间作对马铃薯种植土壤硝化作用和硝态氮供应的影响

土壤硝态氮供应对满足作物氮素需求至关重要,但间作如何影响土壤硝态氮供应及其作用机制尚不清楚。本研究基于4个氮水平(N₀, 0 kg·hm^-2; N₁, 62.5 kg·hm^-2; N₂, 125 kg·hm^-2; N₃, 187.5 kg·hm^-2)的马铃薯单作、马铃薯与玉米间作小区试验,分析土壤硝态氮含量与强度、硝化势和氨氧化功能基因丰度的差异,探讨间作影响土壤硝态氮供应和氮调控的机理。结果表明: 土壤硝态氮含量和强度随施氮量增加而升高,但同一施氮水平下间作均低于单作。施氮提高了土壤硝化势,且单作的响应高于间作。土壤中氨氧化细菌(AOB)的amoA基因丰度大于氨氧化古菌(AOA),二者在间作时均随施氮量增加呈现先增加后降低的趋势;相同施氮量下,间作的AOA和AOB基因丰度(除N₂外)均低于单作。相关分析、回归分析和主成分分析显示,马铃薯间作后,土壤AOB、AOA的amoA基因丰度下降,硝化势减弱,导致土壤硝态氮含量和强度降低。因此,间作导致土壤硝态氮供应降低与土壤氮转化的微生物过程有关,间作条件下的马铃薯种植应注意保障土壤氮素供应。     

Nitrogen contribution of cowpea green manure and residue to upland rice

Cowpea, Vigna unguiculata (L.) Walp., is well adapted to acid upland soil and can be grown for seed, green manure, and fodder production. A 2-yr field experiment was conducted on an Aeric Tropaqualf in the Philippines to determine the effect of cowpea management practice on the response of a subsequent upland rice crop to applied urea. Cowpea was grown to flowering and incorporated as a green manure or grown to maturity with either grain and pods removed or all aboveground vegetation removed before sowing rice. Cowpea green manure accumulated on average 68 kg N ha⁻¹, and aboveground residue after harvest of dry pods contained on average 46 kg N ha⁻¹. Compared with a pre-rice fallow, cowpea green manure and residue increased grain yield of upland rice by 0.7 Mg ha⁻¹ when no urea was applied to rice. Green manure and residue substituted for 66 and 70 kg urea-N ha⁻¹ on upland rice, respectively. In the absence of urea, green manure and residue increased total aboveground N in mature rice by 12 and 14 kg N ha⁻¹, respectively. These increases corresponded to plant recoveries of 13% for applied green manure N and 24% for applied residue N. At 15 d after sowing rice (DAS), 33% of the added green manure N and 16% of the added residue N was recovered as soil (nitrate + ammonium)-N. At 30 DAS, the corresponding recoveries were 20 and 37% for green manure N and residue N, respectively. Cowpea cropping with removal of all aboveground cowpea vegetation slightly increased (p<0.05) soil (nitrate + ammonium)-N at 15 DAS as compared with the pre-rice fallow, but it did not increase rice yield. Cowpea residue remaining after harvest of dry pods can be an effective N source for a subsequent upland rice crop.     

Monoclonal antibodies to a higher-plant nitrate reductase: Differential inhibition of enzyme activities

A set of monoclonal antibodies has been raised against NADH-nitrate reductase (NR; EC 1.6.6.1) from spinach (Spinacea oleracea L.) leaves. Antibodies were screened by enzyme-linked immunosorbent assay and by their ability to inhibit various activities of the enzyme. The six monoclonals selected (AFRC MAC 74 to 79) are all gamma globulins; four (MAC 74 to 77) inhibit all terminal donating activities (NADH-NR; flavin mononucleotide, reduced form (FMNH₂)-NR; and methyl viologen, reduced form (MV)-NR) and two (MAC 78 and 79) inhibit the acceptor activities (NADH-NR, and NADH-cytochrome c reductase). MAC 74 to 77 inhibit the NADH-NR activity of crude extracts of a variety of species (mono- and dicotyledoneae) while MAC 78 and 79 are effective against spinach and marrow, but not oil-seed rape, cucumber, oats, wheat and barley.Abbreviations Cyt c Rase cytochrome c reductase - ELISA enzyme-linked immunosorbent assay - FAD(H₂) flavin adenine dinucleotide (reduced form) - FMN(H₂) flavin mononucleotide (reduced form) - McAb monoclonal antibody - MV methyl viologen reduced form - NR nitrate reductase     

SIMULTANEOUS ASSIMILATION OF AMMONIUM AND NITRATE BY GELIDIUM NUDIFRONS (GELIDIALES: RHODOPHYTA)1

Simultaneous assimilation of NH₄ and NO₃ by Gelidium nudifrons Gardner was observed in culture experiments of 4 possible combinations of NH₄ and NO₃. The combinations tested were those in which the concentration of both N sources were in the range of 3.0–4.0 μg-atN · l^?1; both in the range of 0.5–1.0 μg-atN · l^?1; one in the 3.0–4.0 μg-atN · l^?1 range and the other in the 0.5–1.0 μg-atN · l^?1 range; and, visa versa. The data suggest that the pools of both NH₄ and NO₃ are simultaneously available for algal assimilation.     

Nitrogen-15 balances for urea and neem coated urea applied to lowland rice following two cowpea cropping systems

Little is known about whether the high N losses from inorganic N fertilizers applied to lowland rice (Oryza sativa L.) are affected by the combined use of either legume green manure or residue with N fertilizers. Field experiments were conducted in 1986 and 1987 on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems before rice on the fate and use efficiency of¹⁵N-labeled, urea and neem cake (Azadirachta indica Juss.) coated urea (NCU) applied to the subsequent transplanted lowland rice crop. The pre-rice cropping systems were fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with subsequent incorporation of residue remaining after grain and pod removal. The incorporated green manure contained 70 and 67 kg N ha⁻¹ in 1986 and 1987, respectively. The incorporated residue contained 54 and 49 kg N ha⁻¹ in 1986 and 1987, respectively. The unrecovered¹⁵N in the¹⁵N balances for 58 kg N ha⁻¹ applied as urea or NCU ranged from 23 to 34% but was not affected by pre-rice cropping system. The partial pressure of ammoniapNH₃, and floodwater (nitrate + nitrite)-N following application of 29 kg N ha⁻¹ as urea or NCU to 0.05-m-deep floodwater at 14 days after transplanting was not affected by pre-rice cropping system. In plots not fertilized with urea or NCU, green manure contributed an extra 12 and 26 kg N ha⁻¹, to mature rice plants in 1986 and 1987, respectively. The corresponding contributions from residue were 19 and 23 kg N ha⁻¹, respectively. Coating urea with 0.2g neem cake per g urea had no effect on loss of urea-N in either year; however, it significantly increased grain yield (0.4 Mg ha⁻¹) and total plant N (11 kg ha⁻¹) in 1987 but not in 1986.     

Regulation of nitrate reductase in suspension culture of Silene alba Immunochemical approach

Silene alba cells grown on nitrate, usually develop NADH-nitrate reductase activity only at the beginning of their growth cycle. Immunodiffusion assays, with a specific nitrate reductase antiserum, revealed the presence of cross-reacting material in cells harvested at any time during their culture. Cells grown on ammonium lacked NADH-nitrate reductase activity but contained cross-reacting material. It is suggested that S. alba cells contain an enzymically inactive, antigenic form of nitrate reductase regardless of the nitrogen source.     

Effects of Organic Resource Quality on Soil Profile N Dynamics and Maize Yields on Sandy Soils in Zimbabwe

Optimising the use efficiency of nitrogen (N) derived from different quality organic resources and mineral fertilizers on sandy soils with <100 g clay kg⁻¹ is a major challenge for smallholder farmers in Southern Africa. The dominant sandy soils have a poor capacity to store and supply crop nutrients due to low organic matter contents and inherent infertility. A study was conducted in Zimbabwe to determine the differential N supply effects of different quality and quantities of organic nutrient sources on maize productivity. Crotalaria juncea L., Calliandra calothyrsus Meissn., cattle manure, maize (Zea mays L.) stover and Pinus patula Schiede & Schltdl. & Cham. sawdust which represented high to low quality materials respectively, were each incorporated into soil at 1.2 and 4 t C ha⁻¹ at Makoholi Experiment Station (rainfall: 450–650 mm yr⁻¹) and tested against a sole mineral N fertilizer and control treatments. In a separate experiment conducted in farmers’ fields under different rainfall zones of Zimuto (450–650 mm yr⁻¹), Chinyika (650–750 mm yr⁻¹) and Chikwaka (>750 mm yr⁻¹), commonly available organic materials, including manure and composted miombo leaf litter, applied in varying amounts by farmers were evaluated. Nitrogen release patterns were consistent with differences in resource quality. At 3 weeks after incorporation into soil at the onset of the rains, C. juncea and C. calothyrsus had released as high as 24% and 13% of added N respectively, compared with no more than 5–6% for the rest of the amended treatments. Most of the N released was lost through leaching as evidenced by progressive movement of NO₃⁻-N bulges beyond maize rooting depth following major rainfall events. Maize yields were significantly related to the size of profile mineral N fluxes, with the best linear relationship (R² = 0.86) obtained with N available in the top 30 cm of soil at maize flowering. High grain yields of ~3 t ha⁻¹ were only achieved with C. juncea applied at 4 t C ha⁻¹, which also had highest NO₃⁻-N leaching losses. Conversely, the same application rate increased N immobilization by 30% and 42% under maize stover and sawdust, respectively, relative to the control. Results from farmers’ fields showed that organic resources traditionally used on smallholder farms are invariably of low quality relative to C. juncea and C. calothyrsus. However, they exhibited shorter N immobilization effects than was shown for maize stover and sawdust at Makoholi, suggesting that pre-application treatments, such as composting, employed by farmers enhance seasonal N benefits from these materials. Maize yields increased linearly with total N added in these resources in combination with N fertilizer, justifying the high organic matter loading strategy (e.g. >20 t ha⁻¹ for manure, fresh litter and composted litter) used by farmers who often achieve high crop yields on such coarse sandy soils in Zimbabwe.     

Mechanisms for carbon and nutrient release and retention in beech forest gaps

Nutrient cycling and water balance in forest gaps has received little attention until now, although gap regeneration is important to natural dynamics of temperate forests. Gaps of 30 m diameter, cut in a mature beech forest, exhibited a distinct change in microclimatic conditions in comparison with the surrounding stand. Soil moisture in gaps remained very high throughout the observation period. Disruption of the N cycle in gaps led to substantial nitrate losses; seepage water nitrate concentrations were 10–18 mg NO₃-N L^-1. Excess nitrification was a significant cause of soil acidification and aluminium release. The pH in subsoil seepage water decreased by 0.25. Liming in gaps promoted the establishment of a herbaceous vegetation, which functioned as an important nutrient sink, and thus is recommended for tree regeneration in highly acidified forest ecosystems as it increases the resilience of the ecosystem to nutrient losses.     

Effects of NO3−, NH4+ and urea nutrition on endogenous levels of IAA and four cytokinins in two epiphytic bromeliads

The long-term effects of different nitrogen sources on the endogenous IAA and cytokinin levels in two bromeliad species were investigated. In nature, Vriesea philippocoburgii is a tank-forming epiphytic bromeliad which uses the tank water reservoir as a substitute for soil, whereas Tillandsia pohliana is a tankless atmospheric epiphytic species. A culture was established from seeds germinated in aseptic condictions, and the plantlets were grown for 6 months in a modified Knudson medium to which was added 8 mol m⁻³ of nitrogen in the form of NO₃⁻, NH₄⁺ or urea. The hormonal contents of the bromeliad shoots were determined by means of high-performance liquid chromatography (HPLC), coupled to an enzyme-linked immunosorbent assay (ELISA) for indole-3-acetic acid (IAA), isopentenyladenine (iP), isopentenyladenosine ([9R]iP), zeatin (Z) and zeatin riboside ([9R]Z). Nitrogen supplied in the form of urea gave the highest values of fresh and dry weights for both species, and this was positively correlated to IAA levels. The cytokinin patterns showed that isopentenyladenosine was the predominant form for both species in all samples. However, urea induced the highest level of this riboside form and also the highest level of total cytokinins for V. philippocoburgii, while NH₄⁺ had the same effect on the atmospheric species. These results are discussed in terms of the different growth habits of these two species in nature. It is suggested that urea may be an important source of nitrogen often found inside the tank of V. philippocoburgii. NO₃⁻ treatment increased the IAA/Cks balance, mainly for V. philippocoburgii, while urea and NH₄⁺ shifted this ratio in favour of cytokinins, thus apparently inhibiting root development in both species.     

Evidence that Soil Carbon Pool Determines Susceptibility of Semi-Natural Ecosystems to Elevated Nitrogen Leaching

Deposition of reactive nitrogen (N) compounds has the potential to cause severe damage to sensitive soils and waters, but the process of ‘nitrogen saturation’ is difficult to demonstrate or predict. This study compares outputs from a simple carbon–nitrogen model with observations of (1) regional- and catchment-scale relationships between surface water nitrate and dissolved organic carbon (DOC), as an indicator of catchment carbon (C) pool; (2) inter-regional variations in soil C/N ratios; and (3) plot scale soil and leachate response to long-term N additions, for a range of UK moorlands. Results suggest that the simple model applied can effectively reproduce observed patterns, and that organic soil C stores provide a critical control on catchment susceptibility to enhanced N leaching, leading to high spatial variability in the extent and severity of current damage within regions of relatively uniform deposition. Results also support the hypothesis that the N richness of organic soils, expressed as C/N ratio, provides an effective indicator of soil susceptibility to enhanced N leaching. The extent to which current C/N is influenced by N deposition, as opposed to factors such as climate and vegetation type, cannot be unequivocally determined on the basis of spatial data. However, N addition experiments at moorland sites have shown a reduction in organic soil C/N. A full understanding of the mechanisms of N-enrichment of soils and waters is essential to the assessment of current sensitivity to, and prediction of future damage from, globally increasing reactive nitrogen deposition.