Transworld Research Network 37/661 (2), Fort P.O., Trivandrum-695 023, Kerala, India
Advances in Soil & Tillage Research, 2008: 203-217 ISBN: 978-81-7895-353-3 Editor: Andrea Formato
12
Environmental effects of soil tillage Fagnano Massimo and Quaglietta Chiarandà Fabrizio Dept. Agricultural Engineering and Agronomy, Naples University "Federico II" Via Università, 100 - 80055 Portici (Naples) Italy
Abstract Nowadays to soil tillage it has not only requested to improve crop yield, but also to contribute to mitigate environmental pollution. As regards the main effects of tillage on soil quality, the effects on soil organic matter (SOM) conservation, erosion control and soil compacting are presented. The impact of soil tillage on air quality is mainly related to its effect on oxidative activity into the soil. Tillage techniques that enhance oxidation could increase SOM degradation, thus increasing CO2 emissions. On the contrary, when tillage reduces soil porosity, it could increase anaerobic conditions, thus Correspondence/Reprint request: Prof. Fagnano Massimo, Dept. Agricultural Engineering and Agronomy, Naples University "Federico II" Via Università, 100 - 80055 Portici (Naples) Italy. E-mail:
[email protected]
204
Fagnano Massimo & Quaglietta Chiarandà Fabrizio
increasing denitrification and N2O, emissions. Soil techniques to reduce CH4 and ammonia emissions are also presented. Soil tillage effects on water quality are mainly related to the increase of oxidation and nitrification that lead to nitrate accumulation into the soil in a period characterized by leaching rainfalls and by low crop uptake (i.e. autumn in Mediterranean regions). Nevertheless it is not correct to generalize about the effects of soil tillage on environmental quality because they depend on complex interactions among tillage technique and the other components of cropping system (weather, soil features, crop sequence, ...). Therefore a systemic approach is necessary to understand all the effects of soil tillage in the different agro-climatic conditions.
1. Introduction The traditional function of soil is to allow the chemical, physical and biological conditions for a suitable plant growth and for better yield quantity and quality. In the last years, the worldwide spread of environmental problems has increased the importance of other soil functions related to its capacity to mitigate atmospheric pollution (i.e. greenhouse gases emissions) and water pollution (i.e. nitrate leaching). Among the crop management techniques, soil tillage is the more powerful in affecting soil properties and functions. Therefore, the roles of tillage must be revised in view of the new aims of the agriculture: not only to produce food, but also to produce services for environment. But soil tillage, cannot be considered separately from the other components of the agro-environmental context and of cropping system. The contrasting results reported in literature about the effects of soil tillage on yield and environmental quality are just due to the interactions of soil tillage with soil (texture, structure, SOM content) and weather conditions (rainfall distribution and intensity) and with cropping system structure (features of the species and their sequence, management of crop residues,...) that could affect the relative importance of the different factors in determining the effects [9].
2. Effects of tillage on soil quality The conservation of the productive potential of soils is a key factor in sustainability of agriculture and it is centred on the management of soil organic matter (SOM). SOM degradation is influenced by temperature, moisture, oxygen availability and it is accelerated by cultivation. Soil tillage is the technique that has the major impact on SOM degradation, since it can alter oxidative conditions, soil structure and natural biodiversity. As regards soil physical fertility, soil tillage shows contrasting effects: it is responsible of the
Environmental effects of soil tillage
205
structure degradation, because it enhances SOM degradation and directly destroys the largest aggregates (> 1 mm), but it also represents the main pool to increase soil macroporosity in clayey soils [48]. Conservation tillage (CT) techniques that minimise soil inversion and soil structure disruption, have been able to increase SOM in several world-wide researches [33], but complex strategies of conservation agriculture are necessary to counteract soil degradation due to cultivation. CT could help to reduce soil compacting, mainly if coupled with other husbandry techniques of “conservation agriculture” such as cover cropping and mulching or surface incorporation of crop residues [58]. In long term trials, the use in conjunction of different techniques, such as long rotations including permanent meadows that reduce soil tillage frequency, manuring, conservation tillage and crop residues restitution, allowed to increase SOM content [58]. SOM degradation also depends on soil texture: usually it is slower in clayey and in lime soils, since they promote the formation of more stable humus-clay bonds [71]. Direct drilling or no-tillage, in some cases improved SOM content and microbia biomass as compared with moldboard plow [39, 44], but in other cases these increases were limited to the top layer (10 cm), while SOM content decreased in the deeper ones [32]. In swelling/cracking vertisoils, no tillage caused an increase in SOM content reducing the formation of soil cracks and the consequent oxidation and degradation of SOM [53]. No-tillage can improve physical fertility thanks to its favourable effect on SOM conservation, but in clayey soils these positive effects are limited to the top layer (10 cm), while negative effects have been noticed in deeper layers (lower porosity and higher compaction) so determining reduced root growth and yield losses [42]. In humid environments (Scotland) no tillage caused yield losses due to lower permeability, waterlogging and weed increase [5]. In silty soils too, no-tillage determined porosity reduction and yield losses in wheat and maize [19]. But in sandy soils (sand > 65%), the porosity reduction did not affect root growth in deeper layers and increased yield thanks to a higher soil moisture retention [70]. Minimum tillage is also reported to be able to increase SOM content, but in some experiments it only changed organic matter distribution in the soil profile, increasing its concentration in the top layers and decreasing in the deeper ones, without changing SOM total content [16, 69]. In other experiments, minimum tillage caused an increase of SOM in the top layer, but also soil respiration, mineralization index and phosphatase activity increased. In other words, minimum tillage improves microbial activity both for the increase of SOM concentration and oxidative conditions. Therefore minimum tillage is not able to contribute to SOM conservation in long term, since it also stimulates its mineralization [56].
206
Fagnano Massimo & Quaglietta Chiarandà Fabrizio
In soils with 50% of sand, long term minimum tillage caused porosity increase with positive effects on yield of potato, wheat, burley, oat, but negative effects on yield of different brassica species for the increase in root rot [23]. In some other trials no difference in physical features of soils was noticed among tillage methods [59], probably thanks to the self-mulching activity of swelling soils. Deep moldboard plow is the more harmful tool for SOM conservation, since it increases oxidative conditions and incorporates the organic matter (crop residues or organic fertilizers) in a too deep layer, thereby diluting their effects in a too large soil volume [47]. It is also evident that the differences in the effects of soil tillage are due to the interaction with the soil type. Besides, the self mulching in swelling soils can reduce the effects of the different soil tillage techniques in a few months [3]. Often, soil compacting can affect soil quality so much severely to hide the effects of other techniques such as tillage type, organic fertilization or rotation biodiversity [62]. Therefore a higher attention must be paid to all the solutions that can allow to reduce soil compacting such as more large tools (to reduce the number of tractor passages on the field), more light tractors, tracked tractors, twin wheels or large-section wheels or also reducing the tire pressure.
3. Effects of tillage on erosion Soil tillage techniques are very important in conditioning erosion, since they could affect all the factors of soil erosivity (infiltration rate, soil roughness, aggregate stability). Different tillage methods have been developed (e.g. minimum tillage, sod seeding, ridge tillage), just with the aim to reduce erosion mainly increasing soil covering through crop residues [14]. In well draining soils, minimum tillage reduced turbidity coefficient and erosion, thanks to the effects of the increased SOM concentration in the top layers on the aggregates stability [68], while deep moldboard plow reduced water stable aggregates because of its effects on SOM degradation [47]. In Italian clayey soils, minimum tillage, mainly if made with rotary hoe, determined an increase in runoff and erosion in the more rainy years [15, 21, 25]. This could be ascribed to the increase of soil moisture in the top layers [34] that could be due to a higher and more prolonged soil covering from the crop residues [13, 52], or to the decrease of porosity and infiltration rate [31]. Therefore, in condition of rainy periods and poorly draining clayey soils, the soil moisture surplus caused by minimum tillage in the top layers can stimulate clay dispersion and aggregate stability reduction [47]. In these environments (clayey soils, rainy areas) the best results were obtained with soil tillage techniques that allow good infiltration and avoid sub-soiling
Environmental effects of soil tillage
207
compaction, such as the double-layer tillage made with a deep subsoiler or ripper (50 cm) coupled with harrow or disk in the top layer [10]. A lower effect of soil tillage has been noticed in spring-summer crops and in the more dry years [41] confirming that deep tillage is necessary in rainy years and in clay soils, because it allows water infiltration [17], while minimum tillage is more useful in sandy soils and in rainfed conditions [46]. Tillage direction too can affect runoff and erosion. Where the slopes are limited (
