Short-term effects of conversion to no-tillage on respiration and chemical - physical properties of the soil: a case study in a wheat cropping system in semi-dry environmentRossana Monica Ferraraa,*, Gianluigi Mazzaa,b, Cristina Muschitielloa, Mirko Castellinia , Anna Maria Stel-laccia , Alejandra Navarroa ,c, Alessandra Lagomarsinob , Carolina Vittia, Roberta Rossid , Gianfranco Ranaa

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Abstract: No-tillage (NT) is considered an agricultural practice to preserve soil organic carbon (C), however large uncertainties still affect land-use management measures for reducing carbon dioxide (CO2) emissions from agricultural soil. Short-term changes during the transition between conventional tillage (CT) and NT systems have to be further investigated. In this study, measurements of both actual CO2 fluxes in field and microbial mineralization activity were connected to chemical and physical properties of the soil in a winter wheat cropping system subject to semi-arid climate where NT, performed from only 4 years, has been compared to CT. Results showed no significant differences between CT and NT, confirming a similar turnover time of the organic C between the two treatments, probably due to the young changing in the management (4 years) and the period of measurements (far from ploughing).Keywords: C sequestration, conservative agriculture, CO2 emissions, heterotrophic basal respiration, C mineralization.

Riassunto: La non lavorazione dei terreni (NT) è considerata una pratica agricola utile al fine di preservare il carbonio organico (C) nel suolo, tuttavia c’è ancora una grande incertezza sulle misure agronomiche più idonee a ridurre le perdite di anidride carbonica (CO2) dai suoli agricoli. In questo studio, misure sia di emissioni di CO2 in campo che di attività di mineralizzazione microbica sono state connesse alle proprietà chimiche e fisiche del suolo in un sistema colturale a grano duro soggetto a clima semi-arido in cui la NT, praticata da soli 4 anni, è stata confrontata con la lavorazione convenzionale (CT). I risultati non hanno evidenziato differenze significative tra CT e NT, confermando tempi simili nel turnover del C organico nei due trattamenti, probabilmente dovuti al fatto che il sistema NT era giovane (4 anni) e le misure sono avvenute lontano dalla lavorazione del terreno.Parole chiave: Sequestro del C, agricoltura conservativa, emissioni di CO2, respirazione basale eterotrofa, mineralizzazione del C.

* Corresponding author’s e-mail: rossana.ferrara@crea.gov.ita Council for Agricultural Research and Economics (CREA), Research Unit for Agriculture in Dry Environments, Bari, Italy.b Council for Agricultural Research and Economics (CREA), Research Centre for Agrobiology and Pedology, Firenze, Italy.c Department of Biology, Research Centre of Excellence of Plant and Vegetation Ecology, University of Antwerp, Belgium.d Council for Agricultural Research and Economics (CREA), Research Unit for the Extensive Animal Husbandry, Muro Lucano (PZ).Received 20 March 2016, accepted 16 August 2016

DOI:10.19199/2017.1.2038-5625.047

IntRoductIonLosses of carbon (C) from agricultural soils appear to increase when conventional tillage (CT) is performed because topsoil aeration and mixing of crop residues are higher than in no-tillage (NT), which is considered an agricultural practice to preserve soil C (Balesdent et al., 2000; Six et al., 2002). The adoption of NT has been enhanced in Europe by CAP reform, beginning with the shift from the production-based price support system to the area-based payment for the adoption of environmental-friendly agricultural

practices (Brady et al., 2009). Rural development programs provide specific incentives to farmers shifting from conventional tillage to no-tillage. A period of 5 years is considered the time-frame for reaching a new steady-state characterized by an enhanced soil quality and stabilized production levels. While many studies reviewed the long term environmental effect of no-tillage, there is relatively little information on system behaviour during the transition period as well as on the duration of this possibly detrimental phase (McCarty et al., 1998; Liang et al., 2007). Experimental data collected during this phase can fill a gap, providing basis for designing complementary agronomic strategies and/or appropriate funding schemes to support farmers during the transition toward the new production system.Many factors have to be taken into account for evaluating the effective advantage gained by NT practice when compared to the CT one concerning the C sequestration of agricultural soils: above all, the actual CO2 flux from soil can

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In this study we investigated the effect of the tillage practices, CT and NT, on CO2 emissions by a winter wheat cropping system where NT practice was performed from four years. We used measurements taken both in field and laboratory. In order to analyse the CO2 flux rate when the soil system was in a phase of relative stability, i.e. far from the soil disturbance due to ploughing, the period of investigation started from the phenological stage of jointing, about 5 months after the soil tillage. The total soil CO2 flux in situ and the potential mineralization rate of soil organic matter, for analysing the relative “short-term” effect of changing agronomic practices, were measured and related to physical and chemical soil properties.

MAteRIAlS And MethodS

Study site and the experimentThe experiment was carried out from 14 April to 18 June 2015 in Matera (41°N, 17°54’ E, 122 m a.s.l.) in southern Italy in a private farm cultivated with durum wheat (Triticum durum Desf. cv. Aureo).The field converted to NT four years before the beginning of the experiment can be considered in the so-called transition period. The cropping system during this phase consisted in a short rotation of durum wheat followed by a forage legume-cereal intercropping. In this study the preceding crop was a vetch-barley winter herbage. The two treatments, CT (ploughing depth = 0.25 m) and NT, were replicated two times and arranged in four plots (CT1, CT2, NT1, NT2), each measuring 9 x 10 m. For the chemical and physical characterisation, 4 sub-replicates were collected in each plot (16 samples) in one time, about 5 months after the ploughing (April 2015). Eleven stainless steel collars were installed in the experimental field (3 sub-replicates in each plot, except 2 for one of the CT plots) for measuring actual CO2 fluxes by means of the static/manual chamber method (Livingston e Hutchinson, 1995). Actual CO2 flux measurements were carried out in seven sampling dates between the phenological stages of jointing and physiological maturity, two times for each day (two cycles). In correspondence to the 11 collars, 22 soil samples were also collected (2 samples for each collar) in April the 14th, for the quantification of the potential mineralization rate.Wheat was sown on 28 November 2014 at a rate of 230 kg seed ha-1. The crop was fertilized on 18 November 2014 by applying 200 kg ha-1 of a commercial coated nitrogen (N) fertilizer (31% N). Weed control was performed on 5 February 2015 by applying a chemical herbicide (Axial® Sygenta).

be quantified, which is the second largest C flux in most ecosystems, accounting for 60–90% of total ecosystem respiration (Goulden et al., 1996). Rate of soil respiration is controlled by several factors including temperature and moisture of the soil (Raich and Schlesinger, 1992), quantity and quality of soil organic matter (SOM) (Raich and Tufekcioglu, 2000) and agronomic practices (Balesdent et al., 2000; Paustian et al., 2000; Baker et al., 2007; Ussiri and Lal, 2009) which affect soil properties (biological, chemical and physical). In particular, tillage management plays a key role in SOM turnover, soil aggregation and structure stability, microbial abundance, C storage (Peigné et al., 2007), and thus CO2 emissions. The effect of NT on soil carbon changes is contradictory, especially after short-term periods, because it is affected by soil conditions, climate, residue return, soil depth and their interactive effects (Puget and Lal, 2005). In addition, short and medium-term changes of soil organic carbon (SOC) are difficult to detect due to the high background values linked to soil spatial variability (Lal et al.,1998; Lee et al., 2009). Similarly, contradictory results regarding the effects of tillage on CO2 emissions can be found in literature (among other Kessavalou et al., 1998; Balesdent et al., 2000). For explaining these results, differences in experiment duration and climatic conditions, as well as in soil texture and timing of investigation (hours, weeks or months after soil tillage), have to be taken into account. In particular, several studies report lower CO2 emissions under NT (i.e. Ussiri and Lal, 2009) and, then, NT agriculture is one of the recommended practices for increasing SOC pool in agricultural ecosystems (Ussiri and Lal, 2009; Reicosky, 1997; Al-Kaisi and Yin, 2005), but other authors obtained the opposite result (i.e. Smith et al., 2011) with NT having a negative effect on C sequestration (Six et al., 2004; Regina and Alakukku, 2010). In other words, great uncertainties still exist and, since the efficacy of different soil management strategies has not been established yet, further investigations are essential.CO2 fluxes can be monitored both in the field and in laboratory. In the field these fluxes are easily measured by chamber methods (Livingston e Hutchinson, 1995), while in laboratory studies, microbial respiration is normally analysed using the incubation technique (Wang et al., 2003). The combined use of these two approaches and the monitoring of the chemical and physical soil properties may provide a better understanding of the processes involved in the gaseous C losses by agricultural soils.

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which conducts a catalytic combustion by high temperatures in air environment. Total N was quantifi ed according to the Kjeldahl procedure.

Soil physical characterizationIn agreement with the experimental protocol followed for chemical analysis, structure stability index, SSI (Pieri, 1992), was calculated according to the content of organic carbon (OC), silt and clay [SSI = 1.724 OC (silt + clay)−1], with OC equal to TOC expressed in g 100 g−1 and silt and clay given in percentage. Dry bulk density, BD, and water retention curve, θ(h), of the soil were determined using undisturbed soil cores of 98 cm3. The water retention curve of each undisturbed soil core was obtained using a Buchner funnel apparatus (in the pressure head range -100 ≤ h ≤ 0 cm), EcoTech’s pH laboratory station and the Richard’s chamber for h values of -600 and -15300 cm, respectively (Castellini et al., 2015). Selected volumetric soil water contents close to water saturation (i.e., from zero to –100 cm of pressure head) were determined from the θ(h) relationship in order to estimate a set of capacitive indicators that give into account of water/air availability: macroporosity (PMAC = θs

– θm) (cm3/cm3), air capacity (AC = θs – θFC) (cm3/cm3), and relative fi eld capacity (RFC = θFC/θs) (dimensionless), where θs, θm, θFC are the volumetric water contents corresponding to a pressure head of 0, –10 and –100 cm, respectively. Assessment of soil physical quality (SPQ) was carried out according to classifi cations gathered from literature (Reynolds et al., 2009; Castellini et al., 2016). In particular, the SPQ was considered optimal when 0.9 ≤ BD ≤1.2 g/cm3, 3 ≤ OC ≤ 5%, SSI > 7%, PMAC ≥ 0.07cm3/cm3, AC ≥ 0.14 cm3/cm3, 0.6 ≤ RFC ≤ 0.7. Supplementary information on these indicators can be found in Iovino et al. (2016) and Castellini et al. (2014).

the potential mineralization rate of SoMThe 22 undisturbed soil cores (5 cm diameter and 5 cm height) were collected after litter removal (about 1 cm), using a sample ring kit (Eijkelkamp). Soil cores were stored at 4 °C for 24 h to minimize the decomposition of dead roots and to make root (autotrophic) derived CO2 negligible (Lagomarsino et al., 2009). Undisturbed soil cores were incubated without root removal with the aim to preserve the roots-microbes-soil continuum in the short-term (Högberg and Read, 2006).For measuring microbial respiration the undisturbed soil samples embedded inside the Eijkelkamp-rings were placed in 1 L stoppered glass

The soil was classifi ed as silty clay (42% clay, 44% silt and 14% sand) according to the USDA classifi cation and the terrain was slightly in slope. Soil pH and electrical conductivity (1:2.5, soil:water extract) were on average 8.02 and 0.17 dS m-1. Mean concentrations of total organic carbon (TOC) and total N of 15.25 and 1.29 g kg-1 were recorded. The climate is semi-arid Mediterranean, with hot and dry summer and short and temperate winter. The mean annual temperature is 13.8 °C with a mean annual precipitation of 638 mm. Moreover, following Holdridge et al., (1971), which classify areas with potential evaporation/mean annual precipitation ratio > 1 as dry climates and areas with ratio < 1 as humid climates, this experimental site can be classifi ed as dry since the aridity index is 1.74 on the period 1951-2015.

Soil chemical characterizationIn order to have a deeper understanding of the factors determining CO2 emissions, the following components of the soil carbon and nitrogen pools were quantifi ed: TOC and total N, alkali-extractable carbon (TEC and humic substances), water extractable nitrogen and organic carbon (WEN and WEOC). As for the assessment of the CO2 fl ux rate, soil samples were collected about 5 months after the ploughing. The 16 samples were collected at the depth of 0-0.20 m. WEOC and WEN, indicators for labile organic C and N pools, were extracted from fi eld-moist soil according to a protocol reported in Armenise et al. (2013). In detail, 30 g of fresh-weight soil and 60 ml distilled water (1:2, soil:H2O) were placed in an Erlenmeyer fl ask, stoppered and shaken for 30 min. The extracts were centrifuged at 20,000 g for 10 min and the supernatant was fi ltered through 0.45 mm Millipore fi lter.TOC, TEC, humic and fulvic acids ((HA+FA)-C) and total N were quantifi ed on dried and 2-mm sieved samples. For TOC quantification, soil samples were also ground to a fi ne powder (0.5 mm) using an agate ball mill. TEC was obtained by 0.1M NaOH + 0.1M Na4P2O7 extraction at 65 °C for 48 h. Humic and fulvic acids were separated from the extract by acidifi cation to pH = 2.0 with H2SO4. The purifi cation of FA from non-humic substances was carried out by adsorption onto polyvinylpyrrolidone columns. Total organic carbon in soil samples, as well as C and N fractions in the water and alkali extracts, were quantifi ed through dry combustion using a TOC Vario Select analyzer (Elementar, Germany),

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relationship between the water/air content and the porosity of the soil, for each time of CO2 sampling, soil samples were collected for monitoring gravimetric soil water content (SWC, 1-6 cm) and BD. According to Linn and Doran (1984), the Water Filled Pore Space (WFPS) was calculated as ratio between the gravimetric soil water content and the soil porosity:WFPS (%) = SWC (%) / [1-(BD/2.65)] (1)where 2.65 g cm−3 is the soil particles density.

Statistical analysisFor soil chemical characterization and potential mineralization rate of SOM, data were tested for normality (Kolmogorov-Smirnov test) and homogeneity of variance (Bartlett test). Then, WEC was analysed using a non-parametric test (Kruskal-Wallis). The other variables were analysed with a nested analysis of variance for a randomised complete block design considering the sub-replicates within each plot as pseudo-replicates (McDonald, 2014). The Tukey HSD test was used for multiple comparison of treatments means. Pearson correlation coefficients were also computed to investigate the relationships between the studied chemical variables and the main parameters of microbial mineralization activity (cumulated mineralized C and basal respiration).The statistical significance of SPQ capacitive indicators was tested by calculating the confidence intervals for the average values (p = 0.95). A given indicator was considered indicative of a very good, or very poor, quality when its confidence interval was completely inside, or outside, the range of considered optimal values (see Tab. 2). When the calculated confidence interval included an extreme value of this range, the SPQ was classified good if the mean value of the considered indicator fell inside the optimal range and poor if it fell outside this range. By this approach, the effects due to the spatial variability of the soil were considered and a different statistical confidence could be attributed to the mean values of the selected indicators even if they were obtained by a relatively small number of replicate samples (Iovino et al., 2016). For CO2 flux measurements in situ, the average value of the two cycle measurements of each day was used, after testing for the equality of these measures (two sample t-test p=0.52). One way analyses of variance were conducted for evaluating the difference between treatments (CT and NT) and between plots (CT1, CT2, NT1 and NT2), considering all measures independently from time. Moreover, a nested analysis of variance

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jars and incubated at 28°C; 4 ml NaOH (1 M) were pipetted in the jar and the system was immediately made airtight using a rubber ring and two crossing pegs. Two jars with NaOH (1 M) without soil samples were used as controls. The CO2 evolved was trapped, after 1, 3, 7, 10, 15, 21, 28 and 35 days of incubation, adding 8 ml BaCl2 solution (0.75 M) in 4 ml NaOH (1 M) and determined by titration of the excess NaOH with 0.1 M HCl (Badalucco et al., 1992). C mineralization kinetics was determined following a first-order kinetics model Cm=C0(1-e-kt) as reported in Riffaldi et al. (1996), where C0 is the potentially mineralizable C and k is the rate constant. C0k and Cm described the initial potential rate of C mineralization and the cumulated mineralized C in 35 days, respectively. Cm / C0 can be considered a measure of the microbial capacity to mineralize the available C. The hourly CO2 evolved, afterwards a steady-state condition have achieved, was used as the basal respiration value because, after that period, the soil reached a relatively constant hourly CO2 production rate. According to the definitions proposed by Kuzyakov (2006) our measurements of microbial C mineralization refer to the microbial decomposition of SOM in the bulk soil.

co2 flux measurements in situ The soil CO2 fluxes (FCO2, μmol m-2s-1) were measured using the enclosure method developed by Parkinson (1981), where the soil respiration is determined by measuring the rate of increase in CO2 concentration within a chamber of a known volume placed on the soil surface. The measurements were performed with a portable ADC LCPro+ analyser (Bioscientific Ltd, UK) coupled to a soil hood which is a chamber incorporating an enclosed volume (995 cm3). The chamber consists of a PVC pot containing an air stirrer fan and pressure equalisation vent in order to mix the air and avoid pressure differences, which could affect the evolution of CO2 from the soil surface. The 11 stainless steel collars were positioned in the plots few days before the starting of the CO2 flux measurements. For each of the seven sampling days (April 2015 the 21th, 25th and 29th, May the 5th, 12th and 29th and June the 18th), the two cycles of FCO2 measurements were performed between 10:00 a.m. and 12:00 a.m., when soil temperatures were expected to represent average daily values, closing the chamber for no more than 4 minutes and waiting for CO2 concentration achieved a steady state in the chamber. In an attempt to establish a possible

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was conducted for a mixed-effects design in order to evaluate the trend of the 11 CT and NT sub-replicates with time. In particular, the sub-replicates were considered as random factors nested in treatment groups (CT, NT), which were considered as fixed (Singer and Willet, 2003) and time was considered as a repeated measures factor considered as fixed factor. In other words, in this model the sub-replicates of the two treatments were assumed to be a random sample from the entire population of CT’s and NT’s sub-replicates and measurements of each of these sub-replicate were taken 7 times. Plots were not considered in this model. This model accounts for: the main effect of treatment and time and the interaction between the two. It tells us if there are significative differences between treatments in time.Then, the following non-parametric techniques were applied: (i) Theil-Sen trend line estimation (Theil, 1950; Sen, 1968), which determines a robust linear regression by choosing the median m of the slope among all lines through pairs of two-dimensional sample points and then estimates the intercept d as the median of all those lines; (ii) the Ljung-Box autocorrelation test (Ljung and Box, 1978), which tests whether any of a group of  autocorrelations  of a  time series  are different from zero and is essential for the use of (iii) the Mann-Kendall (τ) non-parametric test (Kendall, 1975), which is used in case of independent data to assess the presence of significant monotonic trends.All the statistical analyses were conducted using the free software environment R (R Development Core Team, 2014. http://cran.r-project.org/).

ReSultS

Soil chemical characterizationTab. 1 reports the effect of soil management on the total and extractable fractions of carbon and nitrogen. The study site was characterised by low average TOC contents (≤ 20 g kg−1), which is typical of southern-Europe soils (Zdruli et al., 2004). The

average TEC and (HA+FA)-C values observed are in agreement with results of other researches carried out in Mediterranean environments, where TEC and (HA+FA)-C values ranged from 7.32 to 12.28 g kg-1 and from 4.96 to 8.65 g kg-1, respectively (Montemurro et al., 2011). Higher values of TOC and TEC were on average observed under NT in comparison to CT soil (see Tab. 1); however differences between treatments were not significant (p=0.6129 and 0.5557). Similar behaviour was recorded for the HA+FA carbon and for the water extractable fractions of carbon and nitrogen. Carbon to nitrogen ratio, a parameter directly affected by soil management (Ussiri and Lal, 2009), showed also close average values. The only significant effect of different soil management was detected for total nitrogen content (p=0.04), with higher values under no-tillage (1.38 vs 1.20 g kg-1; Tab. 1). TOC was highly positively correlated with TEC (p<0.0001), humic and fulvic acids (p<0.0001) and total nitrogen (p=0.0011), and, at a lower extent, with WEOC (p=0.0291). WEOC showed the highest positive correlation with total nitrogen (p=0.0020); water extractable N did not show any significant correlation with the other examined parameters.

Soil physical characterizationThe preliminary soil physical evaluation showed not significant differences between CT and NT. NT soil showed higher OC content, SSI and dry BD than CT one: as a consequence, macroporosity, PMAC, and air capacity, AC, were lower (Tab. 2). Relative field capacity, RFC, that gives an account of the optimal balance between water capacity and air capacity of the soil, was higher in NT than in CT, suggesting major potential risks of anaerobic conditions in NT with respect to CT due to reduced presence of air in the soil porosity (see Tab. 2).According to Reynolds et al., (2009) about physical quality evaluations of agricultural soils, the studied

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Source of variation

toc(g kg-1)

tec(g kg-1)

(hA+FA)-c(g kg-1)

n(g kg-1)

weoc(mg kg-1)

wen(mg kg-1)

nt 16.70 10.84 7.22 1.38 a 43.52 14.98

ct 13.79 9.19 5.97 1.19 b 39.44 14.52

Different letters indicate significant differences between treatment means according to Tukey HSD test.

tab. 1 - Total and extractable fractions of carbon and nitrogen quantified in the 0-0.20 m layer. NT: No-Tillage, CT: Con-ventional Tillage.Tab. 1- Frazioni totali e estraibili di carbonio e azoto nello strato 0 – 0.20 m. NT: non lavorazione, CT: lavorazione conven-zionale.

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silty clay soil showed always very poor OC levels (i.e., OC<3%). Consequently, the observed low values of SSI (SSI<7%) may result in a potential risk of structural degradation of the soil in both treatments, but slightly greater risk in CT system. The remaining soil indicators provided consistent information since they showed too high values of bulk density (>1.2 g cm−3) or too low macroporosity (<0.07 cm3 cm−3) and air capacity (<0.14 cm3 cm−3) values (Tab. 2). These findings were corroborated by the high level of relative field capacity (RFC>0.7). Therefore, since statistically significant differences in terms of capacitive indicators were not observed, comparable GHG emissions (i.e., CO2) between CT and NT are expected.

the potential mineralization rate of SoM The effect of tillage on the heterotrophic respiration, evaluated by the incubation experiment in laboratory, was not significant, as highlighted by the analysis of variance and Tukey HSD test results, showing no significant differences among plots.The hourly soil respiration showed an initial sharp decrease during the first two measurements (day1 and day3); afterwards, soil respiration achieved a steady state that was maintained for the whole period of measurement, without evident downward trends (Fig. 1). The cumulative mineralized C (gC-CO2 m-2) presented a curvilinear relationship with time showing a constant increase of CO2 throughout the 35-day incubation period (Fig. 2). The best fitting was obtained applying the nonlinear models represented by the following equation: y=axb

(p<0.001 and R²=0.99). Under the steady-state conditions, the estimated soil respiration at 28 °C was considered as basal respiration and was 0.674 and 0.549 µmol-CO2 m-2s-1 under NT and CT treatments (p=0.22), respectively. The first-order equation Cm=C0(1-e-kt) provided a good description

Soil physical quality indicator (SPQ) ct nt optimal valuesOC (%) 1.4 a − − * 1.7 a − − 3-5SSI (%) 2.8 a − − 3.3 a − − >7BD (g/cm3) 1.31 a − 1.41 a − − 0.9-1.2PMAC (cm3/cm3) 0.065 a − 0.040 a − − >0.07AC (cm3/cm3) 0.105 a − 0.076 a − − >0.14RFC (−) 0.781 a − 0.835 a − − 0.6-0.7

* For each soil indicator, the mean values followed by the same letter are not statistically different according to a two-tailed t test (P=0.05). A given indicator was considered indicative of a very good (++), or very poor (−−) quality when its confidence interval (P=0.95) was completely inside, or outside, the range of considered optimal values. When the calculated confidence interval included an extreme value of this range, the SPQ was classified good (+) if the mean value of the considered indicator fell inside the optimal range and poor (−) if it fell outside this range.

tab. 2 - Mean value of soil physical quality indicators explained in the text. NT: No-Tillage, CT: Conventional Tillage.Tab. 2 - Valori medi degli indicatori di qualità fisica del suolo dettagliati nel testo. NT: non lavorazione, CT: lavorazione convenzionale.

Fig. 1 - Hourly microbial soil respiration for the whole pe-riod of measurement of 35 days (di, i=1, …, 35). NT: No-Tillage, CT: Conventional Tillage.Fig.1 - Respirazione microbica del suolo a livello orario sull’intero periodo di misura di 35 giorni (di, i=1, …., 35). NT: non lavorazione, CT: lavorazione convenzionale.

Fig. 2 - Cumulated microbial soil respiration in 35 incuba-tion days. NT: No-Tillage, CT: Conventional Tillage.Fig. 2 - Respirazione microbica del suolo cumulata sui 35 giorni di incubazione. NT: non lavorazione, CT: lavorazione convenzionale.

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of the C mineralization kinetics and R² values were always greater than 0.95. Although the values of NT soil respiration were always greater than CT ones, no significant differences between the two treatments were found, both for cumulated and hourly values (p=0.07 and p=0.24, respectively). Moreover, as shown in Fig. 3, the influence of tillage on the main parameters of microbial mineralization activity as the soil respiration during the first 24 hours, the potentially mineralizable C (C0), the mineralized C (Cm), the constant rate of C mineralization (k) and the basal respiration (Rbasal) was not significant.

co2 flux measurements in situ CO2 emissions of CT were, on average, a little lower than NT ones (by 0.1 µmol CO2 m-2s1, 7.2%), but this difference was not significant (p=0.447). Instead, CO2 emissions were significantly different (p=0.01) between the four plots CT1, CT2, NT1 and NT2

(data not shown) indicating soil heterogeneity, probably linked to soil management.As shown in Fig. 4a, CO2 emissions of CT management decreased more rapidly with time, and this difference is confirmed by the nested mixed-effects ANOVA, which showed a significant difference in the behaviours of treatments trough the time (p=0.001). Moreover, CO2 emissions under CT showed a significant downward trend (Mann-Kendall τ = -0.71, p=0.035) with Theil-Sen slope and intercept equal to -0.2 and 2.56, respectively. Water filled pore space (WFPS) showed very similar values under the two considered treatments (Fig. 4b), reaching on average the lowest values in May (21.6%) and the highest ones (31.7%) in June, which resulted the rainiest month during the monitoring period. No significative regression relation between CO2 fluxes and WFPS was found, neither between the two treatments, where the t-test significance of the WFPS regression coefficient (which states

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Fig. 3 - Main parameters of microbial mineralization activity. Mean values are reported with standard deviation bars. NT: No-Tillage, CT: Conventional Tillage.Fig.3 - Principali parametri dell’attività microbica di mineralizzazione. I valori medi sono riportati con le barre di errore standard. NT: non lavorazione, CT: lavorazione convenzionale.

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if the variable is significantly different from 0) was equal to 0.24, nor inside each plot (t-test p=0.26). Moreover, under CT, although air temperature and CO2 emissions seem to show opposite trends (see Figs. 4a and 4b), no significant correlation was found between these variables (Pearson’s correlation r=-0.03).

dIScuSSIon Several studies demonstrated the beneficial effect of NT on soil properties (Six et al., 2000), mainly based on the formation and stabilization of organic matter within micro- and macro-aggregates (Six et al., 1999). Moreover, the frequent soil disturbance in intensively tilled systems compared with NT ones can increase the turnover of soil aggregates, leading to a net loss of C (Paustian et al., 2000). However, the effect of NT on soil carbon changes is sometimes contradictory, especially after short-term periods (Puget and Lal, 2005), and short and medium-term changes are difficult to detect due to the high background values and to soil spatial variability (Lal et al.,1998; Lee et al., 2009). In this context, our results seem to confirm these findings, showing greater, although not significant, concentrations of total organic carbon, alkali-extractable fractions (TEC and (HA+FA)-C) and water extractable C (WEOC) in the top layer of NT soil with respect to CT. A greater, but not significant, C mineralization activity was also found under NT treatment (see Fig. 3). The similar behaviour observed for the main

parameters of the mineralization activity and total and extractable C and N fractions was confirmed by the significant correlations recorded. Among C fractions, humic and fulvic acids showed the highest correlation with cumulated mineralized C (p=0.0168), whereas WEOC with basal respiration (p=0.0006). Significant correlations with these two parameters were also recorded for TEC (p=0.0429 and p=0.0489, respectively), whereas positive but not significant correlations were observed for TOC (p=0.105 and p=0.110). These results are in accordance with other studies. For instance, Wang et al. (2013) observed stronger relationships between soil basal respiration and labile SOC fractions, whereas weaker with total organic carbon. Similar results were found by Iqbal et al. (2010) in different land covers for field CO2 emissions, showing that is the quality of organic matter to affect CO2 fluxes more than the quantity. Higher average mineralization activity and organic carbon concentrations under no-tillage plots could be explained by an increase in the input of crop residues on the soil surface layers, mainly in easily decomposable pools (Kandeler et al., 1999), and by a reduced soil-residue interaction which causes lower rates of mineralization (Ussiri and Lal., 2009). The not significant differences observed can be attributed to the duration of the study and to soil heterogeneity. Indeed, about 10 years are required to obtain stable management effects (Al-Kaisi et al., 2005; Regina and Alakukku, 2010) and soil heterogeneity may confound short-term changes (Lee et al., 2009). Results of

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Fig. 4 - a: Time evolution of CO2 emissions measured in the field; b: air temperature, WFPS and rainfall during the monitoring period. NT: No-Tillage, CT: Conventional Tillage.Fig. 4 - Evoluzione temporale delle emissioni di CO2 misurate in campo (a); temperatura dell’aria, WFPS e precipitazioni durante il periodo di monitoraggio (b). NT: non lavorazione, CT: lavorazione convenzionale.

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mainly caused by the rainfall effects which affected also this study. However, although the soil physical indicators were not evaluated for the whole growing cycle, their use was effective to estimate the air availability for microbial activity and, consequently, for CO2 emissions. The SPQ evaluation suggested a better air availability for CT as compared to NT (respectively, poor and very poor), but this finding may be not linked to the magnitude of the observed CO2 emissions, since this indicator was estimated in a single sampling date. Further study are needed to check the possible similarity about the temporal variability of these soil properties and the dynamic of CO2 emissions.Concerning the “young system” under investigation, converted to NT from only 4 years, the CO2 fluxes measured in the field far from the ploughing practices are in accordance with the results reported by Gerosa et al. (2014) which, in analogous conditions, found that the NT site experienced higher CO2 emissions than conventional one with a higher flux variability among the chambers. These results are in agreement with the results of this study both in terms of magnitude and variability expressed by the coefficient of variation (CV), indicating an expected homogenization of organic matter on the topsoil in the CT plots due to mechanical treatments. In particular, the tillage effect produced a lower variability (27.1% and 14.0% for CT1 and CT2, respectively) compared to NT treatment (33.9% and 34.4% for NT1 and NT2, respectively). Also Álvaro-Fuentes et al. (2008) found that in semi-arid environment CO2 emissions by NT is greater than CT ones. Analogously, Six et al., (2004) reported that in dry zones like our, defined following Holdridge et al., (1971), in the first years of NT implementation (less than 10 years), CO2 emissions by NT are higher than CT ones and this is probably due to a slower incorporation of surface residues into the soil in NT systems. However, since we did not perform measurements during the first period after ploughing treatment, we cannot exclude higher CO2 emissions in CT than NT during the short period after tillage (Reicosky and Lindstrom, 1993; Al-Kaisi and Yin, 2005). Indeed, CO2 fluxes were often larger immediately after tillage probably reflecting a “flush” of microbial CO2 and CO2 released from large voids generated by tillage events (Hendrix et al., 1988; Reicosky, 1997). Anyway, our results confirm that, far from mechanical treatments, CO2 emissions tend to be similar (Ellert and Janzen, 1999). The not significant difference between CT and NT has been confirmed also by Oorts et al. (2007) during 41% of the experimental days of a long-term trial (32 years), while over the entire trial, they found that NT emitted larger

short or long term studies show also not always clear and consistent response in relation to soil C increase. Puget and Lal (2005), comparing 56 different pairs of conventional tillage and no tillage, reported that among the 42 pairs with soil organic carbon gain, statistically significant differences were only observed for 10 pairs. Moreover, the general trend observed in our data is in agreement with other findings on short-term trials documenting that the transformation of a soil profile into that characteristic of no tillage (top-soil SOM build-up, porosity decline) can become detectable within a relatively short time (McCarty et al., 1998), but that developing consistent differences require several years (Kay and VandenBygaart, 2002). Slightly higher cumulated field CO2 emissions were also recorded under NT soil. Higher soil organic carbon contents, in the total or labile pools, are related to greater CO2 emissions (Ussiri and Lal, 2009). Liang et al., (2007) questioned the capacity of NT practices of sequestering more C than CT systems in fine-textured, poorly drained soils arguing that NT may contribute to stratify SOC concentration, but not necessarily to its storage within the soil profile. Same concerns, for fine-textured soils, have been raised by Alvarez et al., (1998). Rates of soil respiration in the field are also controlled by several interacting factors further than quantity and quality of soil organic matter, and particularly by physical soil properties (air-filled porosity, soil moisture, temperature) and long and short-term effects of soil tillage as, for example, the time after soil disturbance (Ussiri and Lal, 2009). For this reason in our study, a set of soil physical indicators, that give account of the soil structure as well as the soil ability to retain air for microbial activity, was selected. With reference to the modelled time evolution of CO2 emissions, the general higher air availability observed in CT system (i.e., relatively lower bulk density or relative field capacity, and a corresponding higher macroporosity or air capacity) at beginning of the field experiment (April) was in agreement with the highest CO2 emissions observed in the field (Fig.4a, first CO2 fluxes sampling date near soil sampling for physical characterization). This general higher air availability would have accelerated the mineralization of readily decomposable residues. Moreover, although CO2 emissions tend to decrease with time (i.e., after the third sampling date of May, the CO2 of CT was lower than NT), a significant (p=0.05) decrease in the air availability is expected especially for the macropores network of CT. Castellini et al. (2014) highlighted a similar behaviour in their evaluation about the temporal evolution of some soil physical quality indicators, with the largest differences

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2007-2013 mis. 124 Codice CUP C32I14000080006; the project “ALForLab” (PON03PE_00024_1) co-funded by the National Operational Programme for Research and Competitiveness (PON RandC) 2007-2013, through the European Regional Development Fund (ERDF) and national resource (Revolving Fund - Cohesion Action Plan (CAP) MIUR); the National Research Project ‘Collections and databases of public interest (Collezioni E-A-OR) Funding: Ministry of Agriculture, Food and Forestry Policies (MiPAAF), Italy. The authors wish to thank the farmer Mr Saverio Dimauro for his support and collaboration.

ReFeRenceSAl- Kaisi M.M. and Yin X., 2005. Tillage and crop

residue effects on soil carbon and carbon dioxide emission in corn-soybean rotations. Journal of Environmental Quality, 34: 437-445.

Álvaro-Fuentes J., López M., Arrúe J. and Cantero-Martínez C., 2008. Management effects on soil carbon dioxide fluxes under semiarid Mediterranean conditions. Soil Science Society of America Journal, 72: 194-200.

Armenise E., Redmile-Gordon M.A., Stellacci A.M., Ciccarese A., Rubino P., 2013. Developing a soil quality index to compare soil fitness for agricultural use under different managements in the Mediterranean environment. Soil And Tillage Research, 130: 91-98.

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amounts of CO2 than CT treatment. Insignificant differences or NT CO2 fluxes greater than CT ones are reported also by Franzluebbers et al. (1995) for various cropping systems in Texas, while Hendrix et al. (1988) reported NT CO2 emissions slightly greater than CT ones in Georgia. These results could be explained considering that Franzluebbers et al. (1995) and Hendrix et al., (1988) performed their measurements 9 and 5 years after tillage treatments, respectively, then, the initial response of CO2 emissions to tillage may have been missed. Similarly, no significant differences in CO2 emissions by unploughed and ploughed soils are reported by Yamulki and Jarvis, (2002), suggesting that the mechanical treatment is not the only factor affecting CO2 flux, but also weather variables control the differences. Probably, CO2 has a relatively long lifetime in the soil and its emission depends mainly on its production within the soil.

concluSIonSIn order to verify the effect of tillage practices on C losses by agricultural soil during the transition to no-till farming, a quantification of CO2 losses was performed, monitoring soil respiration both in the field (actual) and in laboratory (potential). Moreover, chemical and physical properties of the soil were investigated at the beginning of the trial for evaluating the impact of these characteristics on C gaseous emissions. In the selected cropping system under semi-arid climate and subjected to NT from only 4 years (transition time), we did not find significant differences in the CO2 losses, both actual and potential, between the two treatments (NT and CT), with the NT emissions greater than CT ones of about 7.2%. The measured chemical and physical indicators also stress the presence of not significant differences between the two management’s practices, suggesting a similar turnover time of the organic C between the two treatments, probably due to the young changing in the management (4 years) and the period of measurements (far from ploughing). These results are in accordance with results reported in literature by other authors. Even if measurements did not cover the entire cropping season and these results cannot be generalized, they offer a useful set of indicators for evaluating the relationships between tillage and GHG losses and this information can be usefully employed in a longer trial that will start from the ploughing operations and cover the entire cropping season.

AcKnowledgMentS This work was supported by the: the project “Approcci innovativi per il miglioramento delle performances ambientali e produttive dei sistemi cerealicoli no-Tillage - BIOTILLAGE”, PSR Regione Basilicata

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