Nitrous oxide emissions and methane oxidation by soil ... - Springer Link

Biol Fertil Soils (1999) 30 : 52–60

Q Springer-Verlag 1999

ORIGINAL PAPER

T.J. van der Weerden 7 R.R. Sherlock 7 P.H. Williams K.C. Cameron

Nitrous oxide emissions and methane oxidation by soil following cultivation of two different leguminous pastures

Received: 1 November 1998

Abstract Nitrous oxide (N2O) emissions and methane (CH4) consumption were quantified following cultivation of two contrasting 4-year-old pastures. A clover sward was ploughed (to 150–200 mm depth) while a mixed herb ley sward was either ploughed (to 150–200 mm depth) or rotovated (to 50 mm depth). Cumulative N2O emissions were significantly greater following ploughing of the clover sward, with 4.01 kg N2O-N ha –1 being emitted in a 48-day period. Emissions following ploughing and rotovating of the ley sward were much less and were not statistically different from each other, with 0.26 and 0.17 kg N2O-N ha –1 being measured, respectively, over a 55-day period. The large difference in cumulative N2O between the clover and ley sites is presumably due to the initially higher soil NO3– content, greater water filled pore space and lower soil pH at the clover site. Results from a denitrification enzyme assay conducted on soils from both sites showed a strong negative relationship (rp–0.82) between soil pH and the N2O:(N2OcN2) ratio. It is suggested that further research is required to determine if control of soil pH may provide a relatively cheap mitigation option for N2O emissions from these soils. There were no significant differences in CH4 oxidation rates due to sward type or form of cultivation. Key words Nitrous oxide 7 Methane 7 Clover 7 Herb ley 7 Cultivation

T.J. van der Weerden (Y) 7 R.R. Sherlock 7 K.C. Cameron Soil, Plant and Ecological Sciences Division, P.O. Box 84, Lincoln University, Canterbury, New Zealand e-mail: vanderwt6lincoln.ac.nz, Tel.: c64-3-3252811, Fax: c64-3-3253607 P.H. Williams New Zealand Institute for Crop & Food Research Ltd., Lincoln, Canterbury, New Zealand

Introduction The major form of arable production in New Zealand is a mixed cropping system, where arable crops are generally grown for 2–5 years followed by a 2–5 year grazed grass/legume pasture phase. Over 250 000 ha are under this type of agricultural production, most of which occurs on the Canterbury Plains in South Island (Haynes et al. 1991). The pasture phase assists in maintaining good soil structure; however, the period of time given over to either cropping or pasture within the rotation will vary depending upon the relative profitability of the two forms of farming (Haynes et al. 1991). Cultivation of the pasture phase is traditionally performed by ploughing. This typically occurs in the autumn, prior to the sowing of the first crop in the spring. Cultivation generally leads to mineralization of organic N from the residues (Francis and Knight 1993). This in turn nitrifies to nitrate (NO3–) which may be removed from the soil system via a number of pathways including leaching, plant uptake and, when conditions are appropriate, denitrification. Both nitrification and denitrification can result in nitrous oxide (N2O) emissions into the atmosphere. N2O is an important greenhouse gas with an atmospheric lifetime of 120 years and a global warming potential of 320 relative to CO2 for a 100-year time horizon (IPCC 1995). Annual global N2O emissions from the biosphere to the atmosphere were estimated as 15 Tg in 1992 (IPCC 1995), with 70% produced by soil processes (Mosier 1998). N2O also contributes to ozone depletion by acting as a source of NO in the stratosphere (Crutzen 1981). There have been very few studies conducted comparing N2O emissions from different forms of cultivation of grass/legume swards, and how they may contribute to New Zealand’s anthropogenic N2O inventory. Shallow tillage practices such as rotovation may lead to lower mineralization, thus potentially emitting less N2O compared to traditional ploughing practices. Methane (CH4) is another major greenhouse gas which is oxidized in mineral soils by methanotrophic

53

bacteria. Increasing concentrations in the atmosphere may be partly due to decreasing rates of oxidation by soils. Cultivation may reduce the rate of CH4 consumption in two ways. Firstly, ammonium (NHc 4 ) released via mineralization of organic N may inhibit CH4 monooxygenase enzyme activity (Hütsch 1998). Secondly, increased aeration following soil disturbance may lead to a lower soil moisture content. Methanotrophic bacteria are sensitive to water stress, thus affecting their ability to consume CH4 (Cochran et al. 1997). By gaining a greater understanding of the processes controlling this rate of oxidation it may be possible to manipulate the process in order to increase the rate of CH4 consumption or at least offset some of the effects of cultivation. The objectives of this study were to quantify N2O emissions and CH4 consumption following the ploughing of two contrasting 4-year-old pastures (clover sward and mixed herb ley sward) in Canterbury, and also to compare the effect of cultivation (ploughing and rotovation) of the ley sward on these gas fluxes. In addition, the driving variables controlling the N2O and CH4 fluxes were defined. Denitrification enzyme activity (DEA) assays were also conducted to determine the potential denitrification activity.

Materials and methods Site description Two 4-year-old pastures, one a 100% clover (Trifolium repens) sward (clover site) and the other a mixed herb ley consisting of 10 grass and 10 legume species (ley site), were used for this study. Both sites, established on Wakanui silt loam soil (Udic Ustochrept) (Table 1), were monitored from 1 April 1997 until 13 July 1997. Total N and C were determined using a LECO total CNS analyser and organic matter content (OMC) was approximated using the total C content and clay content (Grewal et al. 1991). Dry bulk density was determined at 5 cm increments to 15 cm depth prior to and following cultivation using gamma ray attenuation (CPN, California). The clover sward was sited on a commercial farm 4 km from Lincoln University, Canterbury, while the mixed herb ley was sited on the Lincoln University Biological Husbandry Unit. Prior to cultivation, herbage from the ley site was regularly cut and removed for mulching. At the clover site the pasture was grazed until October 1996, after which the grass component of the sward was removed by herbicide and a clover seed crop was harvested in January 1997. Two treatments were imposed on the clover sward; these were non-cultivation (control) and ploughing (to Table 1 Specific soil properties of the Wakanui silt loam soil (Udic Ustochrept) at the clover and mixed herb ley sites a; np9, except bulk density where np3

150–200 mm depth). Three treatments were imposed on the ley site; non-cultivation (control), ploughing (to 150–200 mm depth) and rotovation (to 50 mm depth). Cultivation was imposed at the clover and ley sites in autumn on 27 May and 20 May, respectively. Both sites were Dutch harrowed twice immediately following cultivation. The rotovated treatment was further chisel ploughed to 50 mm on 16 June to aid the decomposition of the ley residues lying at or near the soil surface. Each treatment was replicated three times. Gas Sampling Gas samples were taken from the field sites as headspace samples beneath cylindrical steel chambers (700 mm diameter, 100 mm height). The chambers were insulated against increased heating of the headspace using 25-mm-thick polystyrene. On each sampling date the chambers were sealed onto 100 mm high metal rims that had been inserted into the soil to a depth of 80 mm at the start of the experiment. Headspace samples were taken at 0, 20 and 40 min within 1 h of mid-day using 60 ml polypropylene syringes connected to the chamber headspace via a three-way stopcock. The chambers showed no sign of gas leakage in preliminary trials using a high concentration of a commercial N2O standard gas. Temperature recorded at 2.5 cm depth in the soil beneath the headspace of selected chambers during sampling showed that after 40 min there was a less than 1 7C change compared to the temperature at the same depth outside the chamber. Sampling occurred, on average, once every 3 days, with greater frequency following tillage and rainfall events. Gas samples were analysed the same day for N2O using a Varian Aerograph ‘series 2800’ gas chromatograph equipped with a Pye-Unicam 63Ni electron capture detector (ECD) operating at 350 7C. CH4 was analysed on the same samples by directing the effluent flow from the analytical column to a flame ionisation detector (FID) installed in a SRI 8610 gas chromatograph. The precolumn, analytical column and switching valves (10 port and 4 port) used, followed the design developed by Mosier and Mack (1980). One difference was the use of oxygen-free nitrogen as the carrier gas instead of argon/CH4. This permitted the analysis of both N2O and CH4 on the same gas sample. The precolumn (1 m!2.0 mm ID) and analytical column (3 m!2.0 mm ID) were filled with Poropak Q (80/100 mesh) operating isothermically at 23 7C. The output signals of the ECD and the FID, in millivolts, were integrated using Peaksimple II software (SRI Instruments). Calculation of gas fluxes Nitrous oxide gas fluxes at the time of sampling (mg N2O-N m –2 h –1) were calculated using the approach of Hutchinson and Mosier (1981), and are depicted in Fig. 1. These single daily values were then modified to estimate the mean daily flux (g N2O-N ha –1 day –1). A correction factor was calculated based on the average daily temperature at 2.5 cm and that measured at the time of sampling with the assumption that Q10p2, that is, for a 10 7C rise in temperature, microbial activity doubles. This assumption is based on previous work by Blackmer et al. (1982), Crill et al. (1994),

Site

Depth (cm)

Total N (%)

Total C (%)

OMC (%)

pH (1 : 2.5 soil : water)

Bulk density (g cm P3)

Clover

0– 5 5–10 10–15 0– 5 5–10 10–15

0.28 0.26 0.25 0.24 0.20 0.18

3.2 2.9 2.8 2.9 2.4 2.2

5.2 4.8 4.6 4.8 4.0 3.5

5.3 5.4 5.4 6.1 6.0 6.0

1.4 1.3 1.4 1.4 1.4 1.5

Herb ley

a

Clay content at both sites approximately 20%

54

Fig. 1 N2O fluxes, water filled pore spaces (0–5 cm), soil pH (0–5 cm), temperature (5 day average at 2.5 cm depth) and rainfall recorded at the clover and herb ley sites. For N2O fluxes the 95% confidence interval, pooled over dates, relates to the data on the log scale. The error bar length is not related to the axis numbers but is correct as a measure of the reliability of data. f indicates time of cultivation. For soil pH: ppP~0.05, 3pP~0.01. Inserted graph within temperature figure illustrates freeze-thaw cycles at soil surface for 5 consecutive days in early July

Müller (1995) and Roslev et al. (1997), who have shown that N2O and CH4 fluxes have a Q10 of approximately 2 for temperatures between 0 and 30 7C. Equation (1) illustrates the procedure used for this modification: FcpFm!e ((T c–T m)!0.0693)!0.24

(1) –1

–1

where Fc is the corrected mean daily flux (g N2O-N ha day ), Fm is the measured flux (mg N2O-N m –2 h –1), Tc is the mean daily temperature at 2.5 cm, Tm is the temperature at the time of sampling, 0.0693 is the correction factor when Q10p2, and 0.24 is the

conversion factor for changing the units from mg N2O-N m –2 h –1 to g N2O-N ha –1 day –1. Cumulative fluxes were calculated using these calculated mean daily fluxes. A similar procedure was used for CH4 fluxes. Soil measurements Temperature measurements were made 5 cm above the soil surface, and at 0, 2.5, 5.0, 10.0 and 15.0 cm below the soil surface on each treatment at both sites. Half-hourly averages were logged on either a CR10 or CR21X datalogger (Campbell Scientific, Inc.). Each treatment replicate had two soil samples taken within 3 m of the gas chambers. Soil cores (5 cm diameter) were taken to 15 cm depth in 5 cm increments. Care was taken to ensure that cores were never taken from the same location more than once. Duplicate cores for each depth were bulked and assessed for the following characteristics. Gravimetric moisture content was determined every time gas samples were taken. Using the bulk density data, this was converted to the percentage of water-filled pore space (WFPS). Mineral N content was determined, on average, once every 2 weeks on 10 g of fresh soil extracted with 50 ml 2 M

55 KCl for 1 h and filtered using Whatman no. 42 filter paper. Extract samples were frozen until analysis using a Tecator Flow Injection Analyser. Soil pH was measured, on average, once every 3 weeks, using a ratio of 10 g soil:25 ml deionized water.

random components such as replicate to replicate differences. Since the gas flux data was log-transformed, in place of a Least significant difference (LSD), the least significance ratio (LSR) is presented. If the ratio of the larger over the smaller of two values is greater than the LSR, then the values are significantly different (at the 5% level).

DEA determinations Short-term DEA were conducted 2 months prior to cultivation in March (autumn) and 1 month following cultivation in late June (winter) to determine the potential denitrification activity before and after cultivation. The method described by Luo et al. (1996) was used, which is based on the method developed by Smith and Tiedje (1979). Briefly, soil is incubated anaerobically in the presence of 10% acetylene to inhibit N2O-reductase activity (Yoshinari et al. 1977). The N2O evolved will therefore be equivalent to the total N2OcN2 production. By ensuring that NO3– and C substrate is non-limiting, the potential denitrification activity is obtained. The denitrifying activity will only then be limited by the denitrifier population in the soil. By excluding one or both substrates, information is obtained on the importance of these substrates at in situ concentrations on denitrification activity. Four replicates of 20 g sieved moist soil were weighed into 180 ml Erlenmeyer flasks, followed by 20 ml deionized water containing NO3– and/or C substrate at 50 mg NO3––N and 300 mg glucose-C g –1 soil, respectively. After sealing the flask with a rubber septum (Suba Seal), the soil slurry was mixed and the headspace was evacuated and replaced with oxygen-free N2 three times. Ten percent of the headspace was then replaced by acetone-free acetylene. Flasks were then incubated at 20 7C in the dark with 6 ml headspace samples being removed after 1 and 5 h and stored in evacuated 5 ml vials. N2O analysis was conducted the same day following the procedure described earlier for field gas samples. Calculation of the denitrification activity included adjustment for dissolved N2O by the use of the Bunsen coefficient, 0.632 at 20 7C (Tiedje 1982). In addition, mineral N, soil moisture content and pH were also determined on DEA soil samples. Statistical analysis Because of the unbalanced experimental design (three treatments at the ley site and two treatments at the clover site) standard ANOVA analysis was not always possible. Therefore residual maximum likelihood analysis (Genstat 1998) was used, as this provides a less biased analysis of both treatment effects and any

Results Prior to and following cultivation, cumulative N2O emissions from the clover sward significantly exceeded those measured from the mixed herb ley (P~0.01) (Table 2). Cultivation and rainfall events stimulated N2O emissions from the ploughed clover sward, whilst a similar, albeit smaller, response was observed following the cultivation of the mixed herb ley (Fig. 1). Emissions increased rapidly from 27 mg N2O-N m –2 h –1 at the clover pasture to 1019 mg N2O-N m –2 h –1 (approximately 245 g N2O-N ha –1 day –1) 8 days following autumn ploughing. This contrasts with the mixed herb ley, where, following ploughing and rotovating, emissions increased from 3 and 2 mg N2O-N m –2 h –1 to only 49 and 16 mg N2O-N m –2 h –1, respectively. The flux measured from the rotovated plots increased further to 44 mg N2O-N m –2 h –1 following chiselling of the topsoil on 16 June. Following cultivation of both sites, cumulative N2O emissions due to both cultivation and site were significantly different (P~0.01) (Table 2). However, there was no significant difference in cumulative N2O emissions between the ploughed and rotovated treatments at the ley site. Both NO3––N and NHc 4 –N were initially significantly lower in the ley soil compared to the clover soil (P~0.01) (Table 3). NO3– concentration in the ley soil following cultivation remained significantly less than that measured in the cultivated clover soil for 2 weeks (P~0.01). Similar NHc 4 –N concentrations in both soils

Table 2 Cumulative flux (g ha P1) and mean daily flux (g ha P1 day P1) of N2O-N and CH4-C prior to and following cultivation (LSR Least significant ratio, 5%) Clover Non-cultivation

LSR a (5%)

Mixed herb ley Plough

Non-cultivation

Plough

Rotovate

Nitrous oxide Pre-cultivation Cumulative flux Mean daily flux

200 3.6

P P

38 0.8

P P

P P

1.50 1.50

Post-cultivation Cumulative flux Mean daily flux

344 7.2

4009 83.5

56 1.0

258 4.7

172 3.1

1.69 1.69

Pre-cultivation Cumulative flux Mean daily flux

P99 P 1.8

P P

P121 P 2.5

P P

P P

1.56 1.56

Post-cultivation Cumulative flux Mean daily flux

P13 P 0.3

P12 P 0.3

P 20 P 0.4

P41 P 0.8

P35 P 0.6

6.66 6.66

Methane

56 Table 3 Ammonium and nitrate content in the top 15 cm prior to and following cultivation (LSD least significant difference of the mean, 5%; nd not determined) Precultivation a

Post-cultivation 1 day

NHc 4 -N (kg N ha P1)

Clover Herb Ley

NO -N (kg N ha P1)

Clover Herb ley LSD (5%)

2 weeks

4 weeks

2 months

Non-cultivation Plough Non-cultivation Plough Rotovate b

5.0 P 2.8 P P 0.5

4.3 8.5 0.4 3.5 4.7 3.0

4.4 3.5 3.2 3.7 9.2 4.5

4.4 3.5 3.9 5.0 6.4 4.1

nd nd 2.3 2.4 6.7 1.8

2.7 4.4 5.1 6.9 4.1 3.2

Non-cultivation Plough Non-cultivation Plough Rotovate

21.9 P 0.7 P P 5.9

3.0 28.3 0.4 3.1 ~0.1 4.5

6.7 34.4 0.1 ~0.1 0.4 7.1

6.7 35.0 0.3 6.1 1.3 7.7

nd nd ~0.1 5.9 5.4 5.4

6.4 6.6 3.7 11.2 8.9 5.0

LSD (5%) P 3

1 week

a Pre-cultivation values calculated as average concentration for the 2 months prior to cultivation

b Rotovated plots were chisel-ploughed 1 day prior to the 4 week data

were measured following cultivation, except for day 1, where concentrations were significantly greater in the clover soil (P~0.01). Heavy rainfall (50 mm) during the week prior to cultivation of the clover site probably leached a large proportion of the 21.9 kg NO3––N ha –1 present out of the top 15 cm of the non-cultivated plots as NO3– levels decreased (Table 3). In contrast, the NO3– content of the ploughed plots remained significantly higher than the non-cultivated control plots for 2 weeks (P~0.01) (Table 3). This will be discussed further. NHc 4 –N content was also significantly greater one day following ploughing of the clover sward compared to the non-cultivated plots (P~0.01) (Table 3); however, there was little difference between the two treatments thereafter. At the ley site the NHc 4 –N content 1 day following ploughing and rotovating was significantly greater than the NHc 4 –N content of the non-cultivated control plots. The rotovated plots exhibited the greatest subsequent concentrations although these were significantly greater than the other two treatments on only two occasions (Table 3). Low NO3– concentrations were measured in the ley soils for the first week following cultivation, after which concentrations increased in the ploughed and rotovated plots and by 4 weeks they were significantly greater than the non-cultivated controls. The clover sward generally showed an increase in WFPS prior to cultivation, whereas the WFPS of the ley sward decreased (Fig. 1). Although herbage dry matter production was not measured, the observed difference in WFPS was probably due to greater water uptake by the tap-rooted species within the ley. Consequently, at the time cultivation was imposed, the WFPS at the two sites were different. Nevertheless, following cultivation, the WFPS declined in the cultivated plots compared to the non-cultivated plots at both sites. The WFPS was generally less overall at the ley site, although both sites received very similar amounts of rainfall (Fig. 1).

Soil pH at the clover site was substantially lower than the ley site (Table 1), a pattern that did not change throughout the experiment (Fig. 1). Nevertheless, cultivation of the clover site significantly reduced the soil pH of the top 5 cm. At the ley site the rotovated treatment showed a significantly higher pH compared to its uncultivated control. This pattern was reversed 2 months following cultivation, with the non-cultivated plots exhibiting the highest pH. The DEA results show that before cultivation was imposed, addition of available C to the clover sward increased denitrification enzyme activity, indicating that this soil was limited in available C (P~0.05) (Table 4). In contrast, addition of NO3– to the ley soil increased the enzyme activity which suggests NO3– content was significantly limiting in this soil (P~0.05) (Table 4). When both C and NO3– were added, the clover and ley soils exhibited similar potential denitrification activities of 7.88 and 9.12 mg N2O-N g –1 dry soil day –1, respectively. The post cultivation assays, conducted in the winter 1 month following cultivation, showed that the non-cultivated control plots gave similar responses to added C and NO3– as observed in the pre-cultivation assays, conducted 3 months earlier in the autumn. However, the potential denitrification activity (measured with added C and NO3–) had nearly doubled. Also, compared to the autumn activity, there was an increase in the proportion of N2 in the denitrification products. Ploughing both swards led to a lower activity compared to the non-cultivated swards, while the rotovated treatment exhibited the highest potential denitrification activity, at 23 mg N2O-N g –1 dry soil day –1. The clover sward, both cultivated and non-cultivated, produced the greater proportion of N2O in the denitrification products compared to the ley sward. Soil pH was found to have a negative linear relationship with the N2O:(N2OcN2) ratio (rp–0.80; P~0.01) (Table 4). In addition, a strong positive linear relationship was found

57 Table 4 Denitrification enzyme activity (mg N2O-N g P1 dry soil day 1) prior to and following cultivation [LSD (5%)p4.75], soil moisture and pH measured at time of sampling, plus the ratio

of N2O : (N2OcN2), as calculated from the soil and soilc10% acetylene (C2H2) treatments

Soil

Soilc10% Soilc10% Soilc10% Soilc10% C2H2 C2H2cC C2H2cCc C2H2c NOP NOP 3 3

Initial moisture content (g g P1 dry soil)

Soil pH

Ratio of N2O : (N2O cN2)

4.64 0.16

5.54 3.61

10.53 3.34

4.94 8.46

7.88 9.12

0.174 0.183

5.4 6.3

0.84 0.04

Post-cultivation b Clover Non-cultivation 0–5 cm 0.92 Plough 0–5 cm 1.40 Plough 15–20 cm 2.00 Herb ley Non-cultivation 0–5 cm 0.01 Plough 0–5 cm 0.42 Plough 15–20 cm 1.23 Rotovate 0–5 cm 0.26

5.45 2.42 6.42 1.20 3.97 5.43 5.30

10.21 4.75 10.39 1.26 4.52 7.91 6.34

5.86 2.20 3.69 12.47 4.65 6.05 9.04

12.62 4.09 8.05 16.50 6.29 8.87 23.10

0.322 0.314 0.331 0.345 0.289 0.294 0.381

5.8 5.6 5.5 6.1 6.1 6.2 6.3

0.17 0.58 0.31 0.01 0.11 0.23 0.05

Pre-cultivation a Clover 0–5 cm Herb ley 0–5 cm

a b

Two months prior to cultivation One month following cultivation

between the soil moisture content at the time of sampling and the potential denitrification activity for the winter DEA assays (rp0.83; P~0.05) (Table 4). There was no significant effect due to cultivation or site on the daily CH4 oxidation rates prior to and following cultivation (Table 2). At both sites, oxidation rates were lower, although not significant, following cultivation compared to rates measured prior to cultivation.

Discussion Influence of type of temporary pasture on N2O emissions following ploughing Ploughing of the clover and herb ley swards resulted in the emission over a 1.5 month period of 4.01 and 0.26 kg N2O-N ha –1, respectively. This difference is due to the more favourable denitrification conditions existing at the clover site following heavy rainfall prior to cultivation. The flush of mineralization that occurred following the ploughing of the clover sward resulted in much higher soil NO3––N concentrations than those in the cultivated herb ley. In addition to higher NO3– concentrations, greater WFPS and lower soil pH also contributed to the large amount of N2O emitted from the ploughed clover sward compared to the ploughed ley sward. A similar flush of mineralization was observed by Francis and Knight (1993) following ploughing of the same soil type in the autumn/winter. A significant decrease in the soil pH compared to the non-cultivated plots (P~0.05) was presumably due to the nitrification which both accompanied and followed the mineralization. The WFPS remained at or below 80% for a few days following ploughing and this would have helped promote nitrification. The dramatic decrease in N2O emissions from the ploughed clover sward in late June

is presumably due to a limited supply of C substrate. Kaiser et al. (1998) suggest that microbial N2O production during winter is limited by available C. Results from the DEA assay, which was conducted in late June (1 month following cultivation), suggest that C was more limiting in the clover soil compared to the ley soil. The increased N2O emission from the ploughed clover sward in early July is attributed to five consecutive days of freeze-thawing of the soil surface from 5 July to 9 July (see insert in clover site temperature figure of Fig. 1). This freezing zone was restricted to soil above 2.5 cm depth. Release of N2O following freeze thaw cycles has been observed by other workers (Christensen and Tiedje 1990; Kaiser et al. 1998). These increased emissions are thought to be due to increased microbial activity induced by the release of available C from microorganisms killed by the freezing process (Christensen and Tiedje 1990). Freeze thaw cycles also occurred throughout June on 11 occasions which may have contributed to the duration of high N2O emissions from the clover site. N2O emissions from the ley site were not enhanced by these freeze thaw cycles. This was because either C was not limiting in this soil or denitrification was enhanced but the additional N2O was completely reduced to N2. The former of these two possible scenarios is more likely, as the DEA study showed the ley site soil to be non-C limiting to denitrifiers. The increasing water filled pore spaces following ploughing will have decreased oxygen concentrations in the ploughed clover soil, as WFPS is directly related to diffusivity and thus the rate of O2 exchange with the atmosphere (Davidson and Schimel 1995). Indeed, the continual rainfall during the cultivation period will have influenced the duration and intensity of the N2O production (Beauchamp et al. 1996). Following cultivation, the WFPS was always greater at the clover site, where it was 80% or above. In contrast, the ploughed ley soil remained at approximately 60–70% WFPS. De-

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nitrification activity has been shown to increase as WFPS increases from 60% to 100%. As anoxic conditions increase, the percentage of N2O in the denitrification products decreases until N2 is the major gas evolved (Davidson 1993). This may partly explain the lower N2O loss from the non-cultivated control plots where the WFPS was greater compared to the ploughed clover sward. This large difference in N2O emissions from the non-cultivated and cultivated clover plots is also partly due to the difference in their NO3– contents (as discussed previously). A decline in soil pH will decrease both denitrification and nitrification activity (Davidson 1993), although the ratio of N2O:(N2OcN2) emitted via denitrification will increase (Granli and Bøckman 1994). The clover site was always more acid (pH ~5.9) than the ley site (pH 1 6.0) throughout the experiment, which may have influenced the amount of N2O released relative to N2 via denitrification. Weier and Gilliam (1986) observed a strong increase in N2O emissions via denitrification when soil pH decreased below 5.8, whilst above 5.8 N2O evolution was minimal. High NO3– concentrations are also known to inhibit the conversion of N2O to N2, especially at low pH (Stevens and Laughlin 1998). This effect typically diminishes with time as the NO3– concentration decreases. So nitrification in the ploughed clover sward may have increased N2O fluxes by two means: by supplying the denitrifier population with sufficient NO3– substrate and by influencing the ratio of N2O:(N2OcN2). Although results from laboratory-based DEA are not easily related to field measurements, this type of assay can assist in identifying important driving variables controlling the observed gaseous emissions. The unamended soil slurry treatment from the DEA assay showed greater N2O evolution from the ploughed clover soil compared to the ploughed ley soil, which is consistent with the observed field emissions. Of greater interest is the relationship between soil pH and the N2O:(N2OcN2) ratio. Although the clover site exhibited greater N2O fluxes, it would appear, based on the laboratory DEA analyses, that a greater proportion of gaseous N emitted via denitrification from the ley site was in the form of N2. Strategies to mitigate N2O emissions are required if its contribution to global warming from agriculture is to be reduced. One such strategy may be the maintenance of soil pH at approximately 6.5 to minimize the total N2O emission from both denitrification and nitrification. At this pH a smaller proportion of the gaseous N emitted will be as N2O compared to more acid conditions (Granli and Bøckman 1994) while emissions via nitrification activity will be limited. Optimum nitrification activity typically occurs at pH 7 (Bremner and Blackmer 1981). Stevens and Laughlin (1998) report on a study of 20 different grassland soils showing soil pH having the greatest effect on the mole fraction of N2O. It was found that an increase of one pH unit lowered the mole fraction by 0.2. These workers proposed that manipulating the interaction of

soil pH and NO3– supply offered the greatest potential for minimizing both N2O and N2 emissions. Results from the present study suggest that the influence of pH on N2O emissions deserves more focused research as it may help to reduce the environmental impact of agriculture. Although soil temperature and N2O emissions appear to follow a similar pattern with time, the influence of temperature on the microbial processes is complicated by the rainfall events. Nevertheless, it is considered for the current investigation that the temperature and emission relationship is relatively weak. By comparing the non-cultivated plots at both sites, the effect of sward type on the N2O fluxes can be observed. Emissions were significantly greater from the clover sward (P~0.05), over the entire experimental period, being nearly four times those measured from the ley sward, and more than 50% greater than those measured from the ploughed ley site. The difference between the two swards is due to the higher mineral N status and WFPS as well as the lower soil pH at the clover site, as already discussed in reference to the ploughed plots. Influence of type of cultivation on the N2O emissions At the ley site, ploughing inverted pasture to approximately 150–200 mm below the soil surface whilst rotovating mixed the top 50 mm of soil and pasture together. Although cumulative N2O emissions were not significantly different following ploughing and rotovating, daily N2O fluxes were generally greater from the ploughed plots. This was most likely due to soil pH influencing the magnitude of the N2O emissions from the rotovated and ploughed leys. In the 6 weeks following cultivation, soil pH in the top 5 cm was greater in the rotovated soil. Groffman et al. (1987) observed an increase in soil pH following ploughing of a clover sward and suggested this was possibly due to the release of cations that had been contained in the legume plant material. The high pH and low N2O emissions from the rotovated soil are consistent with a smaller N2O:(N2OcN2) ratio observed in the DEA assay, suggesting that N2 emissions via denitrification from this treatment were probably greater than from the ploughed ley. Influence of cultivation and type of pasture on CH4 oxidation by soil Cultivation of the soil by either ploughing or rotovating did not result in decreased CH4 oxidation rates (Table 2). This contrasts with observations made from long-term arable systems by other workers (Mosier et al. 1991; Willison et al. 1995). It is likely in this current work that any short-term effect of cultivation on oxidation rates was too small to be observed.

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Although not statistically different, it is interesting to note that the daily CH4 oxidation rates were greater at the ley site. The soil NHc 4 concentrations were similar at both sites over the winter months, so NHc 4 inhibition of CH4 monooxygenase cannot be the mechanism behind this difference, as has been found by Hütsch et al. (1994). It is suggested that the difference in rates at the two sites is due to the lower soil pH at the clover site. Bender and Conrad (1995) found that the optimum soil pH for CH4 oxidation was 7.0–7.5, irrespective of the in situ soil pH which ranged from 4.5 to 8.1 for the four soils used. Hütsch et al. (1994) also observed decreasing CH4 oxidation of a grassland soil with decreasing pH ranging from 6.3 to 5.6. The decreased oxidation rates at both sites following cultivation is presumably due to the WFPS increasing, thus reducing the soil aeration status. CH4 oxidation has been shown to decrease with increasing water content due to lower rates of gas diffusion as soil pores fill with water (Cochran et al. 1997). In conclusion, the work reported here clearly illustrates that the magnitude of N2O emissions from shortterm grass/legume swards can be greatly enhanced by cultivation. The most important driving variables controlling these emissions were considered to be mineral N, soil pH and WFPS. These emissions may contribute significantly to New Zealand’s anthropogenic N2O inventory; therefore further research is required to assess potential mitigation strategies. One relatively cheap mitigation option worthy of further examination is the maintenance of soil pH at approximately 6.5 which might help to maintain a low mole fraction of N2O emitted via denitrification without maximizing nitrification activity. Acknowledgements Thanks are expressed to AGMARDT for a scholarship awarded to Tony van der Weerden. This research was partially funded by the Foundation for Research, Science and Technology (CO2613). The land used for this work was kindly provided by Mr. G. Tucker and the Biological Husbandry Unit, Lincoln University. Statistical assistance was provided by Ruth Butler of Crop & Food Research.

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