Anthony S. R. Juo

Chemical dynamics in slash-and-burn agriculture

Abstract The stability of the slash-and-burn system is more a function of the total nutrient stock of the entire ecosystem than the net gain of the soil after slash-and-burn. During the first cycle of land exploitation, slash-and-burn releases a large portion of nutrients stored in the above-ground biomass into the soil. Therefore, the gain of nutrient stock in the soil is at the expense of the plant biomass. Published data on slash-and-burn systems are mostly concerned with the changes in soil nutrient status during cropping. Studies dealing with the dynamics of total nutrient stock in the primary forest ecosystem and the subsequent cropping and fallow cycles are scarce. Conservation of the total mineral nutrient stock of the slash-and-burn system, starting from the clearing of a primary forest followed by burning, cropping and secondary forest fallow, is possible only in an ideal case of shifting cultivation — the equilibrium model. A significant portion of mineral nutrients released from burning may be lost either through erosion and runoff or through leaching (i.e. K, Mg, Ca, nitrate, and sulfate). A portion of the plant nutrients will be removed in harvested crops. Thus, the total mineral nutrient stock in the whole ecosystem gradually declines during the subsequent cycles of fallow and cropping — the depletion model. For various social and economic reasons, slash-and-burn agriculture is being rapidly replaced by new or modified crop production systems throughout the tropics. In strongly weathered soils, external inputs of mineral nutrients are required, particularly, those aiming at sustaining high levels of crop or pasture productivity. Although nitrogen can be replenished through biological N-fixation, other nutrients, especially P and K, must be supplied by external sources. Therefore, a sustainable alternative to the slash-and-burn system should be judged not only by the stability of crop yield but also by the efficiency of the agroecosystem to recycle mineral nutrients as a function of time.

Carbon and nitrogen mineralization from cowpea plants part decomposing in moist and in repeatedly dried and wetted soil

from 46% of initial plant N in moist -oil to 29% in repeatedly dried and wetted soil at 68 days. Carbon and N mineralization from cowpea were linearly related after an initial phase of rapid C loss. With repeated drying and wetting, a greater amount of N remained in undecomposed plant material, that was retrieved periodically during incubation. Repeated drying and wetting of the soil appeared to increase the resistance of certain N compounds of the plant to microbial decomposition. Further, repeated drying and wetting of the soil severely inhibited growth and/or activity of nitrifiers. Periodic drying of the soil as occurs in the field will reduce N mineralization from legume green manures compared to the decomposition in continuously moist soil, but may contribute to long-term N fertility by increasing soil organic N content.

Changes in soil properties during long-term fallow and continuous cultivation after forest clearing in Nigeria

Abstract Changes in soil chemical properties under natural bush regrowth, planted fallows (i.e. Guinea grass ( Panicum maximum ), Leucaena leucocephala , and pigeon pea ( Cajanus cajan )) and under continuous cropping with minimum tillage (i.e. maize ( Zea mays L.)/cassava ( Manihot esculenta Crantz) intercrop and maize monoculture with residue returned or removed), were monitored for 13 years after clearing of secondary forest on a kaolinitic Alfisol in Nigeria, West Africa. Under bush fallow, soil organic C (SOC) decreased during the first 7 years, then increased to the original level of approximately 20 g kg −1 at 12 years, whereas soil pH, exchangeable Ca and Mg and ECEC remained relatively constant. Chemical soil properties under Guinea grass and leucaena fallows compared favorably to those under bush fallow, whereas chemical properties of the surface soil under pigeon pea fallow deteriorated. Under continuous maize cropping, soil organic C decreased during the first 7 years, then reached a steady-state at about 65% of the level maintained by bush fallow. The surface soil under continuous maize also became acidified. Maize grain yields of the first season declined from 6.0 t ha −1 to 2.5 t ha −1 during 7 years of cropping. Relative yield sustainability of the three cropping systems was maize/cassava intercrop > maize monoculture with residue returned > maize monoculture with residue removed.

Acidification of a kaolinitic Alfisol under continuous cropping with nitrogen fertilization in West Africa

Increased use of N fertilizer and more intensive cropping due to the rising food demand in the tropics requires design and evaluation of sustainable cropping systems with minimum soil acidification. The objectives of this study were to quantify acidification of an Oxic Kandiustalf with different types of N fertilizer in two cropping systems under no-tillage and its effect on crop performance. Chemical soil properties in continuous maize (Zea mays L.) and maize-cowpea (Vigna unguiculata (L.) Walp) rotation were determined with three N sources (urea (UA), ammonium sulfate (AS) and calcium ammonium nitrate (CAN)) in Nigeria, West Africa, during five years. Chemical soil properties were related to grain yield and diagnostic plant nutrient concentrations. For the three N sources, the rate of decline in soil pH in maize-cowpea rotation was 57±7.5% of that in continuous maize, where double the amount of N fertilizer was applied. The rate of soil acidification during the five years was greater for AS than for UA or CAN in continuous maize, and not different for UA and CAN in both cropping systems. With AS, soil pH decreased from 5.8 to 4.5 during five years of continuous maize cropping. Exchangeable acidity increased with N fertilization, but did not reach levels limiting maize or cowpea growth. Return of residues to the soil surface may have reduced soluble and exchangeable Al levels by providing a source of organic ligands. Soil solution Mn concentrations increased with N fertilization to levels likely detrimental for crop growth. Symptoms of Mn toxicity were observed on cowpea leaves where AS was applied to the preceding maize crop, but not on maize plants. Soil acidification caused significant reductions in exchangeable Ca and effective CEC. Main season maize yield with N fertilization was lower with AS than with UA or CAN, but not different between UA and CAN during the six years of cropping. The lower maize grain yield with AS than with the other N sources was attributed to lower pH and a greater extractable Mn concentration with AS. When kaolinitic Alfisols are used for continuous maize cropping, even under no-tillage with crop residues returned as mulch, the soil may become acidifed to pH values of 5.0 and below after a few years. The no-till cereal-legume rotation with judicial use of urea or CAN as N sources for the cereal crop is a more suitable system for these poorly buffered, kaolinitic soils than continuous maize cropping. The use of AS as N source should be avoided. H Marschner Section editor

Tillage, crop residue, legume rotation, and green manure effects on sorghum and millet yields in the semiarid tropics of Mali.

Alternative soil management practices are needed in semi-arid West Africa to sustain soil fertility and cereal production while reducing the need for extended fallow periods and chemical fertilizers. An experiment was conducted at the Cinzana Station near Segou, Mali to assess the effects of tillage, crop residue incorporation and legume rotation on the growth and yield of sorghum (Sorghum bicolor L. Moench) and pearl millet (Pennisetum glaucum L.) for a period of eight years on a loamy sand and a loam soil. The following treatments were compared under tied ridging and the traditional open ridging: continuous cereal with crop residue removed, continuous cereal with crop residue incorporated, cereal in rotation with cowpea (Vigna unguiculata (L.) Waip.), cereal in rotation with sesbania (Sesbania rostrata Bremek. & Oberm.), and cereal in rotation with dolichos (Dolichos lablab L.). Legumes in rotation were incorporated as green manures except cowpea which was removed after each harvest. Tied ridging improved cereal grain yield from 1022 kg ha−1 with open ridging to 1091 kg ha−1 on the loamy sand and from 1554 kg ha−1 to 1697 kg ha−1 on the loam, when averaged across management regimes and years of cropping. Incorporation of cereal residue at the beginning of the rainy season every other year had only small and inconsistent effects on cereal yield. Rotation with cowpea increased cereal grain and stover yields by 18 and 25%, respectively, on the loamy sand, and by 23% and 27%, respectively, on the loam compared to continuous cereal, when averaged across tillage regimes and years. Sesbania and dolichos performed similarly as green manures on both soils. Incorporation of these legumes as green manure at the end of the rainy season increased cereal grain and stover yields by 37% and 49%, respectively, on the loamy sand, and by 27% and 30%, respectively, on the loam, compared to cereal monoculture without organic amendment, when averaged across tillage regimes and years. A significant linear increase in cereal yield was observed during the eight years of the study on the loam soil when sesbania and dolichos green manures were incorporated.

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.

Decomposition of cowpea and millet amendments to a sandy Alfisol in Niger

Current inputs of organic materials to cropped lands on sandy Alfisols and Entisols in Sahelian West Africa are insufficient to arrest soil organic matter (SOM) decline. Crop residues and green manures require proper management in order to maximize their contribution to nutrient supply and SOM maintenance. The objectives of this study were to quantify the rates of C and N mineralization from cowpea (Vigna unguiculata (L.) Walp.) green manure, cowpea residue, and millet (Pennisetum glaucum (L.) R.Br.) residue under field conditions in Niger and to determine the effect of these organic amendments on pearl millet yield. Millet was grown (1) as sole crop, (2) as intercrop with cowpea, (3) as intercrop with cowpea that was incorporated as green manure during the second half of the growing season, (4) with incorporated cowpea residue (2000 kg ha−1), (5) with millet residue mulch (3000 kg ha−1), and (6) with N fertilizer. Carbon loss as CO2 from soil with and without organic amendment was measured three times per week during the growing season. Nitrogen fertilizer increased millet yield only in a year with a favorable rainfall distribution. Cowpea grown in intercrop with millet during the first part of the growing season and subsequently incorporated as green manure between millet rows increased millet grain yield in a year with sufficient early rainfall, which could be attributed to the rapid rate of decomposition and nutrient release during the first 3 weeks after incorporation. In a year with limited early rainfall, however, densely planted green manure cowpeas competed for water and nutrients with the growing millet crop. Incorporated cowpea residue and millet residue mulch increased millet yield. Surface applied millet residue had high rates of decomposition only during the first 3 days after a rainfall event, with 34% of the millet residue C lost as CO2 in one rainy season. Recovery of undecomposed millet residue at the end of the rainy season was related to presence or absence of termites, but not to seasonal C loss. Millet residue mulch increased soil organic C content of this sandy Alfisol in Niger. Cowpea and millet residues had a greater effect on SOM and millet yield than cowpea green manure due to their greater rate of application and slower rate of decomposition.

Mineralization of carbon and nitrogen from cowpea leaves decomposing in soils with different levels of microbial biomass

Soils with greater levels of microbial biomass may be able to release nutrients more rapidly from applied plant material. We tested the hypothesis that the indigenous soil microbial biomass affects the rate of decomposition of added green manure. Cowpea (Vigna unguiculata L.) Walp.] leaves were added to four soils with widely differing microbial biomass C levels. C and N mineralization of the added plant material was followed during incubation at 30°C for 60 days. Low levels of soil microbial biomass resulted in an initially slower rate of decomposition of soil-incorporated green manure. The microbial biomass appeared to adjust rapidly to the new substrate, so that at 60 days of incubation the cumulative C loss and net N mineralization from decomposing cowpea leaves were not significantly affected by the level of the indigenous soil microbial biomass.

Mineralization of labeled N from cowpea [Vigna unguiculata (L.) Walp.] plant parts at two growth stages in sandy soil

Cowpea [Vigna unguiculata (L). Walp.] has great potential as green manure due to its rapid N accumulation and efficient N2 fixation. The objective of this study was to measure the rate of N mineralization from cowpea plant parts harvested at onset of flowering (5 weeks) and mid pod-fill (7 weeks) under near optimum conditions. Cowpeas were grown in a greenhouse and supplied with 15NH415NO3 to isotopically label tissue. Cowpea leaves, stems, and roots were incorporated into a sandy soil (Psammentic Paleustalf) and net N mineralized was measured several times during a 10 week incubation. The amount of N accumulated in 7-week old cowpeas was more than double that in 5-week old cowpeas. The portion of N mineralized after 10 weeks was 24% for 5-week old cowpeas and 27% for 7-week old cowpeas. The rate of N mineralization from leaves and stems increased with plant age, but decreased for roots. The amount of N mineralized from 7-week old cowpeas was more than double (235%) that from 5-week old cowpeas due to greater N accumulation and a more rapid rate of N mineralization of the more mature cowpeas. The greatest amount of N was released from leaves, which amounted to 74 and 65% of total N mineralization from 5- and 7-week old cowpeas, respectively. The percentage of N mineralized by 10 weeks was linearly related to the tissue N concentration of the plant parts and to their C/N ratio. These relationships allow a quick estimation of the amount of N that would mineralize from cowpea residues incorporated into soil based on their N concentration or C/N ratio.

Cropping System Effects and Seasonal Dynamics of Extractable Phosphorus in a Semi‐arid Soil of the Sahel, West Africa

Abstract Phosphorus (P) is often the most limiting nutrient to crop production in the Sahel. Crop rotation and residue management, as well as time of soil sampling, may influence extractable soil P. The objective of this research was to determine the influence of five long‐term crop‐management systems and temporal soil sampling on Bray 1 P concentrations in a clay soil in 1998, near Cinzana, Mali. This information could be used to predict an availability range for single‐sample soil measurements or to adjust laboratory‐measured soil P according to date sampled. Bray 1 P concentration ranged from 2.7 to 4.2 mg P kg−1 soil on an oven‐dry (105°C) basis in soil collected from 27 May until 3 November 1998. In addition, comparison of extractable soil P between crop‐management systems did not reveal a significant increase in P caused by residue return, crop rotation, or green manure compared to the traditional continuous sorghum [Sorghum bicolor (L.) Moench] cropping system. Crop‐management system did not influence sorghum P uptake, and little P (0.3–0.6 kg ha−1) was returned in residue. Continued cropping without larger inputs of fertilizer P will further deplete soil fertility in the Sahel.