Biomass and nutrient cycling in bamboo (Bambusa bambos) plantations of tropical areas

Biomass productivity and nutrient cycling in a Bambusa bambos plantation aged 4, 5, and 6 years were studied. The dry matter production of above-ground biomass increased progressively with age. Nutrient quantities in bamboo stands were in a range of 1–2 t ha^-1 for N and K, 0.5–1 t ha^-1 for Ca and Mg, and 0.1–0.2 t ha^-1 for P. Nutrient concentrations increased with the age of the plantation. About 10% year^-1 the nutrients present in the biomass of the bamboo stand are recycled to the soil by litter fall.     

Influence of management practices on soil organic matter,microbial biomass and cotton yield in Alabama's “Old Rotation”

The Old Rotation cotton experiment was designed to aid farm managers in implementing rotation schemes that not only increase yield, but also improve soil quality. Six different crop rotation treatments were imposed since 1896. Rotations were: IA, cotton (Gossypium hirsutum L.) grown every year without a winter legume and without N fertilization; IB, cotton grown every year with a winter legume and without N fertilization; IC, cotton grown every year without a winter legume and with 134 kg N as NH₄NO₃ ha^-1 year^-1; IIA, 2-year cotton-corn (Zea mays L.) rotation with a winter legume and without N fertilization; IIB, 2-year cotton-corn rotation with a winter legume and with 134 kg N ha^-1 year^-1 as NH₄NO₃; and III, 3-year cotton-corn- alternating soybean [Glycine max (L.) Merr.] or rye (Secale cereale L.) rotation with a winter legume and with 134 g N as NH₄NO₃ ha^-1 year^-1. Crimson clover (Trifolium incarnatum L.) was the winter legume cover crop. The 2-year cotton-corn rotation with a winter legume and with 134 kg N ha^-1 year^-1 (IIB) and the 3-year cotton-corn soybean/rye rotation with a winter legume and with 134 kg N ha^-1 year^-1 (III) had higher amounts of soil organic matter, soil microbial biomass C and crop yield than the other four treatments. The cotton grown every year without a winter legume or N fertilizer (IA) had a lower amount of soil organic matter, soil microbial biomass C and N and cotton seed yield than all other rotations. In 1988 and 1992 cotton seed and legume yield were correlated in positive, curvilinear relationships with soil organic matter (r ² ranged from 0.72 to 0.87). In most months, soil microbial biomass C and N was lower in the cotton grown every year without winter legumes or fertilizer (IA) than the other five rotations. In 1994, microbial biomass C and the C_mic:C_org ratio correlated in positive, curvilinear relationships with seed cotton yield (r ²=0.87 and 0.98, respectively). After 99 years of management the Old Rotation cotton experiment indicates that winter legumes increase amounts of both C and N in soil, which ultimately contribute to higher cotton yields. Microbial biomass C and the C_mic:C_org ratio are poor predictors of annual crop yield but may be an accurate indicator of soil health and a good predictor of long-term crop yield.     

Effect of various mulches on soil microarthropods under a maize crop

Soil microarthropod populations in maize plots mulched with the prunings of three woody agroforestry plant residues (Acioa barteri, Gliricidia sepium and Leucaena leucocephala) and two crop residues [maize (Zea mays) stover (leaves and stems) and rice (Oryza sativa) straw] were monitored throughout the 1991 growing season and compared with those in fallow, bare fallow and unmulched control plots with three rates of nitrogen application. The mean densities of detritivore and phytophage microarthropods in the experimental plots decreased in the following order: rice straw > Gliricidia prunings > Leucaena prunings > maize stover > Acioa prunings > control 2 (90 kg N ha^-1 year^-1) > fallow > control 3 (135 N) > control 1 (45N) > bare fallow. Mulching also affected the population dynamics of predatory microarthropods and omnivorous ants. The chemical composition of plant residues probably influenced the densities of detritivore and phytophage microarthropods, which in addition were also greatly influenced by microclimatic conditons imposed by vegetation cover. Extremely low densities of microarthropods were recorded in the bare fallow plots probably as a result of the combined effects of absence of plant residues and vegetation cover. The potential of mulching as a component of farming practice for increasing the density of microarthropods and for influencing their species composition and population dynamics in agroecosystems is discussed.     

Nitrogen fixation,biomass production,and nutrient uptake by annual Sesbania species in an alkaline soil

Summary In three field trials conducted during the summer season of 1986, 1987 and 1989 in an alkaline soil, 17 accessions of annual Sesbania spp. were evaluated for nodulation, N₂ fixation (acetylene reduction assay), dry weight of roots and shoots, woody biomass production, and nutrient uptake. At 50 days after sowing all the accessions were effectively nodulated (average 36.4 root nodules plant^-1) with a high nodule score (3.4). There was a lot of variation in nodule volume and mass and in acetylene reduction activity but not in N content (5.2%). N uptake in shoots, roots and nodules averaged 639, 31, and 13 mg plant^-1, respectively, and much of the fixed N remained in shoots. Accessions of S. cannabina complex performed better than others. S. rostrata had poor root nodulation but exhibited excellent stem nodulation (300 nodules plant^-1) even though not inoculated with Azorhizobium sp. Average concentrations of N, P, K, S, Ca, and Mg in the shoots were high, at 3.2, 0.28, 1.5, 0.28, 1.5, and 0.4% respectively, and Na was low (0.15%), reflecting the usefulness of Sesbania spp. as an integrated biofertilizer source. Green matter production was 26.0 Mg ha^-1 (5.9 Mg dry matter) and N uptake was 158 kg ha^-1, 54 days after sowing. Average woody biomass of six accessions at maturity, 200 days after sowing, was high (19.9 Mg ha^-1), showing its potential for shortterm firewood production. Total nutrient uptake for production of woody biomass (200 days of growth) was no more demanding than growing the plant to the green-manuring stage of 50–60 days' growth.     

Green manure production of Azolla microphylla and Sesbania rostrata and their long-term effects on rice yields and soil fertility

Summary Azolla spp. and Sesbania spp. can be used as green manure crops for wetland rice. A long-term experiment was started in 1985 to determine the effects of organic and urea fertilizers on wetland rice yields and soil fertility. Results of 10 rice croppings are reported. Azolla sp. was grown for 1 month and then incorporated before transplanting the rice and 3–4 weeks after transplanting the rice. Sesbania rostrata was grown for 7–9 weeks and incorporated only before transplanting the rice. Sesbania sp. grew more poorly before dry season rice than before wet season rice. Aeschynomene afraspera, which was used in one dry season rice trial, produced a larger biomass than the Sesbania sp. The quantity of N produced by the Azolla sp. ranged from 70 to 110 kg N ha^-1. The Sesbania sp. produced 55–90 kg N ha^-1 in 46–62 days. Rice grain yield increases in response to the green manure were 1.8–3.9 t ha^-1, similar to or higher than that obtained in response to the application of 60 kg N ha^-1 as urea. Grain production per unit weight of absorbed N was lower in the green manure treatments than in the urea treatment. Without N fertilizer, N uptake by rice decreased as the number of rice crops increased. For similar N recoveries, Sesbania sp. required a lower N concentration than the Azolla sp. did. Continuous application of the green manure increased the organic N content in soil on a dry weight basis, but not on a area basis, because the application of green manure decreased soil bulk density. Residual effects in the grain yield and N uptake of rice after nine rice crops were found with a continuous application of green manure but not urea.     

Nitrogen and phosphorus use in maize sole cropping and maize/cowpea mixed cropping systems on an Alfisol in the northern Guinea Savanna of Ghana

Summary The use of N and P by mixed and by sole cropping (crop rotation) of maize and cowpeas were compared in a field experiment on an Alfisol at the Nyankpala Agricultural Experiment Station in the northern Guinea Savanna of Ghana, using two levels of N (0 and 80 kg N ha^-1 year^-1 as urea) and P application (0 and 60 kg P ha^-1 year^-1 as Volta phosphate rock). Maize grain yields were significantly reduced in the mixed cropping system. This yield difference became smaller with the application of N and P fertilizer. The N and P concentrations in maize ear leaves at silking indicated that a deficiency in N and P contributed to the maize yield depression in mixed cropping. Competition for soil and fertilizer N between maize and cowpeas was suggested by: (1) A similarity in total N uptake between the two cropping systems; (2) efficient use of soil nitrate by the cowpeas; and (3) low N₂ fixation by the cowpeas, calculated with the aid of an extended-difference method. In general, N₂ fixation was low, with the highest values in the sole cropping (53 kg ha^-1) and a substantial reduction in the mixed cropping system. The application of N fertilizer further reduced N₂ fixation. This was substantiated by nodule counts. The lower N₂ fixation in the mixed cropping system was only partly explained by the lower density of cowpeas in this system. In addition, dry spells during the cropping season and shading by the maize component could have reduced the nodulation efficiency. No N transfer from the legume/rhizobium to the non-legume crop was observed. Impaired P nutrition in the mixed compared with the sole-cropped maize might have been due to less P mobility in the soil. This was indicated by lower soil moisture contents in the topsoil under mixed cropping, especially during the dry year of 1986. The results show that mixed cropping of maize and cowpeas did not lead to improved use of soil and fertilizer N and P or to an enhanced N₂ fixation. On the contrary, an annual rotation of maize and cowpeas was clearly superior.     

Diversity of diazotroph populations in the rhizosphere of maize (Zea mays L.) growing on different French soils

Summary We studied the dominant diazotrophs associated with maize roots and rhizosphere soil originating from three different locations in France. An aseptically grown maize plantlet, the spermosphere model, was used to isolate N₂-fixing (acetylene-reducing) bacteria. Bacillus circulans was the dominant N₂-fixing bacterium in the rhizosphere of maize-growing soils from Ramonville and Trogny, but was not found in maize-growing sandy soil from Pissos. In the latter soil, Enterobacter cloacae, Klebsiella terrigena, and Pseudomonas sp. were the most abundant diazotrophs. Azospirillum sp., which has been frequently reported as an important diazotroph accociated with the maize rhizosphere, was not isolated from any of these soils. The strains were compared for their acetylene-reducing activity in the spermosphere model. The Bacillus circulans strains, which were more frequently isolated, also exhibited significantly greater acetylene-reducing activity (3100 nmol ethylene day^-1 plant^-1) than the Enterobacteriaceae strains (180 nmol ethylene day^-1 plant^-1). This work indicates for the first time that Bacillus circulans is an important maizerhizosphere-associated bacterium and a potential plant growth-promoting rhizobacterium.     

协助自然修复微量元素污染酸性土壤研究：8年田间试验

There are many remediation techniques for organic contaminated soils,but relatively few of them are applicable to trace elementcontaminated soils.A field experiment was carried out to investigate assisted natural remediation（ANR） of an acid soil contaminated by As,Cd,Cu,Zn and Pb using one inorganic amendment,sugar beet lime（SL）,and two organic amendments,biosolid compost（BC）and leonardeite（LE）.The experiment was arranged in a completely randomized block design with four treatments in three replicates:1) a non-amended control（NA）;2) SL amended at 30 Mg ha^-1 year^-1;3) BC amended at 30 Mg ha^-1 year^-1 and 4) LE amended at 20 Mg ha^-1 year^-1 plus SL amended at 10 Mg ha^-1 year^-1（LESL）.The amended plots received two doses of each amendment（DO2）:one in October 2002 and another in October 2003.The plots were then divided in half into two subpolts and one subplot received another two doses of the same amendments（DO4） in October 2005 and October 2006.In 2011,the pH values of the amended soils were greater than that of the NA soil,with the SL-amended soil showing the highest pH.Total organic carbon（TOC） was also greater in the amended soil,and greater with DO4 than with DO2.Amendments reduced the concentrations of 0.01 mol L^-1 CaCl₂-extractable Cd,Cu and Zn,especially in the SL-amended soil.Plant cover of colonizing vegetation was enhanced by amendments,but was independent of amendment doses.Changes in soil properties from 2003 to 2011 showed that the first amendment application of DO2 caused a high differentiation between all the amendment treatments and the NA treatment.After the second application of DO2,soil pH and TOC continued increasing slowly.Further two applications of amendments（DO4） caused differences only in the organic treatments.Plant cover increased over time in all the treatments including NA.It could be concluded that the slow and steady natural remediation of this soil could be enhanced by amendment application（ANR）.     

Amelioration of water stress effects on wetland rice by urea-N,plant growth regulators,and foliar spray of a diazotrophic bacterium Klebsiella sp.

Some of the measures suggested for amelioration of drought effects include application of N fertilizer and plant growth regulators (PGRs). Since N₂-fixing bacteria produce plant growth substances (PGRs), the effect of foliar application of an active strain of Klebsiella sp. (KUPOS) on IR-50 rice was examined using three foliar sprays applied at 10-day intervals. Irrigation once every 3 days was essential for plant growth. Application of KUPOS and 40 kg N ha^-1 improved grain yield of acutely water stressed plants from 330 kg ha^-1 to more than 1300 kg ha^-1 along with an improvement in several growth variables and yield determinants. Indole acetic acid, kinetin and GA₃, in a mixture of 10^-4 M of each, were less effective than KUPOS in alleviating stress effects. The adverse effects of water stress on respiration and photosynthesis as indicated by CO₂ exchange were also alleviated by these treatments. While uptake of K, Mg, Ca, Fe and Mo was increased, Na content decreased, accompanied by an increase in proline content. The order of effectiveness of the treatments was 40 kg N ha^-1 >KUPOS>PGRs.     

Nitrogen use in maize-grain legume cropping systems in semi-arid Kenya

Locally suitable cultivars of maize, beans, and cowpeas were grown in field experiments for four seasons in semi-arid Kenya. For three seasons, the dry matter production and grain yield of maize and beans were not increased by N fertilizer additions up to 120 kg N ha^-1. Fertilizer recoveries measured by ¹⁵N isotope dilution techniques were low, less than 20%. Inoculated and uninoculated beans failed to fix N₂. By contrast the cowpea derived 50% of its N from fixation, equivalent to 197 kg N ha^-1. The N content of the grain generally exceeded 40 kg N ha^-1, and the N content of the seeds from the grain legumes were greater than those from the cereals. Large inputs of N fertilizer or N by fixation are required if maize-grain legume cropping system in semiarid Kenya are to be sustained in the long term.     

Biomass production and N fixation of five Mucuna pruriens varieties and their effect on maize yields in the forest zone of Cameroon

Mucuna has been tested intensively in past years as green manure for intensive maize production in West Africa. However, information is missing about the yield effect of different existing mucuna varieties. Five Mucuna pruriens varieties were grown for 40 weeks followed by sole maize (Zea mays L.) in order to determine differences in biomass production, nitrogen fixation, and effects on maize yield. Mucuna varieties differed in length of growing period, total biomass production (5.9—8.8 Mg ha^—1), seed production (0.65—1.3 Mg ha^—1), nitrogen (N) uptake (147—222 kg ha^—1), N fixation (87—171 kg ha^—1), and the amount of N retained in residues (138—218 kg ha^—1). The grain yield of maize grown immediately after the short mucuna fallow was significantly higher after mucuna vars. jaspaeda (4.60 Mg ha^—1), utilis (3.49 Mg ha^—1), and cochinchinensis (3.44 Mg ha^—1), compared with a non‐fertilized control (1.93 Mg ha^—1) which had a maize crop and vegetation regrowth before. After mucuna vars. ghana and veracruz, 2.90 and 2.65 Mg ha^—1 of maize grain were produced, respectively. No significant correlation between mucuna biomass and its N uptake and maize grain yield was found, whereas maize stover yield showed a significant positive correlation. Application of 30, 60, and 90 kg ha^—1 N as <?tw=98%>urea on sub‐plots of the control yielded 2.20, 3.19, and 3.46 Mg ha^—1 <?tw>of maize grain in the first year. Only the difference between 0 and 90 kg ha^—1 N was significant. Fertilizer N equivalent values for mucuna varieties ranged from 41 to 148 kg ha^—1. The yield advantage of vars. jaspaeda, utilis, and cochinchinensis versus the control without N fertilizer application was confirmed in the following year, with no significant difference in maize grain yield between mucuna and the control with N fertilizer application.<?show $6#>     

Nitrogen and phosphorus fertilization leads to soil arbuscular mycorrhizal fungal diversity changes and rainfed crop yield increase on the Loess Plateau of China: A 37-year study

More than 80% of plants form mutualistic symbiotic relationships with arbuscular mycorrhizal fungi (AMF), and the application of fertilizers, such as nitrogen (N) and phosphorus (P) fertilizers, is a common agricultural management practice to improve crop yield and quality. However, the potential effects of long-term N and P fertilization on the AMF community in the rainfed agricultural system of the Loess Plateau of China are still not well understood. In this study, a long-term field experiment was conducted based on orthogonal design, with three N levels (0, 90, and 180 kg ha^-1 year^-1) and three P levels (0, 90, and 180 kg ha^-1 year^-1) for wheat fertilization. Changes in AMF community and correlations between AMF community composition, soil environmental factors, and wheat yield component traits were analyzed using traditional biochemical methods and high-throughput sequencing technology. The results showed that long-term N and P addition had a significant effect on the AMF community structure and composition. Nitrogen application alone significantly reduced the richness and diversity of AMF community, whereas the combined application of N and P significantly increased the richness and diversity of AMF community. The AMF community was driven mainly by soil available P, total P, and pH. There was a significant positive correlation between Glomus abundance and wheat yield and a significant negative correlation between Paraglomus abundance and wheat yield. Long-term N and P addition directly increased crop yield and affected yield indirectly by influencing soil chemical properties and the AMF community. Combined application of N and P both at 90 kg ha^-1 year^-1 could improve the ecological and physiological functions of the AMF community and benefit the sustainable development of rainfed agriculture.     

Establishment of the Critical Limit of Soil-Available Phosphorous for Maize Production in Low Acidic Ultisols of West Africa

The relationship between yields and the Bray I–extractable phosphorous (P) was studied on a 21-year-old experiment with maize (Zea mays) in an acid Ultisol of West Africa. Chemical fertilizer alone decreased soil organic carbon and increased soil acidity. The applications of cattle manure maintained the highest level of soil organic carbon, and soil acidity did not decrease. The extractable P varied from 4 mg kg^–1 (original soil) to 25 mg P ha^–1 in fertilized treatments. A linear correlation (P < 0.01; r = 0.85) was found between soil extractable P and maize yields. Each unit increase in soil extractable P tallied with 16% of yield increase. A critical limit of soil extractable P of 15.6 mg P kg^–1 was identified. Below this limit, maize produced less than 2500 kg ha^–1, 80% of the maximum yield. Responses to P fertilizer were not expected above this limit.     

Organic residues affect phosphorus availability and maize yields in a Nitisol of western Kenya

 The effects of organic residues and inorganic fertilizers on P availability and maize yield were compared in a Nitisol of western Kenya. Leaf biomass of Calliandra calothyrsus, Senna spectabilis, Croton megalocarpus, Lantana camara, Sesbania sesban, and Tithonia diversifolia were incorporated into the soil at 5 Mg ha^–1 for six consecutive seasons in 3 years and responses compared with those following the application of 120 kg N ha^–1, 0 kg P ha^–1 (0P); 120 kg N ha^–1, 10 kg P ha^–1; and 120 kg N ha^–1 25 kg P ha^–1 as urea and triple superphosphate (TSP); K was supplied in all treatments. Addition of Tithonia, Lantana and Croton increased soil resin-extractable P over that of fertilizer-amended soil throughout the first crop, but the amounts in the former treatments became similar to those for soils amended with inorganic fertilizers for subsequent crops. Addition of Sesbania, Calliandra and Senna had a similar effect on resin P as inorganic fertilizers. Total maize yields after six seasons were tripled by the application of Tithonia compared to 0P, and were higher than those of the Calliandra, Senna, Sesbania and Lantana treatments, and similar only to that of the Croton treatment. P recovered in the above-ground biomass and resin P, immediately after the implementation of the treatments, was higher in the Senna, Sesbania, Croton, Lantana and Tithonia (35–77%) treatments than in the inorganic fertilizer treatments (21–27%). The P content of organic residues, and the soluble C:total P ratio, were the main residue parameters predicting soil P availability and maize yield. All organic residues used in this study can replace inorganic fertilizers for the enhancement of P availability and maize production, while an additional benefit could be obtained from the use of Croton, Lantana and Tithonia. Received: 19 January 2000     

Grain yield response of Zea mays (hybrid AE 703) to Azotobacter chroococcum H23

Summary Field experiments were conducted during 1985 and 1986 to examine the effect of Azotobacter chroococcum on the grain yield of maize. Application of 40 kg N ha^–1 plus A. chroococcum caused a significant increase in maize yield. Azotobacter inoculation was more efficient at lower doses (40 kg N ha^–1) than at high doses (80 kg N ha^–1) of urea.     

Single and double inoculation with Azospirillum/Trichoderma: the effects on dry bean and wheat

A plant growth-promoting rhizobacterium (Azospirillum brasilense Sp7) and a bio-control fungus, which can solubilize insoluble phosphorus (Trichoderma harzianum Rifai 1295-22), were evaluated for their single and combined effects on dry bean (Phaseolus vulgaris) and wheat (Triticum aestivum L.) grown in soil. A pot experiment with bean and a field experiment with both bean and wheat were established. In contrast to single inoculation of Trichoderma, the single inoculation of Azospirillum and the double inoculation did not significantly (P >0.05) increase nodule numbers and nodule mass at 45 days after planting in pot grown beans. However, the Azospirillum inoculation with supplementary phosphorus significantly (P <0.05) increased nodule mass. There were no significant (P >0.05) differences among the inoculation treatments for plant dry weight, total plant nitrogen, and total plant phosphorus at 45 days after planting in both pot and field experiments with bean. However, the combined inoculation and rock phosphate application at 1 Mg ha^–1 significantly (P <0.05) increased bean seed yield, total seed nitrogen and phosphorus in the bean field trial. This treatment more than doubled the mentioned properties compared to the control. The microbial inoculations, with the exception of the combined inoculation, significantly (P <0.05) increased total seed nitrogen, but never affected seed yield in the wheat field trial (P >0.05). The combined inoculation improves many plant and yield parameters and, therefore, has some advantages over single inoculation provided that rock phosphate was supplied at an amount not exceeding 1 Mg ha^–1. Higher rock phosphate application rates decreased many plant and yield parameters in our study.This work was carried out at Gaziosmanpaa University, Tokat, Turkey.     

华北地区采用无机氮测试和植株速测进行夏玉米氮肥推荐

A field experiment with a split-plot design was carried out at Dongbeiwang Farm in Beijing Municipality to establish reliable N fertilizer recommendation indices for summer maize (Zea mays L.) in northern China using the soil Nmin(mineral N) test as well as the plant nitrate and SPAD (portable chlorophyll meter readings) tests. The results showed that Nrnin sollwert (NS) 60 kg N ha^-1 at the third leaf stage and N rate of 40 to 120 kg N ha^-1 at the tenth leaf stage could meet the N requirement of summer maize with a target yield of 5.5-6 t ha^-1. Sap nitrate concentrations and SPAD chlorophyll meter readings in the latest expanded maize leaves at the tenth leaf stage were positively correlated with NS levels, indicating that plant nitrate and SPAD tests reflected the N nutritional status of maize well. Considering that winter wheat subsequently utilized N after the summer maize harvest, the 0-90 cm soil Nmin (74 kg N ha^-1) and apparent N loss (12 kg N ha^-1) in the NS60＋40 treatment were controlled at environmentally acceptable levels. Therefore NS60＋40, giving a total N supply of 100 kg N ha^-1, was considered the optimal N fertilizer input for summer maize under these experimental conditions.     

Gut load and food-retention time in the earthworms Lumbricus festivus and L. castaneus: A field study

Summary Lumbricus festivus and L. castaneus consume dung. In the field, below cow pats, their gut loads were about 0.15 and 0.14 g dry weight g^-1 ash-free dry weight of worm, respectively, but in free soil the loads were higher, about 0.21 and 0.19 g g^-1 ash-free dry weight of worm. The gut contents of dung were lighter than the total ingested material, at about 0.10 and 0.07 g dry weight g^-1 ash-free dry weight of worm, respectively. Field experiments showed that the retention time of dung ranged from >9 to 15 h for L. festivus, and from >3 to 6 h for L. castaneus. The experiments also indicated that L. festivus exploited 20- and 36-day-old dung in different ways, since the gut load was lower in those worms consuming 20-day-old dung than in those consuming 36- to 40-day-old dung. On the basis of these results the calculated consumption rate for L. festivus is 0.08 g dung day^-1 g live weight of worm^-1, and for L. castaneus 0.15 g dung day^-1, with retention times assumed to be at maximum, 15 h, for L. festivus and 6 h for L. castaneus. These calculations indicate that our field population of worms (75 g m^-2) consumes 10–15 t dung ha^-1 180 days^-1, corresponding to the amount of dung produced by 2–3 dairy cows.     

Short- and long-term effects of the endogeic earthworm Millsonia anomala (Omodeo) (Megascolecidæ, Oligochæta) of tropical savannas,on soil organic matter

Summary This study aimed to establish the effects of Millsonia anomala, a tropical geophagous earthworm common in the humid savannas of Lamto (Ivory Coast), on soil organic matter dynamics over different time scales under laboratory conditions. The texture of casts produced by the worms fed on a shrub savanna soil was not significantly different from that of the soil, which showed that M. anomala ingested soil particles without selection. Physical fractionation of soil organic matter showed that the coarse organic fraction (250–2000 m) was depleted by 25–30% in fresh casts compared to the control noningested soil; this was mainly due to a fragmentation of coarse organic debris. Incubation of casts and a 2-mm sieved control soil under laboratory conditions for more than 1 year showed that the C mineralisation rate was almost four times lower in the casts (3% year^-1) than in the control soil (11% year^-1). We therefore concluded that on a long time scale M. anomala populations may significantly reduce the decomposition rate of soil organic matter in Lamto savannas.     

Mineral N dynamics in bare and cropped Leucaena leucocephala and Dactyladenia barteri alley cropping systems after the addition of 15N-labelled leaf residues

In tropical cropping systems with few external inputs, efficient management of mineral N derived from added organic residues is essential for the proper functioning of the system. We studied the dynamics of mineral nitrogen (N) in the top 100 cm of soil with a system of tensiometers and suction cups after applying ¹⁵N-labelled Leucaena leucocephala and Dactyladenia barteri residues to bare and cropped microplots installed in the respective alley cropping systems, and followed the fate of the N for two maize-cowpea rotations (1992 and 1993). Fifty days after applying the residues (DDA), 20% of the added residue N was found in the soil profile of the bare Leucaena treatment, and 5% under Dactyladenia, compared with 5% and 1%, respectively, where cropped. All values decreased to about 1% after 505 days. In the cropped soil, no mineral N derived from the residues was lost by leaching during the first 6 weeks. As the maize grew, the soil profile was gradually depleted of nitrate to near Zero in the Dactyladenia treatment, whereas during the cowpea season the amount of nitrate N increased to 36 kg N ha^?1 for the Leucaena treatment, and 26 kg N ha^?1 for the Dactyladenia treatment. The soil of the bare microplots contained substantially more nitrate N (98 and 47 kg N ha^-1 during the first year on average, under Leucaena and Dactyladenia, respetively) than that of the cropped microplots, except during the 1993 cowpea season. Nitrate residing in the subsoil (80–100 cm) in the bare treatments was not readily leached to deeper soil. The risk of losses of native mineral N was greatest during the first 50 DAA and to a lesser extent during the cowpea seasons. Improved management of the hedgerows could increase the potential of the hedgerow trees to recycle mineral N.