Improving the vitality and productivity of forests on poor sandy soils by introducing rich-litter species.
The provision of forest ecosystem services heavily depends on the ecosystem function of nutrient cycling, in which the belowground ecosystem compartment plays a fundamental role. An essential pathway in such nutrient cycling is the return of nutrients to the soil via leaf litterfall. Although much soil ecological research has been executed throughout the years, our understanding of the mechanisms underlying belowground functioning remains partial, most likely due to the complexity and opacity of this nurturing underworld. In this thesis we aimed at contributing to such understanding in relation to leaf litter effects of different tree species. First, we sought to disentangle the interactions at the plant-soil interface and evaluate how abiotic and biotic factors underlie the context-dependency of litter effects. Second, this thesis further explores the concept of admixing rich litter species as a management measure to restore acidified sandy forest soils.
The first objective was addressed in two fundamental Chapters (2 and 3). Chapter 2 shows that the impact of forest conversion differs depending on the edaphic context, here a pedogenic threshold in soil acid buffering. We found that upon conversion from broadleaved mixed forest to Norway spruce monoculture, a shift in soil buffering domain occurred, triggering multiple concurrent changes including the build-up in the litter layer, an increase in acidity, a pronounced decrease in microbial functional diversity and earthworm biomass, a collapse of base saturation and even a change in the way carbon is sequestered. Subsequently we illustrated how burrowing earthworms can reinforce litter effects in some soils but not in others, using a dataset of European forests. The existence of a positive feedback loop in which burrowing earthworms maintain their own niche – by promoting turnover rate in the forest floor, thereby increasing topsoil pH and creating suitable living conditions for themselves – was evidenced for acidic forest soils. As most of our world’s forests are located on soils vulnerable to acidification, the regulation by pedogenic thresholds (Chapter 2) and the pivotal role of earthworms (Chapter 3) should be taken into account in future research efforts and when making forest management decisions, so that the typical context-dependent conclusions could be supported by more mechanistic and spatially explicit understanding.
In the second, more applied, part of this thesis (Chapter 4, 5 and 6) we built further on the concept of admixing rich litter species as a forest management measure to restore degraded forest soil systems, particularly those located on Pleistocene sandy deposits. In Chapter 4, we re-defined litter quality for West-European forests on Pleistocene aeolian deposits: rich litter is leaf litter that has high concentrations of calcium, magnesium and potassium and thereby promotes earthworm abundance and fast nutrient cycling. We found a negligible impact of leaf litter C/N ratio on the forest floor and belowground acidity status, which could be linked to the overload of nitrogen already present in the studied systems due to the high N-deposition in the region. Moreover, we identified which rich litter species have potential to counteract soil acidification, improve the soil nutrient status and promote fast nutrient cycling and corresponding organic matter turnover. Chapter 5 illustrates, in laboratory conditions, that admixing rich litter of black cherry with poor litter of pedunculate oak may promote the decomposition of oak itself via synergistic effects. Finally, Chapter 6 illustrates that when rich litter is admixed in-situ, large shares of high-quality litter (and thus tree admixture) are needed to have a considerable impact on the soil chemical status. This translates into high amounts of rich litter tree species admixture in terms of share in the basal area (>30%).
Further research is needed to evaluate whether, where and when the improved nutrient availability in the soil, as a consequence of rich litter admixture, feeds back to aboveground nutrient uptake and improved vitality of the overall ecosystem. Additionally, future studies should take into account root litter input, next to leaf litter, and further translate the process of nutrient cycling to soil carbon sequestration.
Taken together, this research shows the pervasive impact litter quality can have on belowground functioning and how site conditions (soil type) and earthworm activity can modulate that impact. Its outcomes also emphasize the potential for forest managers to use tree species selection (as a function of litter quality) for steering belowground functioning and counteracting soil acidification processes. Which could, in turn, affect the services provided by the aboveground forest ecosystem compartment.