Soil enzyme monitoring reveals increasing forest phosphorus demand in central Europe over the last decade

Forest ecosystems in central Europe are currently experiencing various environmental changes like ongoing nitrogen deposition, rising CO₂ levels and more frequent summer droughts, with potential impacts on biogeochemical processes in soils. Monitoring of soil properties, especially sensitive indicators like the activities of extracellular enzymes, enables studying the net effect of different simultaneously ongoing global changes on soil processes. Therefore, we measured the potential activities of four extracellular enzymes related to the C, N, P and S cycle (beta-glucosidase, N-actetyl-glucosaminidase, acid phosphatase and sulfatase)  of topsoils (0-10 cm of the mineral soil) from 150 forest plots under different management in three German regions as part of the Biodiversity Exploratories (https://www.biodiversity-exploratories.de/en/) project (Swabian Alb in the South, Hainich-Dün in the center and Schorfheide Chorin in the North of Germany) in May of 2011, 2014, 2017 and 2021. Analyzed soil samples were mixed samples composed of 14 soil cores (5 cm diameter) per plot, taken along two 40 m transects, sieved to < 2mm, and stored frozen before analyses.  Additional information on soil C, N and pH was obtained for the same samples.
Results revealed that the interannual variation of enzyme activities was about twice as high as that of soil organic carbon contents. Organic carbon, total nitrogen contents or soil pH showed no consistent trend over time across the regions. The same was true for the enzymes beta-glucosidase, N-acetyl-glucosaminidase and sulfates while acid phosphatase activity increased in all regions from 2011 to 2021 with the smallest absolute increase from on average 1290 to 2753 nmol MUF g⁻¹ dw h⁻¹ in the sandy and most acidic region Schorfheide Chorin, and the largest one in the loess-dominated silt-clay soils of the Hainich-Dün region (from 3474 to 5570 nmol MUF g⁻¹ dw h⁻¹).  Accordingly, the ratio of carbon-to-phosphorus acquiring enzymes declined with time. Plots following Moorhead et al. (2016, http://dx.doi.org/10.1016/j.soilbio.2015.10.01) indicated a consistent shift from N- to P-limitation across regions, independent of their total P contents and for both, coniferous and deciduous forests. Forest ecosystems seem to need to acquire more P from organic sources but given that both, plants and microorganisms can produce acid phosphatase, we are currently not able to say, if the increased phosphatase activity was a direct plant response or one (potentially mediated) by microorganisms. However, elevated CO₂ and N-deposition can both potentially lead to nutrient imbalances and thus increasing forest P requirements, and also summer droughts might reduce plant nutrient uptake, so that the observed trend might be an additive effect of all, rather than being attributable to one alone. As next steps we will test if changes in enzyme activities go along with changes in soil microbial communities and with leaf litter P contents.