No-tillage improvement of soil physical quality in calcareous, degradation-prone, semiarid soils
Introduction
Soil physical properties strongly influence soil function and determine potential land uses. This is of special importance in the Ebro Valley of Spain, where they affect both plant-available water content and land degradation processes. Water frequently limits rainfed crop production in this area because of low precipitation (<450 mm) and an uneven interannual distribution. To mitigate that stress it is crucial to enhance plant-available water content which is directly affected by soil structure. This enhancement can be difficult, however, because soils of the Ebro Valley are usually characterized by silty texture, low organic matter content, localized accumulations of both geological and secondary gypsum and other soluble salts; factors affecting soil structure and resulting in a high potential for physical and chemical degradation.
Chisel-ploughing is one management practice that has been introduced for dryland production in the Ebro Valley during the last 20 years to decrease damage to soil structure (Angás et al., 2006). No-tillage is not used as much (10–30% of the land) even though it may be a better option to prevent soil degradation and increase plant-available water. The most widely used practice (i.e. conventional tillage in the area) is reduced vertical tillage with a chisel-plough.
To evaluate effects of management on soil physical quality, aggregate-size distribution, water stability of those aggregates, compaction, water retention and porosity have been widely used as soil quality indicators and are often referred to as dynamic physical quality indicators. Collectively assessing multiple indicators such as these, along with those reflecting biological and chemical properties and processes can be useful for quantifying changes in soil quality due to various management practices (Karlen, 2004).
One reason for suppression of tillage is that it temporarily alters soil structure, breaking apart the largest soil aggregates and disrupting their formation and stabilization cycles (Six et al., 1999). Most of the studies on this topic have been conducted on carbonate-free, neutral to acidic soils, as in Rasmussen (1999) for Scandinavian soils, Castro-Filho et al. (2002) for a Brazilian latosol, Zhang et al. (2007) for an Australian oxisol, and many others. They generally observed a higher mean weight diameter for dry aggregates and a greater percentage of water stable aggregates as tillage intensity decreased. Studies quantifying tillage effects on aggregation in calcium-rich soils, such as those in the Ebro Valley, where calcium plays an important role in the aggregate cycle (Muneer and Oades, 1989), are more scarce in the literature. Recently, Álvaro-Fuentes et al. (2007) observed that organic matter enrichment under no-tillage also enhanced aggregate stability in a loam calcic soil under similar semiarid conditions.
Tillage is often justified because without it compaction can lead to higher bulk density and increased penetration resistance, especially in the top few centimetres of soil. Many authors have found that semiarid no-tillage sites have greater bulk density and penetration resistance than reduced-tillage sites (e.g. Mahboubi et al., 1993, Lampurlanés and Cantero-Martínez, 2003). Such compaction is often characterized by a reduction in size distribution and stability of aggregates; two soil structure factors that provide resistance against external forces (e.g. wheel traffic).
Tillage and the resultant soil structure also influence soil water retention and its availability to plants. This is especially critical for crop production and temporal yield stabilization under semiarid conditions. No-tillage has been shown to increase soil water content through greater infiltration and reduced evaporation (Blevins and Frye, 1993, Cannell and Hawes, 1994, Lampurlanés et al., 2001), and by increasing the proportion of smaller pores (Arshad et al., 1999, Bescansa et al., 2006).
We hypothesized that no-tillage can provide an opportunity to improve soil structure and increase plant-available water content in soils that have lost in some degree of ability to sustain crop production, as a result of decreased physical and/or biochemical quality, under the semiarid conditions in the Mediterranean Ebro Valley of Spain. Our objectives were to quantify soil physical quality indicators and water retention characteristics for two, adjacent farmer-managed fields, one conventional- and one no-tillage, where rainfed barley was being grown. Through this analysis, we wanted to quantify the sustainability of no-tillage for this semiarid area where soils have a little structure development and high potential for degradation due to their physicochemical properties, and where water is the most limiting factor for crop production.
Section snippets
Site description and experimental design
This study was conducted at two adjacent on-farm sites in the Ebro Valley following 7 years of either conventional- or no-tillage practices near the municipality of Santacara (42°23′44″N; 1°32′32″W; altitude 342 m a.s.l.) in the southern portion of Navarre. Conventional tillage (CT, consisting of chisel-ploughing to a depth of 15 cm) has been practiced for decades in the area. No-tillage (NT, using direct-seeding) was implemented in one of the fields in 2000, 7 years before this study was
Soil organic carbon and carbonates, and barley yields
Soil organic carbon (g/kg and Mg/ha) was significantly greater under NT than CT for the 0-5- and 15–30-cm depths. No differences were observed for the 5–15-cm depth (Table 3). Under NT, SOC (g/kg) was significantly greater for the 0–5 cm than 5–15- and 15–30-cm depths, but no differences were observed among 5–15- and 15–30-cm depths. With CT, SOC (g/kg) was significantly greater in the 0–5-cm depth than 5–15-cm depth, which in turn was greater than in the 15–30-cm depth (Table 3). There was no
Soil aggregation
The description and quantification of soil aggregation are important because many agronomic and environmental processes are related to soil structure. At our site where the soil has little structure development, suppression of tillage improved aggregate-size distribution and stability. Dry MWD was greater under NT than CT at all three depth increments (Table 4). This was in agreement with results obtained by Mrabet et al. (2001) in Morocco and Álvaro-Fuentes et al. (2007) in Spain, who both
Scientific conclusions
The suppression of tillage resulted in increased penetration resistance of the tilled layer (0–15 cm) in the studied carbonate-rich degradation-prone soil, with natural low organic matter content, silty texture, and weak structure. However, other physical quality indicators, such as aggregate-size distribution, water stability and water retention characteristics were significantly improved after 7 years of continuous no-tillage (NT). Greater dry mean weight diameter and water stable aggregates
Acknowledgments
This research was supported by the “Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria” (INIA). The authors thank the Basque Government for the pre-doctorate grant to O. Fernández-Ugalde and the USDA-ARS National Soil Tilth Laboratory for the assistance during the research internship of O. Fernández-Ugalde in their laboratories. We also thank Dra Àngela Bosch from Universitat de Lleida for her critical review of our work.
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