Soil properties and crop performance on a kaolinitic Alfisol after 15 years of fallow and continuous cultivation

A long-term field experiment was established on a kaolinitic Alfisol in Ibadan, Nigeria, in 1972. The land was cleared manually from secondary forest and used for (i) continuous no-till cropping with maize (Zea mays L.) and maize/cassava (Manihot esculenta Crantz) intercropping, (ii) planted fallow of guinea grass (Panicum maximum Jacq.), leucaena (Leucaena leucocephala de Wit), and pigeon pea (Cajanus cajan Millsp.), and (iii) natural bush regrowth in a randomized complete block design with three replications. At the end of 15 years, the fallow plots were cleared manually and cropped with maize for three years. The chemical and physical soil properties and crop performance of the newly-cleared plots were compared with those under 15 years of continuous cultivation. A total of 26 woody species were identified on the bush regrowth plots. Above-ground biomass accumulation of the bush plots was 157 Mg ha^-1 containing 1316 kg N ha^-1. Guinea grass, leucaena and natural bush regrowth plots had comparable organic C concentrations (approximately 20 g kg^-1) in the surface soil (0 to 10 cm) after 15 years. The organic C concentration in the surface soil under pigeon pea was the lowest (9.5 g kg^-1) among the four fallow treatments. Soil under 15 years of continuous no-till maize with and without residue mulch, respectively, contained approximately half (10 g kg^-1) and a quarter (5.7 g kg^-1) of the organic C under natural bush or guinea grass fallow. The levels of exchangeable Ca, K, Mg and effective cation exchange capacity (ECEC) were lower in the soils under continuous cultivation than in those under natural bush and planted fallow. Soil acidification occurred in soils under continuous cropping as depicted by the lower pH values and greater exchangeable Al and Mn concentrations compared to the fallow plots. Grain yield of maize (3 to 5 Mg ha^-1) without fertilizer application in the plots newly cleared from natural bush, guinea grass and leucaena fallow was comparable with that of continuous no-till maize with residue mulch and chemical fertilizer (N, P, K, Mg, Zn) applications. Among the four fallow treatments, maize grain and stover yields were the lowest in plots cleared from pigeon pea fallow.     

The effect of fallow and continuous cultivation on the chemical and physical properties of an alfisol in western Nigeria

Summary Soil properties under continuous cropping were compared with those under planted fallows and natural bush regrowth for three years after forest clearing. The cropping treatments consisted of continuous maize with and without stover returned as surface mulch, continuous soybean, and maize and cassava intercropped. The fallow treatments included pigeon pea, leucaena, Guinea grass and natural bush regrowth.In the continuous soybean and unmulched maize plots, soil organic matter and pH declined rapidly; whereas the mulched maize plots maintained a soil organic matter level comparable to the fallow treatments. To maintain soil organic matter in the surface soil at a level comparable to soil under secondary forest, two to three applications of a total amount of 16 MT/ha/annum of dry plant materials (maize stover or grass) are required when the material is applied as surface mulch.In the cropped plots, favorable physical characteristics in the surface soil were also maintained when sufficient plant residue was returned; whereas the deterioration of subsoil structure of the forest soil occurred in all cropping treatments.Guinea grass fallow has a distinct advantage in recycling mineral nutrients and maintaining soil physical properties and organic matter. It is suggested that soils may be planted with a combination of Guinea grass and pigeon pea fallow for one or two years after three or four years of arable cultivation.IITA Journal Paper No. 65 IITA Journal Paper No. 65     

Estimates of the residual nitrogen benefit of groundnut to maize in Northeast Thailand

Four cultivars of groundnut were grown in upland soil in Northeast Thailand to study the residual benefit of the stover to a subsequent maize crop. An N-balance estimate of the total residual N in the maize supplied by the groundnut was made. In addition three independent estimates were made of the residual benefits to maize when the groundnut stover was returned to the land and incorporated. The first estimate (Estimate 1) was an N-balance estimate. A dual labelling approach was used where ¹⁵N-labelled stover was added to unlabelled microplots (Estimate 2) or unlabelled stover was added to ¹⁵N-labelled soil microplots (Estimate 3). The nodulating groundnut cultivars fixed between 59–64% of their nitrogen (as estimated by the ¹⁵N isotope dilution method using non-nodulating groundnut as a non-fixing reference) producing between 100 and 130 kg N ha^-1 in their stover. Although the following maize crop suffered from drought stress, maize grain N and dry weights were up to 80% and 65% greater respectively in the plots where the stover was returned as compared with the plots where the stover was removed. These benefits were comparable with applications of 75 kg N ha^-1 nitrogen in the form of urea. The total residual N estimates of the contribution of the nodulated groundnut to the maize ranged from 16.4–27.5 kg N ha^-1. Estimates of the residual N supplied by the stover and fallen leaves ranged from 11.9–21.3 kg N ha^-1 using the N-balance method (Estimate 1), from 6.3–9.6 kg N ha^-1 with the labelled stover method (Estimate 2) and from 0–11.4 kg N ha^-1 with the labelled soil method. There was closest agreement between the two ¹⁵N based estimates suggesting that apparent added nitrogen interactions in these soils may not be important and that N balance estimates can overestimate the residual N in crops following legumes, even in very poor soils. This work also indicates the considerable ability of local groundnut cultivars to fix atmospheric nitrogen and the potential benefits from returning and incorporating legume residues to the soil in the upland cropping systems of Northeast Thailand. The applicability of the ¹⁵N methodology used here and possible reasons for the discrepancies between estimates 1, 2 and 3 are discussed.     

Nitrogen Release from Decomposing Residues of Leguminous Cover Crops and their Effect on Maize Yield on Depleted Soils of Bukoba District, Tanzania

Nitrogen release patterns from decomposing shoot residues of Tephrosia candida, Crotalaria grahamiana, Mucuna pruriens, Macrotyloma axillare, Macroptillium atropurpureum and Desmodium intortum were studied in the laboratory for a period of 22 weeks in a sandy clay soil and 10 weeks in a clay soil using a leaching tube technique. The residual effect of soil incorporated shoot residues of T. candida, T. vogelii, C. grahamiana, M. pruriens and C. juncea on maize yield was evaluated at four sites each in the high rainfall zone (mean precipitation 2100 mm year⁻¹) and low rainfall zone (mean precipitation 800 mm year⁻¹) of Bukoba District, Tanzania. N mineralised from the legume residues ranged from 24 to 61% of the initial N after 22 weeks in a sandy clay soil and −1 to 34% after 10 weeks in a clay soil. The N mineralisation rates of the residues decreased in both soils in the order M. atropurpureum>M. axillare>C. grahamiana>D. intortum>T.␣candida>M. pruriens and were mostly strongly related to (polyphenols+lignin)-to-N ratio, lignin-to-N ratio and lignin. Relative to the control, legume residues resulted in two and threefold increase in maize grain yield i.e. from 1.1 to 3.2 Mg ha⁻¹ and from 1.4 to 3.8 Mg ha⁻¹ in a high and low rainfall zone respectively. However, maize yield response to legume residues was limited when compared with application of 50 kg N ha⁻¹ of mineral fertiliser. The % fertiliser equivalency (%FE) of legumes ranged between 25 and 59% with higher values recorded with C. grahamiana. At harvest, apparent N recoveries in maize ranged between 23 and 73% of the N applied in the legume residues. Highest recovery was found with application of C. grahamiana and least recovery from T. candida residues. These results suggested that application of legume residues alone might not be sufficient to meet N requirements and to achieve the yield potential of maize crop in Bukoba soils unless supplemented with small doses of mineral fertilisers.     

Yields and accumulations of N and P in farmer-managed intercrops of maize–pigeonpea in semi-arid Africa

Maize (Zea mays L.) is a major staple food in Sub-Saharan Africa but low soil fertility, limited resources and droughts keep yields low. Cultivation of maize intercropped with pigeonpea (Cajanus cajan L. Millsp.) is common in some areas of eastern and southern Africa. The objectives of this study were (1) to investigate dry matter, nitrogen (N) and phosphorus (P) accumulation in different plant components of maize–pigeonpea intercropping systems and (2) to report the effects of the intercrops on soil fertility. Maize–pigeonpea intercrops were compared to sole maize grown using farmersȁ9 practices. Intercropping maize and pigeonpea increased (P < 0.05) total system yield compared to sole maize in terms of biomass, N and P accumulation. Pigeonpea planted in maize did not reduce (P < 0.05) the accumulation of dry matter, N nor P in the maize grain. The harvest indices of maize, calculated on basis dry matter, N or P did not differ either (P < 0.05). Total soil C and N contents and inorganic N content, nitrate and ammonium, were not affected by two seasons of maize–pigeonpea intercropping compared to sole maize (P > 0.11). Nitrate and ammonium levels in soil were still not affected by the treatments after the soils were incubated in anaerobic conditions for 8 days at 37°C (P > 0.11). However, pigeonpea added up to 60 kg of N ha⁻¹ to the system and accumulated up to 6 kg of P ha⁻¹ and only 25% of this N and P were exported in the grain. In conclusion, beside the added grain yield of pigeonpea in the intercropped systems, pigeonpea increased the recirculation of dry matter, N and P, which may have a long-term effect on soil fertility. Furthermore, the stems from pigeonpea contributed to household fuel wood consumption. The intercropped system thus had multiple benefits that gave significant increase in combined yield per unit area without additional labour requirements. The main requirement in order to up-scale the maize–pigeonpea intercropping approach is sufficient supply of high-quality pigeonpea seeds.     

入侵植物飞机草与4种牧草的竞争效应

【背景】飞机草是我国危害最严重的入侵植物之一,目前仍缺乏可持续的控制手段。【方法】运用De Wit取代试验研究法,设置2株·盆-1(42.42株·m~(-2))、4株·盆-1(84.84株·m-2)和8株·盆~(-1)(169.68株·m~(-2))3种密度,分别研究杂交狼尾草、木豆、山毛豆和宽叶雀稗与飞机草的竞争效应,以明确4种牧草对飞机草的替代控制潜力。【结果】3种密度下,杂交狼尾草和木豆均可以显著抑制飞机草的生长,其竞争平衡指数显著大于0,说明杂交狼尾草和木豆的竞争力均大于飞机草;山毛豆和宽叶雀稗的相对产量均显著小于1,其竞争平衡指数均显著小于0,说明山毛豆和宽叶雀稗的竞争力小于飞机草。【结论】杂交狼尾草和木豆可用作飞机草的替代控制植物。     

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.     

Dry season groundnut stover management practices determine nitrogen cycling efficiency and subsequent maize yields

It is generally thought that grain legume residues make a substantial net N contribution to soil fertility in crop rotation systems. However, most studies focus on effects of residues on crops immediately sown after the legume crop while in fact in many tropical countries with a prolonged dry season there is a large gap before planting the next crop with potential for nutrient losses. Thus the objectives of this study were* to improve the efficiency of groundnut (Arachis hypogaea L.) stover-N (100 kg N ha ^–1) recycling by evaluating the effect of dry season stover management, i.e. surface application and immediate incorporation after the legume crop or storage of residues until next cropping in the rainy season. N dynamics (litterbags, mineral N, microbial biomass N, N ₂O emissions) were monitored and ¹⁵N labelled residues were applied to assess the fate of residue N in the plant–soil (0–100 cm) system during two subsequent maize crops. Recycling groundnut stover improved yield of the subsequent maize (Zea mays L.) crop compared to treatment without stover. A higher N recycling efficiency was observed when residues were incorporated (i.e. 55% total ¹⁵N recovery after second maize crop) than when surface applied (43% recovery) at the beginning of the dry season. This was despite the faster nitrogen release of incorporated residues, which led to more mineral N movement to lower soil layers. It appears that a proportion of groundnut stover N released during the dry season was effectively captured by the natural weed population (54–70 kg N ha ^–1) and subsequently recycled particularly in the incorporation treatment. Despite the presence of weeds major leaching losses occurred during the onset of the rainy season while N ₂O emissions were relatively small. There was a good correlation between soil microbial biomass N and first crop maize yield. Incorporation of groundnut residues led to small increases in economic yield, i.e., 3120 versus 3528 kg ha ^–1 over two cropping cycles in the surface versus incorporation treatments respectively, with corresponding residue ¹⁵N uptakes of 4 and 8%, while ¹⁵N recovery in water stable aggregates (9–15%) was not significantly different. In contrast, when stover was removed and applied before the first crop, yield benefits were highest with cumulative maize yields of 4350 kg ha ^–1 and residue utilization of 12%. However, N recycling efficiency was not higher than in the early incorporation treatment due to an asynchrony of N release and maize N demand during the first crop.     

Cultivar variation in the effect of chlorsulfuron in depressing the uptake of copper in wheat

The application of herbicides has induced symptoms of nutrient deficiencies under some circumstances. This glasshouse study examined the effect of chlorsulfuron on the uptake and utilization of copper (Cu) in four cultivars of wheat plants (Triticum aestivum L. cvs. Kulin, Cranbrook, Gamenya and Bodallin) on a Cu-responsive soil. Application of chlorsulfuron depressed the concentration of Cu in wheat plants receiving either inadequate or adequate Cu. In plants with inadequate Cu supply, chlorsulfuron increased the severity of Cu deficiency. Shoot weight was markedly decreased by chlorsulfuron at all levels of Cu, through decreasing the number of tillers and the elongation of leaves. This decreased growth of shoots occurred prior to the effect on Cu concentration in tissues. The retranslocation of Cu in old tissues over time was unaffected by chlorsulfuron. In all wheat cultivars, the decreased growth of shoots were correlated with the concentration of Cu in the youngest fully emerged leaf blade with critical levels of 1.6−1.7 at day 25 and 0.9−1.0 μg g⁻¹ d. wt. at day 60. The application of chlorsulfuron tended to increase the critical level at day 25 but not at day 60. In addition, Kulin seems to be most, and Cranbrook least, sensitive to chlorsulfuron. This sensitivity was associated with the sensitivity of the cultivars to Cu deficiency. It is suggested that chlorsulfuron application induces Cu deficiency in wheat plants mainly due to effects on the uptake of Cu. This revised version was published online in June 2006 with corrections to the Cover Date.     

Quantities of fixed N and effects of grain legumes on following maize,and N and P status of soil as indicated by isotopes

Summary Two consecutive field experiments, using¹⁵N and³²P, were conducted at the National Corn and Sorghum Research Center, Thailand, to quantify N₂ fixed by mungbean, soybean and peanut and to examine effects of the legumes on the yields of succeeding maize and on status of N and P in soils during the following season. An early sorghum, non-nodulating soybean and maize which were used as standard crops in quantifying N₂ fixed by mungbean, soybean, and peanut, respectively, gave statistically comparable A-values for soil N though sorghum tended to give lower value than the other crops did. Amounts of fixed N₂ were 37.5, 119.0 and 150 kg/ha for mungbean, soybean and peanut, respectively. Plots previously grew legumes yielded higher grain and stover weights and higher N and P uptake of maize than those previously grew maize. There were no significant differences among plots previously grew different legumes. A-values, in most cases, did not differentiate the effects of previous legumes from those of previous maize. However, changes in N and P status of soil, in most cases, were too small to produce A-values changes that were large enough to outrun the experimental errors.     

Effect of Drought and Weed Management on Maize Genotypes and the Tensiometric Soil Water Content of an Eutric Nitisol in South Western Nigeria

In the dry savannas of West and Central Africa, where low soil fertility, unpredictable rainfall, weed competition and recurrent drought are major constraints to maize production, the development of tropical maize genotypes with high and stable yields under drought and low-nitrogen condition is very important since access to these improved genotypes may be the only affordable alternative to many small scale farmers. Field trials were conducted in 2002 and 2003 at Ikenne southwestern Nigeria to investigate the effect of weed pressures and drought stress on 2 maize (Zea mays L.) hybrids (9134-14, 9803-9) and 2 open-pollinated varieties (STREVIWD, IYFDCO1). Irrigation was withdrawn 4 weeks after planting (about four weeks to mid-flowering) in the drought stress while the adjacent watered treatment had irrigation throughout the growing period. The weed pressures were the completely weeded plots (hand weeding every week) and weedy plots (weeded once, 2 weeks after planting). The experiment was a split plot in a randomized complete block design with four replicates. Drought stress reduced the stover weight and grain yield of the maize cultivars by 6% and 34% respectively. Weed-free plots had maize with higher agronomic traits than unweeded treatments. Hybrid 9803-9 was more susceptible to drought and weed stress as indicated in the stover weight and grain yield. STREVIWD an open-pollinated variety (OPV) and Hybrid 9134-14 had superior performances in terms of grain yield and shorter anthesis silking interval. Soil moisture content was higher in the unweeded plots while the uptake of moisture was highest in drought susceptible hybrid 9803-9. Irrespective of the genotypes, maize (hybrid and OPV) was more tolerant to drought in a weed-free environment than in unweeded conditions. There existed a negative but significant correlation between weed biomass and chlorophyll content (−0.29, P < 0.01), grain yield (−0.45, P < 0.05), ear plant⁻¹ (−0.27, P < 0.05) and kernel-number (−0.366 P < 0.01).     

Nitrogen fertilizer replacement indexes of legume cover crops in the derived savanna of West Africa

Legume cover crops are a potential means for overcoming N depletion in the derived savanna of West Africa. A 3-year trial was, therefore, conducted near Ibadan, southwestern Nigeria to measure the N contribution of 13 legume cover crops as compared to urea –N, using a N fertilizer replacement index for a maize test crop. Two series of trials involved the following legume cover crop species: Aeschynomene histrix, Centrosema brasilianum, Centrosema pascuorum, Chamaecrista rotundifolia, Cajanus cajan, Crotalaria verrucosa, Crotalaria ochroleuca, Lablab purpureus, Mucuna pruriens, Psophocarpus palustris, Pseudovigna argentea, Pueraria phaseoloides and Stylosanthes hamata. Trials were undertaken using a complete block design. Cover crops were planted in 1994 (Series 1) and 1995 (Series 2) in separate sites and each series was subsequently slashed and planted for one season with maize (Zea mays) in 1995 and 1996. At the 50% flowering stage, N concentration of above-ground vegetation of cover crops ranged from 21 to 38 g N kg^–1. Nitrogen accumulated by 4.5-month old cover crops ranged from 14 to 240 kg N ha^–1, depending on species and year. Cover crops increased grain yield of the subsequent maize crop by 25–136% over the control without N application. Nitrogen uptake by the maize crop was higher following cover crops than after maize or natural grass. The N fertilizer replacement index of cover crops for maize ranged from 11 (A. histrix) to 96 kg N ha^–1 (C. cajan) in Series 2. Perennial (C. brasilianum, S. hamata, C. cajan, P. phaseoloides and C. verrucosa) and annual (C. rotundifolia, M. pruriens, C. ochroleuca and L. purpureus) species could potentially save 50 to 100 kg N ha^–1 for maize crops. The cover crops accumulated more N in the wetter than in the drier year. However, the N fertilizer replacement index was higher for subsequent maize grown in the drier year. The cover crop-N recovery in maize was also higher than the urea-N uptake in the drier year. The N fertilizer replacement indexes can be predicted using the above-ground biomass amount of cover crops at 20 weeks after planting (drier year) or the N concentration at that stage (wetter year).     

Nitrogen fixation by groundnut and soyabean and residual nitrogen benefits to rice in farmers' fields in Northeast Thailand

Nitrogen fixation in groundnut and soyabean and the residual benefits of incoporated legume stover to subsequent rice crops were estimated in farmers' fields using¹⁵N-isotope methods. Three field experiments were conducted, two which examined N₂-fixation in groundnut by¹⁵N-isotope dilution using a non-nodulating groundnut as a reference crop and one in which N₂-fixation in two soyabean genotypes was compared using maize as the non-fixing reference crop. Groundnut fixed 72–77% of its N amounting to 150–200 kg N ha^-1 in 106–119 days and soyabean derived 66–68% of its N from N₂-fixation which amounted to 108–152 kg N ha^-1 under similar conditions. When legume stover was returned to the soil, there was a net contribution of N from N₂-fixing varieties of groundnut in all cases ranging from 13–100 kg N ha^-1, whilst due to the high % N harvest index in soyabean (87–88%) there was a net removal of N of 37–46 kg N ha^-1. In all cases if the legume stover was removed there was a net removal of N in the legume crop which ranged between 54 and 74 kg N ha^-1 in N₂-fixing varieties of groundnut and from 58 to 73 kg N ha^-1 in soyabean, whilst maize removed 66 kg N ha^-1 if its stover was returned and 101 kg N ha^-1 when the stover was removed. Growth of rice was improved in all cases where groundnut stover was returned resulting in increases in grain yield of 12–26% and increases in total dry matter production of 26–31%. Soyabean residues gave no increases in rice grain yield but increased total dry matter production by 12–20%. Rice accumulated more N in all cases where legume stover was returned to the soil, and N yields were larger in all cases after the N₂-fixing legumes than after the non-fixing reference crops. N difference estimates of the total residual N benefits from the N₂-fixing legumes ranged from 11–19 kg N ha^-1 after groundnut and 15–16 kg N ha^-1 after soyabean. The amounts of N estimated directly by application of¹⁵N-labelled stover amounted to 7.2–20.5 kg N ha^-1 with groundnut which represented recovery of 8–22% of the N added in the stover. In soyabean only 3.0–5.8 kg N ha^-1 was estimated to be recovered by¹⁵N-labelling which was 15–23% of the added N, whilst only 1.3 kg N ha^-1 (4% of the N added) was recovered by rice from the maize stover. An indirect¹⁵N-method based on addition of unlabelled stover to microplots where the soil had previously been labelled with¹⁵N gave extremely variable and often negative estimates of residual N benefits. Estimates of residual N from the added stover made by N difference calculations did not correspond with the estimates by direct¹⁵N-labelling in all cases and possible reasons for this are discussed.     

Fate of urea-N in floodwater

One day after application, urea-N remaining in the floodwater and determined as water-soluble N (urea-N + NH₄ ⁺-N) was used to calculate the potential N loss from lowland rice soils. Actual N loss calculated from ¹⁵N balance measurements using forced air exchange (airflow rate: 20 L min^-1) in greenhouse pots. Conditions for variable potential N loss were created by manipulating the method of urea application and duration of presubmergence or by selecting soils with diverse cation exchange capacities (CEC). Potential N loss tended to be lower than actual N loss; the differences were, however, nonsignificant. The method of urea application that led to the lowest potential N loss from a Guthrie silty clay loam (Typic Fragiaquult) also led to the least ¹⁵N loss and vice-versa (r=0.99^**). Duration of presubmergence did not alter the relationship between potential and actual N loss although it influenced the rate of urea hydrolysis in floodwater. The primary depencence of actual N loss on water-soluble N was maintained in soils differing in CEC (r=0.83^**). The association between potential and actual N loss was closer for high-CEC soils ( 20 cmol [+] kg^-1 soil, r=0.91^**) than for low-CEC soils (<20 cmol [+] kg^-1 soil, r=0.85^**). Ammonia volatilization could be more closely predicted by potential N loss than could apparent denitrification.The results of this study suggest that potential N loss calculated from one-time determination of water-soluble N in floodwater can be a good index of actual N loss from flooded, puddled rice soils. Notable exceptions are to be expected for soils in which water-soluble N gets lost from floodwater either before (soils with fast urea hydrolysis in floodwater) or after (soils with steady leaching) determination of potential N loss.     

Source of the soybean N credit in maize production

Nitrogen response trials throughout the United States Corn Belt show that economic optimum rates of N fertilization are usually less for maize (Zea mays L.) following soybean (Glycine max L.) than for maize following maize; however, the cause of this rotation effect is not fully understood. The objective of this study was to investigate the source of the apparent N contribution from soybean to maize (soybean N credit) by comparing soil N mineralization rates in field plots of unfertilized maize that had either nodulated soybean, non-nodulated soybean, or maize as the previous crop. Crop yields, plant N accumulation, soil inorganic N, and net soil mineralization were measured. Both grain yield (6.3 vs. 2.8 Mg ha^–1) and above-ground N accumulation (97 vs. 71 kg ha^–1) were greatly increased when maize followed nodulated soybean compared with maize following maize. A partial benefit to yield and N accumulation was also observed for maize following non-nodulated soybean. Cumulative net soil N mineralization following nodulated soybean, non-nodulated soybean, and maize was 112, 92 and 79 kg N ha^–1, respectively. Net mineralization of soil N appeared to be influenced by both quality (C:N ratio) and quantity of residue from the previous crop. In addition to an increase in plant available N from mineralization, the amount of soil inorganic N (especially in soil 5 cm from the row) was greater following nodulated soybean than non-nodulated soybean or maize. Based on these data, the soybean N credit appears to result from a combination of a decrease in net soil mineralization in continuous maize production and an increase in residual soil N from symbiotic fixation.     

Rotation of maize with cowpea improves yield and nutrient use of maize compared to maize monocropping in an alfisol in the northern Guinea Savanna of Ghana

In the northern Guinea Savanna of Ghana (1984–1987) a field experiment was conducted to study the reasons for beneficial effects of rotating maize (Zea mays) and cowpea (Vigna unguiculata) on yield and N and P use of maize. The treatments included two cropping systems, maize monocropping and maize/cowpea rotation, two levels of nitrogen (0 and 80 kg N ha^-1 as urea) and two levels of phosphorus application (0 and 60 kg ha^-1 P as Volta phosphate rock). Yields and nutrient accumulation of maize were larger in rotation than in monocropping, independent of the N and P level. Fertilizer application (N and P) increased yields of maize in both cropping systems to the same extent. Nitrate contents of the soil after cowpea and after maize monoculture were comparable at the beginning of the cropping period. Also, potential nitrogen mineralization was only slightly larger after cowpea in the unfertilized plots. However, soil nitrate of fertilized plots was similar or even higher under monocropping than under crop rotation, especially in deeper soil layers and at the end of the cropping period. This indicates that in addition to the availability of mineral N, its use by the plants was limiting for the productivity of maize. Root length densities of maize were significant lower in monocropped maize than in maize grown in rotation. Soil physical parameters (infiltration, bulk density, aggregate stability and water capacity) showed a significant deterioration compared to a bush fallow plot, but differed only slightly between the cropping systems. Also in a pot experiment maize growth was much better in the soil from the crop rotation than from the monocropping plots, provided P was eliminated as the main growth-limiting factor. Since this effect persisted in spite of N application and optimization of soil physical properties by mixing the soil with polystyrol it is concluded that the results indicate that yield decline in maize monocropping might be due to allelopathic effects.     

Termite damage to maize grown in agroforestry systems, traditional fallows and monoculture on nitrogen-limited soils in eastern Zambia

Abstract 1 Termites cause significant damage to maize and other crops in southern Africa. Several studies were conducted with the objective of determining the difference in termite damage to maize in various land use systems between monoculture maize, maize grown using traditional fallows and improved fallows. 2 In an experiment conducted at four sites on noncoppicing fallows, maize planted after Tephrosia candida 02971 fallows had lower termite damage compared with fully fertilized monoculture maize. However, the termite suppression was not low enough to warrant rotation of noncoppicing fallows for termite management.. 3 In four experiments comparing termite damage to maize grown in monoculture and in coppicing fallows, fully fertilized monoculture maize had a higher percentage of lodged plants compared with maize grown in pure Leucaena leucocephala, Gliricidia sepium and Acacia anguistissima fallows or in a mixture of A. anguistissima + Sesbania sesban or Tephrosia vogelii + S. sesban. 4 More than 50–75% of the variance in maize yield was explained by preseason inorganic nitrogen and termite damage. However, termite damage to maize was not influenced by inorganic nitrogen, which represents nitrogen readily available to maize. The decomposition rate of biomass (related to lignin + polyphenol to nitrogen ratio) and water retention under fallows also appeared to influence termite damage. 5 It is concluded that maize grown in L. leucocephala, G. sepium, A. anguistissima and S. sesban fallows suffers less termite damage and produces maize yields comparable with conventionally tilled and fully fertilized monoculture maize.     

Monovalent cation transporters; establishing a link between bioinformatics and physiology

Toxic aluminum (Al) ion is a major constraint to plant growth in acid soils. Aluminum tolerance in wheat (Triticum aestivum L.) is strongly related to the Al-triggered efflux of malate from root apices. A role of the secreted malate has been postulated to be in chelating Al and thus excluding it from root apices (malate hypothesis), but the actual process has yet to be fully elucidated. We measured Al content and root growth during and after Al exposure using seedlings of near-isogenic lines [ET8 (Al tolerant) and ES8 (Al sensitive)] differing in the capacity to induce Al-triggered malate efflux. Aluminum doses that caused 50% root growth inhibition during 24-h exposure to Al in calcium (Ca) solution (0.5 mM CaCl₂, pH 4.5) were 50 μM in ET8 and 5 μM in ES8. Under such conditions, the amount of Al accumulated in root apices was approximately 2-fold higher in ET8 than ES8. Al-treated seedlings were then transferred to the Al-free Ca solution for 24 h. Compared to control roots (no Al pretreatment), root regrowth of Al-treated roots was about 100% in ET8 and about 25% in ES8. The impaired regrowth in ES8 was observed even after 24-h exposure to 2.5 μM Al which had caused only 20% root growth inhibition. The addition of malate (100 μM) during exposure to 50 μM Al in ES8 enhanced root growth 1.6 times and regrowth in Al-free solution 7 times, resulting in similar root growth and regrowth as in ET8. Short-term Al treatments of ES8 for up to 5 h indicated that the Al-caused inhibition of root regrowth started after 1-h exposure to Al. The stimulating effect of malate on root regrowth was observed when malate was present during Al exposure, but not when roots previously exposed to Al were rinsed with malate, although Al accumulation in root apices was similar under these malate treatments. We conclude that the malate secreted from root apices under Al exposure is essential for the apices to commence regrowth in Al-free medium, the trait that is not related to the exclusion of Al from the apices.     

The Effects of Crop Rotation and Nitrogen Fertilization on Soil Chemical and Microbial Properties in a Guinea Savanna Alfisol of Nigeria

The impacts of crop rotation and inorganic nitrogen fertilization on soil microbial biomass C (SMBC) and N (SMBN) and water-soluble organic C (WSOC) were studied in a Guinea savanna Alfisol of Nigeria. In 2001, fields of grain legumes (soybean and cowpea), herbaceous legume (Centrosema pascuorum) and a natural fallow were established. In 2002, maize was planted with N fertilizer rates of 0, 20, 40 and 60 kg N ha⁻¹ in a split-plot arrangement fitted to a randomized complete block design with legumes and fallow as main plots and N fertilizer levels as subplots. Surface soil samples were taken at 4 weeks after planting and tasselling stage of the maize. Inorganic N fertilization had no significant (P>0.05) effect on SMBC, SMBN and WSOC, while crop rotation significantly (P<0.0001) affected both SMBC and WSOC. These results demonstrate that crop rotation do not necessarily influence the gross soil microbial biomass, but may affect physiologically distinct subcomponent of the microbial biomass. The soils under the various rotations had a predominance of fungi community as indicated by their wide biomass C/N ratio ranging from 9.2 to 20.9 suggesting fungi to be mainly responsible for decomposition in these soils. Soil microbial biomass and WSOC showed significant (P<0.05) correlation with both soil pH and organic carbon but no relationship with total N. Based on these results, it appears that the soil pH and organic carbon determined the flux of the soil microbial biomass and amount of WSOC in these soils.     

Nitrogen mineralization and potential nitrification at different depths in acid forest soils

Yield decline of cereals grown in monoculture may be alleviated with alternative crop management strategies. Crop rotation and optimized tillage and fertilizer management can contribute to more sustainable food and fiber production in the long-term by increasing diversity, maintaining soil organic matter (SOM), and reducing adverse effects of excessive N application on water quality. We investigated the effects of crop sequence, tillage, and N fertilization on long-term grain production on an alluvial, silty clay loam soil in southcentral Texas. Crop sequences consisted of monoculture sorghum (Sorghum bicolor (L.) Moench,) wheat (Triticum aestivum L.), and soybean (Glycine max (L.) Merr), wheat/soybean double-crop, and rotation of sorghum with wheat/soybean. Grain yields tended to be lower with no tillage (NT) than with conventional tillage (CT) early in the study and became more similar after 11 years. Nitrogen fertilizer required to produce 95% to maximum sorghum yield was similar for monoculture and rotation upon initiation of the experiment and averaged 16 and 11 mg N g^-1 grain with NT and CT, respectively. After 11 years, however, the N fertilizer requirement became similar for both tillage regimes, but was greater in monoculture (17 mg N g^-1 grain) than in rotation (12 mg N g^-1 grain). Crop sequences with double-cropping resulted in greater land use efficiency because similar or lower amounts of N fertilizer were required to produce equivalent grain than with less intensive monoculture systems. These more intensive crop sequences produced more stover with higher N quality primarily due to the inclusion of soybean in the rotation. Large quantities of stover that remained on the soil surface with NT led to greater SOM content, which increased the internal cycling of nutrients in this soil. In southcentral Texas, where rainfall averages nearly 1000 mm yr^-1, more intensive cropping of sorghum, wheat, and soybean with moderate N fertilization using reduced tillage can increase grain production and potentially decrease N losses to the environment by cycling more N into the crop-SOM system.