Routes of Dicyandiamide Uptake in Pasture Plants: A Preliminary Laboratory Study

A consequence of intensification of New Zealand pastures is increased nitrogen (N) inputs to the soil in the form of urine, dung and mineral fertiliser. Dairy cow urine has a high N content that causes large N losses from the grazed system via nitrate (NO3-) leaching, nitrous oxide (N2O) emissions and ammonia volatilization. Dicyandiamide (DCD) is a nitrification inhibitor that has been proven to reduce NO3- leaching and N2O emissions, and increase pasture in New Zealand pastures (De Klein et al., 2014). DCD was commercially available for use in New Zealand pastures until 2013 when its use was suspended due to detection of traces of DCD in exported milk. Although DCD at high doses is relatively non-toxic there is no set maximum residue limit for its consumption. The contamination incident has highlighted the need to understand the pathway by which DCD entered the dairy cow. Nutrients can be absorbed (or taken up) through the leaves via leaf cuticle and stomata of plants (Eichert and Fernández, 2012) and this phenomenon is used to fertilize golf courses and horticultural crops mainly using urea as a spray formulations. Because of the similarity between DCD and urea in terms of molecular weight and structure, we suspected that DCD could similarly be taken up in pasture plants. Few studies have shown the root uptake of DCD but none using pasture plant species. Our objective was therefore to quantify foliar and root uptake of DCD in pasture plants following its application under glasshouse conditions. We hypothesized that DCD can be taken up by both foliar and root uptake pathways

Nitrous oxide and carbon dioxide emission responses to litter incorporated in a grassland soil

While the Intergovernmental Panel on Climate Change (IPCC) guidelines include the possibility of N₂O emissions from crop residues, they do not include grazed pasture or supplementary feed litters. To quantify the relative greenhouse gas (GHG) emissions from pasture litter, ground shoots of clover (Trifolium repens L.), ryegrass (Lolium perenne L.) and maize (Zea mays L.) were incorporated into soil at 1.5, 1.0 and 0.6 g nitrogen (N) (~12.8 g carbon (C), on average) kg⁻¹ soil, respectively. A 42 d incubation at 20°C and either 86% (field capacity) or 54% water-filled pore space (WFPS) was performed. During the first 2 d, 92–95% of the N₂O was emitted. At 86% WFPS, N₂O emissions were 2–3% of the incorporated N with no litter species differences. At 54% WFPS, N₂O emissions were 1.7% > 0.7% = 0.5% of herbage N applied, for clover, ryegrass and maize, respectively (P <0.001). At 86% and 54% WFPS, carbon dioxide (CO₂) emissions averaged 32% and 21%, respectively, with no litter species differences after 38 d. Over 14 d, N₂O emissions expressed as CO₂–eq were 67 and 59% for clover, 59 and 31% for ryegrass and 52 and 18% for maize at 86% and 54% WFPS, respectively, of the total (CO₂+N₂O) greenhouse gas budget. Emissions of N₂O corresponded with the biochemical composition of the litter. At either WFPS, the decomposition rates did not differ due to species. The potential for pasture litter to contribute to N₂O emissions from clover-ryegrass pastures warrants further study in situ

Plant derived nitrous oxide emissions from intensively grazed dairy pastures

The Intergovernmental Panel on Climate Change (IPCC) includes above- and below-ground residues of all non-N and N-fixing crops in its definition of crop residues. Residues from pastures and from perennial forage crops are only accounted for during pasture renewal. The IPCC also confirms that the nitrogen (N) contained in crop residues in arable systems can contribute significantly to N cycling and be a significant source of nitrous oxide (N₂O) emissions. Despite the fact that 70% of the world’s agricultural area and 90% of New Zealand’s total farm area are pastoral systems, the current IPCC methodology does not consider the potential contribution of pasture residues outside of the renewal period with respect to N₂O emissions. Nitrous oxide is an obligate intermediate in the denitrification process and a by-product of nitrification. These microbial processes cause N₂O to be released from soil into the troposphere. Rates of N₂O emission and microbial pathways for production are dependent, amongst other factors, on soil water content and inorganic N in the soil. Therefore, the questions posed here were: Do pasture residues (collectively called ‘litter’) occur in significant quantities during grazing? And what is the role of herbage embodied-N with respect to N2O emissions? The overall objective of the research was to quantify the contribution of such plant-derived N₂O emissions in intensively grazed dairy pastures to New Zealand’s agricultural greenhouse gas emissions inventory. Experiment 1 (Chapter 4), was a field survey performed at Lincoln University Dairy Farm (LUDF), to quantify grazing-induced litter-fall i.e. the fraction of freshly harvested but un-ingested litter dropped by dairy cattle while grazing. Each paddock at the LUDF was grazed 12 times annually. This research showed, for the first time, that the rate of fresh litter fall equated to 53 ± 24 kg DM ha–1 per grazing event in an intensively grazed dairy pasture and was equal to 4% of the apparent dry matter consumption of the dairy cattle. Annually, fresh and senesced litter equated to N application rates of 15.9 kg N ha–1 y–1 and 3.5 kg N ha–1 y–1, respectively. The aforementioned quantities of litter-fall formed the rationale for further experiments. Experiment 2 (Chapter 5), a field study conducted in two parts (A and B), examined the effect of simulated animal treading on herbage decomposition and its implications on N₂O emissions. Presence or absence of herbage did not affect the N₂O emissions with N₂O emissions increasing regardless of the herbage presence. Soil NO3– levels declined due to treading, presumably due to induced anaerobic conditions and denitrification. The results were confirmed using a 15N technique (part B) which showed that a major fraction of the N₂O emitted under herbage-trodden pasture originated from the soil inorganic N pool. However, the 15N enrichment of the inorganic N pool also showed that the size of the soil inorganic-N pool was diluted due to N being released from either the herbage or the soil organic matter pools as a consequence of treading. Experiment 3 (Chapter 6) investigated the effect of incorporating litter of the dominant New Zealand pasture species (clover and ryegrass) and a pasture supplement (maize) with soil, at two soil water contents (54 and 86% water-filled pore space (WFPS)), incubated at 20oC. At field capacity (86% WFPS), the emission factor (EF) of N₂O equated to 2–3% of the litter-N with no differences due to litter species, while at 54% WFPS, the EF was significantly less with 1.7% > 0.7% = 0.5% for clover, ryegrass and maize, respectively. The decomposition rates were also similar at 86% WFPS. The differences in N₂O emissions were attributed to the biochemical properties of the species’ litter, especially cellulose concentrations and their differing C: N ratios. To further investigate the role of biochemical composition, specifically the C: N ratio of the plant litter to contribute to N₂O emissions, increasing amounts of cellulose were mixed with a constant mass of clover litter and incorporated into a pastoral soil (Experiment 4; Chapter 7). Increasing the C: N ratio via cellulose addition enhanced N₂O emissions, indicating that the incorporated cellulose acted as a labile C source favouring denitrification. Higher N₂O emissions from the highest C: N ratio treatments showed that the biochemical availability of C played a critical role in litter-derived N₂O emissions. Therefore higher emissions observed from the clover litter incorporated in Experiment 3 were most likely due to the labile forms of C embodied in the clover leaf tissues and not just attributable to the amount of N in the litter. In Experiment 5 (Chapter 8), 15N-labelled ryegrass was placed on the surface of a pastoral soil in litterbags at the rate of 213 kg N ha–1 (simulating litter-fall) and N₂O and CO₂ emissions were measured. This current study is the first to report soil N dynamics and N₂O emissions using 15N-labelled pasture litter placed in situ. Approximately 70% of the N₂O originated from the litter when surface-applied. Emissions of N₂O likely resulted from ammonification followed by a coupling of nitrification and denitrification during litter decomposition on the soil surface. The litter contributed to both the 15N enrichment of soil NO3– and N₂O emissions which originated from litter-N. The 15N enrichment of the soil NO3– pool showed that litter-N enhanced the soil inorganic N pool, verifying the conclusions drawn in Experiment 2 (part B), where in situ treading of herbage led to an increase in the soil inorganic N pool as evidenced by the decrease in 15N enrichment of the NO3– pool. The EF of the in situ placed litter was 0.9%; similar to the IPCC default EF value of 1%. This suite of experiments has shown that the contribution(s) of herbage-N to N cycling and N2O emissions are significant, yet, not considered within the current IPCC methodology. If the litter-fall data is extrapolated using the various N contents and EFs measured in this thesis, litter-fall accounts for 4.5–10.9% of the total N₂O-N emitted due to dairy cattle. This thesis has also shown that 4% of the total pasture on-offer can be lost as litter-fall resulting in lower dry matter intake (DMI) of dairy cattle. If this is worked through the inventory calculations, the DMI remains unaffected. However, including the litter-fall-derived N₂O emissions in inventory calculations provides a more accurate and refined accounting of the N₂O-N released from grazed pasture N cycling. Before solid recommendations can be made to alter the IPCC inventory methodologies, further data on the effects of different grazing managements, animal and pasture species, and climate are needed

Litter-fall causes nitrous oxide emissions

Our previous study showed that significant quantities of litter-fall (harvested but unconsumed plant material dropped during grazing) can be deposited onto the soil surface during a grazing event. However, the contribution of in situ decomposition of this litter-fall to nitrous oxide (N₂O) emissions is unknown. We applied 15N-labelled ryegrass (Lolium perenne L.) to the surface of a pastoral soil and for up to 139 days thereafter, quantified the contribution of herbage decomposition to N₂O production and soil N dynamics in field conditions. Approximately 70% of the total N₂O originated from the surface-applied litter treatment with 38–75% of the cumulative emissions occurring within 4–10 d of treatment application. After 66 d, dry matter loss from the litterbags equated to 46–82% of the pasture dry matter applied. Emissions of N₂O likely resulted from ammonification followed by a coupling of nitrification and denitrification during litter decomposition. The litter contributed to both the 15N enrichment of the soil NO₃ ⁻−N and N₂O–N pools. The emission factor (EF) of the in situ placed litter was 1.2%; similar to the IPCC default EF value of 1% for crop residues. Further in situ studies using different pasture species and litter-fall rates are required to understand the microbial processes responsible for litter-induced N₂O emissions

Nutrient returns from pasture litterfall during grazing

Soil respiration results in carbon dioxide (CO₂) emissions from pasture ecosystems. This is a function of root respiration, rhizosphere respiration, oxidation of soil organic matter (SOM) and litter decomposition; the latter two make a major contribution. Quantification of litterfall during grazing has not been performed in dairy pastures. In this study, close observation of grazing animal behaviour revealed that a fraction of the animal harvested herbage is not ingested and falls on to the pasture soil surface. To quantify this litter, the pasture soil surface was vacuumed before and after each grazing event. Litterfall yields were 72.4 ± 31.2, 53.0 ± 24.4 and 19.4 ± 17.6 kg DM ha⁻¹ (± SD, n = 150) for total litter, post-grazing-fresh (POGF) and post-grazing-senesced (POGS) litters, respectively. Extrapolating this data on an annual basis indicated that 253 and 92 kg C ha⁻¹ y⁻¹ could be applied to the pasture soil as POGF and POGS. Corresponding values for direct CO₂ emissions were calculated to be 81 and 30 kg CO₂-C ha⁻¹ y⁻¹ when using an emission factor (EF) of 32% generated from a laboratory study using clover (Trifolium repens L.) and ryegrass (Lolium perenne L.). Contributions of nitrous oxide (N₂O), another greenhouse gas, were also investigated, and on a CO₂-equivalents basis accounted for 32-76% of the greenhouse gas emissions. The study shows that a small but important fraction of pasture litter contributes to carbon and nitrogen cycling in pastures

Quantification of hydrogen cyanide as a potential decomposition product of ethanedinitrile during pine log fumigation

Abstract Background The Stakeholders in Methyl Bromide Reduction (STIMBR) are evaluating ethanedinitrile (EDN) as an alternative fumigant to methyl bromide for use as a phytosanitary treatment for pine logs (Pinus radiata D.Don). Ethanedinitrile is hypothesised to decompose into hydrogen cyanide (HCN) in the presence of water. This process, if it occurs, is of particular interest because it may influence the efficacy and emissions data needed for commercialisation. Methods The concentrations of EDN and HCN were measured in the treated space (28 L fumigation chambers) without (n = 1) and with pine log sections (n = 3; 46 ± 1.4% load factor) at 10 or 20 °C in a simulated commercial fumigation. Results On average, the cylinder of EDN tested contained 34.6 g m− 3 HCN (or 3.1%), which corresponds to a concentration of 0.8 g m− 3 (or 0.07%) in the treated space for a 50 g m− 3 EDN dose (commercial rate in Australia). This level of HCN is likely a result of the manufacturing process, whereby HCN is oxidised to produce EDN. During fumigation, HCN was detected in the treated space at relatively low concentrations, which did not significantly change over time. This indicates that HCN is not produced in substantial amounts during fumigation and that, as a result, insect efficacy is unlikely to be affected by low unchanging (P = 0.055) concentrations of this compound in the treated space. Conclusions The results of this work support the statement that EDN is not significantly converted to HCN during the treatment of recently harvested pine logs

Evaluation of prognostic risk models for postoperative pulmonary complications in adult patients undergoing major abdominal surgery: a systematic review and international external validation cohort study

Background Stratifying risk of postoperative pulmonary complications after major abdominal surgery allows clinicians to modify risk through targeted interventions and enhanced monitoring. In this study, we aimed to identify and validate prognostic models against a new consensus definition of postoperative pulmonary complications. Methods We did a systematic review and international external validation cohort study. The systematic review was done in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. We searched MEDLINE and Embase on March 1, 2020, for articles published in English that reported on risk prediction models for postoperative pulmonary complications following abdominal surgery. External validation of existing models was done within a prospective international cohort study of adult patients (≥18 years) undergoing major abdominal surgery. Data were collected between Jan 1, 2019, and April 30, 2019, in the UK, Ireland, and Australia. Discriminative ability and prognostic accuracy summary statistics were compared between models for the 30-day postoperative pulmonary complication rate as defined by the Standardised Endpoints in Perioperative Medicine Core Outcome Measures in Perioperative and Anaesthetic Care (StEP-COMPAC). Model performance was compared using the area under the receiver operating characteristic curve (AUROCC). Findings In total, we identified 2903 records from our literature search; of which, 2514 (86·6%) unique records were screened, 121 (4·8%) of 2514 full texts were assessed for eligibility, and 29 unique prognostic models were identified. Nine (31·0%) of 29 models had score development reported only, 19 (65·5%) had undergone internal validation, and only four (13·8%) had been externally validated. Data to validate six eligible models were collected in the international external validation cohort study. Data from 11 591 patients were available, with an overall postoperative pulmonary complication rate of 7·8% (n=903). None of the six models showed good discrimination (defined as AUROCC ≥0·70) for identifying postoperative pulmonary complications, with the Assess Respiratory Risk in Surgical Patients in Catalonia score showing the best discrimination (AUROCC 0·700 [95% CI 0·683–0·717]). Interpretation In the pre-COVID-19 pandemic data, variability in the risk of pulmonary complications (StEP-COMPAC definition) following major abdominal surgery was poorly described by existing prognostication tools. To improve surgical safety during the COVID-19 pandemic recovery and beyond, novel risk stratification tools are required. Funding British Journal of Surgery Society