DeStefano et al. look at soil C changes for different depths for agroforestry:
doi:10.1007/s10457-017-0147-9
From the abstract:
"The transition from agriculture to agroforestry significantly increased SOC stock of 26, 40, and 34% at 0–15, 0–30, and 0–100 cm respectively. The conversion from pasture/grassland to agroforestry produced significant SOC stock increases at 0–30 cm (9%) and 0–30 cm (10%). Switching from uncultivated/other land-uses to agroforestry increased SOC by 25% at 0–30 cm, while a decrease was observed at 0–60 cm (23%)."
The body of the paper has some confusing caveats:
According to our results and considering a full dataset with forest and uncultivated/other land-uses included, agroforestry revealed a significant and positive effect on SOC stocks at 0–30 and 0–100 cm depths. In the reduced dataset, the significant positive effect of agroforestry was observed at all depths, except for 0 to [greater than] 100 cm."
" Findings suggested that the conversion of agricultural land to agroforestry significantly increased SOC stocks at 0–15, 0–30, 0–100, but not at 0–60 and [greater than] 100 cm. ... the results for 0–60 and 0 to [greater than] 100 cm deviated from the general trend,
"Findings seems to support agroforestry: no significant differences were observed in the conversion from pasture/grassland to agroforestry (0–15, 0–30, 0–100, 0 to [greater than] 100 cm), while a significant increase was observed at 0–60 cm."
" Results indicated no significant difference (0–15, 0–100, 0 to [greater than] 100 cm), significant increase (0–30 cm), and significant decrease (0–60 cm) in SOC stocks. The conversion from uncultivated/other to agrisilviculture and agrosilvopastoral systems in SOC at 0–30 cm, while a significant reduction was observed in the transition from uncultivated/other to agrisilviculture at 0–60 cm."
"No significant differences in SOC stocks were detected in the conversion from forest plantation to agroforestry at 0–60 and 0–100 cm. Similarly, the land-use change from forest plantation to silvopasture did not produce significant results at the same depths. Unfortunately, the database for this category had observations only from a few studies"
" ... decreases in SOC stocks were observed in the conversion of pastures with conifers plantations, while little effects were reported in broadleaf plantations. "
"The conversion from forest to agroforestry lead to losses in SOC stocks in the top layers, while no significant differences were detected when deeper layers were included."
" ... the conversion from agriculture to agroforestry increased SOC stocks in most of the cases. Significant increases were also observed in the transition from pasture/grassland to agroforestry in the top layers, especially with the inclusion perennial in the systems, such as in silvopasture and agrosilvopastoral systems. Finally, the conversion from uncultivated/other land-uses to agroforestry produced inconsistent results ..."
The results are quite ambiguous, especially for larger depths. I attach Fig 10, for 0 to greater than 100cm
exhibiting that error bars are quite large.
sidd