Fine Root Dynamics Of Four Typical Xerophilous Shrubs And Their Effects On Soil Organic Carbon

Present understanding of the terrestrial ecosystem carbon cycle is limited by uncertainty over belowground organic carbon dynamics. Fine root (< 2 mm in diameter) dynamics play an important role in matter cycling and energy flow, mediate the potential uptake and cycling of nutrients and water in terrestrial ecosystems and control carbon inputs from plant to soil. Therefore, the estimation of fine root dynamics are helpful to understand geo-biochemical cycles. In semiarid and arid areas of China, restoration of vegetation on sand land and afforestation on degraded pasture are effective measures for controlling desertification, however, informations about belowground carbon dynamics still remain unclear and the effect of fine roots, acts as the most active root part, on soil organic carbon accumulation is unknown. In this study, we investigated the fine root biomass and distribution, fine root production, fine root turnover, fine root decomposition and soil organic carbon content under four shrublands (Salix psammophila, Artemisia ordosica, Hedysarum mongolicum, and Caragana microphylla) and degraded pasture, using soil cores, ingrowth cores, minirhizotron technique, litterbag technique and soil cores, respectively. The results as follows:(1) This competition was indicated by the influence of the establishment of C. microphylla on the belowground fine root production of the system. However, the rooting strategies of grass fine roots were scarcely affected by established C. microphylla. Belowground interactions between C. microphylla and herbeaouse plants might be explaint by the two-layer hypothesis, which assumes that herbeaous plants are more efficient in using water from surface soil than trees or shrubs. After revegetations on the stable dunes, fine root biomass, distribution and production of S. psammophila were considerably greater than these of A. ordosica and H. mongolicum. The results indicated that different shrubs have significantly different fine root distributions and forage strategies. The roots of S. psammophila fully explored soil patch with developed fine root system. Fine root distribution and forage strategy of A. ordosica were similar to these of herbeaous plants with some fine roots in deeper soils. Additionally, fine roots of H. mongolicum fully occupied the deeper soils and avoided the surface soil layer with periodic drought.(2) Fine root lifespans of H. mongolicum and C. microphylla were significantly higher than these of A. ordosica and S. psammophila, fine root turnover rates showed reverse comparison. Fine root systems of A. ordosica and S. psammophila were renewed for adapt periodical drought, however, H. mongolicum and C. microphylla ensured fine root survival to reduce carbon investments.(3) Fine root necromass of the four shrubs experienced an initial period of very slow loss, a later period of rapid and a final period of slower loss. Fine root decomposition (carbon release) rate of C. microphylla was the fastest, followed by A. ordosica, and S. psammophila and H. mongolicum had the slowest decomposition rate. Fine root decomposition rates were positively related to soil water content and the quality of decomposition substrates, but were not significantly affected by soil temperature.(4) A paired-site experiment showed the conversion of degraded pasture to pea shrubland (C. microphylla) resulted in decreased stable soil organic carbon, which was attributed to differences of fine roots, at least in part. Fine roots of herbeaous plants might be conducive to soil organic carbon accumulation. Over a 10 year period of revegetation, the soil organic carbon content in A. ordosica, H. mongolicum, and S. psammophila plots increased in different degree, in comparision to bare-land plot. Increase in total soil organic carbon content was mainly due to increase in light-fraction soil organic carbon, except for the A. ordosica plot.(5) The mechanisms of SOC accumulation and sequestration differed among shrub plots and the degraded pasture, which were not completely explained by fine root dynamics (distributions, biomass, production, and turnover). The variations in soil organic carbon might be partially explaint by fine root decomposition rates. Therefore, based on soil organic carbon accumulation, A. ordosica may be a better species in the study site, and degraded pasture should be protected for vegetation restoration.In this study, fine root dynamics and soil organic carbon stocks of the four sand-fixed shrubs and degraded pasture were measured and evulated in the Mu Us Desert, and their forage strategies for water and adaptive mechanisms for drought were also assessed. The results suggest the mechanism of SOC accumulation differed among shrub plots, which was partially attributed to the differences in fine roots, and highlight the effectiveness of different shrub species as revegetation materials in terms of soil organic carbon accumulation. Therefore, comparing fine root dynamics and soil organic carbon accumulation under different revegetations may provide new insights into belowground carbon dynamics and regional carbon cycles in the arid and semiarid areas.The results are helpful to understand belowground carbon cycles and provide threoretical supports for regulations and managements of shrublands in semi- and arid areas. In future study, it is necessary to explore the response of fine root dynamics to global warming and their contribution rate to soil organic carbon.