Influence of the order in which low and high C/N residues on soil nutrient availability and wheat nutrient uptake

It is well-known that the C/N ratio of plant residues can influence soil nutrient availability, but the effect of repeated addition of plant residues with different C/N ratio is less explored. In previous studies, we showed that nutrient availability and soil respiration after the second residue addition is influenced not only by the C/N ratio of that residue, but also by the C/N ratio of the previously added residue. These experiments were carried out without plants and it was unclear how the legacy effect would influence plant growth and nutrient uptake. The aim of this experiment was to assess plant growth, nutrient uptake and soil nutrient availability after the second residue addition with different length of time between the first and second residue addition where the first and second residue had the same or a different C/N ratio. High (H) or low C/N (L) residue was added at the start of the experiment, the second residue with either the same or a different C/N ratio was added on days 7, 14, 21 or 28 with a total residue addition of 20 g kg-1 giving four residue treatments: HH, LL, LH and HL. Wheat was planted immediately after the second residue addition and grown for 28 days. N and P availability were measured on days 7, 14, 21 and 28 and at plant harvest. Soil N and P availability after the second residue addition were in the order HH<LH<HL<LL. Wheat biomass generally did not differ between LL, HL and LH, but wheat in HL and LH had a lower shoot/root ratio than in LL suggesting that in HL and LH the plants were able to compensate the lower nutrient availability by increased root growth. In conclusion, the C/N ratio of the previous residue addition influenced nutrient availability after the second residue addition, but plant growth did not differ between HL, LH and L because plants in the former developed a more extensive root system and could therefore access the nutrients released during decomposition of L even in treatments where both H and L were present in the soil

Rapid remediation of sandy sulfuric subsoils using straw-derived dissolved organic matter

When acid sulfate soils dry, oxidation of pyrite can cause acidification and formation of iron (Fe) oxyhydroxy sulfate phases such as jarosite. Remediation via re-establishment of reducing conditions requires submergence and addition of biodegradable organic carbon (OC) to stimulate activity of reducing bacteria. Addition of fresh plant litter has been shown to activate reducing bacteria, likely due to the release of readily available soluble organic matter. However, the effectiveness of soluble organic matter from plant residues has not been tested yet. Here, we tested the potential of wheat straw-derived dissolved OC (DOC) for remediation of a sandy sulfuric (pH < 4) soil. In a second set of experiments, we used combinations of wheat straw-derived DOC with lactate, which is a preferred substrate of sulfate reducing bacteria. All incubation experiments were conducted in the dark at 20 °C. The results showed that addition of DOC from wheat straw induces reduction reactions and rapidly increases the pH by 2–3 units after 3 weeks of incubation under submerged conditions. Mössbauer spectroscopy and X-ray diffraction revealed that jarosite was lost after 200 days of anoxic incubation. Short range-ordered FeIII oxyhydroxides were formed, most likely by FeII-catalysed transformation of jarosite. A second addition of DOC, as well as the addition of lactate, resulted in the almost complete loss of jarosite with increased proportions of FeIII oxyhydroxides in the remaining solids, but not in the formation of FeII sulfides. The formation of FeIII oxyhydroxides reduces the risk of both Fe leaching and renewed acidification in the event of future oxidation. The results suggest that deep injection of wheat straw-derived DOC is a promising approach for rapid and sustainable remediation of sandy sulfuric subsoils. © 2022 The Author

Rhizosphere - perspectives and challenges - a tribute to Lorenz Hiltner 12-17 September 2004 - Munich, Germany

Editorial Correspondance auteur: Marschner P. E-mail: [email protected] audienc

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Not AvailableCrop yields in sandy soils can be increased by addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C/N ratios. A loamy sandy soil (7% clay) was amended with a clay-rich subsoil (73% clay) at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control (sandy soil alone) with no clay addition. The sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg -1: mature wheat (Triticum aestivum L.) straw with a C/N ratio of 68, mature faba bean (Vicia faba L.) straw with a C/N ratio of 39, or their mixtures with different proportions (0%–100%, weight percentage) of each straw. Soil respiration was measured over days 0–45 and microbial biomass C (MBC), available N, and pH on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C/N ratio of the residues or their mixtures, but C use efficiency (cumulative respiration per unit of MBC on day 15) was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of wheat and faba bean in the mixture or more faba bean. Soil N availability and soil pH were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be concluded that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, whereas available N and pH were influenced by clay addition to the sandy soil studied.Not Availabl

Response of microbial activity and community structure to decreasing soil osmotic and matric potential

Low soil water content (low matric potential) and salinity (low osmotic potential) occur frequently in soils, particularly in arid and semi-arid regions. Although the effect of low matric or low osmotic potential on soil microorganisms have been studied before, this is the first report which compares the effect of the two stresses on microbial activity and community structure. A sand and a sandy loam, differing in pore size distribution, nutrient content and microbial biomass and community structure, were used. For the osmotic stress experiment, salt (NaCl) was added to achieve osmotic potentials from -0.99 to -13.13 MPa (sand) and from -0.21 to 3.41 MPa (sandy loam) after which the soils were pre-incubated at optimal water content for 10d. For the matric stress experiment, soils were also pre-incubated at optimal water content for 10d, after which the water content was adjusted to give matric potentials from -0.03 and -1.68 MPa (sand) and from -0.10 to 1.46 MPa (sandy loam). After amendment with 2% (w/w) pea straw (C/N 26), soil respiration was measured over 14d. Osmotic potential decreased with decreasing soil water content, particularly in the sand. Soil respiration decreased with decreasing water potential (osmotic + matric). At a given water potential, respiration decreased to a greater extent in the matric stress experiment than in the osmotic stress experiment. Decreasing osmotic and matric potential reduced microbial biomass (sum of phospholipid fatty acids measured after 14 days) and changed microbial community structure: fungi were less tolerant to decreasing osmotic potential than bacteria, but more tolerant to decreasing water content. It is concluded that low matric potential may be more detrimental than a corresponding low osmotic potential at optimal soil water content. This is likely to be a consequence of the restricted diffusion of substrates and thus a reduced ability of the microbes to synthesise osmolytes to help maintain cell water content. The study also highlighted that it needs to be considered that decreasing soil water content concentrates the salts, hence microorganisms in dry soils are exposed to two stressors

Rewetting Intensity Influences Soil Respiration and Nitrogen Availability

It is not clear how different rewetting intensities (rapid, slow or partial rewetting) influence soil respiration and nitrogen (N) availability. Moist soil left unamended or amended with low C/N faba bean residue and incubated for 21 days was dried to rapid > partial rewetting. From day 4 onwards, moisture treatments differed little in respiration rate. In unamended soils, the rewetting intensity had little effect on available N. But 1 day after rewetting, MBN was lowest in CM, and twofold, fourfold and tenfold higher with rapid, slow and partial rewetting, respectively. In amended soils 1 day after rewetting, available N was about twofold higher in CM and with rapid than slow and partial rewetting, while MBN was about twofold higher with CM and rapid rewetting than slow and partial rewetting. Treatments differed little in available N and MBN 7 and 14 days after rewetting. Slow rewetting induced greater soil respiration and microbial N uptake than rapid rewetting. But effects were short-lived

Biogeochemical, Biophysical, and Biological Processes in the Rhizosphere

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Type of organic carbon amendment influences pH changes in acid sulfate soils in flooded and dry conditions

Purpose Acid sulfate soils (ASS) are common in wetlands and can pose an environmental threat when they dry because oxidation of pyrite may cause strong acidification. Addition of organic matter can stimulate sulfate reduction during wet periods and minimize acidification during dry periods. However, the effect of the organic amendment may depend on its composition. Materials and methods Three wetland acid sulfate (sulfuric, hypersulfidic, and hyposulfidic) soils collected from different depth in one profile were used. The soils, unamended or amended with 10 g C kg−1 as glucose, wheat straw, pea straw, or Phragmites litter, were incubated for 18 weeks under flooded conditions (Bwet period^) followed by 10 weeks during which the soils were maintained at 100 % of maximum water-holding capacity (Bdry period^). Results and discussion During the wet period, the pH decreased in the control and with glucose to pH 3–4, but increased or was maintained in residue-amended soils (pH at the end of the wet period about 7). In the dry period, the pH of the control and glucose-amended soils remained low, whereas the pH in residue-amended soils decreased. However, at end of the dry period, the pH was higher in residue-amended soils than in the control or glucoseamended soils, particularly with pea straw (C/N 50). Conclusions Amendment of acid sulfate soils with plant residues (particularly those with low to moderate C/N ratio) can stimulate pH increase during flooding and reduce acidification under oxidizing conditions

Assessment of the Binding of Protons, Al and Fe to Biochar at Different pH Values and Soluble Metal Concentrations

Biochar can retain large amounts of protons and metals in the drainage water from acid sulfate soils and mine sites. Metal sorption can, however, be influenced by many factors, such as pH and metal composition. This study investigated proton, Al, and Fe retention capacity of eucalyptus biochar (1% w/v) at different pH and metal concentrations. In the absence of metals, the biochar had a high proton binding capacity, (up to 0.035 mmol of H+), whereas its capacity to retain hydroxide ions was limited. A batch experiment was carried out at pH 4 and pH 7 with 10−6, 10−5, 10−4, 10−3, and 10−2 M of added Fe or Al. Added metals precipitated considerably prior to addition of the biochar except that Al remained highly soluble at pH 4. The biochar had a high retention capacity for Al and Fe; at high (>1 mM) concentrations, over 80% of soluble metals were retained. Metal competition for binding sites of both Al and Fe at different ratios was investigated, but increasing concentrations of one metal did not reduce retention of the other. The results confirmed that biochar has high metal binding capacity under both acidic and neutral conditions