Abstract Purpose Afforestation is increasingly recognized as an effective measure to mitigate elevated atmospheric carbon (C) dioxide and combat climate change. While afforestation can increase C sequestration by biomass production with tree growth, little is known about whether and how tree growth affects soil organic carbon (SOC) stocks and stability. Here, we aimed to explore mechanisms underlying changes in SOC stocks and fractions with stand development from the perspective of tree-microbe-mineral interactions. Methods We measured annual litter inputs, soil exchangeable base cations, microbial biomass, hydrolytic and oxidative enzymes, and SOC stocks and fractions along an age-sequence of Mongolian pine ( Pinus sylvestris var. mongolica ) plantations with six age classes ranging from 15- to 61-year-old and adjacent grasslands in the Keerqin Sandy Lands, Northeast China. Results We found that afforestation of grasslands did not significantly affect 0–100 cm SOC stocks. Ecosystem C stocks linearly increased with stand development, and this C accretion was mainly attributed to tree biomass C sequestration. Topsoil (0–10 cm soil layer) mineral-associated C (MAOC) stocks and phenol oxidase activities increased, and particulate organic C (POC) stocks and β-glucosidase activities decreased with increasing stand age, but these changes disappeared in the 61-year-old stand. Structural equation model revealed that topsoil MAOC stocks were directly related to microbial biomass and β-glucosidase and phenol oxidase activities, but not directly to exchangeable calcium concentrations. Moreover, topsoil POC stocks were directly related to β-glucosidase activities, but not directly to annual litter inputs. Conclusions Altogether, our findings suggest that soil microbes play a central role in mediating the dynamics of SOC stocks and stability along stand development.
To realize the sustainable management of forest ecosystems, we should explicitly clarify the types and differences of the ecosystem services provided by different ecosystems under different conditions, with rethinking about the value of forest ecosystems; then solid management strategies and measurements will be enacted and applied to achieve the objects. The broad-leaved Korean pine forest (BLKPF) in Changbai Mountain is a unique and important forest type in China, owing to its many important ecosystem services such as preventing soil erosion, regulating climates, nutrient cycling, providing wood and non-timber forest products, etc. This paper is a preliminary study on the management strategy of BLKPF on the basis of analyzing the characters of the ecosystems and the relative importance of services they provided in this region. Based on the latest research of ecosystem services of BLKPF in Changbai Mountain, an idea of eco-value level (EVL) was introduced, and accordingly, management strategies were summarized by adopting the advanced theories in ecosystem management science and by analyzing field survey data. EVL means the relative amount of the value of ecosystem services provided by certain ecosystem, which can indicate the difference between services in given objects. The EVL classification of BLKPF implies the relative amount of the eco-value of different ecosystems including virgin forest, secondary forest, forest with human disturbance, and man-made forest in the clear-cutting sites. Analytical Hierarchical Processing method was used to formulate the equation for EVL index. Eight factors, namely, slope, soil depth, stability of soil maternal material, coverage of above-ground canopy, species diversity, regeneration rate of the stand, life span of dominant tree species, and intensity of human disturbance were chosen to build the formula. These factors belonged to three aspects affecting ecosystem services including the physical environment, community, and disturbance regime, and their selection and scaling were based on the previous studies on the BLKPF. The equation of EVL index (EI) was expressed as: EI = 0.542A1 + 0.171A2 + 0.072A3 + 0.067B1 + 0.043B2 + 0.014B3 + 0.010B4 + 0.081C1. According to the range of EI, ecosystems were classified into three types: low EVL type with EI from 1.000 to 1.874, medium EVL type with EI 1.874-2.749, and high EVL type with EI 2.749-3.623. Typical plots were surveyed and scaled with EI, and the predominant characters of each EVL type were summarized. Most forests of high EVL type were those in sites at high risk of soil erosion and hard to recover after disrupted. Forests of medium EVL type were those with worse community structure and composition, and were disturbed by human activities in relative steep sites. Forest of low EVL type were those in plane site with serious disruption or some young man-made stands. Based on the analyses of the characters of these three types, different management strategies were put forward. For high EVL type forest, strictly protection is most important to maintain the forest in natural succession and its eco-services. For medium EVL type forest, the key points of management are restoring their health and vigor by regulating their composition and structure in a seminatural way. For low EVL type forest, some area could be used to extensive exploration for economic benefits, and the rests should be reconstructed towards the original stand in composition and structure, based on the 'shadow ecosystem' in a close-to-nature way to promote the capacity of providing more eco-services.
A factorial nitrogen (N) × phosphorus (P) addition experiment was conducted to evaluate responses of leaf nutrient resorption to increased soil N and P availability in a semiarid grassland in Keerqin Sandy Lands, China. Four plant species were selected, among which Artemisia scoparia and Chenopodium acuminatum were dominant species in the control and P-added plots, and Cannabis sativa and Phragmites communis were dominant in the N- and N + P-treated plots. Results showed that N and P resorption varied substantially among species (P < 0.01). A general trend of decrease in N resorption efficiency (NRE) and N resorption proficiency (NRP) was observed in response to increased soil N availability for all species, except P. communis only for NRE. Similarly, P resorption proficiency (PRP) decreased in response to P addition for all species, whereas P resorption efficiency (PRE) was not affected by P addition. Species responded differently in terms of PRE and PRP to N addition, whereas no changes in NRE and NRP occurred in response to P addition except P. communis for NRE. Our results suggest that increased soil nutrient availability can influence plant-mediated nutrient cycling directly by changing leaf nutrient resorption and indirectly by altering species composition in the sandy grassland.
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