piration and heterotrophic microbial respiration. Net ecosystem exchange of CO 2 as an integration of photo-Environmental controls on C cycling in terrestrial ecosystems are synthesis, plant dark respiration, and soil respiration in difficult to define, because (i) C fluxes from plant vs. microbial activity are difficult to separate, and (ii) controlling variables are often inter-grasslands can be obtained with various micrometeorocorrelated. We investigated temporal and spatial determinants of soil logical techniques, which integrate across large land areas respiration and whole-ecosystem respiration using nighttime exposure (Verma, 1990; Norman et al., 1992). Knowing the contriof static chambers to alkali absorption during 2 yr on a tallgrass prairie bution of soil respiration to these fluxes would improve in northeastern Kansas. Soil respiration (mg CO 2-C m Ϫ2 h Ϫ1) was posour understanding of the C cycle and help determine rates itively related to soil organic C (SOC, kg m Ϫ2 0.1 m Ϫ1) through linear of ecosystem C sequestration. Separation of soil resregression [CO 2-C ϭ Ϫ44 ϩ (40 SOC), r 2 ϭ 0.71]. Temporal variations in piration from whole-ecosystem respiration is best suited respiration were related to soil temperature, water-filled pore space during the nighttime, when photosynthetic fixation of (WFPS), and a plant growth rate function, with a combined R 2 of 0.76 CO 2 is not a factor. There is also a need to better underfor soil respiration and of 0.84 for whole-ecosystem respiration. Temstand whole-ecosystem respiration during the nighttime, poral variograms suggested that both soil and whole-ecosystem respiration became increasingly dissimilar the longer the time between mea-since micrometeorological techniques for net ecosystem surements up to 30 d, while dissimilarity in soil temperature and WFPS exchange of CO 2 are generally less suited during the nightleveled between 10 and 20 d of separation. A plant growth rate functime than during the daytime, because of less reliable ention was an important variable that controlled whole-ecosystem respiergy balance, concentration gradients, and wind speeds ration, as well as soil respiration. The ratio of soil respiration to wholeneeded for calculations (Harper, 1989). ecosystem respiration was ≈0.4 during maximum plant growth (July) Previous studies have indicated a high degree of spaand approached a value of 1 during minimal plant growth (November tial and temporal variability in soil respiration that makes to March). We conclude that whole-ecosystem respiration is under simextrapolations of findings to different ecosystems diffiilar environmental controls as soil respiration, the main variables being cult (Buyanovsky et al., 1986; Kiefer, 1990; Rochette et soil organic C, soil temperature, WFPS, and plant growth rate, which al., 1991). Even when attempting to extrapolate results all control the supply of readily mineralizable substrates.
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 AI 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.
ABSTRACTpiration and heterotrophic microbial respiration. Net ecosystem exchange of CO 2 as an integration of photoEnvironmental controls on C cycling in terrestrial ecosystems are synthesis, plant dark respiration, and soil respiration in difficult to define, because (i) C fluxes from plant vs. microbial activity are difficult to separate, and (ii) controlling variables are often intergrasslands can be obtained with various micrometeorocorrelated. We investigated temporal and spatial determinants of soil logical techniques, which integrate across large land areas respiration and whole-ecosystem respiration using nighttime exposure (Verma, 1990;Norman et al., 1992). Knowing the contri- needed for calculations (Harper, 1989).ecosystem respiration was ≈0.4 during maximum plant growth (July)
Smectitic soils of the tropics are medium- to fine-textured alluvial soils containing moderate to large amounts (20% or more) of smectite, a shrinking and swelling clay mineral, in the clay fraction. Small to moderate amounts of other layer silicate minerals, such as illite, chlorite, vermiculite, and kaolinite, are also present in the clay fraction. Smectitic soils have moderate to high values of CEC (10-50 cmol/kg of soil), high base saturation, and high water-retention capacity. These soils are usually developed on alluvial materials rich in basic cations, especially Mg. Smectitic soils commonly occur on alluvial plains in river valleys and deltas as well as in inland depressions. In the wetter tropics, large areas of smectitic soils are found in tropical Asia, especially Vietnam, Thailand, and Myanmar (Burma). These young alluvial soils are rich in nutrient-bearing weatherable minerals, such as micas, feldspars, and hornblende. Smectitic soils on the alluvial plains and inland valleys have a shallow groundwater table, and some soils are flooded during the rainy season. Thus, they are best suited for rice cultivation. For example, in the flood plains along the Mekong and Chao Phraya rivers of the Indo- China peninsula, mineral-rich deposits from annual flooding are able to maintain relatively high rice yields with little or no additional nutrient inputs. Smectitic soils occurring in seasonally flooded coastal mangrove swamps are known as acid sulfate soils. These soils are used for cultivation of swamp rice and floating rice during the rainy season, depending upon the depth of flooding by fresh water. In drier regions, clayey smectitic soils (mainly Vertisols) often exhibit large cracks during the dry season and become very sticky and difficult to work with during the rainy season. In the drier tropics, large areas of clayey smectitic soils are found in central India, central Sudan, southern Ghana, and in the Lake Chad region of central Africa. Clayey smectitic soils are usually found in the inland depressions scattered throughout the drier regions of West, East and Central Africa. Because of their high chemical fertility, these soils are important soils for cropping and grazing in the drier tropics.
No abstract
Cowpea [Vigna unguiculata (L). Walp.] has great potential as green manure due to its rapid N accumulation and efficient N 2 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 ~SNH415NO3 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.
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