3_Bilandzija et al.indd 29Bilandžija, D. et al. Hungarian Geographical Bulletin 66 (2017) (1) 29–35.DOI: 10.15201/hungeobull.66.1.3 Hungarian Geographical Bulletin 66 2017 (1) 29–35. Introduction Agricultural sector has contributed by 9.4 per cent to total Croatian greenhouse gas emis- sions in 2014 (NIR 2016). Agricultural soils can act both as a source or a sink of greenhouse gases. Tillage often accelerates and increases soil CO2 emissions by speeding organic car- bon decomposition i.e. decreasing soil organic matter, changing soil microclimate (tempera- ture and water content), disrupting soil aggre- gates, increasing aeration and increasing con- tact between soil and crop residues (Gebhart, D.L. et al. 1994; Reicosky, D.C. et al. 1995, 1997; Gregorich, E.G. et al. 2005; Bilen, S. et al. 2010; Bilandžija, D. et al. 2016). Tillage may have long-term influence on soil CO2 emissions but also it often increases short-term soil CO2 emissions due to a rapid physical release of CO2 trapped in the soil air pores (Bilandžija, D. et al. 2013). Tillage management can in- crease atmospheric CO2 concentrations but it is uncertain to which extent tillage enhances the transfer of soil CO2 to the atmosphere. The objectives of our research were (1) to determine the effects of ploughing (30 cm depth), very deep ploughing (50 cm depth) and ploughing (30 cm depth) with subsoil- ing (50 cm depth) on short-term soil CO2 emissions relative to no-tillage (NT); (2) to determine the effect of four different tillage systems and time after tillage operation on soil CO2 emissions and soil microclimate; (3) to determine best function of correlation be- tween soil CO2 emissions and microclimatic conditions. Influence of tillage systems on short-term soil CO2 emissions Darija B I L A N D ŽI J A , Željka Z G O R E L E C and Ivica K I S I Ć1 Abstract Agricultural ecosystems can play a significant role in greenhouse gas emissions, specifically, carbon dioxide. Tillage management can increase atmospheric CO2 concentrations and contribute to global warming but it is uncertain to which extent tillage enhances the transfer of soil CO2 to the atmosphere. Our objectives were (1) to determine short-term, tillage-induced soil CO2 emissions; (2) to determine the effect of different tillage systems and time after tillage operation on soil CO2 emissions and soil microclimate and (3) to examine the relations between short-term soil CO2 emissions and microclimate (soil temperature, soil water content; air temperature and relative air humidity). Soil CO2 concentrations were measured on Stagnic Luvisols, in a tem- perate continental climate of the central lowland Croatia in October 2013 before, zero and three hours after tillage operations with in situ closed static chamber method. The four tillage systems were no-tillage (NT), ploughing to 25 cm (P25), very deep ploughing to 50 cm (P50) and subsoiling to 50 cm (PS50). The study showed that tillage has impact on soil CO2 emissions and soil microclimate. Tillage has accelerated the transfer of soil CO2 to the atmosphere but soil CO2 emissions declined sharply within three hours after tillage operations. Soil temperature has decreased after tillage operation and afterwards continued to rise while soil water content has been decreasing during whole study period. Correlations between soil CO2 emissions and microclimatic factors were mostly weak or modest while best type of studied correlations between soil CO2 emissions and microclimate showed to be the second order polynomial correlation. Keywords: short-term soil CO2 emissions, tillage, soil water content, soil and air temperature, relative air humidity 1 Department of General Agronomy, Faculty of Agriculture, University of Zagreb. Svetošimunska cesta 25, 10 000 Zagreb, Croatia. E-mails: dbilandzija@agr.hr; zzgorelec@agr.hr; ikisic@agr.hr. Bilandžija, D. et al. Hungarian Geographical Bulletin 66 (2017) (1) 29–35.30 Materials and methods Experimental site and tillage systems Field experiment with four different tillage sys- tems usually implemented in Croatia was set up in Blagorodovac near Daruvar (elevation: 133 m a.s.l.; N 45°33´54´´, E 17°02´56´´) in cen- tral lowland of Croatia. Field experiment was established in 1994 with the aim of research on determination of soil degradation by wa- ter erosion and later, in 2011, expanded to the research on soil CO2 concentration measure- ments. Soil type at the experimental site is de- termined as Stagnic Luvisols (IUSS 2014). Size of each tillage plot is 22.1 m x 1.87 m. Tillage systems differed in tools that were used, depth and direction of tillage and planting. Tillage was conducted in October 2013 and tillage systems were: a) no-tillage (NT) – planting directly into the mulch along the slope; b) ploughing to 30 cm (P30) – tillage and planting across the slope; c) very deep ploughing to 50 cm (P50) – till- age and planting across the slope; d) ploughing to 30 cm + subsoiling to 50 cm (PS50) – tillage and planting across the slope. Measurement of CO2 concentrations and calculation of soil CO2 emissions Soil CO2 concentrations were measured before, zero and three hours after tillage implementa- tion in three repetitions at each plot. For the measurement of soil carbon dioxide concentra- tions, in situ closed static chamber method was used. The chambers were made of lightproof metal material and they consist of two parts: frames (25 cm in diameter) and caps (25 cm in diameter and 9 cm high) fitted with a gas sam- pling port. The circular frames were inserted about 10 cm into the soil at the beginning of measurements. Before the chambers closure, near the soil surface, the initial CO2 concen- trations inside the frames were measured. Af- terwards, the chambers were closed with caps and the incubation period was 30 minutes after which accumulated CO2 in the chamber was measured (Photos 1–4). Measurements of CO2 concentrations (ppm) were conducted with portable infrared carbon dioxide detector (Ga- sAlertMicro5 IR, 2011). Measurements were conducted on bare soil and when necessary (at no-tillage system), vegetation was removed before the beginning of measurement. The soil CO2 emissions (efflux) were after- wards calculated according to Widen, B. and Lindroth , A. (2003), and Tóth, T. et al. (2005) as: FCO2 = M X P X V (c2 – c1) , R x T x A (t2 – t1) where FCO2 = soil CO2 efflux (kg/ha per day); M = molar mass of the CO2 (kg/mol); P = air pressure (Pa); V = chamber volume (m3); c2 –c1 = CO2 con- centration increase rate in the chamber during incubation period (µmol/mol); R = gas constant (J/mol/K); T = air temperature (K); A = chamber surface (m2); t2 – t1 = incubation period (day). Photos 1–4. Measurement of short-term soil CO2 emissions (from left to right): tillage implementation (1); insert- ed circular frames into the soil (2); incubation period (3); measurement of accumulated CO2 in the chamber (4). 31Bilandžija, D. et al. Hungarian Geographical Bulletin 66 (2017) (1) 29–35. Determination of microclimate Soil temperature, soil water content, air tem- perature and relative air humidity were meas- ured in order to determine their influence on tillage-induced CO2 emissions. Soil tem- perature (°C) and soil water content (%) were determined with IMKO HD2 - probe Trime, Pico64 (2011) at 10 cm depth in the vicinity of each chamber along with measurement of soil CO2 concentrations. The air temperature (°C) and relative air humidity (%) were deter- mined with Testo 610 (2011) and air pressure was determined with Testo 511 (2011) at the height about 1 m above the soil surface. Data analysis Soil CO2 emissions were analyzed using sta- tistical Software SAS (SAS 2002–2004). Vari- ability between tillage systems were evalu- ated with analysis of variance (ANOVA) and tested, if it were necessary, with adequate post-hoc (Bonferroni) t-tests. In all statistical tests significance level was p≤0.05. A linear, exponential, logarithmic and second order polynomial regression proce- dure was used to determine the dependence of each climatological factor on soil surface CO2 emissions. The value of the correlation coefficient was ranked by Roemerk-Orphal scale (0.0–0.10: no correlation; 0.10–0.25: very weak; 0.25–0.40: weak; 0.40–0.50: mod- est; 0.50–0.75: strong; 0.75–0.90: very strong; 0.90–1.00: full correlation) (Vasilj, Đ. 2000). Results Microclimate and short-term tillage-induced soil CO2 emissions During the studied period on October 28, 2013 (between 8.00 and 17.00 hours) it was mostly sunny and warm, air temperature ranged from 23.2 to 28.2 °C and relative air humid- ity varied from 50.9 to 60.4 per cent (Table 1). Soil temperature before the tillage operations varied from 26.9 to 33.0 °C, immediately after the tillage operations it declined sharply up to 10.9 °C and afterwards it mostly continued to Table 1. Soil CO2 emissions and climatologic factors (means ± SD) before, zero and three hours after tillage operation (n = 3) Parameter Tillage system Before tillage operation Zero hours Three hours after tillage operation Air temperature, °C NT P30 P50 PS50 23.2 ± 0.7 23.2 ± 0.9 23.4 ± 1.2 23.4 ± 0.8 25.7 ± 0.5 25.7 ± 0.3 25.8 ± 0.3 25.8 ± 0.2 28.1 ± 0.7 28.1 ± 0.4 28.2 ± 1.1 28.2± 0.9 Relative air humidity, % NT P30 P50 PS50 60.4 ± 2.2 60.4 ± 1.9 56.7 ± 2.2 56.7 ± 2.1 55.6 ± 0.8 55.6 ± 0.7 53.7 ± 0.9 53.7 ± 0.9 50.7 ± 1.1 50.7 ± 0.9 50.9 ± 1.3 50.9 ± 1.1 Soil temperature, °C NT P30 P50 PS50 33.0 ± 1.7 31.5 ± 1.3 26.9 ± 1.3 31.0 ± 3.2 33.4 ± 2.2 10.9 ± 0.7 12.8 ± 2.1 11.6 ± 1.5 33.0 ± 2.0 11.5 ± 2.7 16.2 ± 2.5 10.7 ± 1.5 Soil water content, % NT P30 P50 PS50 23.4 ± 0.1 23.9 ± 0.3 25.7 ± 0.1 25.7 ± 0.1 22.7 ± 0.1 23.0 ± 0.0 23.8 ± 0.1 23.7 ± 0.1 16.2 ± 0.0 16.2 ± 0.1 18.7 ± 0.0 18.6 ± 0.1 Soil CO2 emissions, kg CO2/ha per day -1 NT P30 P50 PS50 114.1 ± 13.3 85.9 ± 4.7 76.5 ± 4.5 85.9± 6.3 100.5 ± 20.1 126.0 ± 10.2 116.6 ± 18.8 123.3± 15.3 122.8 ± 16.3 45.6 ± 6.3 49.6 ± 6.4 34.9± 7.6 Bilandžija, D. et al. Hungarian Geographical Bulletin 66 (2017) (1) 29–35.32 rise while on no-till system, soil temperature was mostly steady. Soil water content ranged from 23.4 to 25.7 per cent before the tillage operations, and after the tillage operations it was continuously declining during the study period up to 16.2 per cent (Table 1). The soil CO2 emissions measured on tilled systems before tillage ranged from 76.5 to 85.9 kg CO2 /ha per day -. Immediately af- ter tillage soil CO2 emissions ranged from 116.6 to 126.0 kg CO2 /ha per day and were on average 47.4 per cent greater than the average emission before tillage operations, while three hours after tillage it was on av- erage 48.6 per cent lower compared to av- erage emission before tillage operation. The exception was no till system where soil CO2 emissions were high and ranged from 100.5 to 122.8 kg CO2 /ha per day during the whole study period (Table 1). Influence of tillage systems and time on soil CO2 emissions and soil microclimate Different tillage systems didn’t have any significant impact on average soil CO2 emis- sions and soil water content while average soil temperature determined at no-till was significantly higher compared to other tilled systems (Table 2). Average soil CO2 emission of the experi- mental plot measured before tillage operation was not significantly different from soil CO2 emissions after tillage but emissions measured immediately after and three hours after till- age operation were significantly different. Soil temperature measured before tillage was sig- nificantly higher compared to those measured after tillage. Soil water content was significant- ly declining within hours after tillage operation Correlation between short-term soil CO2 emissions and microclimate Between soil CO2 emissions and soil tem- perature, very weak positive logarithmic (r = +0.23), modest positive second order polynomial (r = +0.41), weak positive linear (r = +0.25) and exponential (r = +0.35) cor- relation was determined. The values of cor- relation coefficients indicate the presence of positive modest linear (r = +0.40), exponential (r = +0.48) and logarithmic (r = +0.40) corre- lation between soil CO2 emissions and soil water content. An exception is the correlation in the second order polynomial type which is negatively modest and amounts r = -0.41. Between soil CO2 emissions and air tempera- ture, negative weak linear (r = -0.36), negative modest exponential (r = -0.47), negative weak logarithmic (r = -0.35) and negative strong second order polynomial (r = -0.70) correla- tion was determined. Positive weak linear (r = +0.37) and logarithmic (r = +0.38), positive modest exponential (r = +0.46) and negative strong second order polynomial (r = -0.52) correlation was determined between soil CO2 emissions and relative air humidity. Table 2. Influence of different tillage systems and time on soil CO2 emissions and soil microclimate Tillage Soil CO2 emission,kg CO2 /ha per day Soil temperature, °C Soil water content, % vol. NT P30 P50 PS50 112.5 a 85.9 a 80.9 a 81.4 a 33.2 a 18.0 b 18.6 b 17.8 b 20.8 a 21.0 a 22.7 a 22.7 a Time Soil CO2 emission,kg CO2 /ha per day Soil temperature, °C Soil water content, %vol. Before tillage Zero hours after tillage Three hours after tillage 90.6 ab 116.6 a 63.2 b 30.6 a 17.2 b 17.9 b 24.7 a 23.3 b 17.4 c Averages followed by same letter are not significantly different. 33Bilandžija, D. et al. Hungarian Geographical Bulletin 66 (2017) (1) 29–35. Discussion Air temperature was rising and relative air humidity was declining during the meas- urement period. Soil temperature was high and steady at no till during the whole study period while on tilled systems soil tempera- ture declined sharply after tillage operation due to the disruption of soil aggregates and increasing aeration by which the soil climate was changed; after which soil temperature continued to rise. Soil water content was con- tinuously declining, partly due to the tillage operation but also due to the increase of air temperature and an increase in soil water evaporation. Decreased soil water content in tilled treatments just after tillage and the greatest soil water content in NT was ob- served by Alvaro-Fuentes, J. et al. (2007). Lampurlanes, J. et al. (2001) also observed greater water contents in NT and suggested that better infiltration rates in NT promoted greater soil water content as compared to tilled treatments. At no till system, soil CO2 emission was not significantly higher compared to tilled systems and was high and steady during the whole study period. Soil CO2 emissions increased rapidly immediately after tillage operation due to physical release of CO2 from soil pores and solutions at all tilled treatments. A sig- nificant increase of CO2 emission immediately after tillage operations in tillage treatments, except NT, was also observed by Alvaro- Fuentes, J. et al. (2007). Already three hours after tillage operation, soil CO2 emissions de- clined sharply and were lower compared to emissions measured before tillage operation. Reicosky, D.C. (1997), observed a decrease within 2 hours after a pass with plough. Many authors (Reicosky, D.C. and Lindstrom, M.J. 1993; Reicosky, D.C. et al. 1997; Ellert, B.H. and Janzen, H.H. 1999; Al- Kaisi, M.M. and Yin, X. 2005) also obtained in their research that the effect of tillage on soil CO2 emission was short-lived. Reicosky, D.C. and Lindstrom, M.J. (1993), and Prior, S.A. et al. (2000) suggested that initial CO2 emission after tillage was also related to the depth and degree of soil disturbance. In our experiment, similar results were not deter- mined. Within tilled treatments, P30 was the tillage operation with greatest CO2 flux after tillage compared to other tilled treatments although the differences were not significant. In our study, no significant relationships between CO2 emissions and microclimate conditions were found. Microclimatic con- ditions had mostly weak or modest impact on soil CO2 emission. Similar results were reported by Kessavalou, A. et al. (1998); Al- Kaisi, M.M. and Yin, X. (2005); Omonode, R.A. et al. (2007); Jabro, J.D. et al. (2008); Li, C. et al. (2010); Bilandžija, D. et al. (2014) and Bilandžija, D. (2015). Of all tested functions, best type of correlation between soil CO2 emissions and microclimatic factors, showed to be the second order polynomial correla- tion, except for soil water content. According to its determination coefficient, 17 per cent of soil CO2 emissions depended on soil temperature, 17 per cent of soil CO2 emissions depended on soil water content, 27 per cent of soil CO2 emissions depended on relative air humidity and 49 per cent of soil CO2 emissions depended on air tempera- ture. A possible explanation for this lack of relationship with CO2 flux may be related to the fact that soil microclimate conditions were only measured to 10 cm depth and soil tillage was implemented to 30 and 50 cm soil depth. Therefore, a large proportion of the CO2 emission could come from deeper than 10 cm soil layer. Conclusions At no till system soil CO2 emissions were steady and high during whole study pe- riod. Tillage did not have significant, on 3 hours average, short term impact on soil CO2 emissions. However, tillage accelerated the transfer of soil CO2 to the atmosphere and caused an immediate sharp increase in soil CO2 emissions which were on average 40–50 per cent higher compared to those before till- age. This was a relatively short lived process, Bilandžija, D. et al. Hungarian Geographical Bulletin 66 (2017) (1) 29–35.34 lasting less than 3 hours from tillage opera- tion after which the soil CO2 emissions were on average 40–50 per cent lower compared to those measured before tillage. At tilled systems, soil temperature rapidly declined after tillage operation and after- wards continued to rise while at no-till sys- tem it was steady during whole study period. Soil water content was declining with time of measurement during whole study period. The tillage-induced soil CO2 emissions ap- peared to be independent of changes in mi- croclimate as correlations between soil CO2 emissions and microclimatic factors were mostly weak or modest. 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