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Core Ideas There have been suggestions that ruzigrass increases soil P availability.Ruzigrass was grown in rotation with soybean from 2012 to 2016.The observed effect was opposite from the expected under long‐term field conditions.Crop rotation with ruzigrass resulted in a lower soybean grain yield than fallow. Under no‐till farming systems, the use of crop rotations with species adapted to low P soils may enhance soil P availability through P cycling. Growing ruzigrass [Urochloa ruziziensis (R. Germ. and C.M. Evrard) Morrone and Zuloaga] as a cover crop has shown to increase resin extractable P in soils. However, it is not clear how the next crop responds to ruzigrass in the long term. The objective of this study was to evaluate the long‐term effect of growing ruzigrass on soil P availability to soybean [Glycine max (L.) Merr.]. The evaluations were performed over 5 yr on a ruzigrass–soybean crop rotation, in Botucatu, Brazil. The treatments were P rates (0, 13, and 26 kg ha−) applied to soybean seed furrows, and ruzigrass or fallow during the off‐season. Soil samples were taken after ruzigrass desiccation, and soil P was extracted with resin (Presin). The use of ruzigrass increased soil organic matter (SOM) by approximately 20% compared with fallow, regardless of P rates, and increased Presin concentration in the 0‐ to 10‐cm soil depth by approximately 10% with 26 kg ha− of P. Surprisingly, grain yield and soybean leaf P concentration were lower after ruzigrass compared with fallow. Resin seemed to be unsuitable to compare P availability in different cropping systems. In the long‐term, growing ruzigrass as a cover crop in the off‐season decreases P and N availability to soybean, eventually decreasing soybean grain yield. Further studies are needed to understand the mechanisms involved in this unexpected soybean response when cropped in rotation with ruzigrass.

Part of the nitrogen (N) fertilizer applied to crops is lost to the environment, contributing to global warming, eutrophication, and groundwater contamination. However, low N supply stimulates soil organic N turnover and carbon (C) loss, since the soil N/C ratio in soil is quasiconstant, ultimately resulting in land degradation. Grasses such as ruzigrass (Urochloa ruziziensis) grown as winter pasture or a cover crop in rotation with maize (Zea mays) can reduce N leaching, however, this may induce N deficiency and depress yields in the subsequent maize crop. Despite the potential to decrease N loss, this rotation may negatively affect the overall N balance of the cropping system. However, this remains poorly quantified. To test this hypothesis, maize, fertilized with zero to 210 kg N ha -1 , was grown after ruzigrass, palisade grass (Urochloa brizanta) and Guinea grass (Pannicum maximum), and the N inputs, outputs and partial N balance determined. Despite the intrinsically poor soil quality associated with the tropical Ultisol, maize grown after the grasses was efficient in acquiring N, resulting in a negative N balance even when 210 kg ha -1 of N was applied after Guinea grass. Losses by leaching, N2O emission and NH3 volatilization did not exceed 13.8 kg ha -1 irrespective of the grass type. Despite a similar N loss among grasses, Guinea grass resulted in a higher N export in the maize grain due to a higher yield, resulting in a more negative N balance. Soil N depletion can lead to C loss, which can result in land degradation.

Loss of potassium (K) by leaching after potassium chloride (KCl) application is common in light-textured, low cation exchangeable capacity (CEC) soils with predominance of 1 : 1 clay minerals, and is aggravated as soil K concentration increases. Coating of KCl with humic acids may be a strategy to avoid loss and supply K over the plant cycle. The objective of this study was to evaluate the response of maize (Zea mays) and soybean (Glycine max) to regular KCl and KCl coated with humic acid, as well as K leaching as affected by application of these fertilisers in single or split application to soils with different K levels. Field experiments with maize and soybean were conducted on soil with very low, low, and medium exchangeable K levels, in Botucatu, Brazil. Soybean and maize grain yields were higher with a single application of coated KCl compared with regular KCl, in soil with very low K level; however, when the rate was split, yields were higher with regular KCl. This shows the importance of fertiliser K release synchronisation as the plant develops, avoiding possible K losses by leaching in low CEC soils. Potassium leaching was observed in soil with medium K level. Potassium chloride coated with humic acids is an adequate source of K in low CEC soils with very low K level when applied in a single application at planting, as opposed to regular KCl that must be split. However, the coated fertiliser is not effective for avoiding K leaching in soils that are medium or high in K.

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Water deficit may affect the expression of lepidoptera-controlling proteins in cotton. However, it is unknown if there is a differential response of conventional and Bt cotton cultivars to water deficit, what could potentially affect the plant competition with weeds. The objective of this work was to investigate the response of Bt cotton cultivars to water deficit compared with their conventional near-isolines. The experiment was conducted in a greenhouse, where the cotton cultivars FMT 705, FMT 709 and IMACD 8276, with and without the Bt gene, were grown under two water regimens: 100% and 50% (moderate water deficit) of available soil water. Cotton phenology was severely affected by moderate water deficit, with a reduction in shoot and root dry matter production, root length and diameter, plant height and leaf area. No effect of the Bt gene was observed. Water deficit during cotton flowering decrease stomatal conductance, net assimilation of CO2 and transpiration rates. The leaf water potential is lower in plants exposed to a moderate water deficit compared with non-stressed plants. However, the introgression of the Bt gene does not modify cotton physiological and phenotypic response to water deficit.

No abstract

Tropical grasses grown as cover crops can mobilize phosphorus (P) in soil and have been suggested as a tool to increase soil P cycling and bioavailability. The objective of this study was to evaluate the effect of tropical grasses on soil P dynamics, lability, desorption kinetics and bioavailability to soya bean, specifically to test the hypothesis that introducing grass species in the cropping system may affect soil P availability and soya bean development according to soil P concentration. Three grass species, ruzi grass (Urochloa ruziziensis), palisade grass (Urochloa brizantha) and Guinea grass (Megathyrsus maximus), were grown in soils with contrasting P status. Soya bean was grown after grasses to assess soil P bioavailability. Hedley P fractionation, microbial biomass P, phytase‐labile P and the diffusive gradient in thin films were determined, before and after cultivation. It was found that grasses remobilized soil P, reducing the concentration of recalcitrant P forms. The effect of grasses on changing the P desorption kinetics parameters did not directly explain the observed variation on P bioavailability to soya bean. Grasses and microorganisms solubilize recalcitrant organic P (Po) forms and tropical grasses grown as cover crops increased P bioavailability to soya bean mainly due to the supply of P by decomposition of grass residues in low‐P soil. However, no clear advantages in soya bean P nutrition were observed when in rotation with these grasses in high‐P soil. This study indicates that further advantages in soya bean P nutrition after tropical grasses may be impeded by phytate, which is not readily available to plants.