The potential effectiveness of four different options to reduce environmental impacts of grazed pastures. A model-based assessment
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:
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Nitrogen content in animal feed
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Nitrification inhibitors
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Deep rooting
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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:
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Deeper roots allow access to water stored at depth and enable continued photosynthesis and growth during periods without rainfall or irrigation;
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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
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