Factors regulating the contributions of fixed nitrogen by pasture and crop legumes to different farming systems of eastern Australia

On-farm and experimental measures of the proportion (%Ndfa) and amounts of N₂ fixed were undertaken for 158 pastures either based on annual legume species (annual medics, clovers or vetch), or lucerne (alfalfa), and 170 winter pulse crops (chickpea, faba bean, field pea, lentil, lupin) over a 1200 km north-south transect of eastern Australia. The average annual amounts of N₂ fixed ranged from 30 to 160 kg shoot N fixed ha^–1 yr^–1 for annual pasture species, 37–128 kg N ha^–1 yr^–1 for lucerne, and 14 to 160 kg N ha^–1 yr^–1 by pulses. These data have provided new insights into differences in factors controlling N₂ fixation in the main agricultural systems. Mean levels of %Ndfa were uniformly high (65–94%) for legumes growing at different locations under dryland (rainfed) conditions in the winter-dominant rainfall areas of the cereal-livestock belt of Victoria and southern New South Wales, and under irrigation in the main cotton-growing areas of northern New South Wales. Consequently N₂ fixation was primarily regulated by biomass production in these areas and both pasture and crop legumes fixed between 20 and 25 kg shoot N for every tonne of shoot dry matter (DM) produced. Nitrogen fixation by legumes in the dryland systems of the summer-dominant rainfall regions of central and northern New South Wales on the other hand was greatly influenced by large variations in %Ndfa (0–81%) caused by yearly fluctuations in growing season (April–October) rainfall and common farmer practice which resulted in a build up of soil mineral-N prior to sowing. The net result was a lower average reliance of legumes upon N₂ fixation for growth (19–74%) and more variable relationships between N₂ fixation and DM accumulation (9–16 kg shoot N fixed/t legume DM). Although pulses often fixed more N than pastures, legume-dominant pastures provided greater net inputs of fixed N, since a much larger fraction of the total plant N was removed when pulses were harvested for grain than was estimated to be removed or lost from grazed pastures. Conclusions about the relative size of the contributions of fixed N to the N-economies of the different farming systems depended upon the inclusion or omission of an estimate of fixed N associated with the nodulated roots. The net amounts of fixed N remaining after each year of either legume-based pasture or pulse crop were calculated to be sufficient to balance the N removed by at least one subsequent non-legume crop only when below-ground N components were included. This has important implications for the interpretation of the results of previous N₂ fixation studies undertaken in Australia and elsewhere in the world, which have either ignored or underestimated the N present in the nodulated root when evaluating the contributions of fixed N to rotations.     

15N natural abundance in forest and pasture soils of the Brazilian Amazon Basin

The natural abundance of ¹⁵N was examined in soil profiles from forests and pastures of the Brazilian Amazon Basin to compare tropical forests on a variety of soil types and to investigate changes in the sources of nitrogen to soils following deforestation for cattle ranching. Six sites in the state of Rondônia, two sites in Pará and one in Amazonas were studied. All sites except one were chronosequences and contained native forest and one or more pastures ranging from 2 to 27 years old. Forest soil ¹⁵N values to a depth of 1 m ranged from 8 to 23 and were higher than values typically found in temperate forests. A general pattern of increasing ¹⁵N values with depth near the soil surface was broadly similar to patterns in other forests but a decrease in ¹⁵N values in many forest profiles between 20 and 40 cm suggests that illuviation of ¹⁵N-depleted nitrate may influence total soil ¹⁵N values in deeper soil where total N concentrations are low. In four chronosequences in Rondônia, the ¹⁵N values of surface soil from pastures were lower than in the original forest and ¹⁵N values were increasingly depleted in older pastures. Inputs of atmospheric N by dinitrogen fixation could be an important N source in these pastures. Other pastures in Amazonas and Pará and Rondônia showed no consistent change from forest values. The extent of fractionation that leads to ¹⁵N enrichment in soils was broadly similar over a wide range of soil textures and indicated that similar processes control N fractionation and loss under tropical forest over a broad geographic region. Forest ¹⁵N profiles were consistent with conceptual models that explain enrichment of soil ¹⁵N values by selective loss of ¹⁴N during nitrification and denitrification.     

The effectiveness and competitiveness of some Indonesian Rhizobium strains on tropical legumes grown in four soil types of Java

Several Indonesian and some imported Rhizobium strains were assessed for their effectiveness in nodulating four legume species in four soil types of Java. Naturally occurring Rhizobia formed effective symbioses onVigna unguiculata, Macroptilium atropurpureum andDesmodium heterocarpon in all four soils and the applied strains, with some exceptions, did not infect a majority of nodules of these legumes.Centrosema pubescens was more specific in its Rhizobia requirements and applied strains formed effective symbioses in two clay soils, but not in two sandy loam soils.     

Symbiotic effectiveness of Bradyrhizobium strains isolated from Parasponia and tropical legumes on Parasponia host species

The symbiotic effectiveness of Bradyrhizobium strains isolated from three species of Parasponia and from legumes were compared on Parasponia grown in Leonard-jars. Effectiveness of each symbiotic association was estimated from dry weight and total nitrogen of shoots and nodules of plants grown on medium free of combined nitrogen. Twenty strains isolated from three species of Parasponia were found to vary in their effectiveness on P. andersonii, the least effective fixing one fifth of the nitrogen of the most effective strains. The outcome of the symbiosis was not associated with the host source of the test strain. P. andersonii, P. rugosa and P. rigida responded differently to a selection of seven strains of Parasponia Bradyrhizobium; some strains were either ineffective or fully effective on each host, while others varied in their symbiotic performance. P. andersonii fixed significantly (P < 0.001) larger quantities of nitrogen than either P. rugosa or P. rigida with p. rigida being the least effective. In contrast to Bradyrhizobium strains from Parasponia spp. which formed nodules rapidly (within 11–20 days), nine strains isolated from legumes required between 25 and 74 days to form partially effective nodules. The thre Parasponia species formed relatively large quantities of nodule tissue relative to the amount of nitrogen fixed and shoot dry matter produced. The Bradyrhizobium isolated from Parasponia plants growing in Papua New Guinea soils could be grouped together on the basis of their infection characteristics on Parasponia and legumes.     

Optimising biological N2 fixation by legumes in farming systems

Whether grown as pulses for grain, as green manure, as pastures or as the tree components of agro-forestry systems, the value of leguminous crops lies in their ability to fix atmospheric N₂, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N₂ fixing legumes provide the basis for developing sustainable farming systems that incorporate integrated nutrient management. By exploiting the stable nitrogen isotope ¹⁵N, it has been possible to reliably measure rates of N₂ fixation in a wide range of agro-ecological field situations involving many leguminous species. The accumulated data demonstrate that there is a wealth of genetic diversity among legumes and their Rhizobium symbionts which can be used to enhance N₂ fixation. Practical agronomic and microbiological means to maximise N inputs by legumes have also been identified.     

Natural abundance of 15N in tropical plants with emphasis on tree legumes

Natural abundance of ¹⁵N ( ¹⁵N) of leaves harvested from tropical plants in Brazil and Thailand was analyzed. The ¹⁵N values of non-N₂-fixing trees in Brazil were +4.5±1.9, which is lower than those of soil nitrogen (+8.0±2.2). In contrast, mimosa and kudzu had very low ¹⁵N values (–1.4+0.5). The ¹⁵N values of Panicum maximum and leguminous trees, except Leucaena leucocephala, were similar to those of non-N₂-fixing trees, suggesting that the contribution of fixed N in these plants is negligible. The ¹⁵N values of non-N₂-fixing trees in Thailand were +4.9±2.0. Leucaena leucocephala, Sesbania grandiflora, Casuarina spp. and Cycas spp. had low ¹⁵N values, close to the value of atmospheric N₂ (0), pointing to a major contribution of N₂ fixation in these plants. Cassia spp. and Tamarindus indica had high ¹⁵N values, which confirms that these species are non-nodulating legumes. The ¹⁵N values of Acacia spp. and Gliricidia sepium and other potentially nodulating tree legumes were, on average, slightly lower than those of non-N₂-fixing trees, indicating a small contribution of N₂ fixation in these legumes.     

The effects of elevated CO2 on symbiotic N2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems

The response of plants to elevated CO₂ is dependent on the availability of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO₂ concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO₂ stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO₂ in a larger N-sink of the whole grassland ecosystem. One way to maintain the balance between the C and N cycles in elevated CO₂ would be to increase N-import to the grassland ecosystem through symbiotic N₂ fixation. Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N₂ fixation in legumes will be enhanced under elevated CO₂, ii) this enhancement of N₂ fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem. Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO₂ under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO₂, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO₂. Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N₂ fixation usually promotes shoot growth (above-ground C storage) in elevated CO₂. However, the consequences of this larger N input under elevated CO₂ on the below-ground carbon fluxes are not fully understood. On one hand, the positive effect of elevated CO₂ on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon.     

Differential acidity tolerance of tropical legumes grown for green manure in acid sulfate soils

The growth of four tropical legumes (Cajanus cajan, Sesbania aculeata, S. rostrata, and S. speciosa) used as green manures in the tropics was studied in a glasshouse experiment. Two acid sulfate soils (Typic Sulfaquept, Bang Pakong Series; and Sulfic Tropaquept, Rangsit Series) were adjusted to four pH levels: 3.8 or 4.0 (original soil pH), 4.5, 5.5, and 6.5 (amended with lime). Dry weight was determined 49 days after sowing. Concentrations of N, P, K, Ca, Mg, Fe, Mn, and Al were also determined in aerial plant parts at harvest.The legumes responded differently to soil acidity and liming, but not to soil type. Cajanus cajan had the highest biomass production, followed by S. aculeata, S. rostrata and S. speciosa, in this order. The N concentration closely paralleled biomass production, suggesting that the growth of symbiotic rhizobia and nodulation were perhaps more susceptible to soil acidity than were the host plants. Liming to pH 5.5–6.0 was recommended for the legumes' growth based on the quadratic relationships between dry-matter yield and soil pH. In the unlimed soils, the Ca concentration in C. cajan and S. aculeata (0.32%) was twice as high as that in the two low-yielding legumes (0.15%). Furthermore, plant Ca increased exponentially (or quadratically in case of S. speciosa) as lime additions increased. It was estimated that for adequate growth, the Ca requirement in the shoot dry matter was approximately: C. cajan 1.2% Ca, S. aculeata 0.8%, S. rostrata 0.6%, and S. speciosa 0.4%. In contrast with Ca, the concentration of Fe, and to a lesser extent Mn, was significantly lower in C. cajan and S. aculeata than in S. rostrata and S. speciosa. The ratio of Ca to Al in plant tops was used to characterize plant tolerance to soil acidity, and to quantify the critical Al concentration in the plants. It appears that 90% maximum growth was attained only when Ca/Al was 150 for C. cajan and S. speciosa, 200 for S. rostrata, and 300 for S. aculeata. Cajanus cajan tolerated up to 80 mg Al kg^-1 in the shoot dry matter, whereas significant growth reduction occurred in the Sesbania species at levels > 30 mg Al kg^-1.     

The globalization of N deposition: ecosystem consequences in tropical environments

Human activities have more than doubled the inputs of nitrogen (N) into terrestrial systems globally. The sources and distribution of anthropogenic N, including N fertilization and N fixed during fossil fuel combustion, are rapidly shifting from the temperate zone to a more global distribution. The consequences of anthropogenic N deposition for ecosystem processes and N losses have been studied primarily in N-limited ecosystems in the temperate zone; there is reason to expect that tropical ecosystems, where plant growth is most often limited by some other resource, will respond differently to increasing deposition. In this paper, we assess the likely direct and indirect effects of increasing anthropogenic N inputs on tropical ecosytem processes. We conclude that anthropogenic inputs of N into tropical forests are unlikely to increase productivity and may even decrease it due to indirect effects on acidity and the availability of phosphorus and cations. We also suggest that the direct effects of anthropogenic N deposition on N cycling processes will lead to increased fluxes at the soilwater and soil-air interfaces, with little or no lag in response time. Finally, we discuss the uncertainties inherent in this analysis, and outline future research that is needed to address those uncertainties.     

Symbiotic nitrogen fixation in a tropical rainforest: 15N natural abundance measurements supported by experimental isotopic enrichment

* Leguminous trees are very common in the tropical rainforests of Guyana. Here, species-specific differences in N(2) fixation capability among nodulating legumes growing on different soils and a possible limitation of N(2) fixation by a relatively high nitrogen (N) and low phosphorus (P) availability in the forest were investigated. * Leaves of 17 nodulating species and 17 non-nodulating reference trees were sampled and their delta(15)N values measured. Estimates of N(2) fixation rates were calculated using the (15)N natural abundance method. Pot experiments were conducted on the effect of N and P availability on N(2) fixation using the (15)N-enriched isotope dilution method. * Nine species showed estimates of > 33% leaf N derived from N(2) fixation, while the others had low or undetectable N(2) fixation rates. High N and low P availability reduced N(2) fixation substantially. * The results suggest that a high N and low P availability in the forest limit N(2) fixation. At the forest ecosystem level, N(2) fixation was estimated at c. 6% of total N uptake by the tree community. We conclude that symbiotic N(2) fixation plays an important role in maintaining high amounts of soil available N in undisturbed forest.     

Growing vetches (Vicia villosa Roth) in irrigated cotton systems: inputs of fixed N, N fertiliser savings and cotton productivity

We compared symbiotic N₂ fixation by winter forage legumes (clovers, medics and vetches) using the ¹⁵N natural abundance technique in three experiments. Vetches (Vicia spp.) were the most productive legumes, and woollypod vetch fixed (shoot+root) up to 265 kg N ha^–1 (mean 227 kg N ha^–1) during a 4–5 months period over winter and early spring. Balansa and Berseem clovers, and Gama medic were highly productive in the first experiment, but fixed significantly less N than woollypod vetch in the second experiment. A 6-year study (1997–2003) compared cotton (Gossypium hirsutum L.) systems with and without vetch, or with faba beans (Vicia faba L.) to assess the effects of these crops on cotton production. Woollypod vetch was grown either between annual cotton crops, or between wheat (Triticum aestivumL.) and cotton crops. Vetch added 230 kg N ha^–1 (174 kg fixed N ha^–1) to the soil when incorporated as a green manure. Faba bean shoot residues and nodulated roots contributed 108 kg fixed N ha^–1 to the soil, following the removal of 80 kg N ha^–1 in the harvested seed (meaned over three crops). Lablab (Lablab purpureus L. – summer-growing and irrigated) added 277 kg N ha^–1 (244 kg fixed N ha^–1) before incorporation as a green manure in the first year of the experiment. The economic optimum N fertiliser rate for each cropping system was determined every second year when all systems were sown to cotton. Cotton following cotton required 105 kg fertiliser N ha^–1, but only 40 kg N ha^–1 when vetch was grown between each cotton crop. Cotton following wheat required 83 kg fertiliser N ha^–1 but no N fertiliser was needed when vetch was grown after wheat (the highest yielding system). Cotton following faba beans also required no N fertiliser. The vetch-based systems became more N fertile over the course of the experiment and produced greater lint yields than the comparative non-legume systems, and required less N fertiliser. While no cash flow was derived from growing vetch, economic benefits accrued from enhanced cotton yields, reduced N fertiliser requirements and improved soil fertility. These findings help explain the rotational benefits of vetches observed in other regions of the world.     

Natural15N abundance as a method of estimating the contribution of biologically fixed nitrogen to N2-fixing systems: Potential for non-legumes

The¹⁵N abundance of plants usually closely reflects the¹⁵N abundance of their major immediate N source(s); plant-available soil N in the case of non-N₂-fixing plants and atmospheric N₂ in the case of N₂ fixing plants. The¹⁵N abundance values of these sources are usually sufficiently different from each other that a significant and systematic difference in the¹⁵N abundance between the two kinds of plants can be detected. This difference provides the basis for the natural¹⁵N abundance method of estimating the relative contribution of atmospheric N₂ to N₂-fixing plants growing in natural and agricultural settings. The natural¹⁵N abundance method has certain advantages over more conventional methods, particularly in natural ecosystems, since disturbance of the system is not required and the measurements may be made on samples dried in the field. This method has been tested mainly with legumes in agricultural settings. The tests have demonstrated the validity of this method of arriving at semi-quantitative estimates of biological N₂-fixation in these settings. More limited tests and applications have been made for legumes in natural ecosystems. An understanding of the limits and utility of this method in these systems is beginning to emerge. Examples of systematic measurements of differences in¹⁵N abundance between non-legume N₂-fixing systems and neighbouring non-fixing systems are more unusual. In principle, application of the method to estimate N₂-fixation by nodulated non-legumes, using the natural¹⁵N abundance method, is as feasible as estimating N₂-fixation by legumes. Most of the studies involving N₂-fixing non-legumes are with this type of system (e.g., Ceanothus, Chamabatia, Eleagnus, Alnus, Myrica, and so forth). Resuls of these studies are described. Applicability for associative N₂-fixation is an empirical question, the answer to which probably depends upon the degree to which fixed N goes predominantly to the plant rather than to the soil N pool. The natural¹⁵N abundance method is probably not well suited to assessing the contribution of N₂-fixation by free-living microorganisms in their natural habitat, particularly soil microorganisms.This work was supported in part by subcontracts under grants from the US National Science Foundation (DEB79-21971 and BSR821618)     

Fate and effects of phosphorus additions in soils under N2-fixing red alder

Soil phosphorus (P) dynamics are controlled by the interaction of geochemical, biochemical and biological processes. Changes in species composition or management could alter the relative importance of these processes. We examined soil P dynamics in two plantations of N₂-fixing red alder (Alnus rubra) by determining the fate and effects of added fertilizer P. History of the plantations varied such that sites were previously occupied by 60-yr-old stands of alder or non-fixing Douglas-fir (Pseudotsuga menziesii). Without fertilization, the soil with a longer period of alder influence had more organic P (P_o) and less sorbed inorganic P (Hydroxide- and Bicarb-extractable P_i). Fertilization increased soil total P, and 88% of the fertilizer was accounted for in the surface mineral soil (0–15 cm). Sorbed P_i was the major sink for fertilizer P (55–60%), independent of site history. Although P_o was 35–70% of soil P in unfertilized plots, added P did not accumulate as P_o. Neither site history nor P addition influenced phosphatase activity. Fertilization increased decomposition during incubation of the organic horizon, suggesting that late-stage decomposition is P-limited in these N-rich soils. On the time-scale of a few years, geochemical sorption and desorption of inorganic P were the most important processes controlling the distribution of added P. Organic P accumulation is expected to occur over a longer time frame, linked to the production and turnover of organic matter.     

Role of bacteria as food for zooplankton in a eutrophic tropical pond (Ivory Coast)

The aim of the study was to determine whether bacteria could be a substantial source of carbon for zooplankton and whether the grazing pressure of these metazoan filter-feeders could influence the fate of bacterial production. Eight grazing experiments using natural bacteria labelled with³H thymidine were conducted in a tropical pond (Ivory Coast) during various phases of biological colonization (rotifer-dominated and copepod-dominated phases of the colonization). Higher grazing and clearance rates were observed with rotifers (Brachionus plicatilis andHexarthra intermedia), while very low values were obtained when the cyclopoid copepodApocyclops panamensis was dominant. Less than 1% of the bacterial production was harvested when copepods were dominant, whileB. plicatilis consumed up to 36% of this production. However, this consumption of bacteria appeared to contribute only to an insignificant proportion of the daily carbon intake (e.g. 0.9 to 7.1% of body carbon for rotifers). The low contribution of bacteria in the nutrition of zooplankton is discussed in terms of their cell size and their relative abundance in the total amount of seston available.     

Adaptation of methods for evaluating N2 fixation in food legumes and legume cover crops

Methods for partitioning the nitrogen assimilated by nodulated legumes, between nitrogen derived from soil sources and from N₂ fixation, are described as applied in peninsular Malaysia. The analysis of nitrogenous components translocated from the roots to the shoots of nodulated plants in the xylem sap is outlined, with some precautions to be observed for applications in the tropics. Some examples of the use of the technique in surverying apparent N₂ fixation by tropical legumes, in studying interrow cropping in plantation systems and in assessing effects of experimental treatments on N₂ fixation by food legumes, are described. Techniques for assesing N₂ fixation by means of¹⁵N abundance have been used to show that applications of nitrogenous fertilizers commonly used in Malaysia for soybeans depress N₂ fixation, that similar results are obtained with natural abundance and¹⁵N-enrichment methods and that, in at least two locations in Malaysia, differences between the natural abundance of¹⁵N in plant-available soil nitrogen and in atmospheric N₂ are great enough to permit application to measurement of N₂ fixation by leguminous crops.     

Effect of NO3 on N2 fixation and nitrogenous solutes of xylem in two nodulated West African geocarpic legumes,Kersting's bean (Macrotyloma geocarpum L.) and Bambara groundnut (Vigna subterranea L.)

Nodulation, nitrogen (N₂) fixation and xylem sap composition were examined in sand cultured plants of Bambara groundnut (Vigna subterranea L.) and Kersting's bean (Macrotyloma geocarpum L.) inoculated with Bradyrhizobium strain CB756 and supplied via the roots for a 4 week period from the third week onwards with different levels of (¹⁵N)-nitrate (0–15 mM). The separate contributions of nitrate and N₂ to plant nitrogen were measured by isotope dilution. Increasing levels of nitrate inhibited nodule growth (measured as dry matter or nodule N) of both species parallel with decreased dependence on symbiotically-fixed N. Specific nodule activity (N₂ fixed g nodule dry⁻¹ d⁻¹ of nodules) was reduced progressively with time in V. subterranea at higher (5 or 15 mM) levels of NO₃, but this was not so for M. geocarpum. Root xylem bleeding sap of both species showed ureides (allantoin and allantoic acid) as predominant (>90%) solutes of nitrogen when plants were relying solely on atmospheric N. Levels of ureide and glutamine decreased and those of asparagine and nitrate in xylem increased with increasing level of applied nitrate. Relative levels of xylem ureide-N were positively correlated (R²=0.842 for M. geocarpum and 0.556 for V. subterranea), and the ratio of asparagine to glutamine in xylem exudate negatively correlated (R²=0.955 for M. geocarpum and 0.736 for V. subterranea) with plant reliance on nitrogen fixation. The data indicate that xylem sap analyses might be useful for indirect field assays of nitrogen fixation by the species and that Kersting's bean might offer some potential as a symbiosis in which N₂ fixation is relatively tolerant of soil N.     

Soil carbon storage potential of direct seeding mulch-based cropping systems in the Cerrados of Brazil

No‐tillage cropping systems with direct seeding into a mulch of plant residues from cover crops – the so‐called direct seeding mulch‐based cropping (DMC) systems – have been adopted widely over the last 10–15 years in the Cerrado region of Brazil. They are replacing the traditional soybean monoculture with bare fallow using conventional tillage (CT) practices. The objective of this study was to examine how DMC practices affect soil organic carbon (SOC) dynamics and to assess their potential for enhanced soil carbon (C) storage. The approach was to determine soil C stocks along a chronosequence of fields under DMC, and then to apply the generic decomposition and yield (G'DAY) plant–soil model to analyse the soil C storage potential for a number of cropping systems. Forty‐five fields were selected on a plateau of Ferralsols in the central Cerrado region to represent a chronosequence of 0–12 years under continuous DMC. Before DMC the fields had been under CT soybean monoculture following the clearing of the native savannah. An average increase in SOC stocks of 0.83 Mg C ha^?1 yr^?1 in the 0–20 cm topsoil was measured. The corresponding increase in total soil nitrogen was 79 kg N ha^?1 yr^?1. The G'DAY model predicted a net accumulation of 0.70–1.15 Mg C ha^?1 yr^?1 in the 0–40 cm topsoil for the first 12 years, depending on the type of soil and DMC system. Model predictions showed that less soil C was accumulated under DMC systems that commenced immediately after clearing the native savannah. Gains in soil C under DMC were primarily due to the introduction of a second crop that caused higher net primary productivity, leading to higher plant C inputs to soil. A rough estimation shows that the conversion of 6 million ha of CT soybean monoculture to DMC in the Cerrados would enhance soil C storage by 4.9 Tg C yr^?1 during at least the first 12 years following the conversion to DMC.     

Misconceptions and practical problems in the use of 15N soil enrichment techniques for estimating N2 fixation

The ¹⁵N methods are potentially accurate for measuring N₂ fixation in plants. The only problem with those methods is, how to ensure that the ¹⁵N/¹⁴N ratio in the plant accurately reflects the integrated ¹⁵N/¹⁴N ratio (R) in soil which is variable in time and with soil depth. However, the consequences of using an inappropriate reference plant vary with the level of N₂ fixation and the conditions under which the study was made. For example, the errors introduced into the values of N₂ fixation are higher at low levels of fixation, and decrease with increasing rates of fixation. At very high N₂ fixation rates, the errors are often insignificant. Also, the magnitude of error is proportional to the rate of decline of the ¹⁵N/¹⁴N ratio with time. Since N₂ fixation in most plants would be expected to below 60%, the question of how to select a good reference plant is still pertinent. In this paper, we have discussed some of the criteria to adopt in selecting reference plants, e.g. how to ensure that the reference plant is not fixing N₂, is absorbing most of its N from the same zone as the fixing plant, and in the same pattern with time, etc. In addition, we have discussed ¹⁵N labelling materials and methods that are likely to minimize any errors even when the fixing and reference plants don't match well in certain important criteria. The use of slow release ¹⁵N fertilizer or ¹⁵N labelled plant materials results in slow changes in the ¹⁵N/¹⁴N ratio of soil, and is strongly recommended. Where ¹⁵N inorganic fertilizers are used, the application of the fertilizer in small splits at various intervals is recommended over a one-time application. The problem with the reference crop, which has sometimes discouraged potential users of the ¹⁵N methods, is surmountable, as discussed in this paper.     

A simple model of feedback regulation for nitrate uptake and N2 fixation in contrasting phenotypes of white clover

A simple three equation model is proposed for the feedback regulation of nitrate uptake and N2 fixation, based on the concentration of the organic N substrate pool within the plant and two parameters denoting the N substrate concentrations at which half-maximal inhibition occurs. This model simulated three contrasting phenotypes of white clover (Trifolium repens L.) inbred lines with (1) normal rates of nitrate uptake and N2 fixation (NNU); (2) low rates of nitrate uptake (LNU); and (3) very low rates of N2 fixation (VLF). The LNU phenotype was simulated by a decrease in the value of the inhibition parameter for nitrate uptake and the VLF phenotype was simulated by a decrease in the value of the N2 fixation inhibition parameter. The model was tested against nitrate uptake data obtained from white clover plants growing in flowing nutrient culture. There was an accurate prediction of the increase in nitrate uptake caused by N2 fixation activity of the NNU and LNU inbred lines being interrupted by a switch in gas phase from air to Ar : O2. The model was also tested against data for nitrate uptake, N2 fixation and %N from fixation for the three inbred clover lines grown in flowing nutrient culture at 0, 5 or 20 mmol m(-3) N(3-). Again there was accurate prediction of nitrate uptake, although simulated values for N2 fixation were more variable. The simple model has potential use as a sub-routine in larger models of legume growth under field conditions.