The root system of permanent grasslands is of outstanding importance for resource acquisition. Particularly under semi-arid conditions, the acquisition of water and nutrients is highly variable during the vegetation growth period and between years. Additionally, grazing is repeatedly disturbing the functional equilibrium between the root system and the transpiring leaf canopy. However, very few data is available considering grazing effects on belowground net primary productivity (BNPP) and root-shoot dry mass allocation in natural grassland systems. We hypothesise that grazing significantly reduces BNPP due to carbon reallocation to shoot growth. Root biomass and BNPP were estimated by soil coring in 2004, 2005 and 2006 and from ingrowth cores in 2005 and 2006 at one site which has been protected from grazing since 1979 (UG79), at one winter grazing (WG), and one heavily grazed (HG) site. BNPP was estimated from the summation of significant increments of total and live root biomass and from accumulated root biomass of ingrowth cores. Belowground biomass varied from 1,490-2,670 g m −2 and was significantly lower under heavy grazing than at site UG79. Root turnover varied from 0.23 to 0.33 year −1 and was not significantly different between sites. Heavy grazing significantly decreased live root biomass and BNPP compared to site UG79. Taking BNPP estimates from live root biomass dynamics and ingrowth cores as the most Plant Soil (2008) 307:41-50 reliable values, the portion of dry mass allocated belowground relative to total net primary productivity (BNPP/NPP) varied between 0.50-0.66 and was reduced under heavy grazing in 2005, but not in 2006. The positive correlation between cumulative root length density of ingrowth cores and leaf dry matter suggests that the ingrowth core method is suitable for studying BNPP in this semi-arid steppe system. Grazing effects on BNPP and BNPP/NPP should be considered in regional carbon models and estimates of belowground nutrient cycling.

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Abstract:Temporal stability of soil moisture spatial patterns has important implications for optimal soil and water management and effective field monitoring. The aim of this study was to investigate the temporal stability of soil moisture spatial patterns over four plots of 105 m ð 135 m in grid size with different grazing intensities in a semi-arid steppe in China. We also examined whether a time-stable location can be identified from causative factors (i.e. soil, vegetation, and topography). At each plot, surface soil moisture (0-6 cm) was measured about biweekly from 2004 to 2006 using 100 points in each grid. Possible controls of soil moisture, including soil texture, organic carbon, bulk density, vegetation coverage, and topographic indices, were determined at the same grid points. The results showed that the spatial patterns of soil moisture were considerably stable over the 3-y monitoring period. Soil moisture under wet conditions (averaged volumetric moisture contents >20%) was more stable than that under dry (Â j,t < 10%) or moist (Â j,t D 10-20%) conditions. The best representative point for the whole field identified in each plot was accurate in representing the field mean moisture over time (R 2 ½ 0Ð97; p < 0Ð0001). The degree of temporal persistence varied with grazing intensity, which was partly related to grazing-induced differences in soil and vegetation properties. The correlation analysis showed that soil properties, and to a lesser extent vegetation and topographic properties, were important in controlling the temporal stability of soil moisture spatial patterns in this relatively flat grassland. Response surface regression analysis was used to quantitatively identify representative monitoring locations a priori from available soil-plant parameters. This allows appropriate selection of monitoring locations and enhances efficiency in managing soil and water resources in semi-arid environments.

Comb-like polymers with flexible side chains chemically pended onto a polymeric backbone afford some unusual properties due to their hierarchical structures, such as nanoscale confined crystallisation, phase transition and conformational variations, length scale effects, etc. Considerable attention has been paid to these featured polymers, regarding their importance in understanding the correlation between hierarchical structure and the assembled morphologies. In this review, we reviewed the recent research progress on the structure-property correlations of comb-like polymers. This article brings together and highlights the fabrication, structure determination and morphology characterization for comb-like polymers, especially for nanostructured packing patterns and frustrated mobility of chain segments from the selected examples.

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A series of branched poly(ethyleneimine) (PEI) derived polymers with different lengths of n-alkyl side chains, denoted as PEI(n)Cs (n ) 12, 14, 16, 18, 20, number of carbon atoms in alkyl side group), have been prepared by a N-alkylation method, and systematically characterized by differential scanning calorimertry (DSC) and wide-angle X-ray diffraction (WAXD) as well as Fourier transform infrared spectroscopy (FTIR). The side chains grafted on these comblike polymers are long enough to form crystalline phase composed of paraffin-like crystallites. The crystallization of the side chains forces the branched poly(ethyleneimine) molecules to pack into layered structure, between which the crystallites are located. The melting temperatures of the side chain crystallites increase from -12.36 to +51.49 °C with increasing the length of the side chains from n ) 12 to n ) 20, which are a little bit lower than the corresponding pristine n-alkanes. PEI18C was taken as an example in this work for the investigation of phase transition and conformational variation of the side chains with temperature changing. With temperature increasing, the crystalline phase of the side chains undergoes a phase transition process from orthorhombic to hexagonal form and then from hexagonal to melt state. Similarly, increasing temperature leads to the regular trans sequences of the orthorhombic phase transforming to conformationally disordered trans sequences of the hexagonal phase and then to the gauche conformational state (melt state) at higher temperature. The above-described experimental phenomena for both crystalline transition and conformational structure variation are reversible as temperature decreases. Combining the transformations of the crystalline phase and the conformational structure, it is confirmed that hexagonal phase exists as an intermediate phase in the process of temperature variation.