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)
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