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.     

Decreasing Nitrogen Fertilizer Input Had Little Effect on Microbial Communities in Three Types of Soils

In this study, we examined the influence of different nitrogen (N) application rates (0, 168, 240, 270 and 312 kg N ha^-1) on soil properties, maize (Zea mays L.) yields and microbial communities of three types of soils (clay, alluvial and sandy soils). Phospholipid fatty acid analysis was used to characterize soil microbial communities. Results indicated that N fertilization significantly decreased microbial biomass in both clay and sandy soils regardless of application rate. These decreases were more likely a result of soil pH decreases induced by N fertilization, especially in the sandy soils. This is supported by structural equation modeling and redundancy analysis results. Nitrogen fertilization also led to significant changes in soil microbial community composition. However, the change differences were gradually dismissed with increase in N application rate. We also observed that N fertilization increased maize yields to the same level regardless of application rate. This suggests that farmers could apply N fertilizers at a lower rate (i.e. 168 kg N ha^-1), which could achieve high maize yield on one hand while maintain soil microbial functions on the other hand.     

Legumes regulate grassland soil N cycling and its response to variation in species diversity and N supply but not CO2

Legumes are an important component of plant diversity that modulate nitrogen (N) cycling in many terrestrial ecosystems. Limited knowledge of legume effects on soil N cycling and its response to global change factors and plant diversity hinders a general understanding of whether and how legumes broadly regulate the response of soil N availability to those factors. In a 17‐year study of perennial grassland species grown under ambient and elevated (+180 ppm) CO₂ and ambient and enriched (+4 g N m^?2 year^?1) N environments, we compared pure legume plots with plots dominated by or including other herbaceous functional groups (and containing one or four species) to assess the effect of legumes on N cycling (net N mineralization rate and inorganic N pools). We also examined the effects of numbers of legume species (from zero to four) in four‐species mixed plots on soil N cycling. We hypothesized that legumes would increase N mineralization rates most in those treatments with the greatest diversity and the greatest relative limitation by and competition for N. Results partially supported these hypotheses. Plots with greater dominance by legumes had greater soil nitrate concentrations and mineralization rates. Higher species richness significantly increased the impact of legumes on soil N metrics, with 349% and 505% higher mineralization rates and nitrate concentrations in four‐species plots containing legumes compared to legume‐free four‐species plots, in contrast to 185% and 129% greater values, respectively, in pure legume than nonlegume monoculture plots. N‐fertilized plots had greater legume effects on soil nitrate, but lower legume effects on net N mineralization. In contrast, neither elevated CO₂ nor its interaction with legumes affected net N mineralization. These results indicate that legumes markedly influence the response of soil N cycling to some, but not all, global change drivers.     

Taxonomic and ecological patterns in root traits of <Emphasis Type="Italic">Carex</Emphasis> (Cyperaceae)

Background and aims

In Malawi, strategies are being sought to boost maize production through improvements in soil fertility. This study assessed the impact of intercropping maize (Zea mays) with pigeon pea (Cajanus cajan) in Lixisols of Malawi on yield, biological N fixation, soil aggregation, and P forms within soil aggregates.

Methods

Maize and pigeon pea were grown intercropped in pots, with varying degrees of root interaction in order to understand the relative importance of biochemical versus physical rhizospheric interactions. Following harvest, soils were separated into aggregate fractions using wet-sieving, and the nutrient content of all fractions was assessed.

Results

The proportion of macroaggregates and microaggregates increased by 52 and 111%, respectively, in the intercropping treatment compared to sole maize, which significantly increased organic P storage in the microaggregates of intercropped compared to sole maize (84 versus 29 mg P kg^?1, respectively). Biologically fixed N increased from 89% in the sole pigeon pea to 96% in the intercropped system.

Conclusions

Intercropping maize with pigeon pea can have a significant and positive impact on soil structure as well as nutrient storage in these high P-sorbing soils. This is caused primarily by physical root contact and to a lesser degree by biochemical activities.

     

玉米/花生间作对土壤微生物和土壤养分状况的影响

通过大田试验,研究了玉米/花生间作对玉米和花生根区土壤微生物和土壤养分状况的影响.结果表明：与单作相比,间作能显著提高玉米和花生根区的土壤细菌数量;间作花生根区土壤真菌和放线菌数量与单作无显著差异;间作玉米根区土壤真菌和放线菌数量比单作明显提高;间作作物根区微生物群落功能多样性和代谢活性比单作有所改善.玉米/花生间作不同程度提高了整个间作系统根区的土壤碱解氮、速效磷、有机质含量及EC值,其中,间作玉米根区土壤养分的增加更为明显,说明玉米/花生间作可以较明显地改善两种作物根区的微生物和养分状况,土壤微生态环境的改善又会促进作物地上部的生长.     

Biological nitrogen fixation in resource-poor agriculture in South Africa

Farm lands of resource-poor communities in South Africa are depleted of nutrients due to continuous mono-cropping, limited use of fertilisers, and sometimes leaching caused by high rainfall. Despite the well-known advantages of biological nitrogen fixation (BNF) in cropping systems, less than 10% of the grain crops planted annually in these areas are legumes. Using a participatory research and development approach, resource-poor farmers were introduced to conservation agriculture (CA) practices, including BNF, that promoted zero (or reduced) tillage, increased retention of soil cover, as well as crop diversification. Because crop rotation and intercropping of legumes with cereals are known to contribute to soil fertility while enhancing food security, resource-poor farmers from various Provinces in South Africa were trained on the benefits of legume culture for eight years. As a result, these resource-poor farmers did not only get training in inoculation techniques, but were also supplied with inoculants for use on their farms. Data collected from Farmers Demonstration Trials at Belvedere, Dumbarton and Lusikisiki, showed that the grain and fodder yield of maize planted after legumes, and maize intercropped with legumes, were comparable to those of maize receiving high N fertilizer dose (i.e. 54 kg N at planting and 54 kg N as top-dressing). The same data further showed thatRhizobium inoculation, when combined with application of low levels of P and K, significantly increased crop yields within farmers’ trial plots. BNF therefore offers a great opportunity for resource-poor farmers in South Africa to increase their crop yields and thus improve the quality of their livelihoods through the adoption of affordable and sustainable biological technologies that enhance soil fertility.     

Variations of soil structure and microbial population in a compost amended soil

Changes in soil structure and in microbial population were recorded in a long term field experiment over the growing season of maize (June–November). Determinations were made on samples from plots which had received, for two years, the following treatments: mineral fertilizers, farmyard manure and three rates of compost. Seasonal variations were observed for the stability of soil aggregates, total porosity, pore size distribution, mycorrhizal infection and aerobic cellulolytic microorganisms. The stability of the soil aggregates changed in a similar way to that found for both mycorrhizal infection and the number of aerobic cellulolytic microorganisms. Physical characteristics were not affected in any instance by the organic dressings and microbiological populations were generally influenced only by the higher doses of compost.     

种间互作和施氮对蚕豆/玉米间作生态系统地上部和地下部生长的影响

为河西走廊绿洲灌区豆科/禾本科间作体系的养分管理提供科学依据,于2007年在武威绿洲农业试验站应用田间原位根系行分隔技术研究了蚕豆/玉米种间互作和施氮对玉米抽雄期的根系空间分布、根系形态和作物地上部生长的影响。研究结果表明:种间互作和施氮均增加了玉米和蚕豆在纵向和横向两个尺度上的根重密度、根长密度、根表面积、根系体积。根长密度和根表面积与两种作物产量和氮素吸收均呈正相关,而与蚕豆的根瘤重呈负相关;抽雄期的土壤含水量与玉米产量和养分吸收呈显著的负相关。玉米根系可以占据蚕豆地下部空间,但蚕豆的根却较少到间作玉米的地下部空间,也就是间作后增加了玉米根系水平尺度的生态位。蚕豆和玉米根系主要分布分别在0-40 cm浅土层和0-60 cm 土层,且间作玉米根系在60-120 cm比单作和分隔的多。因此,种间互作和施氮扩大了两作物根系纵向和横向的空间生态位,改变了作物根系形态,即扩展了两者水分和养分吸收的生态位,增加了作物吸收养分的有效空间,从而提高了间作生态系统的生产力。     

Response of cereals to nitrogen in sole cropping and intercropping with different legumes

The response of sole and intercropped cereal to nitrogen fertilization was compared in three contrasting cropping systems, sorghum/pigeonpea, maize/groundnut, and sorghum/cowpea. The cereal in these systems responded to nitrogen similarly as in sole cropping, although different legumes affected the cereal differently. There was no current season benefit from the legume, whether it matured earlier or later than the cereal, and for high yields the cereal in intercropping needs fertilizer application. Response to nitrogen varied with the amount and distribution of seasonal rainfall. With increased nitrogen fertilizer applied to the intercropped cereal, the legume yields were suppressed. The optimum dose for the intercropped cereal was similar to that for sole cropping but it was 50% less in a dry year particularly, on a shallow Alfisol. The combined yields of both crops made intercropping more profitable than sole cropping. The relative advantage of intercropping was high in the sorghum/pigeonpea system (40 to 70%) because of the greater temporal difference between species, and moderate in the maize/groundnut (13 to 35%), and sorghum/cowpea (18 to 25%) systems. Although the relative advantage of intercropping (expressed as Land Equivalent Ratio (LER)) decreased with N, the economic value, of the advantage was little affected within the optimum N range because absolute yields increased with fertilization.     

盐渍区农田氮肥施用量对土壤硝态氮动态变化的影响

土壤硝态氮动态变化和残留与农田硝态氮淋溶以及地下水硝态氮污染密切相关。为了促进海河低平原盐渍区农田氮肥合理利用以及农业可持续发展,试验在盐化潮土条件下,通过设计不同施氮量（0,70,140,210kg N hm^-2）处理,重点研究了该区农田氮肥施用量对土壤硝态氮动态、残留以及土壤氮损失的影响。结果表明：（1）0～100cm土壤剖面硝态氮总量随施氮量显著增加,施用氮肥没有改变剖面硝态氮总量随玉米生育进程波状变化趋势,但明显增强了其变化幅度;（2）施氮改变了硝态氮土壤剖面空间分布状态,表现出施氮后上部土层（0～40cm）硝态氮比例显著增加而后迅速降低的趋势;（3）硝态氮残留与氮素损失随施氮量增加而增加,且N210和N140处理下氮素损失量显著高于N70和N0。     

Screening legume green manures as nitrogen sources to succeeding non-legume crops

Legumes managed as green manures provide a good alternative to the use of commercial N-fertilizer for non-legume crop production. A laboratory procedure based on the aerobic incubation (35 °C) of soil samples taken from plots with legumes incorporated was proposed for predicting the N supplying potential of legumes to succeeding non-legume crops. This procedure was evaluated by comparing the amount of inorganic N determined in the soil samples after incubation with N content of aboveground dry matter of maize or with inorganic N found in the soil of an adjacent fallow plot. The soil samples (0.00 to 0.15 m and 0.15 to 0.30 m) were obtained from two field experiments conducted in similar soils of the Cerrado Region of central Brazil during 1984–85 and 1986–87. Although incubation results were affected by the different pretreatments, soil samples prepared and incubated as soon as possible after being taken from the field gave the best correlations. The most convenient incubation procedure was the one-week aerobic incubation of samples previously oven dried at 50°C. The results obtained by this procedure were significantly correlated with N content in aboveground dry matter of maize and with the inorganic N accumulated in an adjacent fallow plot.     

减量施氮对玉米-大豆套作体系土壤氮素残留和氮肥损失的影响

通过田间试验,研究了玉米单作、大豆单作、玉米-豆套作3种种植模式和不施氮、减量施氮(180 kg N·hm^-2)、常量施氮(240 kg N·hm^-2)3种施氮水平对玉米和大豆植株氮素吸收、土壤氮素残留和氮肥损失的影响.结果表明: 玉米-豆套作体系下,施氮提高了玉米土壤中残留的NO₃^--N、NH₄⁺-N含量,但在大豆土壤中则降低.与单作相比,玉米套作的土壤氮素残留量增加,氮肥损失量降低,大豆套作的土壤氮素残留量和氮肥损失量均降低.减量施氮处理下,玉米-豆套作系统的氮肥残留率、损失率和氨挥发损失率分别比玉米单作低17.7%、21.5%和0.4%,比大豆单作高2.0%、19.8%和0.1%.与常量施氮相比,减量施氮降低了玉米-豆套作系统的氮肥残留量、残留率、损失量和损失率,同时还降低了由氨挥发所引起的氮肥损失,其中氮肥残留率、损失率和氨挥发损失率分别降低12.0%、15.4%和1.2%.     

Nostoc cyanobacterial inoculation in South African agricultural soils enhances soil structure, fertility, and maize growth

Many soils in South Africa have low nutrient supply, poor structural stability and are prone to soil erosion due to susceptibility to surface sealing and crusting. Two crusting soils from the Eastern Cape Province, South Africa were used to evaluate the effects of inoculation with a strain of Nostoc on soil structure, fertility and maize growth. The Nostoc suspension was uniformly applied over potted soils at a rate of 6g (dry weight) per square meter soon after maize germination. Nostoc inoculation increased soil N by 17% and 40% in Hertzog and Guquka soils, respectively. Soil C was also increased significantly and this increase was strongly associated with that of soil N (R ² = 0.838). The highest contents of soil C, soil N and mineral N, however, were found in non-cropped Nostoc inoculated soils. Nostoc inoculation increased maize dry matter yields by 49% and 40% in Hertzog and Guquka soils, respectively. Corresponding increases in maize tissue N were 23% and 14%, respectively. Scanning electron microscopy (SEM) revealed that soil particles and fragments of non-cropped inoculated soils had coatings of extracellular polymeric substances (EPS) with other particles enmeshed in networks of filaments, whilst by contrast little or no EPS and/or filaments were observed on cropped and/or non-inoculated soils. This was consistent with chemical analysis which showed that Nostoc caused significant increases in the EPS and soil C contents of non-cropped soils. The proportion of very stable aggregates was increased by inoculation with Nostoc possibly due to the greater quantities of soil C and EPS observed in inoculated soils. Inoculated soils cropped with maize had a lower proportion of stable aggregates presumably due to their low soil C and EPS contents compared to non-cropped soils. The results suggested that Nostoc could improve the fertility and structural stability of the studied degraded soils.     

间作对植株生长及养分吸收和根际环境的影响

通过盆栽实验研究了线辣椒和玉米间作对其植株生长、矿质养分吸收、根际环境以及铁载体分泌的影响,以探索间作促进铁、磷等养分吸收利用的可能生理机制.结果表明:(1)与单作相比,间作线辣椒地上部干重降低23.0%,根系干重增加44.2%,玉米地上部和根系的干重分别增加8.7%和22.9%;间作线辣椒根冠比和根系活力分别显著提高86.4%和29.8%;间作线辣椒、玉米叶绿素含量分别显著提高12.6%和7.8%.(2)与单作相比,间作线辣椒的铁、锌、锰含量分别增加1.50倍、1.39倍和1.34%,而间作玉米则无显著变化;间作线辣椒和玉米的钙含量都显著低于相应单作,氮含量没有显著变化,但磷、钾含量显著增加.(3)间作线辣椒和玉米的根际土、非根际土的酸性磷酸酶活性及根系酸性磷酸酶活性都显著高于相应单作,而其根际土和非根际土的pH值无显著变化;间作玉米根系的铁载体分泌比单作减少32.8%,间作线辣椒根系的铁还原酶活性是单作的1.10倍.研究发现,线辣椒/玉米间作能通过影响根际生物学特征和化学过程提高植株的铁、锌、磷和钾养分水平,缓解养分胁迫,是一种很有推广价值的种植模式.     

Soil physical properties and carbon/nitrogen relationships in stable aggregates under legume and grass fallow

Short-season fallow with legumes and/or grasses can restore the soil organic C and nitrogen (N) and improve soil structure. In this study, we accessed the effects of 2-season legume and grass fallow on structural properties and C/N relationships in aggregates of a sandy loam soil. Two legumes (Calopogonium mucunoides and Centrosema pubescens), and two grasses (Guinea grass (Panicum maximum) and goose grass (Eleusine indica) were used. Results showed that Calopogonium and Centrosema increased soil total porosity and reduced soil bulk densities, while goose grass increased bulk density and reduced total porosity of the soils at 0–15 and 15–30?cm depths. Guinea grass significantly increased the saturated hydraulic conductivity (50.4?cm?h^?1) and water holding capacity of the soils. Aggregates, 4.75 to 0.5?mm were greater in Guinea grass and least in goose grass fallowed soils. Calopogonium increased macro-aggregates at 0–15?cm soils by 48%, and mean weight diameter (MWD) by 44%. Organic carbon in 0.5–0.25?mm and <0.25?mm aggregate sizes was higher in Guinea grass soils. Generally, grasses had 4-fold increases of C:N contents in dry aggregates. In conclusion, short-season fallow with Guinea grass, Calopogonium and Centrosema, increased soil C and N and protected them from losses in stable aggregates.     

Leguminous plants significantly increase soil nitrogen cycling across global climates and ecosystem types

Leguminous plants are an important component of terrestrial ecosystems and significantly increase soil nitrogen (N) cycling and availability, which affects productivity in most ecosystems. Clarifying whether the effects of legumes on N cycling vary with contrasting ecosystem types and climatic regions is crucial for understanding and predicting ecosystem processes, but these effects are currently unknown. By conducting a global meta-analysis, we revealed that legumes increased the soil net N mineralization rate (R_min) by 67%, which was greater than the recently reported increase associated with N deposition (25%). This effect was similar for tropical (53%) and temperate regions (81%) but was significantly greater in grasslands (151%) and forests (74%) than in croplands (−3%) and was greater in in situ incubation (101%) or short-term experiments (112%) than in laboratory incubation (55%) or long-term experiments (37%). Legumes significantly influenced the dependence of R_min on N fertilization and experimental factors. The R_min was significantly increased by N fertilization in the nonlegume soils, but not in the legume soils. In addition, the effects of mean annual temperature, soil nutrients and experimental duration on R_min were smaller in the legume soils than in the nonlegume soils. Collectively, our results highlighted the significant positive effects of legumes on soil N cycling, and indicated that the effects of legumes should be elucidated when addressing the response of soils to plants.     

Effect of crop and soil management practices on soil compatibility in maize and groundnut plots in a Paleustult in Southeastern Nigeria

Frequent occurrences of soil compaction damage resulting from high raindrop impact energy, and from human and animal trafficking during field operations pose a problem to farmers around the tropics. We studied the effect of some crop and soil management practices (manure, mulch, NPK applications, tillage and crop type) on some soil compactibility indices (dry bulk density, cone index, total soil porosity, gravimetric soil water content) in a Typic Paleustult in southeastern Nigeria. The study was carried out for three consecutive planting seasons using two tillage systems and four other soil management practices (poultry droppings + NPK, mulch + NPK, NPK alone and no amendment). These were laid out as split-plot in a RCB design replicated three times and using maize (Zea mays L.) and groundnut (Arachis hypogea) as test crops. Results indicate that the different soil management techniques adopted influenced dry bulk density, penetration resistance, total soil porosity and gravimetric soil water content at 44 and 66 days after planting (DAP) whereas only gravimetric soil water content was affected at 90 DAP. The dry bulk density of tilled maize and groundnut plots increased significantly (P<0.05) by between 2 and 14% relative to no-till plots at 44 and 66 DAP. In both maize and groundnut plots, dry bulk density decreased significantly (P<0.05) in plots amended with poultry droppings +NPK relative to the control plots by 3–10% at 44 and 66 DAP. Tilled maize and groundnut plots had 37–45% lower (P< 0.05) penetration resistance than their corresponding no-till plots at both 44 and 66 DAP. Penetration resistance measurements were lower by 16.5–25% in plots amended with poultry droppings + NPK relative to unamended plots at 44 and 66 DAP. Cumulative (1996, 1997, 1998) data indicate that gravimetric soil water content in maize and groundnut plots generally increased significantly (P<0.05) in no-till plots relative to tilled plots by 18–27% at both 44 and 66 DAP. Plots amended with poultry droppings + NPK had between 24 and 111% increase (P<0.05) in soil gravimetric soil water content at both 44 and 66 DAP. Results are indicative that all soil compactibility indices measured were not affected at 90 DAP except for soil gravimetric soil water content in 1996 and 1998. Results from this work demonstrate that some crop and soil management practices could be used to reduce soil compactibility problems thus increasing productivity of such soils.     

长期施肥对水稻土有机碳分布及化学结合形态的影响

采用湖南省4个23年连续施肥的稻田长期定位试验,研究了施肥对湖南省水稻土有机碳分布及化学键合形态的影响。试验设不施肥(CK)、化肥(NPK)、中量有机肥(MOM)和高量有机肥(HOM)4个处理。结果表明:在所有施肥处理中,水稳性团聚体均以0.25～1mm和2～5mm粒径含量最高,分别达全土总量的18%～43%和13%～48%。中、高量有机肥处理显著增加了>1mm大团聚体含量以及有机碳在大团聚体中的分配,其中0.25～1mm和1～2mm粒径团聚体中有机碳含量均略高于其余粒径组。与不施肥比较,钙结合态有机碳(Ca-SOC)占总有机碳的比例在2%～4%左右且随有机肥施用呈下降趋势,而铁铝结合态有机碳Fe(Al)-SOC占总有机碳的18%～33%呈上升趋势。有机肥施用条件下,有机碳在大团聚体中的分布的增加、Fe(Al)-SOC的提升以及Ca-SOC的降低意味着土壤有机碳物理和化学保护作用的增强,有利于稻田土壤有机碳的积累,是有机肥施用条件下保持稻田土壤较高固碳速率的重要原因。     

Carbon accumulation at depth in Ferralsols under zero‐till subtropical agriculture

Conservation agriculture can provide a low‐cost competitive option to mitigate global warming with reduction or elimination of soil tillage and increase soil organic carbon (SOC). Most studies have evaluated the impact of zero till (ZT) only on surface soil layers (down to 30 cm), and few studies have been performed on the potential for C accumulation in deeper layers (0–100 cm) of tropical and subtropical soils. In order to determine whether the change from conventional tillage (CT) to ZT has induced a net gain in SOC, three long‐term experiments (15–26 years) on free‐draining Ferralsols in the subtropical region of South Brazil were sampled and the SOC stocks to 30 and 100 cm calculated on an equivalent soil mass basis. In rotations containing intercropped or cover‐crop legumes, there were significant accumulations of SOC in ZT soils varying from 5 to 8 Mg ha^?1 in comparison with CT management, equivalent to annual soil C accumulation rates of between 0.04 and 0.88 Mg ha^?1. However, the potential for soil C accumulation was considerably increased (varying from 0.48 to 1.53 Mg ha^?1 yr^?1) when considering the soil profile down to 100 cm depth. On average the estimate of soil C accumulation to 100 cm depth was 59% greater than that for soil C accumulated to 30 cm. These findings suggest that increasing sampling depth from 30 cm (as presently recommended by the IPCC) to 100 cm, may increase substantially the estimates of potential CO₂ mitigation induced by the change from CT to ZT on the free‐draining Ferralsols of the tropics and subtropics. It was evident that that legumes which contributed a net input of biologically fixed N played an important role in promoting soil C accumulation in these soils under ZT, perhaps due to a slow‐release of N from decaying surface residues/roots which favored maize root growth.     

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