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Dissolved organic matter (DOM) plays a key role in soil organic matter (SOM) decomposition via the priming effect (PE). The DOM-induced soil PE is closely related to nutrient availability, especially nitrogen (N). Regardless of the widespread of chronic N addition, how elevated N deposition affects DOM-induced PEs remains poorly understood. To fill this knowledge gap, we studied the effects of N addition, 13C-labeled leaf-DOM (herein DOM) addition, and leaf-DOM plus N addition (DOM+N) on soil PEs in soils of a subtropical Chinese-fir (Cunninghamia lanceolata) plantation and a natural Castanopsis carlesii forest (hereafter referred to as Chinese-fir soil and Castanopsis soil, respectively). Soil properties (e.g., soil organic C, total N, available phosphorus, and ratio of C and N), dissolved organic C (DOC), soil microbial biomass C (MBC), phospholipid fatty acid (PLFA), and enzyme activities were also investigated, because these parameters predominantly affect the intensity and direction of soil priming. The addition of DOM induced positive PEs in the Castanopsis soil but negative PEs in the Chinese-fir soil. In addition, DOM addition increased MBC and fungal abundance and the activities of phenol oxidase (PhOx) and peroxidase (Perox) in the Castanopsis soil but not in the Chinese-fir soil. Compared with DOM-only addition, DOM+N addition significantly enhanced PEs in the Chinese-fir soil but not in the Castanopsis soil. Furthermore, compared with DOM-only addition, DOM+N addition significantly increased MBC, abundance of fungi and AMF, fungi to bacteria ratio (F:B), and activities of four enzymes [β-glucosidase (βG), N-acetyl glucosaminidase (NAG), PhOx, and Perox] in the Chinese-fir soil but not in the Castanopsis soil. The DOM+N addition also had a significant effect on composition of main microbial groups in the Chinese-fir soil but not in the Castanopsis soil. These results suggest the enhanced PE following DOM+N in the Chinese-fir soil was likely mediated by enhanced enzyme production associated with increased fungal abundance. Our study highlights that the effects of increases of DOM on soil C cycling is largely affected by N availability and mediated by the effects on the abundance of soil microbial groups and enzyme activities. Our result also demonstrated a case in which effects of DOM and N addition on soil C cycling differ between a Castanopsis forest and a Chinese-fir plantation forest, with Chinese-fir soil being more sensitive to N addition. This is an important finding that needs to be taken into consideration in estimating the soil C pools.

Changes in soil abiotic and biotic properties can be powerful drivers of feedback between plants and soil microbial communities. However, the specific mechanisms by which seasonal changes in environmental factors shape soil microbial communities are not well understood. Here, we collected soil samples from three sites along an elevational gradient (200–1200 m) in subtropical forests with unvarying canopy vegetation. We used an elevation gradient with similar annual precipitation but a clear temperature gradient, and phospholipid fatty acids (PLFAs) were measured to determine the seasonal variations in the composition of soil microbial communities in response to rising temperatures. Our results showed that the abundance of Gram-negative bacteria and total PLFAs were the lowest at low elevations in winter, and the ratio of Gram-positive to Gram-negative bacteria decreased with increasing elevation. However, the biomass of other microbial groups was the highest at medium elevations in summer, with the exception of actinomycetes species and fungi. Regardless of seasonal changes, soil fungal biomass tended to increase with increasing elevation. Moreover, in summer, microbial carbon use efficiency (CUE) increased with increasing elevation, whereas an opposite trend was observed in winter. Redundancy analysis and structural equation modeling showed that the dissolved organic carbon in soil was the main factor affecting the microbial communities along the elevation gradient in winter, whereas in summer, the microbial community structure was driven by shifting nitrogen availability, with both being associated with changing microbial CUE. As such, this study demonstrates distinct seasonal changes in the soil microbial community composition across an elevation gradient that are driven by carbon and nitrogen resource availability and shifts in microbial CUE. Furthermore, our results suggest that the interaction of underground plant roots and microbes drives changes in resource availability, thereby resulting in seasonal variation in soil microbial community composition across an elevation gradient.

The original version of this article, unfortunately, contained an error. In Figure 2 - panel d, incorrect image was published and this is now presented correctly in this article.

Forests constitute a critical component of terrestrial carbon reservoirs, with a substantial amount of carbon stored in soil as organic carbon, holding significant potential for climate change mitigation [...]

Aims Increasing temperature and nitrogen (N) deposition are major drivers of global change that will influence plant-soil systems. We aimed to understand how plant stoichiometry and nutrient limiting types could change with continued warming and N inputs in subtropical regions. Methods In 2014, the experiments were established in 30 mini-plots (2 × 2 m) with the following treatments: control, high N addition, low N addition, warming, warming + high N addition, and warming + low N addition. We sampled the leaf and root of Cunninghamia lanceolata and soils to assess their elemental and stoichiometric variables and δ¹⁵N under all six conditions. Results Both experimental warming and N fertilization consistently induced an increase in fine-root N, P, and N:P. The N:P ratio of the mature green-leaf and soil was 7.24-11.63 and 4.79-6.56, respectively. On average, C. lanceolata showed higher proportional P resorption, but lower N resorption. The δ¹⁵N enrichment factor significantly increased in the warming and N addition treatments. Conclusions N addition decrease leaf N content, and increased the plant growth, which was due to the effect of the N dilution of C. lanceolata. In subtropical regions, N-limitation affects the growth of C. lanceolata, and the concurrent increase in warming and N fertilization should help relieve N-limiting conditions.

Forest litter inputs to soil can stimulate the decomposition of older soil organic matter (SOM) via a priming effect (PE). The magnitude and underlying mechanisms driving PE are poorly understood, with especially little know about how litter quality and site conditions affect PE in situ. Further, very few studies have examined PE in tropical and subtropical soils. Here, we established low and high elevation sites (600 vs. 1,400 m a.s.l.) in the subtropical Wuyishan National Park, China, that differed with respect to mean annual temperature (MAT; ∆MAT = 4.2°C), vegetation, soil texture and soil moisture. We conducted a 1‐year field incubation study at these two sites to compare PE induced by adding low‐ and high‐quality 13C‐labelled leaf litter to soils. At the low elevation site, additions of high‐quality (low C/N) litter caused a PE that was 140% greater than the PE observed following additions of low‐quality (high C/N) litter. In contrast, we saw no significant differences in PE between litter types at the high elevation site, perhaps because PE was not limited by substrate quality at this cooler, finer textured and higher soil moisture coniferous site. In addition, we found a negative relationship between home‐field advantage (HFA) for litter decomposition and PE, indicating that specialized litter decomposer community driving HFA may not accelerate SOM decomposition via PE in the same way. In line with our observed strong relationship between PE and the efficiency of priming (PE size per unit of mineralized litter C), PEs induced by the high‐ and low‐quality litters were directed to microbial phosphorus (P) mining rather than nitrogen (N) mining. This interpretation aligns with observed increases in the activity of P acquiring extracellular enzymes, often described as phosphatases (P‐tases), as well as the positive relationship between the PE, P‐tase activity and the activity of C acquiring extracellular enzymes. Overall, this PE study across two contrasting sites highlights the important role of site characteristics and litter quality in regulating PE size. Further, we suggest that MAT may be a dominant driver of soil priming, through both the direct effects of litter quantity on labile substrate supply and the indirect effects of litter quality changes on downstream decomposer communities. A free Plain Language Summary can be found within the Supporting Information of this article. A free Plain Language Summary can be found within the Supporting Information of this article.

The conversion of natural forests to tree plantations alters the quality and decreases the quantity of litter inputs into the soil, but how the alteration of litter inputs affect soil organic matter (SOM) decomposition remain unclear. We examined SOM decomposition by adding 13C-labeled leaf-litter of Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) to soils from a natural evergreen broad-leaved forest and an adjacent Chinese fir plantation converted from a natural evergreen broad-leaved forest 42 years ago. Over 195 days, we monitored CO2 efflux and its δ13C, microbial biomass, and the composition of microbial groups by phospholipid fatty acids (PLFAs). To distinguish priming mechanisms, partitioning of C sources in CO2 and microbial biomass was determined based on δ13C. Leaf-litter addition to natural forest increased microbial biomass and induced up to 14% faster SOM decomposition (positive priming) than that in soil without litter. In contrast, negative priming in soils under plantation indicated preferential use of added leaf-litter rather than recalcitrant SOM. This preferential use of leaf-litter was supported by an increased fungal to bacterial ratio and litter-derived (13C) microbial biomass, reflecting increased substrate recalcitrance, the respective changes in microbial substrate utilization and increased C use efficiency. The magnitude and direction of priming effects depend on microbial preferential utilization of new litter or SOM. Concluding, the impact of coniferous leaf-litter inputs on the SOM priming is divergent in natural evergreen broad-leaved forests and plantations, an important consideration in understanding long-term C dynamics and cycling in natural and plantation forest ecosystems.

Aims Net primary productivity is expected to increase in many forests as Earth warms, which can increase litter inputs to soils and affect carbon (C) and nitrogen (N) dynamics. Understanding how increasing litter inputs affect soil C and N cycling in tropical and subtropical forests is important because they represent some of the most productive ecosystems on Earth, suggesting that small changes in these cycles can have large effects. Methods To test the effects of increased litter inputs and the interactive effect between microbes and roots on soil C and N stocks and dynamics, we manipulated litter inputs and used trenching to exclude roots in a 40-year-old Cunninghamia lanceolata Lamb. (Chinese fir) plantation. At the site, we measured soil C and N pools, soil 13 C and 15 N natural abundance, and potential activities for C-, N-, and phosphorus-acquiring enzymes. Results After four years of experimental treatment, we found that increasing litter inputs reduced total soil N content by 26% relative to background litter inputs, but that increasing litter inputs did not affect soil C content in the plots with roots. In the plots without roots, both soil N and C did not change in response to litter inputs. In the plots with roots, soil δ 15 N increased with increasing litter inputs, but there was no effect in the plots without roots. We found a strong interactive effect between root and litter treatment on soil N pools and δ 15 N. The decline in soil N pools and increase in soil δ 15 N were associated with elevated potential enzyme activity for N-acquisition (N-acetyl glucosaminidase). Conclusions Adding litter did not have a significant effect on soil C pools, likely because potential soil C losses were offset by increasing litter-derived C inputs. In contrast to C, adding litter decreased N availability, likely through multiple pathways including gaseous N losses, NO 3 − leaching, root N uptake, and interactions between saprotrophic microbes and roots during the four-year litter addition experiment. Global changes that increase litter production may lower N pools and imbalance C and N cycling in subtropical coniferous forest ecosystems.

Estimation of forest biomass is of great significance for determining the most effective way to analyze carbon storage and dynamics. To enhance the accuracy of such estimations, the development of locally not pantropically—derived reliable allometric models is advised whenever possible, especially for dominant and widely distributed tree species, such as in the Castanopsis carlesii H. ( C. carlesii ) forests in subtropical China. Here, C. carlesii allometric equations were developed and applied to examine for three subtropical forests: a natural primary forest, an artificial-assisted naturally regenerated secondary forest and a C. carlesii plantation. To develop these allometric equations, destructive measurements of architecture and biomass of above- and below-ground components were undertaken for 33 sample trees from 10 dominant species, with stem diameters (at 1.30 m, or above buttresses) ranging from 4 to 67 cm. The mixed-species regressions with only the diameter at breast height as a predictor gave the best-fitting allometric relationship for biomass estimation of foliage, branch and coarse roots; the inclusion of tree height gave the best-fitting allometric relationship for estimation of stem and total biomass. Adding wood density did not improve model performance. Dominant-species regression for C. carlesii was able to predict biomass well using only diameter at breast height as a metric, but adding height and wood density slightly improved the goodness-of-fit, indicating that wood density and tree height may be crucial factors in above- and below-ground biomass models of these subtropical forests. The mixed-species regressions were able to predict well the total biomass of both primary and secondary forests, while the dominant-species models gave a better fit for estimating biomass of the C. carlesii plantation. The biomass estimates of the secondary forest was significantly higher than those of the C. carlesii plantation, indicating that intense forest regeneration practices might cause the reduction of above- and below-ground forest biomass. Therefore, distinct models for the biomass estimation of mixed forests and pure plantation are likely to be needed to improve the accuracy of both biomass assessment protocols and estimations of C sequestration for subtropical forests.

As a consequence of changing global rainfall patterns, frequent extreme droughts will significantly affect plant growth and ecosystem functions. Fine roots and arbuscular mycorrhizal fungi (AMF) both facilitate Chinese fir nutrient uptake. However, how the growth of fine roots and AMF is regulated for the Chinese fir under drought conditions is unclear. This study used a precipitation reduction treatment (−50% throughfall) to study the seasonal effects of drought on a subtropical Chinese fir plantation. The effects measured included the fine root production, root diameter, specific root length, specific surface area, root tissue density, mycorrhizal hyphal density, spore number, mycorrhizal infection rate and total glomalin. Drought had no significant effect on Chinese fir fine root production but decreased the diameter and tissue density of primary and secondary roots while increasing the specific surface area of secondary roots. Additionally, drought significantly decreased the arbuscular mycorrhizal infection rate and significantly increased hyphal density. The results showed that drought caused the decrease in root diameter, which decreased the surface area available for AMF infection and led to the increase in mycorrhizal hyphal density. Redundancy analyses showed that soil-dissolved organic carbon and nitrogen were the key factors affecting AMF. Our results show that drought could enhance the cooperative strategy of nutrient and moisture absorption by roots and mycorrhizae of the Chinese fir, improving the resistance of Chinese fir growth to drought.