Impact of past and present land‐management on the C‐balance of a grassland in the Swiss Alps

Abstract

Grasslands cover about 40% of the ice-free global terrestrial surface, but their quantitative importance in global carbon exchange with the atmosphere is still highly uncertain, and thus their potential for carbon sequestration remains speculative. Here, we report on CO 2 exchange of an extensively used mountain hay meadow and pasture in the Swiss pre-Alps on high-organic soils (7-45% C by mass) over a 3-year period (18 May 2002-20 September 2005, including the European summer 2003 heat-wave period. During all 3 years, the ecosystem was a net source of CO 2 (116-256 g C m À2 yr À1 ). Harvests and grazing cows (mostly via C export in milk) further increased these C losses, which were estimated at 355 g C m À2 yr À1 during 2003 (95% confidence interval 257-454 g C m À2 yr À1 ). Although annual carbon losses varied considerably among years, the CO 2 budget during summer 2003 was not very different from the other two summers. However, and much more importantly, the winter that followed the warm summer of 2003 observed a significantly higher carbon loss when there was snow (133 AE 6 g C m À2 ) than under comparable conditions during the other two winters (73 AE 5 and 70 AE 4 g C m À2 , respectively). The continued annual C losses can most likely be attributed to the long-term effects of drainage and peat exploitation that began 119 years ago, with the last significant drainage activities during the Second World War around 1940. The most realistic estimate based on depth profiles of ash content after combustion suggests that there is an 500-910 g C m À2 yr À1 loss associated with the decomposition of organic matter. Our results clearly suggest that putting efforts into preserving still existing carbon stocks may be more successful than attempts to increase sequestration rates in such high-organic mountain grassland soils.

Peat decomposability in managed organic soils in relation to land use, organic matter composition and temperature

“…They also indicate that soils of our study sites were characteristic of European fens and resembled typical properties of managed organic soils (Berglund, 1995;Kechavarzi et al, 2010;Eickenscheidt et al, 2015;Krueger et al, 2015;Wüst-Galley et al, 2016;Brouns et al, 2016). Several studies assume that deeper layer peat of managed organic soils is less decomposed (Ewing and Vepraskas, 2006;Rogiers et al, 2008;Leifeld et al, 2011a, b;Krueger et al, 2015;Wüst-Galley et al, 2016). We therefore interpret the different SOM characteristics found in the topsoils of our samples as indicators of advanced decomposition triggered by drainage.…”

“…The samples were stored at 4 • C for up to 2 months until analysis. We applied the method of Rogiers et al (2008) to account for soil compaction during sampling for any sample and divided the cores into segments corresponding to 5-10 cm depth increments. This corresponded, depending on the type of soil corer used and length of the increment, to sample volumes of between 31 and 196 cm 3 per segment.…”

“…When peat becomes exposed to oxic conditions, mineralization seems to lead to relative enrichment of N, explaining why decreased C / N ratios are found in organic topsoils compared to undrained peat layers or bottom layers of drained organic soils (Malmer and Holm, 1984;Kuhry and Vitt, 1996;Krueger et al, 2015). While undisturbed organic soils have a low bulk density, drainage leads to subsidence processes and increasing bulk densities in the topsoils (Rogiers et al, 2008;Leifeld et al, 2011a, b).…”

Mitigating the greenhouse gas balance of ruminant production systems through carbon sequestration in grasslands

“…Therefore, net CO 2 release (F CO 2 , 0) is associated with organic-rich soils and heat stress. Indeed, a net CO 2 release was found with drained organic soils subjected to grazing in Switzerland and in New Zealand (Nieveen et al, 2005;Rogiers et al, 2008), and these sites were found to lose C (i.e. negative NCS; Table 1).…”

“…Carbon flux studies show that NCS is affected by a number of site-specific factors, including grassland type (newly established v. permanent, Byrne et al, 2005), N fertiliser supply , grazing pressure , drainage (Nieveen et al, 2005;Rogiers et al, 2008) and burning (Suyker and Verma, 2001) (Table 1). In addition, annual rainfall, temperature and radiation (Hunt et al, 2004;Ciais et al, 2005;Gilmanov et al, 2007;Soussana et al, 2007) play an important role for the variability in NCS between years and between sites.…”