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Fine-root traits mediate the capacity of plants to acquire soil resources in different environments. This study aimed to examine the changes of fine-root traits when roots proliferate into the litter layer vs. mineral soils, and to determine fine-root trait plasticity of these roots in response to nitrogen (N) addition. A one-year N addition experiment was conducted in a 22-year-old broadleaf Mytilaria laosensis (Hamamelidaceae) plantation in subtropical China. Newly produced fine roots were collected monthly from the litter layer and upper mineral soil (0–10 cm) layer to measure root morphological traits and nutrient concentrations. Fine-root production was determined using ingrowth mesh screens in the litter layer. Fine-root production in the litter layer in the Mytilaria laosensis plantation was 2.6 g m−2 yr.−1 but increased 3- to 5-fold with N addition. Significant differences in fine-root morphological traits and nutrient concentrations were found between the litter layer and 0–10 cm mineral soil layer. Fine roots in the litter layer were thinner, with higher specific root length (SRL), higher specific root area (SRA), a higher proportion of fine-root biomass in lower, more absorptive root orders, and lower root tissue density (RTD) than those in 0–10 cm mineral soil layer. Higher carbon (C), N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) concentrations and lower C:N ratio (C/N) were also observed in fine roots in the litter layer, compared to the 0–10 cm mineral soil layer. Nitrogen addition significantly increased root P, K, and Ca concentrations, but had no effect on Mg concentration. Nitrogen addition did not affect most fine-root morphological traits but did result in decreased root diameter. Compared with the mineral soil, roots produced in the litter layer generally reflected a more absorptive strategy with smaller root diameter and lower RTD and with higher SRL, SRA, and nutrient concentrations which together are generally associated with more metabolically active, but shorter lived roots. Strong responses of fine-root production and nutrient concentrations to N addition also suggest that N may be a driving factor for fine-root growth into the litter layer. Further studies are required to identify the effect of fine-root growth into the litter layer on microbial activity.

Large-scale land use changes have remarkably influenced the global carbon (C) and nitrogen (N) cycling. Soil microorganisms are known to be the key drivers of these processes and act as susceptive indicators of changes in ecosystem functioning due to land use changes. In forest ecosystems, differences in the stability and turnover of soil C and N pools are mainly associated with the variation in the above- and belowground litter/root inputs to the soil from the tree species. However, the impact of soil C and N pool differences caused by tree species on soil microbial community structure has not been fully investigated in subtropical China. This study aimed to assess the effects of tree species conversion from a coniferous to broad-leaved plantation on the soil microbial biomass and community structure associated with C and N transformations within the plant-soil system. The microbial biomass and composition (reflected by 28 phospholipid fatty acid profiles), soil C and N pools in the top soils, and C and N contents of certain litter and fine root profiles were measured 19 years after the reforestation of Chinese fir ( Cunninghamia lanceolata ) woodland with the same species or a native broadleaf species Mytilaria laosensis . The results suggested that soil microbial biomass was significantly higher in the M. laosensis than in the C. lanceolata plantations, and non-metric multidimensional scaling ordination plots showed distinct patterns of soil microbial community structure between these two species. Soil microbial biomass showed negative correlations with litter N or mineral N content, i.e., ammonium N (NH 4 + -N) and nitrate N (NO 3 − -N), but was positively correlated with soil C content and litter C: N ratio. Further, there were negative correlations between soil microbial biomass C and mineral N pools. These results indicated that tree species transition from M. laosensis to C. lanceolata might have improved the soil labile C and N pools and their availability, leading to an increase in the soil microbial biomass. Redundancy analysis conducted to elucidate the relationships between the microbial community and C or N parameters also showed that the soil C: N ratio, soil total N, and NH 4 + -N might be the major factors influencing the soil microbial community. However, soil microbial diversity and richness were not significantly altered by the tree species transition. These results suggested that the potential process rates mediated by litter-derived C and N availabilities might not always be accompanied by a remarkable response from community diversity, but might affect microbial biomass. In conclusion, long-term tree species transition from coniferous to broad-leaved plantations significantly improved soil C and N pools and their availabilities, thereby increasing the soil microbial biomass and changing the composition of in situ soil microbial community. Previous events (e.g., land use history) might have considerable long-lasting impacts on soil microbial diversity and richness than the contemporary environment variables caused by the tree species transition 19 years after reforestation.

Soil organic carbon (SOC) is a valuable resource for mediating global climate change and securing food production. Despite an alarming rate of global plant diversity loss, uncertainties concerning the effects of plant diversity on SOC remain, because plant diversity not only stimulates litter inputs via increased productivity, thus enhancing SOC, but also stimulates microbial respiration, thus reducing SOC. By analysing 1001 paired observations of plant mixtures and corresponding monocultures from 121 publications, we show that both SOC content and stock are on average 5 and 8% higher in species mixtures than in monocultures. These positive mixture effects increase over time and are more pronounced in deeper soils. Microbial biomass carbon, an indicator of SOC release and formation, also increases, but the proportion of microbial biomass carbon in SOC is lower in mixtures. Moreover, these species-mixture effects are consistent across forest, grassland, and cropland systems and are independent of background climates. Our results indicate that converting 50% of global forests from mixtures to monocultures would release an average of 2.70 Pg C from soil annually over a period of 20 years: about 30% of global annual fossil-fuel emissions. Our study highlights the importance of plant diversity preservation for the maintenance of soil carbon sequestration in discussions of global climate change policy.

Litter decomposition is a key process for ecosystem fertility and carbon (C) balance, key uncertainties remain about how this fundamental process is affected by microbial community composition. Evidence is growing that plant litter generally decays fastest at the site from which it was derived, owing to the presence of specialized microbial communities that can decompose specific types of litter. The objectives were to determine the impact of sites on litter decomposition and to examine the relationships among microbial community composition, litter chemistry, and decomposition rates of coniferous Cunninghamia lanceolata litter of higher lignin content and broadleaved Mytilaria laosensis litter of lower lignin content at different stages of decomposition under plantations of the respective species. The study was conducted for 16 months using a randomized split-plot design experiment with four replications of all combinations of treatments, the treatments being litter type and site (plantation species). The results showed that decomposition rates were the same for all combinations of amendments and sites, meaning that both sites had microbial communities equally capable or adapted to decompose plant substrates it had not previously encountered, despite marked differences in soil microbial communities between sites and the chemistry of the two litter types. Initial M. laosensis litter was of lower lignin content and C:N ratio and decomposed faster in the first 8 months than C. lanceolata litter under either M. laosensis or C. lanceolata forest. Litter decomposition was significantly slower in the environment from which it was derived between month 8 and month 16. This could be attributed to the exceptionally poor decomposition of M. laosensis litter which was significantly higher lingnin content at month 8 under M. laosensis than under C. lanceolata due to the impact of site on preferential degradation of litter C. Decomposers under C. lanceolata forest were more efficient in degrading alkyl C and/or less efficient in degrading O-alkyl C than those under M. laosensis forest during the experimental period, which might be related to the microbial community composition in the decomposing litter. Our study clearly showed interactions between changing litter chemistry and litter microbial communities and their impacts on litter decomposition. Site was not important in impacting decomposition rates, but played an important role in the preferential degradation of C components. However, further studies are needed to examine the conditions in forests where more rapid litter decomposition beneath the parent species than another species is considered to be common, in order to improve our ability to model decomposition globally.

Nitrogen (N) exerts strong effects on litter decomposition through altering microbial abundance and community composition. However, the effect of N addition on plant–soil interactions such as home-field advantage (HFA: enhanced decomposition at a home environment compared to a guest environment) in relation to litter decomposition remains unclear. To fill this knowledge gap, we conducted a reciprocal litter transplant plus N addition experiment in Mytilaria laosensis and Cunninghamia lanceolata plantations for two years in subtropical China where anthropogenic N input is amongst the highest in the world. We found positive HFA effects (in which the calculation incorporates litter of both species) with litter mass loss 11.2% faster at home than in the guest environment in the N addition (50 kg N ha−1 yr−1) treatment, but no significant HFA effects were found in the control treatment. The magnitude of the HFA effect on carbon (C) release increased with N addition, while that on N release decreased. The HFA effects on phosphorus, potassium, calcium, sodium, and magnesium release were positive overall, but varied through time and the magnitude of the effects were different among elements. The greater HFA effects in the N addition treatment were associated with greater differences in microbial biomass and community composition between home and guest environments than in the control treatment. Our results indicate that anthropogenic N enrichment could lead to enhanced HFA effects, through modification of microbial communities, and thereby affect C sequestration and N cycling in subtropical forests.

Soil acidity plays a central role in the diversity and function of terrestrial ecosystems. Recent studies have revealed that acid deposition has acidified topsoil over time. However, uncertainties relating to how the acidity of the entire soil profile, including deep soil, responds to multiple global change drivers make it challenging to predict the effects of the ongoing global change on soil functions. Using data from 2952 observations of 200 montane sites in subtropical China, we show that the soil pH decreased over the last 60 years across the whole soil profile (0–150 cm), though there was less reduction in deep soils. The contents of exchangeable Ca2+ and Mg2+ decreased at the same rate, or more quickly, in the deep soil than topsoil. Soil pH and base cations decreased more in forests and shrublands at low elevations, but less in mountain meadows at high elevations. Our sensitivity analysis indicated that regional N deposition, S deposition, warming, and decreasing water availability have contributed to the temporal decreases in pH and base cations in natural ecosystems across tropical and subtropical China. The results extend the previous findings of changes in acidity in surface soil layers and demonstrate that deep soils of natural systems across a large area can be acidified over a few decades. Our results suggest that ongoing global changes are reducing the base nutrients across the entire soil profile, and thus, the diversity and functionality of subtropical forests.

This study aimed to determine the influence of tree species on soil microbial community structure. We conducted a litter and root manipulation and a short-term nitrogen (N) addition experiment in 19-year-old broadleaf Mytilaria laosensis (Hamamelidaceae) and coniferous Chinese fir (Cunninghamia lanceolata) plantations in subtropical China. Phospholipid fatty acid (PLFA) analysis was used to examine treatment effects on soil microbial community structure. Redundancy analysis (RDA) was performed to determine the relationships between individual PLFAs and soil properties (soil pH, carbon (C) and N concentration and C:N ratio). Soil C:N ratio was significantly greater in M. laosensis (17.9) than in C. lanceolata (16.2). Soil C:N ratio was the key factor affecting the soil microbial community regardless of tree species and the litter, root and N treatments at our study site. The fungal biomarkers, 18:1ω9 and 18:2ω6,9 were significantly and positively related to soil C:N ratio and the abundance of bacterial lipid biomarkers was negatively related to soil C:N ratio. N addition for 8 months did not change the biomass and structure of the microbial community in M. laosensis and C. lanceolata soils. Soil nutrient availability before N addition was an important factor in determining the effect of N fertilization on soil microbial biomass and activity. PLFA analysis showed that root exclusion significantly decreased the abundance of the fungal biomarkers and increased the abundance of the Gram-positive bacteria. Rootless plots had a relatively lower Gram-positive to Gram-negative bacteria ratio and a higher fungi to bacteria ratio compared to the plots with roots under both M. laosensis and C. lanceolata. The response of arbuscular mycorrhizal fungi (16:1ω5) to root exclusion was species-specific. These observations suggest that soil C:N ratio was an important factor in influencing soil microbial community structure. Further studies are required to confirm the long-term effect of tree species on soil microbial community structure.

Controlled experiments have shown that global changes decouple the biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P), resulting in shifting stoichiometry that lies at the core of ecosystem functioning. However, the response of soil stoichiometry to global changes in natural ecosystems with different soil depths, vegetation types, and climate gradients remain poorly understood. Based on 2,736 observations along soil profiles of 0-150 cm depth from 1955 to 2016, we evaluated the temporal changes in soil C-N-P stoichiometry across subtropical China, where soils are P-impoverished, with diverse vegetation, soil, and parent material types and a wide range of climate gradients. We found a significant overall increase in soil total C concentration and a decrease in soil total P concentration, resulting in increasing soil C:P and N:P ratios during the past 60 years across all soil depths. Though average soil N concentration did not change, soil C:N increased in topsoil while decreasing in deeper soil. The temporal trends in soil C-N-P stoichiometry differed among vegetation, soil, parent material types and spatial climate variations, with significantly increased C:P and N:P ratios for evergreen broadleaf forest and highly weathered Ultisols, and more pronounced temporal changes in soil C:N, N:P, and C:P ratios at low elevations. Our sensitivity analysis suggests that the temporal changes in soil stoichiometry resulted from elevated N deposition, rising atmospheric CO2 concentration and regional warming. Our findings revealed that the responses of soil C-N-P and stoichiometry to long-term global changes have occurred across the whole soil depth in subtropical China and the magnitudes of the changes in soil stoichiometry are dependent on vegetation types, soil types, and spatial climate variations. This article is protected by copyright. All rights reserved.

Plantation management can affect ecosystem soil carbon (C) and nitrogen (N) cycles. However, how different harvest residue management strategies impact soil C and N processes over the long term is largely unclear. In this study, we examined effects of harvest residue management on soil C and N concentrations, labile soil C and N pools and soil CO2 efflux (Rs) at different stages after Chinese fir (Cunninghamia lanceolata) was replanted in subtropical China. The residue management treatments were slash and burning, whole-tree harvesting, stem-only harvesting and stem only harvesting with double residue retention. Our results showed that the harvest residue management treatments did not differ significantly in their effect on soil C and N, mineral N (NH4+-N plus NO3−-N), dissolved organic C or total dissolved N concentrations, except for soil N concentrations in surface soil (0–10 cm) at year 3 and soil total dissolved N concentrations at year 12, which were significantly lower where the slash was burnt than in the double residue retention treatment. Similarly, Rs did not differ significantly among the four residue management strategies at year 15. Topsoil temperature and topsoil moisture were also unaffected by harvest residue management treatment. Soil temperature was found to be the most important factor controlling the temporal pattern of Rs, accounting for 65.8% of seasonal variation of Rs. There was no significant difference in temperature sensitivity of Rs (Q10) or annual Rs among the four treatments. These results indicated that harvest residue management may not significantly cause long-term effects on soil C cycling and N availability in subtropical Chinese fir plantations.

The knowledge of individual tree species impacts on soil respiration based on rigorous experimental designs is limited, but is crucial to help guide selection of species for reforestation and carbon (C) management purposes. We assessed monthly soil respiration and its components, litterfall input, fine root production and mortality under 19-year-old native coniferous Cunninghamia lanceolata and broadleaved Mytilaria laosensis plantations in sub-tropical China. Total soil respiration from October 2011 to March 2013 was significantly lower under the C. lanceolata than the M. laosensis plantation. The difference in respiration rates derived from fine roots and the litter layer explained much of the variation of total soil respiration between the two tree species. We used an exponential equation and base temperature (10 °C) to normalize soil respiration rate and its components (R10) and determined the correlation between R10 and soil moisture. Although soil moisture had a positive relationship with R10 derived from roots or litter under both C. lanceolata and M. laosensis forests, these positive correlations were masked by negative relationships between soil moisture and R10 derived from root-free soil, which resulted in a neutral correlation between total R10 and soil moisture under C. lanceolata forests. Monthly litterfall input was associated with variation in concurrent total soil respiration rate under the M. laosensis plantation and respiration rate lagging 3 months behind under the C. lanceolata plantation, which may suggest that litterfall input from M. laosensis can more rapidly produce C substrates for microbial respiration than litterfall from C. lanceolata. This study highlighted that tree species-induced variation in the quality and quantity of fine roots and litterfall can impact not only the soil respiration rate but also the seasonal variation model of forest soil respiration.

We investigated microbial biomass and composition (lipid profile), mineral N pools and soil physicochemical parameters in the top 5-cm soils 19 years after reforestation of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) woodland with itself or a native broadleaf species, Mytilaria laosensis. The results suggested that tree species transition had a large impact on microbial biomass and a small impact on the composition of the microbial community as indicated by the relative abundance of individual lipid biomarkers. Between November 2011 and October 2012, there was on average 50% greater microbial biomass carbon (C) measured by the fumigation extraction procedure under M. laosensis than under C. lanceolata. A one-time measurement of phospholipid fatty acids in soil samples collected in May 2012 suggested M. laosensis plots had greater content of individual lipid biomarkers than C. lanceolata plots. Using a litter manipulation experiment, we found that the increases in content of lipid biomarkers under M. laosensis can be attributed to changed litter chemistry. Analysis of soil mineral N pools indicated that there were significantly lower NH 4 + and NO 3 − pools as well as potential net N mineralization rates in M. laosensis soil than in C. lanceolata soil. The relationships among N dynamics, soil chemistry and microbial properties were analysed. The results suggested tree species induced differences in soil N mineralization rates and mineral N pools were related to labile C availability, soil C:N ratio and the composition of the microbial community. Our data of mineral N pools and soil δ15N implied that the transition of land use from C. lanceolata to M. laosensis leads to an enhanced N retention in the plantation.