Kandiudalfic Eutrudox Research Articles

Inorganic nitrogen dynamics in fallows and maize on an Oxisol and Alfisol in the highlands of Kenya

Fallows with naturally regenerated or planted vegetation are important in many subsistence agricultural systems of tropical regions, but the underlying soil processes in fallows are not properly understood. We investigated N dynamics under different fallow vegetation on a Kandiudalfic Eutrudox (2372-mm rain in 16 months) and a Kandic Paleustalf (1266-mm rain in 15 months) in the Kenyan highlands. The treatments, which extended for three cropping seasons (15–16 months), were Zea mays (maize), natural regrowth of vegetation (natural fallow), planted Sesbania sesban (sesbania fallow) and uncultivated soil without vegetation (bare fallow). Inorganic N (nitrate+ammonium-N) to 2-m depth under bare fallow increased by 242 kg N ha −1 year −1 on the Oxisol and 54 kg N ha −1 year −1 on the Alfisol, indicating that N mineralization exceeded N losses. Subsoil inorganic N (0.5–2.0 m) remained relatively unchanged after three crops of unfertilized maize, which produced limited total biomass because of P deficiency. Inorganic N decreased during natural and sesbania fallows, and both fallows similarly depleted subsoil inorganic N. The fallows depleted inorganic N at 0.5–2.0 m by 75–125 kg N ha −1 year −1 down to a minimum N content between 40 and 80 kg N ha −1. After slashing sesbania and incorporating the above-ground biomass with 154–164 kg N ha −1, soil inorganic N increased within 2 months by 136 kg N ha −1 on the Oxisol and 148 kg N ha −1 on the Alfisol. Inorganic N decreased after cropping the bare fallow on the Oxisol with maize, indicating that inorganic N was prone to leaching during heavy rains when the maize was small. A considerable part of the N in biomass of the natural fallow was recycled. Much of the total N accumulated by the sesbania fallow was removed with the wood and the amount of N recycled was similar on the Oxisol and Alfisol. We conclude that sesbania fallows can retrieve considerable subsoil inorganic N on deep soils with high subsoil N and effectively cycle this N through its rapidly decomposable biomass to subsequent crops.

Aplicações da curva de resistência no controle da qualidade física de um solo sob pastagem

A degradação da qualidade física do solo pode estar associada com a compactação causada pelo pisoteio dos animais. A resistência do solo à penetração (RP) é um parâmetro físico utilizado para estabelecer o grau de compactação do solo. Contudo, esta varia com a umidade (teta) e densidade do solo (Ds). O objetivo deste trabalho foi obter a curva de resistência do solo e utilizá-la na avaliação da qualidade física do solo num sistema de pastejo intensivo rotacionado de capim-elefante. A curva de resistência do solo foi determinada por meio de 48 amostras indeformadas, obtidas na profundidade de 0-10 cm numa Terra Roxa Estruturada utilizada com pastagem em sistema intensivo de exploração. Os resultados demonstraram correlação negativa entre a RP e teta, e correlação positiva entre RP e Ds. Estimativas indicaram que no potencial de -0,01 MPa a RP não atinge valores considerados restritivos ao crescimento radicular. Entretanto, no potencial de -0,3 MPa, a RP atinge níveis limitantes em toda a área. Quanto ao sistema de manejo e a espécie estudada, os resultados sugerem que a curva de resistência do solo pode ser utilizada para orientar as práticas de manejo visando à manutenção de uma qualidade física do solo adequada para o crescimento das plantas.

Spatial heterogeneity of soil properties in areas under elephant-grass short-duration grazing system

Soil properties can be changed by several factors, such as plant roots and animal trampling. The identification of spatial heterogeneity of these properties depends on the sampling scale. This study was developed to test the hypothesis that soil chemical and physical properties beneath elephant-grass plants are different from those between them. The research was carried out in a soil classified as Kandiudalfic Eutrudox. Forty-eight soil samples were collected from 0-10 cm depth (24 beneath plants and 24 between plants). The following properties were measured: pH, organic matter, S, available P, K, Ca and Mg exchangeable, sum of bases, cation exchange capacity, base saturation percentage, dry-aggregate distribution, bulk density and soil penetration resistance. Statistical analyses (t test) indicated that there were no significant differences in soil chemical properties in relation to spatial position. However, significant differences were observed in soil physical properties, with higher values of bulk density and soil resistance to penetration between the plants than beneath the plants.

Nutrient uptake and growth of young trees in a P-deficient soil: Tree species and phosphorus effects

Phosphorus deficiency is widespread in the subhumid highlands of eastern Africa but there are few data on the effect of P deficiency on the growth of agroforestry tree species. We studied the effect of P application on growth, nutrient uptake and dry matter partitioning in young trees of Calliandra calothyrsus, Cedrela serrulata, Eucalyptus grandis, Grevillea robusta, Markhamia lutea, Senna spectabilis, and Sesbania sesban on a P-deficient soil (Kandiudalfic Eutrudox, bicarbonate-EDTA extractable P = 1 mg kg −1) in western Kenya. The trees were grown at two P levels (control and 500 kg added P ha −1) at 1 m 2 spacing in a randomized complete block design with three replications. Leaf K concentrations were in the low range for all species (5–9 mg g −1) and K deficiency may have limited responses to P. Averaged over species, P addition increased aboveground shoot dry matter by a factor of 2.6 at 62 and 124 days, but the response decreased to 1.3 at 325 days. The increases at 62 days were large in sesbania (5.4) and eucalyptus (3.2) but small in calliandra (1.4) and markhamia (1.1). Relative response to P was more strongly correlated with shoot growth rate per unit root length among species than with shoot growth rate alone. Calliandra, which had high early growth rate but low response to added P, had an exceptionally high root length (6.0 km m −2) compared with the other species (0.3–2.1 km m −2). P addition increased N and P content but decreased final shoot K content in sesbania and calliandra, and had little effect on K content in the other species. The high-yielding species (eucalyptus, sesbania and calliandra) accumulated more than 30 g N and 2 g P m −2 in shoots in 325 days of growth. The proportion of total shoot N in wood (branch + stem) was in a higher range (67–75%) in the shrubby species (sesbania, calliandra, senna) than in the upperstorey tree species (38–43%). Slow early shoot growth relative to total root length, and high specific root length (root length per unit root mass) are proposed as criteria for the selection of species and provenances that are well adapted to P deficient soils.

Soil Phosphorus Fractions in Unfertilized Fallow‐Maize Systems on Two Tropical Soils

AbstractThe rotation of crops with either legumes or natural regrowth of native vegetation is known to supply N to crops in tropical land‐use systems, but little is known about the effect of such rotations on P supply to crops. An experiment was conducted on two soils in Kenya, an Oxisol (very fine, isohyperthermic Kandiudalfic Eutrudox), and an Alfisol (mixed, isothermic Kandic Paleustalf), to determine the effect of unfertilized, organic‐based land‐use systems on fractions of soil inorganic P (Pi) and organic P (Po). The land‐use systems involved growth of three maize (Zea mays L.) crops (18 mo total duration) after 17 mo of either (i) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (ii) natural regrowth of vegetation without cultivation (natural fallow), (iii) three crops of unfertilized maize (maize monoculture), or (iv) bare uncultivated soil (bare fallow). Soil (0–15 cm deep) was collected before post‐fallow maize and after the three post‐fallow maize crops. Sesbania and natural fallows increased the amount of P in light fraction soil organic matter (OM) (>150 µm, <1.13 Mg m‐3) and macroorganic matter (250–2000 µm) on each soil and increased chloroform extractable P on the Oxisol. Land‐use systems had no effect on extractable Pi (bicarbonate, Bray‐1, iron‐oxide impregnated paper), except for resin P on the Oxisol. Unfertilized sesbania and natural fallows cycled soil P through plant residues, resulting in increased P in soil microbial biomass immediately after the fallows and increased P in light fraction soil OM for three post‐fallow cropping seasons.

Sesbania Tree Fallows on Phosphorus‐Deficient Sites: Maize Yield and Financial Benefit

AbstractRotation of Sesbania sesban (L.) Merr., a fast‐growing N2‐fixing tree, with maize (Zea mays L.) has potential for increasing fertility of tropical soils, where fertilizer use by resource‐poor farmers is limited. At two sites in Kenya (Ochinga, with a Kandiudalfic Eutrudox soil, and Muange, with a Kandic Paleustalf), we compared maize yields and financial returns for (i) sesbania grown for three or four seasons followed by three maize crops (sesbania fallow), (ii) one maize crop followed by natural regrowth of vegetation for three seasons and then three maize crops (natural fallow), and (iii) maize monoculture for seven seasons. After the fallows, plots were split with and without added P. Maize responded to P at both sites. Cumulative grain yields for seven seasons of maize monoculture were 8.4 Mg ha−1 at Ochinga and 5.6 Mg ha−1 at Muange. They were comparable to cumulative maize yields for sesbania fallow (Ochinga, 10.6 Mg ha−1 Muange, 4.5 Mg ha−1) and natural fallow (Ochinga, 7.7 Mg ha−1; Muange, 4.2 Mg ha−1), even though maize was grown for only three or four seasons in the fallow treatments. Sesbania fallow was financially attractive at Ochinga (≥500 mm rain in each season) but not at Muange, where low rainfall (<300 mm in each postfallow season) limited maize yield. Phosphorus fertilization of maize at Ochinga increased (P < 0.2) net benefit for sesbania fallow. Improved fallows have potential to supply nutrients to crops, but they are unlikely to eliminate the need for P fertilizers on P‐deficient soils.

Vertical Distribution of Roots and Soil Nitrate: Tree Species and Phosphorus Effects

AbstractWe hypothesized that trees can rapidly root into subsoil and capture NO3, which can accumulate in the subsoil of agricultural soils with high anion sorption. The vertical distribution of root length and inorganic N (NO3 and NH4) to 3.95‐m soil depth was compared for 11‐mo‐old stands of eucalyptus (Eucalyptus grandis W. Hill ex Maiden), sesbania [Sesbania sesban (L.) Merr.], calliandra (Calliandra calothyrsus Meissner), markhamia [Markhamia lutea (Benth.) Schumann], and grevillea (Grevillea robusta A. Cunn. ex R. Br.) grown at two P levels (no added P and 500 kg added P ha−1) on a Kandiudalfic Eutrudox in Kenya. The trees were planted at a 1 by 1 m spacing in a randomized complete block with three replications. Added P had no effect on root length, soil NO3, and soil NH4 even though the soil was low in available P. Total root length was greater for calliandra (15.5 km m−2) than other trees (1.2–5.6 km m−2). The slope for the model of natural logarithm root length density (cm cm−3) as a function of soil depth was affected by tree species (P < 0.01), indicating differences among trees in the tendency for deep rooting. Root length densities, averaged for the two P levels, were ≥0.1 cm cm−3 to depths of 2.2 m with calliandra, 1.8 m with sesbania, 1.2 m with eucalyptus, 0.45 m with grevillea, and 0.3 m with markhamia. Calliandra and sesbania reduced soil NO3 in the top 2 m by about 150 to 200 kg N ha−1 within 11 mo after establishment and effectively captured subsoil NO3. Fast‐growing trees with high root length densities can rapidly reduce subsoil NO3.

Root and inorganic nitrogen distributions in sesbania fallow, natural fallow and maize fields

One hypothesis for a benefit of integrating trees with crops is that trees with deep root systems can capture and “pump up” nutrients from below the rooting zone of annual crops. Few studies have compared both root and nutrient distribution for planted trees, crops and grassland vegetation. A field study was conducted on a Kandiudalfic Eutrudox in the highlands of western Kenya to measure rooting characteristics and distribution of inorganic N and water in three land-use systems (LUS): (i) Sesbania sesban (L.) Merr. fallow, (ii) uncultivated natural weed fallow and (iii) unfertilized maize (Zea mays L.) monoculture. The maximum rooting depth was 1.2 m in the maize LUS, 2.25 m in a 13-month-old natural fallow, and > 4 m in a 15-month-old sesbania fallow. Total root length was 1.26 km m-2 for the maize LUS, 5.98 km m-2 for the natural fallow, and 4.56 km m-2 to 4 m for the sesbania fallow. Root length to 1.2 m was greater (p < 0.01) for natural fallow than for maize and sesbania fallow. A considerable portion of the sesbania root length to 4 m was in the subsoil; 47% was at 1.2 to 4 m and 31% was at 2.25 to 4 m. Deep rooting of sesbania coincided with lower soil water below 2 m in the sesbania fallow than the natural fallow. Nitrate-N, but not ammonium-N, to 4 m was affected by LUS. Total nitrate to 4 m was 199 kg N ha-1 for the maize LUS, 42 kg N ha-1 for the natural fallow and 51 kg N ha-1 for the sesbania fallow. Soil nitrate in the maize LUS was highest at 0.3 to 1.5-m depth on this Oxisol with anion sorption capacity. No such accumulation of subsoil nitrate was present under sesbania and natural fallow.

Soil Nitrate and Water Dynamics in Sesbania Fallows, Weed Fallows, and Maize

We hypothesized that the integration of trees into agricultural land-use systems can reduce NO3 leaching and increase subsoil N utilization. A field study was conducted on a Kandiudalfic Eutrudox (Ochinga site) and a Kandic Paleustalf (Muange site) in the subhumid highlands of Kenya to measure changes in soil NO3 and water to 200-cm depth for one rainy season in four land-use systems (LUS): (i) planted tree fallow using Sesbania sesban (L.) Merr., (ii) unfertilized maize (Zea mays L.), (iii) weed fallow, and (iv) bare fallow. Subsoil (50-200 cm) NO3-N at the start of the season ranged from 58 to 87 kg ha-1 for the four LUS and two sites. In maize, subsoil NO3-N differed by <5 kg ha-1 between planting and harvest at both sites. In sesbania, subsoil NO3-N decreased by 22 kg ha-1 at both sites, whereas in weed fallow subsoil NO3-N decreased by 26 and 38 kg ha-1 at Ochinga and Muange, respectively. At both sites, subsoil water contents at the start of the season were similar in the four LUS; but at the end of the season, soil water at 100 to 200 cm was significantly lower for sesbania than for maize. Adsorption of NO3 increased with soil depth. Sorbed NO3 at 100 to 200 cm was about 60% in the Kandiudalfic Eutrudox and about 15% in the Kandic Paleustalf. Rotation of maize with either a sesbania fallow or a weed fallow can result in more effective subsoil NO3 and water utilization than maize monoculture.