Nitrogen mineralization and crop uptake of N from decomposing 15N labelled red clover and yellow sweetclover plant fractions of different age

Nitrogen mineralization from 15N labelled red clover (Trifolium pratense L.) and yellow sweetclover (Melilotus officinalis Lam.) plant fractions of three different ages (8-, 14- and 20-week old) was studied in an out-door pot experiment during 8.5 months. Individual plant fractions (leaves/stems/roots/flowers), 23 g dry matter pot-1 (corresponding to 7300 kg ha-1), were incorporated into a sandy soil. The net mineralization of N was measured as 15N recovery in spring wheat (Triticum æstivum L.), perennial ryegrass (Lolium perenne L.) following the wheat and in the soil mineral N pool. Dry matter and N yields of the wheat crop were largest in pots receiving the legume leaf materials and the oldest root fractions. The largest amount of net N mineralized was obtained after application of sweetclover leaves, 381 mg N pot-1 (38% of added N), and a smaller amount was measured from red clover leaves, 215 mg N pot-1 (26% of added N). The N release was much smaller from the stems, being on average 63 mg N pot-1 (15% of added N), with intermediate values obtained from roots, 152 mg N pot-1 (26% of added N). The effects of age of the legume fractions on net N mineralization were more pronounced for sweetclover than for red clover materials. Greater net N release was obtained from sweetclover leaves and roots with inceasing age, but the opposite was valid for stems. At final harvest of the ryegrass, an additional 2.8% of added legume N was mineralized, compared with at wheat harvest. The net N mineralized proportion of added N was significantly related to concentrations of N and cell wall constituents in the plant material. Differences in net N mineralization estimates were generally larger between plant fractions than between materials of different age, implying that leaf proportion of the above-ground biomass is of great importance when predicting net N mineralization from green-manure plant materials. In addition, the contribution from roots to net mineralized legume N could be substantial.     

Barley response to nitrogen and non-nutritional benefits of legume green manure

Green manure application may benefit subsequent crops not only by improving nitrogen (N) fertility but also via non-nutritional mechanisms. The quantification of the latter effect, however, is complicated by the confounding effect of N fertility. Two experiments were conducted in controlled environments to partition the yield response of barley to green manure between N and non-nutritional effects. Each experiment included a factorial of fertilizer N application rates and green manure application rates. The fertilizer was labelled with ¹⁵N to facilitate discrimination between N sources. Approximately 24% of the N applied in green manure was assimilated by barley after 45 days (Experiment 1) and 32% was recovered by barley grown to maturity (Experiment 2). Apparent recovery of green manure-N by barley was not appreciably affected by fertilizer application. Regression analysis of the relationship between dry matter yield and plant N uptake demonstrated that yield responses to green manure application were not entirely attributable to improved N fertility. For a given amount of N assimilated by the crop, yields were higher in green manure-amended treatments than in those receiving no green manure. In barley grown to maturity, barley response to N and non-nutritional effects were estimated to be 5.3 and 2.2g pot⁻¹, respectively. The relationship between dry matter yield and N uptake is suggested as a method for distinguishing nutritional and non-nutritional yield responses. This approach assumes that no other nutrient is limiting growth. The presence of non-nutritional benefits observed in this study demonstrates that the agronomic value of green manure is not limited to N release and casts doubt on the assumptions inherent to calculation of fertilizer equivalents. Contribution 3879132 Contribution 3879132     

Nitrogen mineralization ofSesbania sesban used as green manure for lowland rice in Sri Lanka

Sesbania sesban was evaluated as green manure crop for lowland rice in the Dry Zone of Sri Lanka. The legume was grown during a fallow period before lowland rice (Oryza sativa) and ploughed under just before transplanting. Weight loss and nitrogen content in litterbags containing leaves, stems and roots of the legume were monitored. Comparisons were made between rice yields from 20 m² plots after green manuring in combination with different nitrogen fertilizer levels (0, 2.4, 4.8 and 7.2 gm⁻²) and nitrogen fertilizer (9.6 gm⁻²) alone. Above-ground biomass ofS. sesban was 440 gm⁻² (dry wt) when ploughed under after 84 days growth. N-content in leaves, stems and roots was 3.76%, 0.41% and 0.73%, respectively. This gave a N-input fromS. sesban of 9.2 gm⁻² (8.3 g from above-ground parts and 0.9 g from roots). The corresponding K and P inputs were 7.3 and 0.6 gm⁻² respectively. The nitrogen rich leaves, which contained 88% of the nitrogen in the above-ground parts, decomposed and released its nitrogen much more rapidly than the stems and roots. After only four days the leaves had released 5.3 g Nm⁻² and after 14 days they had released 6.4 g Nm⁻². The highest rice yield (505 gm⁻²) was obtained usingS. sesban and 4.8 gm⁻² of N-fertilizer. The yields with only N-fertilizer or onlyS. sesban were 442 gm⁻² and 396 gm⁻², respectively. Due to the rapid decomposition of the nitrogen rich leaves,S. sesban did not behave as a slow release fertilizer. Thus, it is not necessary to apply nitrogen fertilizers as a basal dose.     

The fate of spring applied fertilizer N during the autumn-winter period: comparison between winter-fallow and green manure cropped soil

A field experiment was carried out at a pilot plot that was cropped with oilseed rape, and then left partly fallow and partly cropped with a green manure (mustard) during the autumn after harvest of the oilseed rape. The rape residues were incorporated in the soil. Methods used to quantify the N fluxes from harvest until sowing of the next crop were (1) ¹⁵N balance method, (2) total mineral N analysis and (3) NO emission measurements. Losses of spring applied fertilizer N were negligible in cropped plots and minimal in fallow plots during the following autumn-winter period. Most of the plant-N residues was retained by the organic N pool of the upper 30-cm soil layer. The green manure contributed slightly to soil available N at sowing of the next crop. However, the incorporation of plant material resulted in a nitrate flux that was at risk of leaching on the fallow plots, and on the green manure plots after incorporation of the green manure. This nitrate was largely derived from soil organic N, not from unused fertilizer applied in spring or from immobilized fertilizer. The NO emissions from the green manure plots were significantly higher than emissions from the fallow plots. The plants had a stimulating effect on the NO emission. A relationship between the NO emission and the soil nitrate concentration could not be established. No emissions were measured after green manure incorporation due to the low temperatures at the pilot plot. However, a greenhouse experiment showed an increased emission after incorporation. The NO emissions seemed to be related with the soil ammonium concentration.     

Nitrogen mineralization and potential nitrification at different depths in acid forest soils

Yield decline of cereals grown in monoculture may be alleviated with alternative crop management strategies. Crop rotation and optimized tillage and fertilizer management can contribute to more sustainable food and fiber production in the long-term by increasing diversity, maintaining soil organic matter (SOM), and reducing adverse effects of excessive N application on water quality. We investigated the effects of crop sequence, tillage, and N fertilization on long-term grain production on an alluvial, silty clay loam soil in southcentral Texas. Crop sequences consisted of monoculture sorghum (Sorghum bicolor (L.) Moench,) wheat (Triticum aestivum L.), and soybean (Glycine max (L.) Merr), wheat/soybean double-crop, and rotation of sorghum with wheat/soybean. Grain yields tended to be lower with no tillage (NT) than with conventional tillage (CT) early in the study and became more similar after 11 years. Nitrogen fertilizer required to produce 95% to maximum sorghum yield was similar for monoculture and rotation upon initiation of the experiment and averaged 16 and 11 mg N g^-1 grain with NT and CT, respectively. After 11 years, however, the N fertilizer requirement became similar for both tillage regimes, but was greater in monoculture (17 mg N g^-1 grain) than in rotation (12 mg N g^-1 grain). Crop sequences with double-cropping resulted in greater land use efficiency because similar or lower amounts of N fertilizer were required to produce equivalent grain than with less intensive monoculture systems. These more intensive crop sequences produced more stover with higher N quality primarily due to the inclusion of soybean in the rotation. Large quantities of stover that remained on the soil surface with NT led to greater SOM content, which increased the internal cycling of nutrients in this soil. In southcentral Texas, where rainfall averages nearly 1000 mm yr^-1, more intensive cropping of sorghum, wheat, and soybean with moderate N fertilization using reduced tillage can increase grain production and potentially decrease N losses to the environment by cycling more N into the crop-SOM system.     

Mineralization of labeled N from cowpea [Vigna unguiculata (L.) Walp.] plant parts at two growth stages in sandy soil

Cowpea [Vigna unguiculata (L). Walp.] has great potential as green manure due to its rapid N accumulation and efficient N₂ fixation. The objective of this study was to measure the rate of N mineralization from cowpea plant parts harvested at onset of flowering (5 weeks) and mid pod-fill (7 weeks) under near optimum conditions. Cowpeas were grown in a greenhouse and supplied with ¹⁵NH₄ ¹⁵NO₃ to isotopically label tissue. Cowpea leaves, stems, and roots were incorporated into a sandy soil (Psammentic Paleustalf) and net N mineralized was measured several times during a 10 week incubation. The amount of N accumulated in 7-week old cowpeas was more than double that in 5-week old cowpeas. The portion of N mineralized after 10 weeks was 24% for 5-week old cowpeas and 27% for 7-week old cowpeas. The rate of N mineralization from leaves and stems increased with plant age, but decreased for roots. The amount of N mineralized from 7-week old cowpeas was more than double (235%) that from 5-week old cowpeas due to greater N accumulation and a more rapid rate of N mineralization of the more mature cowpeas. The greatest amount of N was released from leaves, which amounted to 74 and 65% of total N mineralization from 5- and 7-week old cowpeas, respectively. The percentage of N mineralized by 10 weeks was linearly related to the tissue N concentration of the plant parts and to their C/N ratio. These relationships allow a quick estimation of the amount of N that would mineralize from cowpea residues incorporated into soil based on their N concentration or C/N ratio.     

Effect of rainfall pattern on nitrogen mineralization and leaching in a green manure experiment in South Rwanda

The effects of green manures, sorghum residues and farmyard manure on N dynamics and crop yields were studied during three dry and wet seasons on a Typic Sombriudox in South Rwanda. In addition, a resin core study was conducted within a 4-year green manure field experiment to follow the seasonal pattern of N mineralization and leaching after application of residues from Tephrosia vogelii, Sorghum bicolor, a mixture of both materials, and farmyard manure.During the dry season, topsoil (0–20 cm) mineral N remained constant. At the beginning of the wet season, the rainfall pattern determined N availability. With low rainfall intensities a mineralization flush occurred, doubling topsoil mineral N concentrations within 5 days after wetting. In contrast, under heavy rains at the onset of the rainy season, topsoil mineral N decreased by 50–70% within the first two weeks.The application of organic fertilizers has a strong influence on N availability, but the effects can be negated by heavy rainfall. Incorporation of leaves from Tephrosia vogelii (2.7 t dm ha^-1) and farmyard manure (7 t dm ha^-1) doubled the mineralization flush after the first rains. During the rest of the wet season, N release by the green manure was small, whereas the farmyard manure was found to mobilize N after a period of N immobilization. Incorporation of sorghum residues had only a small effect, while mixing the straw with green and farmyard manure immobilized N temporarily.Nitrogen leaching, measured by exchange resins at a depth of 20 cm, was increased up to 50% by the incorporation of green and farmyard manure. This points to rapid N translocation of easily mineralizable N. The additional incorporation of sorghum residues reduced N leaching of both materials significantly. Since rainfall is often unpredictable, the synchronization of N released from crop residues with crop N demand may require additional management practices.     

Vertical and horizontal development of the root system of carrots following green manure

Cover crops grown as green manure or for other purposes will affect nitrogen (N) distribution in the soil, and may thereby alter root growth of a succeeding crop. During two years, experiments were performed to study effects of nitrogen supply by green manure on root development of carrots (Daucus carota L). Total root intensity (roots cm⁻² on minirhizotrons) was significantly affected by the green manures, and was highest in the control plots where no green manure had been grown. Spread of the root system into the interrow soil was also affected by green manure treatments, as the spread was reduced where spring topsoil N_min was high. Although N supply and distribution in the soil profile differed strongly among the treatments, no effect was observed on the rooting depth of the carrot crops. Across all treatments the rooting front penetrated at a rate of 0.82 and 0.68 mm day⁻¹ °C⁻¹ beneath the crop rows and in the interrow soil, respectively. The minirhizotrons only allowed measurements down to 1 m, and the roots reached this depth before harvest. Extrapolating the linear relationship between temperature sum and rooting depth until harvest would lead to rooting depths of 1.59 and 1.18 m under the crop rows and in the interrow soil respectively. Soil analysis showed that the carrot crop was able to reduce N_min to very low levels even in the 0.75 to 1.0 m soil layer, which is in accordance with the root measurements. Still, where well supplied, the carrots left up 90 kg N ha⁻¹ in the soil at harvest. This seemed to be related to a limited N uptake capacity of the carrots rather than to insufficient root growth in the top metre of the soil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Yield responses of two ryegrass and two white clover varieties grown in contrasting mixtures with applications of phosphate at different depths and on different soils

To validate earlier findings using ³²P in upland soil and at different depths in lowland soil, ryegrass varieties S.23 or Titania were grown in mixtures with white clover varieties S.184 or Olwen. Unlabelled phosphate (0 or 43 kg P/ha) was applied to the surface or at 30 cm depth in lowland soil, and dry matter yields of grass and clover were measured. In June, on lowland soil, the greatest yield of clover was obtained by surface application. The most productive mixture was S.23/Olwen, although in August Titania outyielded S.23. By October S.23 again outyielded Titania. These results validated the isotopically discovered interactions between phosphate depth and grass/clover mixtures. On upland peat and mineral soil, 0 or 43 kg P/ha was surface applied to the same varietal mixtures. In May, phosphate application to mineral soil slightly decreased clover presence, while clover was increased by mixture with Titania rather than S.23. In July, no significant phosphate effects were found, but in October phosphate increased clover yield and, at least on mineral soil, S.23/Olwen responded most.     

Net mineralization of N at deeper soil depths as a potential mechanism for sustained forest production under elevated [CO2]

Elevated atmospheric carbon dioxide concentrations [CO₂] is projected to increase forest production, which could increase ecosystem carbon (C) storage. This study contributes to our broad goal of understanding the causes and consequences of increased fine‐root production and mortality under elevated [CO₂] by examining potential gross nitrogen (N) cycling rates throughout the soil profile. Our study was conducted in a CO₂‐enriched sweetgum (Liquidambar styraciflua L.) plantation in Oak Ridge, TN, USA. We used ¹⁵N isotope pool dilution methodology to measure potential gross N cycling rates in laboratory incubations of soil from four depth increments to 60 cm. Our objectives were twofold: (1) to determine whether N is available for root acquisition in deeper soil and (2) to determine whether elevated [CO₂], which has increased inputs of labile C resulting from greater fine‐root mortality at depth, has altered N cycling rates. Although gross N fluxes declined with soil depth, we found that N is potentially available for roots to access, especially below 15 cm depth where rates of microbial consumption of mineral N were reduced relative to production. Overall, up to 60% of potential gross N mineralization and 100% of potential net N mineralization occurred below 15 cm depth at this site. This finding was supported by in situ measurements from ion‐exchange resins, where total inorganic N availability at 55 cm depth was equal to or greater than N availability at 15 cm depth. While it is likely that trees grown under elevated [CO₂] are accessing a larger pool of inorganic N by mining deeper soil, we found no effect of elevated [CO₂] on potential gross or net N cycling rates. Thus, increased root exploration of the soil volume under elevated [CO₂] may be more important than changes in potential gross N cycling rates in sustaining forest responses to rising atmospheric CO₂.     

Effectiveness of Sesbania rostrata and Phaseolus calcaratus as green manure for upland rice grown in acidic soil

Two field experiments on green manuring were conducted under upland acidic soil (pH = 4.35) conditions with the following objectives: (1) to determine the influence of inoculation site, P fertilization, and liming on the biomass production, N content, N accumulation, and N availability of S. rostrata grown in an acidic soil, (2) to compare the effectiveness of S. rostrata, P. calcaratus and urea as N sources for upland rice as affected by liming and N source-sowing time combination, and (3) to assess the effect of liming and N source-sowing time combination on % Ndff (N derived from the fertilizer), % Ndfs (N derived from soil), % FNU (fertilizer N utilization), and FNY or fertilizer N yield (kg N ha^–1) of upland rice grown in acidic soil. At 2 weeks after incorporating S. rostrata (95 days after lime application), liming significantly increased N availability by more than 2-fold suggesting that the decomposition of S. rostrata by soil microflora was stimulated by lime. Liming, phosphorus application, and inoculation site improved significantly the dry biomass production, N content and N accumulation of S. rostrata; thus, enhancing its green manuring potential. Regardless of liming, S. rostrata whether applied at 0 week or 2 weeks before sowing was superior to urea in improving grain and straw yields. P. calcaratus when applied at 2 weeks before sowing also produced higher grain yield than urea. Immediate sowing of upland rice after green manure incorporation did not affect negatively the growth and development of upland rice; hence, farmers could save at least 2 weeks in their cropping calendar. N source-sowing time combination had a highly significant influence on % Ndff, % Ndfs, % FNU, N uptake, and fertilizer N yield of upland rice. However, only N uptake was influenced significantly by liming. The rice plant obtained significantly higher % Ndfs from the soils treated with green manure than those treated with urea regardless of liming. The % FNU and % Ndff from the green manures were 11-37% and 9-25%, respectively. These values are much lower than those obtained under continuously flooded soil conditions possibly because of the differences in the organic matter decomposer populations and N loss mechanisms between sloping upland conditions and continuously flooded conditions.     

The fate of nitrogen (15N) released from different plant materials during decomposition under field conditions

Release of N, retention in soil, availability to a subsequent crop and total recovery of N derived from different¹⁵N-labelled plant materials decomposing in soil was investigated in two field experiments. In the first experiment five different plant species (white clover, red clover, subterranean clover, field bean and timothy) and in the second subterranean clover of different maturity (2,3 and 4 months old) were buried in mesh bags in the soil and allowed to decompose for 10 and 4 months, respectively. Most of the N released from the decaying plant materials was retained in the soil (27–46% of input). The subsequent crop (barley) took up 6–25% of input. The uptake correlated with the amount of N released from the decomposing material (r=0.936^*, I experiment). Similar amounts of subterranean clover N were taken up by barley regardless to whether the material was buried in soil in the previous autumn or just before sowing of the crop. At the end of the experiments, the total recovery of the introduced plant-derived N varied between 89 and 102%. The results present evidence that the ability of the soil to retain plant-derived N is strong in comparison with the ability of the subsequent crop and different loss mechanisms to remove it.     

Modelling the change in soil organic C and N and the mineralization of N from soil in response to applications of slurry manure

A computer simulation model of the turnover of organic matter in soil was adapted to simulate the change in soil organic C and N contents of soil during several years following annual additions of farm slurry to maize fields. The model proved successful in estimating the build-up of both C and N in soil and the leaching of N to ground-water in response to applications of slurry ranging from 50 to 300 tons per hectare per year. The model was then used to estimate the build-up of organic matter in soil under crops of fodder maize that were grown using the excess of manure produced during the last 20 years in the Netherlands. The build-up of organic matter from these applications was estimated to lead to about 70 kg extra nitrogen mineralized ha^-1 yr^-1. As a result of legislation manure applications have decreased and are expected to decrease further in the immediate future. Calculations suggest that after 10 years of manure applied at rates no longer exceeding the amount needed to replace the phosphorus removed by crops, the extra mineralization of N will still be between 45 and 60 kg ha^-1 yr^-1. If manure applications cease altogether then the extra mineralization will be about 25–30 kg N ha^-1 yr^-1.     

Microbial dynamics and carbon and nitrogen cycling following re-wetting of soils beneath two semi-arid plant species

Sporadic summer rainfall in semi-arid ecosystems can provide enough soil moisture to drastically increase CO₂ efflux and rates of soil N cycling. The magnitudes of C and N pulses are highly variable, however, and the factors regulating these pulses are poorly understood. We examined changes in soil respiration, bacterial, fungal and microfaunal populations, and gross rates of N mineralization, nitrification, and NH₄⁺ and NO₃^– immobilization during the 10 days following wetting of dry soils collected from stands of big sagebrush (Artemisia tridentata) and cheatgrass (Bromus tectorum) in central Utah. Soil CO₂ production increased more than tenfold during the 17 h immediately following wetting. The labile organic C pool released by wetting was almost completely respired within 2–3 days, and was nearly three times as large in sagebrush soil as in cheatgrass. In spite of larger labile C pools beneath sagebrush, microbial and microfaunal populations were nearly equal in the two soils. Bacterial and fungal growth coincided with depletion of labile C, and populations peaked in both soils 2 days after wetting. Protozoan populations, whose biomass was nearly 3,000-fold lower than bacteria and fungi, peaked after 2–4 days. Gross N mineralization and nitrification rates were both faster in cheatgrass soil than in sagebrush, and caused greater nitrate accumulation in cheatgrass soil. Grazing of bacteria and fungi by protozoans and nematodes could explain neither temporal trends in N mineralization rates nor differences between soil types. However, a mass balance model indicated that the initial N pulse was associated with degradation of microbial substrates that were rich in N (C:N <8.3), and that microbes had shifted to substrates with lower N contents (C:N =15–25) by day 7 of the incubation. The model also suggested that the labile organic matter in cheatgrass soil had a lower C:N ratio than in sagebrush, and this promoted faster N cycling rates and greater N availability. This study provides evidence that the high N availability often associated with wetting of cheatgrass soils is a result of cheatgrass supplying substrates to microbes that are of high decomposability and N content.     

Simulation of the decomposition and nitrogen mineralization of aboveground plant material in two unfertilized grassland ecosystems

A simple model of the decomposition and nitrogen mineralization of plant material from two unfertilized grassland ecosystems has been developed, with only the proportion of leaves and stems in the original material, the initial nitrogen contents of these plant parts and temperature as input data. The model simulates carbon losses from stems and leaves, using a double exponential decay function, with the temperature sum as independent variable. Mineralization of nitrogen is not calculated via microbial growth rates, but simulated on the basis of the carbon utilization efficiency of the microorganisms and the critical C/N ratio, i.e. the C/N ratio of the litter at which the microbial demand for nitrogen is met exactly. The parameter values for leaching fractions of carbon and nitrogen, relative decay rates, microbial carbon utilization efficiencies and critical C/N ratios were derived from a litter bag experiment with 12 litter types (species) including both green and dead materials, carried out in two unfertilized grassland ecosystems differing in production level. The model was evaluated using a cross-validation method, in which one species was omitted from the parametrization procedure, and its decomposition and mineralization were predicted by the resulting model. In general there was good agreement between the observed and predicted amounts of carbon and nitrogen remaining for all green litter types/species, but carbon and nitrogen dynamics in the dead material of Festuca rubra were poorly predicted. This disparity has been attributed to the proportion of leaves in the material of Festuca rubra (95%) being far beyond the range of leaf proportions in the three litter types the calibration set consisted of (8–35%). When the data of all litter types were used to determine the model parameters, good agreement was obtained between measured and simulated values for the changes in nitrogen and carbon in all litter types of both the green and dead material series. Optimization yielded parameter values for microbial carbon utilization efficiencies of 0.30 for microorganisms associated with green litter and 0.35 for those associated with dead litter. The critical C/N ratios for green and dead material were found to be 29 and 36, respectively.     

Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils

In the next decades, many soils will be subjected to increased drying/wetting cycles or modified water availability considering predicted global changes in precipitation and evapotranspiration. These changes may affect the turnover of C and N in soils, but the direction of changes is still unclear. The aim of the review is the evaluation of involved mechanisms, the intensity, duration and frequency of drying and wetting for the mineralization and fluxes of C and N in terrestrial soils. Controversial study results require a reappraisal of the present understanding that wetting of dry soils induces significant losses of soil C and N. The generally observed pulse in net C and N mineralization following wetting of dry soil (hereafter wetting pulse) is short‐lived and often exceeds the mineralization rate of a respective moist control. Accumulated microbial and plant necromass, lysis of live microbial cells, release of compatible solutes and exposure of previously protected organic matter may explain the additional mineralization during wetting of soils. Frequent drying and wetting diminishes the wetting pulse due to limitation of the accessible organic matter pool. Despite wetting pulses, cumulative C and N mineralization (defined here as total net mineralization during drying and wetting) are mostly smaller compared with soil with optimum moisture, indicating that wetting pulses cannot compensate for small mineralization rates during drought periods. Cumulative mineralization is linked to the intensity and duration of drying, the amount and distribution of precipitation, temperature, hydrophobicity and the accessible pool of organic substrates. Wetting pulses may have a significant impact on C and N mineralization or flux rates in arid and semiarid regions but have less impact in humid and subhumid regions on annual time scales. Organic matter stocks are progressively preserved with increasing duration and intensity of drought periods; however, fires enhance the risk of organic matter losses under dry conditions. Hydrophobicity of organic surfaces is an important mechanism that reduces C and N mineralization in topsoils after precipitation. Hence, mineralization in forest soils with hydrophobic organic horizons is presumably stronger limited than in grassland or farmland soils. Even in humid regions, suboptimal water potentials often restrict microbial activity in topsoils during growing seasons. Increasing summer droughts will likely reduce the mineralization and fluxes of C and N whereas increasing summer precipitation could enhance the losses of C and N from soils.     

Seed germination at different temperatures and water stress levels,and seedling emergence from different depths of Ziziphus lotus

Ziziphus lotus (L.) Lam. is a deciduous shrub with intricately branched stems in the Rhamnaceae family. It's a dominant and economically important species widely distributed in active sand dunes in the southern desert of Tunisia. To provide basic information for its conservation and reintroduction, we studied the influence of environmental factors on seed germination patterns. The germination responses of seeds were determined over a wide range of constant temperatures (10–50 °C), polyethylene glycol (PEG)-6000 solutions of different osmotic potentials (0 to − 1 MPa) and burial depths (1–10 cm). Temperatures between 15 and 45 °C seem to be favorable for the germination of this species. Germination was inhibited by either an increase or decrease in temperature from the most suitable temperature found (35 °C). The highest germination percentages (100%) were obtained under control conditions without PEG, and increasing moisture stress progressively inhibited seed germination, which was less than 5% at − 1 MPa. When tested for germination in distilled water, after PEG treatments, seeds germinated to the same extent as when fresh. When seeds buried deeply, there was a significant decrease in seedling emergence percentage and rate. Seedlings of Z. lotus emerged well at depths of 1–2 cm and could not emerge when sand burial depth was > 4 cm.     

Simulating the effects of N availability, straw particle size and location in soil on C and N mineralization

Predicting the C and N mineralization of straw added to soil is important for forecasting subsequent soil N availability during and between crop growth cycles. The decomposition module of the STICS model, parameterized under optimal conditions, was used to predict straw decomposition in sub-optimal conditions, i.e. when contact between soil and residue was poor (due to large size residues or surface placement) or when mineral N availability was restricted. The data used in the simulations were obtained from published studies of effects of residue size, location and N availability on C and N mineralization from straw under controlled laboratory conditions. We selected studies in which the dynamics of C and N mineralization were measured simultaneously. The dynamics of straw mineralization could be well predicted by the model under optimal conditions with standard parameter values as derived from measured C/N ratios of the residues, but not under sub-optimal conditions which required a new parameterization. A good fit could be obtained on these treatments by a marked reduction in the rate constants of residue and microbial biomass decomposition and a marked increase in the microbial biomass C/N ratio. Our results show the need to include in decomposition models routines for simulating effects of spatial heterogeneity of residue distribution, different particle sizes and limiting N availability.     

Effects of red clover (Trifolium pratense) green manure and compost soil amendments on wild mustard (Brassica kaber) growth and incidence of disease

Using a soil bioassay technique, seedling growth and incidence of disease of wild mustard (Brassica kaber) and sweet corn (Zea mays) were assessed in soil from field plots that received either of two treatments: incorporated red clover (Trifolium pratense) residue plus application of compost (`amended soil'), or application of ammonium nitrate fertilizer (`unamended soil'). Soils were analyzed for percent moisture, dissolved organic carbon, conductivity, phenolics, and nutrient content. A trend toward greater incidence of Pythium spp. infection of wild mustard seedlings grown in amended soil was observed during the first 40 days after incorporation (DAI) of red clover and compost, with significant differences ( = 0.05) at two out of four sampling dates in 1997, and four out of four sampling dates in 1998. Incidence of Pythium infection was 10–70% greater in the amended soil treatment during that period. Asymptomatic wild mustard seedlings grown in amended soil were also on average 2.5 cm shorter ( = 0.05) at 5 DAI than those grown in unamended soil in one year out of two. Concentration of phenolic compounds in soil solution was weakly correlated with decreased shoot and root growth (r = 0.50, 0.28, respectively) and increased incidence of disease (r = 0.48) in wild mustard seedlings in one year out of two. Dissolved organic carbon concentration was weakly correlated with increased disease in wild mustard seedlings in both years (r = 0.51, 0.33, respectively). Growth of corn seedlings did not differ between the two soil treatments, suggesting that red clover green manure and compost may selectively reduce density and competitive ability of wild mustard in the field. Bioassay results corresponded well with emergence and shoot weight results from a related field study, indicating that this technique may be useful for screening potential soil treatments prior to field studies.     

Nitrogen uptake and recovery from urea and green manure in lowland rice measured by15N and non-isotope techniques

In the recent past considerable attention is paid to minimize dependence on purchased inputs such as inorganic nitrogen fertilizer. Green manure in the form of flood-tolerant, stem-nodulatingSesbania rostrata andAeschynomene afraspera is an alternative N source for rice, which may also increase N use efficiency. Therefore research was conducted to determine the fate of N applied to lowland rice (Oryza sativa L.) in the form ofSesbania rostrata andAeschynomene afraspera green manure and urea in two field experiments using¹⁵N labeled materials.¹⁵N in the soil and rice plant was determined, and¹⁵N balances established. Apparent N recoveries were determined by non-tracer method. ¹⁵N recoveries averaged 90 and 65% of N applied for green manure and urea treatments, respectively. High partial pressures of NH₃ in the floodwater, and high pH probably resulted from urea application and favoured losses of N from the urea treatment. Results show that green manure N can supply a substantial proportion of the N requirements of lowland rice. Nitrogen released fromSesbania rostrata andAeschynomene afraspera green manure was in synchrony with the demand of the rice plant. The effect of combined application of green manure and urea on N losses from urea fertilizer were also investigated. Green manure reduced the N losses from¹⁵N labeled urea possibly due to a reduction in pH of the floodwater. Positive added N interactions (ANIs) were observed. At harvest, an average of 45 and 25% of N applied remained in the soil for green manure and urea, respectively.Contribution from IRRI, Los Baños, Philippines and Justus-Liebig-University, Giessen, GermanyContribution from IRRI, Los Baños, Philippines and Justus-Liebig-University, Giessen, Germany