Microbial responses and nitrous oxide emissions during wetting and drying of organically and conventionally managed soil under tomatoes

The types and amounts of carbon (C) and nitrogen (N) inputs, as well as irrigation management are likely to influence gaseous emissions and microbial ecology of agricultural soil. Carbon dioxide (CO₂) and nitrous oxide (N₂O) efflux, with and without acetylene inhibition, inorganic N, and microbial biomass C were measured after irrigation or simulated rainfall in two agricultural fields under tomatoes (Lycopersicon esculentum). The two fields, located in the California Central Valley, had either a history of high organic matter (OM) inputs (“organic” management) or one of low OM and inorganic fertilizer inputs (“conventional” management). In microcosms, where short-term microbial responses to wetting and drying were studied, the highest CO₂ efflux took place at about 60% water-filled pore space (WFPS). At this moisture level, phospholipid fatty acids (PLFA) indicative of microbial nutrient availability were elevated and a PLFA stress indicator was depressed, suggesting peak microbial activity. The highest N₂O efflux in the organically managed soil (0.94 mg N₂O-N m⁻² h⁻¹) occurred after manure and legume cover crop incorporation, and in the conventionally managed soil (2.12 mg N₂O-N m⁻² h⁻¹) after inorganic N fertilizer inputs. Elevated N₂O emissions occurred at a WFPS >60% and lasted <2 days after wetting, probably because the top layer (0–150 mm) of this silt loam soil dried quickly. Therefore, in these cropping systems, irrigation management might control the duration of elevated N₂O efflux, even when C and inorganic N availability are high, whereas inorganic N concentrations should be kept low during times when soil moisture cannot be controlled.     

Genetic diversity in the Chamaecytisus proliferus complex (Fabaceae: Genisteae) in the Canary Islands in relation to in situ conservation

Summary Electrophoresis was carried out for six isozyme systems on 175 accessions of the seven morphological forms of Chamaecytisus proliferus (L. fil.) Link (Fabaceae: Genisteae) from the Canary Islands. Previous studies have shown that there is clear differentiation into morphological and ecological forms. The underlying genetic variation as visualised by studying isozymes does not reflect this. While substantial genetic diversity can be identified, this was found to be as much within as between morphological forms. This genetic diversity was found to decrease from east to west, and reflects the pattern of variation remaining after colonisation of the individual islands, which is assumed to have progressed in the same direction. Subsequently, adaptive radiation has given rise to the overlying morphological variation as exhibited in the seven morphological forms.In terms of in situ conservation, overall genetic diversity can be easily conserved in the abundant populations occurring in the east of the archipelago, while more attention is required for conservation of the much rarer morphological forms found in the west, despite their relative lack of isozyme diversity.Abbreviations ACO aconitase - ADH alcohol dehydrogenase - IDH Asocitrate dehydrogenase - MDH malate dehydrogenase - PGD 6-phosphogluconate dehydrogenase - PGM phosphoglucomutase - CA cluster analysis - PCA principal component analysis     

Potassium release on drying of soil samples from a variety of weathering regimes and c clay mineralogy in China

Thirty two soil samples from China were analyzed for exchangeable K before and after drying. Most soil samples were higher in exchangeable K after air-drying and ovendrying (60°C) than when wet. Soil clay minerals, especially clay-size mica, affected K released in air-dry and oven-dry samples. According to composition of clay minerals of air-dried samples, five classes were recognized. Soil samples with high mica and montmorillonite have the highest exchangeable K. Samples that contained higher kaolinite than mica had lower exchangeable K. Samples with higher kaolinite than quartz and mica had still lower exchangeable K. Samples of sandy soils contained very low exchangeable K. When quartz was the main mineral, the samples that had kaolinite and gibbsite as the main clay minerals had very low exchangeable K. The degree of weathering (weathering mean) bore an inverse relationship to the amount of K released on drying in air or at 60°C. Udults and Udalfs (Red Earths) of southeastern China, because of their high K release on being dried, are inferred to have received fine mica from the Western Desert dust rainout, reported to Liu et al. (1981).     

Quantity and potential biological activity of caffeic acid in sweet potato [Ipomoea batatas (L.) Lam.] storage root periderm

The caffeic acid content of storage root periderm and cortex tissues of genetically diverse sweet potato [Ipomoea batatas (L.) Lam.] cultivars and breeding clones was quantified by high-performance liquid chromatography. Periderm caffeic acid content of the clones ranged from 0.008 to 7.97 mg/g dry weight, whereas the highest cortex content was 0.047 mg/g. Clones varied greatly in periderm caffeic acid content in all experiments, but there were also differences between experiments in content averaged for all clones. This indicates that periderm caffeic acid content is subject to genetic and environmental influences. Caffeic acid inhibited the growth of four sweet potato pathogenic fungi and germination of proso millet seeds in bioassays. Inhibitory activity in the bioassays suggests that high periderm caffeic acid levels contribute to the storage root defense chemistry of some sweet potato genotypes.     

Potassium fertilization effects on isoflavone concentrations in soybean [Glycine max (L.) Merr.

Soybean isoflavone concentrations vary widely, but the contribution of soil fertility and nutrient management to this variability is unknown. Field experiments from 1998 to 2000 on soils with low to high exchangeable potassium (K) concentrations evaluated K application and placement effects on isoflavone concentrations and composition of soybean in various tillage and row-width systems. Soybean seed yield and concentrations of daidzein, genistein, glycitein, leaf K, and seed K were measured. Significant increases in daidzein, genistein, and total isoflavone were observed with direct deep-banded K or residual surface-applied K on low-K soils. Positive effects of K fertilization on isoflavones were less frequent on medium- to high-testing K soils. Both individual and total isoflavones were often positively correlated with seed yield, leaf K, and seed K on low-K soils. Appropriate K management could be an effective approach to increase isoflavone concentrations for soybeans produced on low- to medium-K soils.     

Measured and modelled rainfall interception loss from an agroforestry system in Kenya

Rainfall interception losses from an agroforestry system in semi-arid Kenya comprising Grevillea robusta and maize were measured over a period of 33 months. These measurements showed that interception was slightly higher (10.2%) under trees with no maize understorey, than in the intercropped treatment (9.8%), and was directly related to the degree of tree canopy cover. Interception estimates using the reformulated version of the Gash analytical model were 4% lower than measured totals, and were strongly dependent on both canopy cover and on monthly variations in the mean rainfall rate ( , mm h⁻¹).