Field N2O, CO2 and CH4 fluxes in relation to tillage, compaction and soil quality in Scotland

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Abstract

Tillage practices and weather affect the release of greenhouse gases but there have been few integrated studies of the quantities released or the mechanisms involved. No-tillage may increase emissions of nitrous oxide (N2O) and the fixation of carbon by decreasing carbon dioxide (CO2) emissions. Tillage may also decrease the oxidation rate of atmospheric methane (CH4) in aerobic soil. These effects are partly due to compaction and to the lack of both soil disturbance and residue incorporation. Our objective was to investigate how tillage practices, soil conditions and weather interact to influence greenhouse gas emissions. Here we present early measurements of N2O and CO2 emission and CH4 oxidation in two field experiments in Scotland under a cool moist climate, one involving soil compaction plus residue incorporation and the other involving no-tillage and two depths of mouldboard ploughing of a former grass sward. The experiments were located 10–15 km south of Edinburgh on a cambisol and a gleysol. In order to monitor emissions regularly, at short intervals and over long periods, a novel automatic gas sampling system which allows subsequent automated determination of both N2O and CO2 fluxes was used. Both N2O and CO2 fluxes were episodic and strongly dependent on rainfall. Peak N2O emissions were mainly associated with heavy rainfalls after fertilisation, particularly with no-tilled and compact soils. In the tillage experiment, N2O fluxes and treatment differences were greater under spring barley (Hordeum vulgare L.) (up to 600 g N ha−1 per day) than under winter barley. CO2 emissions in the few weeks after sowing were not strongly influenced by tillage and diurnal variations were related to soil temperature. However, periods of low or zero CO2 fluxes and very high N2O fluxes under no-tillage were associated with reduced gas diffusivity and air-filled porosity, both caused by heavy rainfall. Early results show that CH4 oxidation rates may best be preserved by no-tillage. The quality of the loam/clay-loams and the climate in these experiments makes ploughing, preferably to 300 mm depth, and the control of compaction necessary to minimise soil N2O and CO2 losses. The gas exchange response of different soil types to tillage, particularly methane oxidation rate which is affected by long-term soil structural damage, is a potentially useful aspect of soil quality when taken in conjunction with other qualities.

Introduction

Gas exchange between soils and the atmosphere is an important contributory factor to global change due to increasing release of greenhouse gases (Bouwman, 1990). Three of the principal gases of interest are nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4). Both N2O and CO2 are emitted from the soil whereas CH4 is normally oxidised by aerobic soils making them sinks for atmospheric CH4 (Goulding et al., 1995). The N2O emissions are normally associated with N (as fertiliser or manure) application under wet conditions (Clayton et al., 1994) and CO2 emissions with respiration which is often stimulated by tillage (Roberts and Chan, 1990). Reduced or no-tillage systems may decrease CO2 emission, thereby increasing the storage of soil C (Kern and Johnson, 1993), but may increase N2O emission (Aulakh et al., 1984). Soil compaction by tractor wheels can also increase N2O emission above that associated with zero traffic levels (Douglas and Crawford, 1993). Soil conditions, particularly near the soil surface, have an important influence on N2O emission (Arah et al., 1991). The production, consumption and transport of N2O and CO2 are strongly influenced by the changes in soil structural quality and in water content associated with tillage and compaction. One such soil quality influencing gas transport in soil is gas diffusivity. The influence of tillage and compaction on soil conditions (including gas diffusivity) and on consequent gaseous emissions may be the important aspects of soil quality.

The objective of this paper was to investigate how tillage practices, soil conditions and weather interact to influence greenhouse gas emissions. We show some early measurements of N2O and CO2 emission in two field experiments in south-east Scotland, one involving soil compaction and residue incorporation and the other involving no-tillage and two depths of mouldboard ploughing. Emissions were related to soil qualities and weather conditions.

Section snippets

Study sites

The compaction experiment was located on an imperfectly drained loam of Macmerry series (Cambisol) in 1995. The tillage experiment was located on an imperfectly drained clay loam of Winton series (Eutric Gleysol).

Tillage and compaction experiments

The design of the compaction experiment was a randomised complete block with fourfold replication. The plots were 24 m × 2.4 m. Further details were given by Ball and Ritchie (1999). Treatments were applied to soil which had been mouldboard ploughed the previous day, incorporating the

Nitrous oxide fluxes

Nitrous oxide is emitted by soils as a result of denitrification in anaerobic soil and nitrification in aerobic soil with the anaerobic production considered more important. Thus emissions generally increase with increasing soil moisture. In the compaction experiment, although the soil at treatment and fertiliser application under the spring barley was quite wet (22–30% w/w), no treatment effects on N2O emission were found, probably because of insufficient soil compaction to the target depths.

Conclusions

  • 1.

    Temporal variability of gas fluxes was marked in tillage and compaction treatments and was readily assessed using an automated gas sampling system at frequent measurement intervals. Episodic N2O fluxes were mainly associated with the period after fertilisation and were strongly dependent on rainfall, particularly in no-tilled and in compact soils. In the tillage experiment, N2O fluxes and treatment differences were greater under spring barley than under winter barley.

  • 2.

    Carbon dioxide emissions in

Acknowledgements

We are grateful to I.J. Crichton for technical assistance. The project was supported by the Scottish Office Agriculture, Environment and Fisheries Department.

References (19)

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    These tradeoffs of tillage factors influencing N2O emissions resulted in inconsistent reports in the literature. The literature indicates N2O emissions increased (MacKenzie et al., 1997; Ball et al., 1999; Baggs et al., 2003; Krauss et al., 2017), decreased (Venterea et al., 2005; Lehman et al., 2017), or remained unchanged (Robertson et al., 2000; Elmi et al., 2003; Snyder et al., 2009) by switching from conventional tillage to NT practices. Studies that are completely randomized and are more than 50-yr old are scant, and data on N2O emissions from such long-term tillage practices are less reported.

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