Mineralisation, leaching and stabilisation of <sup>13</sup>C-labelled leaf and twig litter in a beech forest soil

Abstract

Abstract. Very few field studies have quantified the different pathways of C loss from decomposing litter even though the partitioning of C fluxes is essential to understand soil C dynamics. Using 0.75 kg m −2 of 13 Cdepleted leaf (δ 13 C = −40.8 ‰) and 2 kg m −2 of twig litter (δ 13 C = −38.4 ‰), we tracked the litter-derived C in soil CO 2 effluxes, dissolved organic C (DOC), and soil organic matter of a beech forest in the Swiss Jura. Autotrophic respiration was reduced by trenching. Our results show that mineralisation was the main pathway of C loss from decomposing litter over 1 yr, amounting to 24 and 31 % of the added twig and leaf litter. Contrary to our expectations, the leaf litter C was mineralised only slightly (1.2 times) more rapidly than the twig litter C. The leaching of DOC from twigs amounted to half of that from leaves throughout the experiment (2 vs. 4 % of added litter C). Tracing the litter-derived DOC in the soil showed that DOC from both litter types was mostly removed (88-96 %) with passage through the top centimetres of the mineral soil (0-5 cm) where it might have been stabilised. In the soil organic C at 0-2 cm depth, we indeed recovered 4 % of the initial twig C and 8 % of the leaf C after 1 yr. Much of the 13 C-depleted litter remained on the soil surface throughout the experiment: 60 % of the twig litter C and 25 % of the leaf litter C. From the gap in the 13 C-mass balance based on C mineralisation, DOC leaching, C input into top soils, and remaining litter, we inferred that another 30 % of the leaf C but only 10 % of twig C could have been transported via soil fauna to soil depths below 2 cm. In summary, over 1 yr, twig litter was mineralised more rapidly relative to leaf litter than expected, and much less of the twig-derived C was transported to the mineral soil than of the leaf-derived C. BothCorrespondence to: A. Kammer (adrian.kammer@wsl.ch) findings provide some evidence that twig litter could contribute less to the C storage in these base-rich forest soils than leaf litter.

Transformation and stabilization of pyrogenic organic matter in a temperate forest field experiment

“…We recovered >99% PyOM-C but only half of wood-C in the soil mesocosms after 10 months in situ averaged across all treatments, which is consistent with the results from the same site for wood (Kammer & Hagedorn, 2011;Jones et al, 2011). The loss of pine wood C corresponds to annual to decadal turnover times, which is similar to that estimated by a 180 days laboratory incubation of the same wood in two Alfisols of different parents material and by a 1 year litter experiment using 13 C labeled twigs (Beech tree, 1-8 mm) in the same field study area (Kammer & Hagedorn, 2011;Jones et al, 2011).…”

“…The PyOM application rate was based on a previous estimate of PyOM inputs to soil after a fire in a similar forest type (Eckmeier et al, 2007b). The amount of wood was based on estimation of twig and other wood contribution to the litter (Kammer & Hagedorn, 2011;Jones et al, 2011). Unamended-control mesocosms were similarly disturbed to those that received wood or PyOM.…”

Influence of land use and tillage depth on dynamics of soil microbial properties, soil carbon fractions and crop yield after conversion of short‐rotation coppices

“…Nevertheless, a decrease of the C:N ratio of harvest residues at all sites (Table ) indicates an advanced stage of decomposition of this fraction after 4 yrs. Likewise, fast mineralization rates of labelled twigs during the first year of decomposition in the forest floor of two different soils and a concurrent decrease of the C:N ratio of twigs from 95 to 51 was observed by Kammer & Hagedorn (). C:N ratio of the harvest residue under maize at site W‐P was less than that for grassland; this may be due to a greater amount of mineral N. Wachendorf () showed in an incubation experiment with poplar roots mixed in the soil of site W‐P, that small differences in mineral N content in soil decreased the C:N ratio of particulate organic matter, although no effect was observed on the total amount of carbon in that fraction.…”

“…Furthermore, a positive effect of reduced tillage on total microbial biomass and fungal biomass was observed in the topsoil (Murugan et al ., ). Woody plant residues with high C:N ratio and high lignin concentration are supposed to decompose at a slower rate than leaf litter, but high mineralization rates of labelled twigs were observed in a beech forest floor layer by Kammer & Hagedorn (). In this study, a first attempt was made to differentiate the origin of C between woody harvest residues of the SRC and the succeeding arable crop by planting maize after SRC and performing a 13 C analysis of harvest residues.…”

Fate and stability of dissolved organic carbon in topsoils and subsoils under beech forests

“…Even though DOC fluxes reaching the subsoil are small, their contribution to build up stabilised SOC in subsoils may be large (Hagedorn et al 2012;Kalbitz et al 2007). Isotopic labelling techniques are useful tools to follow the fate of DOC (Fröberg et al 2009;Hagedorn et al 2015;Kammer and Hagedorn 2011). Furthermore, laboratory experiments may be useful to identify distinct processes participating in the DOC turnover, but the combined effect of microbial turnover and flux conditions on the role and fate of DOC in subsoils can be only realistically quantified under field conditions.…”