How does tillage intensity affect soil organic carbon? A systematic review
The loss of carbon (C) from agricultural soils has been, in part, attributed to tillage, a common practice providing a number of benefits to farmers. The promotion of less intensive tillage practices and no tillage (NT) (the absence of mechanical soil disturbance) aims to mitigate negative impacts on soil quality and to preserve soil organic carbon (SOC). Several reviews and meta-analyses have shown both beneficial and null effects on SOC due to no tillage relative to conventional tillage, hence there is a need for a comprehensive systematic review to answer the question: what is the impact of reduced tillage intensity on SOC?
We systematically reviewed relevant research in boreo-temperate regions using, as a basis, evidence identified within a recently completed systematic map on the impacts of farming on SOC. We performed an update of the original searches to include studies published since the map search. We screened all evidence for relevance according to predetermined inclusion criteria. Studies were appraised and subject to data extraction. Meta-analyses were performed to investigate the impact of reducing tillage [from high (HT) to intermediate intensity (IT), HT to NT, and from IT to NT] for SOC concentration and SOC stock in the upper soil and at lower depths.
A total of 351 studies were included in the systematic review: 18% from an update of research published in the 2 years since the systematic map. SOC concentration was significantly higher in NT relative to both IT [1.18 g/kg ± 0.34 (SE)] and HT [2.09 g/kg ± 0.34 (SE)] in the upper soil layer (0–15 cm). IT was also found to be significant higher [1.30 g/kg ± 0.22 (SE)] in SOC concentration than HT for the upper soil layer (0–15 cm). At lower depths, only IT SOC compared with HT at 15–30 cm showed a significant difference; being 0.89 g/kg [± 0.20 (SE)] lower in intermediate intensity tillage. For stock data NT had significantly higher SOC stocks down to 30 cm than either HT [4.61 Mg/ha ± 1.95 (SE)] or IT [3.85 Mg/ha ± 1.64 (SE)]. No other comparisons were significant.
The transition of tilled croplands to NT and conservation tillage has been credited with substantial potential to mitigate climate change via C storage. Based on our results, C stock increase under NT compared to HT was in the upper soil (0–30 cm) around 4.6 Mg/ha (0.78–8.43 Mg/ha, 95% CI) over ≥ 10 years, while no effect was detected in the full soil profile. The results support those from several previous studies and reviews that NT and IT increase SOC in the topsoil. Higher SOC stocks or concentrations in the upper soil not only promote a more productive soil with higher biological activity but also provide resilience to extreme weather conditions. The effect of tillage practices on total SOC stocks will be further evaluated in a forthcoming project accounting for soil bulk densities and crop yields. Our findings can hopefully be used to guide policies for sustainable management of agricultural soils.
Agriculture, Conservation, Till, Plough, Farming, Land management, Climate change, Land use change, Carbon sequestration
Soils contain the greatest terrestrial carbon (C) pool on the planet. Since approximately 12 % of soil C is held in cultivated soils, management of these agricultural areas has a huge potential to affect global carbon cycling; acting sometimes as a sink but also as a source. Tillage is one of the most important agricultural practices for soil management and has been traditionally undertaken to mechanically prepare soils for seeding and minimize effects of weeds. It has been associated with many negative impacts on soil quality, most notably a reduction in soil organic carbon (SOC), although still a matter of considerable debate, depending on factors such as depth of measurement, soil type, and tillage method. No tillage or reduced intensity tillage are frequently proposed mitigation measures for preservation of SOC and improvement of soil quality, for example for reducing erosion. Whilst several reviews have demonstrated benefits to C conservation of no till agriculture over intensive tillage, the general picture for reduced tillage intensity is unclear. This systematic review proposes to synthesise an extensive body of evidence, previously identified through a systematic map.
This systematic review is based on studies concerning tillage collated in a recently completed systematic map on the impact of agricultural management on SOC restricted to the warm temperate climate zone (i.e. boreo-temperate). These 311 studies were identified and selected systematically according to CEE guidelines. An update of the original search will be undertaken to identify newly published academic and grey literature in the time since the original search was performed in September 2013. Studies will be critically appraised for their internal and external validity, followed by full data extraction (meta-data describing study settings and quantitative study results). Where possible, studies will be included in meta-analyses examining the effect of tillage reduction (‘moderate’ (i.e. shallow) and no tillage relative to ‘intensive’ tillage methods such as mouldboard ploughing, where soil is turned over throughout the soil profile). The implications of the findings will be discussed in terms of policy, practice and research along with a discussion of the nature of the evidence base.