Influence of land-use management on CO
2
emissions from a silt loam
soil in New Zealand
T. Aslam
a,c, M.A. Choudhary
a, S. Saggar
b,∗aInstitute of Technology and Engineering, Massey University, Palmerston North, New Zealand bLandcare Research, Private Bag 11052, Palmerston North, New Zealand
cNational Agricultural Research Centre, Pakistan Agricultural Research Council, Islamabad, Pakistan
Received 14 October 1998; received in revised form 8 June 1999; accepted 29 July 1999
Abstract
Effects of land-use management on agricultural sustainability and greenhouse gas emissions are major issues for researchers, regional councils and farmers in New Zealand. This study was undertaken to investigate the environmental impact of land-use management on field-CO2 emissions in Ohakea silt loam soil (Typic andoaqualf) that had been converted from permanent pasture to forage cropping for 2 years. The treatments were plow tillage (PT) and no-tillage (NT), with summer fodder maize (Zea mays L.) double-cropped in rotation with winter oat (Avena sativa L.); permanent pasture (PP) was used as control. Field-CO2 emissions measured every 3–4 weeks during 12 months, were significantly highest in the PP. Overall, results indicate that reduction in tillage had only a minor effect on field-CO2emission throughout the crop growth period. During one year CO2emissions ranged from 55–132 kg C ha−1per day in the PP treatment, 43–91 kg C ha−1per day in the NT treatment and 36–81 kg C ha−1per day in the PT treatment. Daily field-CO2emissions measured within a few days after cultivation were significantly highest in the PP treatment and were in the order of PP > NT > PT. Conversion of PP to NT cropping had no effect on surface organic C levels, but conversion to cropping with tillage markedly reduced C levels. ©2000 Elsevier Science B.V. All rights reserved.
Keywords: Plow tillage; No-tillage; Permanent pasture; Field CO2emission; Soil moisture; Maize; Oats
1. Introduction
Soils are the largest reservoirs of C in agriculture ecosystems. Release of CO2 from soil to the atmo-sphere is an important part of C cycling in nature and provides a useful index for the C budget of an agricul-tural production system. Recent concerns with global changes in atmospheric CO2 concentrations have
∗Corresponding author. Tel.:+64-63-56-71-54; fax:+ 64-63-55-92-30.
E-mail address: saggars@landcare.cri.nz (S. Saggar).
emphasized the possibility of increasing the storage of CO2–C in the soil by changes in land-use and management practices that increase the synthesis and retention of soil organic matter (SOM) (Carter and Hall, 1995). In New Zealand, as a result of the Re-source Management Act (RMA) of 1991 and growing environmental awareness, the effects of land manage-ment on soil degradation and on water and air quality are important issues for farmers, regional councils and researchers. Recent strategies to reduce CO2 emis-sions by the New Zealand government have focused on the likelihood of introducing a C tax. Research on
the reduction of CO2emissions from the agricultural sector has been a recent phenomenon in New Zealand, and the planting of trees to absorb and store CO2–C emissions has been a major focus. However, no em-phasis has been placed on measuring CO2 emissions from arable crops sown with plow tillage.
Field respiration has been used to obtain a better understanding of soil C mineralization and stabiliza-tion (Anderson, 1982). Soil respirastabiliza-tion is influenced by climate, soil physical, chemical, biological prop-erties and management practices (Saggar et al., 1997, 1998), and has been widely used to assess the func-tioning of the soil system (Hendrix et al., 1988).
Soil tillage plays an important role in C cycling in nature and can bring about environmental change. Car-bon losses have been shown to occur within minutes after plow tillage fractures the soil forcefully with the release of CO2stored in soil pores and water (Reicosky et al., 1997). In contrast, conservation tillage, espe-cially no-tillage, retains surface residues, enhances soil moisture retention and reduces soil losses by erosion (Choudhary et al., 1997), improves soil structure (Ross and Hughes, 1985), and increases organic matter and nutrient levels (Francis and Knight, 1993). Choudhary and Baker (1994) reported that no-tillage is an effi-cient method of crop establishment. However, the en-vironmental benefits of no-tillage are not fully under-stood. Therefore, this study was initiated to determine tillage and no-tillage impacts on CO2 emissions by measuring field CO2emissions and changes in basal soil respiration of soils under PP and within the first 2–3 years of conversion of PP to field crops.
2. Methods and materials
2.1. Experimental site
Full description of the site, soils, experimental de-sign and treatments used in this study is presented elsewhere (Aslam et al., 1999). Briefly, experimen-tal site (latitude 40◦23′S, 175◦38′E) was established in 1995 at Massey University Turitea Campus, on an Ohakea silt loam soil (Typic andoaqualf) classified as Gleyic luvisol (FAO), where permanent pasture land was converted to a double-crop rotation using plow and no-tillage. The site has a 30-years average rain-fall of 963 mm per annum. The three treatments were
ryegrass (Lolium perenne L.) and clover (Trifolium sp.) based permanent pasture (PP), plow tillage (PT) and no-tillage (NT). The PT involved mouldboard plough-ing at 20 cm depth followed by rollplough-ing, and two passes of a power harrow for seedbed preparation. In the NT, there was no cultivation and weeds were controlled by using 4 l ha−1 of glyphosate herbicide spray. In both the treatments, a no-till drill with Baker Boot (Choud-hary and Baker, 1994) was used for sowing.
In November 1996, a summer crop of fodder maize (Zea mays L.) and in April, 1997 winter oats (Avena
sativa L.) were grown in the PT and NT treatments.
Nitrophoska fertilizer, containing 12% N, 10% P, 10% K and 1% S, was applied at the rate of 120 kg ha−1. Both crops were grazed at maturity by sheep. The pasture was grazed by sheep every time grass was approximately 2000 kg DM ha−1.
Replicated soil samples collected from the three treatments, for 0–100 mm depth, were analyzed for pH (1 : 2.5 soil : water), total C and total N.
2.2. Field CO2emissions over time
Field-CO2 emissions were measured once every 3–4 weeks by installing static chambers (110 mm dia. 135 mm high) in each plot, with one blank as con-trol, after cutting above-ground vegetation to 10 mm height. Vials containing 20 ml 1 M NaOH were placed within the chambers and sealed with air-tight screw cap lids. After 24 h, the vials of NaOH were re-moved. The total CO2 absorbed was measured by titration of aliquots (2 ml) of alkali against 0.1 M HCl to determine the residual alkali, after first precipitat-ing carbonates by the addition of 10 ml 10% BaCl2; phenolphthalein was used as an indicator (Anderson, 1982). Soil cores were taken at 0–5 and 5–10 cm depths and moisture contents measured throughout the observations by drying the soil at 105◦C.
2.3. Field CO2emissions immediately after tillage
primary tillage. In the PT treatment, cumulative CO2 effluxes were measured for the first 4 h, 4–15 h and 15–20 h to obtain the amount respired over 24 h fol-lowing tillage. These data were compared with those obtained at the same time from the NT and PP treat-ments. Soil moisture content at 0–50 mm depth was obtained in close proximity to the CO2-emission mea-surement areas.
2.4. Statistics
A General Linear Models Procedure (GLM) with the statistical analysis system (SAS) programme was used (SAS Institute Inc., 1985) to analyze all experi-mental data. ANOVA, using the t-test for least signifi-cant differences (LSD) at the 5% confidence level, was used to determine differences between the treatments.
3. Results and discussion
Cropping caused marked decline in soil surface organic matter. After 2 year continuous cropping the total C contents at 100 mm depth in PT and NT treatments were 34.0 and 27.6 Mg ha−1, respec-tively, reflecting a decline in total C as compared with PP (35.3 Mg ha−1). The higher amount in NT (19%) as compared to PT apparently resulted from previous pasture organic residues and substantial input of crop residues left intact on soil surface. To-tal N contents were also significantly higher in PP (3.93 Mg ha−1) and NT (3.78 Mg ha−1) as compared to PT (3.27 Mg ha−1). The average soil pH (5.4) was common of clover-based New Zealand pasture soils and did not differ between the three treatments sug-gesting that 2 years of tillage and no-tillage practices had no impact on soil pH.
3.1. Tillage effects on long-term field-CO2emissions
Monthly measurements during 1 year showed sig-nificantly higher field-CO2 emissions from the PP than from the NT or PT treatments (Table 1). The CO2emissions ranged from 55 to 132 kg C ha−1 per day in the PP treatment, 43–91 kg C ha−1per day in the NT treatment and 36–81 kg C ha−1per day in the PT treatment. Overall results have shown that tillage
practices had only minor effects (PP > NT=PT) on field CO2-emission throughout the crop-growth pe-riod. These results are similar to those of Schimel (1986), who found that soil CO2 emissions were higher under grassland than under cereals and were lowest in fallow land.
During the growth of fodder maize from Decem-ber to March, tillage practices had little influence on CO2 emissions. The CO2 emissions were highest in January reflecting peak biological activities during the middle maize-growth period. In February, irrespec-tive of tillage intensity, CO2 emissions decreased by an average of 28% with the decrease in soil mois-ture content, February was a low rainfall period dur-ing that summer (Table 2). However, high CO2 emis-sions were sustained in the PP treatment, even with the decrease in soil moisture content. In March, when the crop was fully mature and soil moisture remained low, CO2emission levels decreased further. In the NT treatment, soil moisture in March was similar to the value in January, perhaps because the decomposable substrate had declined with crop maturity.
Following land preparation and the establishment of the oat crop, CO2 efflux from the PT treatment was slightly higher than from the NT treatment, the differences were, however, not significant. These data suggest that freshly ploughed land may not enhance CO2emissions as compared with untilled soil because once the cultivated soil was re-compacted with a heavy roller, the CO2emissions returned to levels similar to those in the untilled soil. These data gave some insight into the potential effects of recent cultivation on CO2 efflux, as suggested by Reicosky et al. (1997).
Table 1
Monthly field CO2 emission measurements in the PP, NT and PT treatments during November 1996 to October 1997a
Treatment Field CO2 emissions (kg CO2–C ha−1 per day)
November December January February March April May June July August September October 10.11.96 12.12.96 9.1.97 11.2.97 3.3.97 11.4.97 27.5.97 24.6.97 5.7.97 29.8.97 25.9.97 17.10.97
PP 71 a 102 a 117 a 122 a 108 a 132 a 90 a 76 a 66 a 55 a 68 a 93 a
NT 56 a 43 b 91 b 72 b 50 b 57 b 70 a 82 a 62 a 47 ab 44 a 76 b
PT 51 b 36 b 81 b 62 b 49 b 75 b 82 a 79 a 58 a 38 b 36 c 73 b
LSD0.05 11.09 11.09 12.96 27.68 13.91 22.90 26.16 20.88 11.09 12.61 6.12 9.85 aValues followed by the same letter in each column show no significant differences (P<0.05); PP: Permanent pasture; NT: No-tillage;
PT: Plow tillage.
3.2. Tillage induced short-term CO2emission
The data show that lack of tillage for seedbed prepa-ration generally had little impact on CO2 emissions. However, CO2 emissions a week after land prepara-tion and sowing of winter oats (April 1997) in the PT treatment were 24% higher than in the NT treatment (Table 1). This led to a belief that tillage may have en-hanced soil respiration and encouraged CO2emissions as reported in a number of studies (Dugas et al., 1997; Reicosky and Lindstrom, 1993; Reicosky et al., 1997). The short-term CO2 emissions were, once again, higher from the pasture plots than from the two tillage practices (Table 3) probably additional contributions from the standing pasture biomass and residue. Soil tillage reduced CO2 emissions compared with those in the untilled soil, within the first 3 days of culti-vation. These data were similar to those of Hendrix et al. (1988), who observed greater soil CO2 efflux from NT than from PT soils. However, they contra-dict those of Reicosky et al. (1997), who suggested that soil tillage encouraged oxidation of C, resulting
Table 2
Soil moisture content at 0–5 cm depth during the CO2 emission measurements in the PP, NT and PT treatmentsa
Treatment Moisture content (kg kg−1)
December January February March April May June July August September October 12.12.96 9.1.97 11.2.97 3.3.97 11.4.97 27.5.97 24.6.97 5.7.97 29.8.97 25.9.97 17.10.97
PP 0.23 b 0.25 ab 0.16 a 0.21 b 0.37 a 0.31 b 0.28 a 0.34 a 0.48 a 0.38 a 0.46 a NT 0.28 a 0.26 a 0.16 a 0.25 a 0.36 ab 0.35 a 0.27 a 0.35 a 0.44 a 0.39 a 0.41 a PT 0.24 b 0.23 b 0.18 a 0.18 b 0.32 b 0.31 ab 0.26 a 0.30 a 0.35 b 0.31 b 0.32 b LSD0.05 0.028 0.031 0.036 0.041 0.041 0.044 0.044 0.017 0.063 0.047 0.053
aValues followed by the same letter in each column show no significant differences (P<0.05); PP: Permanent pasture; NT: No-tillage;
PT: Plow tillage.
in higher CO2emissions than in no-till treatment. A probable explanation for the present results could be that bottom layer of the ploughed soil had low number of micro-organisms, which did not encourage much soil respiration. Another possible reason could be that ploughing fractured soil pores, and so released CO2 within minutes as suggested by Reicosky et al., 1997). In this study, the first CO2measurements were taken 4 h after ploughing, and it is possible that most of the CO2 had effluxed before the measurements began. Such findings were earlier reported by Steensel (1995), who suggested that, as the soil was turned over, sub-strates for the micro-organisms disappeared from the top 5 cm and resulted in a significant drop in soil respiration.
Table 3
Short-term field CO2emissionsa
Treatment CO2 emissions (kg CO2–C ha−1per day)
Days 1–4 after ploughing the Days 5–11after power harrowing
Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 11
PP 118 a 124 a 109 a 113 a n.d.b n.d. 98 a 85 a
NT 92 b 97 b 88 b 90 b n.d. n.d. 80 b 38 b
PT 35 c 50 c 40 c 73 c 80 81 62 c 26 c
LSD0.05 11.33 21.74 13.27 9.11 – – 6.80 9.86
aValues followed by the same letter in each column show no significant differences (P<0.05); PP: Permanent pasture; NT: No-tillage;
PT: Plow tillage.
bn.d.: Not determined.
Table 4
Soil moisture content at 0–5 cm depth during short-term field CO2 emissionsa
Treatment Moisture content (kg kg−1)
Days 1–4 after ploughing and Days 5–11 after power harrowing
Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 Day 11
PP 0.37 0.38 0.35 0.33 0.31 0.28 0.28 0.23
NT 0.36 0.36 0.33 0.32 0.30 0.27 0.26 0.21
PT 0.30 0.25 0.23 0.24 0.20 0.20 0.19 0.18
aValues in each columns are means of two replicates, and therefore no statistical analysis was performed on these data; PP: Permanent
pasture; NT: No-tillage; PT: Plow tillage.
power harrow had increased soil aeration and imme-diately stimulated soil biological activities. However, degassing from soil pores is also probable.
Soil moisture content (SMC) declined over each day of measurement in all three treatments (Table 4). No rainfall occurred during these days, and generally, as SMC decreased from Day 1 to Day 11, field CO2 emissions also declined significantly. In the PP treat-ment, with a decrease of 25% in SMC, there was a proportional drop in CO2 emission levels. In the NT treatment, with a 25% drop in SMC, CO2 emissions levels dropped by 58%, and in PT treatment, with a SMC reduction of 36%, a 25% drop in CO2emissions was observed.
Regression analysis showed a strong linear rela-tionship between short-term daily CO2emissions and SMC in both the PP (r=0.96) and NT (r=0.90), in which no soil disturbance occurred and uniform con-ditions prevailed. On the other hand, only a moderate relationship (r=0.59) existed between CO2efflux and SMC in the PT treatment in which there was a lack of surface organic matter and loose soil conditions.
4. Conclusion
Acknowledgements
Our thanks are extended to Landcare Research for analytical facilities, to Carolyn Hedley, Gareth Salt and Dexter McGhie for technical assistance, and to the New Zealand Overseas Development Assistance (NZODA) programme for financial assistance to T. Aslam.
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