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No-till systems: gains and drawbacks for carbon sequestration, ecosystem services and environmental health a
Ilan Stavi & Eli Argaman a
b
Dead Sea & Arava Science Center, Ketura 88840, Israel
b
Soil Erosion Research Station, Ministry of Agriculture & Rural Development, Hamakbim St., Rishon Lezion P.O.B. 30, Bet Dagan 50250, Israel Published online: 12 Aug 2014.
To cite this article: Ilan Stavi & Eli Argaman (2014) No-till systems: gains and drawbacks for carbon sequestration, ecosystem services and environmental health, Carbon Management, 5:2, 123-125 To link to this article: http://dx.doi.org/10.1080/17583004.2014.912828
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Editorial
No-till systems: gains and drawbacks for carbon sequestration, ecosystem services and environmental health
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Carbon Management (2014) 5(2), 123–125
Ilan Stavi1* & Eli Argaman2 Keywords: best management practices conservation agriculture crop residue management food security land use change pest control soil erosion soil organic carbon weed control
Changes in land use from native woodlands, shrublands or grasslands to agricultural lands have generated considerable modifications in hydrological systems, biodiversity [1] and nutrient cycling [2]. The conversion from natural lands to croplands has also had a detrimental effect on soil quality and functioning, specifically through the breakdown of macro-aggregates and the increased decomposition rates of soil organic carbon (SOC). First and foremost, these changes are attributed to tillage action, which leads to deformation of soil structure and exposes the previously protected SOC to oxidation. Also, excluding crop residues from the field decreases the replenishment of organic matter back into the system [3]. Because SOC determines soil quality and fertility as well as regulates a range of related ecosystem services, the diminished SOC pools result in both onsite and offsite degradation processes [4]. In the first decade of the 21st century, global cropland cover has been ~1.6 billion Ha [5]. Soils in most of these land areas have lost between 30 and 75% of their original SOC pools to the atmosphere in the form of CO2 [6]. Many times tillage has also resulted in a sharp increase in emissions of N2O from cropland soil [7], and as a result, these soils have become a net source
of GHGs. In addition to tillage, other farming practices, such as nutrient management and pest control, increase offsite and onsite emissions of GHGs related to agriculture. Together with CO2 emissions related to energy consumption in agricultural machinery work, emissions of GHGs have resulted in the agricultural sector being one of the main drivers of climate change [8]. Globally, agricultural lands account for ~25% of CO2 emissions, 50% of CH4 emissions and 70% of N2O emissions [9]. Also, conventional farming practices, of which crop residue exclusion and tillage are among the major factors, have led to land degradation due to the loss of enormous amounts of soil through erosion processes [3] and have had detrimental effects on related ecosystem services. A recent study estimated the global soil loss from agricultural lands to have reached 35 Pg year-1 [2]. Although still under debate, several studies have demonstrated that SOC lost during erosion processes is being oxidized and emitted as CO2 [10]. Also, agricultural run-off leads to redistribution of herbicides, pesticides and fertilizers over wide areas, thus increasing the risks of contamination to natural habitats and eutrophication at offsite water sources [11].
1
Dead Sea & Arava Science Center, Ketura 88840, Israel Soil Erosion Research Station, Ministry of Agriculture & Rural Development, Hamakbim St., Rishon Lezion P.O.B. 30, Bet Dagan 50250, Israel *Author for correspondence: Tel.: +972-8-630-6319; Fax: +972-8-635-6634; E-mail:
[email protected] 2
© 2014 Taylor & Francis
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The rise of conservation farming practices in several regions around the world has emerged in order to halt processes of onsite soil and land degradation and to contribute to soil restoration. Of the most prevalent conservation practices, the onsite retention of crop residues and the practice of no-till (NT) are especially notable. Soil conservation professionals widely accept retaining crop residues as an effective management tool. This method reduces soil-water evaporation and, at the same time, minimizes raindrop splash impact and diminishes the formation of sealed mechanical crusts on the ground surface. This results in greater infiltration capacity of water into the soil and smaller run-off generation and soil erosion [12]. Also, the decomposition of crop residues considerably replenishes the SOC pools. At the same time, following NT practice halts the disturbance of the soil structure, thus sustaining its hydraulic conductivity and water-retention capacity. Moreover, the lack of disturbance to the soil aggregates enables the build-up of considerable SOC pools, with the associated increase in soil microbial biomass and activity, and the improvement in soil health. As a consequence, over time, soils from these systems have the potential to become a net sink of carbon, with the potential capacity of mitigating climatic change [3]. NT systems, particularly, have been reported to efficiently control run-off and soil erosion [13]. The combined effect of these processes results in greater functioning and larger net primary productivity of the agro-ecosystem, with the related decreased environmental footprint of food production. Moreover, the overall reduced need for agricultural machinery work under NT systems than that under conventional tillage systems results in lesser energy consumption and lower onsite emissions of GHGs related to conservation farming activities [8]. Despite the positive effects of NT on SOC sequestration, soil erosion control and a range of ecosystem services, these farming conservation systems have been associated with some major drawbacks. For example, NT practice in Mediterranean and semi-arid regions may be beneficial in ‘normal’ years, but can considerably inhibit water access for crops in relatively dry years [14]. At the same time, NT may be undesired in poorly drained soils in temperate biomes, as it might result in a decrease of soil biological activity and a delay in seed germination [15]. This lowered biological activity in the soil may increase infestation of pathogens and pests [16], considerably augmenting the need to use fungicides and pesticides. For example, a recent study in Israel revealed that in rain-fed wheat (Triticum aestivum L. emend. Thell.) systems, NT tended to increase infestation by the pest, Phyllopertha nazarena [17]. Also, weed infestation under NT has been reported to be a serious competitor for crop productivity, necessitating the
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intensive application of herbicides [18]. Depending on the prevailing physical conditions, pesticides and herbicides may be prone to disperse over large areas, increasing the risk of environmental pollution. In addition, livestock manures or composts, applied to the ground surface as means of organic- or integrated-nutrient management, are also prone to spatial redistribution, risking the quality of down-slope water sources [8]. Furthermore, while considering the offsite emissions of GHGs related to production processes of pesticides and herbicides, the total net carbon sequestration under conservation farming systems becomes considerably smaller. Several agro-technical methodologies could be relevant for mitigating some of the adverse effects of NT systems, for instance, crop rotation, as well as winter cropping for regulating weed infestation in temperate biomes [18]. Devices for reduced tillage have been developed, aimed at, among others, reducing weed density. For example, occasional tillage, where inversion cultivation occasional tillage, where inversion cultivation is practiced to a frequency of between 3 and 5 years approximately, has been suggested as an effective means of controlling infestation by both pests and weeds. In properly managed agro-systems, livestock manure can be applied in conjunction with occasional tillage, augmenting nutrient availability in the rhizosphere. Regardless, however, the effect of the occasional tillage system on SOC pools is still controversial. For example in Nebraska, USA, it was reported that occasional tillage resulted in the vertical redistribution of SOC, but did not cause a decrease in the total SOC throughout the tilled layer [19]. However, in a recent study in Ohio, USA, occasional tillage was reported to decrease SOC at the shallow soil layer but not to increase SOC in deeper layers. In this same study, the physical and hydrological characteristics of the occasional tillage system were degraded compared to those of a continuous NT system [20]. Regardless, wise implementation of complementary conservation practices, either in conjunction with NT systems, reduced tillage systems or conventional farming systems, could potentially decrease the environmental risks imposed by croplands. Among these practices, particularly notable are the implementation of crop rotation, inter-cropping, cover cropping, perennial forages, agroforestry systems and biochar application in soil. Along with such conservation practices, implementation of precision agriculture procedures in nutrient and pest management would considerably reduce the utilization of excess inputs, further reducing risks for ecosystem services and environmental health. Research efforts therefore need to focus on: implementing comprehensive lifecycle assessments
No-till systems Editorial
and economic evaluations for different combinations of conservation agricultural practices, aimed at assessing both their impact on the environment and economic cost-effectiveness; and formulating user friendly models that would help farmers in determining the specific best management practices, which take into consideration a range of physical conditions and biotic
parameters at the very local scale. Funding of conversion to conservation practices should be supported by governments and rely on payments for accomplishing ecosystem services. Judicious support of such agricultural sector conversion would help governments to reduce the environmental footprint of agriculture, while strengthening food security.
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