Spatially divergent trends of nitrogen versus phosphorus limitation across European forests

Soil phosphorus compared to nitrogen limitation increases the uncertainty of subsoil organic carbon sequestration in Pinus massoniana mixed forests

Limited nitrogen (N) and phosphorus (P) availability will constrain terrestrial carbon sinks in the 21st century. Mixed forests improve the plant community composition and productivity of pure coniferous forests. Nevertheless, it is uncertain whether and to what extent changes in soil N and P dynamics caused by mixed forests can affect forest soil organic carbon (SOC) stocks. The research purpose is to demonstrate and evaluate the effects of soil N and P on SOC stocks in Pinus massoniana mixed forests. Our meta-analysis, which included 616 paired observations, revealed that the coupling of soil N, P, and plant species richness (PSR) contributed 22.6%, 28.8%, and 28.4%, respectively, to SOC stock accumulation in the topsoil (0-20 cm), subsoil (20-100 cm), and whole profile (0-100 cm). The interactions between soil total N and P concentrations (TN:TP ratios) dominated the increase in SOC stocks in the mixed forest topsoil and whole profile, explaining 35.6% and 20.3% of the variation, respectively. Conversely, independent TN and TP concentrations were the primary contributors (explained by 17.5% and 12.3%, respectively) to subsoil SOC stocks increase. The TN and TP concentrations limit SOC stock accumulation in mixed forests for the next 60 years (2025-2085), with the TP concentration and TN:TP ratio having a greater effect in subsoil. Fortunately, the TN and TP limitations on SOC stock increase in mixed forests can be mitigated and balanced by altering soil TN:TP ratio by increasing or decreasing the PSR (PSR thresholds for topsoil, subsoil, and whole profile were 15, 8, and 8, respectively). Overall, plant mixing fails to enhance plant availability of pure forest soil N and P. Accelerated N cycling and increased P reabsorption efficiency are optimal strategies for balancing N and P supply in mixed forests, promoting biomass accumulation, and ensuring SOC stocks increase.

Complex imprint of air pollution in the basal area increments of three European tree species

The impact of atmospheric pollution on the growth of European forest tree species, particularly European beech, Silver fir and Norway spruce, is examined in five mesic forests in the Czech Republic. Analyzing of basal area increment (BAI) patterns using linear mixed effect models reveals a complex interplay between atmospheric nitrogen (N) and sulphur (S) deposition, climatic variables and changing CO2 concentrations. Beech BAI responds positively to N deposition (in tandem with air CO2 concentration), with soil phosphorus (P) availability emerging as a significant factor influencing overall growth rates. Fir BAI, on the other hand, was particularly negatively influenced by S deposition, although recent growth acceleration suggests growth resilience in post-pollution period. This fir growth surge likely coincides with stimulation of P acquisition following the decline of acidic pollution. The consequence is the current highest productivity among the studied tree species. The growth dynamics of both conifers were closely linked to the stoichiometric imbalance of phosphorus in needles, indicating the possible sensitivity of exogenous controls on nutrient uptake. Furthermore, spruce BAI was positively linked to calcium availability across sites. Despite enhanced water-use efficiency under elevated CO2, spruce growth is constrained by precipitation deficit and demonstrates weakening resilience to increasing growing season air temperatures. Overall, these findings underscore the intricate relationships between atmospheric pollution, nutrient availability, and climatic factors in shaping the growth dynamics of European forest ecosystems. Thus, incorporating biogeochemical context of nutrient availability is essential for realistic modelling of tree growth in a changing climate.

Microbial and environmental medium-driven responses to phosphorus fraction changes in the sediments of different lake types during the freezing period

The comparative study of the transformation among sediment phosphorus (P) fractions in different lake types is a global issue in lake ecosystems. However, interactions between sediment P fractions, environmental factors, and microorganisms vary with the nutrient status of lakes. In this study, we combine sequential extraction and metagenomics sequencing to assess the characteristics of P fractions and transformation in sediments from different lake types in the Inner Mongolian section of the Yellow River Basin. We then further explore the response of relevant microbial and environmental drivers to P fraction transformation and bioavailability in sediments. The sediments of all three lakes exhibited strong exogenous pollution input characteristics, and higher nutritional conditions led to enhanced sediment P fraction transformation ability. The transformation capacity of the sediment P fractions also differed among the different lake types at the same latitudes, which is affected by many factors such as lake environmental factors and microorganisms. Different drivers reflected the mutual control of weakly adsorbed phosphorus (WA-P), potential active phosphorus (PA-P), Fe/Al-bound phosphorus (NaOH-P), and Ca-bound phosphorus (HCl-P) with the bio-directly available phosphorus (Bio-P). The transformation of NaOH-P in reducing environments can improve P bioavailability, while HCl-P is not easily bioavailable in weakly alkaline environments. There were significant differences in the bacterial community diversity and composition between the different lake types at the same latitude (p < 0.05), and the role of P fractions was stronger in the sediments of lakes with rich biodiversity than in poor biodiversity. Lake eutrophication recovery was somewhat hindered by the microbial interactions of P cycling and P fractions within the sediment. This study provides data and theoretical support for exploring the commonalities and differences among different lake types in the Inner Mongolian section of the Yellow River Basin. Besides, it is representative and typical for promoting the optimization of ecological security patterns in ecologically fragile watersheds.

Response of C:N:P stoichiometry to long-term drainage of peatlands: Evidence from plant, soil, and enzyme

Drought induced by climate warming and human activities regulates carbon (C) cycling of peatlands by changing plant community composition and soil properties. Estimating the responses of peatlands C cycling to environmental changes requires further study of C: nitrogen (N): phosphorus (P) stoichiometric ratios of soil, plants, and enzyme activities. However, systematic studies on the stoichiometry of above-ground and below-ground ecosystems of peatlands post drainage remain scarce. This study compared stoichimetric ratios of plant and soil and stoichimetric ratios of enzyme activities with different functions in two different parts of a minerotrophic peatland, a natural undisturbed part and a part that had been drained for almost 50 years, in Northern China. For the shrub plants, the average C:N:P ratios of leaf in natural and drained peatland were 448:17:1 and 393:15:1, respectively. This indicated that the growth rate of shrub plants is higher in the drained peatland than in the natural peatland, which makes P element more concentrated in the photosynthetic site. However, from the perspective of the dominant plant, the mean C:N:P ratio of Carex leaf was 650:25:1 in the natural peatland, but was 1028:50:1 for Dasiphora fruticosa in drained peatland. This indicated that the intensification of P-limitation of plant growth after drainage. Soil C:N:P ratios of above water table depth (AWT) were 238:15:1 and 277:12:1, but were 383:17:1 and 404:19:1 for below water table depth (BWT) in the natural and the drained peatland, respectively. Soil C:P ratios were greater than the threshold elemental ratio of C:P (174:1), but the soil C:N ratios were less than the threshold elemental ratio of C:N (23:1), which suggested that P was the most limiting nutrient of soil. The soil microbial activities were co-limited by C&P in Baijianghe peatlands. However, the microbial metabolic P limitation was intensified, but the C limitation was weakened for the above water table depth soil after long-term drainage. There are connection between plant-microbe P limitation in peatlands. The P limitation of microbial metabolism was significant positively correlated with soil C:N but negatively with soil moisture. The increase in the lignocelluloses index suggested considerable decomposition of soil organic matter after peatland drainage. These results of stoichiometric ratios from above- to below ground could provide scientific base for the C cycling of peatland undergone climate change.

Shifting of nutrient limitation dominates the recovery of aboveground net primary productivity of mixed forests in northeastern China after selective logging

The primary productivity of temperate forests is commonly limited by nitrogen (N) supply, which may be aggravated by the removal of trees. After selective logging, whether and the mechanism by which the N limitation can be alleviated by the rapidly increasing nutrient turnover during the recovery processes, which is important for improving carbon sequestration in temperate forests, remain unclear. We investigated the effect of nutrient limitation (leaf N:Pcom: the leaf N:P ratio at the community level) on plant community productivity by selecting 28 forest plots including seven forest recovery periods (at the sites logged 6, 14, 25, 36, 45, 55, and 100 years ago) following low-intensity selective logging (13-14 m3/ha) and one unlogged treatment by measuring the soil N concentration, soil phosphorus (P) concentration, leaf N concentration, leaf P concentration, and the aboveground net primary productivity (ANPP) of 234 plant species. The plant growth in temperate forests was limited by N, but the P limitation was observed at the sites logged 36 years ago, which showed a transition pattern of plant growth from N limitation to P limitation during the forest recovery process. Meanwhile, a robust linear trend in the community ANPP was observed with the increase in the community leaf N:P ratio, which suggests the enhancement in community ANPP with the release of N limitation after selective logging. Nutrient limitation (leaf N:Pcom) had a significant direct effect (56.0 %) on the community ANPP and showed a higher independent contribution (25.6 %) to the variation in the community ANPP than the soil nutrient supply and even the changes in species richness. Our results suggested that selective logging alleviated the N limitation, but a shift toward P limitation should also be highly regarded in learning the changes in carbon sequestration during the recovery processes.

Does long-term soil warming affect microbial element limitation? A test by short-term assays of microbial growth responses to labile C, N and P additions

Increasing global temperatures have been reported to accelerate soil carbon (C) cycling, but also to promote nitrogen (N) and phosphorus (P) dynamics in terrestrial ecosystems. However, warming can differentially affect ecosystem C, N and P dynamics, potentially intensifying elemental imbalances between soil resources, plants and soil microorganisms. Here, we investigated the effect of long-term soil warming on microbial resource limitation, based on measurements of microbial growth (18 O incorporation into DNA) and respiration after C, N and P amendments. Soil samples were taken from two soil depths (0-10, 10-20 cm) in control and warmed (>14 years warming, +4°C) plots in the Achenkirch soil warming experiment. Soils were amended with combinations of glucose-C, inorganic/organic N and inorganic/organic P in a full factorial design, followed by incubation at their respective mean field temperatures for 24 h. Soil microbes were generally C-limited, exhibiting 1.8-fold to 8.8-fold increases in microbial growth upon C addition. Warming consistently caused soil microorganisms to shift from being predominately C limited to become C-P co-limited. This P limitation possibly was due to increased abiotic P immobilization in warmed soils. Microbes further showed stronger growth stimulation under combined glucose and inorganic nutrient amendments compared to organic nutrient additions. This may be related to a prolonged lag phase in organic N (glucosamine) mineralization and utilization compared to glucose. Soil respiration strongly positively responded to all kinds of glucose-C amendments, while responses of microbial growth were less pronounced in many of these treatments. This highlights that respiration-though easy and cheap to measure-is not a good substitute of growth when assessing microbial element limitation. Overall, we demonstrate a significant shift in microbial element limitation in warmed soils, from C to C-P co-limitation, with strong repercussions on the linkage between soil C, N and P cycles under long-term warming.

Long-term soil warming decreases microbial phosphorus utilization by increasing abiotic phosphorus sorption and phosphorus losses

Phosphorus (P) is an essential and often limiting element that could play a crucial role in terrestrial ecosystem responses to climate warming. However, it has yet remained unclear how different P cycling processes are affected by warming. Here we investigate the response of soil P pools and P cycling processes in a mountain forest after 14 years of soil warming (+4 °C). Long-term warming decreased soil total P pools, likely due to higher outputs of P from soils by increasing net plant P uptake and downward transportation of colloidal and particulate P. Warming increased the sorption strength to more recalcitrant soil P fractions (absorbed to iron oxyhydroxides and clays), thereby further reducing bioavailable P in soil solution. As a response, soil microbes enhanced the production of acid phosphatase, though this was not sufficient to avoid decreases of soil bioavailable P and microbial biomass P (and biotic phosphate immobilization). This study therefore highlights how long-term soil warming triggers changes in biotic and abiotic soil P pools and processes, which can potentially aggravate the P constraints of the trees and soil microbes and thereby negatively affect the C sequestration potential of these forests.

Linking Leaf Functional Traits with Soil and Climate Factors in Forest Ecosystems in China

Plant leaf functional traits can reflect the adaptive strategies of plants to environmental changes. Exploring the patterns and causes of geographic variation in leaf functional traits is pivotal for improving ecological theory at the macroscopic scale. In order to explore the geographical variation and the dominant factors of leaf functional traits in the forest ecosystems of China, we measured 15 environmental factors on 16 leaf functional traits in 33 forest reserves in China. The results showed leaf area (LA), carbon-to-nitrogen ratio (C/N), carbon-to-phosphorus ratio (C/P), nitrogen-to-phosphorus ratio (N/P), phosphorus mass per area (Pa) and nitrogen isotope abundance (δ15N)) were correlated with latitude significantly. LA, Pa and δ15N were also correlated with longitude significantly. The leaf functional traits in southern China were predominantly affected by climatic factors, whereas those in northern China were mainly influenced by soil factors. Mean annual temperature (MAT), mean annual precipitation (MAP) and mean annual humidity (MAH) were shown to be the important climate factors, whereas available calcium (ACa), available potassium (AK), and available magnesium (AMg) were shown to be the important climate factors that affect the leaf functional traits of the forests in China. Our study fills the gap in the study of drivers and large-scale geographical variability of leaf functional traits, and our results elucidate the operational mechanisms of forest-soil-climate systems. We provide reliable support for modeling global forest dynamics.

Phosphorus Limitation of Trees Influences Forest Soil Fungal Diversity in China

Fungal-biogeography studies have shown global patterns of biotic interactions on microbial biogeography. However, the mechanisms underlying these patterns remain relatively unexplored. To determine the dominant factors affecting forest soil fungal diversity in China, soil and leaves from 33 mountain forest reserves were sampled, and their properties were measured. We tested three hypotheses and established the most realistic one for China. The results showed that the soil fungal diversity (Shannon index) varied unimodally with latitude. The relative abundance of ectomycorrhizae was significantly positively correlated with the leaf nitrogen/phosphorus. The effects of soil available phosphorus and pH on fungal diversity depended on the ectomycorrhizal fungi, and the fungal diversity shifted by 93% due to available phosphorus, potassium, and pH. Therefore, we concluded that latitudinal changes in temperature and the variations in interactions between different fungal guilds (ectomycorrhizal, saprotrophic, and plant pathogenic fungi) did not have a major influence. Forest soil fungal diversity was affected by soil pH, available phosphorus, and potassium, which are driven by the phosphorus limitation of trees.