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Previous studies have shown that carbon (C) mineralization in saline or sodic soils is affected by various factors including organic C content, salt concentration and water content in saline soils and soil structure in sodic soils, but there is little information about which soil properties control carbon dioxide (CO) emission from saline-sodic soils. In this study, eight field-collected saline-sodic soils, varying in electrical conductivity (EC, a measure of salinity, ranging from 3 to 262 dS m) and sodium adsorption ratio (SAR, a measure of sodicity, ranging from 11 to 62), were left unamended or amended with mature wheat or vetch residues (2% w/ w). Carbon dioxide release was measured over 42 days at constant temperature and soil water content. Cumulative respiration expressed per gram SOC increased in the following order: unamended soil

Plant roots and microorganisms play an important role in the soil N cycle and plant N nutrition through the release of extracellular enzymes. In the present greenhouse pot experiment, wheat ( Triticum aestivum) seedlings were grown in a fluvo-aquic soil (Udifluvent) to investigate N mineralization and utilization in the rhizosphere of wheat. The soil received chemical fertilizer (N-labeled urea), chemical fertilizer plus manure (common urea + N-labeled swine manure) or no N. Plant roots were separated from the soil with a nylon cloth, and 1-mm increments of soil moving laterally away from roots were analyzed for N, microbial C, and the activities of invertase, urease and protease. Chemical fertilizer plus manure promoted wheat growth and N absorption significantly compared with chemical fertilizer. N from both chemical fertilizer and swine manure accumulated significantly in the rhizosphere soil within 5 mm of the roots. Fertilized N could thus move easily laterally towards roots and there was no indication that movement through the soil limited plant N supply. A large proportion of fertilizer N was lost from the soil during the wheat growing period, and N utilization efficiency was 24% for chemical fertilizer and 30% for swine manure. In addition, faster rates of N mineralization, larger amounts of microbial C, and increased activities of invertase, urease and protease occurred in the rhizosphere compared with other parts of the soil. There was a significant correlation between microbial C and N mineralization rate ( r = 0.968, P < 0.01) in the whole soil. Microbial C also showed significant positive correlations with activities of invertase ( r = 0.892, P < 0.01) and protease ( r = 0.933, P < 0.01). Further study showed that adding manure into soil increased microbial C and the activities of invertase and protease; adding urea stimulated urease activity in the same soil. Changes in soil enzyme activities in response to N fertilizers could be considered indicators for different fertilizer managements. [ABSTRACT FROM AUTHOR]

biological (3 and 6 weeks aerobic incubation) and a chemical method [successive extractions with cold 0.1 (H1-N) and 0.5M HCl (H2-N)] were applied to 21 soils to determine: a) the potentially mineralizable-N; b) the most useful soil variables for predicting soil-N availability; and c) their usefulness for predicting N uptake by a greenhouse wheat crop. At t = 3, both net N mineralized (NNM) and net N mineralization rate (NNMR) were correlated: a) positively with SOM- and CEC-related variables; and b) positively with soil δN and negatively with soil pH, suggesting that N-mineralization, dominated by nitrification, is associated with NO-N losses and soil acidification. At t = 6, all previously discussed variables were important for NNM, but not for NNMR, mainly controlled by the available-P content. The importance of H1-N increased with N-inputs and decreased with NO losses and soil-N. Relationships of H1-N and H2-N with soil CEC and texture showed the strong relations among nutrients content, biological activity and N mineralization, and the recalcitrance of clay-bounded SOM. Soil-N correlations with wheat-N in absolute amount (positive) and as percentage of soil-N (negative) showed an important available-N supply by N-rich soils, despite their slow N turnover. All best regression models for wheat-N included 1-2 main available nutrients. The percentage of soil-N exported to plant biomass was negatively correlated with non-crystalline Al compounds and soil δN. Mineralized-N and wheat-N pools did not share many correlations with soil properties and seemed to come from different sources; consequently, the mineralized-N, which only explained a quarter of wheat-N variance, was not more useful than soil-N for predicting it. Wheat-N and hydrolysable-N only shared a positive correlation with soil-N, highlighting that they are two mainly unrelated N pools. Nevertheless, half of wheat-N variation was explained by its negative relationship with the percentage of soil-N as (H1+H2)-N; a possible explanation is that (H1+H2)-N is biologically labile and it constitutes an important soil-N fraction only when a limiting factor for microbial N mineralization or plant growth allowed its cumulation. [ABSTRACT FROM AUTHOR]

A soil microcosm experiment was conducted to evaluate the influence of copper contamination on the dynamics and diversity of bacterial communities actively involved in wheat residue decomposition. In the presence of copper, a higher level of CO2 release was observed, which did not arise from greater wheat decomposition but from a higher level of stimulation of soil organic matter mineralization (known as the priming effect). Such functional modifications may be related to significant modifications in the diversity of active bacterial populations characterized using the DNA stable-isotope probing approach. [ABSTRACT FROM AUTHOR]

The silica accumulation in orchard grass (Dactylis glomerata L.) and wheat (Triticum aestivum L.) has been studied in plant samples grown under defined conditions in a pot trial. The plant habit and the quantity of biomineralised silica within the selected Gramineae depend to a remarkable extent on the soil. The plants grew with different soil pH values and silica additives. By means of atomic absorption spectrometry, the silicon enrichment in different plant parts was determined. In dried plant parts the silica bodies can be well distinguished by variable pressure scanning electron microscopy in the back scattering mode. They are located in silica cells below the epidermis and in epidermal appendices (bristles, prickle hairs). The silica bodies showed a defined morphology, structure and composition which was elucidated by the combined performance of scanning electron microscopy in combination with X-ray spectroscopy, solid-state nuclear resonance, X-ray diffraction and Raman spectroscopy. The composition was near to stoichiometric SiO2 (41 weight% silicon, 56 weight % oxygen), and the SiO4/2 tetrahedra were arranged preferentially in three-dimensional networks; a smaller proportion was in chains and layers. The silica bodies with an overall amorphous structure contained crystalline precipitates, which could be indexed by α-quartz. [ABSTRACT FROM AUTHOR]

N mineralisation and immobilisation were quantified in field conditions in the presence or in the absence of wheat residues. The incubation study was conducted in cylinders placed in microplots (no plants were grown in cylinders), and the rest of each microplot was sowed with the wheat crop (Triticum durum var. Massa). N mineralisation and immobilisation depend on the presence or the absence of wheat residues. In absence of residues, a linear model of regression was developed to follow the clear nitrogen mineralisation at different soil levels. Nitrogen mineralisation (mg kg–1), during the five months of wheat development, showed the following decreasing order: 0–15 cm (132.6) > 15–30 cm (120.6) > 30–45 cm (91.3). The mineralisation rate was 24.1, 22.9 and 18.9 mg kg–1 d–1 for 0–15, 15–30 and 30–45 cm levels, respectively. The supply of wheat residues resulted in a five months N immobilisation process. At level 0–15 cm the immobilisation (mg kg–1) showed the following decreasing order: (61.6) > (46.4) > (30.0) for the supply of wheat residues at seeding time, and 15 and 30 d before seeding respectively. At the other levels, the same decreasing order was recorded. The supply of 8 t ha–1 of wheat residues at seeding time, and 15 or 30 d before seeding, decreased the dry matter yield and N accumulation in wheat crop. In consequence, there was no synchronism between the nitrogen liberated by wheat residues decomposition and the wheat growth. [Copyright &y& Elsevier]