Leguminous Cover Crop Effects on Nitrogen Mineralization Rates and Kinetics in Soils

Soils were collected from an experimental site (initiated in 1991) at which leguminous crops were grown as natural soil cover in the interspaces of a 19‐year‐old coconut plantation. Atylosia scarabaeoides, Centrosema pubescens, Calopogonium mucunoides and Pueraria phaseoloides were grown in separate plots during the rainy season and ploughed into the soil towards the end of the monsoon (in December every year). Soil samples were collected from this site at the end of the 7th year and incubated in PVC columns at 35 ± 1°C and 0.01 MPa moisture content for 36 weeks. The soils were then leached at periodic intervals for up to 36 weeks and nitrogen (N) mineralization rates and kinetics were determined by the double exponential model. The N mineralization rates were highest during the first week and decreased with time in all soils. Soils amended (in situ ploughing) with cover crops leached 191 mg kg^–1 more NO₃^– + NO₂^–‐N than the unamended control. The per cent organic N mineralized (total and net) and the cumulative inorganic N mineralized (NO₃^– + NO₂^–‐N) varied with the amount (biomass) and type of cover crop incorporated into the soil. In general, soils amended with cover crops had greater N mineralization potentials and rate constants than the unamended control. The kinetic parameters N_oS and N_o(1 – S) and their respective rate constants h and k also varied with the amount and type of cover crops incorporated into the soil. The results further indicated that the lignin + polyphenol : N ratio of the cover crops is extremely important in predicting the rate of decomposition and N mineralization in soils.     

Nitrogen Requirements at Different Growth Stages of Short-duration Pigeonpea (Cajanus cajan L. Millsp)1

The response to N fertilization of a short-duration pigeonpea genotype, ICPL 87, was studied in the field to assess the scope for genetically improving symbiotic N₂ fixation by pigeonpea. The field study was undertaken during 1985, 1986 and 1987 growing seasons on Vertisol and Alfisol at ICRISAT Center (peninsular India), Inceptisol at Gwalior (central India) and Entisol at Hisar (northern India) in as non-limiting environmental conditions as possible. Nitrogen fertilizer was applied to the soil at various growth stages to determine when N becomes most limiting. There was a significant response in grain yield to fertilizer N applied at flowering in Vertisol but not in Alfisol, Inceptisol or Entisol. This suggests that biological N₂ fixation by short-duration pigeonpea was not adequate to meet N requirements of the crop grown in Vertisol but that it was probably adequate in the other three soil types. These results are discussed in relation to the nodulation and acetylene reductase activity of pigeonpea and also N mineralization potential of different soils. It can be concluded that there is a need for genetic improvement of N₂ fixing ability of short-duration pigeonpea grown on heavy textured soils such as Vertisols.     

Relationship between Biomass Production and Nitrogen Fixation under Drought-Stress Conditions in Peanut Genotypes with Different Levels of Drought Resistance

The relationship between biomass production and N₂ fixation under drought‐stress conditions in peanut genotypes with different levels of drought resistance is not well understood. The objective of this study was to determine the effect of drought on biomass production and N₂ fixation by evaluating the relative values of these two traits under well watered and water‐stress conditions. Twelve peanut genotypes were tested under field conditions in the dry seasons of 2003/2004 and 2004/2005 in north‐east Thailand. A split‐plot design with four replications was used. Main‐plot treatments were three water regimes [field capacity (FC), 2/3 available soil water (AW) and 1/3 AW], and sub‐plot treatments were 12 peanut lines. Data were recorded on biomass production and N₂ fixation under well watered and water‐stress conditions. Genotypic variations in biomass production and N₂ fixation were found at all water regimes. Biomass production and N₂ fixation decreased with increasing levels of drought stress. Genotypes did not significantly differ in reductions for biomass production, but did differ for reductions in N₂ fixation. High biomass production under both mild and severe drought‐stress conditions was due largely to high potential biomass production under well‐watered conditions and, to a lesser extent, the ability to maintain high biomass production under drought‐stress conditions. High N₂ fixation under drought stress also was due largely to high N₂ fixation under well‐watered conditions with significant but lower contributions from the ability to maintain high nitrogen fixation under drought stress. N₂ fixation at FC was not correlated with the reduction in N₂ fixation at 2/3 AW and 1/3 AW. Positive relationships between N₂ fixed and biomass production of the tested peanut genotypes were found at both levels of drought stress, and the relationship was stronger the more severe the drought stress. These results suggested that the ability to maintain high N₂ fixation under drought stress could aid peanut genotypes in maintaining high yield under water‐limited conditions.     

Dinitrogen Fixation by Winter Chickpea Across Scales in Waterlogged Soil in a Mediterranean Climate

Studies on N₂ fixation by grain legumes during periods of winter waterlogging prone Mediterranean regions have rarely been performed across scales. Here, we quantified the spatial variability of N₂ fixation by rain‐fed chickpea (Cicer arietinum L.) at the field‐ and micro‐scales (0.15 m spacing) after waterlogging during the vegetative growth phase in the winter. We also determined effects of tillage (standard and minimum) and crop and soil variables on N₂ fixation in water stressed conditions. After waterlogging, yield was greatly reduced but there were no visible signs of water stress or tillage effects on N₂ fixation. At the field scale, percent N derived from N₂ fixation (%Ndfa) ranged from 51 to 93 % and was related to the amount of soil‐derived N in the plant. Total grain N did not increase when N₂ fixation increased and the amount of N derived from the soil was replaced with fixed N. In contrast, %Ndfa at the micro‐scale, ranging between 0 to 72 %, was primarily related to yield and total plant N whereas available soil N or any of the other measured soil properties were not significant predictors of %Ndfa. Total N in the grain increased solely due to N₂ fixation as the contribution from soil N remained constant. Although %Ndfa had a nearly pure nugget variance across the scales, total N derived from N₂ fixation (gNdfa) showed a relatively high level of spatial correlation. The range of available soil N pools was likely different at the two scales, leading to differences in the responses of chickpea N₂ fixation to available soil N.     

Selection for Increased Seed Nitrogen Accumulation in Common Bean: Implications for Improving Dinitrogen Fixation and Seed Yield

Estimates of N₂ fixation in segregating populations of bean plants based on ¹⁵N-isotope methods are technically demanding and expensive; therefore, indirect measures based on related traits including total seed N were used to select for improved N₂ fixation and yield. In 1985, six populations of F₂-derived F₃ families resulting from six parental lines crossed to a common tester were grown in field trials on a low-N soil. In 1986, 25 selected half-sib families and two populations of full-sib F₄ families were grown under similar conditions. Parents and a non-nodulating soybean line were included both years. Narrow sense (H_NS) heritability estimates based on parent-offspring regressions ranged from 0.57 for total seed N to 0.39 for shoot biomass in one population, but were near zero for all measured traits in a second population. Among the criteria used to identify parental lines with superior potential for producing progenies with high total seed N, testcross population means combined with estimates of realized heritability were the most reliable. Selection of the best F₃ families for total seed N resulted in F₄ families with increased total seed N and higher seed yields, while seed protein percentage was unchanged. When plants are grown on low-N soils, selection for total seed N offers a useful alternative to selection for increased N₂ fixation based on ¹⁵N-isotopic or total-N-difference method.     

Nitrogen efficiency components of winter cereals

A model to describe the importance of different physiological processes to explain grain yield differences (ΔG_w) between cropping systems (Huggins and Pan, 1993) was modified to evaluate the nitrogen use efficiency of different cereals. The method uses measurements of grain yield (G_w), grain N (N_g), above-ground plant biomass (B), above-ground plant N (N_t), applied fertilizer N (N_f), and post-harvest inorganic soil N in control plots without fertilizer (N_h). The components are N supply (N_s), N uptake efficiency (N_t/N_s), assimilation efficiency (B/N_t), harvest index (G_w/B) and N harvest index (N_g/N_t). For a first verification of the model different winter cereal species, i.e., one genotype of winter wheat, one of winter rye and one of spelt wheat, were compared in a 2-year field experiments at two sites with different soil fertility and climate. The modified nitrogen efficiency component analysis provided a good understanding of yield differences at different levels of applied N and soil fertility. The method could be useful for selection of genotypes with a high N use efficiency in breeding programmes.     

Interaction Amongst Soybean [Glycine max (L.) Merrill] Genotype, Soil Type and Inoculant Strain with Regard to N2 Fixation

The nitrogen (N₂)‐fixing bacterial inoculant strain for soybean [Glycine max (L.) Merrill] is not indigenous to South African soils. The interaction between soybean genotype, soil type and inoculant strain, however, has a definite influence on soybean production and compatibility should be optimized. This paper reports a growth chamber study using three different soybean genotypes (Barc‐9, Avuturda and Talana), three Bradyrhizobium japonicum inoculant strains (WB108, WB112 and WB1) and three soil types (Avalon, Arcadia and sand) to evaluate the effectiveness of N₂ fixation by different genotype × soil type × inoculant strain combinations, using different measuring parameters. These parameters included nodule fresh mass (NFM), amount of N₂ fixed (P_fix), as determined by the ureide method, seed protein content (SPC), average seed mass per plant (SMP) and average foliar N content (FNC). The comparison amongst the three‐way interactions, genotype × soil type × inoculant strain, did not differ significantly for the parameters used. Significant two‐way interactions were soil × inoculant for FNC, P_fix and SMP; soil × genotype for FNC and SMP, and inoculant × genotype for FNC (P < 0.05). The soil × inoculant strain interaction was significant for P_fix (P < 0.05). NFM, P_fix, FNC, SMP and SPC correlated positively with soil pH and negatively with soil clay content and soil NO₃^– and NH₄⁺ content (P < 0.05). SPC was significantly different (P < 0.05) for soil type, genotype and inoculant strain. P_fix and NFM did not reflect the protein content of the seeds, indicating that nodule evaluation should be used with caution as a N₂ fixation parameter. Low soil pH and high mineral N content inhibited N₂ fixation. NFM correlated negatively with the clay content of the soil. This finding confirms that soybean production in South Africa can be improved by appropriate selection of genotypes and inoculant strains for their compatibility in different soils.     

Organic fertilizer effects on growth,crop yield,and soil microbial biomass indices in sole and intercropped peas and oats under organic farming conditions

In a field experiment, peas (Pisum sativum L.) and oats (Avena sativa L.) were grown as sole crops and intercrops, fertilized with horse manure and yard-waste compost derived from shrub and garden cuttings at 10 t C ha⁻¹ each. The objectives were to compare the effects of these organic fertilizer and cropping system in organic farming on (a) yield of peas and oats, grown as the sole crop or intercropped, as well as N₂ fixation and photosynthetic rates, (b) the yield of wheat as a succeeding crop, (c) microbial biomass indices in soil and roots, and (d) microbial activity estimated by the CO₂ evolution rate in the field and the amount of organic fertilizers, recovered as particulate organic matter (POM). In general, organic fertilizer application improved nodule dry weight (DW), photosynthetic rates, N₂ fixation, and N accumulation of peas as well as N concentration in oat grain. Averaged across fertilizer treatments, pea/oat intercropping significantly decreased nodule DW, N₂ fixation and photosynthetic rate of peas by 14, 17, and 12%, respectively, and significantly increased the photosynthetic rate of oats by 20%. However, the land equivalent ratio (LER) of intercropped peas and oats exceeded 1.0, indicating a yield advantage over sole cropping. Soil microbial biomass was positively correlated with pea dry matter yields both in sole and intercropped systems. Organic fertilizers increased the contents of microbial biomass C, N, P, and fungal ergosterol in soil and CO₂ production, whereas the cropping system had no effects on these microbial indices. According to the organic fertilizer recovered as POM, 70% (manure) and 64% (compost) of added C were decomposed, but only 39% (manure) and 13% (compost) could be attributed to CO₂–C during a 101-day period. This indicated that horse manure was more readily available to soil microorganisms than compost, leading to increased grain yields of the succeeding winter wheat.     

Beet vinasse applied to wheat under dryland conditions affects soil properties and yield

The effect of six doses of beet vinasse (0, 3, 6, 10, 20 and 40 t ha⁻¹, respectively) on wheat (Triticum aestivum cv. Cajeme) yield in dryland conditions (Guadalquivir Valley, Andalusia, Spain) for 3 years on a Typic Xerofluvent was studied. The results showed that at low doses, beet vinasse is of agricultural interest due mainly to its organic matter concentration. The application of this byproduct to the soil increased soil microbial biomass and mineralization of its organic matter increased NO₃⁻–N concentrations in soil. This caused an increase in grain yield in the three seasons. When the vinasse was applied with high doses, NO₃⁻–N concentrations in soil, soil microbial biomass, soil structure, bulk density, electric conductivity, nutrient uptake, crop yield and grain quality were negatively affected. We assume that the high amounts of monovalent cations, particularly Na⁺, and of fulvic acids, which had been transported into the soil by the vinasse, destabilized the soil structure. This may have led to anaerobic soil conditions being presumably responsible for restricted N mineralization or even for denitrification. This explains the lower N supply to the crops reflected by the low N concentrations in the leaves of treatments A4 and A5.     

Effect of Deep Placement of N Fertilizers and Different Inoculation Methods of Bradyrhizobia on Growth,N2 Fixation Activity and N Absorption Rate of Field‐grown Soybean Plants

The effects of deep placement (supplied at 20 cm depth from soil surface below plants) of 100 kg N ha^?1 of N fertilizers, urea, coated urea or calcium cyanamide (lime nitrogen) on the growth, nitrogen fixation activity, nitrogen absorption rate and seed yield of soybean (Glycine max L. Merr.) plants were examined by comparing them with control plots without deep placement of N fertilizer in sandy dune field. In addition, three different inoculation methods of bradyrhizobia were used for each N treatment: (1) transplantation of 10‐day‐old seedling in a paper pot with vermiculite inoculated with Bradyrhizobium japonicum USDA110, (2) direct transplantation of inoculated 10‐day‐old seedlings, and (3) transplantation of 10‐day‐old seedlings in a non‐inoculated paper pot. The deep placement of N fertilizers, especially calcium cyanamide and coated urea, markedly increased the growth and total N accumulation in shoot, roots and nodules, which resulted in an increase in seed yield. Daily N₂ fixation activity and N absorption rate were estimated by relative abundance of ureide‐N analysed from the concentration of N constituents (ureide‐N, amide‐N and nitrate‐N) in root bleeding xylem sap and increase in total N accumulation in whole plants at R1, R3, R5 and R7 stages. The total amount of N₂ fixation was about 50 % higher in the plants with calcium cyanamide and coated urea deep placements compared with control plants. Deep placement of slow release fertilizers kept nodule dry weight higher in the maturing stage of seed, possibly through abundant supply of photoassimilate to the nodules by supporting leaf area and activity until late reproductive stages. The results indicate that deep placement of calcium cyanamide or coated urea enhances N₂ fixation activity, which ultimately increases the seed yield. The promotive effect was observed with the seedlings transplanted in paper pot with inoculum of bradyrhizobia within any treatments, although nodulation by indigenous rhizobia was observed in the plants transplanted with non‐inoculated paper pot.     

Physiological and Growth Responses of Sugarcane Genotypes to Nitrogen Rate on a Sand Soil

Yields of sugarcane (a complex hybrid of Saccharum spp.) in FL, USA, are lower on sand soils than on organic (muck) soils. Nitrogen (N) supply may limit sugarcane growth and yields on these sand soils. A 2‐year pot study was conducted to determine sugarcane genotypic variation in response to N rate on a sand soil. Treatments included four N rates (0, 75, 150 and 225 kg ha^?1) and three sugarcane genotypes (CP 80‐1743, CP 01‐2390 and TCP 87‐3388). Nitrogen fertilizer was equally split and applied at about 55 and 125 days after planting (DAP) for each treatment. During the experiment, the number of nodes and length of the primary stalks and tillers were recorded. Leaf relative chlorophyll (soil plant analysis development (SPAD)) and net photosynthetic rate (Pn) were measured biweekly. All plants were harvested at 183 DAP to measure green leaf area (GLA), shoot biomass accumulation and partitioning, and fertilizer N use efficiency (NUE). Genotypes differed significantly in leaf SPAD, Pn, GLA, and shoot biomass accumulation and partitioning. CP 01‐2390 had the highest leaf Pn and shoot biomass, and CP 80‐1743 had the lowest GLA, shoot biomass and NUE among genotypes. Nitrogen rate affected leaf SPAD, GLA, shoot biomass and NUE, but had much less effect on leaf Pn. Green leaf area and biomass increased with increasing N rates. Our results suggest that a two‐pronged approach, selection of genotypes with high NUE while working to optimize N rates and delivery can improve sugarcane yields on sand soils.     

氮沉降对北方森林土壤微生物的影响研究进展

大气氮沉降导致土壤有效氮含量增加,将改变作为陆地生态系统重要组成部分的土壤微生物群落结构,尤其在高纬度氮限制地区,土壤微生物对这种变化更为敏感。北方森林地处高纬度地区,氮沉降将改变其土壤微生物的结构、功能和动态。为全面了解近年来氮沉降对北方森林土壤微生物的影响,笔者综述了氮沉降对北方森林土壤微生物量、群落结构和生物多样性、功能和酶活性等方面的影响。结果表明:(1)氮沉降减少了土壤微生物量;(2)氮沉降改变土壤中真菌与细菌,革兰氏阴性细菌(G-)与革兰氏阳性细菌(G+)之间的比值,而这种改变大多数是趋向于减小;(3)氮沉降加剧,将导致土壤微生物群落中贫营养微生物处于劣势地位,富营养微生物处于优势地位,间接地影响了微生物群落结构和生物多样性;(4)氮沉降抑制了微生物呼吸,但对于土壤酶的影响尚无统一规律;(5)氮沉降改变了微生物底物利用模式,导致土壤微生物对复杂有机物质的分解能力下降;(6)氮沉降导致固氮基因等功能基因相对丰度下降。     

Effect of Deep Placement of Slow-release Fertilizer (Lime Nitrogen) Applied at Different Rates on Growth, N2 Fixation and Yield of Soya Bean (Glycine max [L.] Merr.)

A new fertilization method with deep placement of slow‐release N fertilizers, such as coated urea and lime nitrogen (LN) (calcium cyanamide) at 20 cm depth was found to promote soy bean seed yield. In the present study, the effect of deep placement of LN was investigated on different parameters such as growth, N accumulation, N₂ fixation activity and yield of soy bean by applying LN at different rates in the rotated paddy field of Niigata, Japan. In addition to the basal fertilizer, ammonium sulphate (16 kg N ha^?1), deep placement of LN was conducted by applying various amounts such as 50 kg N ha^?1 (A50), 100 kg N ha^?1 (A100) and 200 kg N ha^?1 (A200) at 20 cm depth in separate plots. A ¹⁵N‐labelled LN fertilizer was also employed for each of the above treatments to calculate N utilization from LN in separate plots. Soya bean plant growth and N₂ fixation activity were periodically analysed. Both plant growth and N accumulation were found to increase with LN treatment compared with control plants. An increase in N₂ fixation activity was found in the A100 plots. The total seed yield was the highest in the deep placement of LN with A100 (73 g per plant) compared with other treatments. The visual quality of harvested seeds also showed that A100 enhanced the quality of seeds compared with other treatments. Thus, it is suggested that N fertilization management with particular reference to optimum amount of fertilizers is important for maximum growth, N₂ fixation and enhancement of seed yield of soy bean.     

Soil nitrogen availability indices as predictors of sugarcane nitrogen requirements

Nitrogen recommendation systems for sugarcane (Saccharum spp.) generally does not consider the N supply from soil. Identifying a reliable soil test for estimating N availability is crucial to avoid yield losses or environmental pollution. Therefore, the objective of this study was to correlate and calibrate N availability indices with field–based measures of soil N supply. Between 2006 and 2013, 15 trials for rate–response to N fertilizer by sugarcane ratoons were performed in São Paulo, the main sugarcane–producing state in Brazil. The following indices were tested: KMnO₄ oxidizable C, hot KCl extractable N, phosphate–borate buffer distillable N, NaOH distillable N, Illinois Soil N Test, organic C, total N, mineral N, anaerobic incubation, soil respiration, substrate–induced respiration, microbial biomass C, metabolic quotient, microbial quotient, and gross N mineralization. The indices were then correlated with sugarcane yield (Y_0N) and N content of the crop (N_0N) in N–unfertilized plots, relative yield (RY), and the N rate predicted to achieve 90% of the RY (NR 90%RY). Although weak correlations were found between Y_0N with anaerobic incubation, total N, and soil respiration, as well as between N_0N and anaerobic incubation, no index correlated with RY or NR 90%RY. Grouping sites based on soil texture or byproduct management did not improve prediction of RY. Therefore, we concluded that none of the fifteen laboratory indices is a reliable predictor of soil N supply, and hence could not be used to adjust N fertilization rate for sugarcane.     

Genotype x Bradyrhizobium japonicum strain interactions in dinitrogen fixation and agronomic traits of soybean (Glycine max L. Merr.)

Summary Bradyrhizobium japonicum strain G49 has been the only inoculum used in French soils. Soybean (Glycine max L. Merr.) cultivars were selected and tested according to their performances with this rhizobial strain. The aim of the present study was to determine the consequences of strain substitution on N₂ fixation abilities of various genotypes. Three genotypes and cultivar Weber, in combination with B. japonicum strain G49 or SMGS1, were cultivated in pots and tested for nitrogenase activity under differing nitrogen nutrition conditions. The reliability of ARA (acetylene reduction activity) measurement for assessing symbiotic nitrogen fixation under the experimental conditions used was checked. Genotypic variability for symbiotic fixation activity was observed with each strain under soil culture conditions; important genotype x strain interactions were also involved. These results were corroborated for the protein yield and other yield component performances of the various genotype-strain associations. Thus, in France, the replacement of strain G49 with another one might result in the alteration of the relative agronomic performances of the soybean cultivars, since N₂ fixation is considered as a major factor of soybean productivity.     

Long‐Term Chemical Fertilization Along with Farmyard Manure Enhances Resistance and Resilience of Soil Microbial Activity against Heat Stress

The effect of fertilization on resistance and resilience of soil microbial activity against heat stress in the tropical soils is largely unknown. We investigated the impact of long‐term (36 years) application of chemical fertilizers and farmyard manure (FYM) on substrate‐induced respiration (SIR) and dehydrogenase activity (DHA) and their resistance and resilience against heat stress in a sandy clay loam soil (Typic Haplustept). Surface soils from five selected treatments (Control, N, NP, NPK, NPK + FYM) under maize (Zea mays) crop were assessed immediately after sampling (0 Day) and at 1, 14, 28 and 56 day(s) after heat stress (48 °C for 24 h). The heat stress significantly decreased soil respiration and dehydrogenase activity by 20–80 %. Recovery after stress was up to 100 % within 56 days. The combined application of NPK (balanced) and FYM was most effective in enhancing resistance and resilience (stability) of soil microbial activity against heat stress. Correlation between resistance of dehydrogenase activity and substrate‐induced respiration revealed a significant relationship (R² = 0.85). However, after stress, this correlation was initially weak but subsequently improved with time (R² = 0.38–57), indicating different time lags to restore the normalcy of these parameters.     

长期稻草还田对烟田土壤氮素矿化特征的影响

为探明稻草还田对烟田土壤氮素释放的影响，进而有效调控并为配套的施肥技术提供理论依据，采用室内培养和田间原位培养试验，研究了长期稻草还田对烟田土壤有机氮矿化特征和进程的影响。结果表明：稻草还田提高了土壤氮矿化潜力（N0值）和矿化速率（k0），连续还田5年土壤的N0值为92.11 kg/hm2，比对照增加了23.78%；稻草还田促进了烟田土壤氮的矿化，烟株生长期内还田5年的土壤矿化氮量为94.94 kg/hm2，比对照高17.67%。在烟株生长中后期，稻草还田的烟田表层土壤无机氮数量明显增加。稻草还田增加了土壤微生物量氮，移栽时，还田5年土壤微生物量氮比对照增加20.18%，采烤后则高出40.71%。稻草还田1年后土壤微生物量氮处于调整平衡阶段。长期实行稻草还田提高了烟田土壤氮供应能力，增加了上部烟叶全氮和烟碱含量过高的风险。     

White lupine as a beneficial crop in Southern Europe: I. Potential for N mineralization in lupine amended soil and yield and N2 fixation by white lupine

Two experiments were carried out at Pegões (central Portugal) to determine the potential N mineralization in a pulse amended disturbed and undisturbed soil incubated at several temperatures, and to evaluate for 2 years the yield and N₂ fixation capacity of sweet lupine (Lupinus albus L. cv. Estoril) inoculated with a mixture of rhizobia strains or nodulated by indigenous soil bacteria and submitted to conventional tillage or no-till practices. A completely randomized block design for soil mobilization with three replicates was used for the laboratory study, and completely randomized blocks for inoculation and tillage treatments with four replicates were used for the lupine yield and N₂ fixation experiment. Residue N immobilization occurred immediately after mature residue return to the soil independent of temperature, and was greater at 7 °C especially in the disturbed topsoil. Greater immobilization was also observed by doubling the amount of mature residue incorporated in the soil. This was expected since soil microorganisms would be in direct contact with the fresh organic matter and would be provided with more organic carbon under these circumstances. Nitrogen mineralization proceeded after 5 days of amendment. Potential N mineralization was higher at 25 °C than at 18 or 7 °C, for both conventional and no-till practices. At 25 °C, 42% of buried residue-¹⁵N was released over 210 days, at a smaller rate than 18 °C (49%) over 81 days. Lupine yield and N₂ fixation capacity were similar in plots that were not inoculated and those receiving the mixture of three rhizobia strains. White lupine had an efficient symbiosis with indigenous soil rhizobia at pod-filling (>99%, >100 kg N ha⁻¹ year⁻¹) which was not affected by tillage. At this stage, plant residue including visible roots and nodules accounted for a soil N input of +96 kg ha⁻¹ year⁻¹ (91% of crop N), showing the large soil N benefit by the crop at this stage. The lupine residue at pod-filling stage can be used as a green manure under the conditions of organic farming systems. The apparent N “harvest” index of the pulse at pod-filling was only 9% though at maturity phase it should result in a higher value and the legume will show a lower fertilizer N value.     

Screening techniques and improved biological nitrogen fixation in cool season food legumes

Dinitrogen fixation and legume productivity are greatly influenced through the interactions of legume host, Rhizobium, and the above- and below-ground environment. The benefits of improving legume N₂ fixation include reduced reliance on soil N, leading to more sustainable agricultural systems and reduced requirements for fertilizer N, enhanced residual benefits to subsequent crops, and increased legume crop yields. Most of the gains in N₂ fixation to date have been derived from management of legume cropping systems and through inoculation of legume seed with competitive and symbiotically effective rhizobia. Further gains are possible by developing plant cultivars with tolerance to soil abiotic factors, increased plant yield, and a broader and more effective matching of plant host and rhizobia. Techniques for screening material for superior N₂ fixation and examples of programs to increase fixed N, with attention to the major abiotic stresses, are discussed.     

Chickpea and faba bean nitrogen fixation in a Mediterranean rainfed Vertisol: Effect of the tillage system

Efficient management of legumes in order to maximize benefits depends on a correct field assessment of N₂ fixation. A field experiment was conducted during a 6-year period (2001–2002 to 2006–2007) in Córdoba (Southern Spain) on a rainfed Vertisol within the wheat-chickpea and wheat-faba bean rotation framework of a long-term experiment started in 1986. The aim was to determine the effect of tillage systems [no tillage (NT) and conventional tillage (CT)] on chickpea and faba bean N₂ fixation. Fixation was calculated using the ¹⁵N isotopic dilution (ID) and ¹⁵N natural abundance (NA) methods with the reference being the wheat crop. The strong inter-annual rain variation caused great differences in the behaviour of both leguminous plants with regard to grain yield, nodule biomass and N₂ fixation. The NT system showed more nodule biomass than the CT system in both legumes. The ID method was more accurate than the NA method in determining N₂ fixation. The average amount of fixed N in faba bean (80 kg ha^?1 year^?1) was much greater than that in chickpea (31 kg ha^?1 year^?1). The Vertisol under the NT system offered more favourable conditions for the stimulation of the N₂ fixation, with fixed N values that were significantly higher than under CT. The N added to the system through N₂ fixation was low in faba bean and virtually nonexistent in chickpea, only in terms of above-ground biomass.