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According to the slash-and-burn technique used in Eastern Province, Zambia, cut trees are piled and burned in only a part of the cleared fields, because adequate tree biomass is not available to burn the entire field. Due to a recent decrease in emergent trees, not only emergent tree piles but also bush tree piles may exist. Therefore, our objective was to evaluate the changes in soil organic matter followed by nutrient release occurring immediately after burning in spots unburned and burned with emergent and bush trees. Fire intensity was significantly higher where emergent tree piles were present. Total carbon (C) decreased by 25.1% and 14.7% in spots burned with emergent and brush tree piles, respectively, while total nitrogen (N) decreased by 15.0% only at spots burned with emergent tree piles and did not change significantly elsewhere. Additionally, the mortality of microbes with soil heating caused an increase in C mineralization after fire. The levels of available nutrients, such as ammonium nitrogen (NH 4 -N), available phosphorus (P), and exchangeable potassium (K) and calcium (Ca), increased following the decomposition of soil organic matter and microbial mortality that occurred with an increase in fire intensity. Net N mineralization did not occur, especially in spots burned with emergent tree piles, because the N content of labile organic matter decreased. Maize (Zea mays L.) grain yield increased with fire intensity, because fire increased nutrient availability and limited weed biomass. Although the burned emergent and bush tree piles occupied only 6.9 and 7.5% of total cleared field, respectively, the grain yield in spots burned with emergent and bush trees accounted for 21% and 15% of the total yield, respectively. Therefore, the burning of bush trees, which is increasing because of the decreased number of emergent trees, could result in a decrease in grain yield but could also alleviate the overall severe loss of soil organic matter.

Pearl millet (Pennisetum glaucum), a diploid outcrossing crop widely grown in semiarid tropics, provides a unique extant material for the study of crop-weed interactive evolution. Co-occurrence of a weedy, shattering type of pearl millet with the cultivated one is the rule in the traditional agro-ecosystem in the Sahel zone of Africa. Selfed progeny of weed-type plants invariably segregated into distinct weed and crop types in an approximately 3:1 ratio. Genetic analysis using a cleaved amplified polymorphic sequence (CAPS) marker strongly suggested that a series of differences between the crop and the weed types are determined by a single putative supergene that has two allelic types, C and W. The crop-type plants are CC homozygotes, and the weed-type plants are CW heterozygotes. WW homozygotes are sterile and rare in the field. Thus, the CW weed plants recurrently arise from crosses between the crop and the weed, as well as from crosses among the weed-type plants. The weed type appears to have a sufficiently high fitness to maintain the W allele in the pearl millet population, resulting in the perpetuation of this unique crop-weed polymorphism.

The severe degradation of grasslands caused by overgrazing and other diverse human operations has become widespread in Inner Mongolia, China. In this study, the vegetations of several grasslands under different land-use conditions were compared to examine the relationships between the vegetation and the disturbance of the grassland. Floristic and life-form compositions of the stands representing the six different sites were researched by the quadrat method: a non-grazed grassland (G1), a lightly grazed grassland (G2), a path (P), an abandoned field (AF) and two artificial meadows (M1 and M2). P, AF, M1 and M2 were located in a part of the grazed grassland. AF, M1 and M2 had been tilled when planted. The dominant species found in G1 and G2 were Potentilla fragarioides, Poa annua, and Cleistogenes squarrosa. Artemisia annua, Chenopodium album, Cannabis sativa f. ruderalis and Calystegia hederacea were found in P and AF, being scarce in the grassland areas. Amaranthus retroflexus, Kochia scoparia and Setaria viridis, which are cosmopolitan weeds, were dominant only in AF. Leymus chinensis, Agropyron cristatum, Potentilla bifurca, and Potentilla tanacetifolia were widely observed across all the surveyed sites. The land-use change from grassland to path caused a decrease of perennial species and an invasion of annual species by heavy trampling. A similar vegetational change occurred with the land-use change from grassland to abandoned field due to the destruction of original vegetation by tillage. Human disturbance such as tillage and trampling also resulted in a decrease in the species with the tufted-and branchedform and an increase in the species with the erect-form. Tillage could produce a more serious impact on the grassland vegetation as compared to grazing. Flora of Inner Mongolian grasslands M. Ao et al.

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