Multi-ecosystem services differently affected by over-canopy and understory nitrogen additions in a typical subtropical forest.

Obtaining a holistic understanding of the impacts of atmospheric nitrogen deposition on multiple ecosystem services of forest is essential for developing comprehensive and sustainable strategies, particularly in heavy N deposition regions such as subtropical China. However, such impacts remain incompletely understood, with most previous studies focus on individual ecosystem function or service via understory N addition experiments. To address this knowledge gap, we quantified the effects of over-canopy and understory N additions on multiple ecosystem services based on a 7-year large-scale field experiment in a typical subtropical forest. Our results showed continued over-canopy N addition with 50 kg ha-1 year-1 over a period of 4-7 years significantly increased plant nutrient retention, but did not affect the services of soil nutrient accumulation, water yield, C sequestration (in plants and soil), or oxygen release. There were trade-offs between the soil and plant on providing the services of nutrient accumulation/retention and C sequestration under over-canopy N addition. However, without uptake and retention of tree canopy, the trade-off between soil and plant were more weaken under the understory N addition with 50 kg ha-1 year-1 , and their relationships were even synergetic under the understory N addition with 25 kg ha-1 year-1 . The results suggest that understory N addition cannot accurately simulate the effects of atmospheric N deposition on multiple services, along with mutual relationships. Interestingly, the services of plant N, P retention, and C sequestration exhibited a synergetic increase under the over-canopy N addition but a decrease under the understory N addition. Our results also found tree layer plays a primary role in providing plant nutrient retention service and is sensitive to atmospheric N deposition. Further studies are needed to investigate the generalized effects of forest canopy processes on alleviating the threaten of global change factors in different forest ecosystems.

Dynamics of community structure and bio-thermodynamic health of soil organisms following subtropical forest succession.

Soil organisms play essential roles in maintaining multiple ecosystem processes, but our understanding of the dynamics of these communities during forest succession remains limited. In this study, the dynamics of soil organism communities were measured along a 3-step succession sequence of subtropical forests (i.e., a conifer forest, CF; a mixed conifer and broad-leaved forest, MF; and a monsoon evergreen broad-leaved forest, BF). The eco-exergy evaluation method was used as a complement to the classic community structure index system to reveal the holistic dynamics of the bio-thermodynamic health of soil organism communities in a forest succession series. Association between the self-organization of soil organisms, soil properties, and plant factors were explored through redundancy analyses (RDA). The results indicated that the biomass of soil microbes progressively increased in the dry season, from 0.75 g m-2 in CF to 1.75 g m-2 in BF. Microbial eco-exergy showed a similar pattern, while the community structure and the specific eco-exergy remained constant. Different trends for the seasons were observed for the soil fauna community, where the community biomass increased from 0.72 g m-2 to over 1.97 g m-2 in the dry season, but decreased from 3.94 g m-2 to 2.36 g m-2 in the wet season. Faunal eco-exergies followed a similar pattern. Consequently, the average annual biomass of the soil faunal community remained constant (2.17-2.39 g m-2) along the forest succession sequence, while the significant seasonal differences in both faunal biomass and eco-exergy observed at the early successional stage (CF) were insignificant in the middle and late forest successional stages (MF and BF). Both the dynamics of soil microbes and soil fauna were tightly correlated with tree biomass and with soil physicochemical properties, especially soil pH, moisture, total nitrogen, nitrate nitrogen, and organic matter content.

Nitrogen budgets of a lower subtropical forest as affected by 6 years of over-canopy and understory nitrogen additions.

Although tropical and subtropical regions have replaced temperate regions as the global-change hotspots for increased atmosphere nitrogen (N) deposition, whether the regional forests reach N saturation is still unclear. Understory or floor N addition has been commonly used in N-deposition studies, but the results of such studies have recently been challenged because they fail to account for canopy interception, assimilation, and leaching processes. Here, we conducted a field experiment to quantify the effects of over-canopy and understory N addition on N budgets in a lower subtropical monsoon evergreen broadleaved (LSMEB) forest. We found that the LSMEB forest was not N saturated after receiving additional N at 25 and 50 kg ha-1 yr-1 for 6 years. Plants were able to absorb the added N by increasing the N concentrations in their organs, with 120-412 % increasing trend of plant N pools under N-addition treatments. Canopy absorption of N resulting from over-canopy N addition led to increases in N concentrations in tree organs but not to increases in tree biomass. Understory N addition could underestimate the effects of N deposition in forests due to neglecting canopy N interception and canopy effects on N redistribution. Additional experiments using over-canopy N addition are needed to assess the true effects of N deposition on different forest ecosystems in different climate zones.

Earthworm Pontoscolex corethrurus stimulated soil CO2 emission by enhancing substrate availability rather than changing microbiota community structure.

Earthworms may regulate carbon (C) mineralization through a top-down process by altering microbiota community and/or a bottom-up process by providing basic conditions such as mineralizable substrates. However, these two roles performed by earthworms have not been separately quantified. Here, we focused on how the pantropical widespread earthworm, Pontoscolex corethrurus, affected CO2 emission by changing soil microbiota community and substrate availability in infertile subtropical soils. Two experiments were performed. Firstly, we conducted a 3-year field experiment wherein P. corethrurus population was manipulated by electrical shocking. The two treatments were earthworm reduction (ER) and earthworm addition (EA). Thereby, we tried to understand CO2 emission pattern through the earthworm-induced changes in soil microbiota community and C-related enzyme activities in field plots. Secondly, a cross-soil inoculation microcosm experiment was conducted to partition the contributions of earthworm-regulated microbiota community and substrate quality to CO2 emission. The four treatments were 1) autoclaved ER soil + ER microbiota, 2) autoclaved EA soil + ER microbiota, 3) autoclaved ER soil + EA microbiota, and 4) autoclaved EA soil + EA microbiota. We found that, in the field experiment, earthworm addition changed soil microbiota community structure, but increased CO2 emission despite decreasing soil ß-glucosidase activity by 12%. In the cross-inoculation experiment, the ß-glucosidase activity and CO2 emission (1-day incubation) in EA soils was 65.5% and 35.5% greater than that in the ER soils, respectively; the cumulative CO2 emission (30-day incubation) in EA soils was also significantly greater than that in ER soils. However, no significant effects of microbiota inoculation on soil CO2 emissions were observed. These results suggested that the earthworm-enhanced substrate availability, rather than the earthworm-induced changing in the microbiota community structure and enzymatic activities, played a key role in C mineralization. This study implies that P. corethrurus occupies a "bottom niche" in infertile subtropical soils.

An alternative approach to reduce algorithm-derived biases in monitoring soil organic carbon changes.

Quantifying soil organic carbon (SOC) changes is a fundamental issue in ecology and sustainable agriculture. However, the algorithm-derived biases in comparing SOC status have not been fully addressed. Although the methods based on equivalent soil mass (ESM) and mineral-matter mass (EMMM) reduced biases of the conventional methods based on equivalent soil volume (ESV), they face challenges in ensuring both data comparability and accuracy of SOC estimation due to unequal basis for comparison and using unconserved reference systems. We introduce the basal mineral-matter reference systems (soils at time zero with natural porosity but no organic matter) and develop an approach based on equivalent mineral-matter volume (EMMV). To show the temporal bias, SOC change rates were recalculated with the ESV method and modified methods that referenced to soils at time t1 (ESM, EMMM, and EMMV-t1) or referenced to soils at time zero (EMMV-t0) using two datasets with contrasting SOC status. To show the spatial bias, the ESV- and EMMV-t0-derived SOC stocks were compared using datasets from six sites across biomes. We found that, in the relatively C-rich forests, SOC accumulation rates derived from the modified methods that referenced to t1 soils and from the EMMV-t0 method were 5.7%-13.6% and 20.6% higher than that calculated by the ESV method, respectively. Nevertheless, in the C-poor lands, no significant algorithmic biases of SOC estimation were observed. Finally, both the SOC stock discrepancies (ESV vs. EMMV-t0) and the proportions of this unaccounted SOC were large and site-dependent. These results suggest that although the modified methods that referenced to t1 soils could reduce the biases derived from soil volume changes, they may not properly quantify SOC changes due to using unconserved reference systems. The EMMV-t0 method provides an approach to address the two problems and is potentially useful since it enables SOC comparability and integrating SOC datasets.

Consistent effects of canopy vs. understory nitrogen addition on the soil exchangeable cations and microbial community in two contrasting forests.

Anthropogenic N deposition has been well documented to cause substantial impacts on the chemical and biological properties of forest soils. In most studies, however, atmospheric N deposition has been simulated by directly adding N to the forest floor. Such studies thus ignored the potentially significant effect of some key processes occurring in forest canopy (i.e., nitrogen retention) and may therefore have incorrectly assessed the effects of N deposition on soils. Here, we conducted an experiment that included both understory addition of N (UAN) and canopy addition of N (CAN) in two contrasting forests (temperate deciduous forest vs. subtropical evergreen forest). The goal was to determine whether the effects on soil exchangeable cations and microbial biomass differed between CAN and UAN. We found that N addition reduced pH, BS (base saturation) and exchangeable Ca and increased exchangeable Al significantly only at the temperate JGS site, and reduced the biomass of most soil microbial groups only at the subtropical SMT site. Except for soil exchangeable Mn, however, effects on soil chemical properties and soil microbial community did not significantly differ between CAN and UAN. Although biotic and abiotic soil characteristics differ significantly and the responses of both soil exchangeable cations and microbial biomass were different between the two study sites, we found no significant interactive effects between study site and N treatment approach on almost all soil properties involved in this study. In addition, N addition rate (25 vs. 50 kg N ha(-1) yr(-1)) did not show different effects on soil properties under both N addition approaches. These findings did not support previous prediction which expected that, by bypassing canopy effects (i.e., canopy retention and foliage fertilization), understory addition of N would overestimate the effects of N deposition on forest soil properties, at least for short time scale.

CAN Canopy Addition of Nitrogen Better Illustrate the Effect of Atmospheric Nitrogen Deposition on Forest Ecosystem?

Increasing atmospheric nitrogen (N) deposition could profoundly impact community structure and ecosystem functions in forests. However, conventional experiments with understory addition of N (UAN) largely neglect canopy-associated biota and processes and therefore may not realistically simulate atmospheric N deposition to generate reliable impacts on forest ecosystems. Here we, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. UAN and reviewed the merits and pitfalls of the two approaches. The following hypotheses will be tested: i) UAN overestimates the N addition effects on understory and soil processes but underestimates those on canopy-associated biota and processes, ii) with low-level N addition, CAN favors canopy tree species and canopy-dwelling biota and promotes the detritus food web, and iii) with high-level N addition, CAN suppresses canopy tree species and other biota and favors rhizosphere food web. As a long-term comprehensive program, this experiment will provide opportunities for multidisciplinary collaborations, including biogeochemistry, microbiology, zoology, and plant science to examine forest ecosystem responses to atmospheric N deposition.

Bird community comparisons of four plantations and conservation concerns in South China.

Plantations of non-native, fast-growing trees are increasing in the tropics and subtropics, perhaps with negative consequences for the native avifauna. We studied bird diversity in 4 types of plantations in South China to determine which plantation types are especially detrimental, and compared our findings with studies in nearby natural forests to assess the magnitude of the negative impact. A total of 57 species was recorded. The mean capture rate of understory birds was 1.7 individuals 100-net-h(-1). Bird richness and capture rate were lower in plantations than in nearby natural forests. Babblers (Timaliidae), primarily forest-dependent species in South China, were particularly under-represented in plantations. Species richness, composition and bird density, particularly of understory birds, differed between plantation types. Plantations of Schima, which is native to South China, had the highest species richness according to point count data. Plantations of Acacia (non-native) supported the highest understory species richness and produced the highest capture rate of understory birds, probably because of their complex structure and high arthropod abundance. If bird diversity is to be considered, we strongly recommend that future re-afforestation projects in South China should, as far as possible, use mixed native tree species, and especially Schima, ahead of the other species.

Maintenance of a living understory enhances soil carbon sequestration in subtropical orchards.

Orchard understory represents an important component of the orchards, performing numerous functions related to soil quality, water relations and microclimate, but little attention has been paid on its effect on soil C sequestration. In the face of global climate change, fruit producers also require techniques that increase carbon (C) sequestration in a cost-effective manner. Here we present a case study to compare the effects of understory management (sod culture vs. clean tillage) on soil C sequestration in four subtropical orchards. The results of a 10-year study indicated that the maintenance of sod significantly enhanced the soil C stock in the top 1 m of orchard soils. Relative to clean tillage, sod culture increased annual soil C sequestration by 2.85 t C ha(-1), suggesting that understory management based on sod culture offers promising potential for soil carbon sequestration. Considering that China has the largest area of orchards in the world and that few of these orchards currently have sod understories, the establishment and maintenance of sod in orchards can help China increase C sequestration and greatly contribute to achieving CO2 reduction targets at a regional scale and potentially at a national scale.

[Changes of soil nutrient contents after prescribed burning of forestland in Heshan City, Guangdong Province].

A comparative study was conducted to analyze the changes of soil nutrient contents in Eucalyptus forestland and in shrubland after three years of prescribed burning. In Eucalyptus forestland, soil organic carbon, total nitrogen, available potassium contents and soil pH decreased significantly; soil available phosphorus and exchangeable magnesium contents, net nitrogen mineralization rate and ammonification rate also decreased but showed no significant difference. In shrubland, soil exchangeable calcium content increased significantly, but the contents of other nutrients had no significant change. The main reason of the lower soil net nitrogen mineralization rate in Eucalyptus forest could be the decrease of available substrates and the uptake of larger amount of soil nutrients by the fast growth of Eucalyptus. The soil nutrients in shrubland had a quick restoration rate after burning.

[Characteristics of CO2, CH4 and N2O emissions from winter-fallowed paddy fields in hilly area of South China].

With closed static chamber and modified gas chromatograph (HP5890 II), the in situ measurements were made on the CO2, CH4 and N2O emissions from winter-fallowed paddy fields in the hilly area of South China. Gas samples were taken simultaneously from the fields with and without rice stubble. The results showed that both of the fields had the peak value of CO2 flux in the late afternoon. In the fields with and without rice stubble, the CH4 flux was positive in the day time while negative in the night, and the N2O flux in the day time was 1.79 and 1.58 times as much as that in the night, respectively. The diurnal average CO2 flux in the field with rice stubble was significantly higher than that in bare field (P < 0.05). Correlation analysis demonstrated that the CO2 flux in winter-fallowed paddy fields had significant correlations with soil temperature, aboveground temperature, and air temperature, suggesting that temperature was the main factor affecting the CO2 emission from rice field after harvesting. During the observation time (from 2003-11-10 to 2004-01-18), the average CO2, CH4 and N2O fluxes in the field with rice stubble were (180.69 +/- 21.21) mg x m(-2) x h(-1), (-0.04 +/- 0.01) mg x m(-2) x h(-1) and (21.26 +/- 19.31) microg x m(-2) x h(-1), respectively. Compared with bare field, the CO2 flux in the field with rice stubble was 13.06% higher, CH4 absorption increased by 50%, while N2O flux was 60.75% lower. It was concluded that the winter fallowed paddy field in hilly area of South China was the source of atmospheric CO2 and N2O, and the sink of atmospheric CH4.

[Artifical improvement of soil fertility in a regraded forest ecosystem by using municipal sewage sludge].

The increasing occurrence of forest ecosystem degradation is a serious problem in tropical and subtropical regions. Field experiments showed that the application of sludge from a sewage treatment plant could not only promote the growth and reproduction of trees, including the increase in the height and diameter of trees and thus being advantageous to the growth of shrub and herb of trees, but also improve soil fertility such as increasing soil organic matter and available nitrogen and phosphorus. The test of residual heavy metals of soil indicated that the application of sludge increased the content of Pb in the soil and the increment of Pb was varied with the increase of sludge usage. There was no significant increase in other heavy metals.