Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage

Abstract

There is a strong trend toward reforestation of abandoned grasslands in alpine regions which may impact the carbon balance of alpine ecosystems. Here, we studied the effects of afforestation with Norway spruce (Picea abies L.) on an extensively grazed subalpine pasture in Switzerland on soil organic carbon (SOC) cycling and storage. Along a 120-year long chronosequence with spruce stands of 25, 30, 40, 45, and [120 years and adjacent pastures, we measured tree biomass, SOC stocks down to the bedrock, natural 13 C abundances, and litter quality. To unravel controls on SOC cycling, we have monitored microclimatic conditions and quantified SOC decomposability under standardized conditions as well as soil respiration in situ. Stocks of SOC were only moderately affected by the afforestation: in the mineral soil, SOC stocks transiently decreased after tree establishment, reaching a minimum 40-45 years after afforestation (-25 %) and increased thereafter. Soils of the mature spruce forest stored the largest amount of SOC, 13 % more than the pasture soils, mainly due to the accumulation of an organic layer (23 t C ha-1). By comparison, C accumulated in the tree biomass exceeded the SOC

Winter greenhouse gas fluxes (CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O) from a subalpine grassland

“…The significantly higher winter CO 2 fluxes in the grassland compared to the forest (Fig. 8) are consistent with the observation of decreasing summer soil respiration rates during afforestation of a subalpine pasture (Hiltbrunner et al, 2013). In their study the lower rates in the forest were explained with a smaller root turnover, a lower litter quality and a less favorable microclimate in the forest than in the adjacent grassland.…”

Above- and belowground patterns in a subalpine grassland-shrub mosaic

“…Other studies also reported higher contents of organic C in forest than in grasslands (Guo et al 2007;Guidi et al 2014). As a consequence of reduced soil temperature and moisture following forest succession (Hiltbrunner et al 2013), the decomposition of organic matter is slowed down, leading to a decreased incorporation of C into soil aggregates (Gregorich et al 2006). As discussed by Guidi et al (2014), such an increase of C content in the soil is a typical positive effect of transitional phases between grasslands and forest.…”

Shifts in the microbial community structure explain the response of soil respiration to land-use change but not to climate warming

“…While afforestation of moorland increased Rs, afforestation of grassland significantly reduced Rs. A number of previous studies reported reductions in Rs upon afforestation of pasture and cropland ScottDenton et al, 2006;Hiltbrunner et al, 2012Hiltbrunner et al, , 2013Wang et al, 2013;Zhang et al, 2013) that was attributed to low C inputs from tree to belowground communities. This observation is consistent with a previous report of considerably lower primary production and soil respiration in forest compared with grassland at the field scale (Schulze et al, 2010).…”