G. Cadisch

Future benefits from biological nitrogen fixation: An ecological approach to agriculture

Strategies for the enhancement and exploitation of biological nitrogen fixation are assessed with attention to the likely timescales for realization of benefits in agriculture. Benefits arising from breeding of legumes for N2-fixation and rhizobial strain selection have less potential to increase inputs of fixed N than alleviation of environmental stresses or changes in farming systems to include more legumes. Genetic engineering may result in substantial enhancement of N2-fixation, particularly if the ability to fix N2 is transferred to other crops but these are long-term goals. Immediate dramatic enhancements in inputs from N2-fixation are possible simply by implementation of existing technical knowledge. Apart from the unfortunate political and economic barriers to the use of agricultural inputs, better communication between researchers and farmers is required to ensure proper focus of research and development of appropriate technologies. Legumes must be considered within the context of the farming systems within which they are grown and not in isolation. Proper integration of legumes requires a good understanding of the role of the legume within the system and a better understanding of the relative contributions of N sources and of the fates of fixed N.

Interactions between residues of maize and pigeonpea and mineral N fertilizers during decomposition and N mineralization.

Nitrogen mineralization patterns of maize and pigeonpea (Cajanus cajan) residues were examined in leaching tubes, both in isolation and mixtures, in Malawian soils of varying texture. Senesced pigeonpea leaves (C-to-N ratio 24) induced a short period of nitrogen immobilization which was followed by steady net nitrogen mineralization in all of three soils. The immobilization period lasted between 14 and 28 days and was longer in soils with larger clay contents. Maize residues contained 30% of their N in the form of water-soluble nitrate. Both the sole maize residue (C-to-N ratio 75 after adjustment for nitrate which constituted 28% of the N) and the mixture of maize and senesced pigeonpeas leaves revealed a similar prolonged strong net N immobilization up to 130 days before the two treatments started to diverge slightly. Mixing maize with pigeonpea residues with equal amounts of N failed to substantially alleviate the N immobilization capacity of the maize residues. N immobilization in the mixture was much greater than that predicted from the mineralization patterns of the individual components. When increasing amounts (50, 100 and 150 mg N kg ˇ1 soil) of green pigeonpea leaves, senesced pigeonpea leaves and ammonia-N were added to 50 mg N kg ˇ1 soil of maize residues, N released in the mixtures increased with the increasing amounts of N added to the maize residues with greater increases from residues with larger N concentrations. There was evidence that microbial degradation of maize carbon was limited by N availability. The implications of the results for management of crop residues and mineral N fertilizers in the field are discussed. 7 2000 Elsevier Science Ltd. All rights reserved.

Nitrogen release from prunings of legume hedgerow trees in relation to quality of the prunings and incubation method

Three methods: litterbags, incubation of materials in pots and incubation in leaching-tubes, were compared to determine the effects of N, lignin and polyphenols of legume tree prunings on their decomposition and N release rates in a Red-yellow Podzolic soil (Ultisol). A protein-binding capacity assay was used to measure the content of active polyphenols in pruning materials in relation to their effect on decomposition. Prunings of Calliandra calothyrsus, Peltophorum pterocarpa, Gliricidia sepium, Leucaena leucocephala, and a mixture of Peltophorum and Gliricidia from an alley cropping experiment in Lampung, Indonesia, were used for the study. Decomposition and N release rates of the prunings were in the order Gliricidia > Leucaena > Calliandra > Peltophorum in all three incubation methods, however, the patterns of N release varied between incubation methods and species. The (lignin + polyphenol):N ratio was consistently among the best quality descriptions to predict weight losses and N released from the prunings in the litterbag and leaching-tube experiments but not in the pot experiment. In the latter the lack of a good correlation with quality factors maybe due to the presence of soluble polyphenols with a greater capacity to bind protein under non-leaching conditions.

REGULATING N RELEASE FROM LEGUME TREE PRUNINGS BY MIXING RESIDUES OF DIFFERENT QUALITY

Abstract In an attempt to regulate the rate of N mineralization from legume tree prunings by manipulating the quality of the prunings, a low-quality legume tree pruning (Peltophorum dasyrrachis) was mixed at various proportions with a high quality legume tree pruning (Gliricidia sepium). N mineralization behaviour of the pruning mixtures was tested upon application to the soil in the laboratory under controlled leaching and non-leaching conditions over a 14-week period. Mixing the two species resulted in complex, non-linear interactions in quality, particularly with regard to the activity of extractable polyphenols. N mineralization rate of the pruning mixtures decreased with increasing proportion of Peltophorum prunings in the mixtures, indicating that N mineralization rate of prunings can be manipulated by mixing different quality materials. The initial lignin-to-N ratio and the (lignin + polyphenol)-to-N ratio were strongly correlated with the N mineralization rate constant of the prunings mixtures under both leaching and non-leaching conditions. Under leaching conditions, however, the N mineralization rate constant was best correlated with the initial protein-binding capacity of the pruning mixtures. In agreement with patterns of N mineralized from the pruning mixtures, the N recovery by maize from the pruning mixtures increased with increasing proportion of Gliricidia prunings in the mixtures. The strong relationship between N recovery and the protein-binding capacity suggested that protein-binding by polyphenols was the cause of the reduced N recovery of individual prunings in the mixtures. The results also showed that by mixing with slow N release legume prunings, such as Peltophorum, possible losses of N mineralized from the rapid N release legume prunings (Gliricidia) can be minimized. In this study, when the proportion of Peltophorum prunings in the mixture was more than 50%, the N recovery from Gliricidia was strongly reduced, probably because of the high protein-binding capacity of polyphenols in the Peltophorum prunings.

Carbon turnover (δ13C) and nitrogen mineralization potential of particulate light soil organic matter after rainforest clearing

Abstract Soil samples from under a rainforest, a papaya plantation, a pure Brachiaria humidicola pasture and a B. humidicola/Desmodium ovalifolium sward from the humid tropical Bahia, Brazil, were analyzed for fertility parameters, litter quality, particulate light fraction [ PLF > 100 53 -100 μ m , 100 μm) PLF was of younger age than the finer PLF or whole soil samples. The age of the 53–100 μm PLF was very close to that of stabilized organic matter as indicated by 13C data, its low C-to-N ratio (15–17:1) and its turnover time was about 30 y. The PLF appeared not to consist of one uniform pool and thus may have to be subdivided to be useful for modelling purposes (e.g. active, passive pools and charcoal). The contribution of PLF and above ground litter to total soil N mineralization (anaerobic incubation) was highest in the undisturbed rainforest ( 12 16 % for the PLF/litter, respectively, in the 0–2 cm layer), but much less in the other systems. However, mineralization of PLF from different sources was unpredictably altered by the density agent. Introducing D. ovalifolium into these pastures increased, significantly, the amounts of N in PLF, decreased the C-to-N ratio of PLF and tended to increase, the mineralization potential despite its high lignin and polyphenol content. The combination of δ13C analysis and size/density separations, of surface samples in particular, allowed sensitive detection of changes in soil organic matter dynamics and soil fertility.

Manipulation of quality and mineralization of tropical legume tree prunings by varying nitrogen supply

The effect of N supply on the quality of Calliandra calothyrsus and Gliricidia sepium prunings was studied in a glasshouse over a 7-month growing period. Increasing the concentration of N supplied from 0.625 to 10.0 mM NO3-N resulted in increased N concentration but decreased polyphenol concentration, protein-binding capacity and C:N ratio of prunings from both species. Lignin concentration was not consistently altered by the N treatment. Mineralization of N from the prunings was measured over a 14-week period under controlled leaching and non-leaching conditions. The results indicated a strong interaction between legume species and concentration of N supply in their influence on N mineralization of the prunings applied to the soil. Differences in the %N mineralized were dictated by the quality of the prunings. The (lignin + polyphenol):N ratio was the pruning quality factor which could be used most consistently and accurately to predict N mineralization of the legume prunings incubated under leaching conditions, and the relationship was best described by a linear regression. Under non-leaching conditions, however, the protein-binding capacity appeared to be the most important parameter in determining the patterns of N release from the prunings studied. The relationship between the N mineralization rate constant and the protein-binding capacity was best described by a negative exponential function, y=0.078 exp(−0.0083x). The present study also indicated that the release of N from legume prunings containing a relatively high amount of polyphenol could be enhanced by governing the N availability conditions under which the plant is grown, for example whether or not it is actively fixing nitrogen. Estimates of pruning N mineralization after 14 weeks with the difference method averaged 6% (leaching conditions) and 22% (nonleaching conditions) more than with the 15N method for all legume prunings studied. The recovery of pruning by maize (4–38%) was well correlated with the % pruning N mineralized suggesting that incubation data closely reflect the pruning N value for a given catch crop under non-leaching conditions.

N recovery from legume prunings and priming effects are governed by the residue quality

Nitrogen recovery from 15N-labelled prunings of Gliricidia sepium, Peltophorum dasyrrachis, Calliandra calothyrsus and Leucaena leucocephala, each of two different chemical qualities, was followed over three cropping cycles in a growth room. Half of the pots of each treatment received a further addition of unlabelled pruning material, from the same species as that previously applied, before the second and third crop cycle. The cumulative maize total N accumulation revealed the largest benefit from N rich, low lignin and polyphenols Gliricidia prunings followed by Leucaena, Calliandra and Peltophorum. Cumulative N recovery measured using 15N over the three crop cycles ranged from 9% from Calliandra prunings to 44% from Gliricidia prunings. The vast majority of this N was recovered during the first crop cycle which agreed well with estimates using the N difference method. Recoveries in the second and third crops ranged from 0.4–5% (15N method) and 6–14% (N difference method) of the N initially applied. The protein binding capacity of polyphenols was the best predictor of N recovery at both initial and later crop cycles. Treatments which led to a large N recovery initially, continued to provide greater N benefits in subsequent cycles although with increasing harvest time this trend decreased. Thus, there was no compensation in initial N release from low quality prunings at later harvests and the agronomic implications of this are discussed. Addition of unlabelled Gliricidia prunings before the second and third cycle led to a positive apparent priming effect on previously applied 15N labelled prunings. By contrast, repeated additions of Peltophorum residues, rich in lignin and active polyphenols, resulted in a reduced recovery of initially applied pruning-15N. However, the maximum positive or negative effects on recovery of pruning N amounted to less than 2% recovery of the initial amount of N added over 14 weeks. Thus the scope for regulation of N release from tree prunings during these later stages of decomposition appears to be limited.

Soil organic matter and nitrogen transformation mediated by plant residues of different qualities in sandy acid upland and paddy soils.

Organic matter management is believed to solve many of the chemical and physical problems of coarse-textured, low fertility soils of NE Thailand. The influence of different plant residues available in this area on soil C and N dynamics in upland (Oxic Paleustult) and lowland (Aeric Paleaquult) soils was tested. Residues included groundnut (upland) or Sesbania rostrata stover (lowland), rice straw, Tamarindus indica and Dipterocarpus tuberculatus leaves applied at 10 t ha-1 (dry matter). For the former three residues additional application rates of 20 t ha-1 were included as well as a mixture (50:50) of groundnut/ Sesbania - rice straw treatment. Groundnut stover and Sesbania had C:N ratios 17%) and polyphenol (>4.5%) contents. These latter residues, despite slow decomposition, apparently resulted in only moderate soil C ( 1 mm). Thus the mixture of groundnut/ Sesbania with straw was among those residue treatments that led to the highest soil C ( Sesbania with rice straw with a high C:N ratio residue N mineralization could be delayed and prolonged, improving potentially the synchrony of N release and plant demand. Additions of dipterocarp and tamarind resulted in an initial N immobilization phase and net mineral N release remained low thereafter. Dynamics of microbial biomass N were closely related to N mineralization and immobilization cycles in both upland and lowland experiments. Residue N concentration was the most significant factor controlling N release in both systems. While extractable polyphenols exhibited a significant influence on N release in upland conditions their effect was not evident in the lowland.

Substrate amendments can alter microbial dynamics and N availability from maize residues to subsequent crops.

The release and recovery of N from poor-quality (0.35% N), 15N-labelled maize residues was studied over five cropping cycles in a pot experiment in a sandy (7% clay) Quarzipsamment and a sandy–clay (31% clay) Acrorthox from Brazil. During the cropping cycles the soils were amended with bean residues high in N (4.4%), cellulose, or KNO3. Total recovery of maize-N in plants at the end of the experiment was greater in the sandy soil (23–37%) compared with the sandy–clay soil (19–30%) in all treatments. Additions of bean residues increased the recovery of maize-N consistently in both soils compared with control treatments, especially when applied from the first cropping cycle onwards. At the end of the five cropping cycles, sequential bean residue additions resulted in a 38% increase in maize residue-N recovery in the sandy soil and a 32% increase in the sandy–clay soil. When applied from the second cycle onwards they resulted in only 27% and 9% increases respectively. The increased maize-N recovery resulted from greater microbial activity due to the bean-C application, followed by fast turnover of the newly-formed and largely unprotected microbial biomass. However, rather than producing a “real priming effect” on maize residue decomposition, this seems to have produced a “pool substitution” effect, where microbial metabolites of bean instead of maize origin were stabilized in soil. KNO3–N applications had the opposite effect to that of bean applications (16% average decrease in maize-N recovery compared with control treatments) because available-C was not added to stimulate microbial activity, and NH4–N from the decomposing residues was probably recycled in preference to NO3–N by the soil microorganisms. Cellulose application considerably reduced availability of both soil-N and residue-N to plants but the effect lasted for only one cropping cycle. The results clearly showed the potential for manipulation of long-term recoveries of N from recalcitrant plant residues, particularly when better quality residues are also applied to satisfy part of the N demand of plants and microbial biomass, but this probably occurs at the expense of the recovery of the N from the other substrate.

Short and medium term plant litter decomposition in a tropical Ultisol elucidated by physical fractionation in a dual 13C and 14C isotope study

Size–density fractionation in combination with the use of 13C analysis yielded detailed information on soil organic matter (SOM) dynamics and stabilisation in a Brachiaria humidicola pasture (C4), established after rain-forest (C3) by slash and burn 16 year prior to soil sampling. The medium term 13C marker was supplemented with a short-term 14C marker by incubating the soil over 75 days with homogeneously-labelled Lolium perenne. The use of the medium-term marker (13C) showed a potential for density fractionation that would not have become apparent using the short-term marker (14C) on its own. To a large extent this disparity has to do with limiting characteristics of the used 14C-labelled material (small particle size and high content of water soluble carbon). The 14C-labelled material was immediately present in all fractions but mainly corroborated the results from the 13C determination. Both 14C and 13C measurements indicated the transfer of carbon from large light fractions to fine heavy organomineral fractions. The ‘youngest’ C4 fractions floated on water, whereas the ‘intermediately aged’ C4/C3 fractions were intimately associated with the mineral phase through biological processes, and came out as heavy. The ‘oldest’ C3 fraction consisted of stabilised free organic material that had not attained high mineral affinity, and therefore had intermediate density. The changes in 14C specific activity of the isolated fractions in the short term indicated that these fractions were not homogeneous. This suggests that there will be limited scope for tying the size–density fractions and the conceptual homogenous pools used in SOM models together without further characterisation of these pools or by using alternative decay models.