Biogeochemical cycles of phosphorus (P) and iron (Fe) are tightly interlinked, especially in highly weathered acidic subtropical and tropical soils rich in iron (oxyhydr)oxides (Fe(III)). The quantitative contribution of the reductive dissolution of Fe(III)-bound inorganic P (Pi) (Fe–P) in low-redox paddy soils may cover the demands of rice plants (Oryza sativa L.) and microorganisms. We hypothesized that microbially-driven Fe(III) reduction and dissolution can cover the P demand of microorganisms but not of the young rice plants when the plants’ P demand is high but their root systems are not sufficiently developed. We grew pre-germinated rice plants for 33 days in flooded rhizoboxes filled with a paddy soil of low P availability. 32P-labeled ferrihydrite (30.8 mg kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots from directly mobilizing Fe–P (pellets-in-mesh bag treatment), or (2) in the same pellet form but without a mesh bag to enable roots accessing the Fe–P (pellets-no-mesh bag treatment). Without mesh bags, Pi was more available leading to increases in microbial biomass carbon (MBC) by 18–55% and nitrogen (MBN) by 4–108% in rooted soil as compared to Pi pellets not directly available to roots. The maximum enzyme activities (Vmax) of phosphomonoesterase and β-glucosidase followed this pattern. During rice root growth, day 10 to day 33, MBC and microbial biomass phosphorus (MBP) contents in both rooted and bottom bulk (15–18 cm) soil gradually decreased by 28–56% and 47–49%, respectively. In contrast to our hypothesis, the contribution of Fe–P to MBP remarkably decreased from 4.5% to almost zero from 10 to 33 days after rice transplantation, while Fe–P compensated up to 16% of the plant P uptake at 33 days after rice transplantation, thus outcompeting microorganisms. Although both plants and microorganisms obtained Pi released by Fe(III) reductive dissolution, this mechanism was not sufficient for the demand of either organism groups.
The degree of atomization of the element is the principal factor defining the sensitivity of the atomic-absorption determination. An attempt is made in this paper to establish its values for yttrium, lanthanum, and neodymium by the method of linear absorption, and also to estimate the effect of the degree of dissociation of the compounds, the degree of ionization, and the relative population of the ground energy level of the atoms on the efficiency of atomization of these elements.
Addition of easily available organic substances to soil often increases the CO2 efflux from pre-existing soil carbon (C). This phenomenon is often explained in terms of the Nitrogen (N)-Mining Hypothesis. According to this proposed – but never conclusively proven – mechanism, increased C availability induces N limitation in microbes, which then access N by degrading soil organic matter (SOM) – a priming effect. This is supported by some experiments demonstrating reduced CO2 efflux after mineral N addition. However, amino acids cause priming, despite their very low C:N ratios and rapid deamination to mineral N. To explore this contradiction, we applied 14C- and 15N-labelled C and N sources (glucose, alanine and ammonium sulfate) to rigorously test two key predictions of the N-Mining Hypothesis: (i) an amino acid should stimulate much less priming than glucose, and (ii) priming should be similarly suppressed for an amino acid or a stoichiometrically equivalent addition of glucose plus mineral N. Both of these key predictions of the N-Mining Hypothesis failed. Efflux of CO2 from native C was essentially determined by the type and amount of C added, with alanine stimulating more priming than glucose (16–50% cumulative increase relative to control, versus 0–25%, respectively). Higher C additions caused more priming than low additions. Mineral N reduced native-C-derived CO2 efflux when added alone or with organic substrates, but this effect was independent of the organic C additions and did not influence C-induced priming. These results were inconsistent with the hypothesized role of N mining in priming. We conclude that the N-Mining Hypothesis, at least in its current form, is not a universal explanation for observed priming phenomena. Instead, we observed a strong correlation between the rates of priming and the mineralization of the added substrates, especially during the first 8 days. This indicated that priming was best explained by energy-induced synthesis of SOM-degrading exoenzymes, possibly in combination with apparent priming from accelerated turnover of microbial biomass.
The labile fraction of heavy metals (HM) in soils is the most important for toxicity for plants and microorganisms. Thus, it is crucial to reduce this fraction in contaminated soils to decrease the negative effect of HM. In a greenhouse experiment, the effects of several additives on the labile fractions of Zn, Cd, Cu, Ni, and Pb were investigated in a soil contaminated during long‐term sewage‐sludge application. The accumulation of HM was studied in the aboveground biomass of wheat ( Triticum aestivum L.). The additives used were the clay minerals Na‐bentonite, Ca‐bentonite, and zeolite; the Fe oxides hematite and goethite; the phosphate fertilizers superphosphate and Novaphos. Wheat was planted three times during 5 months, allowed to grow for 7 w, and harvested. Dry matter and HM content of shoots were determined after each harvest. Soil samples were taken after the first and third harvest, and the NH 4 NO 3 ‐extractable HM contents were determined. After the addition of 2% Na‐bentonite as well as 2% Ca‐bentonite, a strong reduction of the labile HM soil fraction and shoot HM concentration was observed. At the end of the experiment, the labile fraction was reduced due to the addition of Na‐bentonite and Ca‐bentonite by 24% and 31% for Zn, by 37% and 36% for Cd, by 41% and 43% for Cu, by 54% and 61% for Ni, and by 48% and 41% for Pb, respectively. Furthermore, the shoot HM concentrations with the exception of Zn were reduced below the phytotoxicity range. Accordingly, the shoot dry‐matter production was significantly increased. The addition of phosphate fertilizers (notably Novaphos) strongly reduced the bioavailability of Pb for wheat plants. By addition of 0.05% Novaphos, the labile fraction and the shoot concentration of Pb were lowered by 39% and 64%, respectively. However, the addition of Fe oxides and zeolite resulted only in a small reduction in HM bioavailability to wheat plants. Among the studied additives, Na‐bentonite and Ca‐bentonite have the most promising potential to reduce the bioavailability for the studied HM.
Carbon rhizodeposition and root respiration at eight developmental stages of Lolium perenne were studied on a loamy Gleyic Cambisol by 14C02 pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total 14C02 efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first eight days after labelling decreased during plant de velopment from 14 to 6.5 % of 14C input. The average of total CO2 efflux from the soil with Lolium perenne was approximately 21 J.lg C-COi(d . g). It increased during plant development. The contribution of plant roots to total CO2 efflux from the soil increased from 10-20 % at the beginning and to more than 90 % towards the end of plant development. The root exudates led to a enormous reduction in humus decomposition. This interaction can be termed as 'negative priming effect' caused by Lolium perenne.
