Elsevier

Agricultural Systems

Volume 186, January 2021, 102960
Agricultural Systems

The potential effectiveness of four different options to reduce environmental impacts of grazed pastures. A model-based assessment

https://doi.org/10.1016/j.agsy.2020.102960Get rights and content

Highlights

  • We modelled the effects of 3 plant traits and 1 management practice on GHG balances.

  • Reducing N content in animal feed reduced leaching and gaseous N losses.

  • Nitrification inhibitors reduced leaching and N2O non-linearly but increased NH3.

  • Deeper rooting did not reduce leaching when total root mass remained constant.

  • Frequent pasture renewal could increase SOC but at the cost of reduced production.

Abstract

Pastoral agriculture can have negative environmental impacts. These include greenhouse gas emissions (such as enteric CH4 and soil N2O emissions), NH3 volatilisation, and leaching of nitrogen into waterways. We used a modelling approach to assess the effect of three plant traits and one management practice on N2O emissions, nitrogen (N) losses via leaching and NH3 volatilisation, pasture production and soil organic carbon (SOC) changes (as applicable). The aim was to identify traits/management practices that could potentially be environmentally beneficial and could then inform future research to find or breed plants with those traits.

The traits and practice investigated were: 1) N content in animal feed; 2) plant-excreted nitrification inhibitors; 3) deep rooting; and 4) frequency of pasture renewal. Of these, the N content in animal feed provided the most promising results, with low N content resulting in lower urine N excretion, and consequently reduced leaching losses and emissions of N2O and NH3. Modelling the effect of plant-excreted nitrification inhibitors showed mixed results, with reduced leaching rates but increased NH3 emissions. N2O emissions could be reduced if nitrification rates were strongly reduced. However, at lower rates of inhibition, there was little effect on N2O emissions. In the deep rooting scenarios, we found that N leaching losses were minimised if roots grew predominantly in upper soil layers where any mineral N was likely to spend more time before being leached. Nitrogen was, therefore, more effectively intercepted and prevented from leaching by greater root proliferation in the upper rather than deeper layers. For pasture renewal, we unexpectedly found that SOC could be increased by more frequent pasture renewal. However, this would come at the cost of reduced pasture production as renewal events reduced grazing off-takes proportionately more than the reduction in carbon gain by pasture plants. A renewal period of about 10 years was found to be optimal for pasture production and could be maintained with minimal SOC changes.

Introduction

Grazed grasslands contribute to global greenhouse gas emissions via enteric CH4 emissions from grazing animals and N2O emissions from the microbial breakdown of animal excreta and nitrogen (N) fertilisers in the soil. N can also be lost from the soil via NH3 volatilisation and NO3 leaching, which can contribute to secondary N2O emissions and affect local air and water quality (Cameron et al., 2013). In addition, changes to soil organic carbon (SOC) can contribute to changes in atmospheric CO2 concentrations.

In New Zealand, agriculture (predominantly grazed pastures) accounts for 48% of the country's gross greenhouse gas emissions (MfE, 2019). Of these emissions, CH4 from enteric fermentation accounts for 71%, and (direct and indirect) N2O emissions from managed soils for 22%, with the remainder coming from manure management, burning of field residues and CO2 emissions from urea and lime. Even though it has been recognised that there can be SOC changes within pastures in New Zealand (Schipper et al., 2017), such changes are not included in the national inventory because of the technical difficulty of assessing these changes for the country as a whole, and because it has not been mandatory to account for such changes under the rules of the Kyoto Protocol.

There is much interest in methods to reduce agricultural greenhouse gas emissions while maintaining productivity (Luo et al., 2008; de Klein and Eckard, 2008; Eckard et al., 2010; Dijkstra et al., 2013; de Klein et al., 2020; Bryant et al., 2020). While there are numerous potential mitigation technologies, conducting laboratory and field trials to assess their efficacy is expensive and time consuming. However, in many cases it may be possible to pre-screen proposed mitigation technologies using process-based modelling (e.g. Lehuger et al., 2011; White and Snow, 2012; Snow and White, 2013; Kirschbaum et al., 2017; Bryant et al., 2020) to determine which technologies are likely to have the highest mitigation potential.

Here, we investigate the potential of different possible plant-based mitigation strategies. Instead of testing specific available actual plants or management options, we start with a theoretical analysis of the possible scope of different hypothetical traits. We are trying to ask what could hypothetically be available, and what could be achieved with such hypothetical traits. Future research can search for actual plants that may possess the desired traits or could be bred to express them, or management approaches could be devised and operationalised towards those trait goals.

We primarily used a process-based model of soil and plant C, N, and water cycles (CenW vers. 5.0) to quantify any changes in net greenhouse gas emissions through modifications of selected traits. This allowed us to keep everything constant other than a selected trait and explore the overall system response to changes in the selected trait. These system responses included feedback effects that could reduce or amplify any initial response. The aim was to assess the ultimate net response of the system to the modification of one specific initial trait after inclusion of all system-internal feedbacks. However, for some traits we used simplified models focusing on the key processes when modelling of the whole system was not required.

Specifically, we studied four different plant or management traits. They were:

  • Nitrogen content in animal feed

  • Nitrification inhibitors

  • Deep rooting

  • Pasture renewal

To investigate the effects of different plant N contents in animal feed, we considered varying the N content of herbage grazed by cattle between 1 and 5% and assessed the effects on N excretion in animal dung and urine and its subsequent effect on N losses (N2O emissions, leaching, and NH3 volatilisation). Our focus was solely on the effect of N intake on N excretion by the animals. We did not consider any other interactions between the plant and soil.

In general, reducing the amount of surplus N consumed by animals should reduce the amount of N excreted and therefore the subsequent losses. Previous studies have confirmed that excretal N losses are reduced with decreasing feed N content (e.g. Castillo et al., 2000; Kebreab et al., 2001; Misselbrook et al., 2005; Dijkstra et al., 2013; Arndt et al., 2015; de Klein et al., 2020; Bryant et al., 2020). Luo et al. (2008), for example, found a 22% reduction in N2O emissions per unit of milk production for cows receiving a low-N maize supplement compared to a control group that grazed only pasture. However, the higher stocking rate in the supplement-fed system meant that total N2O emissions per area increased by about 4%. In general, most studies have found N excretion to increase with N ingestion (from plant diets and supplements) by grazing animals (Castillo et al., 2000; Kebreab et al., 2001; de Klein et al., 2020; Bryant et al., 2020). In the present work, we tried to underpin these various findings with a generic theoretical analysis.

Nitrification inhibitors slow down the rate of nitrification (the transformation of NH4+ to NO3) in the soil (Cameron et al., 2013). NH4+, being positively charged, is less prone to leaching as it is attracted to the negatively charged clay surfaces in the soil. Reducing the amount of NO3 in the soil also reduces N2O losses via denitrification (the reduction of NO3 under anaerobic conditions). However, when N is kept for longer in the form of NH4+, it increases the risk of losses via NH3 volatilisation. While nitrification inhibitors can only slow down, but not completely stop the nitrification process, they can reduce leaching and N2O losses by giving plants more time to take up N before nitrification occurs, or for NH4+ to be adsorbed to clay and soil organic matter surfaces.

Artificial nitrification inhibitors, such a dicyandiamide (DCD), have therefore been used as a mitigation option to reduce N2O and leaching losses from fertiliser application and animal excreta (Cameron et al., 2013). Di and Cameron (2016) estimated that DCD could reduce N2O emissions from urine patches by an average of 57% and leaching losses by 30–50%. However, there are costs involved with the purchase and application of artificial inhibitors, and concerns about contamination of animal products have resulted in the suspension of the use of DCD in New Zealand since 2013 (MPI, 2013). Alternatively, some plants can naturally produce compounds, such as aucubin, that inhibit nitrification (Subbarao et al., 2007; Zakir et al., 2008; Dijkstra et al., 2013; Gardiner et al., 2018, Gardiner et al., 2020; de Klein et al., 2020) and could overcome many of the problems inherent in the use of artificial nitrification inhibitors. In the scenarios presented here, we quantified potential changes in NH3, N2O and leaching losses from a urine patch by different levels of nitrification inhibition.

In the present work, we investigated the potential benefits of growing plants with root systems that penetrate deeper into the soil (Thorup-Kristensen and Kirkegaard, 2016; Horne and Scotter, 2016; Rosolem et al., 2017). We constrained the simulation by maintaining constant total resource allocation for root growth and only changing its vertical distribution. Any enhanced root growth at one depth in the soil therefore had to come at the expense of lowered root growth at other depths.

Within that defined constraint, deep rooting of plants could have two potential benefits:

  • 1.

    Deeper roots allow access to water stored at depth and enable continued photosynthesis and growth during periods without rainfall or irrigation;

  • 2.

    Deeper roots can access NO3 that has been leached to deeper soil horizons and prevent it from leaching beyond the root zone where it could contaminate downstream water ways.

The first benefit was already explored by Kirschbaum et al. (2017), and we further elaborate on that work here. The second potential benefit had not previously been explored, and our work here involved the modelling of root distribution in the soil and quantification of NO3 uptake per unit of root mass. It tried to assess the likelihood of NO3 uptake by plants, preventing it from leaching beyond the root zone, under different relative root distributions.

Pasture renewal (pasture renovation) is commonly practised by dairy farmers to maintain productivity and economic returns from their pastures (Kerr et al., 2015). Pasture renewal involves killing an old pasture either mechanically or with herbicides, followed by a fallow period before a new pasture is sown (e.g. Velthof et al., 2010). Consequently, pasture renewal leads to a net C loss during the fallow period (Ammann et al., 2013; Rutledge et al., 2015, Rutledge et al., 2017a), but grazing offtake is also halted until the new pasture has grown to a sufficient size to be ready to be harvested again. Empirical studies have shown that these factors together apparently have little effect on SOC at annual or longer time scales (Gál et al., 2007; Carolan and Fornara, 2016; Fornara et al., 2020).

To better understand the effect of pasture renewal on the site C balance, we have explored the effects of renewal frequency, pasture deterioration rates over time, lengths of fallow periods, renewal timing, and associated environmental factors on the C balance of grazed temperate pastures. A more detailed account of this study has been given by Liáng et al. (2020). Here, we only focus on the effects of renewal frequency and deterioration status on the site C balance.

Section snippets

Overview of the approach

The work here used a combination of conceptual explorations as appropriate for the different questions. This included detailed process simulations using the process-based model CenW (Kirschbaum et al., 2015, Kirschbaum et al., 2017) where modelling of whole-system responses, including various feedback processes, was warranted. For modelling the effects of feed N content and the effect of rooting depth on nitrate leaching, we used simpler models of the interactions between the relevant factors

N losses from a urine patch

Fig. 2 shows the rates of N exported in urine, dung and animal products for a constant intake of 10,000 kgDM ha−1 yr−1. An N content of about 1.9% in animal feed would be just sufficient to match N exports, including a base-level urinary loss of 25 kgN ha−1 yr−1 required as part of normal nitrogen turnover in the animals' metabolism. A feed-intake N content of less than 1.9% would be insufficient to balance the three principal losses from the system and result in a negative N balance and loss

Conclusions

In the work shown here, we assessed three plant traits and one increasingly common management option for their potential to affect net greenhouse gas balances positively or negatively. Modification to the N content in animal feed provided the most promising results, with reduced N content resulting in lower urine N excretion and consequently reduced N2O and NH3 emissions, and N-leaching losses. In principle, feed N contents could be modified through changes in pasture N contents, either by

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) NZAGRC-PGGRC 2.1 and the Ministry of Business, Innovation and Employment (MBIE) strategic science investment fund. We would like to express our thanks to the members of the NZAGRC advisory groups for advising us on the details of plant traits on which to focus, to Louis Schipper, Aaron Wall, Cecile de Klein, Surinder Saggar, Sandra Lavorel and Johannes Laubach for advice and specific feedback on this

References (80)

  • D.J. Horne et al.

    The available water holding capacity of soils under pasture

    Agric. Water Manag.

    (2016)
  • D.G. Kim et al.

    Linear and nonlinear dependency of direct nitrous oxide emissions on fertilizer nitrogen input: a meta-analysis

    Agric. Ecosyst. Environ.

    (2013)
  • M.U.F. Kirschbaum

    CenW, a forest growth model with linked carbon, energy, nutrient and water cycles

    Ecol. Model.

    (1999)
  • M.U.F. Kirschbaum et al.

    Modelling carbon and nitrogen dynamics in forest soils with a modified version of the CENTURY model

    Soil Biol. Biochem.

    (2002)
  • M.U.F. Kirschbaum et al.

    Modelling carbon and water exchange of a grazed pasture in New Zealand constrained by eddy covariance measurements

    Sci. Total Environ.

    (2015)
  • M.U.F. Kirschbaum et al.

    The trade-offs between milk production and soil organic carbon storage in dairy systems under different management and environmental factors

    Sci. Total Environ.

    (2017)
  • J.J. Landsberg et al.

    A generalised model of forest productivity using simplified concepts of radiation-use efficiency, carbon balance and partitioning

    For. Ecol. Manag.

    (1997)
  • S. Lehuger et al.

    Predicting and mitigating the net greenhouse gas emissions of crop rotations in Western Europe

    Agric. For. Meteorol.

    (2011)
  • L.L. Liáng et al.

    Nitrous oxide fluxes determined by continuous eddy covariance measurements from intensively grazed pastures: temporal patterns and environmental controls

    Agric. Ecosyst. Environ.

    (2018)
  • K.L. McGeough et al.

    Evidence that the efficacy of the nitrification inhibitor dicyandiamide (DCD) is affected by soil properties in UK soils

    Soil Biol. Biochem.

    (2016)
  • T.H. Misselbrook et al.

    Dietary manipulation in dairy cattle: laboratory experiments to assess the influence on ammonia emissions

    J. Dairy Sci.

    (2005)
  • D.S. Powlson et al.

    Soil management in relation to sustainable agriculture and ecosystem services.

    Food Policy

    (2011)
  • C.A. Rosolem et al.

    Enhanced plant rooting and crop sstem management for improved N use efficiency

    Adv. Agron.

    (2017)
  • S. Rutledge et al.

    Carbon balance of an intensively grazed temperate dairy pasture over four years

    Agric. Ecosyst. Environ.

    (2015)
  • S. Rutledge et al.

    The carbon balance of temperate grasslands part I: the impact of increased species diversity

    Agric. Ecosyst. Environ.

    (2017)
  • S. Rutledge et al.

    The carbon balance of temperate grasslands part II: the impact of pasture renewal via direct drilling

    Agric. Ecosyst. Environ.

    (2017)
  • M.G. Ryan et al.

    Age-related decline in forest productivity

    Adv. Ecol. Res.

    (1997)
  • S. Saggar et al.

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

    Total Environ.

    (2013)
  • G. Waghorn

    Beneficial and detrimental effects of dietary condensed tannins for sustainable sheep and goat production - Progress and challenges

    Anim. Feed Sci. Technol.

    (2008)
  • C. Ammann et al.

    Effect of grassland renovation on the greenhouse gas budget of an intensive forage production system

    Adv. Anim. Biosci.

    (2013)
  • N.J. Barrow et al.

    Partition of excreted nitrogen, sulphur, and phosphorus between the faeces and urine of sheep being fed pasture.

    Aust. J. Agric. Res.

    (1962)
  • J.G. Benjamin et al.

    Modelling corn rooting patterns and their effects on water uptake and nitrate leaching

    Plant Soil

    (1996)
  • J.E. Bertram et al.

    Hippuric acid and benzoic acid inhibition of urine derived N2O emissions from soil

    Glob. Chang. Biol.

    (2009)
  • N.S. Bolan et al.

    Gaseous emissions of nitrogen from grazed pastures: processes, measurements and modelling, environmental implications, and mitigation

    Adv. Agron.

    (2004)
  • R.H. Bryant et al.

    Can alternative forages substantially reduce N leaching? Findings from a review and associated modelling

    NZ J. Agric. Res.

    (2020)
  • K.C. Cameron et al.

    Nitrogen losses from the soil/plant system: a review

    Ann. Appl. Biol.

    (2013)
  • A.R. Castillo et al.

    A review of efficiency of nitrogen utilisation in lactating dairy cows and its relationship with environmental pollution

    J. Anim. Feed Sci.

    (2000)
  • J.R. Crush et al.

    Adventitious root mass distribution in progeny of four perennial ryegrass (Lolium perenne L.) groups selected for root shape

    NZ J. Agric. Res.

    (2010)
  • D. Dalley et al.

    Productivity and environmental implications of fodder beet and maize silage as supplements to pasture for late lactation dairy cows

    NZ J. Agric. Res.

    (2020)
  • C.A.M. de Klein et al.

    Targeted technologies for abatement from animal agriculture

    Aust. J. Exp. Agr.

    (2008)
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