Spatial and temporal variation in the Argentine ant edge effect: Implications for the mechanism of edge limitation

One consequence of human land-use is the exposure of native communities to invasive human commensal species along edges. Argentine ants (Linepithema humile) invade a variety of habitats in California with consequent dramatic declines in native ants. In coastal southern California, USA Argentine ants appear to be an edge effect in small habitat fragments [Suarez, A.V., Bolger, D.T., Case, T.J., 1998. Effects of fragmentation and invasion on native ant communities in coastal southern California. Ecology 79, 2041-2056]. They invade fragments from the urban edge, but only penetrate coastal sage scrub (CSS) habitat to a distance of approximately 200 m. Using pitfall sampling in edge (<250 m from the urban edge) and interior (>600 m from the edge) habitat I tested whether there is also an Argentine ant edge effect in the largest blocks of habitat in the landscape and investigated patterns of spatial and temporal variation in native and Argentine ants. Argentine ants were common in coastal sage scrub habitat within 250 m of urban edges, but rare in interior areas. Correspondingly, native ants were significantly less abundant and diverse in edge areas as compared to interior. Over the period 1997-2000 Argentine ants did not become more abundant in interior habitat suggesting it will remain a refuge for native ants. Argentine ant abundance in edge plots varied greatly among years and sites. Annual variation in abundance was positively related to annual rainfall. Increased soil moisture near edges due to urban runoff has often been suggested as the mechanism that allows the invasion of edge but not interior habitat. This hypothesis predicts that edge habitat downslope of the urban edge should support invasions of higher abundance and greater spatial penetration than habitat upslope. However, I found that edge slope did not predict the extent of invasion, whereas, soil type did. Coarse, well-drained soils supported an Argentine ant invasion of lower abundance and lesser spatial penetration than soils that should retain more moisture. These patterns of spatial and annual variation are more consistent with a biotic flow mechanism where ants move from urban habitat into CSS sites that are temporarily favorable, rather than an abiotic flow where urban runoff causes a physical change to CSS habitat near edges. This invasion affects a substantial area of habitat, however, the inability of Argentine ants to invade interior habitat suggests that refugia for native ants will persist if large unfragmented blocks of habitat are maintained.     

南方红壤丘陵区水土流失现状与综合治理对策

在南方红壤区水土流失与生态安全综合科学考察的基础上,分析南方红壤丘陵区水土流失现状、特点、成因、危害以及近50年来的演变趋势。结果表明：南方红壤考察区目前有水土流失面积13．12万km^2,占红壤考察区土地面积的15．06％。近50年来,南方8省的水土流失面积在1985年前呈增加趋势,1985年以后呈现递减趋势。针对水土流失的特点与危害,提出水土流失治理的综合治理对策。认为：《中华人民共和国水土保持法》是水土流失综合治理的根本保证;科学规划、分区治理、实施战略性推进是综合治理的前提,推行生态补偿机制、实施重大工程是综合治理的契机;水土保持科技进步是综合防治水土流失的催化剂。     

Negative and positive contributions of arbuscular mycorrhizal fungal taxa to wheat production and nutrient uptake efficiency in organic and conventional systems in the Canadian prairie

Improving technologies and the challenge of producing more bio-products while reducing the environmental footprint of humans are shifting paradigms in agricultural research. Harnessing the microbial resources of arable soils is a new avenue to improve the efficiency of nutrient use in agriculture. The objective of this study was to define how crop management influences the contribution of resident AM fungi to nutrient efficiency and crop productivity. The AM fungal communities of 72 organically and 78 conventionally managed wheat fields of the Canadian prairie were described by 454 pyrosequencing and related to crop productivity and N and P use efficiency. Conventional management reduces soil pH and increases the fluxes of all soil nutrients except S, B, and K. Organic management increased the abundance of Claroideoglomus reads. The efficiency of N and P uptake from soil by organic wheat was 2.3 and 1.8 times higher than that of conventional systems. This high N and P uptake efficiency in organic wheat crops was mainly attributable to the low soil fertility of organic fields, as wheat biomass production was 1.44 times greater in conventional than organic systems. Overall, the amounts of N and P taken up by conventional and organic wheat crops were similar. Plant nutrient balance and the abundance of Paraglomus drove conventional wheat production, whereas organic production depended mainly on soil moisture, plant nutrient balance, and abundance of Glomus, which was associated with reduced and nutrient-inefficient wheat production. The high nutrient concentrations at maturity and the low productivity of organic wheat fit a model of limiting CO₂-assimilation. The trade-off between nutrient use efficiency and productivity in low input wheat production could be relieved by reducing the abundance of Glomus species, increasing soil moisture and early N availability, or by improving the inherent CO₂ assimilation capacity of wheat.     

太行山山前平原近50 年气候变暖特征及其对冬小麦-夏玉米作物系统的影响

气候变暖及其对作物系统的影响是农业应对全球变化领域的主要研究命题之一。本文以河北省石家庄为例, 通过对近50 年气温数据以及作物系统热量资源变化特征的分析, 探讨了太行山山前平原区气候变暖对冬小麦-夏玉米作物系统的影响。山前平原区近50 年来明显变暖, 增温速率为0.35 ℃·10a^-1, 其中冬季增温幅度最大, 为0.51 ℃·10a^-1。气候变暖对作物的影响主要体现在有效积温的增加。近20 年来, >10 ℃积温较基准时段增加明显, 农业热量资源条件改善, 相当于农作物有效生育期延长10~20 d。由于较大的增温幅度及季节不均衡性, 冬小麦-夏玉米作物系统受到显著影响: 1990 年以来, 冬小麦生长季积温较基准时段上升幅度超过10%, 冬前生长期积温增加易造成旺长, 影响其安全越冬能力, 需推迟冬小麦播种期以适应气候变暖导致的不利影响; 气候变暖改善了夏玉米生育期热量条件, 综合考虑其收获期因小麦晚播而相应推迟的影响,夏玉米生长季>10 ℃积温可超过2 900 ℃, 满足中晚熟玉米品种平播的热量条件。