Responses of soil enzyme activity to freeze-thaw alternation in Greater Khingan Mountains, China.

We examined the effects of freeze-thaw cycles (0, 1, 3, 5, 7, 15 cycles) on the activities of urease, invertase, and proteinase in the different layers of the soil under the four typical stands in the cold temperate zone, i.e., Pinus pumila stand, Rhododendron-Betula platyphylla stand, Rhododendron-Larix gmelinii stand, Ledum-Larix gmelinii stand, with the indoor freeze-thaw simulation culture method being used. The relationship between soil enzyme activity and multiple physicochemical variables was analyzed during freeze-thaw alternation. The results showed that the activity of soil urease was firstly increased and then inhibited during freeze-thaw alternation. After freeze-thaw, urease activity did not differ from that without freeze-thaw. Invertase activity was firstly inhibited and then increased during freeze-thaw alternation, and was significantly increased by 8.5%-40.3% after freeze-thaw. Proteinase activity was firstly increased and then inhibited during freeze-thaw alternation, and was significantly decreased by 13.8%-68.9% after freeze-thaw. After freezing and thawing, there was significant positive correlation between urease activity and ammonium nitrogen and soil water content in Ledum-L. gmelinii stand and P. pumila stand, respectively, and proteinase activity had significant negative correlation with inorganic nitrogen concentration in P. pumila stand, Rhododendron-B. platyphylla stand, and Ledum-L. gmelinii stand. Invertase activity had significant positive correlation with organic matter in Rhododendron-L. gmelinii stand and Ledum-L. gmelinii stand.

The shift of soil microbial community induced by cropping sequence affect soil properties and crop yield.

Rational cropping maintains high soil fertility and a healthy ecosystem. Soil microorganism is the controller of soil fertility. Meanwhile, soil microbial communities also respond to different cropping patterns. The mechanisms by which biotic and abiotic factors were affected by different cropping sequences remain unclear in the major grain-producing regions of northeastern China. To evaluate the effects of different cropping sequences under conventional fertilization practices on soil properties, microbial communities, and crop yield, six types of plant cropping systems were performed, including soybean monoculture, wheat-soybean rotation, wheat-maize-soybean rotation, soybean-maize-maize rotation, maize-soybean-soybean rotation and maize monoculture. Our results showed that compared with the single cropping system, soybean and maize crop rotation in different combinations or sequences can increase soil total organic carbon and nutrients, and promote soybean and maize yield, especially using soybean-maize-maize and maize-soybean-soybean planting system. The 16S rRNA and internal transcribed spacer (ITS) amplicon sequencing showed that different cropping systems had different effects on bacterial and fungal communities. The bacterial and fungal communities of soybean monoculture were less diverse when compared to the other crop rotation planting system. Among the different cropping sequences, the number of observed bacterial species was greater in soybean-maize-maize planting setup and fungal species in maize-soybean-soybean planting setup. Some dominant and functional bacterial and fungal taxa in the rotation soils were observed. Network-based analysis suggests that bacterial phyla Acidobacteria and Actinobacteria while fungal phylum Ascomycota showed a positive correlation with other microbial communities. The phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) result showed the presence of various metabolic pathways. Besides, the soybean-maize-maize significantly increased the proportion of some beneficial microorganisms in the soil and reduced the soil-borne animal and plant pathogens. These results warrant further investigation into the mechanisms driving responses of beneficial microbial communities and their capacity on improving soil fertility during legume cropping. The present study extends our understanding of how different crop rotations effect soil parameters, microbial diversity, and metabolic functions, and reveals the importance of crop rotation sequences. These findings could be used to guide decision-making from the microbial perspective for annual crop planting and soil management approaches.

[Co-accumulation characters of soil organic carbon and nitrogen under broadleaved Korean pine and Betula platyphylla secondary forests in Changbai Mountain, China.] / 长白山阔叶红松林和杨桦次生林土壤有机碳氮的协同积累特征.

The retrogressive succession is an important driver for dynamics of soil organic carbon (SOC) and total nitrogen (TN). We studied the quantitative distribution and synergistic accumulation characteristics of soil organic carbon and nitrogen in the primary broadleaved Korean pine (KP) forest and Betula platyphylla (BP) secondary forest in Changbai Mountain through paired plot approach. Further, we analyzed the changes of carbon pool and carbon sink effect in temperate forest soil caused by secondary succession and their carbon-nitrogen coupling mechanism. The results showed that the BP forest accumulated more organic carbon and nitrogen in the surface and subsurface soil (0-20 cm) than the KP forest, with relatively low soil C/N. Compared with KP forest, soil organic carbon storage in BP forest (0-20 cm) was higher by 14.7 t·hm-2, equivalent to a soil carbon sink gain of 29.4 g·m-2·a-1. SOC and TN concentrations were positively correlated in each soil layer of all forest types, causing a co-accumulative relationship between SOC and TN. The coefficient of determination (R2) between SOC and TN in the upper soil layers of BP forest was significantly higher than that of the KP forest, indicating that SOC accumulation under the relatively N-rich BP forest was more dependent on the accumulation of organic nitrogen. In the upper soil layers (0-10 cm) where organic matter concentrated, there was no significant difference in light fraction organic carbon and nitrogen stock between the two forest types, whereas the content, stock, and allocation percentage of heavy fraction organic carbon and nitrogen of BP forest were all significantly higher than that of the KP forest, with an average increment of 8.5 t·hm-2 in heavy fraction organic carbon stock. Those results indicated that the increase of soil organic carbon and nitrogen during secondary succession was mainly due to the increases of soil organic carbon and nitrogen pools in mineral-bound stability. The carbon-nitrogen coupling mechanisms in litter decomposition and soil organic matter formation was an important driving mechanisms underlying the changes of soil organic carbon and nitrogen pools during secondary succession.

[Soil bacterial community structure and predicted functions in the larch forest during succession at the Greater Khingan Mountains of Northeast China]. / å¤§å ´å®‰å²­è½å¶æ¾æž—ä¸åŒæ¼”æ›¿é˜¶æ®µåœŸå£¤ç»†èŒç¾¤è½ç»“æž„ä¸ŽåŠŸèƒ½æ½œåŠ¿.

To reveal soil bacterial community structure and potential functions in larch forest during succession at Greater Khingan Mountains (Hanma National Nature Reserve), 16S rDNA was sequencing by Illumina Miseq. The results showed that the Proteobacteria, Acidobacteria, Verrucomicrobia, Bacteroidetes, Actinobacteria, Planctomycetes and Chloroflexi were the most dominant phyla in soils of larch forests at various successional stages. Along forest succession, Acidobacteria increased, while Chloroflexi decreased. Relative abundance of dominant phyla was different at various successional stages. The α diversity, Chao1, Shannon index and Simpson index of soil bacterial community had no significant difference among five succession stages, while significant differences in soil bacterial community structure were observed between young and medium larch, between young and over mature larch, and between near mature and mature larch. Bacterial community structure was mainly influenced by redox potential, pH and available phosphorus. The redox potential was the most important factor influencing soil bacterial community structure. Along the succession of larch forest, N-fixation, denitrification, ammonia oxidation and lignin breakdown decreased, dissimilatory sulfate reduction had down-up trend, carbon fixation had up-down trend, and alkaline phosphatase had no apparent trend. Bacterial community potential function was mainly influenced by redox potential and available phosphorus.

Effects of exogenous organic acids on the resistance of Changbai larch (Larix olgensis) seedlings to mixed Pb and Cd stress.

Mixed Pb and Cd soil contamination is an issue in Northeast China. We examined the effects of exogenous organic acids on the resilience of Changbai larch (Larix olgensis) seedlings, a pioneering forestry species in afforestation and vegetation restoration in Northeast China, under such stress. Mixed Pb and Cd stress led to significantly higher Pb and Cd content in the leaves and fine roots, malondialdehyde content in the leaves, superoxide dismutase activity, and soluble protein content in the leaves. Lower biomass of the roots, stems, and leaves was observed, with the roots showing the sharpest reduction in biomass. However, the application of organic acids mitigated or reversed these effects. This was most pronounced following treatment with 0.2 mmol·L-1 or 1.0 mmol·L-1 organic acids for 20 days. Citric acid had the greatest positive effect compared with succinic acid and oxalic acid. We suggest that exogenous organic acids have the potential to alleviate Pb and Cd-induced oxidation injury symptoms in Changbai larch seedlings, and may enhance resilience to mixed Pb and Cd stress.

[Black carbon content and distribution in different particle size fractions of forest soils in the middle part of Great Xing'an Mountains, China.] / å¤§å ´å®‰å²­ä¸­æ®µæ£®æž—åœŸå£¤çš„é»‘ç¢³å«é‡åŠå ¶åœ¨ä¸åŒç²’çº§ä¸­çš„åˆ†å¸ƒ.

Soil black carbon (BC) is considered to be the main component of passive C pool because of its inherent biochemical recalcitrance. In this paper, soil BC in the middle part of Great Xing'an Mountains was quantified, the distribution of BC in different particle size fractions was analyzed, and BC stabilization mechanism and its important role in soil C pool were discussed. The results showed that BC expressed obvious accumulation in surface soil, accounting for about 68.7% in the whole horizon (64 cm), and then decreased with the increasing soil depth, however, BC/OC showed an opposite pattern. Climate conditions redistributed BC in study area, and the soil under cooler and moister conditions would sequester more BC. BC proportion in different particle size fractions was in the order of clay>silt>fine sand>coarse sand. Although BC content in clay was the highest and was enhanced with increasing soil depth, BC/OC in clay did not show a marked change. Thus, the rise of BC/OC was attributed to the preservation of BC particles in the fine sand and silt fractions. Biochemical recalcitrance was the main stabilization mechanism for surface BC, and with the increasing soil depth, the chemical protection from clay mineral gradually played a predominant role. BC not only was the essential component of soil stable carbon pool, but also took up a sizable proportion in particulate organic carbon pool. Therefore, the storage of soil stable carbon and the potential of soil carbon sequestration would be enhanced owing to the existence of BC.

[Seasonal dynamics of soil organic carbon mineralization for two forest types in Xiaoxing'an Mountains, China].

To investigate the seasonal dynamics of soil organic carbon (SOC) mineralization in Xiaoxing'an Mountain, we incubated soil samples collected from virgin Korean pine forest and broad-leaved secondary forest in different seasons in the laboratory and measured the SOC mineralization rate and cumulative SOC mineralization (Cm). We employed simultaneous reaction model to describe C mineralization kinetics and estimated SOC mineralization parameters including soil easily mineralizable C (C1), potentially mineralizable C (C0). We also analyzed the relations between Cm, C1and their influencing factors. Results showed that the incubated SOC mineralization rate and Cm for 0-5 cm soil layer decreased from early spring to late autumn, while for 5-10 cm soil layer the seasonal variation was not statistically significant for both forest types. The C1 in 0-5 and 5-10 cm soil layers varied from 42.92-92.18 and 19.23-32.95 mg kg⁻¹, respectively, while the C0 in 0-5 and 5-10 cm soil layers varied from 863.92-3957.15 and 434.15-865.79 mg · kg⁻¹, respec- tively. Both C1 and C0 decreased from early spring to late autumn. The proportions of C0 in SOC for two forest types were 0.74%-2.78% and 1.11%-1.84% in 0-5 and 5-10 cm soil layers, respectively, and decreased from early spring to late autumn, indicating that SOC tended to become more stable as a whole from spring to autumn. The Cm and C0 were significantly positively correlated to in situ soil water content and hot water-extractable carbohydrate content, but were not correlated to in situ soil temperature and cool water-extractable carbohydrate content. We concluded that soil labile organic carbon, soil physical and chemical properties contributed to the seasonal dynamics of SOC mineralization in the forests.

[Soil organic carbon pools and their turnover under two different types of forest in Xiao-xing'an Mountains, Northeast China].

Soil samples collected from virgin Korean pine forest and broad-leaved secondary forest in Xiaoxing'an Mountains, Northeast China were incubated in laboratory at different temperatures (8, 18 and 28 °C) for 160 days, and the data from the incubation experiment were fitted to a three-compartment, first-order kinetic model which separated soil organic carbon (SOC) into active, slow, and resistant carbon pools. Results showed that the soil organic carbon mineralization rates and the cumulative amount of C mineralized (all based on per unit of dry soil mass) of the broad-leaved secondary forest were both higher than that of the virgin Korean pine forest, whereas the mineralized C accounted for a relatively smaller part of SOC in the broad-leaved secondary forest soil. Soil active and slow carbon pools decreased with soil depth, while their proportions in SOC increased. Soil resistant carbon pool and its contribution to SOC were both greater in the broad-leaved secondary forest soil than in the virgin Korean pine forest soil, suggesting that the broad-leaved secondary forest soil organic carbon was relatively more stable. The mean retention time (MRT) of soil active carbon pool ranged from 9 to 24 d, decreasing with soil depth; while the MRT of slow carbon pool varied between 7 and 24 a, increasing with soil depth. Soil active carbon pool and its proportion in SOC increased linearly with incubation temperature, and consequently, decreased the slow carbon pool. Virgin Korean pine forest soils exhibited a higher increasing rate of active carbon pool along temperature gradient than the broad-leaved secondary forest soils, indicating that the organic carbon pool of virgin Korean pine forest soil was relatively more sensitive to temperature change.