Do cover crops change the lability of phosphorus in a clayey subtropical soil under different phosphate fertilizers?

Plants have developed different mechanisms to absorb and solubilize phosphorus (P) in the soil, especially in environments with low P availability. This study evaluated the effects of different winter cover crops on soil P availability in a clayey subtropical (Hapludox) soil receiving soluble P fertilizer and a rock phosphate applied to the summer crop, under no‐tillage. The experiment was carried out over 3 yrs (2009–2011) with five different cover crop species: common vetch, fodder radish, ryegrass, black oat, white clover and fallow as control. The soil was sampled after the third year of cover crop cultivation and analysed for inorganic and organic P forms according to the well‐established Hedley fractionation procedure. Phosphate fertilizers promoted accumulation of both labile and nonlabile P pools in soil in the near surface layer, especially under rock phosphate. Fertilizer applications were not able to change P fractions in deeper layers, emphasizing that the Brazilian clayey soils are a sink of P from fertilizer and its mobility is almost nil. Although the cover crops recycled a great amount of P in tissue, in a short‐term evaluation (3 yrs) they only changed the content of moderately labile P in soil, indicating that long‐term studies are needed for more conclusive results.     

Limited effects of land use on soil dissolved organic matter chemistry as assessed by excitation–emission fluorescence spectroscopy and molecular weight fractionation

Dissolved organic matter (DOM) in soil solution represents a complex mixture of organic molecules and plays a central role in carbon and nitrogen cycling in plant–microbial–soil systems. We tested whether excitation–emission matrix (EEM) fluorescence spectroscopy can be used to characterize DOM and support previous findings that the majority of DOM is of high molecular weight (MW). EEM fluorescence spectroscopy was used in conjunction with MW fractionation to characterize DOM in soil solution from a grassland soil land management gradient in North Wales, UK. Data analysis suggested that three distinct fluorescence components could be separated and identified from the EEM data. These components were identified as being of humic‐like or fulvic‐like origin. Contrary to expectations, the majority of the fluorescence signal occurred in the small MW (<1 kDa) fraction, although differences between soils from the differently managed grasslands were more apparent in larger MW fractions. We conclude that following further characterization of the chemical composition of the fluorophores, EEM has potential as a sensitive technique for characterizing the small MW phenolic fraction of DOM in soils.     

The combined effects of cover crops and symbiotic microbes on phosphatase gene and organic phosphorus hydrolysis in subtropical orchard soils

P deficiency is a major obstacle for crop production in subtropical red soils in South China, and the hydrolysis of organic P (Po) is of great significance in these soils due to the immobilization of P by Fe and Al. Cover cropping in orchards and symbiotic microbial inoculation are considered to improve soil quality, including P status, however, their effects on the hydrolysis of Po is little known. In this study, five soil managements were established in a guava orchard in South China for two and a half years, including clean culture (CC), cover cropping with Paspalum natatu (PN), PN with arbuscular mycorrhizal fungal inoculation (PNA), cover cropping with Stylosanthes guianensis (SG), SG with rhizobial inoculation (SGR). Soil chemical, biochemical and microbial properties were analyzed. Results indicate that soil pH and SOM content tended to increase following cover cropping alone or with microbial inoculation. Po content was significantly elevated in PNA. Po fractionation revealed that cover cropping alone or with microbial inoculation significantly affected the contents of moderately labile Po (MLPo) and moderately resistant Po (FAPo). Enzyme assay indicated that cover cropping with microbial inoculation increased the activities of acidic phosphomonoesterase (ACP), neutral phosphomonoesterase (NP) and alkaline phosphomonoesterase (ALP), with ALP the most sensitive, although ACP activity dominated in red soils. Correlation analysis suggested a significantly positive relationship between ALP activity and MLPo or FAPo. PCR-DGGE profile of the alp-harboring bacterial community showed that cover cropping with S. guianensis and mycorrhizal inoculation to P. natatu promoted the bacterial diversity and/or species richness. For almost all the measured parameters, PN and SG were comparable, however, PNA was superior to SGR, indicating the stronger additive effect of arbuscular mycorrhizal fungus than that of rhizobia. Cat-PCA indicated that MLPo was the most influential factor on phosphomonoesterase. In general, this study suggests that, in subtropical orchards with red soil, cover cropping with microbial inoculation can improve the Po hydrolysis via the promoted alp-harboring bacterial community and then ALP activity. Our results also suggest that the combination of P. natatu and arbuscular mycorrhizal fungus is better than S. guianensis and rhizobia, which possesses practical significance for sustainable production in these orchards.     

稳定碳、氮同位素在牛肉加工过程中的分馏效应

为了探究牛肉及副产物中稳定碳、氮同位素在加工过程中的变化规律,确证稳定碳、氮同位素在牛肉加工制品产地溯源中的稳定性和有效性。本试验通过对牛肉进行不同时间的水煮、烤制和油炸3种处理,其中水煮和烤制加工时间分别为5、10、15、20、25和30 min,油炸加工时间分别为1、2、3、4和5 min;采用元素分析仪-稳定同位素比率质谱仪(EA-IRMS)测定脱脂牛肉、粗脂肪及副产物中δ¹³C和δ¹⁵N值。结果表明,脱脂牛肉中δ¹³C值在水煮、烤制和油炸3种加工方式不同加工时间之间均无显著差异,水煮和烤制后粗脂肪中δ¹³C值无显著变化,油炸后的牛肉粗脂肪δ¹³C值主要受植物油的影响,加工方式及加工时间对其无显著影响;水煮脱脂牛肉δ¹⁵N值在加工25、30 min时与对照组牛肉存在显著差异,但平均差值仅为0.3‰~0.9‰。牛肉稳定碳、氮同位素在不同加工过程中分馏效应较小,可有效用于牛肉加工半成品及成品的原产地溯源。     

Can differences in 15N natural abundance be used to quantify the transfer of nitrogen from legumes to neighbouring non-legume plant species?

Natural variations in the stable isotope ¹⁵N are often exploited in studies of N cycling in ecosystems. Lower ¹⁵N natural abundance in non-legume plants growing in association with legumes, compared with the non-legume grown alone in pure stands have been observed in cropping, forage, and agroforestry systems. Such observations have frequently been attributed to the transfer of biologically-fixed nitrogen (N) from the legume to the companion non-legume, and various methodologies have been employed to calculate the extent of the N transfer. While some of these ¹⁵N natural abundance-based estimates of N transfer were within the range previously reported using equivalent ¹⁵N-enriched techniques (<20% of non-legume plant N and <10 kg N ha⁻¹ derived from fixed N contributed by neighbouring legumes), many of the values obtained using natural abundance were much higher (30%–83% of the non-legume N derived from fixed N representing up to 30–40 kg N ha⁻¹) than generally measured by ¹⁵N-enriched methods; with even greater estimates being determined where data were available to allow N transfer to be re-calculated on the basis of total legume N rather than fixed N (42% to >100%, and up to 110 kg N ha⁻¹ per year). This review raises concerns about the assumptions behind the natural abundance approach, and provides some alternative interpretations for the observed differences in natural ¹⁵N abundance between plants grown in the presence and absence of legumes. It was concluded that simple comparative measures of non-legume δ¹⁵N alone cannot provide a quantitative estimate of N transfer between plant species if the dominant source and the isotopic identity of the transferred N cannot be validated, and if the extent of any isotopic fractionation associated with relevant N transformations occurring during transfer cannot be defined. To date this information is not forthcoming. There is a need to greatly improve our understanding of the transfer processes before the real value of the δ¹⁵N technology can be realized. In the first instance this will primarily be achieved by carefully executed experiments under controlled conditions, and in the field, employing both ¹⁵N natural abundance and enrichment approaches so estimates of transfer can be compared, and the data interrogated using modelling approaches to explore isotopic fractionation.     

Importance of Noncrystalline Hydroxide Phases in Sequential Extractions to Fractionate Antimony in Acid Soils

Abstract

Sequential extraction techniques have been used to make inferences about speciation of phosphorus (P) and to a lesser extent arsenic (As) in soils. However, sequential extraction studies on the less‐abundant group V element, antimony (Sb), are limited. In this work, a widely used P sequential extraction scheme was modified and used to extract P, As, and Sb from two acidic soils from the Macleay River floodplain, NSW, that were enriched with Sb (26.9 and 23.0 mg kg^?1). An ammonium oxalate–oxalic acid step was included in the extraction sequence to dissolve the noncrystalline iron (Fe) and aluminium (Al) hydroxide phase. It extracted 30 to 47% of Sb, indicating the importance of this fraction, which may be mobilized in the floodplain by acid sulfate soil processes and periodic waterlogging. The original method overestimated P, As, and Sb in the residual fraction (30–71%). Relative efficiency values of extracts for P, As, and Sb were compared, and inferences about phase distributions were made. The results suggest some potential in using extractions to assess bioavailability of Sb in soil.     

Specific polyphenol oxidase activity of red clover (Trifolium pratense) and its relation with forage quality in field experiments

Abstract

In feeding studies, red clover (RC) influenced positively the N utilization by ruminants. A relationship between polyphenol oxidase (PPO) activity and forage quality has not been established. Our objective was to investigate seasonal, site, genotype, and management effects on specific PPO activity in RC in three experiments under field conditions, and relate the activity to forage-quality parameters. In Experiment 1, six RC genotypes at two study sites were submitted to a 3-cut system. Specific PPO activity, forage quality, and vegetation stage were determined. PPO activity varied between harvest and study sites, with genotypes differing up to 3-fold in PPO activity within harvests. The specific PPO activity, forage quality, and vegetation stage in RC subjected to 5-cut system and grazing (Experiment 2) were determined. Additionally, in Experiment 3, cutting frequency in RC swards including mechanical stress (rolling) was investigated. The induction of PPO activity in RC by grazing or mechanical stress (Experiments 2 and 3) increased the activity up to 2.5-fold compared with RC at similar vegetation stage submitted to the 5-cut system. Mechanical stress induced by grazing or rolling, and seasonal differences, seem to have a larger influence on specific PPO activity than does the genotype effect observed in Experiment 1. For forage quality, an increased specific PPO activity explained 29–46% of the reduction in protein fraction ‘A’ content (non-protein N) in the cutting systems in Experiments 2 and 3. Other CP fractions achieved a lower relation. Furthermore, the precipitation-to-temperature ratio preceeding a harvest explained 63% of the variation in the specific PPO activity. In conclusion, the PPO activity in RC is induced by grazing and rolling. Whereas weather conditions preceeding a harvest showed a large influence, genotype influence had only minor relevance. These results may have implications for regional harvest management towards efficient N utilization by ruminants.     

Soil organic matter fractionation as a tool for predicting nitrogen mineralization in silty arable soils

After decades of searching for a practical method to estimate the N mineralization capacity of soil, there is still no consistent methodology. Indeed it is important to have practical methods to estimate soil nitrogen release for plant uptake and that should be appropriate, less time consuming, and cost effective for farmers. We fractionated soil organic matter (SOM) to assess different fractions of SOM as predictors for net N mineralization measured from repacked (disturbed) and intact (undisturbed) soil cores in 14 weeks of laboratory incubations. A soil set consisting of surface soil from 18 cereal and root‐cropped arable fields was physically fractionated into coarse and fine free particulate OM (coarse fPOM and fine fPOM), intra‐microaggregate particulate OM (iPOM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation‐resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF‐res OM). Stepwise multiple linear regression yielded a significant relationship between the annual N mineralization (kg N/ha) from undisturbed soil and coarse fPOM N (kg N/ha), silt and clay N (kg N/ha) and its C:N ratio (R² = 0.80; P < 0.01). The relative annual N mineralization (% of soil N) from disturbed soils was related to coarse fPOM N, HF‐res OC (% of soil organic carbon) and its C:N ratio (R² = 0.83; P < 0.01). Physical fractions of SOM were thus found to be the most useful predictors for estimating the annual N mineralization rate of undisturbed soils. However, the bioavailability of physical fractions was changed due to the disturbance of soil. For disturbed soils, a presumed stable chemical SOM fraction was found to be a relevant predictor indicating that this fraction still contains bio‐available N. The latter prompted a revision in our reasoning behind selective oxidation and extraction as tools for characterizing soil organic N quality with respect to N availability. Nonetheless, the present study also underscores the potential of a combined physical and chemical fractionation procedure for isolating and quantifying N fractions which preferentially contribute to bulk soil N mineralization. The N content or C:N ratio of such fractions may be used to predict N mineralization in arable soils.