Opportunities for reducing environmental emissions from forage-based dairy farms

Modern dairy production is inevitably associated with impacts to the environment and the challenge for the industry today is to increase production to meet growing global demand while minimising emissions to the environment. Negative environmental impacts include gaseous emissions to the atmosphere, of ammonia from livestock manure and fertiliser use, of methane from enteric fermentation and manure management, and of nitrous oxide from nitrogen applications to soils and from manure management. Emissions to water include nitrate, ammonium, phosphorus, sediment, pathogens and organic matter, deriving from nutrient applications to forage crops and/or the management of grazing livestock. This paper reviews the sources and impacts of such emissions in the context of a forage-based dairy farm and considers a number of potential mitigation strategies, giving some examples using the farm-scale model SIMSDAIRY. Most of the mitigation measures discussed are associated with systemic improvements in the efficiency of production in dairy systems. Important examples of mitigations include: improvements to dairy herd fertility, that can reduce methane and ammonia emissions by up to 24 and 17%, respectively; diet modification such as the use of high sugar grasses for grazing, which are associated with reductions in cattle N excretion of up to 20% (and therefore lower N losses to the environment) and potentially lower methane emissions, or reducing the crude protein content of the dairy cow diet through use of maize silage to reduce N excretion and methane emissions; the use of nitrification inhibitors with fertiliser and slurry applications to reduce nitrous oxide emissions and nitrate leaching by up to 50%. Much can also be achieved through attention to the quantity, timing and method of application of nutrients to forage crops and utilising advances made through genetic improvements

The amount but not the proportion of N2 fixation and transfers to neighboring plants varies across grassland soils

Biological nitrogen fixation (BNF) is an important nitrogen source for both N2-fixers and their neighboring plants in natural and managed ecosystems. Biological N fixation can vary considerably depending on soil conditions, yet there is a lack of knowledge on the impact of varying soils on the contribution of N from N2-fixers in mixed swards. In this study, the amount and proportion of BNF from red clover were assessed using three grassland soils. Three soil samples, Hallsworth (HH), Crediton (CN), and Halstow (HW) series, were collected from three grassland sites in Devon, UK. A pot experiment with 15N natural abundance was conducted to estimate BNF from red clover, and the proportion of N transferred from red clover to the non-N2 fixing grass in a grass-clover system. The results showed that BNF in red clover sourced from atmosphere in the HH soil was 2.92 mg N plant−1, which was significantly lower than that of the CN (6.18 mg N plant−1) and HW (8.01 mg N plant−1) soils. Nitrogen in grass sourced from BNF via belowground was 0.46 mg N plant−1 in the HH soil, which was significantly greater than that in CN and HW soils. However, proportionally there were no significant differences in the percentage N content of both red clover and grass sourced from BNF via belowground among soils, at 65%, 67%, 65% and 35%, 27%, 31% in HH, CN, and HW, respectively. Our observations indicate that the amount of BNF by red clover varies among grassland soils, as does the amount of N sourced from BNF that is transferred to neighboring plants, which is linked to biomass production. Proportionally there was no difference among soils in N sourced from BNF in both the red clover plants and transferred to neighboring plants

Effect of soil saturation on denitrification in a grassland soil

Nitrous oxide (N2O) is of major importance as a greenhouse gas and precursor of ozone (O3) destruction in the stratosphere mostly produced in soils. The soil-emitted N2O is generally predominantly derived from denitrification and, to a smaller extent, nitrification, both processes controlled by environmental factors and their interactions, and are influenced by agricultural management. Soil water content expressed as water-filled pore space (WFPS) is a major controlling factor of emissions and its interaction with compaction, has not been studied at the micropore scale. A laboratory incubation was carried out at different saturation levels for a grassland soil and emissions of N2O and N2 were measured as well as the isotopocules of N2O. We found that flux variability was larger in the less saturated soils probably due to nutrient distribution heterogeneity created from soil cracks and consequently nutrient hot spots. The results agreed with denitrification as the main source of fluxes at the highest saturations, but nitrification could have occurred at the lower saturation, even though moisture was still high (71% WFSP). The isotopocules data indicated isotopic similarities in the wettest treatments vs. the two drier ones. The results agreed with previous findings where it is clear there are two N pools with different dynamics: added N producing intense denitrification vs. soil N resulting in less isotopic fractionation

An Inventory of Mitigation Methods and Guide to their Effects on Diffuse Water Pollution, Greenhouse Gas Emissions and Ammonia Emissions from Agriculture

Prepared as part of Defra Project WQ010

Effect of the application of cattle urine with or without the nitrification inhibitor DCD, and dung on greenhouse gas emissions from a UK grassland soil

Emissions of nitrous oxide (N2O) from soils from grazed grasslands have large uncertainty due to the great spatial variability of excreta deposition, resulting in heterogeneous distribution of nutrients. The contribution of urine to the labile N pool, much larger than that from dung, is likely to be a major source of emissions so efforts to determine N2O emission factors (EFs) from urine and dung deposition are required to improve the inventory of greenhouse gases from agriculture. We investigated the effect of the application of cattle urine and dung at different times of the grazing season on N2O emissions from a grassland clay loam soil. Methane emissions were also quantified. We assessed the effect of a nitrification inhibitor, dicyandiamide (DCD), on N2O emissions from urine application and also included an artificial urine treatment. There were significant differences in N2O EFs between treatments in the spring (largest from urine and lowest from dung) but not in the summer and autumn applications. We also found that there was a significant effect of season (largest in spring) but not of treatment on the N2O EFs. The resulting EF values were 2.96, 0.56 and 0.11% of applied N for urine for spring, summer and autumn applications, respectively. The N2O EF values for dung were 0.14, 0.39 and 0.10% for spring, summer and autumn applications, respectively. The inhibitor was effective in reducing N2O emissions for the spring application only. Methane emissions were larger from the dung application but there were no significant differences between treatments across season of application

Mechanisms behind the inhibition of autotrophic nitrification following rice-straw incorporation in a subtropical acid soil

The effectiveness of rice-straw incorporation to alleviate environmental deterioration and increase soil fertility is widely accepted, whereas, the effect of this management on stimulating soil nitrogen (N) transformation is not fully understood. This study was conducted to investigate the effect of rice-straw incorporation on soil N transformation. An incubation experiment was conducted with rice-straw incorporated at rates of 0 (RS0), 1.67 (RS1),3.33(RS2)and 6.67 gkg−1 soil(RS3).Tracing experiments with 15NH4NO3 and NH415NO3 was conducted in the first (Week 1) and tenth week (Week 10) after straw incorporation, and a numerical model was used to calculate gross rates of N transformations. Incorporation of rice-straw increased gross rates of soil organic N mineralization, ammonium (NH4+) and nitrate (NO3−) immobilization and oxidized organic-N to NO3−, by 0.2–1.7 times,4.6–11.6 times,20.4–74.9 times and 6.2–20.3 times,respectively. However, the stimulation of soil N transformation via rice-straw incorporation was insignificant by week 10. Over the incubation period, the stimulation of soil inorganic N production pathways (organic N mineralization and oxidized organic-N to NO3−) via rice-straw incorporation was less than on consumption pathways (NH4+ and NO3− immobilization), leading to soil inorganic N supply capacity decreasing with straw incorporation rates. Dissimilatory NO3− reduction to NH4+ was stimulated by rice-straw incorporation, as observed in both the first and tenth week. Compared with RS0, autotrophic nitrification decreased by 14%, 25% and 46% in RS1, RS2 and RS3, respectively, but this effect disappeared by week 10. However, nitrification capacity (NC, the ratio of autotrophic nitrification rate to total mineralization rate) was constrained following rice-straw incorporation both in the first and tenth weeks. Decreasing autotrophic nitrification was the most important factor contributing to decreased NO3− content with straw incorporation, followed by increasing NO3− immobilization. The gross rate of autotrophic nitrification was negatively correlated with NH4+immobilization, indicating that autotrophic nitrification inhibition may be attributed to increased NH4+ immobilization. Therefore, based on the observations of this study, rice-straw incorporation is recommended for reducing nitrification capacity and reducing risks of N losses in subtropical acid soil