Dr. Frank Hagedorn

Wälder sind eine wichtige Kohlenstoffsenke, stehen jedoch durch sich häufende Klimaextreme wie Hitze und Trockenheit zunehmend unter Druck. Nach 25 Jahren Monitoring an den 19 Standorten des Programms «Langfristige Waldökosystem-Forschung» zeigen sich Veränderungen in der Waldstruktur und im Wachstum. Die Wälder wurden generell dichter und älter. Das Wachstum nahm sowohl in den Tief- als auch in den Hochlagen ab, vor allem bei Fichte, Tanne und Buche. Das Wachstum korreliert auf Einzelbaum- und auf Bestandesebene mit Bestandesstruktur (Bestandesdichte und Alter) und negativ mit Temperatur und Trockenheit. Verglichen mit den 1990er-Jahren weisen die Wälder an den untersuchten Standorten heute eine geringere Produktivität bei gleicher Bestandesdichte auf, was auf eine Abnahme der Standortskapazität hinweist.

25 years of growth development in 19 Swiss forests under increasing drought
Forests are an important carbon sink but are under increasing pressure due to more frequent climate extremes such as heat and drought. After 25 years of monitoring at the 19 sites of the Long-Term Forest Ecosystem Research programme (LWF), changes in forest structure and growth are evident. The forests have generally become denser and older. Growth decreased in both low- and high-altitude sites, especially for spruce, fir and beech. Growth was correlated with stand structure (stand density and age) at individual tree and stand level, as well as negatively with temperature and drought. Compared to the 1990s, the LWF forests today show lower productivity at the same stand density, which indicates a decrease in site capacity.

Schweizer Waldböden speichern mit 143 Tonnen pro Hektar die grössten Mengen an Kohlenstoff aller europäischer Wälder. Dieser hohe Vorrat hat sich über Jahrtausende aufgebaut und ist wahrscheinlich auf die feuchten und kühlen Klimaverhältnisse zurückzuführen. Auf den Flächen des Projekts «Langfristige Waldökosystem-Forschung» besteht eine negative Beziehung zwischen Waldbiomasse und Bodenkohlenstoff, weil bei günstigen Wuchsverhältnissen, etwa in einem warmen Klima, mehr Bodenkohlenstoff abgebaut wird. Bodenminerale, die die Stabilisierung von Kohlenstoff steuern, haben die grösste Bedeutung für den Kohlenstoffvorrat im Boden. Schweizer Waldböden weisen unter trockeneren und wärmeren Klimabedingungen niedrigere Kohlenstoffvorräte auf, was auf potenzielle Kohlenstoffverluste in einem zukünftigen Klima hindeutet.

Das Programm «Langfristige Waldökosystem-Forschung» in der Schweiz liefert detaillierte Ergebnisse zu den Auswirkungen von Luftverschmutzung und Klimaänderung auf die Waldböden und Baumernährung. Seit den 1980er-Jahren sind die Emissionen von Schwefeldioxid und Stickoxiden in Mitteleuropa signifikant zurückgegangen. Diese Trends können auch für die Immissionen in der Schweiz bestätigt werden. Die Monitoringdaten zeigen, dass die Auswaschung von Sulfat aus dem Bodenprofil zurückgegangen ist. Auch die Stickstoffauswaschung nahm grösstenteils ab, jedoch nicht an Standorten mit weiterhin hohen Stickstoffeinträgen, wo sogar ansteigende Trends beobachtet wurden. Daher bleibt die Stickstoffbelastung trotz der insgesamt positiven Entwicklungen ein Problem. Ein weiterer kritischer Befund ist die fortschreitende Bodenversauerung, die sich in abnehmenden pH-Werten und niedrigen Verhältnissen von basischen Kationen zu Aluminium zeigt. Die Böden puffern die sauren Depositionen durch die Auswaschung von Nährstoffen und die Freisetzung von Aluminium. Diese Veränderungen in der Bodenlösungschemie können lange anhalten und die Nährstoffverfügbarkeit für die Bäume beeinträchtigen. Dies ist einer der Faktoren, die zu einem signifikanten Rückgang wichtiger Nährstoffe wie Stickstoff, Phosphor und Schwefel in den Blättern geführt haben. Dies weist auf eine Verschlechterung der Baumernährung hin. Zusammengefasst zeigen die Ergebnisse, dass trotz des Rückgangs saurer Depositionen die langfristigen Auswirkungen auf die Waldböden und die Baumernährung bestehen bleiben, was sich auf die Gesundheit und Vitalität der Schweizer Wälder auswirken kann.

Trends in atmospheric deposition and effects on soil water quality and tree nutrition
Long-term forest ecosystem research (LWF) in Switzerland provides detailed results on the impacts of air pollution and climate change on forest soils and tree nutrition. Since the 1980s, sulphur dioxide and nitrogen oxides emissions have significantly decreased in Central Europe. These trends can also be confirmed for emissions in Switzerland. Monitoring data show that sulphate leaching from the soil profile has decreased. Nitrogen leaching has also mostly declined, but not at sites with persistently high nitrogen inputs, where even increasing trends have been observed. Therefore, nitrogen deposition remains a problem despite the overall positive developments. Another critical finding is the ongoing soil acidification, reflected in decreasing pH values and low ratios of base cations to aluminium. The soils buffer the acidic deposition through nutrient leaching and aluminium release. These changes in soil solution chemistry can persist for a long time and impair nutrient availability for trees. This is one of the factors contributing to a significant decline in key nutrients such as nitrogen, phosphorus, and sulphur in the leaves. In summary, the results show that despite the reduction in acidic deposition, the long-term impacts on forest soils and tree nutrition persist, potentially affecting the health and vitality of Swiss forests.

Purpose: The purpose of the study was to clarify the relevance of colloidal phosphorus (P_coll) to total P (TP) leaching and its link to dissolved organic P (DOP) leaching in forest topsoils under field conditions. Effects of heavy rainfall after a drought and the influence of P and N inputs to forest soils on P_coll leaching from the topsoil were assessed to better understand the causes of P_coll leaching.
Methods: We carried out an irrigation experiment with zero-tension lysimeters collecting leachate from the organic forest floor layers and mineral topsoils of a sandy, low-P and a loamy, high-P soil.
Results: The contribution of P_coll to TP concentrations was on average around 10±14% irrespective of sampling date, season and soil. The highest P_coll concentrations were recorded upon irrigation after a summer dry period with the increase in concentrations being most pronounced for the 20–70 nm size fraction with low median C:P ratios (29 to 31) typical for microbial residues. Significant relationships between P_coll and DOP concentrations suggested that 27% (low P soil) and 17% (high P soil) of DOP were of colloidal nature. Independent of the soil P stock, the addition of P and P + N affected neither P_coll concentrations nor their share of TP significantly.
Conclusion: While P_coll hardly affected the overall estimates of beech forest P budgets, it provides important further insights into processes governing the mobilization, transport, and cycling of P in forest ecosystems.

• The absolute carbon stock of forest biomass has remained almost constant over the past decade.
• At 269 tonnes of carbon per hectare, Swiss forests have the highest relative carbon stock in Europe. The carbon stock is around 119 tonnes per hectare in living trees and around 150 tonnes per hectare in deadwood, the organic layer and soil.
• Forests and wood use will continue to play a role in tackling climate change, but this will require resilient and climate-adapted forests.

• Der absolute Kohlenstoffvorrat der Waldbiomasse ist in der letzten Dekade nahezu konstant geblieben.
• Mit 269 Tonnen Kohlenstoff pro Hektare hat der Schweizer Wald den höchsten relativen Kohlenstoffvorrat in Europa. In den lebenden Bäumen beläuft er sich auf rund 119 Tonnen pro Hektare. In Totholz, organischer Auflage und Boden sind es rund 150 Tonnen pro Hektare.
• Auch in Zukunft sollen der Wald und die Verwendung von Holz zur Bewältigung des Klimawandels beitragen. Um dies zu gewährleisten, sind resiliente und klimaangepasste Wälder nötig.

• In termini assoluti, le riserve di carbonio legate alla biomassa del bosco sono rimaste costanti nell'ultimo
decennio.
• Con 269 tonnellate di carbonio per ettaro, il bosco svizzero ha il valore relativo di riserva di carbonio più alto di tutta Europa. Gli alberi vivi ne contengono circa 119 tonnellate per ettaro; nel legno morto, nell'orizzonte organico e nel suolo questo valore sale a circa 150 tonnellate.
• Anche in futuro il bosco e l'utilizzazione di legname devono contribuire a ridurre le conseguenze del cambiamento climatico. Per questo sono necessari boschi resilienti e adattati al clima.

• Les réserves de carbone absolues de la biomasse forestière sont restées quasi constantes durant la dernière décennie.
• Avec 269 tonnes de carbone par hectare (t C/ha), la forêt suisse contient les stocks relatifs les plus élevés d'Europe. Environ 119 t C/ha sont stockées dans les arbres vifs, et 150 t C/ha dans le bois mort, l'horizon organique et le sol.
• La forêt et l'utilisation du bois devront encore contribuer à l'avenir à la gestion des changements climatiques. Seules des forêts résilientes et adaptées au climat peuvent le garantir.

Wälder tragen zur Erneuerung des Grundwassers in Trinkwasserqualität bei. Wiederholte Untersuchungen zeigen, dass die hohe Stickstoffdeposition immer noch zu erhöhten Konzentrationen von Nitrat im Sickerwasser aus Wäldern führen kann. Zu hohe Nitratwerte im Trinkwasser können die Gesundheit für Mensch und Umwelt beeinträchtigen. Die Bodeninventur auf fünf ausgewählten Flächen des Programms «Langfristige Waldökosystem-Forschung» zeigt, dass die Stickstoffsättigung auf einem Teil der Flächen, trotz Rückgang der Einträge, eher zugenommen hat. Dadurch sinkt die Fähigkeit von Böden, Stickstoff aufzunehmen, womit das Risiko erhöhter Nitratauswaschung tendenziell steigt.

Reflecting recent advances in our understanding of soil organic carbon (SOC) turnover and persistence, a new generation of models increasingly makes the distinction between the more labile soil particulate organic matter (POM) and the more persistent mineral-associated organic matter (MAOM). Unlike the typically poorly defined conceptual pools of traditional SOC models, the POM and MAOM soil fractions can be directly measured for their carbon content and isotopic composition, allowing for fraction-specific data assimilation. However, the new-generation model predictions of POM and MAOM dynamics have not yet been validated with fraction-specific carbon and ¹⁴C observations. In this study, we evaluate five influential and actively developed new-generation models (CORPSE, MEND, Millennial, MIMICS, SOMic) with fraction-specific and bulk soil ¹⁴C measurements of 77 mineral topsoil profiles in the International Soil Radiocarbon Database (ISRaD). We find that all five models consistently overestimate the ¹⁴C content (Δ¹⁴C) of POM by 69 ‰ on average, and two out of the five models also strongly overestimate the Δ ¹⁴C of MAOM by more than 80 ‰ on average, indicating that the models generally overestimate the turnover rates of SOC and do not adequately represent the long-term stabilization of carbon in soils. These results call for more widespread usage of fraction-specific carbon and ¹⁴C measurements for parameter calibration and may even suggest that some new-generation models might need to restructure or further subdivide their simulated carbon pools in order to accurately reproduce SOC dynamics.

Forests are under pressure and going through rapid changes. However, current inventorying and monitoring (IM) programs are often either disjointed, too narrow in their scope and/or do not operate at fine enough temporal resolutions, which may hinder scientific understanding, the timely supply of information, fast decision making, and may result in the sub-optimal use of resources. For these reasons, there is an urgent need for Advanced Forest Inventorying and Monitoring (AIM) programs to (i) achieve expanded relevance (by augmenting data/information across ecosystem properties and trophic levels), (ii) have increased temporal resolution (by tailored data collection frequency), and (iii) make use of technological advances (by incorporating novel tools and technologies). The Advanced Inventorying and Monitoring for Swiss Forests (SwissAIM) initiative was launched in 2020 to address these needs. SwissAIM builds upon the foundation offered by the existing programs (e.g., national forest inventory, long-term forest ecosystem research, biodiversity monitoring). It aims to offer a collaborative and adaptive framework to enable integrated data collection, evaluation, interpretation, analysis, and modeling. Ideally, it will result in a more responsive system with respect to current and predicted biotic/abiotic stressors that will challenge Swiss forests. Developing such a system implies identifying the information needs of different stakeholders (e.g., science, policy, practice), related technical requirements, and governance frameworks. Here, we present (i) the main features of the SwissAIM initiative (vision, scientific questions and variables, governance and engagement), (ii) the main outcomes of the participatory design process (measurements, sampling, and plot design), (iii) the potential transferability of AIM initiatives outside Switzerland (timing, relevance, practicability), and (iv) the key messages that emerged (i.e., need for advancement, integration and transdisciplinarity, statistical underpinning). Since similar needs related to forest inventorying and monitoring are emerging throughout Europe and elsewhere, the objective of this opinion paper is to share our experience and promote a dialog with those interested in developing AIM initiatives in other countries and regions.

Treelines advance due to climate warming. The impacts of this vegetation shift on plant-soil nutrient cycling are still uncertain, yet highly relevant as nutrient availability stimulates tree growth. Here, we investigated nitrogen (N) and phosphorus (P) in plant and soil pools along two tundra-forest transects on Kola Peninsula, Russia, with a documented elevation shift of birch-dominated treeline by 70 m during the last 50 years. Results show that although total N and P stocks in the soil-plant system did not change with elevation, their distribution was significantly altered. With the transition from high-elevation tundra to low-elevation forest, P stocks in stones decreased, possibly reflecting enhanced weathering. In contrast, N and P stocks in plant biomass approximately tripled and available P and N in the soil increased fivefold toward the forest. This was paralleled by decreasing carbon (C)-to-nutrient ratios in foliage and litter, smaller C:N:P ratios in microbial biomass, and lower enzymatic activities related to N and P acquisition in forest soils. An incubation experiment further demonstrated manifold higher N and P net mineralization rates in litter and soil in forest compared to tundra, likely due to smaller C:N:P ratios in decomposing organic matter. Overall, our results show that forest expansion increases the mobilization of available nutrients through enhanced weathering and positive plant-soil feedback, with nutrient-rich forest litter releasing greater amounts of N and P upon decomposition. While the low N and P availability in tundra may retard treeline advances, its improvement toward the forest likely promotes tree growth and forest development.

The microbial quotient of soil, defined as the proportion of soil elements present in soil microorganisms, reflects the capacity of microbial communities to regulate processes like organic matter decomposition and nutrient cycling. These processes are vital for ecosystem functioning, supporting plant growth, and maintaining soil fertility. However, the global scale's magnitude and drivers of soil microbial quotients remain elusive. This study complied a comprehensive dataset comprising ratios of microbial biomass carbon to soil organic carbon (MBC/SOC), microbial biomass nitrogen to total nitrogen (MBN/TN), and microbial biomass phosphorus to total phosphorus (MBP/TP) from 1394 published studies until 2022 to investigate global patterns of soil microbial quotients using a random forest model. The data showed that the modeled global means for MBC/SOC, MBN/TN, and MBP/TP at 0–30 cm soil depth were 2.2 %, 3.5 %, and 7.0 %, respectively. Climatic factors, including relative humidity, mean annual temperature (MAT), and mean annual precipitation (MAP), were identified as the most important drivers of MBC/SOC at the global scale. Modeled MBC/SOC showed higher values in tropical/subtropical biomes than in temperate and boreal biomes. Conversely, MBN/TN and MBP/TP were higher in boreal forests and tundra than in tropical/subtropical biomes, possibly reflecting nitrogen and phosphorus limitations as well as nutrient recycling in microbial biomass in cold regions. In addition, MBC/SOC displayed obvious seasonal variations, with the highest values in spring (2.4) and the lowest in winter (1.9). This variation is likely a result of the higher temperature sensitivity of soil microbial biomass compared to soil organic carbon. Overall, the findings of this study significantly enhance our understanding of the mechanisms driving soil organic matter and nutrient cycling across terrestrial ecosystems. It also supplies critical parameters that could improve the modeling of soil nutrient cycling.

The link between above- and belowground communities is a key uncertainty in drought and rewetting effects on forest carbon (C) cycle.
In young beech model ecosystems and mature naturally dry pine forest exposed to 15-yr-long irrigation, we performed ¹³C pulse labeling experiments, one during drought and one 2 wk after rewetting, tracing tree assimilates into rhizosphere communities.
The ¹³C pulses applied in tree crowns reached soil microbial communities of the young and mature forests one and 4 d later, respectively. Drought decreased the transfer of labeled assimilates relative to the irrigation treatment. The ¹³C label in phospholipid fatty acids (PLFAs) indicated greater drought reduction of assimilate incorporation by fungi (−85%) than by gram-positive (−43%) and gram-negative bacteria (−58%).
¹³C label incorporation was more strongly reduced for PLFAs (cell membrane) than for microbial cytoplasm extracted by chloroform. This suggests that fresh rhizodeposits are predominantly used for osmoregulation or storage under drought, at the expense of new cell formation. Two weeks after rewetting, ¹³C enrichment in PLFAs was greater in previously dry than in continuously moist soils. Drought and rewetting effects were greater in beech systems than in pine forest.
Belowground C allocation and rhizosphere communities are highly resilient to drought.

Soil microbial growth rate and microbial carbon use efficiency (CUE) are critical parameters of soil microbial carbon metabolism, moderating soil organic carbon (SOC) dynamics. However, global patterns of soil microbial growth rate and microbial CUE are still unresolved. Here, we show that the metabolic theory of ecology (MTE) can be applied to model soil microbial growth rate at the global scale as a function of microbial biomass, temperature, and SOC contents. Rates of soil microbial growth were modeled at depths of 0–30 cm and calibrated against rates measured with the ¹⁸O-labeled H₂O incubation method. The modeled soil microbial growth rates were strongly driven by temperature. They decreased with latitude and had greater seasonal variations in tundra and boreal forest compared to tropical biomes. Soil microbial CUE (0–30 cm) ranged from 0.25 to 0.63 among global biomes, averaging at 0.43. Modeled annual soil microbial growth rates followed the same global patterns and were on the same order of magnitude as other key ecosystem C fluxes such as net primary productivity, litterfall, and heterotrophic respiration. This indicates a strong functional linkage of aboveground and belowground communities at the global scale. Our MTE-based approach provides the first estimates of global patterns for soil microbial growth rate and microbial CUE and potentially provides a powerful mechanistic framework to incorporate soil microbes into Earth System Models.

Low temperatures near alpine treelines limit microbial release of soil nitrogen and tree growth. Ectomycorrhizal fungi can increase nitrogen supply for trees, but the importance of this exchange of carbon for nitrogen at the treeline remains unclear. Our bomb radiocarbon measurements indicated that trees transferred carbon fixed <2 years previously to fungi. The allocated carbon likely included sugars involved in starch synthesis, as δ¹³C in fungal caps closely resembled that of fine-root starch. Mass balance of nitrogen isotopes along the plant-fungi-soil continuum revealed that Larix decidua trees relied less on fungal nitrogen (0–35% of N uptake), compared to Pinus mugo trees (up to 41%). We estimated that treeline trees allocate up to 18% of photoassimilates to fungi. Our findings suggest that at alpine treelines, trees allocate to ectomycorrhizal symbionts relatively high amount of carbon compared to the reverse nitrogen flux, but the exact exchange is tree species-specific.

Understanding the dynamics of treeline ecotones under global change requires long-term ecological and environmental data. The Stillberg ecological treeline research site in the Swiss Alps was established in 1975 by planting 92,000 seedlings of Larix decidua, Pinus cembra and Pinus mugo ssp. uncinata, and has been continuously monitored since then. Here, we present curated long-term data acquired over almost 50 years at the Stillberg site, and we synthesise the major research findings. The long-term datasets comprise 6.5 million ecological and environmental records from the 40-year afforestation experiment, as well as from a 9-year free-air CO₂ enrichment experiment crossed with a 6-year soil warming experiment, a 12-year nutrient addition experiment and an 8-year multifactorial tree seedling recruitment experiment. Our datasets further include 38 million records of 25 meteorological parameters measured at an hourly resolution from 1975 to 1996, and at a 10 min resolution since 1997. We provide all datasets and the corresponding metadata as open research data. Almost five decades of research in this treeline ecotone showed high mortality after tree establishment that was closely related to microclimatic variability. The two Pinus species survived at a much lower rate than L. decidua, due to indirect pathogen interactions. Furthermore, CO₂ enrichment only increased growth of L. decidua, while warming increased growth of P. mugo ssp. uncinata and two Vaccinium shrub species. Enhanced nutrient availability stimulated growth in tree and understorey shrub species. In addition, soil warming and CO₂ enrichment stimulated microbial activity and decreased soil carbon stocks. These findings improve our understanding of ecological processes in the treeline ecotone under global change and confirm the importance of tree growth and establishment limitations. The enhanced availability and quality of these long-term data are expected to foster whole-system approaches and transdisciplinary research syntheses, supporting the development of effective global change adaptation strategies.

Aim: Forest disturbances are increasing around the globe due to changes in climate and management, deteriorating forests' carbon sink strength. Estimates of global forest carbon budgets account for losses of plant biomass but often neglect the effects of disturbances on soil organic carbon (SOC). Here, we aimed to quantify and conceptualize SOC losses in response to different disturbance agents on a global scale.
Location: Global.
Time Period: 1983–2022.
Major Taxa Studied: Forest soils.
Methods: We conducted a comprehensive global analysis of the effects of harvesting, wildfires, windstorms and insect infestations on forest SOC stocks in the surface organic layer and top mineral soil, synthesizing 927 paired observations from 151 existing field studies worldwide. We further used global mapping to assess potential SOC losses upon disturbance. Results: We found that both natural and anthropogenic forest disturbances can cause large SOC losses up to 60 Mg ha⁻¹. On average, the largest SOC losses were found after wildfires, followed by disturbances from windstorms, harvests and insects. However, initial carbon stock size, rather than disturbance agent, had the strongest influence on the magnitude of SOC losses. SOC losses were greatest in cold-climate forests (boreal and mountainous regions) with large accumulations of organic matter on or near the soil surface. Negative effects are present for at least four decades post-disturbance. In contrast, forests with small initial SOC stocks experienced quantitatively lower carbon losses, and their stocks returned to pre-disturbance levels more quickly.
Main Conclusions: Our results indicate that the more carbon is stored in the forest's organic layers and top mineral soils, the more carbon will be lost after disturbance. Robust estimates of forest carbon budgets must therefore consider disturbance-induced SOC losses, which strongly depend on site-specific stocks. Particularly in cold-climate forests, these disturbance-related losses may challenge forest management efforts to sequester CO₂.

Understanding the temporal dynamics of soil microbial biomass is crucial for assessing soil ecosystem functions and services, yet these dynamics are globally uncertain. Here, we compiled a data set of soil microbial biomass carbon (MBC) and nitrogen (MBN) from 1493 studies between 1988 and 2019 to elucidate their temporal trends and potential drivers. Results showed that global MBC and MBN significantly decreased by 0.033 Mg C ha⁻¹ yr⁻¹ and 0.007 Mg N ha⁻¹ yr⁻¹ at 0–30 cm soil depth, between 1988 and 2019, respectively, which might be primarily attributed to the warming of the climate, the increase in global precipitation, and reduction of soil organic carbon (SOC) stock. The rate of decline in MBC and MBN showed a non-linear trend: following a decline from 1988 to 1999, it slowed down until 2014, likely due to the global warming hiatus. Afterward, the pace of decline increased again from 2015 to 2019. Boreal biomes experienced the largest decrease in soil microbial biomass with the reduction rate of MBC being 4.3 times higher than in temperate biomes, showing a higher sensitivity in boreal biomes to climate change. Grassland ecosystems also exhibited greater reductions, possibly driven by their degradation. These findings shed valuable insights on the long-term dynamics of soil microbial biomass on a global scale over the last three decades. Furthermore, this study underscores the importance of preserving soil microbial biomass as a key strategy to mitigate the adverse effects of future climate change, thereby sustaining ecosystem health and resilience.

Anthropogenic inputs of nitrogen (N) and phosphorus (P) to terrestrial ecosystems alter soil nutrient cycling. However, the global-scale responses of soil P fractions to N and P inputs and their underlying mechanisms remain elusive. We conducted a global meta-analysis based on 818 observations of soil P fractions from 99 field N and P addition experiments in forest, grassland, and cropland ecosystems ranging from temperate to tropical zones. Our global meta-analysis revealed distinct responses of soil P fractions to N and P enrichment. For studies using the Chang and Jackson inorganic (Pi) method, we found that high N addition promoted the transformation of immobile Pi fractions into Ferrum/Aluminum-bound Pi and available Pi in surface soils through soil acidification. However, this acid-induced transformation of Pi fractions by N addition was observed only in Calcium-rich soils, while in acidic soils, further acidification led to increase P binding. In contrast, additions of P alone or combined with N significantly increased all soil Pi fractions. Regarding the Hedley P fractions, N addition generally decreased labile organic P by enhancing soil acid phosphatase activity. The responses of other P fractions were influenced by soil pH, fertilization rates, ecosystem type, and other factors. P addition increased most soil P fractions. Overall, both P fractionation methods consistently demonstrate that N inputs deplete soil P and accelerate P cycling, while P inputs increase most soil P fractions, alleviating P limitation. These findings are crucial for predicting the effects of future atmospheric N and P deposition on P cycling processes.