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 N 2 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 N 2 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 legume grain may be relatively small. Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. While some of these rotational effects may be associated with improvements in availability ofN 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 hydrogen emitted from nodules as a by-product of'N, fixation.

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Continuous sugarcane (Saccharum spp.) culture in Brazil, with low N inputs and almost total removal of plant biomass at each harvest, has not depleted soil N reserves. This, and high numbers of N2‐fixing bacteria associated with the plants, suggests that the crop may be obtaining considerable N from biological nitrogen fixation (BNF). This 3‐yr study assessed the importance of such contributions to three sugarcane species (S. officinarum L., S. barberi Jesw., and S. spontaneum L.), and seven commercial Brazilian hybrids. The plants were grown in a concrete tank containing 15N‐labeled soil in order to use 15N isotope dilution to estimate the BNF contributions. A grass, Brachiaria arrecta (cv. IRI 442), was included as a non‐N2‐fixing control. All aerial tissue was harvested annually, and the roots and stem bases were removed at the end of the experiment. For all 3 yr, the commercial hybrids and the S. spontaneum cultivar (Krakatau) accumulated more N at significantly lower 15N enrichments than the control. These data suggest that the plants obtained considerable BNF contributions, but interpretation of the 15N data was prejudiced by (i) the steadily declining 15N enrichment of the soil mineral N, (ii) carry‐over of N from one harvest to the next in the roots and stem bases, and (iii) shading of the control crop by the tall cane plants. Several of the sugarcane cultivars had significantly positive N balances, however, and there was good agreement between the estimates of BNF contributions derived from N balance and isotope dilution. Krakatau and the commercial hybrids CB 45‐3 and SP 70‐1143 obtained the largest contributions from BNF, but methodological problems did not allow exact determinations.

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Brazil has the largest herd of beef cattle in the world, estimated at approximately 200 million animals. Production is predominantly pasture-based and low input and hence time to slaughter is long, which promotes high methane (CH 4) emissions per kg of product. The objective of this study was to investigate the impact of increasing animal productivity using fertilizers, forage legumes, supplements and concentrates, on the emissions of greenhouse gases (GHGs) in five scenarios for beef production in Brazil. A life cycle analysis (LCA) approach, from birth of calves to mature animals ready for slaughter at the farm gate, was utilized using Tier 2 methodologies of the IPCC and the results expressed in equivalents of carbon dioxide (CO 2 eq) per kg of carcass produced. Fossil CO 2 emitted in the production of supplements, feeds and fertilizers was included using standard LCA techniques. The first four scenarios were based solely on cattle production on pasture, ranging from degraded Brachiaria pastures, through to a mixed legume/Brachiaria pasture and improved N-fertilized pastures of Guinea grass (Panicum maximum). Scenario 5 was the most intensive and was also based on an N-fertilized Guinea grass pasture, but with a 75-day finishing period in confinement with total mixed ration (TMR). Across the scenarios from 1 to 5 the increase in digestibility promoted a reduction in the forage intake per unit of animal weight gain and a concomitant reduction in CH 4 emissions. For the estimation of nitrous oxide (N 2 O) emissions from animal excreta, emission factors from a study in the Cerrado region were utilized which postulated lower emission from dung than from urine and much lower emissions in the long dry season in this region. The greatest impact of intensification of the beef production systems was a 7-fold reduction of the area necessary for production from 320 to 45 m 2 /kg carcass. Carcass production increased from 43 to 65 Mg per herd across the scenarios from 1 to 5, and total emissions per kg carcass were estimated to be reduced from 58.3 to 29.4 kg CO 2 eq/kg carcass. Even though animal weight gain was lower in the mixed grass-legume scenario (3) than for the N-fertilized Guinea grass pastures (scenarios 4 and 5) GHG emissions per kg carcass were similar as the legume N 2 fixation input had no fossil-fuel cost. A large source of uncertainty for the construction of such LCAs was the lack of data for enteric CH 4 emissions from cattle grazing tropical forages.

Resumo -O objetivo deste trabalho foi avaliar os efeitos dos cultivos isolado e consorciado dos adubos verdes de verão crotalária (Crotalaria juncea) e milheto (Pennisetum americanum) na produção de fitomassa, nos teores e acúmulo de nutrientes e na fixação biológica de nitrogênio (FBN). O delineamento experimental adotado foi blocos ao acaso, com quatro repetições, em que os tratamentos constaram dos adubos verdes crotalária, milheto, crotalária + milheto e vegetação espontânea. A crotalária apresentou maior produção de fitomassa, que foi 108% maior que a da vegetação espontânea e 31% superior a do milheto. No consórcio crotalária + milheto, a leguminosa contribuiu com 65% da massa de matéria seca total. A presença da crotalária resultou em maiores teores de N e Ca, enquanto o milheto e as ervas espontâneas apresentaram maiores teores de potássio. O acúmulo de P e Mg foi fortemente influenciado pela produção de fitomassa, atingindo valores elevados com a presença da crotalária, ao passo que o acúmulo de N e Ca resultou tanto dos maiores teores quanto da maior produção de fitomassa nos tratamentos com a leguminosa. A FBN foi 61% na leguminosa quando consorciada e 57% quando isolada, incorporando ao solo via FBN 89 e 173 kg/ha de N, respectivamente, constituindo-se excelente estraté-gia de incremento de N ao solo.Termos para indexação: Crotalaria juncea, Pennisetum americanum, planta de cobertura, matéria seca. Phytomass yield, nutrients accumulation and biological nitrogen fixation by single and associated green manuresAbstract -The objective of this work was to evaluate the effects of sole and intercropping systems of the summer green manures sunnhemp (Crotalaria juncea) and millet (Pennisetum americanum) in phytomass yield, content and accumulation of nutrients and biological nitrogen fixation (BNF). The experimental design was a randomized complete block with four replicates, and treatments consisted of green manures sunnhemp, millet, sunnhemp + millet and spontaneous vegetation. The sunnhemp stood out in the phytomass yield, being 108% greater than the spontaneous vegetation and 31% superior to millet. In the sunnhemp + millet intercropping, the leguminous contributed with 65% of the total dry matter. The presence of sunnhemp resulted in larger contents of N and Ca, while millet and the spontaneous vegetation showed larger content of potassium. The accumulation of P and Mg was strongly influenced by the phytomass yield, reaching high values in the presence of sunnhemp, while the accumulation of N and Ca was a result of the largest contents as well as the largest phytomass yield in the treatments with the leguminous. The BNF was 61% in the intercropping and 57% in sole cropping, incorporating to soil via BNF 89 and 173 kg/ha of N, respectively, being an excellent strategy for increasing soil nitrogen.