Soil organic carbon and total nitrogen gains in an old growth deciduous forest in Germany.

Marion Schrumpf,Ernst-Detlef Schulze,Klaus Kaiser,Jose Luis Balcazar

doi:10.1371/journal.pone.0089364

Marion Schrumpf, Ernst-Detlef Schulze + Show 2 more

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https://doi.org/10.1371/journal.pone.0089364

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Abstract

Temperate forests are assumed to be organic carbon (OC) sinks, either because of biomass increases upon elevated CO2 in the atmosphere and large nitrogen deposition, or due to their age structure. Respective changes in soil OC and total nitrogen (TN) storage have rarely been proven. We analysed OC, TN, and bulk densities of 100 soil cores sampled along a regular grid in an old-growth deciduous forest at the Hainich National Park, Germany, in 2004 and again in 2009. Concentrations of OC and TN increased significantly from 2004 to 2009, mostly in the upper 0–20 cm of the mineral soil. Changes in the fine earth masses per soil volume impeded the detection of OC changes based on fixed soil volumes. When calculated on average fine earth masses, OC stocks increased by 323±146 g m−2 and TN stocks by 39±10 g m−2 at 0–20 cm soil depth from 2004 to 2009, giving average annual accumulation rates of 65±29 g OC m−2 yr−1 and 7.8±2 g N m−2 yr−1. Accumulation rates were largest in the upper part of the B horizon. Regional increases in forest biomass, either due to recovery of forest biomass from previous forest management or to fertilization by elevated CO2 and N deposition, are likely causes for the gains in soil OC and TN. As TN increased stronger (1.3% yr−1 of existing stocks) than OC (0.9% yr−1), the OC-to-TN ratios declined significantly. Results of regression analyses between changes in OC and TN stocks suggest that at no change in OC, still 3.8 g TN m−2 yr−1 accumulated. Potential causes for the increase in TN in excess to OC are fixation of inorganic N by the clay-rich soil or changes in microbial communities. The increase in soil OC corresponded on average to 6–13% of the estimated increase in net biome productivity.

Highlights

Terrestrial ecosystems are facing a number of environmental shifts, such as rising CO2 in the atmosphere, increasing temperature, changes in precipitation, increasing nitrogen deposition, or increased frequency of extreme weather events
The closer spatial correlation of organic carbon (OC) and total nitrogen (TN) stocks in deeper than in upper soil layers can be explained by the closer spatial dependence of the stones, and the fine earth contents in deeper than in upper soil layers (Table 2)
The location dependence of soil OC stocks demonstrates the validity of the used paired sampling approach

Summary

Introduction

Terrestrial ecosystems are facing a number of environmental shifts, such as rising CO2 in the atmosphere, increasing temperature, changes in precipitation, increasing nitrogen deposition, or increased frequency of extreme weather events. These factors can affect ecosystem carbon storage and thereby feed back to climate change [1,2]. A number of ecosystem manipulation experiments have been carried out to determine effects of changing environmental conditions on ecosystem processes [3]. Few results on repeated forest soil OC inventories exist to date [10]. Additional repeated inventories of forest soil OC stocks that include determination of all contributing variables are needed

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