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Working towards re-establishing chemical fertility in forest soils?

Quentin Ponette (Cahtolic University of Louvain) takes the floor

Acidification and eutrophication: towards a reestablishment of chemical fertility in forest soils?

Summary

The soil is a reservoir of mineral elements where trees and vegetation tape the resources they need. Changes in soil mineral content therefore closely reflect the risks of nutrient imbalance that atmospheric pollution and nutrient loss due to tree harvesting can cause or exacerbate.

The acid rain crisis at the beginning of the 1980s drew attention to the impact atmospheric pollutants were having on forests, often hundreds of kilometres from their emission points. In effect, the acidifying effect of sulphur and nitrogen deposits was causing soil impoverishment in terms of exchangeable elements, with real deficiencies appearing in the poorest, most acidic soils. Notably, the tree foliage yellowing widely observed in the Vosges Mountains reflected a magnesium deficiency (Landmann and Bonneau, 1995). Furthermore, an increase in atmospheric nitrogen deposits had a fertilisation effect which accentuated the risk of deficiency by creating and imbalance in tree mineral nutrition and by increasing the trees' need for other nutrient elements.

In the 1990s, the first studies were launched to measure changes in forest soil properties by re-sampling sites in north eastern France which had been previously sampled, up to 20 years before (Dupouey et al., 1998). Despite the fact that polluting sulphur and nitrogen emissions had already been perceptibly reduced, these studies found that there was a trend toward impoverishment in terms of exchangeable elements (particularly magnesium and calcium), that acidification had increased (lower rates of saturation in exchangeable base cations) and that nitrogen enrichment had occurred (a lower C/N ratio).

Since then, atmospheric sulphur and nitrogen deposits have continued to decrease, yet the question remains as to whether or not chemical soil fertility has been able to recover in forested areas.

When RENECOFOR was established in 1992, for the first time, measurements of changes in the physical/chemical properties of forest soils became available at the national scale. The 102 RENECOFOR sites include a wide range of ecological contexts in mainland France and soil analyses have been carried out twice, first between 1993 and 1995, then between 2007 and 2012. Strict methodological precautions were taken to ensure data comparability. Forest floor litter was sampled according to morphological horizon (OL, OF and OH) and the underlying mineral soil was sampled in successive layers (0-10 cm, 10-20 cm, 20-40 cm). Spatial variations in soil type within plots was accounted for with the same sampling design for each field campaign: at each plot, 25 samples divided equally among five permanent sub-plots (or "clusters") were taken, then combined into a composite sample. The physical and chemical properties of these composites were then analysed at the soil analysis laboratory (INRA, Arras) following identical procedures for both sampling campaigns.

The changes observed in the soil were surprising: they mainly occured in parameters associated with organic matter (organic carbon and total nitrogen). Organic carbon stocks increased significantly between the two sampling campaigns (+0.34 tC/ha/y on average). This increase occurred mostly in the surface layers (litter and mineral soil down to 10 cm in depth) while organic carbon levels remained stable in the deeper soil layers (mineral soil from 10 to 40 cm in depth). On the other hand, though total nitrogen levels increased slightly in the surface layers (litter and mineral soil down to 10 cm in depth), they clearly dropped in the deeper soil layers (mineral soil from 10 to 40 cm in depth). There was, therefore, a global decrease in nitrogen stocks, which, while only slightly significant statistically, was still more than the decline in atmospheric nitrogen deposits alone could explain (-11 kg/ha/y on average). This unexplained decrease in total nitrogen may have been caused by leaching or by greater nitrogen immobilisation in the biomass. Due to these changes in carbon (C) and nitrogen (N) stocks, the C/N ratio increased in a highly significant manner in all the soil layers whatever the ecological context (+2.6 units on average for the litter and mineral soil layers down to 40 cm combined). This indicates a change in the quality of the organic matter in forest soils, and possibly indicates that decomposition slowed and favoured storage stability over time.

As for acidification, the phenomenon continued in the most acidic soils and in those most sensitive to acidification (pH H20 < 4.5). For these soils, pH decreased in each mineral layer, and the base saturation percentage also fell for global stocks of exchangeable cations in the 0 to 40 cm layers. However, this acidification did not bring about an impoverishment in absolute terms of these most acidic soils. Their exchangeable magnesium and potassium reserves actually increased during the same time period. This apparently contradictory trend appears essentially in the 0-to-10-cm mineral layer where the global increase in cation exchange capacity paralleled an increase in stored organic matter.

These results highlight the importance of organic matter dynamics, not only in terms of carbon sequestration but also for trends in forest soil chemical fertility. When associated with the other parameters measured on the same plots, these analyses make up an exceptional data set which can help us improve our understanding of the carbon and nutrient cycles and build more accurate models to reflect them.

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