DiscussionTop

Soil temperature and moisture

No-tillage system recorded low temperature during all different studied seasons, suggesting that crop residues in NT plots diminished the effect of high air temperatures and the solar radiation (Fig.1). However, from December to March 2015, the increase of soil temperature under NT could be explained by the fact that the mean air temperature during these months was generally lower than the reported ones during the same months of the other years (Table 2). In this context, the crop residues left on the soil surface could have been involved in buffering the impact of the low air temperature on the soil surface. This result coincides with the one found by Ussiri & Lal (2009), who reported higher soil temperature under NT system from November to March. Soil moisture content was higher under NT than CT (Fig. 2), the presence of crop residues on the soil surface in NT plots minimised water losses due to evaporation and surface runoff and increased soil moisture, and this was similar to the results found by Ussiri & Lal (2009) in a cultivated maize crop. The low soil temperature and high moisture obtained under NT (Figs. 1 and 2) are in accordance with the results reported by Moitinho et al. (2013), who stated that the absence of residues and the greater surface exposure of the tilled plots enhanced water evaporation and decreased soil moisture in CT system.

SOC distribution and accumulation

The results obtained in Table 4 showed an increase of the SOC throughout the studied years under both tillage systems. This increase could be explained by the fact that OM acts as a reservoir of nutrients for the crop, participating in the soil biological activity, which lead to a quantitative and qualitative changes of the structure due to tillage (Roldan et al., 2005). Generally, the biological and biochemical parameters of the soil play an important role as early sensitive indicators to ecological stress and soil restoration (Izquierdo et al., 2003; Roldan et al., 2003). During all the studied years, NT system presented significantly higher values than CT at 0-10 cm soil depth, except in 2013 where tillage system did not affect significantly the SOC values at the same depth. Actually, in November 2013, when the soil samples were collected (Table 4), low temperature and precipitation could have caused a low activity of the soil microorganisms, which decreases the OM degradation, and this could explain the absence of the significant difference between tillage systems. The differences found among tillage system was also reported by Huang et al. (2015), who studied the long-term effects of tillage system on different parameters of soil quality in a maize monoculture and found that concentration of OM increased by 18% in the first few centimeters of soil with NT and the amount was associated with the soil aggregates. The crop residues left on the soil surface acted as a protective layer that prevented from losses through evapotranspiration, water and wind erosion and increased the OM content that contributed to improve the soil quality (Traore et al., 2007; Blanco-Canqui, 2013; Zhang et al., 2018). Basamba et al. (2006) and Zhang (2012) pointed out the importance of the accumulation of OM in the upper soil horizon as it improved the quality of the interface between soil and atmosphere and gave the soil greater resistance to different degradation processes that occur on the surface. The results reported by Varvel & Wilhem (2008), Wen-Guang et al. (2015) and Nie et al. (2016) supported the ones obtained in this study, as they found that NT maize led to an accumulation of SOC at or near the soil surface while different tillage treatments including chisel, disk or plough displayed lower SOC values. The crop residues left on the soil surface under NT had a considerable influence on SOC increase at 10 cm depth in 2015 and 2017 (Table 4). In CT plots, the mouldboard plough broke the soil structure and SOC content could have been lost due to mineralization. This process did not occur under NT system where the absence of soil disturbance promoted the soil stabilization and the greater accumulation of SOC on the surface. Tillage affects SOC stocks in the ploughed layer by distributing crop residues mechanically throughout the tillage zone (Yang & Wander, 1999) which caused low rates for mineralization distribution of crop residues and homogenization of SOC stocks in the ploughed layers.

In this area, maize crop grows with high temperature and soil moisture (from irrigation) which could lead to high activity of the soil microorganisms promoting a rapid degradation of SOC. However, because of the lack of water during the whole crop cycle in 2017, soil microorganisms’ activity decreased and the SOC content was significantly higher than other years. These results coincided with Dimassi et al. (2014) who concluded that the climate interacted with tillage, leading to a greater C sequestration in dry than in wet regions. These authors found that SOC changes under the reduced tillage over time were negatively correlated with the water balance, indicating that sequestration rate was positive in dry periods and negative in wet conditions. De Bona et al. (2006) reported that irrigation increased the decomposition rate of OM by 19% and 15% under CT and NT systems respectively after 8 years of research. Luo et al. (2010) found an increase of the soil C in the topsoil (0-10 cm) under NT but no significant difference was reported over the soil profile to 40 cm, because of the C redistribution through the soil profile under CT system.

Tillage effects on short and long term CO2 emissions

In CT plots, the CO2 emissions were significantly higher than in NT plots due to soil inversion by mouldboard ploughing that activated the rapid oxidation processes, decreasing the levels of OM in the soil, releasing a large amount of CO2 into the atmosphere, and contributing to a greater global warming than in NT system. The results obtained in this study (Fig. 3) coincide with those obtained by Reicosky et al. (1997) who recorded a CO2 flux that ranged from 0.7 to 2.2 g CO2 m-2 h-1 under NT and CT systems respectively in a sorghum monoculture after the mouldboard ploughing. Al-Kaisi & Yin (2005) reported lower soil CO2 emissions in NT compared with mouldboard plough with the greatest differences occurring immediately after tillage operations in maize-soybean rotation. CO2 emissions displayed an important increase in the tilled soil after the mouldboard plough use (Fig. 3) compared to the non-tilled. These results are in accordance with the ones obtained by Prior et al. (2000), who indicated that CO2 flux increases after the soil ploughing and that it depends on both tillage depth and the degree of soil alteration. The mouldboard ploughing caused aggregates disruption leading to the exposure of the C, previously protected within these aggregates, to the microbial action (Six et al., 2000), resulting in higher CO2 emissions values under CT when compared to NT treatment. In addition, aggregates disintegration improves soil aeration, so that higher soil CO2 emissions under CT were related to the higher number of macropores under this management (Silva et al., 2019). The CO2 f lux decreased more than four times considerably in the f irst 2 hours after the mouldboard pass under CT in all the studied years (Fig. 4). Reicosky et al. (1997) observed a decrease in the first two hours after the mouldboard pass. After 2 hours, the flux began to decrease until reaching similar values in both treatments at 24 h. Other studies observed that the measurements of CO2 emissions during short periods after tillage were significantly lower under NT than CT (Alvarez et al., 2001; Alvaro-Fuentes et al., 2007; Carbonell-Bojollo et al., 2011). The results obtained showed that soil tillage operations accelerate CO2 emissions and the soil C losses (Rakotovao et al., 2017).

The CO2 emissions reached their maximum in July and August, from the vegetative phenological stages of the crop (V3-V10) to flowering- filling stages (Fig. 4). At these growth stages, CO2 emissions were greater under CT than in NT system. The absence of crop residues under CT system induced the increase of soil temperatures and promoted the rapid oxidation of OM. In addition, the microbiological and radicular activity in the soil increased, generating oxidation reactions that resulted in higher CO2 emissions. Aon et al., (2001) reported that high temperatures resulted in higher decomposition rates of OM. Kuzyakov & Domanski (2000) found that the crop growth had a significant impact on microbial activity through root exudates, and soil microorganisms easily broke them down. This could explain the increase in soil CO2 emissions observed from the leaf development to the silking stage (R1). Hanson et al. (2000) pointed out that for annual crop the contribution of the root to soil respiration are higher during the crop growth and lower during the periods of inactivity. In maize crop, the contribution of rhizosphere respiration (root respiration plus decomposition of root exudates) to total soil respiration can be significant with values close to 50% around the period of maximum crop activity (Rochette et al., 1999). The CO2 f lux ranged from 10.7 (2013) to 21.6 (2014) Mg ha-1 under CT tillage and from 7.8 (2017) to 16.95 (2014) Mg ha-1 in NT plots from sowing to phenological maturity (Table 7). The lower CO2 emissions under NT system compared to CT could be attributed to a greater surface of crop residues, which could serve as barrier for CO2 emissions from soil to the atmosphere and reducing soil temperature (Omonode et al., 2007). The slower decomposition of crop residues placed on the soil surface under NT could lead to lower CO2 fluxes in NT soil (Curtin et al., 2000). The lower CO2 emissions in NT plots results agree with those obtained by Reicosky & Archer (2007) and Almaraz et al. (2009). Because of the earlier maturity of maize crop and the lower amount of irrigation water in 2017, CO2 emissions decreased compared to the other years. The differences of CO2 emissions reported among the studied years could be caused by the SOC different concentrations in the upper soil layer between years, changes in soil physical processes or soil temperatures and moisture variability.

Rochette et al. (1999) indicated that CO2 emissions were related to temperature and crop growth and that the highest CO2 emissions in the warmer months could be associated with root respiration, as the plant growth was also much higher during these months. A significant relationship between CO2 emissions and both soil temperature and moisture (R2=0.60) was detected under both tillage systems in 2015 and could be caused by the high amount of water applied during irrigation. Omonode et al. (2007) found a weak significant relationship between CO2 emission and soil temperature and moisture when applying different tillage treatments. Under NT, the surface residues provide a barrier between the soil and the atmosphere, which reduces soil evaporation leading to the increase of soil moisture and affects the microbial mobility and gas diffusion in the soil. The general low relationship between CO2 emissions with soil temperature and moisture found in this study was consistent with other reports (Alkaisi & Yin, 2005; Omonode et al., 2007).

In the six studied years, there were significant differences between tillage systems, CO2 flux was higher under CT than under NT, except in 2013 (Table 7). In this year, the non-significant difference between treatments could be explained by three hypotheses: (i) the combination of the higher amount of crop residues left on the soil surface of 2012 compared to other years, and of the significant increase of grain yields in NT obtained in 2013 (Table 3); (ii) in 2013, despite the lower water amount irrigated, soil moisture was higher under NT (Rodríguez-Bragado, 2015), which displayed higher yield than CT and led to similar CO2 fluxes in both treatments; (iii) there was a strong correlation between CO2 emission and soil temperature in 2013 (R2=0.78**) across both tillage systems, while in other years this correlation was lower.

The ratio of CO2 emissions to grain yield was low under NT system, which means that the amount of CO2 emissions per unit grain decreased under this practice. Under this soil management, CO2 emissions decreased due to the amount of crop residues left on the soil surface which led to low soil temperatures and high soil moisture (Figs. 1 and 2). In these conditions, the results obtained could be explained by the changes occurring in the root level and microorganism activities, thus enhancing the increase of SOC sequestration at 0-30 cm laying and the decrease of CO2 emissions.

In conclusion, this study was initiated to assess SOC stocks, to observe the grain yield response and to determine short-term and seasonal soil CO2 fluxes under CT and NT practices in continuous maize cropping system during six years in a semiarid region of Castile and Leon. The results showed that SOC stock was 36% greater under NT (18.3 Mg C ha-1) than under CT (13.5 Mg C ha-1) with a rate of 1.61 and 1.13 Mg ha-1 yr-1 respectively at 0-10 cm depth. In the lower layers, SOC values were 7 and 3% higher in NT plots than in CT plots at 10-20 and 20-30 cm depths. These results suggest that tillage accelerated the decomposition in the 0-10 cm depth but had minimal influence in the 10-30 cm depth. In 2013 and 2017, maize grain yield reached higher values under NT system than CT, this demonstrates that the non-disturbance of the soil promotes moisture retention for a longer period compared to soil disruption. Generally, this study confirmed that NT management could lead to equal and even higher grain yield than CT system. Short-term CO2 emissions measured for 48 h after ploughing and cultivator labor were higher under CT than for NT for all measurements. On a seasonal basis, mean CO2 emissions during the growing season were affected by tillage systems and were 3.32 Mg CO2 ha-1 greater under CT than NT system. Soil temperature and moisture effects on CO2 flux did not show any significant difference except in 2013 and 2014 under NT and 2015 under CT. Generally specific correlations of soil temperature and moisture with CO2 emission was insignificant and depended on the moment when the measurements were carried out. From the obtained results in this study, it can be pointed out that the conversion from CT to NT system in irrigated maize would increase the sequestration of OC in the soil and reduce CO2 emissions to the atmosphere, which can contribute positively to the reduction of GHGs emissions by the agricultural sector without compromising the grain yield.