What controls fine root C turnover and stabilization in temperate ecosystems?

Background/Question/Methods

Soils are the largest active terrestrial reservoir of the global carbon (C) cycle. Fine roots (here defined as < 2 mm in diameter) significantly contribute to the formation of soil C, but their decomposition rates have been hardly studied in most ecosystem types. Changes in environmental site conditions, such as climate, land management and biodiversity as well as variations in litter quality influence fine root decomposition rates. However, the partial effects of these factors on fine root decomposition have been poorly quantified across soil profiles. Therefore, to answer these research gaps we performed large scale fine root decomposition experiments located in three German regions. Each region differs in climate and soil properties. In total 5112 litterbags with a mesh size of 100 micrometres were distributed between 5 and 35 cm soil depth. Moreover, we were able to quantify the contribution of fine root C to the bulk soil, the mineral associated and dissolved organic matter in temperate grasslands. This was quantified by adding 1) isotopically depleted in ¹⁴C and 2) unlabeled -control- fine root litter in field mesocosms.

Results/Conclusions

Our results show that after one year, fine root decomposition in temperate grasslands (24 ± 6 %) is twice as high as in temperate forests (12 ± 4 %, p<0.001). Soil moisture, soil temperature and soil nutrient contents explain most of the variability in fine root decomposition (34% in grasslands and 24% in forests). Additional variation is explained by the root lignin:N ratio (15% in grasslands and 11% in forests). Fine root decomposition rates over two years did not vary with soil depth. In grasslands about 1/3 of the total root C added with the isotopically depleted ¹⁴C root litter resided in the soil and of this between 16 and 26 % was stabilized in the mineral associated organic matter. These findings indicate that organic C may turnover faster in grasslands than in forests. They also demonstrate that climatic or other large-scale differences are more important for initial stages of litter decomposition than changes in environmental conditions with soil depth. Therefore a similar potential to decompose fine root litter may occur in top and subsoil horizons. A significant portion of mineral associated organic C was observed to be formed from fine root organic substrates in grasslands.