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Nitrous oxide emissions from grazed grasslands: interactions between the N cycle and climate change — a New Zealand case study
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ScienceDirect Nitrous oxide emissions from grazed grasslands: interactions between the N cycle and climate change — a New Zealand case study Cecile AM de Klein1, Mark A Shepherd2 and Tony J van der Weerden1 Nitrous oxide (N2O) is the third most important greenhouse gas globally. In grazed livestock systems, excreta deposited by grazing animals are the largest source of N2O. Understanding the effect of climate change on N cycling within the plant–soil–animal continuum is critical for future-proofing N2O mitigation strategies for these systems. We describe the impacts of elevated CO2 (eCO2), elevated temperatures (eTemp) and elevated or reduced rainfall (eRain or rRain, respectively) on: first, the rates of N cycling processes that produce N2O and second, the amount of urinary N supply as affected by pasture growth and composition, and the N content of the pasture. We use climate change predictions for New Zealand as a case-study example to assess climate change implications for mitigating N2O emissions from grazed grasslands. Climate change effects on soil N processes will most likely result in an increase in nitrification, NH3 volatilisation and denitrification. The effects on mineralisation and N leaching are relatively uncertain. However, overall N2O emissions are likely to increase in grazed grasslands in New Zealand as a result of primarily eTemp and eRain effects on N cycling process rates. The effects of eCO2 on net primary production, legume growth and biological N fixation are likely to increase the amount of urine N deposited to pastures in the short term, thus also increasing N2O emissions. Although we are more certain about increased N2O emissions due to the effects on N cycling processes, we are less certain about climate change effects on the size of the urinary N source. Development of future-proof N2O mitigation options will require a detailed understanding of the interactions between CO2 and N fertilisation, C/N ratios and microbial activity and integration of component studies at a field scale to fully understand farm systems’ responses to climate change. Addresses 1 AgResearch, Invermay, Private Bag 50034, Mosgiel 9053, New Zealand 2 AgResearch, Ruakura, Private Bag 3123, Hamilton 3240, New Zealand Corresponding author: de Klein, Cecile AM ([email protected]) Current Opinion in Environmental Sustainability 2014, 9–10:131–139 This review comes from a themed issue on System dynamics and sustainability Edited by Carolien Kroeze, Wim de Vries and Sybil Seitzinger For a complete overview see the Issue and the Editorial Received 26 March 2014; Revised 12 September 2014; Accepted 26 September 2014 Available online 22nd October 2014 http://dx.doi.org/10.1016/j.cosust.2014.09.016 1877-3435/# 2014 Elsevier B.V. All rights reserved.
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Introduction Nitrous oxide (N2O) is a potent greenhouse gas and is the third most important greenhouse gas (GHG) emitted globally, after carbon dioxide (CO2) and methane (CH4) [1]. The majority of these N2O emissions derive from agriculture and with the growing demand for agricultural products, N2O concentrations in the atmosphere are expected to double by 2050 [1]. The ongoing use of reactive nitrogen for agricultural production will continue to increase the risk of N2O emissions and much effort is being put into developing mitigation strategies that will enable us to meet global food demands whilst lowering N2O emissions (e.g. [2,3]). Climate change impacts in response to elevated atmospheric CO2 concentrations, increasing temperatures and changing rainfall and drought patterns, affect the N cycle in grazed grasslands through changes in N inputs and demands (e.g. biological N fixation, recycling of plant residues, root litter and/or excretal N, changes to growth). Furthermore, climate change also impacts on the rates of soil N cycling processes that are direct sources of N2O (nitrification and denitrification) or those that are commonly referred to as ‘indirect’ sources of N2O (volatilisation or leaching) [1]. Understanding the net effect of climate change on N cycling within the plant–soil–animal continuum, and its many feedback loops and interactions is critical for the development of efficient and effective N2O mitigation strategies for grazed livestock systems. However, predicted climate change scenarios vary across geographical regions [4] and the net effect of climate change on N cycling and N2O emissions will depend on the expected climate change impacts for each specific region. This paper therefore reviews firstly our current understanding of the likely impacts of individual drivers of climate change, that is elevated CO2 (eCO2), elevated temperatures (eTemp) and elevated or reduced rainfall (eRain or rRain, respectively), on N cycling inputs and processes in grazed pastures. Specifically we review their effect on N2O emissions and on the key drivers and pathways within the nitrogen cycle that are mostly likely to affect N2O emissions. We then present an assessment of their combined effects using climate change predictions for New Zealand as a case study. The implications for the development of future-proof N2O mitigation options for grazed livestock systems are also discussed. Current Opinion in Environmental Sustainability 2014, 9–10:131–139
132 System dynamics and sustainability
Sources of N2O in grazed pasture systems Animal excreta are the largest source of N2O in grazed pastures, contributing up to 80% of N2O emissions from these systems (e.g. [5]). These emissions occur from discrete small patches that, in any 1 year, cover as little as 5% of grazed pasture in very extensively managed systems to up to 30% in intensively grazed pastures [6,7]. Because of the large amounts of readily available N, urine patches are a particularly important source of N2O [5] and this paper therefore focuses on this source of animal excreta. Depending on the system and on the animal type, the N loading rates of urine patches can range between 200 and 2000 kg N/ha [6]. The N2O emissions from these patches are determined by first, the amount of urine N deposited and second, the rate of soil N cycling processes following deposition. Key factors in the amount of urinary N return are the amount of above ground dry matter (DM) grazed and its crude protein content [6], which are in turn affected by climate effects on the availability of N (from biological N fixation (BNF), soil N mineralisation or fertiliser inputs), plant species composition and the animals’ physiological state [8]. Urine patch overlap can result in high N loading rates, with the risk of overlap increasing with increased stocking rate. However, limited evidence suggests that urine patch overlap is unlikely to increase total N2O emissions, as the rate of N applied does not appear to affect the N2O emission factor from urine (i.e. the proportion of urine N emitted as N2O) when N2O emissions are high [9]. Once deposited onto soil, urinary N is rapidly hydrolysed to soil ammonium (NH4+). It can then be lost through ammonia (NH3) volatilisation or nitrified to nitrate (NO3 ). In turn, this NO3 can then be denitrified to N2O (and dinitrogen gas, N2) or leach down the soil profile [6]. The proportion of urinary N that is nitrified and/or denitrified to N2O is largely affected by localised biotic and abiotic conditions, such as the size and activity of microbial population, soil temperature and aeration status as affected by changes soil physical characteristics (e.g. compaction or treading effects and soil porosity) and weather patterns (temperature and rain). Urinary N that is initially lost via NH3 volatilisation or NO3 leaching can subsequently be emitted as N2O following re-deposition of NH3 to land or from NO3 leached to surface waters. These processes are therefore commonly referred to as indirect sources of N2O emissions [10].
(urinary) N supply (via effects on e.g. DM production, N content or BNF) (Figure 1). Direct effects on N cycling processes will impact mostly on N2O emissions at the individual urine patch level (i.e. the N2O emission factor of any N deposited) whereas indirect effects on urinary N source will mainly influence N2O emissions at a systems level (e.g. through changes in DM production and N inputs or turn-over within the grazing system). Effects on N cycling processes
We could not find any literature documenting climate change effects on N2O emissions from grazed field or urine patches per se. However, it is well established that N2O emissions generally increase with temperature and soil moisture content [11,12]. N2O emissions typically following a bell-shape curve with increasing soil moisture, with emissions peaking between field capacity and nearsaturation [5,13,14,15]. Flechard et al. [15] also showed that the peak in N2O emissions increased with soil temperature. Therefore, if climate change results in increased warming (eTemp) and increased rainfall (eRain), then N2O emissions from urine patches are likely to increase. In grassland exposed for 4 years to eCO2, eTemp and rRain, Cantarel et al. [16] found that eTemp (+3.5 8C) alone and in combination with eCO2 and rRain (summer) significantly increased N2O emissions. In contrast, Dijkstra et al. [17] and Niboyet et al. [18] only found small effects of eTemp on N2O emissions, but this was most likely due to the relatively modest increases in soil surface temperature (0.8–1.0 8C) used in these studies. Ross et al. [19] also found that long-term exposure to eCO2 resulted in an increase of soil water content and soil pH, both of which could result in enhanced denitrification and N2O emissions [9,11].
Impacts of climate change on N2O losses
eTemp has been shown to increase nitrification rates in cut/ ungrazed grassland ecosystems [20,21,22]. In addition, Bowatte et al. [23] showed that eCO2 may result in greater nitrification rates in ryegrass pastures due to changes in microbial associations and functions in the rhizosphere. These authors showed that soil containing ryegrass plants from seeds grown under eCO2 for several years had higher nitrification rates than plants grown from browntop seeds that were produced under eCO2. Ryegrass and browntop plants grown from seeds produced under ambient CO2 had similar nitrification rates. Although further work is required to determine the mechanism behind the effect of eCO2 on nitrification rates in ryegrass pasture, Bowatte et al. [23] suggest that competition for nutrients, the amount and quality of root exudates, and/or differences in rhizosphere environment are the most likely explanations.
The impacts of climate change on N cycling and N2O losses in grazed pastures are likely to be complex and may result from either direct effects of eCO2, eTemp and eRain or rRain on N cycling processes (i.e. nitrification, denitrification, mineralisation, volatilisation and leaching), and/or their indirect effects on the amount of
eRain may increase denitrification and N2O emissions due to increasing soil moisture contents [20,24]. eRain may also increase the risk of soil compaction by animal treading, which could in turn reduce soil aeration and thus the risk of increased N2O emissions [11,13]. Brown et al.
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Climate change impacts on N2O emissions de Klein, Shepherd and van der Weerden 133
Figure 1
Legume abundance
Biological N fixation
eCO2 rRain
Below ground DM production
Above ground DM production
Box1
Box2
Plant N uptake
Direct and indirect N2O emissions
N content DM N intake
eCO2 eTemp eRain rRain
Litter/root exudates Dung N excreted Box3
Urine N excreted Urea hydrolysis
N and C mineralisation
Nitrification NH3 volatilisation
Denitrification NO3 leaching
eTemp eRain rRain
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Overview of the key climate change impacts on the N cycle and N2O emissions from grazed livestock systems. Boxes 1 and 2 represent the effects that lead to increased urinary and dung N excretion and box 3 the direct effects on process rates. Words in italics refer to the climate change impacts that are affecting the components of the specific boxes.
[20] measured the effects of eCO2, eRain, eTemp and added N on N2O emissions from an annual grassland ecosystem. They showed that eCO2 (alone or in combination with the other treatments) did not affect N2O emissions but that eRain, especially in combination with eTemp and added N, significantly increased N2O emissions. However, if eRain results in soil moisture contents close to saturation, then N2O emissions may reduce due to an increase in N2O reduction to N2, leading to a decrease in the N2O:N2 ratio from denitrification [25]. rRain or increased incidences of drought may also reduced N2O emissions, as shown by Hartmann et al. in a 3-year field experiment on grassland soils [26,27]. Yet, nitrification and denitrification potentials and the abundance of functional nitrifying and denitrifying genes were not affected by drought conditions. Hartmann et al. [27] therefore concluded that the drought effects on N2O emissions were largely driven by environmental limitations rather than by changes in potential process rates. In contrast, Evans and Burke [28] found that N2O emissions from soils under prolonged drought increased following rewetting because soil mineral N had accumulated under the drought conditions. Similarly, rewetting of dry soil can also induce a rapid pulse of N (and C) mineralisation in pasture soils known as the ‘Birch effect’ [29], which increases the amount of N available nitrification and denitrification to N2O. It would, therefore, seem likely that where climate change results in a pattern of increased drought combined with increased rainfall pulses and warming, N2O emissions may increase. www.sciencedirect.com
The direct effects of climate change on NH3 volatilisation are most likely through eTemp, eRain and/or rRain. eTemp and rRain will increase the risk of NH3 volatilisation, while eRain will reduce it [24,30]. The direct effect of climate change on N leaching will be driven by changes in rainfall patterns that will affect the transport of N through the soil profile. Although Larsen et al. [21] found that eRain increased N leaching, estimates of the effect of climate change on leaching are uncertain due to the competing processes of N source generation versus transport [31]. For example, Torbert et al. [32] observed lower leaching losses below soybean and sorghum grown in elevated compared to ambient CO2 and attributed this largely to increased uptake of soil N. Similarly, Dijkstra et al. [33] found that eCO2 reduced N leaching in grassland soils, especially when leaching losses were high. In contrast, increased N mineralisation induced by rewetting of dry soil (i.e. the ‘Birch effect’ [29]) could increase N leaching if drainage occurs before pasture has recovered sufficiently from drought to use the mineralised N [34]. Effects on urinary N supply
Climate change can indirectly affect N2O emissions from grazed pastures through changes in the factors that determine the amount of urinary N deposited to pasture and/or though changes in other N inputs such as BNF and N mineralisation. As mentioned earlier, the amount of urinary N excreted is determined by N intake by the grazing animals, as Current Opinion in Environmental Sustainability 2014, 9–10:131–139
134 System dynamics and sustainability
influenced by the amount of above ground dry matter (DM) grazed, and the protein content of that DM. In general, eCO2 and/or eTemp enhance biomass production, as was found in two meta-analyses of results from 150 and 127 experimental sites across a range of ecosystems and climates [35,36]. If this extra biomass is utilised through an increase in DM intake per animal or through an increase in stocking rate, then urine N deposition in grazed grasslands is likely to increase, assuming that the extra DM production is also associated with extra N intake. However, the effect of an increase in biomass production on N excretion could be off-set by reductions in the N content of plants (through N dilution) and/or of the pasture (through changes in plant composition). For example, in their meta-analysis, Dieleman et al. [35] concluded that foliar N concentrations in plants decrease significantly in combined eCO2 and warming treatments. The potential hypotheses for the mechanism(s) of this N dilution effect were reviewed by Taub and Wang [37], who tended to favour a combination of dilution of N in plant material, due to increased compounds derived from photosynthate, and decreases in N uptake. However, this dilution effect in individual plants has not always been observed at a field scale because the composition of the plant community also responds to eCO2. It is generally accepted that eCO2 leads to increased growth in plants whose physiology and growth rate can benefit most from the extra CO2, that is plants with higher N content such as legumes [38]. However, Newton et al. [39] found that the effect of eCO2 on plant composition can be modified by grazing animals. These authors showed that although the effects in the first 5 years of eCO2 in a ryegrass/clover pasture system were as expected, that is more legumes and forbs, and less C3 grasses, after 12 years of exposure to eCO2 there was little difference in plant composition as the animals selectively grazed the legumes and forbs. Climate change effects on other inputs of N, such as BNF or N mineralisation, can also affect plant growth and dietary N intake, and thus urinary N excretion, by grazing animals. When legumes are present in agricultural systems, eCO2 can increase the legume biomass [19] and increase N inputs via BNF by increasing the number and mass of root nodules [40] and influencing the rhizobial population composition. In contrast, Watanabe et al. [41] found that BNF in clover plants grown under eCO2 for 13 years was significantly lower than in plants grown under ambient CO2, due to a reduction in the number of rhizobial nifH genes in the nodules. Lam et al. [36] observed that eCO2 increased N fixation in legumes as well as N2O emissions from grain crops and legume pasture systems. However, in two long-term experiments the effect of eCO2 on increased BNF was short-lived [42,43]. In these studies the decline in N fixation was associated with shortages of other nutrients: molybdenum [42]; or low soil P availability [43]. Current Opinion in Environmental Sustainability 2014, 9–10:131–139
In terms of N mineralisation effects on N2O emissions, it has been observed that eTemp typically increases mineralisation rates and thus N availability [22,35,44], especially when external N is added [22]. This increase in N availability is particularly important in grassland systems that rely on low external N inputs as these systems are at risk of a yield decline due to progressive N limitation (PNL). PNL is a biogeochemical feedback mechanism that results in a reduction in N availability under eCO2 (e.g. [45,46,47]). Newton et al. [46] found that eCO2 in a grazed system initially increased N harvested in above ground biomass, but this effect was no longer evident after 5 years, suggesting the occurrence of PNL. This N limitation was temporarily broken following a drought breaking rain providing a flush of mineral N (i.e. Birch effect) in year 6 of the experiment, but there was evidence that PNL started to occur again in the following years. However, in a combined eCO2 plus eTemp study in temperate perennial grassland, Hovenden et al. [48] found that the effect of eCO2 on reduced soil N availability was offset by eTemp (+2 8C), suggesting that warming could potentially overcome PNL-induced growth restrictions. Increased incidences of drought (rRain) may also reduce biomass production [26,27] unless this is offset by irrigation. Hartmann et al. [27] also suggested that rRain increased N mineralisation rates and thus N availability, and that drought combined with increased rainfall pulses and warming, are likely to increase N2O emissions. The above analysis suggests that the indirect effects of eCO2, eTemp and rRain on N cycling and N2O emissions are uncertain and will depend on whether the CO2 fertilisation effect can be sustained, either through an increase in N mineralisation under eTemp and/or external inputs such as N fertiliser (to combat PNL) or irrigation (to combat rRain). In ecosystems that receive substantial external N inputs, PNL is unlikely to occur [45]. Furthermore, even if an increase in above ground biomass is limited by PNL, eCO2 stimulation of photosynthesis often results in increased allocation of C below ground [49] due to increases in root growth, root exudates and/or C supply to symbionts. This in turn is likely to increase soil biological activity, and thus affect N turnover rates and N2O emissions.
Implications for mitigation of N2O emissions from grazed grasslands — a New Zealand case study In this section we assess the likely impacts of contrasting future climate change scenarios in New Zealand (‘high carbon’ versus ‘rapidly decarbonising’ world scenarios; Table 1) on direct and indirect N2O emissions and on N process rates and inputs into intensively and extensively grazed pastures (Table 2). We have assumed that intensive systems will rely on ongoing or increasing www.sciencedirect.com
Climate change impacts on N2O emissions de Klein, Shepherd and van der Weerden 135
Table 1 Predicted ranges for key climate variables in New Zealand, based on contrasting scenarios of carbon dioxide increase from the ‘high carbon’ or ‘rapidly decarbonising’ world scenarios [56]. The range in values provided a guide for the estimated climate change effects. Changes are relative to 1980–1999 levels Variable Carbon dioxidea (ppm) Temperature (8C) Change in rainfall (%)
Season All All Summer and autumn Winter and spring
Hot days Frosts Heavy rainfall
Summer half-year Winter half-year All
Drought
Summer half-year
Strong winds
Winter, Spring Summer/ autumn
Region of NZ
Range predicted for year 2049
Level of confidence in predicted values
All All South and West South Island Rest of NZ North and east North Island North and east South Island Rest of NZ All lowland areas
480–530 ppm 0.7–0.9 Zero to +5% Up to 5%
Moderate to high High Moderate
Zero to 10% Zero to +10%
High
Up to 100% increase
High
Central North Island and South Island West of both Islands and south of South Island Eastern South Island and all of North Island
Up to 50% reduction
High
Extremes occur up to 50% as often Up to 5–10% more of year
High
All North Island South Island
Increase of few % Little change Decrease of few %
Moderate for type of change; low for magnitude Moderate for type of change; low for magnitude
Adapted from [57]. a
Current level of carbon dioxide is 395 ppm.
external N inputs, while in extensive systems external N inputs are assumed to be low, with most provided through BNF. It should be noted that this assessment does not take account of any effects of climate change on animal performance. The research on primary production impacts (as the ultimate source of urinary N) points to a number of possible scenarios for the effects of climate change. The amount of available pasture for consumption could increase due to eCO2 fertilisation effects or, conversely, could decrease if PNL is not addressed or if drought conditions prevail. The nature and magnitude of external N inputs, the initial N status of the ecosystem and changes in soil C stocks will critically determine if and when PNL occurs. In addition, farmers may increase N fertiliser use and/or irrigation to combat PNL and drought conditions, and maximise the benefits of eCO2 and eTemp. However, increased use of irrigation will increase the demand for, and consequently the pressures on, access to water. Similarly, the N content of pasture could increase or decrease based on a number of factors, such as dilution of N through increased growth, changes in species mixes (more legumes), and selective grazing pressure. However, based on the work reviewed in this paper we conclude that, on balance, climate change impacts are likely to increase N2O emissions from grazed grasslands in New Zealand. This can occur both directly, through increasing nitrification, denitrification and NH3 www.sciencedirect.com
volatilisation rates, and indirectly through increased urinary N deposition. These increased urinary N inputs are largely a result of increased net primary production and, to some extent, BNF. While research into individual aspects or components of the ecosystem, gives us building blocks to assess these likely impacts of climate change on N2O emissions, an assessment of the integrated effects at a field or farm scale is much more difficult given that the system’s response will result from complex interactions of these individual building blocks. As a result, we can perhaps place some certainty around increased N2O emissions due to direct factors, that is there is an overall likelihood of increased process rates. However, we are less certain about indirect effects at a paddock or farm scale due to interactions with, for example, animal grazing behaviour or management decisions by farmers. For example, Newton and coworkers demonstrated some of the systems’ interactions in the long-term eCO2 study in grazed pasture where selective grazing pressure modified the commonly observed responses of plant composition to eCO2 (e.g. [37,38,39,46]). If the risk of N2O emissions will increase due to climate change impacts, the question then becomes ‘can we manage or mitigate these increased N2O emissions’? Current Opinion in Environmental Sustainability 2014, 9–10:131–139
136 System dynamics and sustainability
Table 2 Summary of the potential impacts of NZ climate change scenarios and certainty of effect on N cycling in intensively and extensively grazed pastures under temperate climates (adapted from [58]) Soil nitrogen factor
Intensively grazed pastures
Extensively grazed pastures
N2O losses Direct
Indirect via NH3 volatilisation Indirect via NO3 leaching
Increase driven by eTemp, milder winters and periods of eRain. Sources may also increase: manure, urine, effluent driven by increased forage production; fertiliser driven by increased inputs to capitalise on eCO2 fertilisation effect or PNL Increase driven by eTemp and periods of drought. Sources (manure, urine, effluent, fertiliser etc.) may also increase, as described above. More rain could decrease losses, but more winter rain expected, that is when emissions are lower. Nitrate leaching will be a balance of other processes affecting the amount of mineral N remaining in the soil at times of drainage. Will increase with increasing occurrence of drought and droughtbreaking rain.
?
Process rates Denitrification Nitrification Mineralisation
?
?
N inputs Atmospheric inputs Biological N fixation
Likely to increase due to eTemp and eRain. Assumes that irrigation will increase in areas where rRain may occur. Increased due to increased temperature eTemp and increased substrate availability will generally increase microbial activity, however net effects of mineralisation-immobilisation turnover are uncertain. Increase slightly as a result of slightly more rainfall and small increases in ammonia volatilisation. Increase due to balance of evidence suggesting eCO2 favours legume growth. But could remain unchanged due to selective grazing pressure. eTemp, eRain, and rRain will also affect N fixation adding further uncertainty. Increased inputs will be used to mitigate against PNL, unless limited by regulation. Assumes BNF will be sufficient to mitigate PNL in extensive systems. Increased due to increased net primary production and legume growth.
Fertiliser N Litter/root exudation Excreta N
Justification
?
Increase as a result of more pasture production where rain/irrigation and N inputs are sufficient to capitalise on the eCO2 fertilisation effect; more likely in dairy systems.
Direction of change and certainty in science knowledge: overall, most likely to increase; overall, most likely to decrease; overall, most likely to remain unchanged; ? reasonably uncertain of effects, therefore direction cannot be predicted. Colour of arrows: & reasonably certain of effects; neither certain nor uncertain.
There may be some scope to manage N2O losses through controlling nitrification. This is one of the key controlling process of N2O emissions from urine patches, as it determines the amount of NO3 available for denitrification to N2O (and, under complete denitrification, N2). In addition, nitrification itself can also be a source of N2O. Nitrification rates are expected to increase slightly due to eTemp (Table 2), and eCO2 may increase soil nitrification rates under a ryegrass pasture [23]. The nitrification inhibitor dicyandiamide (DCD) has been shown to be an effective strategy for reducing N2O emissions from urine patches, with average reductions of 57% observed in New Zealand field studies conducted over 10 years [50]. However, DCD degrades in soil, with the degradation rate increasing with temperature [51]. DCD can also leach down the soil profile especially under increased rainfall [52]. Impacts of climate change on DCD degradation and leaching rates remain to be tested in field experiments.
incidences of warm/wet soil conditions (Table 1) that are conducive to denitrification [11]. In addition, if farmers use irrigation and external N inputs to combat drought and PNL, the risk of N2O losses through denitrification may further increase. Careful spatial and temporal management of urine patch returns to avoid high-risk hot spots (wet times and areas [53]) may therefore become increasingly important. Management options to avoid hot spots includes the use of ‘off-paddock’ systems (feedpads, stand-off pads, animal shelters), which provide farmers with the ability to adapt to climate change (eRain), particularly increased storm events delivering more intense rainfall. However, these systems need to be optimised with respect to manure management, as there is a risk of enhanced N emissions during manure storage due to eTemp and because more time off-pasture will increase the amount of manure held in storage [54].
Denitrification rates in New Zealand grassland soils are also likely to be enhanced (Table 2) due to increased
Increased use of irrigation under rRain conditions requires development of management guidelines for irrigation consultants and farmers. N2O emissions are reduced when the return period for irrigators is
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Climate change impacts on N2O emissions de Klein, Shepherd and van der Weerden 137
de Klein CAM, Luo J, Woodward KB, Styles T, Wise B, Lindsey S, Cox N: The effect of nitrogen concentration in synthetic cattle urine on nitrous oxide emissions. Agric Ecosyst Environ 2014, 188:85-92.
lengthened, due to lower average soil moisture contents (van der Weerden, unpublished data). Likewise, ensuring irrigation does not increase soil water content up to or beyond field capacity will reduce average soil moisture content and associated N2O emissions via denitrification.
9.
In conclusion, current predictions of the direct impacts of climate change on N2O production rates from N cycling processes at a urine patch level are relatively certain. However, the indirect effects of climate change on the size of the urine N source in grazed systems are highly uncertain due to our lack of detailed understanding of the complex interactions between CO2 and N fertilisation, soil and plant C/N ratios, and microbial activity [55]. Understanding the full implications of climate change effects on N2O emissions will require ongoing research efforts and linkages between N and C cycling studies, and integration of individual component studies at a field or farm scale.
11. Bolan NS, Surinder S, Luo J, Rita B, Jagrati S: Gaseous emissions of nitrogen from grazed pastures: processes, measurements and modelling, environmental implications, and mitigation. Adv Agron 2004, 84:37.
Acknowledgement This review draws on a project funded by the Ministry for Primary Industries, New Zealand on Climate Change impacts on N cycling (AGR30693).
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12. de Klein CAM, Pinares-patino C, Waghorn GC: Greenhouse gas emissions. In Environmental Impacts of Pasture-based Farming. Edited by McDowell RW. CAB International; 2008:1-32. 13. van der Weerden TJ, Kelliher FM, de Klein CAM: Influence of pore size distribution and soil water content on nitrous oxide response curves. Soil Res 2012, 50:125-135. 14. Saggar S, Jha N, Deslippe J, Bolan NS, Luo J, Giltrpa DL, Kim D-G, Zaman M, Tillman RW: Denitrification and N2O:N2 production in temperate grasslands: processes, measurements, modelling and mitigating negative impacts. Sci Total Environ 2013, 465:173-195. 15. Flechard CR, Ambus P, Skiba U, Rees RM, Hensen A, van Amstel A, van den Pol-van Dasselaar A, Soussana JF, Jones M, Clifton-Brown J, Raschi A, Horvath L, Neftel A, Jocher M, Ammann C, Leifeld J, Fuhrer J, Calanca P, Thalman E, Pilegaard K, Di Marco C, Campbell C, Nemitz E, Hargreaves KJ, Levy PE, Ball BC, Jones SK, van de Bulk WCM, Groot T, Blom M, Domingues R, Kasper G, Allard V, Ceschia E, Cellier P, Laville P, Henault C, Bizouard F, Abdalla M, Williams M, Baronti S, Berretti F, Grosz B: Effects of climate and management intensity on nitrous oxide emissions in grassland systems across Europe. Agric Ecosyst Environ 2007, 121:135-152. This paper reports on results from 3-year study on N2O emissions, and its driving variables, from grasslands sites across 10 sites in Europe. Highlights the need for climate sensitive emission factors for N2O instead of default values. 16. Cantarel AAM, Bloor JMG, Deltroy N, Soussana JF: Effects of climate change drivers on nitrous oxide fluxes in an upland temperate grassland. Ecosystems 2011, 14:223-233. This paper presents results of combined effects of warming, summer drought, and elevated CO2 concentrations on N2O fluxes from an extensively managed grassland. Concluded that temperature effects may be primarily driving future N2O emissions from temperate, extensively managed grasslands. 17. Dijkstra FA, Prior SA, Runion GB, Torbert HA, Tian H, Lu C, Venterea RT: Effects of elevated carbon dioxide and increased temperature on methane and nitrous oxide fluxes: evidence from field experiments. Front Ecol Environ 2012, 10:520-527. 18. Niboyet A, Brown JR, Dijkstra P, Blankinship JC, Leadley PW, Roux X, le Barthes L, Barnard RL, Field CB, Hungate BA: Global change could amplify fire effects on soil greenhouse gas emissions. PLoS ONE 2011:e20105. 19. Ross D, Newton PCD, Tate K: Elevated [CO2] effects on herbage production and soil carbon and nitrogen pools and mineralization in a species-rich, grazed pasture on a seasonally dry sand. Plant Soil 2004, 260:183-196. 20. Brown JR, Blankinship JC, Niboyet A, van Groenigen KJ, Dijkstra P, Le Roux X, Leadley PW, Hungate BA: Effects of multiple global change treatments on soil N2O fluxes. Biogeochemistry 2012, 109:85-100. 21. Larsen KS, Andresen LC, Beier C, Jonasson S, Albert KR, Ambus PER, Arndal MF, Carter MS, Christensen S, Holmstrup M, Ibrom A, Kongstad J, Van Der Linden L, Maraldo K, Michelsen A, Mikkelsen TN, Pilegaard KIM, Ro-Poulsen H, Priem E´A, Schmidt IK, Selsted MB, Stevnbak K: Reduced N cycling in response to elevated CO2, warming, and drought in a Danish heathland: synthesizing results of the CLIMATE project after two years of treatments. Global Change Biol 2011, 17:18841899. 22. Ma L-N, Lu X-T, Liu Y, Guo J-X, Zhang N-Y, Yang J-Q, Wang R.-Z.: The effects of warming and nitrogen addition on soil nitrogen Current Opinion in Environmental Sustainability 2014, 9–10:131–139
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cycling in a temperate grassland, Northeastern China. PLoS ONE 2011, 6:e27645. 23. Bowatte S, Newton PCD, Hill A, Theobald P, Luo D, Hovenden M, Osanai Y: Offspring of plants exposed to elevated or ambient CO2 differ in their impacts on soil nitrification in a common garden experiment. Soil Boil Biochem 2013, 62:134-136. 24. Butterbach-Bahl K, Dannenmann M: Denitrification and associated soil N2O emissions due to agricultural activities in a changing climate. Curr Opin Environ Sustain 2011, 3:389-395. 25. Smith K: The potential for feedback effects induced by global warming on emissions of nitrous oxide by soils. Global Change Biol 1997, 3:327-338. 26. Hartmann AA, Niklaus PA: Effects of simulated drought and nitrogen fertilizer on plant productivity and nitrous oxide (N2O) emissions of two pastures. Plant Soil 2012, 361:411-426. 27. Hartmann AA, Barnard RL, Marhan S, Niklaus PA: Effects of drought and N-fertilization on N cycling in two grassland soils. Oecologia 2013, 171:705-717. 28. Evans SE, Burke IC: Carbon and nitrogen decoupling under an 11-year drought in the shortgrass steppe. Ecosystems 2013, 16:20-33. Paper reports on plant production, plant tissue chemistry, soil trace gas fluxes and soil inorganic N dynamics after 11 years of simulated drought. Long-term drought decoupled C and N cycles (reduced biomass production yet increased soil N), resulting in changes in plant N and increased vulnerability to N loss. 29. Unger S, Ma´guas C, Pereira JS, David TS, Werner C: The influence of precipitation pulses on soil respiration – assessing the ‘‘Birch effect’’ by stable carbon isotopes. Soil Biol Biochem 2010, 42:1800-1810. 30. Meisinger JJ, Jokela WE: Ammonia volatilisation from dairy and poultry manure. Managing Nutrients and Pathogens from Animal Agriculture (NRAES-130). Ithaca, NY: Natural Resource, Agriculture and Engineering Service; 2000, 334-354. 31. Stuart ME, Gooddy DC, Bloomfield JP, Williams AT: A review of the impact of climate change on future nitrate concentrations in groundwater of the UK. Sci Total Environ 2011, 409:28592873. 32. Torbert HA, Prior SA, Rogers HH, Schlesinger WH, Mullins GL, Runion GB: Elevated atmospheric carbon dioxide in agroecosystems affects groundwater quality. J Environ Qual 1996, 25:720-726. 33. Dijkstra FA, West JB, Hobbie SE, Reich PB, Trost J: Plant diversity, CO2, and N influence inorganic and organic N leaching in grasslands. Ecology 2007, 88:490-500. 34. Whitehead PG, Wilby RL, Butterfield D, Wade AJ: Impacts of climate change on in-stream nitrogen in a lowland chalk stream: an appraisal of adaptation strategies. Sci Total Environ 2006, 365:260-273.
Authors tended to favour a combination of dilution of N in plant material, due to increased compounds derived from photosynthate, and decreases in N uptake. 38. Allard V, Newton PCD, Lieffering M, Clark H, Matthew C, Soussana JF, Gray YS: Nitrogen cycling in grazed pastures at elevated CO2: N returns by ruminants. Global Change Biol 2003, 9:1731-1742. 39. Newton PCD, Lieffering M, Parsons AJ, Brock SC, Theobald PW, Hunt CL, Luo D, Hovenden MJ: Selective grazing modifies previously anticipated responses of plant community composition to elevated CO2 in a temperate grassland. Global Change Biol 2014, 20:158-169. Paper reports on effects of long-term eCO2 exposure on plant community composition in a grazed system. It shows how animal grazing behaviour off-set eCO2-induced changes in sward composition that are observed in non-grazed systems (i.e. increased proportions of legumes and forbs), as animals selectively grazed these species. After 11 years of eCO2 exposure in this sheep-grazed FACE facility, there was no difference in plant community composition between ambient and elevated CO2. 40. Zanetti S, Hartwig UA, Luscher A, Hebeisen T, Frehner M, Fischer BU, Hendrey GR, Blum H, Nosberger J: Stimulation of symbiotic N2 fixation in Trifolium repens L. under elevated atmospheric pCO2 in a grassland ecosystem. Plant Physiol 1996, 112:575-583. 41. Watanabe T, Bowatte S, Newton PCD: A reduced fraction of plant N derived from atmospheric N (%Ndfa) and reduced rhizobial nifH gene numbers indicate a lower capacity for nitrogen fixation in nodules of white clover exposed to longterm CO2 enrichment. Biogeosciences 2013, 10:8269-8281. 42. Hungate BA, Dukes JS, Shaw MR, Luo Y, Field CB: Nitrogen and climate change. Science 2003, 302:1512-1513. 43. Niklaus PA, Ko¨rner C: Synthesis of a six-year study of calcareous grassland responses to in situ CO2 enrichment. Ecol Monogr 2004, 74:491-511. 44. Reich PB, Hobbie SE: Decade-long soil nitrogen constraint on the CO2 fertilization of plant biomass. Nat Clim Change 2012, 3:278-282. 45. Hu S, Tu C, Chen X, Gruver JB: Progressive N limitation of plant response to elevated CO2: a microbiological perspective. Plant Soil 2006, 289:47-58. 46. Newton PCD, Lieffering M, Bowatte WMSD, Brock SC, Hunt CL, Theobald PW, Ross DJ: The rate of progression and stability of progressive nitrogen limitation at elevated atmospheric CO2 in a grazed grassland over 11 years of free air CO2 enrichment. Plant Soil 2010, 336:433-441. Paper presents the results of a long-term (11 years) experiment in a sheep-grazed free air CO2 enrichment facility on soil N availability and PNL. It is providing, for the first time, evidence of PNL in a grazed system. It also shows that PNL can be reduced by perturbation (in this case a drought breaking rain providing N input through N mineralisation) but that the underlying trend is for PNL to occur at eCO2.
35. Dieleman WIJ, Vicca S, Dijkstra FA, Hagedorn F, Hovenden MJ, Larsen KS, Morgan JA, Volder A, Beier C, Dukes JS, King J, Leuzinger S, Linder S, Luo Y, Oren R, De Angelis P, Tingey D, Hoosbeek MR, Janssens IA: Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature. Global Change Biol 2012, 18:2681-2693. In-depth review and synthesis of published results of eTemp and eCO2, separately or in combination, on biomass production, soil respiration and N mineralisation of terrestrial ecosystems. The results suggest that C and N cycling may be more sensitive to the effects of eCO2 and to eTemp, which the authors attribute to the larger imposed changes in eCO2 compared eTemp as per projections.
47. Reich PB, Hungate BA, Luo Y: Carbon–nitrogen interactions in terrestrial ecosystems in response to rising atmospheric carbon dioxide. Annu Rev Ecol Evol Syst 2006, 37:611-636.
36. Lam SK, Chen D, Norton R, Armstrong R, Mosier AR: Nitrogen dynamics in grain crop and legume pasture systems under elevated atmospheric carbon dioxide concentration: a metaanalysis. Global Change Biol 2012, 18:2853-2859.
50. de Klein CAM, Cameron KC, Di HJ, Rys G, Monaghan RM, Sherlock RR: Repeated annual use of the nitrification inhibitor dicyandiamide (DCD) does not alter its effectiveness in reducing N2O emissions from cow urine. Anim Feed Sci Technol 2011, 166–167:480-491.
37. Taub DR, Wang X: Why are nitrogen concentrations in plant tissues lower under elevated CO2? A critical examination of the hypotheses. J Integr Plant Biol 2008, 50:1365-1374. Review and critical analysis of a range of hypotheses on the mechanisms responsible for the effect of eCO2 on the N concentration in plant tissue. Current Opinion in Environmental Sustainability 2014, 9–10:131–139
48. Hovenden MJ, Newton PCD, Carran RA, Theobald P, Wills KE, Van der Schoor JK, Williams AL, Osanai Y: Warming prevents the elevated CO2-induced reduction in available soil nitrogen in temperate, perennial grassland. Global Change Biol 2008, 14:1018-1024. 49. Hu S, Zhang W: Impact of global change on biological processes in soil: implications for agroecosystem management. J Crop Improvement 2004, 12:289-314.
51. Kelliher FM, van Koten C, Kear MJ, Sprosen MS, Ledgard SF, de Klein CAM, Letica SA, Luo J, Rys G: Effect of temperature on dicyandiamide (DCD) longevity in pastoral soils under field conditions. Agric Ecosyst Environ 2014, 186:201-204. www.sciencedirect.com
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52. Shepherd M, Wyatt J, Welten B: Effect of soil type and rainfall on dicyandiamide concentrations in drainage from lysimeters. Aust J Soil Res 2012, 50:67-75. 53. Monaghan RM, de Klein CAM: Integration of measures to mitigate reactive N losses to the environment from grazed pastoral dairy systems. J Agric Sci 2014 http://dx.doi.org/ 10.1017/S 0021859613000956. 54. Montes F, Meinen R, Dell C, Rotz A, Hristov AN, Oh J, Waghorn G, Gerber PJ, Henderson B, Makkar HPS, Dijkstra J: Mitigation of methane and nitrous oxide emissions from animal operations: II. A review of manure management mitigation options. J Anim Sci 2013, 91:5070-5094. 55. Gruber N, Galloway JN: An Earth-system perspective of the global nitrogen cycle. Nature 2008, 451:293-296. Paper provides interesting concepts on global N cycling with strong emphasis on the interactions and close coupling with the C cycle. It suggests that two processes are of key interest with respect to N, C and climate interactions: decoupling of N and C cycling and changes in the global reactive N inventory.
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56. Reisinger A, Mullan AB, Manning M, Wratt DW, Nottage RAC: Global and local climate change scenarios to support adaptation in New Zealand. In Climate Change Adaptation in New Zealand: Future Scenarios and Some Sectoral Perspectives. Edited by Nottage RAC, Wratt DS, Bornman JF, Jones K. Wellington: New Zealand Climate Change Centre; 2010: 26-43. 57. Review of the impacts of climate change on soil processes and the consequences for ecosystem services Soil and Land Use Alliance (SLUA) report prepared for the Ministry for Primary Industries, New Zealand, 2013. 58. van der Weerden TJ, Shepherd M, Davies M, Thomas S, Stevenson B, Curtin D, de Klein CAM: Impacts of climate change on soil nitrogen cycling. Review of the Impacts of Climate Change on Soil Processes and the Consequences for Ecosystem Services Soil and Land Use Alliance (SLUA) Report Prepared for the Ministry for Primary Industries, New Zealand. 2013:99-123.
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ScienceDirect Nitrous oxide emissions from grazed grasslands: interactions between the N cycle and climate change — a New Zealand case study Cecile AM de Klein1, Mark A Shepherd2 and Tony J van der Weerden1 Nitrous oxide (N2O) is the third most important greenhouse gas globally. In grazed livestock systems, excreta deposited by grazing animals are the largest source of N2O. Understanding the effect of climate change on N cycling within the plant–soil–animal continuum is critical for future-proofing N2O mitigation strategies for these systems. We describe the impacts of elevated CO2 (eCO2), elevated temperatures (eTemp) and elevated or reduced rainfall (eRain or rRain, respectively) on: first, the rates of N cycling processes that produce N2O and second, the amount of urinary N supply as affected by pasture growth and composition, and the N content of the pasture. We use climate change predictions for New Zealand as a case-study example to assess climate change implications for mitigating N2O emissions from grazed grasslands. Climate change effects on soil N processes will most likely result in an increase in nitrification, NH3 volatilisation and denitrification. The effects on mineralisation and N leaching are relatively uncertain. However, overall N2O emissions are likely to increase in grazed grasslands in New Zealand as a result of primarily eTemp and eRain effects on N cycling process rates. The effects of eCO2 on net primary production, legume growth and biological N fixation are likely to increase the amount of urine N deposited to pastures in the short term, thus also increasing N2O emissions. Although we are more certain about increased N2O emissions due to the effects on N cycling processes, we are less certain about climate change effects on the size of the urinary N source. Development of future-proof N2O mitigation options will require a detailed understanding of the interactions between CO2 and N fertilisation, C/N ratios and microbial activity and integration of component studies at a field scale to fully understand farm systems’ responses to climate change. Addresses 1 AgResearch, Invermay, Private Bag 50034, Mosgiel 9053, New Zealand 2 AgResearch, Ruakura, Private Bag 3123, Hamilton 3240, New Zealand Corresponding author: de Klein, Cecile AM ([email protected]) Current Opinion in Environmental Sustainability 2014, 9–10:131–139 This review comes from a themed issue on System dynamics and sustainability Edited by Carolien Kroeze, Wim de Vries and Sybil Seitzinger For a complete overview see the Issue and the Editorial Received 26 March 2014; Revised 12 September 2014; Accepted 26 September 2014 Available online 22nd October 2014 http://dx.doi.org/10.1016/j.cosust.2014.09.016 1877-3435/# 2014 Elsevier B.V. All rights reserved.
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Introduction Nitrous oxide (N2O) is a potent greenhouse gas and is the third most important greenhouse gas (GHG) emitted globally, after carbon dioxide (CO2) and methane (CH4) [1]. The majority of these N2O emissions derive from agriculture and with the growing demand for agricultural products, N2O concentrations in the atmosphere are expected to double by 2050 [1]. The ongoing use of reactive nitrogen for agricultural production will continue to increase the risk of N2O emissions and much effort is being put into developing mitigation strategies that will enable us to meet global food demands whilst lowering N2O emissions (e.g. [2,3]). Climate change impacts in response to elevated atmospheric CO2 concentrations, increasing temperatures and changing rainfall and drought patterns, affect the N cycle in grazed grasslands through changes in N inputs and demands (e.g. biological N fixation, recycling of plant residues, root litter and/or excretal N, changes to growth). Furthermore, climate change also impacts on the rates of soil N cycling processes that are direct sources of N2O (nitrification and denitrification) or those that are commonly referred to as ‘indirect’ sources of N2O (volatilisation or leaching) [1]. Understanding the net effect of climate change on N cycling within the plant–soil–animal continuum, and its many feedback loops and interactions is critical for the development of efficient and effective N2O mitigation strategies for grazed livestock systems. However, predicted climate change scenarios vary across geographical regions [4] and the net effect of climate change on N cycling and N2O emissions will depend on the expected climate change impacts for each specific region. This paper therefore reviews firstly our current understanding of the likely impacts of individual drivers of climate change, that is elevated CO2 (eCO2), elevated temperatures (eTemp) and elevated or reduced rainfall (eRain or rRain, respectively), on N cycling inputs and processes in grazed pastures. Specifically we review their effect on N2O emissions and on the key drivers and pathways within the nitrogen cycle that are mostly likely to affect N2O emissions. We then present an assessment of their combined effects using climate change predictions for New Zealand as a case study. The implications for the development of future-proof N2O mitigation options for grazed livestock systems are also discussed. Current Opinion in Environmental Sustainability 2014, 9–10:131–139
132 System dynamics and sustainability
Sources of N2O in grazed pasture systems Animal excreta are the largest source of N2O in grazed pastures, contributing up to 80% of N2O emissions from these systems (e.g. [5]). These emissions occur from discrete small patches that, in any 1 year, cover as little as 5% of grazed pasture in very extensively managed systems to up to 30% in intensively grazed pastures [6,7]. Because of the large amounts of readily available N, urine patches are a particularly important source of N2O [5] and this paper therefore focuses on this source of animal excreta. Depending on the system and on the animal type, the N loading rates of urine patches can range between 200 and 2000 kg N/ha [6]. The N2O emissions from these patches are determined by first, the amount of urine N deposited and second, the rate of soil N cycling processes following deposition. Key factors in the amount of urinary N return are the amount of above ground dry matter (DM) grazed and its crude protein content [6], which are in turn affected by climate effects on the availability of N (from biological N fixation (BNF), soil N mineralisation or fertiliser inputs), plant species composition and the animals’ physiological state [8]. Urine patch overlap can result in high N loading rates, with the risk of overlap increasing with increased stocking rate. However, limited evidence suggests that urine patch overlap is unlikely to increase total N2O emissions, as the rate of N applied does not appear to affect the N2O emission factor from urine (i.e. the proportion of urine N emitted as N2O) when N2O emissions are high [9]. Once deposited onto soil, urinary N is rapidly hydrolysed to soil ammonium (NH4+). It can then be lost through ammonia (NH3) volatilisation or nitrified to nitrate (NO3 ). In turn, this NO3 can then be denitrified to N2O (and dinitrogen gas, N2) or leach down the soil profile [6]. The proportion of urinary N that is nitrified and/or denitrified to N2O is largely affected by localised biotic and abiotic conditions, such as the size and activity of microbial population, soil temperature and aeration status as affected by changes soil physical characteristics (e.g. compaction or treading effects and soil porosity) and weather patterns (temperature and rain). Urinary N that is initially lost via NH3 volatilisation or NO3 leaching can subsequently be emitted as N2O following re-deposition of NH3 to land or from NO3 leached to surface waters. These processes are therefore commonly referred to as indirect sources of N2O emissions [10].
(urinary) N supply (via effects on e.g. DM production, N content or BNF) (Figure 1). Direct effects on N cycling processes will impact mostly on N2O emissions at the individual urine patch level (i.e. the N2O emission factor of any N deposited) whereas indirect effects on urinary N source will mainly influence N2O emissions at a systems level (e.g. through changes in DM production and N inputs or turn-over within the grazing system). Effects on N cycling processes
We could not find any literature documenting climate change effects on N2O emissions from grazed field or urine patches per se. However, it is well established that N2O emissions generally increase with temperature and soil moisture content [11,12]. N2O emissions typically following a bell-shape curve with increasing soil moisture, with emissions peaking between field capacity and nearsaturation [5,13,14,15]. Flechard et al. [15] also showed that the peak in N2O emissions increased with soil temperature. Therefore, if climate change results in increased warming (eTemp) and increased rainfall (eRain), then N2O emissions from urine patches are likely to increase. In grassland exposed for 4 years to eCO2, eTemp and rRain, Cantarel et al. [16] found that eTemp (+3.5 8C) alone and in combination with eCO2 and rRain (summer) significantly increased N2O emissions. In contrast, Dijkstra et al. [17] and Niboyet et al. [18] only found small effects of eTemp on N2O emissions, but this was most likely due to the relatively modest increases in soil surface temperature (0.8–1.0 8C) used in these studies. Ross et al. [19] also found that long-term exposure to eCO2 resulted in an increase of soil water content and soil pH, both of which could result in enhanced denitrification and N2O emissions [9,11].
Impacts of climate change on N2O losses
eTemp has been shown to increase nitrification rates in cut/ ungrazed grassland ecosystems [20,21,22]. In addition, Bowatte et al. [23] showed that eCO2 may result in greater nitrification rates in ryegrass pastures due to changes in microbial associations and functions in the rhizosphere. These authors showed that soil containing ryegrass plants from seeds grown under eCO2 for several years had higher nitrification rates than plants grown from browntop seeds that were produced under eCO2. Ryegrass and browntop plants grown from seeds produced under ambient CO2 had similar nitrification rates. Although further work is required to determine the mechanism behind the effect of eCO2 on nitrification rates in ryegrass pasture, Bowatte et al. [23] suggest that competition for nutrients, the amount and quality of root exudates, and/or differences in rhizosphere environment are the most likely explanations.
The impacts of climate change on N cycling and N2O losses in grazed pastures are likely to be complex and may result from either direct effects of eCO2, eTemp and eRain or rRain on N cycling processes (i.e. nitrification, denitrification, mineralisation, volatilisation and leaching), and/or their indirect effects on the amount of
eRain may increase denitrification and N2O emissions due to increasing soil moisture contents [20,24]. eRain may also increase the risk of soil compaction by animal treading, which could in turn reduce soil aeration and thus the risk of increased N2O emissions [11,13]. Brown et al.
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Climate change impacts on N2O emissions de Klein, Shepherd and van der Weerden 133
Figure 1
Legume abundance
Biological N fixation
eCO2 rRain
Below ground DM production
Above ground DM production
Box1
Box2
Plant N uptake
Direct and indirect N2O emissions
N content DM N intake
eCO2 eTemp eRain rRain
Litter/root exudates Dung N excreted Box3
Urine N excreted Urea hydrolysis
N and C mineralisation
Nitrification NH3 volatilisation
Denitrification NO3 leaching
eTemp eRain rRain
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Overview of the key climate change impacts on the N cycle and N2O emissions from grazed livestock systems. Boxes 1 and 2 represent the effects that lead to increased urinary and dung N excretion and box 3 the direct effects on process rates. Words in italics refer to the climate change impacts that are affecting the components of the specific boxes.
[20] measured the effects of eCO2, eRain, eTemp and added N on N2O emissions from an annual grassland ecosystem. They showed that eCO2 (alone or in combination with the other treatments) did not affect N2O emissions but that eRain, especially in combination with eTemp and added N, significantly increased N2O emissions. However, if eRain results in soil moisture contents close to saturation, then N2O emissions may reduce due to an increase in N2O reduction to N2, leading to a decrease in the N2O:N2 ratio from denitrification [25]. rRain or increased incidences of drought may also reduced N2O emissions, as shown by Hartmann et al. in a 3-year field experiment on grassland soils [26,27]. Yet, nitrification and denitrification potentials and the abundance of functional nitrifying and denitrifying genes were not affected by drought conditions. Hartmann et al. [27] therefore concluded that the drought effects on N2O emissions were largely driven by environmental limitations rather than by changes in potential process rates. In contrast, Evans and Burke [28] found that N2O emissions from soils under prolonged drought increased following rewetting because soil mineral N had accumulated under the drought conditions. Similarly, rewetting of dry soil can also induce a rapid pulse of N (and C) mineralisation in pasture soils known as the ‘Birch effect’ [29], which increases the amount of N available nitrification and denitrification to N2O. It would, therefore, seem likely that where climate change results in a pattern of increased drought combined with increased rainfall pulses and warming, N2O emissions may increase. www.sciencedirect.com
The direct effects of climate change on NH3 volatilisation are most likely through eTemp, eRain and/or rRain. eTemp and rRain will increase the risk of NH3 volatilisation, while eRain will reduce it [24,30]. The direct effect of climate change on N leaching will be driven by changes in rainfall patterns that will affect the transport of N through the soil profile. Although Larsen et al. [21] found that eRain increased N leaching, estimates of the effect of climate change on leaching are uncertain due to the competing processes of N source generation versus transport [31]. For example, Torbert et al. [32] observed lower leaching losses below soybean and sorghum grown in elevated compared to ambient CO2 and attributed this largely to increased uptake of soil N. Similarly, Dijkstra et al. [33] found that eCO2 reduced N leaching in grassland soils, especially when leaching losses were high. In contrast, increased N mineralisation induced by rewetting of dry soil (i.e. the ‘Birch effect’ [29]) could increase N leaching if drainage occurs before pasture has recovered sufficiently from drought to use the mineralised N [34]. Effects on urinary N supply
Climate change can indirectly affect N2O emissions from grazed pastures through changes in the factors that determine the amount of urinary N deposited to pasture and/or though changes in other N inputs such as BNF and N mineralisation. As mentioned earlier, the amount of urinary N excreted is determined by N intake by the grazing animals, as Current Opinion in Environmental Sustainability 2014, 9–10:131–139
134 System dynamics and sustainability
influenced by the amount of above ground dry matter (DM) grazed, and the protein content of that DM. In general, eCO2 and/or eTemp enhance biomass production, as was found in two meta-analyses of results from 150 and 127 experimental sites across a range of ecosystems and climates [35,36]. If this extra biomass is utilised through an increase in DM intake per animal or through an increase in stocking rate, then urine N deposition in grazed grasslands is likely to increase, assuming that the extra DM production is also associated with extra N intake. However, the effect of an increase in biomass production on N excretion could be off-set by reductions in the N content of plants (through N dilution) and/or of the pasture (through changes in plant composition). For example, in their meta-analysis, Dieleman et al. [35] concluded that foliar N concentrations in plants decrease significantly in combined eCO2 and warming treatments. The potential hypotheses for the mechanism(s) of this N dilution effect were reviewed by Taub and Wang [37], who tended to favour a combination of dilution of N in plant material, due to increased compounds derived from photosynthate, and decreases in N uptake. However, this dilution effect in individual plants has not always been observed at a field scale because the composition of the plant community also responds to eCO2. It is generally accepted that eCO2 leads to increased growth in plants whose physiology and growth rate can benefit most from the extra CO2, that is plants with higher N content such as legumes [38]. However, Newton et al. [39] found that the effect of eCO2 on plant composition can be modified by grazing animals. These authors showed that although the effects in the first 5 years of eCO2 in a ryegrass/clover pasture system were as expected, that is more legumes and forbs, and less C3 grasses, after 12 years of exposure to eCO2 there was little difference in plant composition as the animals selectively grazed the legumes and forbs. Climate change effects on other inputs of N, such as BNF or N mineralisation, can also affect plant growth and dietary N intake, and thus urinary N excretion, by grazing animals. When legumes are present in agricultural systems, eCO2 can increase the legume biomass [19] and increase N inputs via BNF by increasing the number and mass of root nodules [40] and influencing the rhizobial population composition. In contrast, Watanabe et al. [41] found that BNF in clover plants grown under eCO2 for 13 years was significantly lower than in plants grown under ambient CO2, due to a reduction in the number of rhizobial nifH genes in the nodules. Lam et al. [36] observed that eCO2 increased N fixation in legumes as well as N2O emissions from grain crops and legume pasture systems. However, in two long-term experiments the effect of eCO2 on increased BNF was short-lived [42,43]. In these studies the decline in N fixation was associated with shortages of other nutrients: molybdenum [42]; or low soil P availability [43]. Current Opinion in Environmental Sustainability 2014, 9–10:131–139
In terms of N mineralisation effects on N2O emissions, it has been observed that eTemp typically increases mineralisation rates and thus N availability [22,35,44], especially when external N is added [22]. This increase in N availability is particularly important in grassland systems that rely on low external N inputs as these systems are at risk of a yield decline due to progressive N limitation (PNL). PNL is a biogeochemical feedback mechanism that results in a reduction in N availability under eCO2 (e.g. [45,46,47]). Newton et al. [46] found that eCO2 in a grazed system initially increased N harvested in above ground biomass, but this effect was no longer evident after 5 years, suggesting the occurrence of PNL. This N limitation was temporarily broken following a drought breaking rain providing a flush of mineral N (i.e. Birch effect) in year 6 of the experiment, but there was evidence that PNL started to occur again in the following years. However, in a combined eCO2 plus eTemp study in temperate perennial grassland, Hovenden et al. [48] found that the effect of eCO2 on reduced soil N availability was offset by eTemp (+2 8C), suggesting that warming could potentially overcome PNL-induced growth restrictions. Increased incidences of drought (rRain) may also reduce biomass production [26,27] unless this is offset by irrigation. Hartmann et al. [27] also suggested that rRain increased N mineralisation rates and thus N availability, and that drought combined with increased rainfall pulses and warming, are likely to increase N2O emissions. The above analysis suggests that the indirect effects of eCO2, eTemp and rRain on N cycling and N2O emissions are uncertain and will depend on whether the CO2 fertilisation effect can be sustained, either through an increase in N mineralisation under eTemp and/or external inputs such as N fertiliser (to combat PNL) or irrigation (to combat rRain). In ecosystems that receive substantial external N inputs, PNL is unlikely to occur [45]. Furthermore, even if an increase in above ground biomass is limited by PNL, eCO2 stimulation of photosynthesis often results in increased allocation of C below ground [49] due to increases in root growth, root exudates and/or C supply to symbionts. This in turn is likely to increase soil biological activity, and thus affect N turnover rates and N2O emissions.
Implications for mitigation of N2O emissions from grazed grasslands — a New Zealand case study In this section we assess the likely impacts of contrasting future climate change scenarios in New Zealand (‘high carbon’ versus ‘rapidly decarbonising’ world scenarios; Table 1) on direct and indirect N2O emissions and on N process rates and inputs into intensively and extensively grazed pastures (Table 2). We have assumed that intensive systems will rely on ongoing or increasing www.sciencedirect.com
Climate change impacts on N2O emissions de Klein, Shepherd and van der Weerden 135
Table 1 Predicted ranges for key climate variables in New Zealand, based on contrasting scenarios of carbon dioxide increase from the ‘high carbon’ or ‘rapidly decarbonising’ world scenarios [56]. The range in values provided a guide for the estimated climate change effects. Changes are relative to 1980–1999 levels Variable Carbon dioxidea (ppm) Temperature (8C) Change in rainfall (%)
Season All All Summer and autumn Winter and spring
Hot days Frosts Heavy rainfall
Summer half-year Winter half-year All
Drought
Summer half-year
Strong winds
Winter, Spring Summer/ autumn
Region of NZ
Range predicted for year 2049
Level of confidence in predicted values
All All South and West South Island Rest of NZ North and east North Island North and east South Island Rest of NZ All lowland areas
480–530 ppm 0.7–0.9 Zero to +5% Up to 5%
Moderate to high High Moderate
Zero to 10% Zero to +10%
High
Up to 100% increase
High
Central North Island and South Island West of both Islands and south of South Island Eastern South Island and all of North Island
Up to 50% reduction
High
Extremes occur up to 50% as often Up to 5–10% more of year
High
All North Island South Island
Increase of few % Little change Decrease of few %
Moderate for type of change; low for magnitude Moderate for type of change; low for magnitude
Adapted from [57]. a
Current level of carbon dioxide is 395 ppm.
external N inputs, while in extensive systems external N inputs are assumed to be low, with most provided through BNF. It should be noted that this assessment does not take account of any effects of climate change on animal performance. The research on primary production impacts (as the ultimate source of urinary N) points to a number of possible scenarios for the effects of climate change. The amount of available pasture for consumption could increase due to eCO2 fertilisation effects or, conversely, could decrease if PNL is not addressed or if drought conditions prevail. The nature and magnitude of external N inputs, the initial N status of the ecosystem and changes in soil C stocks will critically determine if and when PNL occurs. In addition, farmers may increase N fertiliser use and/or irrigation to combat PNL and drought conditions, and maximise the benefits of eCO2 and eTemp. However, increased use of irrigation will increase the demand for, and consequently the pressures on, access to water. Similarly, the N content of pasture could increase or decrease based on a number of factors, such as dilution of N through increased growth, changes in species mixes (more legumes), and selective grazing pressure. However, based on the work reviewed in this paper we conclude that, on balance, climate change impacts are likely to increase N2O emissions from grazed grasslands in New Zealand. This can occur both directly, through increasing nitrification, denitrification and NH3 www.sciencedirect.com
volatilisation rates, and indirectly through increased urinary N deposition. These increased urinary N inputs are largely a result of increased net primary production and, to some extent, BNF. While research into individual aspects or components of the ecosystem, gives us building blocks to assess these likely impacts of climate change on N2O emissions, an assessment of the integrated effects at a field or farm scale is much more difficult given that the system’s response will result from complex interactions of these individual building blocks. As a result, we can perhaps place some certainty around increased N2O emissions due to direct factors, that is there is an overall likelihood of increased process rates. However, we are less certain about indirect effects at a paddock or farm scale due to interactions with, for example, animal grazing behaviour or management decisions by farmers. For example, Newton and coworkers demonstrated some of the systems’ interactions in the long-term eCO2 study in grazed pasture where selective grazing pressure modified the commonly observed responses of plant composition to eCO2 (e.g. [37,38,39,46]). If the risk of N2O emissions will increase due to climate change impacts, the question then becomes ‘can we manage or mitigate these increased N2O emissions’? Current Opinion in Environmental Sustainability 2014, 9–10:131–139
136 System dynamics and sustainability
Table 2 Summary of the potential impacts of NZ climate change scenarios and certainty of effect on N cycling in intensively and extensively grazed pastures under temperate climates (adapted from [58]) Soil nitrogen factor
Intensively grazed pastures
Extensively grazed pastures
N2O losses Direct
Indirect via NH3 volatilisation Indirect via NO3 leaching
Increase driven by eTemp, milder winters and periods of eRain. Sources may also increase: manure, urine, effluent driven by increased forage production; fertiliser driven by increased inputs to capitalise on eCO2 fertilisation effect or PNL Increase driven by eTemp and periods of drought. Sources (manure, urine, effluent, fertiliser etc.) may also increase, as described above. More rain could decrease losses, but more winter rain expected, that is when emissions are lower. Nitrate leaching will be a balance of other processes affecting the amount of mineral N remaining in the soil at times of drainage. Will increase with increasing occurrence of drought and droughtbreaking rain.
?
Process rates Denitrification Nitrification Mineralisation
?
?
N inputs Atmospheric inputs Biological N fixation
Likely to increase due to eTemp and eRain. Assumes that irrigation will increase in areas where rRain may occur. Increased due to increased temperature eTemp and increased substrate availability will generally increase microbial activity, however net effects of mineralisation-immobilisation turnover are uncertain. Increase slightly as a result of slightly more rainfall and small increases in ammonia volatilisation. Increase due to balance of evidence suggesting eCO2 favours legume growth. But could remain unchanged due to selective grazing pressure. eTemp, eRain, and rRain will also affect N fixation adding further uncertainty. Increased inputs will be used to mitigate against PNL, unless limited by regulation. Assumes BNF will be sufficient to mitigate PNL in extensive systems. Increased due to increased net primary production and legume growth.
Fertiliser N Litter/root exudation Excreta N
Justification
?
Increase as a result of more pasture production where rain/irrigation and N inputs are sufficient to capitalise on the eCO2 fertilisation effect; more likely in dairy systems.
Direction of change and certainty in science knowledge: overall, most likely to increase; overall, most likely to decrease; overall, most likely to remain unchanged; ? reasonably uncertain of effects, therefore direction cannot be predicted. Colour of arrows: & reasonably certain of effects; neither certain nor uncertain.
There may be some scope to manage N2O losses through controlling nitrification. This is one of the key controlling process of N2O emissions from urine patches, as it determines the amount of NO3 available for denitrification to N2O (and, under complete denitrification, N2). In addition, nitrification itself can also be a source of N2O. Nitrification rates are expected to increase slightly due to eTemp (Table 2), and eCO2 may increase soil nitrification rates under a ryegrass pasture [23]. The nitrification inhibitor dicyandiamide (DCD) has been shown to be an effective strategy for reducing N2O emissions from urine patches, with average reductions of 57% observed in New Zealand field studies conducted over 10 years [50]. However, DCD degrades in soil, with the degradation rate increasing with temperature [51]. DCD can also leach down the soil profile especially under increased rainfall [52]. Impacts of climate change on DCD degradation and leaching rates remain to be tested in field experiments.
incidences of warm/wet soil conditions (Table 1) that are conducive to denitrification [11]. In addition, if farmers use irrigation and external N inputs to combat drought and PNL, the risk of N2O losses through denitrification may further increase. Careful spatial and temporal management of urine patch returns to avoid high-risk hot spots (wet times and areas [53]) may therefore become increasingly important. Management options to avoid hot spots includes the use of ‘off-paddock’ systems (feedpads, stand-off pads, animal shelters), which provide farmers with the ability to adapt to climate change (eRain), particularly increased storm events delivering more intense rainfall. However, these systems need to be optimised with respect to manure management, as there is a risk of enhanced N emissions during manure storage due to eTemp and because more time off-pasture will increase the amount of manure held in storage [54].
Denitrification rates in New Zealand grassland soils are also likely to be enhanced (Table 2) due to increased
Increased use of irrigation under rRain conditions requires development of management guidelines for irrigation consultants and farmers. N2O emissions are reduced when the return period for irrigators is
Current Opinion in Environmental Sustainability 2014, 9–10:131–139
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Climate change impacts on N2O emissions de Klein, Shepherd and van der Weerden 137
de Klein CAM, Luo J, Woodward KB, Styles T, Wise B, Lindsey S, Cox N: The effect of nitrogen concentration in synthetic cattle urine on nitrous oxide emissions. Agric Ecosyst Environ 2014, 188:85-92.
lengthened, due to lower average soil moisture contents (van der Weerden, unpublished data). Likewise, ensuring irrigation does not increase soil water content up to or beyond field capacity will reduce average soil moisture content and associated N2O emissions via denitrification.
9.
In conclusion, current predictions of the direct impacts of climate change on N2O production rates from N cycling processes at a urine patch level are relatively certain. However, the indirect effects of climate change on the size of the urine N source in grazed systems are highly uncertain due to our lack of detailed understanding of the complex interactions between CO2 and N fertilisation, soil and plant C/N ratios, and microbial activity [55]. Understanding the full implications of climate change effects on N2O emissions will require ongoing research efforts and linkages between N and C cycling studies, and integration of individual component studies at a field or farm scale.
11. Bolan NS, Surinder S, Luo J, Rita B, Jagrati S: Gaseous emissions of nitrogen from grazed pastures: processes, measurements and modelling, environmental implications, and mitigation. Adv Agron 2004, 84:37.
Acknowledgement This review draws on a project funded by the Ministry for Primary Industries, New Zealand on Climate Change impacts on N cycling (AGR30693).
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
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2.
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3.
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Selbie DR, Buckthought LE, Shepherd MA: The challenge of the urine patch for managing nitrogen in grazed pasture systems. Adv Agron 2014:129. (in press). This paper reviews the current state of knowledge of urine N dynamics in grazed grasslands and provides an update on the earlier review by Haynes and Williams (1993). It reviews urine patch characteristics, implications for N cycling at the farm and paddock-scale and strategies to mitigate N losses from the urine patch.
7.
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