Miguel Angel Ayarza

Texture and land-use effects on soil organic matter in Cerrado Oxisols, Central Brazil

Abstract Due to the continuous agricultural expansion in the Brazilian Cerrados and the known adverse effects of agriculture on soil organic matter (SOM), SOM dynamics of typical Cerrado land-use systems were studied. Bulk soil samples and particle-size separates from clayey and loamy Oxisols under crop, pasture, and reforestation sites were compared with the natural Cerrado savanna in order to characterise texture and treatment effects on SOM and to follow the alterations of organic compounds in the different particle-size fractions. The analyses included an assessment of SOM, particulate organic matter (POM), polysaccharides, lignin oxidation products (VSC-lignin), and scanning electron microscopy (SEM) of different particle-size fractions. The amounts of SOM, polysaccharides, and VSC-lignin were clearly higher in the clayey soils due to the increasing stabilisation of organic substances to the clay fraction. Conversely, the amounts of POM were comparable between the substrates resulting in a higher proportion of labile organic matter, and thus an overall greater lability of the loamy soils. Continuous cropping and reforestation with pine led to a clear reduction of SOM contents, whereas pasture and eucalyptus reforestation might have increased both the amount and quality of SOM in relation to the Cerrado control. The analysis of particle-size separates showed the decline of lignin and plant-derived polysaccharides from the sand to the clay fraction, and an accumulation of microbially metabolised polysaccharides in the clay fraction, suggesting that humification and mineralisation of organic matter in Cerrado soils follow along a biogeochemical gradient similar to that of temperate soils and climates.

Land-use effects on phosphorus fractions in Cerrado oxisols

Abstract Differently managed oxisols from Central Brazil were studied using a sequential P fractionation procedure because the effects of land use on the distribution of P forms in these soils are barely known. Therefore levels of labile and recalcitrant P forms in the natural Cerrado savanna were compared with those of crop, pasture and reforestation sites on differently textured oxisols. Under natural conditions of strong P deficiency, >60% of labile P was organic P (Po), reflecting the high contribution of Po to plant nutrition. Fertilisation after land-use change only increased levels of inorganic P (Pi) forms, the increase being most accentuated in the labile Pi fraction. At the crop and the pasture sites P tended to accumulate as recalcitrant P forms in the clayey soils, while in the loamy soils there was only a minor enrichment, probably due to the lower amounts of Fe- and Al-(hydr)oxides. In the reforestation sites, labile P was maintained at high levels, most likely through efficient recycling of the litter. The P fractionation procedure was also applied to particle-size fractions which reflected P transformations along an organic and mineralogical gradient. The clay fraction corresponded to 69–87% of total P and appeared to be both a sink for highly recalcitrant P and a source for labile P, especially labile Po. The sequential P fractionation procedure was also an effective method with which to detect the presence of fertiliser-P residues in the coarse-silt fraction.

Distribution of water-stable aggregates and aggregating agents in Cerrado Oxisols

Abstract The effects of structural changes on Cerrado Oxisols in Brazil after land-use change are still insufficiently studied. Therefore, topsoil samples of loamy and clayey Cerrado Oxisols under crop, pasture and reforestation were fractionated using a wet-sieving procedure to obtain the distribution of water-stable aggregates and compared to samples from natural savanna. The results were correlated to organic and inorganic soil compounds to identify the main aggregating agents and discussed in relation to changes in the pore-size distribution of the soils. Clayey soils showed a significantly higher macro-aggregation than loamy soils. Compared to natural savanna, macro-aggregation was clearly reduced at the crop sites, whereas aggregation of soils under pasture and reforestation was only slightly affected. In both the clayey and the loamy soils, bonding of polysaccharides was the main aggregating agent. In the clayey soils, liming was also very important for disaggregation by weakening the electrostatic forces between positively and negatively charged soil compounds, whereas in the loamy soils, binding of macro-aggregates by roots was significant. The introduction of crop–pasture rotations was therefore proposed to take advantage of the strong rooting and polysaccharide production of pastures. Management-induced disaggregation strongly affected the pore-size distribution by compacting the soils. Thereby, macro-porosity was reduced and the amount of meso-pores was increased, while micro-porosity was unaffected from management and only differed between the two substrates. Considering the low pore space at plant-available matrix potentials typical of Cerrado Oxisols, the increase might be important for annual crops during the frequent dry spells in the rainy season.

Adaptive attributes of tropical forage species to acid soils. III. Differences in phosphorus acquisition and utilization as influenced by varying phosphorus supply and soil type

Abstract Low phosphorus (P) supply is a major limitation of forage production in acid soils of the tropics. A glasshouse experiment was conducted to determine the differences in P acquisition and utilization among three legumes (Arachis pintoi, Stylosanthes capitata and Centrosema acutifolium) and one grass (Brachiaria dictyoneura). The plants were grown under monoculture or in grass + legume associations in two acid Oxisols of contrasting texture (sandy loam or clay loam). The soils were amended with soluble phosphate at rates ranging from 0 to 50 kg‐ha‐1. After 80 days of growth, total P and nitrogen (N) content in different plant parts, inorganic P content and acid phosphatase activity in roots and leaves, vesicular‐arbuscular mycorrhizal root infection, total P acquisition from soil, P acquisition efficiency, P transport index, and P use efficiency were determined. Total P acquisition (per unit soil surface) of A. pintoi was higher than that of the other two legumes at 50 kg‐ha‐l of P supply. Phosphor...

Adaptive attributes of tropical forage species to acid soils II. Differences in shoot and root growth responses to varying phosphorus supply and soil type

Abstract Identification of plant attributes that improve the performance of tropical forage ecotypes when grown as monocultures or as grass+legume associations in low fertility acid soils will assist the development of improved forage plants and pasture management technology. The present work compared the shoot and root growth responses of four tropical forages: one grass and three legumes. The forages were grown in monoculture or in grass+legume associations at different levels of soil phosphate. Two infertile acid soils, both Oxisols, were used: one sandy loam and one clay loam. They were amended with soluble phosphate at rates ranging from 0 to 50 kg ha‐1. The forages, Brachiaria dictyoneura (grass), Arachis pintoi, Stylosanthes capitata and Centrosema acutifolium (legumes), were grown in large plastic containers (40 kg of soil per container) in the glasshouse. After 80 days of growth, shoot and root biomass production, dry matter partitioning, leaf area production, total chlorophyll content in leaves,...

Adaptive attributes of tropical forage species to acid soils I. Differences in plant growth, nutrient acquisition and nutrient utilization among C4 grasses and C3 legumes

Abstract Low supply of nutrients is a major limitation of forage adaptation and production in acid soils of the tropics. A glasshouse study was conducted to find differences in plant growth, nutrient acquisition and use, among species of tropical forage grasses (with C4 pathway of photosynthesis) and legumes (with C3), when grown in two acid soils of contrasting texture and fertility. Twelve tropical forage legumes and seven tropical forage grasses were grown in sandy loam and clay loam Oxisols at low and high levels of soil fertility. After 83 days of growth, dry matter distribution among plant leaves, stems, and roots, leaf area production, shoot and root nutrient composition, shoot nutrient uptake, and nutrient use efficiency were measured. Soil type and fertility affected biomass production and dry matter partitioning between roots and shoots. The allocation of dry matter to root production was greater with low soil fertility, particularly in sandy loam. The grasses responded more than the legumes to ...

Somaclonal variation in plant adaptation to acid soil in the tropical forage legume Stylosanthes guianensis

Somaclonal variation offers the possibility to obtain changes in one or a few characters of an otherwise outstanding cultivar without altering the remaining, and often unique, part of the genotype. It has been shown to be heritable for some species. A check line of Stylosanthes guianensis (Aubl.) Sw., CIAT 2243 and 14 somaclones in the R4 generation, selected after three generations from the original 114 plants regenerated from callus cultures, were used in a glasshouse trial. The main objective of the study was to evaluate the physiological basis of the differences in agronomic performance of certain somaclones over the check genotype when grown in a sandy loam acid soil at low or high fertility level. Measurements at the time of harvest (170 days of plant age) included dry matter distribution between shoot and roots, leaf area production, nutrient levels in soil and plant parts, and uptake of nutrients from soil. Somaclones differed with the check genotype in terms of (i) partitioning of fixed carbon between the shoot and roots; (ii) root biomass production and (iii) uptake of nitrogen and phosphorus. Positive relationships were found between total nitrogen uptake and total biomass, and total phosphorus uptake and total biomass, and total phosphorus uptake and total nitrogen uptake. The results of this study provide an insight into the potential use of somaclonal variation for the improvement of plant adaptation to acid soil conditions.

The use of carbon isotope ratios to evaluate legume contribution to soil enhancement in tropical pastures

Soil carbon distribution with depth, stable carbon isotope ratios in soil organic matter and their changes as a consequence of the presence of legume were studied in three 12-year-old tropical pastures (grass alone —Brachiaria decumbens (C4), legume alone —Pueraria phaseoloides (C3) and grass + legume) on an Oxisol in Colombia. The objective of this study was to determine the changes that occurred in the13C isotope composition of soil from a grass + legume pasture that was established by cultivation of a native savanna dominated by C4 vegetation. The13C natural abundance technique was used to estimate the amount of soil organic carbon originating from the legume. Up to 29% of the organic carbon in soil of the grass + legume pasture was estimated to be derived from legume residues in the top 0–2-cm soil depth, which decreased to 7% at 8–10 cm depth. Improvements in soil fertility resulting from the soil organic carbon originated from legume residues were measured as increased potential rates of nitrogen mineralization and increased yields of rice in a subsequent crop after the grass + legume pasture compared with the grass-only pasture. We conclude that the13C natural abundance technique may help to predict the improvements in soil quality in terms of fertility resulting from the presence of a forage legume (C3) in a predominantly C4 grass pasture.

Adsorção de nitrato em solos de cerrado do Distrito Federal

This work aimed to assess the rate of nitrate adsorption and identify the soil properties that most affect it, in a Dark-Red Latosol, with 56% clay content, in Planaltina, DF, Brazil. Soil samples were collected from areas with four different management systems, at depths of 0-20, 20-40, 40-80 and 80-100†cm. Three of the areas were obtained from an on-going five-year experiment, i.e., native grass savannah land and two areas cultivated with soybeans and maize that received different amounts of liming, sulphate and other nutrients and whose soils had inverted charges in depth. The fourth area was a woody savannah land, with negative net electric charge in depth. The relationship between nitrate adsorbed and nitrate in solution, was described by Freundlich equation, and the relationship between nitrate adsorbed and some soil physical-chemical properties was studied by multiple regression. Ni- trate adsorption increased with soil depth in all treatments. Nitrate adsorption was higher in the native grass and woody savannah than in the other treatments. The DpH as an index of net electric charge did not affect nitrate adsorption. There was a negative correlation between nitrate adsorption, organic matter and extractable sulphate. Nitrate adsorption increased with organic matter reduction in the deep layers of the soil and with decrease in extractable sulphate.

Use of undisturbed soil cores for evaluation of Rhizobium strains and methods for inoculation of tropical forage legumes in a Colombian Oxisol

SummaryThree experiments were conducted in an acid infertile Oxisol from the Llanos Orientales of Colombia. It was shown that greater increases in nitrogen yield in the tops (N yield) due to N fertilization ofPueraria phaseoloides (CIAT germplasm accession no. 9900),Stylosanthes capitata no. 1019,Centrosema macrocarpum no. 5065 andDesmodium ovalifolium no. 350 occurred in undisturbed soil cores than in pots of disturbed soil. Inoculation significantly increased N yield ofC. macrocarpum in soil cores, but not in pots.In screening trials where a range of Rhizobium strains was used to inoculateD. ovalifolium, P. phaseoloides andC. macrocarpum grown in undisturbed soil cores, strains CIAT 2335, 2434 and 1780, respectively, caused the greatest increases in N yield. Inoculation caused greater increases of nodule numbers relative to the uninoculated control inC. macrocarpum than inD. ovalifolium orP. phaseoloides.When each legume was inoculated with the most effective strain by different methods, and grown in soil cores, it was found that granulated inoculant (0.5 g/seed) was more effective than seeds pelleted with 50 g inoculant/kg for the small-seededD. ovalifolium, but was similar in effectiveness to pelleted inoculant for the larger-seededP. phaseoloides. With the relatively large-seededC. macrocarpum, granulated inoculant was less effective than pelleted inoculant. No clear differences between different types of pellets were observed.The results show that undisturbed cores of acid infertile Oxisol can be used to screen for Rhizobium strains tolerant to these adverse soil conditions, although selected strains should then be subjected to further screening in the field, to determine whether they would make suitable commercial inoculants. It may not be possible to obtain maximum responses to inoculation by all legumes, unless the inoculation methods used here are improved.