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

It is widely accepted that phosphorus (P) limits microbial metabolic processes and thus soil organic carbon (SOC) decomposition in tropical forests. Global change factors like elevated atmospheric nitrogen (N) deposition can enhance P limitation, raising concerns about the fate of SOC. However, how elevated N deposition affects the soil priming effect (PE) (i.e., fresh C inputs induced changes in SOC decomposition) in tropical forests remains unclear. We incubated soils exposed to 9 years of experimental N deposition in a subtropical evergreen broadleaved forest with two types of 13 C-labeled substrates of contrasting bioavailability (glucose and cellulose) with and without P amendments. We found that N deposition decreased soil total P and microbial biomass P, suggesting enhanced P limitation. In P unamended soils, N deposition significantly inhibited the PE. In contrast, adding P significantly increased the PE under N deposition and by a larger extent for the PE of cellulose (PEcellu ) than the PE of glucose (PEglu ). Relative to adding glucose or cellulose solely, adding P with glucose alleviated the suppression of soil microbial biomass and C-acquiring enzymes induced by N deposition, whereas adding P with cellulose attenuated the stimulation of acid phosphatase (AP) induced by N deposition. Across treatments, the PEglu increased as C-acquiring enzyme activity increased, whereas the PEcellu increased as AP activity decreased. This suggests that P limitation, enhanced by N deposition, inhibits the soil PE through varying mechanisms depending on substrate bioavailability; that is, P limitation regulates the PEglu by affecting soil microbial growth and investment in C acquisition, whereas regulates the PEcellu by affecting microbial investment in P acquisition. These findings provide new insights for tropical forests impacted by N loading, suggesting that expected changes in C quality and P limitation can affect the long-term regulation of the soil PE.

Interannual variation in rainfall modulates temperature sensitivity of carbon allocation and flux in a tropical montane wet forest.

Tropical forests exert a disproportionately large influence on terrestrial carbon (C) balance but projecting the effects of climate change on C cycling in tropical forests remains uncertain. Reducing this uncertainty requires improved quantification of the independent and interactive effects of variable and changing temperature and precipitation regimes on C inputs to, cycling within and loss from tropical forests. Here, we quantified aboveground litterfall and soil-surface CO2 efflux ("soil respiration"; FS ) in nine plots organized across a highly constrained 5.2°C mean annual temperature (MAT) gradient in tropical montane wet forest. We used five consecutive years of these measurements, during which annual rainfall (AR) steadily increased, in order to: (a) estimate total belowground C flux (TBCF); (b) examine how interannual variation in AR alters the apparent temperature dependency (Q10 ) of above- and belowground C fluxes; and (c) quantify stand-level C allocation responses to MAT and AR. Averaged across all years, FS , litterfall, and TBCF increased positively and linearly with MAT, which accounted for 49, 47, and 46% of flux rate variation, respectively. Rising AR lowered TBCF and FS , but increased litterfall, with patterns representing interacting responses to declining light. The Q10 of FS , litterfall, and TBCF all decreased with increasing AR, with peak sensitivity to MAT in the driest year and lowest sensitivity in the wettest. These findings support the conclusion that for this tropical montane wet forest, variations in light, water, and nutrient availability interact to strongly influence productivity (litterfall+TBCF), the sensitivity of above- and belowground C fluxes to rising MAT (Q10 of FS , litterfall, and TBCF), and C allocation patterns (TBCF:[litterfall+TBCF]).

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.

Understory ferns promote the restoration of soil microbial diversity and function in previously degraded lands.

Microorganisms facilitate the recovery of previously degraded soils, such as degraded lands experiencing vegetation restoration and understory expansion, through vital soil functions like nutrient cycling and decomposing organic matter. Despite the role of microorganisms in recovery, little is known about the effects of the process on microbial diversity and function. Here, we performed an understory fern, Dicranopteris dichotoma (Thunb.) Berhn removal treatments nested within three Masson pine (Pinus massoniana L.) plantations with different restoration years in subtropical China. Three ferns treatments including no ferns cover, with ferns cover, and the ferns removal treatments were established to assess the impact of the ferns on soil microbial diversity and function during revegetation and drivers of observed changes. We combined high-throughput sequencing, network structure modeling, and function prediction of soil bacterial and fungal communities to determine microbial diversity and functions. Our results showed that soil bacterial and fungal diversity increased with restoration time. Understory ferns significantly increased soil microbial diversity in the un-restored land but the effect became smaller in two restored sites. Understory ferns significantly increased the relative abundance of bacterial phyla Proteobacteria and Acidobacteria, but decreased that of Chloroflexi and Firmicutes. Furthermore, the presence of ferns increased the abundance of Basidiomycota, but increased the abundance of Ascomycota. Co-occurrence network analysis revealed that the presence of ferns leads to more complex of bacterial networks with more connections, nodes, average degrees, betweenness, and degrees. The functional predictions indicate that aerobic chemoheterotrophy, chemoheterotrophy, and nitrogen fixation functional groups play key roles in the nutrient cycling of soils with ferns cover. The bacterial and fungal community compositions were strongly affected by revegetation and understory ferns as litter biomass and soil nitrogen were identified as the key environmental factors. Our study highlights the role of understory in facilitating microbial diversity and function recovery during degraded lands restoration.

[Responses of soil microbial biomass and carbon source utilization to simulated nitrogen deposition and drought in a Cunninghamia lanceolata plantation]. / 杉木人工林土壤微生物生物量和碳源利用能力对模拟氮沉降和干旱的响应.

Chinese fir (Cunninghamia lanceolata) plantation is a dominant forest type and carbon sink in the subtropical region in China. An experiment with simulated nitrogen deposition (addition of 40 kg N·hm-2·a-1) and drought (50% of precipitation exclusion, PE) was established in Chinese fir plantation in 2018. Soil samples (0-15 cm) were collected in summer (July 2020) and winter (January 2021). Soil microbial biomass, colony forming units (CFUs) and carbon source utilization were determined through phospholipid fatty acids (PLFAs), plate count, and Biolog methods, respectively. The results showed significant seasonal variations of PLFAs-related microbial biomass and composition. Soil bacterial and fungal CFUs tended to be decreased by nitrogen addition or precipitation exclusion treatment, and bacterial CFUs were more sensitive to the two treatments than fungal CFUs. Soil microbial function (i.e. carbon source utilization) was not affected by nitrogen addition, but significantly decreased by precipitation exclusion. There was a significant positive correlation between bacterial CFUs and microbial function, indicating the crucial roles of culturable bacteria in microbial carbon transformation. Our results highlight the critical effects of nitrogen deposition and 50% reduced precipitation on microbes in topsoil of fir plantation, with implications for unraveling soil microbial ecological function of subtropical forest ecosystem under global changes in future.

[Effects of soil moisture on priming effect of soil organic carbon in meadow in Wuyi Mountain, China.] / 水分对武夷山草甸土壤有机碳激发效应的影响.

Moisture is an important factor affecting the priming effect of soil organic carbon (SOC). However, empirical evidence for its effect in mountain meadows soil is lacking. We conducted a 126-day laboratory incubation experiment with the high altitude (2130 m) mountain meadow soil in Wuyi Mountain, by adding 13C-labelled glucose combined with controlling soil moisture (30% and 60% of field water capacity, FWC). The CO2 concentration and 13C-CO2 abundance were measured regularly to examine the differences of SOC mineralization and priming effects under different water conditions and the driving factors. Our results showed that SOC mineralization rate increased with increasing soil water content. The priming effect of meadow soil with different soil moisture showed a decreasing trend with the increases of incubation time. The priming effect in soils with low FWC soil was significantly greater than that with high FWC. At the end of incubation, the cumulative priming effect of low FWC soil was 61.4% higher than that of high FWC soil. Compared with low FWC soil, high FWC soil released more CO2 from glucose, and the ratio of cumulative primed carbon to glucose mineralization under low FWC was significantly higher than that under high FWC soil, indicating that soil microorganisms under the high FWC condition might preferentially mineralize more glucose than SOC and consequently lower priming effect. Therefore, the priming effect under high FWC was smaller than that under low FWC. There was a significant positive relationship between priming effect and microbial biomass carbon, microbial biomass carbon/microbial biomass nitrogen, and NH4+-N, indicating that soil microbial biomass and composition could be changed under low FWC condition. The improved microbial "nitrogen-mining" would increase priming effect. Consequently, the decline of soil moisture of mountain meadow induced by global climate change may increase the priming effect of carbon, with consequences on carbon loss.

Decline in the contribution of microbial residues to soil organic carbon along a subtropical elevation gradient.

There has been an increasing interest in studying microbial necromasses and their contribution to soil organic carbon (SOC) accumulation. However, it remains unclear how the interaction among climate, plants, and soil influence the microbial anabolism and how microbial necromass contribute to SOC formation. Here, we assessed the relative contribution of microbial residues to SOC pool across a subtropical elevation gradient (ranged from 630 to 2130 m a.s.l.) representing a subtropical ecosystem on Wuyi Mountain in China, by using amino sugars as tracers. Analysis of topsoil (0-10 cm) amino sugars and the composition of microbial community across this gradient revealed that the soil total amino sugars accounting for 12.2-25.7% of the SOC pool, decreased with increasing elevation. Moreover, the linear reduction in the bacterial-derived carbon (C) and an increase in the ratio of fungal- to bacterial-derived C with increasing elevation suggested the reduction in the contribution of bacterial-derived C to SOC pool across this elevation gradient. The divergent changes in the contribution of the microbial residues to SOC infer a potential change in SOC composition and stability. The microbial-derived SOC formation and its climatic responses are influenced by the interaction of vegetation types and soil properties, with soil amorphous Fe being the determiner of soil amino sugar accrual. Our work highlights the importance of understanding ecosystem type and mineral composition in regulating microbial-mediated SOC formation and accumulation in responses to climate change in subtropical ecosystems.