Quality over quantity – primeval forests store carbon in the soil sustainably

Xin Xiong and two other researchers have investigated how leaf litter affects soil organic carbon. Currently, the assumption is that higher litter fall implies higher soil carbon. Litter quality is rarely taken into account.

In a 15-year (2000-2015) field phase in Guandong Province (China), the ratio of stable carbon isotopes (δ13C) in plants and soil organic carbon was determined in a pioneer and a virgin forest area. The primeval forest area has been strictly protected from anthropogenic influences for 400 years and consists of an evergreen monsoon deciduous forest. In the pioneer forest area, on the other hand, a plantation with the pine species Pinus massoniana was artificially established in the 1930s. Since then, there have been no more interventions in the area. Both forests are close to each other, are the same in soil slope and elevation, but differ in their successional stage and thus in soil type. The δ13C values were used to quantify the effects of vegetation on soil carbon and soil properties. Temporal changes in δ13C were used to determine the proportion of new (created 2000-2015) and old (created before 2000) carbon in total soil organic carbon.

Differences in soil

The carbon to nitrogen (C/N) ratio of litter in the virgin forest was lower than that in the pioneer forest. This means that more nitrogen is available to the plants in the primeval forest. Carbon is released during the decomposition of the litter. Some of this is carried into the soil as organic carbon. Although the difference between the annual litter fall in the forests was not significant, the absolute amount of carbon entered into the soil was significantly higher in the virgin forest (148.7±8.8 g C per m2 and year) than in the pioneer forest (99.7±4.5 g C per m2 and year). Furthermore, the carbon input in the pioneer forest was concentrated in the upper soil layers, whereas it was more evenly distributed in the virgin forest soil profile. The virgin forest soil also had a more favourable soil environment for carbon storage. Of importance for these results was not the amount of litter fall itself, but the amount of carbon introduced as an intermediate product of litter decomposition, which caused the significant differences in soil organic carbon.

The research group concludes that it is not the quantity of litter but its quality (i.e. lower C/N ratio and lignin content) that controls soil organic carbon accumulation. This could explain why old-growth forests sustainably store carbon in the soil. The results call into question carbon models for calculating soil carbon that consider biomass and litter fall alone.

Comment

In their study, Xiong et al. (2019) highlight the importance of differentiated considerations of biological processes. There is an interaction between soil and vegetation. After a period of 400 years, during which no human influence has been exerted on the composition and structure of a forest, it can be assumed that the potential natural vegetation has set in. An ecosystem with interactions at all levels, which ultimately leads to an even incorporation of carbon into the soil. It is generally known that soils are significantly larger carbon stores than the vegetation above them. This also applies to forests. In future, it should be questioned whether the equilibrium state of carbon storage through tree growth and death, which is feared and propagated in many quarters, is not indeed a desirable state for forests, since they can then store quantitatively more carbon in the soil. However, the study by Xiong et al. (2019) also makes it clear that when comparing commercial forests with virgin forests, it is important to use old virgin forest-like forests as a comparison and not former commercial forests that have only been declared a national park for a few years.