Long-term no-till as a means to maintain soil surface structure in an agroecosystem transformed into irrigation
Introduction
Soil management practices affect both soil surface characteristics and crop productivity. Tillage exposes soil to erosive agents such as wind and water, inducing its degradation. Under severe erodible forces, soils are exposed to the impact of water-drops, either produced by irrigation or by rainfall. This last process results in the release of organic matter and, generally, in soil crusting (Awadhwal and Thierstein, 1985). In bare soils, structural crusts are a major problem facing many agricultural areas worldwide (Mbuvi et al., 2009). Structural crusts, developed on soil surface, negatively affect seedling emergence and reduce infiltration, favoring runoff and soil erosion (Fox et al., 2004). Furthermore, crusting is closely related to soil aggregation. In that sense, Bouaziz et al. (1990) found a linear relationship between soil aggregate size and the proportion of non-emerged wheat seedlings due to soil crusting.
In Mediterranean climate regions, an increasing number of rainfed areas are transformed into irrigation to stabilize or increase crop yields (Apesteguía et al., 2015). This conversion generates significant consequences in agroecosystems. Greater biomass production by irrigation leads to an increase in crop residues which can be returned to the soil. The increase of organic C inputs to the soil usually entails an increase in soil organic carbon (SOC) (Franzluebbers, 2005) and, concomitantly, an improvement in soil quality (Wick et al., 1998, Dexter et al., 2008). Moreover, C inputs play an essential role in the formation of soil aggregates, which physically protect SOC from microbial degradation (Beare et al., 1994) boosting SOC sequestration and climate change mitigation (Lal, 2011).
C-enriched aggregates are more stable to alterations such as rainfall, irrigation or tillage. Furthermore, crop residues protect the soil surface, preventing the formation of crusts (Jordán et al., 2010). Besides its importance in the Mediterranean climate regions, the impact of rainfed into irrigation transformation on soil surface characteristics (e.g., soil aggregation, soil organic carbon, bulk density, infiltration, penetration resistance and soil porosity) has been scarcely studied. Regarding to this, Apesteguía et al. (2015) observed an increase of the proportion of large macroaggregates under corn and wheat cropping systems managed under conventional tillage (chisel plow) when transforming a Mediterranean rainfed area into irrigation in north of Spain. Also, in Central Great Plains, Denef et al. (2008) found greater SOC storage in the surface soil layer (0–20 cm) in pivot-irrigated areas compared to dryland areas.
Tillage operations that incorporate crop residues into the soil increase soil susceptibility to degradation. When intensive tillage systems are adopted, soil remains bare until the next planting. Bare soils are more exposed to erosive agents and to drop impact promoting soil surface sealing and crusting and, at the end, water runoff (Pagliai et al., 2004). Tillage generally decreases soil bulk density compared to no-tillage (NT) (Lal, 1999) and it can negatively influence soil water infiltration, depending on soil type and properties (Dexter et al., 2004). For instance, Chan and Heenan (1993) and McGarry et al. (2000) reported lower infiltration rates under conventional tillage (CT) compared to NT. The adoption of NT systems has been identified as an optimal practice to reduce soil degradation and to improve soil aggregation in rainfed Mediterranean areas (Álvaro-Fuentes et al., 2009; Plaza-Bonilla et al., 2010). Moreover, it has been proved that long-term use of NT increases soil organic carbon (SOC) sequestration (Plaza-Bonilla et al., 2015). Similarly, Follett et al. (2013) showed that CT induced greater losses of old organic matter than NT in irrigated corn systems influencing soil physical properties. Soil organic matter plays a fundamental role in the formation and maintenance of aggregates, positively influencing the soil water retention capacity, water infiltration, and avoiding the formation of superficial crusts which improves seed germination and crop emergence.
In Mediterranean irrigated agroecosystems, typical soil management strategies include intensive tillage with deep subsoilers and mouldboard ploughs. However, unlike in irrigated systems, in Mediterrenan rainfed areas an increasing adoption of reduced tillage (RT) or NT techniques has been taking place over the last 30 years (Lampurlanés et al., 2016). In Mediterranean irrigated areas, the limited knowledge associated to the use of MT or NT systems, makes farmer adoption difficult and jeopardizes the soil quality benefits attained with long-term NT. As a consequence, the aim of this study was to determine to what extend soil management practices affect soil surface characteristics and crop establishment when transforming a rainfed area into irrigation in Mediterranean conditions.
Section snippets
Experimental design
A field experiment was conducted in Agramunt, NE Spain (41°48′ N, 1°07′ E, 330 m asl), where the soil was classified as Typic Xerofluvent (Soil Survey Staff, 2014). Soil characteristics are presented in Table 1. The climate is semiarid Mediterranean with a mean annual precipitation of 430 mm and a potential evapotranspiration of 855 mm. Mean annual air temperature is 13.8 °C.
A rainfed long-term field experiment (LTE) was established in 1996 to compare three tillage systems (no-tillage, NT; reduced
Results
Rainfall, irrigation events and air temperature during the entire experimental period are shown in Fig. 1. Air temperature increased from the beginning of the experimental period, reaching a maximum in summer months (July-August), to decrease later during autumn months. Total rainfall during the crop cycle was 140 mm with the greatest rainfall recorded in May (43 mm), which was far from the evapotranspiration needs. The amount of water applied by irrigation was 672 mm, 80% of this considered
Effect of long-term and short-term management practices on soil surface and corn development
The different historical management of the two experiments tested had a great impact on the results obtained. In the LTE, soil inversion with moldboard plough for the last 20 years (CT treatment) led to soil crusting (Fig. 6b). However, NT for 20 years provided greater resilience to soil degradation and crust formation, enhancing water infiltration, with almost two-fold greater water infiltration in NT compared to CT.
In the STE, tillage treatments significantly affected PR between corn rows,
Conclusions
Our study shows that the long-term use of intensive tillage in areas recently transformed into irrigation leads to a greater susceptibility to soil crust formation, and structural degradation. The results of this study have shown that the main process behind soil crusting was the breakdown of dry-sieved aggregates.
Although the proportion of dry-sieved aggregates increased after tillage (even reaching similar values than NT at the end of corn growing season) their water stability was lower.
Acknowledgements
We would like to thank the field and laboratory technicians Javier Bareche, Carlos Cortés and Silvia Martí. This research work is financially supported by the Ministerio de Economía y Competitividad of Spain (project AGL2013-49062-C4-1-R; PhD fellowship BES-2014-070039). Daniel Plaza-Bonilla received a Juan de la Cierva postdoctoral grant from the Ministerio de Economía y Competitividad of Spain (FJCI-2014-19570).
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