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The processes of N mineralization and immobilization which can occur in agricultural soils during decomposition of plant residues are briefly reviewed in this paper. Results from different incubation studies have indicated that the amounts of N immobilized can be very important and that the intensity and kinetics of N immobilization and subsequent remineralization depend on the nature of plant residues and the type of decomposers associated. However, most of the available literature on these processes refer to incubations where large amounts of mineral N were present in soil. Incubations carried out at low mineral N concentrations have shown that the decomposition rate of plant residues is decreased but not stopped. The immobilization intensity, expressed per unit of mineralized C, is reduced and N remineralization is delayed. Nitrogen availability in soil can therefore strongly modify the MIT kinetics (mineralization-immobilization turnover) by a feed-back effect. The mineralization and immobilization kinetics have been determined in a two-years field experiment in bare soil with or without wheat straw. Mineralization in plots without straw seemed to be realistically predicted by accounting for variations in soil temperature and moisture. Immobilization associated with straw decomposition was clearly shown. It was increased markedly by the addition of mineral N throughout decomposition. It is concluded that mineral N availability is an important factor controlling plant residues decomposition under field conditions. A better prediction of the evolution of mineral N in soil may therefore require description and modelling of the respective localization of both organic matter and mineral N in soil aggregates.

Lime (calcium hydroxide) was used as a pretreatment agent to enhance the enzymatic digestibility of two common crop residues: bagasse and wheat straw. A systematic study of pretreatment conditions suggested that for short pretreatment times (1-3 h), high temperatures (85-135 degrees C) were required to achieve high sugar yields, whereas for long pretreatment times (e.g., 24 h), low temperatures (50-65 degrees C) were effective. The recommended lime loading is 0.1 g Ca(OH)2/g dry biomass. Water loading had little effect on the digestibility. Under the recommended conditions, the 3-d reducing sugar yield of the pretreated bagasse increased from 153 to 659 mg Eq glucose/g dry biomass, and that of the pretreated wheat straw increased from 65 to 650 mg Eq glucose/g dry biomass. A material balance study on bagasse showed that the biomass yield after lime pretreatment is 93.6%. No glucan or xylan was removed from bagasse by the pretreatment, whereas 14% of lignin became solubilized. A lime recovery study showed that 86% of added calcium was removed from the pretreated bagasse by ten washings and could be recovered by carbonating the wash water with CO2 at pH 9.5

Reports on the effect of organic matter addition on soil pH have been contradictory. This study examined the effect of applying legume residues differing in concentrations of N (4.3-45.5 mg g⁻¹) and excess cations/organic anions (0.22-1.56 mmol g⁻¹) on pH change of five soils differing in initial pH (3.60-5.58 in 0.01 M CaCh) under sterile and non-sterile conditions. Addition of the legume residues at a level of 1% soil weight increased the pH of all soils by up to 2 units after incubation for 35 and 100 d under non-sterile conditions. Exceptions were the Lancelin (initial pH 5.06) and Kellerberin (pH 5.58) soils with addition of clover roots (excess cations 22 cmol/kg) for 100 d where soil pH decreased by 0.13-0.15 units as compared to the control. The amounts of alkalinity produced in soil correlated positively with concentrations of excess cations and total nitrogen of the added legume residues, and negatively with the initial pH of the soil. When soil was fumigated with chloroform during incubation, similar trends of soil pH changes and alkalinity production, due to legume residues addition, were displayed but the effects of the residue on alkalinity production in the Wodjil and Lancelin soils were much less than under non-sterile conditions. Direct shaking of soil with the residues under sterile conditions increased the amount of alkalinity in the soils with initial pH of 3.60-4.54, but not in the soils with initial pH of 5.06 and 5.58. The maximal alkalinity production was less than one third of that produced in the soil after 100 d of incubation under non-sterile conditions. The results suggest that the direction and the magnitude of pH change depend largely on the concentration of organic anions in the residues, initial soil pH and the degree of residue decomposition. The incorporation of crop residues, especially those with high concentrations of excess cations, is recommended in minimizing soil acidification in farming systems.

▪ Abstract  Farmers increasingly leave crop residues on the soil surface rather than incorporating them into the soil. This practice helps reduce soil erosion, conserve energy, increase soil moisture, and increase crop yields. However, many soilborne plant pathogens survive in the previous year's crop residue, making diseases more problematic under reduced-tillage conditions. Reduced tillage can favor pathogens by such mechanisms as protecting the pathogen's refuge in the residue from microbial degradation, lowering soil temperature, increasing soil moisture, and leaving soil undisturbed. In order for reduced tillage to become more popular, additional controls are needed for pathogens. The four major control tactics (disease-control chemicals, biological control, host resistance, and cultural controls) can be used to limit damage from diseases. It is highly recommended, however, that crop rotation be coupled with reduced tillage. This practice controls many diseases and yet allows as much of the crop residue as possible to be retained on the soil surface.

Cover crops help control erosion, prevent nutrient leaching, fix nitrogen, improve sail conditions, and protect seedlings, but also use water, thus affecting soil water relationships far the next crop. Effects are positive when cover crops are managed to improve infiltration and decrease evaporation, or to remove water from a wet soil to allow timely establishment of the next crop. Effects are negative when they limit water for the next crop or aggravate a wet soil condition. Cover crops are better suited to humid and subhumid regions where precipitation is more reliable than to semiarid regions where precipitation is limited. Where cover crops are not used, use of conservation tillage that involves crop residue retention on the soil surface helps conserve soil water and provides many of the benefits of cover crops, except for nitrogen fixation, soil nutrient (especially nitrate) uptake to prevent leaching, excess water removal, and additional organic matter inputs.

Effects of contact between the soil and crop residues on the processes of residue decomposition are still poorly understood. The objective of this study was to investigate the effects of residue particle size on the decomposition of wheat (Triticum aestivum L.) straw (C/N= 270) and green rye (Secale cereale) residues (C/N= 9). Residue particle size was used as a means to vary the contact between crop residues and the soil. Carbon mineralization was measured during 102 d for straw and 65 d for rye, on residues ranging in sizes from laboratory model (0.03 cm) to field-scale (10 cm). The soil was a silt (Typic Hapludalf) and the incubation was performed at 15 °C. The effects of particle size on C mineralization varied for the two residues. In the first two days of incubation, decomposition rate of rye increased with decreasing particle size but thereafter, the trend was reversed. In 65 days, 8% more C was decomposed in the 7-cm residues than in the 0.03-cm ones. For wheat straw, early decomposition (3-17 days) was faster for the small-sized particles (0.06 and 0.1 cm). Thereafter, the largest size classes (5 and 10 cm) decomposed faster. After 102 days, the very fine particles (< 0.1 cm) showed the greatest and the intermediate size classes (0.5 and 1 cm), the lowest amount of C mineralized. We hypothesized that greater availability and accessibility of N was responsible for the higher rates of decomposition observed for finely-ground wheat straw while a physical protection of finely ground residues was probably involved in the observed reverse effect for rye.

We investigated the effects of dietary carbohydrates an the composition and pH of fecal material and on the ammonia emission from the slurry of growing pigs. Thirty-four barrows (BW approximately 40 kg) were randomly allotted to 1 of 10 diets. A basal diet was formulated to meet all requirements for protein, amino acids, minerals, and vitamins. The control diet was composed of the basal diet plus heat-treated cornstarch. In the other diets, the cornstarch in the control diet was replaced with three levels of either coconut expeller, soybean hulls, or dried sugar beet pulp. Feces were collected separately from urine in a balance experiment. Feces were mixed with a standardized urine (ratio of 1:2.5, wt/wt) to form a slurry. A sample of this slurry was placed in an in vitro system to determine the pH and the ammonia emission for 16 d at 20 degrees C. The fecal and slurry DM contents decreased (P .001) and the total VFA concentrations increased (P .001) when the level of dietary carbohydrates increased. The pH and the ammonia emission decreased as the level of carbohydrates increased (P .001). The addition of soybean hulls to the diet had the greatest effect on reducing the pH and ammonia emission (P .001), and the effects of sugar beet pulp and coconut expeller were approximately the same. A linear relationship was found between the intake of dietary nonstarch polysaccharides (NSP) and the ammonia emission (P .001). For each 100-g increase in the intake of dietary NSP, the slurry pH decreased by approximately .12 unit and the ammonia emission from slurry decreased by 5.4%. We conclude that replacing cornstarch in the diet with components that have a high concentration of fermentable carbohydrates increases the VFA concentration of feces and slurry and reduces the pH and ammonia emission from the slurry of growing pigs

A pot experiment was conducted to study interactive effects of arbuscular mycorrhizae (AMs) and maize (Zea mays L.) straws on wheat (Triticum aestivum L.) growth and organic carbon (C) storage in a sterilized sandy loam soil. The experiment included four treatments: control, inoculation with AM fungus Glomus caledonium (M), amendment with maize straw (S), and amendment with maize straw plus inoculation with G. caledonium (S + M). The inoculation of G. caledonium significantly (P < 0.05) increased wheat root biomass and root-to-straw ratio, but had no significant effects on shoot biomass, grain yield, and soil parameters. The amendment of maize straw significantly (P < 0.05) decreased soil pH, wheat root biomass, and root-to-straw ratio, and significantly (P < 0.05) increased soil invertase and alkaline phosphatase activities, but had no significant effects on shoot biomass, grain yield, soil organic C content, and urease activity. The combined application of G. caledonium and maize straw had no significant effects on root mycorrhizal colonization rate compared to the M treatment, while significantly (P < 0.05) increased wheat root biomass and significantly (P < 0.05) decreased soil pH compared to the S treatment, and also significantly (P < 0.05) increased grain yield, soil organic C content, and urease activity compared to the control. The Two-Way ANOVA also showed interactive effects of G. caledonium and maize straw on soil pH (P < 0.05) and wheat grain yield (P < 0.01), and the redundancy analysis result indicated the potential application of AM fungi in straw-returned fields.

The effects of different contents of impurities and seed hulls in the raw material on the sensory characteristics, chemical quality, and oxidative stability of sunflower oil prepared by the procedure of cold pressing on a screw press were investigated. It was found that the presence of impurities (up to 10%) and hulls (up to 32%) had an adverse effect on the sensory and chemical characteristics of the oil. The adverse influence on the oils colour was also evidenced from the results of measuring their transparency, which ranged from 14.75% to 43.60%. The presence of impurities and seed hulls caused also a decrease in the oxidative stability of oils, as the values of the induction period ranged from 3.63 h to 4.63 h, while the Totox values were in the range from 2.25 to 5.87.

Soil organic matter (SOM) takes part in many environmental functions and, depending on the conditions, it can be a source or a sink of the greenhouse gases. Presently, the changes in soil organic carbon (SOC) stock can arise because of the climatic changes or changes in the land use and land management. A promising method in the estimation of SOC changes is modelling, one of the most used models for the prediction of changes in soil organic carbon stock on agricultural land being the RothC model. Because of its simplicity and availability of the input data, RothC was used for testing the efficiency to predict the development of SOC stock during 35-year period on agricultural land of Slovakia. The received data show an increase of SOC stock during the first (20 years) phase and no significant changes in the course of the second part of modelling. The increase of SOC stock in the first phase can be explained by a high carbon input of plant residues and manure and a lower temperature in comparison with the second modelling part.