Hydraulic conductivity, residue cover and soil surface roughness under different tillage systems in semiarid conditions

Abstract

The objective of this study was to investigate the effect of tillage and cropping system on near-saturated hydraulic conductivity, residue cover and surface roughness to improve soil management for moisture conservation under semiarid Mediterranean conditions. Three tillage systems were compared (subsoil tillage, minimum tillage and no-tillage) under three field situations (continuous crop, fallow and crop after fallow) on two soils (Fluventic Xerochrept and Lithic Xeric Torriorthent). Soil under no-tillage had lower hydraulic conductivity (5.0 cm day⁻¹) than under subsoil tillage (15.5 cm day⁻¹) or minimum tillage (14.3 cm day⁻¹) during 1 of 2 years in continuous crop due to a reduction of soil porosity. Residue cover at sowing was greater under no-tillage (60%) than under subsoil or minimum tillage (<10%) in continuous crop. Under fallow, residue cover was low (10%) at sowing of the following crop for all tillage systems in both soils. Surface roughness increased with tillage, with a high value of 16% and decreasing following rainfall. Under no-tillage, surface roughness was relatively low (3–4%). Greater surface residue cover under no-tillage helped conserve water, despite indications of lower hydraulic conductivity. To overcome the condition of low infiltration and high evaporation when no-till fallow is expected in a cropping sequence, either greater residue production should be planed prior to fallow (e.g. no residue harvest) or surface tillage may be needed during fallow.

Introduction

In rainfed agriculture of arid and semiarid environments the ability of soil to store water plays an important role in the success of crops. Infiltration and evaporation are the most significant processes determining soil water storage.

Surface conditions play a major role in determining the rates of water infiltration and evaporation from soil. Tillage is the most effective way to modify the soil surface characteristics due to its effect on pore space (shape, volume and continuity of pores), residue cover and surface roughness. The specific soil management system that optimizes soil water storage is dependent on soil and climate (Godwin, 1990), because soil characteristics that define surface conditions can have contradictory effects on soil processes involved in water balance (infiltration, redistribution and evaporation). For example, high porosity and pore continuity are good characteristics for increasing soil water storage capacity and deep infiltration, but they also favour water evaporation from deeper soil layers. Residue cover increases infiltration and reduces evaporation but under extended dry conditions there are no differences from bare soil (Godwin, 1990). Surface roughness produced by tillage increases surface ponding, thus reducing surface runoff and also increasing the soil surface area exposed to evaporation. Soil mulch produced by tillage reduces evaporation from deep layers, but moist soil is drawn to the soil surface, producing water losses. Therefore, the effect of a specific soil management system on water balance will depend on soil characteristics and meteorological conditions.

The impact of different tillage systems on soil water dynamics, specifically infiltration has been well investigated (Ehlers and van der Ploeg, 1976, Ankeny et al., 1990, Pelegrin and Moreno, 1994, Logsdon and Kaspar, 1995) using rainfall simulators and ponded or tension infiltrometers. In general, infiltration is reported to be greater under no-tillage (NT) than in tilled soils (Ehlers and van der Ploeg, 1976, Radcliffe et al., 1988, Chan and Heenan, 1993, Azooz et al., 1996, McGarry et al., 2000) due to the larger number of macropores (Moran et al., 1988, Logsdon et al., 1990, Chan and Heenan, 1993, McGarry et al., 2000), increased fauna activity, which is responsible for many of these macropores (Blevins et al., 1983, Moreno et al., 1997, Logsdon and Kaspar, 1995, Suwardji and Eberbach, 1998) and accumulated organic matter forming a litter of residues (Radcliffe et al., 1988, Pikul and Zuzel, 1994, Golabi et al., 1995, Logsdon and Kaspar, 1995, Arshad et al., 1999). Disruption of macropore continuity by tillage is reported to reduce infiltration and hydraulic conductivity (Ehlers and van der Ploeg, 1976, Godwin, 1990, Logsdon et al., 1990). However, in other studies, infiltration and/or hydraulic conductivity was found to be lower under NT than inversion tillage (Pelegrin et al., 1988, Ferreras et al., 2000). Reasons for this may be increased bulk density (small porosity) found in NT soils and increased porosity produced by tillage (Pelegrin et al., 1990, Hubbard et al., 1994, Pelegrin and Moreno, 1994), which affects large pores in particular (Tebrügge and Düring, 1999).

Hydraulic conductivity was found to decrease during the growing season in tilled soils (Messing and Jarvis, 1993, Mwendera and Feyen, 1993, Logsdon et al., 1993) due to soil structural breakdown and surface sealing, and root growth that progressively blocks pores (Ankeny et al., 1990, Suwardji and Eberbach, 1998). For these reasons, saturated hydraulic conductivity can be greater for tilled than untilled soils at the beginning of the growing season due to increased porosity caused by tillage (Radcliffe et al., 1988, Hill, 1990, Suwardji and Eberbach, 1998), whereas it can be lower at the end of the season (López and Arrúe, 1997, Suwardji and Eberbach, 1998).

The use of crop residues to conserve soil and water in arid and semiarid areas is becoming more and more important. In fact, the decisive criterion for classifying tillage as conservation tillage is that 30% of soil is covered by residues after sowing (Unger et al., 1991, Blevins and Frye, 1993, Gilley, 1995). Crop residues protect the soil from raindrop impact (Unger and McCalla, 1980, Smika and Unger, 1986, Unger et al., 1991, Blevins and Frye, 1993, Gilley, 1995) reduce slaking of surface aggregates and prevent pore sealing and crust formation. Crop residues also increase soil aggregation and structural stability (Cannell and Hawes, 1994, Singh et al., 1994). Residues left over the soil slow the flow of surface runoff (Smika and Unger, 1986, Blevins and Frye, 1993, Gilley, 1995), increasing the opportunity for water to infiltrate (Godwin, 1990). The result of these factors is an increase in infiltration (McCalla and Army, 1961, Unger and McCalla, 1980, Potter et al., 1995). Crop residues also slow the rate of evaporation during the first stage (Bond and Willis, 1971, Smika and Unger, 1986, Godwin, 1990, Unger et al., 1991, Blevins and Frye, 1993) by isolating the soil from sun heating and air temperature, and increasing resistance to water vapor flux by reducing wind speed (Smika and Unger, 1986, Blevins and Frye, 1993). The increase in infiltration and the decrease in evaporation generally result in greater soil water storage, depending on amount of crop residues left on the soil surface (Unger and McCalla, 1980, Smika and Unger, 1986) and the duration of the dry period (McCalla and Army, 1961, Bond and Willis, 1971, Unger and McCalla, 1980, Godwin, 1990, Unger et al., 1991, Blevins and Frye, 1993).

The amount of crop residues on the soil surface varies with time (Ghidey and Alberts, 1993, Singh et al., 1994). The most important reasons for this variation are tillage and residue decay. Tillage modifies residue cover instantaneously and the percentage of residues left or buried after tillage according to tillage system and intensity has been reported (Blevins and Frye, 1993, Kok and Thien, 1994, Gilley, 1995). Residues decay with time following an exponential function (Steiner et al., 1994, Schomberg and Steiner, 1999) controlled basically by temperature and moisture (McCalla and Army, 1961, Steiner et al., 1994, Steiner et al., 1999, Schomberg and Steiner, 1999) as well as residue properties (McCalla and Army, 1961, Schomberg and Steiner, 1999) and number and variety of microorganisms in soil (McCalla and Army, 1961). Stroo et al. (1989) observed a rapid loss of residues in summer. Standing biomass seems to decompose more slowly than flat residues (Steiner et al., 1999).

Soil surface roughness is another soil property that influences water balance because it increases the depression storage capacity of soil (Mwendera and Feyen, 1993, Hansen et al., 1999), extending the time for infiltration to take place before runoff starts (Blevins and Frye, 1993). On agricultural lands, surface roughness is influenced by tillage, vegetation, soil type and previous amount and intensity of rainfall (Hansen et al., 1999). Tillage influences two of the four types of surface roughness stated by Römkens and Wang (1986): random roughness (non-directional surface variations due to cloddiness as a result of soil break-up by tillage implements) and oriented roughness (one-directional systematic differences in elevation due to farm implements). Though in general tillage increases surface roughness (Unger et al., 1991, Singh et al., 1994, Gilley, 1995), repeated tillage operations can also reduce it (Römkens and Wang, 1986). Rainfall reduces surface roughness (Singh et al., 1994, Gilley, 1995), especially the first rains after tillage due to breakdown and sloughing of soil clods upon wetting during rainstorms, consolidation of loosely tilled soil upon drying and soil erosion by drop impact and deposition into depressions (Römkens and Wang, 1986). Hyperbolic (Römkens and Wang, 1987) and exponential functions (Römkens and Wang, 1987, Eltz and Norton, 1997) have been used to model the decrease of surface roughness with cumulative rainfall. A large variation in surface roughness with time has been observed, especially in tilled soils. Under NT, surface roughness is low and runoff control depends on surface residues (Singh et al., 1994).

In summary, hydraulic conductivity, residue cover and surface roughness interact to determine the soil water balance, and can be modified by tillage and rainfall. The resulting effect of tillage on water balance depends on the site specific soil and climate characteristics and can only be assessed by local experiments. Few studies have measured these variables, especially in semiarid Mediterranean conditions.

The objective of this work was to investigate the effect of different tillage and cropping systems on hydraulic conductivity, residue cover and surface roughness to improve soil management for moisture conservation under semiarid Mediterranean conditions.