Long-term nitrogen deposition inhibits soil priming effects by enhancing phosphorus limitation in a subtropical forest

Carbon and nitrogen addition-derived enzyme activities in topsoil but nitrogen availability in subsoil controls the response of soil organic carbon decomposition to warming

Subsoil stores the majority of soil organic carbon (SOC), and plays a vital role in the global carbon cycle in terrestrial ecosystems and in regulating climate change. Response of SOC decomposition to temperature warming (TR) is a crucial parameter to predict SOC dynamics under global warming. However, it remains unknown how TR varies across the whole soil profile and responds to exogenous C and N inputs. To assess this, we designed a novel incubation system to measure SOC-derived CO2 efflux across the whole soil column (i.e., 60 cm length), allowing manual addition of 13C-labeled glucose and ammonium nitrate, and incubated it under ambient or warmed temperatures (+4 °C). We found that C addition significantly increased TR in 0-20 cm, 20-40 cm and 40-60 cm by 64.3 %, 68.1 % and 57.2 %, respectively. However, the combined addition of C and N decreased TR by 11.1 % - 15.3 % compared to without anything addition (CK) in the whole soil profile. The effect of N on TR ranged from -22.8 % to -40.4 % in the whole soil profile, and was significantly lower in topsoil than in subsoil. Furthermore, sole N addition significantly promoted TR compared to CK by 79.0 % and 94.7 % in 20-40 cm and 40-60 cm subsoil, only 9.8 % in 0-20 cm topsoil. These results together suggested that TR is sensitive to increasing C availability in the whole soil profile and increasing N availability in 20-60 cm subsoil. Random forest model indicated that soil enzyme activities (explained 21.3 % of the variance) and DOC (explained 11.1 % of the variance) dominantly governed TR in topsoil, but N availability displayed a predominant control of TR in subsoil. Overall, our results suggested that increased C and N availability under climate warming scenarios could further increase the risk of carbon loss especially in subsoil with substrate deficiency, but labile C (e.g., root exudation) input under climate warming and N enrichment could reduce SOC decomposition and benefit for C sequestration by decreasing TR.

Greater influences of nitrogen addition on priming effect in forest subsoil than topsoil regardless of incubation warming

Subsoil (below 20 cm), storing over 50 % of soil organics carbon (SOC) within the 1 m depth, plays a critical role in regulating climate and ecosystem function. However, little was known on the changes in SOC decomposition induced by exogenous C input (i.e., priming effect) across the whole soil profile under nitrogen (N) enrichment and climate warming. We designed an incubation system of soil columns with minor physical disturbance, which allows the manual additions of exogenous C and N and incubation under ambient or elevated temperature. A negative priming effect by glucose was observed in all layers of ambient soil, while the negative priming effect was enhanced by soil depth but inhibited by warming. Nitrogen addition shifted the priming effect from negative to positive under ambient temperature, and decreased the magnitude of negative priming effect under elevated temperature. Nitrogen uplift effect on priming effect was more pronounced in subsoil compared to topsoil, while this effect diminished with rising temperature. Soil microbial activity (e.g., the CO2 production within 3 days) and acid phosphatase activity had important roles in regulating the variations in priming effect across the soil profile. Our results indicated that increase in labile substrate (e.g., exogenous C input) input would not lead to native SOC destabilization in subsoil, N addition shifted the priming effect from negative to positive, increasing the SOC decomposition under ambient temperature, while labile C input together with N addition benefited SOC sequestration by inducing negative priming effects in forest soil under warming climate.

Divergent responses of rhizosphere soil phosphorus fractions and biological features of Salix psammophila to fertilization strategies under cadmium contamination

Soil oligotrophy in areas heavily contaminated with heavy metals poses a significant challenge to vegetation establishment and phytoremediation processes. Phosphorus (P) cycling plays a critical role in global biogeochemical cycles, but there is limited understanding of its response to varying fertilization strategies and its correlation with phytoremediation effectiveness. This study primarily investigated the effects of various fertilization strategies, including nitrogen (N, 300 mg·kg-1), P (100 mg·kg-1), NP (combined N and P at 300 mg·kg-1 and 100 mg·kg-1, respectively), and HP (high P, 300 mg·kg-1) application, on rhizosphere soil P fractions and P-solubilizing microbial community (harboring phoD and phoC genes, respectively) of Salix psammophila under cadmium contamination. Application of NP significantly enhanced plant growth and cadmium accumulation, whereas HP inhibited cadmium bioaccumulation but promoted its translocation. Compared to untreated soil, N application promoted P cycling, leading to increases of 141.9 %, 60.4 %, and 10.3 % in Resin-Pi, diluted HCl-Pi, and conc.HCl-Pi, respectively. P application decreased organic phosphorus (Po) fractions by 24.4 % - 225.8 %, but N incorporation mitigated the declining trend in Po and augmented alkaline phosphatase activity. Fertilization strategies significantly regulated phoC- or phoD-harboring bacterial community structure, but their differential nutrient demands resulted in distinct responses. The phoD-harboring bacteria exhibited higher diversity and network complexity, with numerous biomarkers and fertilizer-sensitive OTUs discovered across treatments. Structural equation modeling (SEM) analysis indicated that phytoremediation efficiency was directly affected by Pi fractions, and phoD-harboring bacteria exhibited stronger associations with Pi fractions than phoC-harboring bacteria. In conclusion, our results reveal potential pathways through which fertilization strategies influence phytoremediation by affecting the structure of P-solubilizing microbial community. Furthermore, our study emphasizes the importance of combined N and P application in promoting Cd accumulation in plants, with high P levels appearing as an ideal fertilization strategy for phytoremediation targeting the harvest of aboveground biomass.

Soil microbes deal with the nitrogen deposition enhanced phosphorus limitation by shifting community structure in an old-growth subtropical forest

Elevated atmospheric nitrogen (N) deposition potentially enhances the degree of phosphorus (P) limitation in tropical and subtropical forests. However, it remains elusive that how soil microorganisms deal with the N deposition-enhanced P limitation. We collected soils experienced 9 years of manipulative N input at various rates (0, 40, and 80 kg N ha-1 y-1) in an old-growth subtropical natural forest. We measured soil total and available carbon (C), N and P, microbial biomass C, N and P, enzyme activities involved in C, N and P acquisition, microbial community structure, as well as net N and P mineralization. Additionally, we calculated element use efficiency and evaluated microbial homeostasis index. Our findings revealed that N input increased microbial biomass C:P (MBC:P) and N:P (MBN:P) ratios. The homeostasis indexes of MBC:P and MBN:P were 0.68 and 0.75, respectively, indicating stoichiometric flexibility. Interestingly, MBC:P and MBN:P correlated significantly with the fungi:bacteria ratio (F:B), not with N and P use efficiencies, net N and P mineralization, and enzyme C:P (EEAC:P) and N:P (EEAN:P) ratios. Furthermore, EEAC:P and EEAN:P correlated positively with F:B but did not negatively correlate with the C:P and N:P ratios of available resources and microbial biomass. The effects of N deposition on MBC:P, MBN:P and EEAN:P became insignificant when including F:B as a covariate. These findings suggest that microbes flexibly adapted to the N deposition enhanced P limitation by changing microbial community structure, which not only alter microbial biomass C:N:P stoichiometry, but also the enzyme production strategy. In summary, our research advances our understanding of how soil microorganisms deal with the N deposition-enhanced soil P limitation in subtropical forests.

Chemical composition of soil carbon is governed by microbial diversity during understory fern removal in subtropical pine forests

Understory vegetation has an important impact on soil organic carbon (SOC) accumulation. However, little is known about how understory vegetation alters soil microbial community composition and how microbial diversity contributes to SOC chemical composition and persistence during subtropical forest restoration. In this study, removal treatments of an understory fern (Dicranopteris dichotoma) were carried out within pine (Pinus massoniana) plantations restored in different years in subtropical China. Soil microbial community composition and microbial diversity were measured using phospholipid fatty acids (PLFAs) biomarkers and high-throughput sequencing, respectively. The chemical composition of SOC was also measured via solid-state 13C nuclear magnetic resonance (13C NMR). Our results showed that fern removal decreased alkyl C by 4.2 % but increased O-alkyl C by 15.6 % on average, leading to a decline of alkyl C/O-alkyl C ratio, suggesting altered chemical composition of SOC and lowered SOC recalcitrance without fern. Fern removal significantly lowered the fungi-to-bacteria ratio, and it also reduced fungal and bacterial diversity. Partial correlation analysis revealed that soil nitrogen availability was a key factor influencing microbial diversity. Bacterial diversity showed a close relationship with the Alkyl C/O-alkyl C ratio following fern removal. Furthermore, the microbial community structure and bacterial diversity were responsible for 18 % and 55 % of the explained variance in the chemical composition of SOC, respectively. Taken together, these analyses jointly suggest that bacterial diversity exerts a greater role than microbial community structure in supporting SOC persistence during understory fern removal. Our study emphasizes the significance of understory ferns in supporting microbial abundance and diversity as a means of altering SOC persistence during subtropical forest restoration.

Changing soil available substrate primarily caused by fertilization management contributed more to soil respiration temperature sensitivity than microbial community thermal adaptation

Substrate depletion and microbial community thermal adaptation are major mechanisms that regulate the temperature sensitivity (Q10) of soil microbial respiration. Traditionally, the Q10 of soil microbial respiration is measured using laboratory incubation, which has limits in the continuous input of available substrates and the time scale for microbial community thermal adaptation. How the available substrate and the soil microbial community regulate the Q10 of soil microbial respiration under natural warming conditions remains unclear. To fill this gap in knowledge, a long-term field experiment was conducted consisting of two years of soil respiration observations combined with a soil available substrate and microbial community thermal adaptation analysis under seasonal warming conditions. The Q10 of soil respiration was calculated using the square root method, and it was more affected by the available substrate than by microbial community thermal adaptation. Fertilization management has a stronger effect on soil available substrate than temperature. As the temperature increased, NH4-N proved itself to be important for the bacterial community in the process of Q10 regulation, while dissolved organic carbon and nitrogen were key factors for the fungal community. Based on the niche breadth of microbial community composition, the changing Q10 of the soil respiration was not only closely associated with the specialist community, but also the generalist and neutralist communities. Furthermore, bacterial community thermal adaptation primarily occurred through shifts in the abundances of specialists and neutralists, while changes in species richness and species replacement occurred for the fungal generalists and neutralists. This work indicates that changing available nitrogen and DOC primarily caused by fertilization management contributed more in regulating the Q10 of soil microbial respiration than microbial community thermal adaptation, and there are different mechanisms for bacterial and fungal community thermal adaptation under warming.