Forest liming in the face of climate change: the implications of restorative liming on soil organic carbon in mature German forests

Abstract. Forest liming is a management tool that has and continues to be used extensively across northern Europe to counteract acidification processes from anthropogenic sulfur and nitrogen (N) deposition. In this study, we quantified how liming affects soil organic carbon (SOC) stocks and attempt to disentangle the mechanisms responsible for the often-contrasting processes that regulate net soil carbon (C) fluxes. Using a paired-plot experimental design we compared SOC stocks in limed plots with adjacent unlimed control plots at 28 experimental sites to 60-cm soil depth in mature broadleaf and coniferous forests across Germany. Historical soil data from a subset of the paired experiment plots was analyzed to assess how SOC stocks in both control and limed plots had changed between 1990 and 2019.  Overall, we found that forest floor C stocks have been accumulating over time, particularly in the control plots. Liming however largely offsets this organic layer buildup, which means that nutrients remain mobile and are not bound in soil organic matter complexes. Results from the paired plot analysis showed that forest floor C stocks were significantly lower in limed plots than the control ( 34 %,  8.4 ± 1.7 Mg C ha-1), but did not significantly affect SOC stocks in the mineral soil, when all sites are pooled together. In the forest floor layers, SOC stocks exhibited an exponential decrease with increasing pH, highlighting how lime-induced improvements in the biochemical environment stimulate organic matter (OM) decomposition. Nevertheless, for both forest floor and mineral soils, the magnitude and direction of the belowground C changes hinged directly on the inherent site characteristics, namely, forest type (conifer versus broadleaf), soil pH, soil texture and the soil SOC stocks. On the other hand, SOC stock decreases were often offset by other processes that fostered C accumulation, such as improved forest productivity or increased carbon stabilization, which correspondingly translated to an overall variable response by SOC stocks, particularly in the mineral soil.  Lastly, we measured soil carbon dioxide (CO2) and soil methane (CH4) flux immediately after a re-liming event at three of the experimental sites. Here, we found that (1) liming doubles CH4 uptake in the long-term, (2) highlighted that soil organic matter mineralization processes respond quickly to liming, though the duration and size of the CO2 flush varied between sites, and (3) lime-derived CO2 contributed very little to total CO2 emissions over the measurement period.