| Phyllostachys praecox Z.D. Chu et C.S. Chao f. prevelnalis, cultivated in Zhejiang,Jiangxi, Jiangsu, Anhui, Fujian province, et al., is an excellent bamboo used for bamboo shoots.With the implementation of cultivation techniques of winter soil-surface mulching andfertilization for early shooting and high yielding, the yield of bamboo shoots and economicoutput efficiency had been greatly increased. However, with the current annual soil-surfacemulching and the long-term application of chemical fertilizer, the stand structure becameirrational, the nutrient metabolism become disorder, the quality and yield of bamboo shootsdecreased substantially. As a result, the phenomenon of degradation became more and moreserious. Therefore, it is crucial to ensure the steady increment of the economic benefit andmaintain the stability of the structure and function in the meantime so as to realize thesustainable management for P. praecox. In this paper, the evaluation of vegetation degradationdegree was conducted for experimental forests of P. praecox in the city of Lin'an, Zhejiangprovince and Wannian County in Jiangxi province. Based on the scientific evaluation ofvegetation degradation, the characteristics of nutrient and its ecological stoichiometry werestudied for different components in the soil vegetation system (SVS) of different degradationdegrees of P. praecox. Temporal-spatial variability of soil, bamboo leaf and bamboo rhizomenutrients was discussed in the annual growth cycle of refoliation and branch developmentperiod, rhizome growing period, bud differentiation period, pregnancy and emergence forshoots period. The response characteristics of internal and external nutrient cyclings forbamboo leaf, nutrient stock of fine root in the nutrient cycling of different degradation degreesof P. praecox were also given consideration, which was tested through the characteristic ofnutrients ecological stoichiometry in the system of different degradation degrees of P. praecox.The aiming of the research was to distinguish the limiting nutrient element and its sensitive responding periods which could lay a theoretical foundation for remediation. The main resultswere as follows:1The evaluation of vegetation degeneration for P. praecox of different degrees ofmanagement was conducted based on the factors of stand structure and bamboo shoot yield.The P. praecox testing forests were classified into four categories in terms of vegetationdegradation degree, namely, non-degraded, light-degraded, moderate-degraded andheavy-degraded. With the increasing of degradation degree, average DBH and bamboo shootyield decreased gradually. The proportion of two-year-old standing bamboo decreased whilethat of three-year-old-and-over standing bamboo increased. The heavy degradation degrees of P.praecox had the lowest proportion of new bamboo which had low self-renewal capacity. Theage structure,average diameter of breast height (DBH) and bamboo shoot yield were mainresponding indexes for different degradation degrees of P. praecox.2There were obvious differences among different degradation degrees of P. praecox forthe nutrients responding characteristics. Different components in SVS showed differentnutrient characteristics for different functions. With the extension of implementation years, theenrichment of nutrients made the metabolism lose balance and the degradation degree becomeaggravating. The enrichment of P, Zn elements was the critical factors for the degradation of P.praecox. The levels of soil available nutrients were affected signicificantly by the cultivationtechniques. The effect of P element appeared obviously in the period of bud differentiationwhile that of Zn element was revealed in the period of refoliation and branch development.(1) Soil available nutrient content show significant difference among different degradationdegrees of P. praecox. Soil organic matter, soil available P and Zn contents were increasedsignificantly while available Mn decreased for heavy-degraded forests with the extension ofyears for soil-surface mulching. Light-degraded forests of P. praecox without soil-surfacemulching had the lowest soil available P content, which were54.82mg·kg-1in the period ofrefoliation and branch development,50.96mg·kg-1in the period of rhizome growing,61.92mg·kg-1in the period of bud differentiation,13.04mg·kg-1in the period of pregnancy and emergence for shoots, while soil available P content for heavy-degraded forests of P. praecoxwere477.19mg·kg-1,705.83mg·kg-1,544.63mg·kg-1and402.92mg·kg-1in the correspondingperiods, which were8.71,13.85,8.80and30.89times as much as that for light-degradedforests of P. praecox. Therefore, the significant increment of soil available P content caused bysoil-surface mulching was the distinguishing feature for degraded forests of P. praecox. In theprocess of degradation, soil acidification had the significant positive correlation with theaccumulation of soil available P, Zn and other contents.(2) Soil available P, Zn contents were the key factors to restrict the changing of bambooleaf and bamboo rhizome nutrient contents. P, Zn contents of both bamboo leaf and rhizomecould reflect the changing of soil nutrient contents for different degradation degrees of P.praecox. In the period of bud differentiation, P content of bamboo leaf and rhizome had thesignificant correlation with soil available Zn content, while in the period of refoliation andbranch development, Zn content of bamboo rhizome was restricted by the soil available Pcontent. There was obvious differentiation between the nutrient contents of bamboo leaf andrhizome in different growing periods. The influence of the enrichment of soil P content to thenutrient metabolic level of P. praecox had a significant positive correlation with the high levelof soil Zn.(3) The internal and external nutrient cyclings for bamboo leaf, an adaptive mechanismunder nutrient stressed environments, were also the important feature representing thedegradation degrees of P. praecox. Non-degraded forests, with a large nutrient absorption andexuberant metabolic level, made the best of nutrient resources in a manner of "conservation";while heavy-degraded forests of P. praecox, which had a low P nutrient resorption efficiency,showed an "extravagance" pattern of consumption under P nutrient stress. The existence ofadaptive mechanism for bamboo resulted in the non-significant correlation between thenutrient resorption efficiency and the corresponding soil nutrient availability level. With theincreasing of degradation degree, the quantity of litter leaf decreased gradually. Nutrient returnof litter leaf had a significant negative correlation with the soil P, Zn content. (4) Fine root biomass and nutrient stock were the vital indicators representing thedegradation degrees of P. praecox. With the increasing of degradation degree, fine root biomassshowed a gradually decreasing trend. Non-degraded forests reached the highest level of4.68t·hm-2while heavy-degraded forests got to the lowest level of0.95t·hm-2in the period ofpregnancy and emergence for shoots, which showed an obvious degradation feature. The N, P,K, Mn, Zn stock of fine root for heavy-degraded forests decreased significantly, which revealeda low rate of fine root turnover. The enrichment of soil available P and Zn contents were thekey restricting indicators for the production of fine root.3C, N, P stoichiometric characteristics for different components in SVS of differentdegradation degrees of P. praecox could reflect the degradation degree and the influence oflimiting nutrient element effectively for P. praecox. Compared with non-degraded forests, N: Pstoichiometric ratios of soil, leaf and fine root for heavy-degraded forests of P. praecox reducedsignificantly, while N: P of litter leaf rose in the periods of refoliation and branch developmentand emergence for shoots, which reflected the degradation degree; however, in the period ofbud differentiation, the stoichiometric characteristic of P element of litter leaf reduced whichrevealed the feature of reduction for P absorption and conservation capacity resulted from theexcessive accumulation of P element in degraded forests. N: P stoichiometric ratios of soil andleaf could reveal the characteristic of P enrichment for soil in degraded forests, while that ofleaf and litter leaf could represent leaf N resorption level of degraded forests. N: Pstoichiometric ratio of fine root gave expression to leaf P resorption level.4The responding characteristics of limiting nutrient element could provide a theoreticalbasis for the management countermeasures of remediation for degraded forests of P. praecox.In the early period of the implementation, cultivation techniques for early shooting and highyielding had effects of slowing down the process of vegetation degradation and increasing theeconomic output. With the extension of implementation years, the enrichment of nutrientsespecially P element made the degradation degree become aggravating. Thus, it is necessary toreduce the input of P fertilizer properly, especially in the period of bud differentiation when the level of growth and metaboly is slow for forests. In view of the effect of the soil acidificationto the changing of soil nutrient content, it is proper to adjust soil pH to improve the situation ofnutrient stress. Meanwhile, the interaction of P, Zn and other elements should be given highconsideration in the process of nutrients formulation management for degraded forests so as torealize the sustainable management for P. praecox effectively.
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