Conversion of drylands to paddy fields on former wetlands restores soil organic carbon by accumulating labile carbon fractions in the Sanjiang Plain, northeast China.

BACKGROUND: Since the 1990s, drylands have been extensively converted to rice paddy fields on the former wetlands in the Sanjiang Plain of northeast China. However, the influence of this successiveland-use change from native wetlands to drylands to rice paddy fields on soil organic carbon (C) dynamics remains unexplored. Here, we compared the difference in soil organic C stock among native wetlands, drylands, and paddy fields, and then used a two-step acid hydrolysis approach to examine the effect of this land-use change on labile C I (LPI-C), labile C II (LPII-C), and recalcitrant C (RP-C) fractions at depths of 0-15 cm, 15-30 cm, and 30-50 cm. RESULTS: Soil organic C stock at a depth of 0-50 cm was reduced by 79% after the conversion of wetlands to drylands but increased by 24% when drylands were converted to paddy fields. Compared with wetlands, paddy fields had 74% lower soil organic C stock at a depth of 0-50 cm. The conversion of wetlands to drylands reduced the concentrations of LPI-C, LPII-C, and RP-C fractions at each soil depth. However, land-use change from drylands to paddy fields only increased the concentrations of LPI-C and LPII-C fractions at the 0-15 cm and 30-50 cm depths. CONCLUSION: The conversion of drylands to paddy lands on former wetlands enhances the soil organic C stock by promoting labile C fraction accumulation, and labile C fractions are more sensitive to this successive land-use change than recalcitrant C fractions in the Sanjiang Plain of northeast China. © 2022 Society of Chemical Industry.

Alder encroachment alters subsoil organic carbon pool and chemical structure in a boreal peatland of Northeast China.

Boreal peatlands have been experiencing increased abundances of symbiotic dinitrogen-fixing woody plants (mainly alder species). However, how alder encroachment alters soil organic carbon (C) pool and stability is unclear. To examine the effects of alder encroachment on soil organic C, we measured soil organic C pool, phenol oxidase (POX) activity, organic C mineralization rate, and organic C chemical structure (alkyl C, O-alkyl C, aromatic C, and carbonyl C) using solid-state 13C nuclear magnetic resonance spectroscopy in the 0-10 cm, 10-20 cm, and 20-40 cm depths in the Alnus sibirica islands and adjacent open peatlands in the north of Da'xingan Mountain, Northeast China. A. sibirica islands had 28 %, 25 %, and 30 % greater POX activity and 36 %, 31 %, and 100 % higher organic C mineralization than open peatlands in the 0-10 cm, 10-20 cm, and 20-40 cm soil depths, respectively. Despite no significant changes in the 0-10 cm and 10-20 cm depths, alder encroachment reduced soil organic C pool in the 20-40 cm depth. Soil organic C pool in the 0-40 cm depth was lower in A. sibirica islands (298 Mg ha-1) than in the open peatlands (315 Mg ha-1). Moreover, alder encroachment increased alkyl (7 %) and carbonyl (57 %) C fractions but reduced O-alkyl C fraction (16 %) in the 20-40 cm depth, resulting in increased aliphaticity and recalcitrance indices. These findings suggest that alder encroachment will reduce soil organic C accumulation by accelerating microbial decomposition, and highlight that increased biochemical stabilization would attenuate soil organic C loss after alder expansion in boreal peatlands. Our results will help assess and project future C budgets in boreal peatlands.

Response of individual sizes and spatial patterns of Deyeuxia angustifolia to increasing water level gradient in a freshwater wetland.

The wetland plants are very sensitive to hydrological regimes. In this study, the individual sizes of a widely distributed species (i.e., Deyeuxia angustifolia) at three typical marshes with different water table depths (i.e., wet meadow (WM) marsh; seasonal inundated (SI) marsh; perennial inundated (PI) marsh) were investigated in the Sanjiang Plain of Northeast China. Concurrently, three primary point pattern processes (homogeneous Poisson (HP) process, homogeneous Thomas (HT) process, and inhomogeneous Thomas (IT) process) were used to model spatial patterns in the distribution at 0-50 cm scale for this tillering-cloning species. The plant height, diameter at breast height (d.b.h), internode number, branches number, and individual aboveground biomass of D. angustifolia decreased sharply with rising water level; however, its density and coverage increased first and then decreased as water level increases. The distribution of D. angustifolia totally diverged from the complete spatial randomness (CSR) model (i.e., HP process) suggesting strong aggregation at 0-50 cm scale in all marshes, and aggregated intensity enhanced with increasing water level. Interestingly, the spatial distribution of D. angustifolia fits better with the nested double-cluster model (i.e., IT process) at all scales in WM and SI marshes, indicating that there is a series of clustered patterns under the slight flood stress. However, the spatial pattern fits well with the Poisson cluster model (i.e., HT process) at all scales in PI marsh, implying the small-scale clustering disappeared with the intensification of flooding stress. Our results highlight that the D. angustifolia population could adapt to flooding stress in a certain degree via individual miniaturization strategies and multi-aggregation mechanisms in the freshwater wetlands.

[Difference in intra- and inter-specific competition of two endangered plant species (Toona ciliate var. pubescens and Taxus chinensis var. mairei)in the middle subtropical zone of China]. / ä¸­äºšçƒ­å¸¦æ¿’å±æ¤ç‰©æ¯›çº¢æ¤¿å’Œå—æ–¹çº¢è±†æ‰ç§å† ä¸Žç§é—´ç«žäº‰å·®å¼‚.

Endangered plant species are an important part of global biodiversity. To understand the competition patterns and mechanisms of endangered tree species from plant growth forms in the middle subtropical forest ecosystems, we examined the differences in intra- and inter-specific competitions between Toona ciliate var. pubescens (an intolerant of shade, deciduous species) and Taxus chinensis var. mairei (a tolerant of shade, evergreen species) in the Jiulingshan National Nature Reserve, Jiangxi Province. The results showed that intra-specific competition was dominant in the T. ciliate var. pubescens population, accounting for 66.4% of the total competition intensity. In contrary, the competitive intensity of T. chinensis var. mairei was dominated by the inter-specific competition, which accounted for 68.7% of the total competition intensity. The intra- and inter-specific competition intensity of both species decreased gradually with increasing tree diameter, indicating that competitive pressure was prevalent in small trees. T. ciliate var. pubescens was mainly affected by self-thinning due to intra-specific competition, whereas T. chinensis var. mairei was dominated by alien-thinning due to inter-specific competition. The small individuals of both species could develop into mature stage only after experiencing intense competitive selection during stand regeneration. Considering the substantial difference in the sources of competition pressures, different biodiversity conservation measures should be taken for the two endangered species with contrasting growth forms in the middle subtropical regions.

Alnus sibirica encroachment promotes dissolved organic carbon biodegradation in a boreal peatland.

Symbiotic dinitrogen (N2)-fixing trees have been expanding to boreal peatlands, yet its influence on dissolved organic carbon (DOC) biodegradation is unclear. Here, we measured DOC, ammonium­nitrogen (NH4+-N), nitrate­nitrogen (NO3--N), dissolved inorganic nitrogen (DIN), and dissolved total nitrogen (DTN) concentrations, specific ultraviolet absorbance at 254 nm (SUVA254), and humification index in the extracts obtained from peats in the 0-10 cm, 10-20 cm, and 20-40 cm depths in the open peatlands and Alnus sibirica islands in a boreal peatland, Northeast China. Afterwards, the peat extracts were used to assess the effect of N2-fixing woody plant expansion on DOC biodegradation with a 42-day incubation experiment. The expansion of A. sibirica significantly increased NH4+-N, NO3--N, DIN, and DTN concentrations, but did not produce a significant effect on SUVA254 and humification index in the extracts in each depth. Following A. sibirica expansion, DOC biodegradation was enhanced by 24.5%, 15.4%, and 38.3% at 0-10 cm, 10-20 cm, and 20-40 cm depths, respectively. Furthermore, DOC biodegradation was significantly and negatively correlated with DOC:DIN and DOC:DTN ratios, but exhibited no significant relationship with SUVA254 and humification index. This implied that improved N availability and associated shifts in C:N stoichiometry determined the increase in DOC biodegradation following A. sibirica expansion. Our findings suggest that N2-fixing tree encroachment promotes microbial decomposition of DOC through improved N availability in boreal peatlands, which may cause organic C loss from soils in these C-enriched ecosystems.

Effects of P addition on plant C:N:P stoichiometry in an N-limited temperate wetland of Northeast China.

Phosphorus (P) enrichment induced by anthropogenic activities results in modified plant nutrient status, which potentially alters the stoichiometry of carbon (C), nitrogen (N), and P in plants. However, how increased P availability changes plant C:N:P stoichiometry at different hierarchical scales is unclear in N-limited ecosystems. In this study, we conducted a four-level P addition experiment (0, 1.2, 4.8, and 9.6gPm(-)(2)year(-1)) to elucidate the effect of P enrichment on plant C:N:P stoichiometric ratios at both the species and community levels in a freshwater wetland in the Sanjiang Plain, Northeast China. We found that species- and community-level plant C:N:P stoichiometry responded consistently to six years of P addition, although there was a shift in species dominance. Phosphorus addition increased plant N and P concentrations and thus decreased C:N, C:P, and N:P ratios irrespective of the P addition levels. These similar change trends at different scales resulted from the identical responses of plant N and P concentrations in different species to P addition. Moreover, plant N concentration exhibited an increasing trend with increasing P addition levels, whereas plant C:N ratio showed a declining trend. At the community level, P addition at the rates of 1.2, 4.8, and 9.6gPm(-2)year(-1) decreased the C:N ratio by 24%, 27%, and 34%; decreased the C:P ratio by 33%, 35%, and 38%; and decreased the N:P ratio by 12%, 10%, and 6%, respectively. Our results indicate that the stoichiometric responses to P addition are scale-independent, and suggest that altered plant C:N:P stoichiometry induced by P enrichment would stimulate organic matter decomposition and accelerate nutrient cycles in N-limited temperate freshwater wetlands.