Carbon and nitrogen mineralization in temperate forest soils at low temperatures

In the past, carbon (C) and nitrogen (N) mineralization in temperate forest soils was often considered negligible at low temperatures during the dormant season, which is questioned by recent findings. Climate models predict warmer winters but little is known about the temperature sensitivity of C and N mineralization at low temperatures. Temperature rises in the lower range are of critical importance as the temperature dependency of microbial processes is enhanced at low temperatures. Besides microbial parameters, substrate availability and quality are known to influence the temperature dependency of decomposition. Around freezing point, soil frost may also affect the soil microbial biomass and reduce their activity, especially during a subsequent (dry) vegetation period. This study aimed at quantifying the contribution of overwinter net N mineralization to the annual net N mineralization in a temperate European beech and Norway spruce forest soil. Gross ammonification, gross nitrification, net N and C mineralization as well as the temperature response of these processes (Q10 value) were determined at low temperatures. The influence of substrate quality and availability on the temperature dependence was investigated. Furthermore, the effects of soil frost on C mineralization and microbial biomass during a subsequent desiccation period were investigated. Overwinter net N mineralization was determined in a in situ study using the sequential coring method over six months. Homogenized soil samples of Oi/Oe, Oa (spruce) and A (beech) horizons were incubated at -4, -1, +2, +5 and +8°C during laboratory incubations, gross ammonification and nitrification was determined with the 15N pool dilution technique and the Arrhenius equation was used to calculate temperature dependencies (laboratory experiment 1). Addition of glycine (in case of gross ammonification) and ammonium (in case of gross nitrification) enhanced the substrate availability in homogenized soil samples in laboratory experiment 2. The quantification of C mineralization and microbial biomass after soil samples were exposed to soil frost was conducted by GC measurements twice per week (C mineralization) and the substrate-induced respiration method (microbial biomass) on two time points in the 3rd laboratory experiment. During the dormant season, 44 kg N ha-1 6 months-1 were mineralized under beech and 11 kg N ha-1 6 months-1 under spruce, thereby contributing 30% (beech) and 15% (spruce) to the annual net N mineralization. Results from the laboratory incubations confirmed that considerable gross ammonification, nitrification and net N and C mineralization take place at low temperatures. Gross ammonification and C mineralization in beech Oi/Oe exceeded that of spruce Oi/Oe by a factor of 9 and 5. In deciduous forests, the leaf fall in autumn provides a huge amount of easily decomposable organic matter directly before the winter period, which is mineralized at winter temperatures. Apparent Q10 values of C and gross N mineralization were in the range of 2.4 to 11 and higher in substrates of high quality. This gives evidence that, besides substrate quality, substrate availability largely determines the temperature response of decomposition in our soils. The substrate availability experiment could affirm this assumption, as the addition of glycine raised the Q10 values of gross ammonification by a factor of 2. However, after glycine addition gross rates were erratically high. Likely, the glycine addition induced microbial growth which biased Q10 values and thus, they do not reflect “pure” temperature responses. Likewise, homogenization of soil samples increased substrate availability and Q10 values of net N mineralization were higher in homogenized than in undisturbed soil samples. The ratio of C mineralization to gross ammonification was narrow at low temperatures (~1), suggesting preferential mineralization of N rich organic substrates or rapid turnover of the N pool in microbial biomass. Gross and net nitrification were low at low temperatures and rates under spruce slightly exceeded rates under beech, suggesting a moderate risk of nitrate leaching in the spruce site. Microbial biomass and C mineralization quickly recover from soil frost (within 1-7 days) and all frost-related effects disappeared until day 90. Freeze-thaw cycles have no effects on C mineralization during a subsequent moderate desiccation phase. However - under optimal soil moisture conditions - frost-related effects may occur belated (after 90 days), impacting certain microbial groups and leading to a reduction of CO2 emissions in previously frozen soils. Generally, this work underlines the great importance of overwinter C and N mineralization as well as their large temperature sensitivity at low temperatures. Increasing winter temperatures are expected to have a huge effect on the C and N cycle in temperate forest soils but effects will decrease with soil depth, likely due to the decreasing substrate quality of the organic matter. Projected temperature changes in winter will particularly affect the C and N cycle in deciduous forests, in which the leaf fall in autumn provides a huge amount of easily decomposable organic matter directly before the dormant season.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Schütt, Marianne

Contributors dc:contributor

• Matzner, Egbert

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1860/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1860