Soil organic carbon and root distribution in a temperate arable agroforestry system
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To determine, for arable land in a temperate area, the effect of tree establishment and intercropping treatments, on the distribution of roots and soil organic carbon to a depth of 1.5 m.
A poplar (Populus sp.) silvoarable agroforestry experiment including arable controls was established on arable land in lowland England in 1992. The trees were intercropped with an arable rotation or bare fallow for the first 11 years, thereafter grass was allowed to establish. Coarse and fine root distributions (to depths of up to 1.5 m and up to 5 m from the trees) were measured in 1996, 2003, and 2011. The amount and type of soil carbon to 1.5 m depth was also measured in 2011.
The trees, initially surrounded by arable crops rather than fallow, had a deeper coarse root distribution with less lateral expansion. In 2011, the combined length of tree and understorey vegetation roots was greater in the agroforestry treatments than the control, at depths below 0.9 m. Between 0 and 1.5 m depth, the fine root carbon in the agroforestry treatment (2.56 t ha-1) was 79% greater than that in the control (1.43 t ha−1). Although the soil organic carbon in the top 0.6 m under the trees (161 t C ha−1) was greater than in the control (142 t C ha−1), a tendency for smaller soil carbon levels beneath the trees at lower depths, meant that there was no overall tree effect when a 1.5 m soil depth was considered. From a limited sample, there was no tree effect on the proportion of recalcitrant soil organic carbon.
The observed decline in soil carbon beneath the trees at soil depths greater than 60 cm, if observed elsewhere, has important implication for assessments of the role of afforestation and agroforestry in sequestering carbon.
KeywordsAgroforestry Roots Soil Carbon Carbon fractions Populus Carbon sequestration
The authors gratefully acknowledge the support of William Stephens in securing funding for the research and the help of Francois Clavagnier, Pascal Pasturel, and Julius Nkomaula in undertaking important fieldwork. The fractionation of the soil organic carbon was undertaken by Andy Gregory at Rothamsted Research. We also acknowledge support from Forest Research and the Scottish Forestry Trust during the writing up of this work.
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