非平衡系统的自组织与农业

耗散结构理论对当代科学和哲学的影响日益显著。本文运用其中的非平衡系统的自组织理论,结合农业系统中的一些问题进行了讨论。     

论高铝锌合金的组织形成过程

本文论述了高铝锌合金一次结晶过程中枝晶偏析、非平衡共晶体的形成和包晶反应,固态下的调幅分解和共析转变以及某些合金元素对铸态组织和固态相变的影响。     

Nitrate Sorption in a Mexican Allophanic Andisol using Intact and Packed Columns

Abstract

Contamination of groundwater by nitrate is a worldwide environmental issue. A better knowledge of nitrate sorption characteristics by soils contributes to efficient fertilizer use and prevents aquifer contamination. In volcanic soils, nitrate sorption is induced by variable charges due to the presence of amorphous materials and aluminum (Al) and iron (Fe) oxides. Anion transport in packed and intact columns was investigated in a Mexican Allophanic Andisol, under different permanent flow regimes in unsaturated conditions and several NO₃ ^?‐N and Br^? input concentrations. In the packed columns, the NO₃ ^?‐N adsorption in the soil was nonlinear. In the intact columns, the retardation coefficient variation was directly correlated to the increase of amorphous material with depth. The presence of preferential flow in the intact columns significantly increased the mobility and velocity of nitrate moving through the columns, whereas in the packed columns, NO₃ ^?‐N fate was only affected by soil chemical composition and mineralogy.     

草原管理的生态学理论与概念模式进展

20世纪初期CLEMENTS的演替学说被引入草原科学,标志着草原科学理论的一大进步。在此后相当长的一个时期,以演替学说为代表的平衡生态学理论对全世界草原科学研究和草原资源管理模式产生了深远的影响。然而,20世纪70和80年代以来,平衡生态学和稳态管理模式日益受到批评。一方面,在草原生态系统的研究中发现植被动态表现出非线性特征,而这与传统演替理论和稳态管理观点不一致。另一方面,在世界其他地区开展的研究发现,在美国建立起来的草原科学理论和管理模式,在亚洲、非洲等环境随机干扰比较频繁的生态系统并不适用,因而呼吁研究新的框架模型,以便更准确地解释实际的草原植被动态,更有效地为草原管理提供科技支撑。在此背景下,非平衡生态学应运而生。特别是在东非草原生态系统的研究,提出了生态系统是非平衡的但又是可持续的（持续非平衡模型）的观点。进一步的研究揭示了干（冷）季关键资源相对比例对家畜数量和草原植被的潜在影响。20世纪80年代提出的状态与过渡模型是一个具有重要意义的替代性概念框架,20世纪90年代在美国被应用于草原管理实践,取代了盛行多年的草原基况评价。而非平衡理论则逐渐被后出现的恢复力理论所替代,因为研究发现过去认为的“非平衡态”,被解释为“多平衡态”似乎更为合理。生态恢复力在涉及多稳态生态系统管理的领域得到了广泛应用,特别是复杂的社会生态系统的适应性管理。草原生态学理论的进步,也伴随着草原资源管理模式的平行发展,其中一个极为显著的特征是人类的角色从自然资源的利用者转变成了自然资源的管理者。20世纪草原管理的主要模式被概括为稳态管理,其目标是获得牧草和家畜产品的最大可持续产量。恢复力理论代表了当代生态学最新、最重要的进展,其应用进一步扩展到社会系统与生态系统的耦合──社会生态系统,并促进了适应性管理等概念和方法的发展。目前,上述几种管理模式在草原生态学领域都有不同程度的应用。随着时间的推移,草原生态学理论和管理模式必将得到进一步的发展和整合,从而形成更完善的框架模型,能更加有效地应用于兼具复杂性、适应性和社会性的草原生态系统。     

A hierarchical framework for the analysis of scale

Landscapes are complex ecological systems that operate over broad spatiotemporal scales. Hierarchy theory conceptualizes such systems as composed of relatively isolated levels, each operating at a distinct time and space scale. This paper explores some basic properties of scaled systems with a view toward taking advantage of the scaled structure in predicting system dynamics. Three basic properties are explored:(1) hierarchical structuring, (2) disequilibrium, and (3) metastability. These three properties lead to three conclusions about complex ecological systems. First, predictions about landscape dynamics can often be based on constraints that directly result from scaled structure. Biotic potential and environmental limits form a constraint envelope, analogous to a niche hypervolume, within which the landscape system must operate. Second, within the constraint envelope, thermodynamic and other limiting factors may produce attractors toward which individual landscapes will tend to move. Third, because of changes in biotic potential and environmental conditions, both the constraint envelope and the local attractors change through time. Changes in the constraint structure may involve critical thresholds that result in radical changes in the state of the system. An attempt is made to define measurements to predict whether a specific landscape is approaching a critical threshold.Research supported in part by the Ecological Research Division Office of Health and Environmental Research, US Department of Energy under contract No. DE-AC05-840R21400 to Martin Marietta Energy Systems Inc., and in part by the Ecosystem Studies Program, National Science Foundation, under Interagency Agreement No. NSF BSR 8315185 to the Department of Energy. Publication No. 3381, Environmental Sciences Division, Oak Ridge National Laboratory.Dr. Johnson's participation was supported in part by an appointment to the Postgraduate Research Training program under contract DE-AC05-76OR00033 between the U.S. Department of Energy and Oak Ridge Associated Universities, Oak Ridge, TN 37831.     

Is Microcredit a Form of Risk for Pastoral Households of Inner Mongolia’s Semiarid Rangelands?

Microcredit loans are now common for Inner Mongolian pastoralists and are encouraged by government policy on the basis of their previous success for poverty alleviation. However, the effects of the highly variable weather characteristics of many semiarid rangelands on the efficacy of microcredit have not been fully examined. Pastoralists in our study area are often trapped in a vicious cycle of borrowing more each year to pay for previous debt and the next year’s production. Instead of helping to maintain herds through bad years, microcredit has often led to reduced herds and assets. To understand why, a qualitative, interview-based approach was used to determine the kinds of loans taken out and why they are taken out, as well as to assess household livestock sales, income, and costs in three villages. In poor years, 82% of households used loans to purchase winter forage. However, borrowers sold more livestock because the standard 1-yr loan term, combined with weather and market conditions, often forced sales for repayment. Weather and market variation made annual income and costs difficult to anticipate. Loans became an added household risk, another way that environment can influence the social and economic interactions of a rangeland social-ecological system. Longer-term loans could smooth the uncertainty of weather and market conditions, and supplementary measures such as government subsidies or forage insurance could buffer the inevitable but unpredictable bad years. Globally, study of the impacts of nonequilibrial ecological dynamics on economic and policy institutions would help to understand why many development initiatives have failed in such systems.     

六价铬在土壤中吸持和迁移的试验研究

运用批平衡试验和土柱试验,研究了六价铬在土壤中的吸持反应,并进行了动力学模型拟合和吸持等温线平衡模型研究。试验共设3种土壤、6个浓度和2个温度,同时运用6个动力学模型和4个平衡模型进行了拟合。结果表明,由于六价铬的阴离子特性,六价铬在土壤中的物理化学特性非常复杂,而且不易于被土壤所吸持,吸持量较小,吸持效果不稳定。大部分动力学模型和全部平衡模型拟合效果较好。通过土柱试验研究了六价铬在土壤中运移的非平衡运移规律,应用HYDRUS-1D模型进行拟合,结果准确、可靠。     

不同质地土壤中镉运移的数值模拟与预测

通过室内土柱实验,采用溶质运移的对流-弥散方程,模拟了饱和条件下重金属镉在不同质地土壤中的运移.结果表明,化学非平衡单点模型(One-site model,OSM)能较好地模拟镉在粉壤土、壤砂土中的运移(R2分别为0.810和0.901),而对镉在砂壤土中运移的模拟效果较差(R2=0.542).OSM模型准确地模拟了土柱中镉运移的穿透时间,但是对穿透曲线拖尾部分的拟合效果欠佳.土壤对镉运移的阻滞作用与土壤理化性质有关,但是其机理还需要进一步研究.由于上层砂壤土对镉的强烈吸附,使镉的穿透时间较晚(10 pv),且不同土层界面处镉的浓度和吸附量变化明显.