Differential responses and mechanistic controls of soil phosphorus transformation in Eucalyptus plantations with N fertilization and introduced N2 -fixing tree species

Trees show higher resilience than herbs under phosphorus deficit induced by 12-year simulated acid rain

Acid rain, with 60% deposition in Asia, may exacerbate plant phosphorus (P) limitation; however, its long-term effects on different plant life-forms remain largely undetermined. Understanding these effects is essential for predicting ecosystem resilience and promoting forest health under environmental change. Herein, we investigated the P status in two tree and two herb species and their rhizosphere soils after 12 years of acid treatment at three pH levels (pH: 4.0, 3.5, and 3.0) in a tropical forest in Southern China. We found that leaf, litter, and root P; leaf N and P resorption efficiency; and their ratios remained stable in trees; however, herb leaf and litter P levels declined. Acid addition reduced inorganic P in tree rhizosphere soil and inorganic and organic P in herb rhizosphere soil. Rhizosphere soil P fractions were more regulated by soil physicochemical properties and less regulated by microbial community in trees than in herbs. Under long-term simulated acid rain, stable tree P status benefited from soil inorganic P depletion, and herbs partially met their P requirements via biological mineralization of soil organic P. These distinct P-associated responses and acquisition strategies provide insights into safeguarding forest health among plants of different functional types under long-term acid rain events.

Field Experiments and a Meta-Analysis Reveal a Minor Influence of Nitrogen Addition on Phosphorus Fractions in Forests

Anthropogenic nitrogen (N) inputs can significantly impact nutrient cycling and ecosystem functioning in terrestrial ecosystems. However, the effects of N addition on phosphorus (P) cycling processes in forest ecosystems remain unclear. In this study, we combined data from a long-term (11-year) N addition experiment across seven different forests ranging from temperate to tropical biomes, with a global meta-analysis from 88 relevant publications, to investigate the responses of P cycling-related variables to N inputs in forest ecosystems. We found that N addition had little effect on most P cycling-related variables (e.g., leaf P, soil total P, soil available P, soil P fractions, and microbial biomass P) across the studied forest ecosystems. The meta-analysis highlighted that N-induced changes in P cycling were highly variable. Only a few variables, such as the leaf P concentration and the activity of soil acid phosphatase, presented significant responses to N addition and changed with climatic zone and the amount and duration of N inputs. Our study suggests that P cycling processes in forest ecosystems remain largely unaffected by N inputs. Our findings contribute to a better understanding and prediction of biogeochemical cycles in the context of N deposition related to anthropogenic activities and global climate change.

Microbial mechanisms of mixed planting in regulating soil phosphorus availability across different stand ages in Chinese fir plantations

The mixed planting of Chinese fir with broadleaf species to increase soil phosphorus (P) availability has been widely adopted in subtropical China. As soil P availability is significantly influenced by tree growth, the microbial mechanisms underlying the effects of mixed planting on soil P availability across different stand ages are not fully understood. In this study, we collected soil samples from mixed-species plantations of Chinese fir and Schima superba (MCP) and pure Chinese fir plantations (PCP) at young (5 years), middle-aged (20 years), and mature (32 years) stages in southeastern China. We determined the soil P fractions, organic P (Po) mineralizing ability, and dynamics of the microbial community associated with Po mineralization in the samples. We hypothesized that the influence of mixed planting on soil P availability is modulated by stand age. Compared with the PCP stands, the young and mature MCP stands exhibited significantly greater contents of labile and moderately labile P, with increases of 13.22% and 8.18%, respectively, in the young stands and 22.20% and 30.52%, respectively, in the mature stands. Conversely, the middle-aged MCP stands exhibited lower contents of labile and moderately labile P, with decreases of 20.93% and 18.16%, respectively. The communities of Po-mineralizing fungi (Pmin-F) and bacteria (Pmin-B) changed not only among the different plantation types but also across the various stand ages. The Pmin-F community contributed mainly to labile P, whereas the Pmin-B community was the primary driver of moderately labile P. Additionally, mixed planting mediated labile P availability through soil pH, accounting for 71% of the variation in this P fraction. Conversely, stand age affected the availability of moderately labile P through soil nitrogen availability and the Pmin-F community, explaining 81% of the variation in this P fraction. Overall, we revealed that the impact of mixed planting on soil P availability is modulated by stand age, with fungi and bacteria fulfilling distinct ecological roles in the process. Our results are highly important for maintaining soil P availability for the sustainable management of Chinese fir plantations.

Organic fertilizer improved the lead and cadmium metal tolerance of Eucalyptus camaldulensis by enhancing the uptake of potassium, phosphorus, and calcium

Phytoremediation is a strategy for the amelioration of soil heavy metal contamination that aligns with ecological sustainability principles. Among the spectrum of phytoremediation candidates, woody plants are considered particularly adept for their substantial biomass, profound root systems, and non-participation in the food chain. This study used Eucalyptus camaldulensis-a tree species characterized for its high biomass and rapid growth rate-to assess its growth and metal uptake in mining tailings. The results were as follows: exposure to heavy metals reduced the E. camaldulensis uptake of potassium (K), phosphorus (P), and calcium (Ca). Heavy metal stress negatively affected the biomass of E. camaldulensis. Lead (Pb) primarily accumulated in the roots, while cadmium (Cd) predominantly accumulated in the stems. The application of organic fertilizers bolstered the stress tolerance of E. camaldulensis, mitigating the adverse impacts of heavy metal stress. A synergistic effect occurred when organic fertilizers were combined with bacterial fertilizers. The plant's enrichment capacity for Cd and its tolerance to Pb was augmented through the concurrent application of bacterial and organic fertilizers. Collectively, the application of organic fertilizers improved the heavy metal tolerance of E. camaldulensis by enhancing the uptake of K, P, and Ca and elevating the content of glutathione peroxidase (GPX) and gibberellin acid (GA) in roots. These findings provided nascent groundwork for breeding E. camaldulensis with enhanced heavy metal tolerance. Moreover, this proved the potentiality of E. camaldulensis for the management of heavy metal-contaminated tailings and offers a promising avenue for future environmental restoration.

Enhanced Soil Fertility and Carbon Dynamics in Organic Farming Systems: The Role of Arbuscular Mycorrhizal Fungal Abundance

Arbuscular mycorrhizal fungi (AMF) are critical for soil ecosystem services as they enhance plant growth and soil quality via nutrient cycling and carbon storage. Considering the growing emphasis on sustainable agricultural practices, this study investigated the effects of conventional and organic farming practices on AMF diversity, abundance, and ecological functions in maize, pepper, and potato-cultivated soils. Using next-generation sequencing and quantitative PCR, we assessed AMF diversity and abundance in addition to soil health indicators such as phosphorus content, total nitrogen, and soil organic carbon. Our findings revealed that, while no significant differences in soil physicochemical parameters or AMF diversity were observed across farming systems when all crop data were combined, organic farming significantly enhances AMF abundance and fosters beneficial microbial ecosystems. These ecosystems play vital roles in nutrient cycling and carbon storage, underscoring the importance of organic practices in promoting robust AMF communities that support ecosystem services. This study not only deepens our understanding of AMF's ecological roles but also highlights the potential of organic farming to leverage these benefits for improving sustainability in agricultural practices.

Phosphorus availability regulates nitrogen fixation rate through a key diazotrophic assembly: Evidence from a subtropical Moso bamboo forest subjected to nitrogen application

Biological N fixation (BNF) is an important N input process for terrestrial ecosystems. Long-term N application increases the availability of N, but may also lead to phosphorus (P) deficiency or an imbalance between N and P. Here, we performed a 5-year N application experiment in a subtropical Phyllostachys heterocycla forest in site and a P application experiment in vitro to investigate the effect of N application on the BNF rate and its regulatory factor. The BNF rate, nifH gene, free-living diazotrophic community composition and plant properties were measured. We found that N application suppressed the BNF rate and nifH gene abundance, whereas the BNF rate in soils with added P was significantly higher overall than that in soils without added P. Moreover, we identified a key diazotrophic assembly (Mod#2), primarily comprising Bradyrhizobium, Geobacter, Desulfovibrio, Anaeromyxobacter, and Pseudodesulfovibrio, which explained 77 % of the BNF rate variation. There was a significant positive correlation between the Mod#2 abundance and soil available P, and the random forest results showed that soil available P is the most important factor affecting the Mod#2 abundance. Our findings highlight the importance of soil P availability in regulating the activities of key diazotrophs, and thus increasing P supply may help to promote N accumulation and primary productivity through facilitating the BNF process in forest ecosystems.