Abstract
Introduction of temporary grasslands into cropping cycles could be a sustainable management practice leading to increased soil organic carbon (SOC) to contribute to climate change adaption and mitigation. To investigate the impact of temporary grassland management practices on SOC storage of croplands, we used a spatially resolved sampling approach combined with geostatistical analyses across an agricultural experiment. The experiment included blocks (0.4- to 3-ha blocks) of continuous grassland, continuous cropping and temporary grasslands with different durations and N-fertilizations on a 23-ha site in western France. We measured changes in SOC storage over this 9-year experiment on loamy soil and investigated physicochemical soil parameters. In the soil profiles (0–90 cm), SOC stocks ranged from 82.7 to 98.5 t ha−1 in 2005 and from 81.3 to 103.9 t ha−1 in 2014. On 0.4-ha blocks, the continuous grassland increased SOC in the soil profile with highest gains in the first 30 cm, while losses were recorded under continuous cropping. Where temporary grasslands were introduced into cropping cycles, SOC stocks were maintained. These observations were only partly confirmed when changing the scale of observation to 3-ha blocks. At the 3-ha scale, most grassland treatments exhibited both gains and losses of SOC, which could be partly related to soil physicochemical properties. Overall, our data suggest that both management practices and soil characteristics determine if carbon will accumulate in SOC pools. For detailed understanding of SOC changes, a combination of measurements at different scales is necessary.
Highlights
With 1500 Pg organic carbon stored in the first meter [1,2], soils are the largest terrestrial carbon reservoir
Lower fine and coarse silt content was found for soil organic carbon (SOC) gain sampling plots as compared to sampling plots showing SOC loss, while no differences were found for soil clay content (Table 5)
Spatial differences of SOC changes have been observed before and were found to reflect both topography and geological pattern [57]. While in this 23-ha experiment, climate, topography and vegetation may be excluded as controlling factors for contrasting SOC stock changes at the two different spatial scales, we found that differences in soil properties separated sampling points with SOC gain and loss (Figure 4)
Summary
With 1500 Pg organic carbon stored in the first meter [1,2], soils are the largest terrestrial carbon reservoir. Increasing soil organic carbon (SOC) storage has recently been promoted as negative emission technology, able to contribute significantly to climate change mitigation [3,4]. The C stored in soils can be a source of CO2 depending on soil management practices [2]. Soils have lost 116 Gt of organic carbon since the beginning of agriculture [5], and currently, agricultural activities are responsible for a quarter of all greenhouse gas emissions [6], which is partially due to the mineralization of SOC pools. One agricultural practice able to increase SOC storage may be the establishment of a grassland phase in crop rotations [9,10,11]. Grassland soils can sequester SOC at a rate of 0.5 Pg SOC yr−1 [12]
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