Bruno José Rodrigues Alves

The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems

Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30–40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineral N (intercropping legumes with cereals) can also be critical. Much of the N2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legumegrain may be relatively small.Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. Whilesome of these rotational effects may be associated with improvements in availability of N in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogenemitted from nodules as a by-product of N2, fixation.

The success of BNF in soybean in Brazil

Approximately forty years after commercial cropping of soybean in Brazil began, the total area under this crop has reached over 13 M ha with a mean productivity of 2400 kg ha−1. Soybean varieties introduced from the USA and varieties rescued from early introductions in Brazilian territory were part of the Brazilian soybean-breeding programme which spread the crop from high to low latitudes. Disease-resistance, pest-resistance, tolerance to low fertility soils, as well as production of plants with pods sufficiently high above the ground for efficient mechanical harvesting, were all aims of the programme. Although BNF was not explicitly considered as a trait for selection in the breeding/selection programme, maximisation of biological nitrogen fixation (BNF) was favoured by conducting selection and breeding trials on soils low in N, in which the seeds were inoculated with efficient Bradyrhizobium inoculants but without N fertiliser application. Several efficient imported Bradyrhizobium strains were found to be unable to compete with native soil micro-flora and other previously-introduced Bradyrhizobium strains. Surprisingly, after being in the soil for many years one or two of these strains had become more competitive while maintaining their high BNF capacity. Today, these strains are included amongst the recommended Brazilian inoculants and have promoted significant improvements in grain yields. The breeding of soybeans in conditions that made grain yield highly dependent on BNF, and the continuous attention paid to the selection of Bradyrhizobium strains appropriate for the newly released varieties, have been the main contributors to todays high yields and their great benefit to the Brazilian economy. There seems to be no reason why this ongoing research programme should not serve as an appropriate model to improve BNF inputs to grain legumes in other countries of the world.

The contribution of biological nitrogen fixation for sustainable agricultural systems in the tropics

Abstract The pressing need to increase food production in the tropics to feed the burgeoning population of the Third World requires that crop yields ha−1 must be increased without prejudicing the resource base for future generations. Biological nitrogen fixation (BNF), especially that associated with legumes, has great potential to contribute to productive and sustainable agricultural systems for the tropics, but more research is required to investigate how biologically fixed N, and the increased BNF contributions resulting from research innovations, can be incorporated into viable agricultural systems to increase crop or pasture yields and to substitute N fertiliser inputs. A majority of the soils of the humid and semi-humid tropics have mineral fractions composed of 1:1 lattice clays or sesquioxides of relatively low capacity to retain nutrients (CEC) and water (WHC). It is the soil organic matter (SOM) which has high CEC (after liming) and WHC, and soils under undisturbed climax vegetation are usually high in organic matter which is responsible for their fertility. The key to the long term fertility of such soils is to maintain their soil organic matter by the preservation of crop residues and the selection of suitable crop rotations or fallows. In this review we examine several types of agricultural systems utilised in the tropics ranging from pastures, ley cropping, zero-till rotations as well as green manuring and discuss the management options that can be adopted to preserve their agricultural productivity through the strategic use of legumes in these systems, and their effects on pasture and crop yields. The introduction of forage legumes into tropical pastures can increase and sustain their productivity, with only modest inputs of lime and P and K fertilisers. Similarly, crop and pasture rotations (ley cropping) maintain SOM and soil fertility and crop yields can benefit greatly from the introduction of pasture legumes into the ley. Continuous cropping under minimum or zero tillage can maintain soil cover, and stimulate the retention of SOM, such that nutrient losses are often minimal, and legume derived N can be efficiently transferred to subsequent crops. The options for the resource-poor small-holder to efficiently utilise biologically fixed N as a N supply for cereal grains are more limited and need more attention from researchers as well as less neglect from government organisations. The addition of lime and P fertiliser in modest quantities in many under-developed regions could make large contributions to increased crop yields. If such modest fertiliser inputs were to be combined with suitable crop rotations including green manure or grain legume crops, larger increases in crops yields could be achieved on a sustainable basis, but in many regions agricultural extension services are non-existent and poor farmers have little access to even these basic chemical inputs.

Management swing potential for bioenergy crops

Bioenergy crops are often classified (and subsequently regulated) according to species that have been evaluated as environmentally beneficial or detrimental, but in practice, management decisions rather than species per se can determine the overall environmental impact of a bioenergy production system. Here, we review the greenhouse gas balance and ‘management swing potential’ of seven different bioenergy cropping systems in temperate and tropical regions. Prior land use, harvesting techniques, harvest timing, and fertilization are among the key management considerations that can swing the greenhouse gas balance of bioenergy from positive to negative or the reverse. Although the management swing potential is substantial for many cropping systems, there are some species (e.g., soybean) that have such low bioenergy yield potentials that the environmental impact is unlikely to be reversed by management. High‐yielding bioenergy crops (e.g., corn, sugarcane, Miscanthus, and fast‐growing tree species), however, can be managed for environmental benefits or losses, suggesting that the bioenergy sector would be better informed by incorporating management‐based evaluations into classifications of bioenergy feedstocks.

Quantification of the contribution of biological nitrogen fixation to tropical green manure crops and the residual benefit to a subsequent maize crop using 15N-isotope techniques.

In this study the contribution of biological N2 fixation (BNF) to leguminous green manures was quantified in the field at different sites with different 15N methodologies. In the first experiment, conducted on a Terra Roxa soil in Cuba, the BNF contribution to three legumes (Crotalaria juncea, Mucuna aterrima and Canavalia ensiformis) was quantified by applying 15N-labelled ammonium sulphate to the soil. The second experiment was planted in a very low fertility sandy soil near Rio de Janeiro, and the 15N natural abundance technique was applied to quantify BNF in C. juncea, M. niveum and soybean. In both studies the advantages of using several non-N2-fixing reference plants was apparent and despite the much greater accumulation of the C. juncea in the experiment performed on the fertile soil of Cuba, the above ground contributions of BNF at both sites were similar (40-80 kg N x ha(-1)) and greater than for the other legumes. In a further experiment the possible contribution of root-derived N to the soil/plant system of two of the legumes was quantified using a 15N-leaf-labelling technique performed in pots. The results of this study suggested that total below-ground N could constitute as much as 39 to 49% of the total N accumulated by the legume crops.

The effect of the presence of a forage legume on nitrogen and carbon levels in soils under Brachiaria pastures in the Atlantic forest region of the South of Bahia, Brazil

The impact of forest clearance, and its replacement by Brachiaria pastures, on soil carbon reserves has been studied at many sites in the Brazilian Amazonia, but to date there appear to be no reports of similar studies undertaken in the Atlantic forest region of Brazil. In this study performed in the extreme south of Bahia, the changes in C and N content of the soil were evaluated from the time of establishment of grass-only B. humidicola and mixed B. humidicola/Desmodium ovalifolium pastures through 9 years of grazing in comparison with the C and N contents of the adjacent secondary forest. The decline in the content of soil C derived from the forest (C3) vegetation and the accumulation of that derived from the Brachiaria (C4) were followed by determining the 13C natural abundance of the soil organic matter (SOM). The pastures were established in 1987, 10 years after deforestation, and it was estimated that until 1994 there was a loss in forest-derived C in the top 30 cm of soil of approximately 20% (9.1 Mg C ha−1). After the establishment of the pastures, C derived from Brachiaria accumulated steadily such that at the final sampling (1997) it was estimated 13.9 Mg ha−1 was derived from this source under the grass-only pasture (0–30 cm). Samples taken from all pastures and the forest in 1997 to a depth of 100 cm showed that below 40 cm depth there was no significant contribution of the Brachiaria-derived C and that total C reserves under the grass/legume and the grass-only pastures were slightly higher than under the forest (not significant at P=0.05). The more detailed sampling under the pastures showed that to a depth of 30 cm there was significantly (P<0.05) more C under the mixed pasture than the grass-only pasture. It was estimated that from the time of establishment the apparent rate of C accumulation (0–100 cm depth) under the grass/legume pastures (1.17 Mg ha−1 yr−1) was almost double that under the grass-only pastures (0.66 Mg ha−1 yr−1). The data indicated that newly incorporated SOM derived from the Brachiaria had a considerably higher C:N ratio than that present under the forest.

Field application of the15N isotope dilution technique for the reliable quantification of plant-associated biological nitrogen fixation

To apply the isotope dilution (ID) technique, it is necessary to grow the “N2-fixing” crop in a soil where the mineral N is labelled with15N. Normally the “N2-fixing” crop and a suitable non-N2-fixing control crop are grown in the same labelled soil and the15N enrichment of the control crop is assumed to be equal to the15N enrichment of the nitrogen (N) derived from the soil in the “N2-fixing” crop. In this case the proportion of unlabelled N being derived from the air via biological N2 fixation (BNF) in the “N2-fixing” crop will be proportional to the dilution of the enrichment of the N derived from the labelled soil.To label the soil, the technique most often used is to add a single addition of15N-labelled N fertilizer shortly before, at, or shortly after, the planting of the crops. Data in the literature clearly show that this technique results in a rapid fall in the15N enrichment of soil mineral N with time. Under these conditions, if the control and the “N2-fixing” crops have different patterns of N uptake from the soil they will inevitably obtain different15N enrichments in the soil-derived N. In this case the isotope dilution technique cannot be applied, or if it is, there will be an error introduced into, the estimate of the contribution of N derived from BNF.Several experiments are described which explore different strategies of application of the ID technique to attempt to attenuate the errors involved. The results suggest that it is wise to use slow-release forms of labelled N, or in some cases, multiple additions, to diminish temporal changes in the15N enrichment of soil mineral N. The use of several control crops produces a range of different estimates of the BNF contributions to the “N2-fixing” crops, and the extent of this range gives a measure of the accuracy of the estimates. Likewise the use of more than one15N enrichment technique in the same experiment will also give a range of estimates which can be treated similarly. The potential of other techniques, such as sequential harvesting of both control and test crops, are also discussed.

Influence of decomposition of roots of tropical forage species on the availability of soil nitrogen

Abstract Immobilization of mineral N induced by decomposition of roots of four tropical forage species (Stylosanthes guianensis, Centrosema sp., Andropogon gayanus and Brachiaria decumbens) in an Oxisol was studied under laboratory conditions. Root materials had a high lignin content (12–20%) but total polyphenol content was small ( 0.99) a double exponential equation defining two compartments of root carbon of differing susceptibility to decomposition. The equation predicted that between 43% (Centrosema) and 62% (Brachiaria) of root carbon would not be decomposed even at infinite time under incubation conditions. Mineral N in the soil was immobilized rapidly at the start of the incubation, and the immobilization was greatest with the higher rate of application of root material. Although the C-to-N ratio of legume roots was narrower their higher degradability stimulated greater immobilization of soil mineral N than the grass roots. The results are discussed with reference to N immobilization and carbon sequestration in planted pastures of tropical South America.

Influência da cultura antecessora e da adubação nitrogenada na produtividade de milho em sistema plantio direto e solo preparado

In the Cerrado region a proportion of the top dressed N for corn might be applied to the previous cover crop. This could accelerate the decomposition rate and increase overall N availability to corn. Therefore, the objectives of this experiment were: a) to evaluate maize productivity after oil radish and millet grown in winter with and without nitrogen applied and b) to determine the efficiency of recovery of N fertilizer by corn and quantify the losses by volatilization of ammonia of N fertilizers in no-tillage (NT) and conventional tillage (CT). The experimental design was a randomized complete block in strips with four replications. At flowering, the dry matter production and N accumulation of the oil radish were, respectively, 2,274 and 53.0kg ha-1 under NT, and 2,546 and 61.6kg ha-1 under CT. For millet the results obtained were 5,202 and 107.8kg ha-1 under NT, and 5,101 and 104.1kg ha-1 under CT. Until the seeding of the maize under NT, after desiccation by knife rolling the winter crops, 77.3 and 130.7kg N ha-1 were released in the sequences of oil radish - maize and of millet - maize, respectively. Under CT the winter crops were incorporated into the soil before the seeding of the maize. In the growth cycle of corn the losses of N via ammonia volatilization were less than 2.0% of the N applied at pre-seeding (71.3kg N ha-1 in a mixture urea:ammonium sulphate of 4:1) and 14% of the N at the 6-leaf stage (35.7kg N ha-1 of ammonium sulphate), evaluated in NT and CT in the sequence oil radish - corn. Under NT the fertilizer-N-use-efficiency was 57.1 and 42.1% of the N applied in the sequence millet - corn and oil radish-maize, respectively. Under CT these values were 46.8 and 46.3%, respectively. The application of N fertilizer caused a mean yield increase of 2,396kg grain ha-1 in the sequence millet-corn under NT compared to the non-fertilized control. These increase under CT was 895kg grain ha-1 in the sequence oil radish-corn under NT, and 1,166 and 166kg grain ha-1 in the sequences millet-corn and oil radish-corn, respectively.

Elephant grass genotypes for bioenergy production by direct biomass combustion

O objetivo deste trabalho foi avaliar genotipos de capim-elefante (Pennisetum purpureum Schum.) quanto ao potencial para a producao de bioenergia por combustao direta da biomassa. Avaliaram-se cinco genotipos de capim-elefante, em dois solos com baixa fertilidade. Os experimentos foram conduzidos na estacao experimental da Embrapa Agrobiologia, em Seropedica, RJ. O delineamento experimental foi o de blocos ao acaso, em parcelas subdivididas, com quatro repeticoes. Os genotipos estudados foram Cameroon, Bag 02, Gramafante, Roxo e CNPGL F06-3. Determinaram-se a producao de biomassa, o acumulo de nitrogenio na biomassa, o nitrogenio da biomassa proveniente da fixacao biologica, as relacoes carbono/nitrogenio e talo/folha, e os teores de fibra, lignina, celulose e cinzas da biomassa. Aproducao de materia seca variou de 45 a 67 Mg ha-1. A menor producao foi do genotipo Roxo, e as maiores, dos genotipos Bag 02 e Cameroon. O nitrogenio total acumulado na materia seca variou de 240 a 343 kg ha-1. Em media, 51% do nitrogenio foram provenientes da fixacao biologica. As relacoes carbono/nitrogenio e talo/folha e os conteudos de fibra, lignina, celulose e cinzas nao variaram entre os genotipos. Os cinco genotipos constituem materiais adequados para producao de energia pela queima.