lost as N2O-N) for dairy cows wintered on a kale forage crop on a poorly drained soil. Urine was collected from non-lactating dairy cows on a forage kale diet and applied at 550 kg N ha−1 to artificially compacted soil to simulate trampling and non-compacted soil in a kale field. Cumulative N2O losses over four months were 7.38 and 2.64 kg N2O-N ha−1 from urine applied to, respectively, compacted and non-compacted soil. The corresponding EF3 values 0.75% and 0.30%, respectively, differed (P = .003) due to compaction. Combining our results with previous studies, where brassica-fed livestock urine was applied to soils supporting a forage brassica crop, suggested a significant relationship between soil water-filled pore space (WFPS) and brassica-derived urine EF3 (P = .005).