Nitrogen uptake by ryegrass from organic wastes applied to a sandy loam soil

Sandy‐textured Mediterranean soils are invariably depleted in organic matter and supply only small amounts of N to crops. To compensate for these deficiencies, we tested the N supply from six organic wastes applied to a Cambic Arenosol in pots growing ryegrass. The results showed that the behaviour of the wastes in supplying N to a ryegrass crop grown in this soil can be predicted by observing their performance in laboratory aerobic incubations. The N made available during these incubations fitted well to a one‐pool kinetic model.     

Effect of tillage and crop on arbuscular mycorrhiza colonization of winter wheat and triticale under Mediterranean conditions

Large‐scale inoculation with arbuscular mycorrhizal fungi (AMF) is generally impractical in most regions and we have little understanding of the factors that determine inoculation success. Nevertheless, the ability to take full advantage of indigenous AMF for sustainable production needs to be developed within cropping systems. We used part of a long‐term field experiment to understand the influence of tillage and the preceding crop on AMF colonization over the growing season. Arbuscular mycorrhiza colonization rate was more affected by treatment (tillage or the combination of crop and preceding crop) than by the total number of AMF spores in the soil. Conventional tillage (CT) had a statistically significant negative effect (P ≤ 0.05) on spore numbers isolated from the soil, but only in the first year of study. However, the AMF colonization rate was significantly reduced by CT, and the roots of wheat, Triticum aestivum, L, cv. Coa after sunflower, Helianthus annuus L., were less well colonized than were those of triticale, X Triticosecale Wittmack, cv. Alter after wheat, but the affect of tillage was more pronounced than was the effect of crop combination. Under no‐till there was a significant increase in AMF colonization rate throughout the sampling period in both wheat and triticale, indicating that the extraradical mycelium previously produced acted as a source of inoculum. In general, triticale showed greater AMF colonization than wheat, despite the preceding crop being less mycotrophic. Under these experimental conditions, typical of Mediterranean agricultural systems, AMF colonization responded more strongly to tillage practices than to the combination of crop and preceding crop.     

Soil and weed management for enhancing arbuscular mycorrhiza colonization of wheat

Tillage and weed control are critical components of cropping systems that need to be combined such that crops benefit from reduced competition. However, weeds may also contribute to the biological diversity within the agro‐environment. This greenhouse study investigated whether common weeds of arable cropping systems were suitable host plants for arbuscular mycorrhizal fungi (AMF), allowing the development of extraradical mycelium (ERM) that can contribute to the early colonization of a following wheat crop, especially in the absence of soil disturbance. Weeds were allowed to grow for up to 2 months before being controlled by soil disturbance or herbicide application (glyphosate or paraquat). Pregerminated wheat seeds were then planted. Chemical control of the weeds prior to sowing enhanced the early arbuscular mycorrhiza (AM) colonization rate of wheat roots, whereas mechanical disturbance was less acceptable as a method of weed control for rapid AM colonization. The type of herbicide (contact or systemic) had no impact on colonization of the wheat crop. Enhanced AM colonization promoted early P acquisition and growth of the crop. Appropriate management of weeds emerging between two consecutive cropping seasons coupled with no‐till soil management could ensure a quick and efficient AM colonization of the following wheat plants.     

The Tripartite Symbiosis Formed by Indigenous Arbuscular Mycorrhizal Fungi, Bradyrhizobium japonicum and Soya Bean Under Field Conditions

The tripartite symbiosis formed by indigenous arbuscular mycorrhizal fungi (AMF), Bradyrhizobium japonicum (Kirchner) Jordan and soya bean (Glycine max L. Merr. cv. Evans) was investigated under field conditions to test the hypotheses that: (i) the tripartite symbiosis enhances nodulation and nodule activity; and (ii) its establishment does not rely on improved phosphorus (P) uptake through the fungal partner. Soil tillage was used to produce treatments with contrasting AMF colonization potentials while the amount of B. japonicum inoculum was kept constant. Nodulation, AMF colonization and the P and nitrogen (N) nutrition of plants were evaluated at 10 and 51 (full‐bloom) days after emergence. N₂ fixation was estimated by the difference method and by the isotopic dilution method. At the early stage of plant growth, AMF hyphal colonization and nodulation were, respectively, 16 % and 33 % greater in plants from untilled than from rototilled soil. The establishment of the tripartite symbiosis was observed under field conditions, and factors other than P nutrition were critical to its formation. However, the tripartite symbiosis did not promote N₂ fixation under the high soil P conditions of this study.     

Nitrogen conservation in soil and crop residues as affected by crop rotation and soil disturbance under Mediterranean conditions

We investigated conservation and cycling of N under oat–oat and lupine–oat rotations in disturbed and undisturbed soil, when roots or roots plus aboveground residues were retained. Crop residues were labelled with ¹⁵N in Year 1, and differential soil disturbance was imposed after harvest. In Year 2, plant growth, N transfer from residue into the various sinks of the second crop (plant, soil, and residual residues), and changes in microbial activity and numbers were determined. Oat biomass was greater after lupine than after oat due to differences in supply of N from these residues. Buried residues of both crops appeared to decompose faster than when left on the soil surface. Lupine residues decomposed faster than oat residues. Oat biomass was not affected by soil disturbance if grown after lupine but decreased when oat straw was buried in the soil. More residue N was recovered from soil than from the crop. Most ¹⁵N was recovered from disturbed soil, which also had greater dehydrogenase activity and more culturable fungi. At the end of the oat–oat rotation, 20 and 5 kg N ha⁻¹ were derived from the roots of the first crop in undisturbed or disturbed soil, respectively. Equivalent values for the lupine–oat rotation were 18 and 44 kg N ha⁻¹. Returning aboveground residues provided an extra 52–80 kg N ha⁻¹ for oat and 61–63 kg N ha⁻¹ for lupine relative to treatments where they were removed. Over a year, lupine contributed 9 to 20 kg N ha⁻¹ more to the agroecosystem than did oat.     

Accumulation of specific flavonoids in soybean (Glycine max (L.) Merr.) as a function of the early tripartite symbiosis with arbuscular mycorrhizal fungi and Bradyrhizobium japonicum (Kirchner) Jordan

This study is the first report assessing the effect of soil inoculation on the signalling interaction of Bradyrhizobium japonicum, arbuscular mycorrhizal fungi (AMF) and soybean plants throughout the early stages of colonisation that lead to the tripartite symbiosis. In a study using soil disturbance to produce contrasting indigenous AMF treatments, the flavonoids daidzein, genistein and coumestrol were identified as possible signals for regulating the establishment of the tripartite symbiosis. However, it was unclear whether soil disturbance induced changes in flavonoid root accumulation other than through changing the potential for AMF colonization. In this study, soil treatments comprising all possible combinations of AMF and B. japonicum were established to test whether (1) modifications in root flavonoid accumulation depend on the potential for AMF colonization, and (2) synthesis and accumulation of flavonoids in the roots change over time as a function of the early plant-microbial interactions that lead to the tripartite symbiosis. The study was comprised of two phases. First, maize was grown over 3-week periods to promote the development of the AM fungus Glomus clarum. Second, the interaction between soybean, G. clarum and B. japonicum was evaluated at 6, 10, 14 and 40 days after plant emergence. Root colonization by G. clarum had a positive effect on nodulation 14 days after emergence, producing, 30% more nodules which were 40% heavier than those on roots solely inoculated with B. japonicum. The tripartite symbiosis resulted in 23% more N₂ being fixed than did the simpler symbiosis between soybean and B. japonicum. The presence of both symbionts changed accumulation of flavonoids in roots. Daidzein and coumestrol increased with plant growth. However, development of the tripartite symbiosis caused a decrease in coumestrol; accumulation of daidzein, the most abundant flavonoid, was reduced in the presence of AMF.     

Light penetration in soil and particulate minerals

Knowledge of light penetration in soils is of particular interest for photolytic degradation of pesticides, for laser-induced fluorescence spectroscopy and remote sensing, and for understanding better the germination of seeds. To date little information has been available in the literature on this topic. In this paper light penetration in soils is determined successfully using diffuse reflectance and transmittance spectroscopy and the relatively simple Kubelka–Munk model. Using the latter model of light propagation in turbid media, the optical properties of kaolinite, montmorillonite, barium sulphate, goethite and 19 different soils were determined in the wavelength range 275–700 nm. In particular, the light absorption coefficient, k, and light scattering coefficient, s, were determined. The depth at which the light intensity at the surface is reduced by 99% (light penetration depth) and the depth of the sample contributing to the measured reflected radiation (information depth) could also be calculated from k and s. For kaolinite, the light penetration depth ranged between 10 and 200 µm for wavelengths between 275 and 700 nm, respectively; the information depth was between 5 and 80 µm. For soils, the penetration depth was in the range 17–110 µm at 275 nm and 120–300 µm at 700 nm, and the information depth was in the range 8–60 µm at 275 nm and 60–175 µm at 700 nm. Hence, the information depth is about half the penetration depth for media with reflectance smaller than 0.7 (e.g. soils). For dry soils, an empirical relationship was established between the light absorption coefficient and the amount of particle-size fractions. The effect of water content was also investigated: addition of water to kaolinite reduced its scattering coefficients, whereas the absorption coefficient was hardly affected. For the soils, addition of water had a more complex mode of action affecting both absorption and scattering coefficients. With the measured optical properties of dry minerals and soils it is possible to calculate light intensity profiles with depth and to quantify photochemical processes occurring in these media.