Soil physical properties often regulate aeration-dependent microbial activities important to nutrient cycling, soil fertility and environmental quality. Microbial activity depends on soil water content and is maximum at a water content where the limiting effects of substrate diffusion and O 2 supply are equal. The mechanism whereby this occurs and predictions of the soil water content where aerobic microbial activity is a maximum were the objectives of this study. In particular, this study predicted the shape of the microbial activity vs. water content function from soil physical concepts. Soil physical processes are assumed to influence microbial activity by limiting the steady flux of a required substrate or O 2 to sites of microbial activity. Steady-state flux relations are used to define the activity function. The dependence of diffusion coefficient on water content or air-filled porosity is assumed. With these assumptions, it is possible to show that a maximum in the activity function exists. The predicted shape of the activity curve is consistent with experimental observations. The relationship between aeration-dependent microbial activity and soil water content facilitates evaluating the indirect effects of soil management practices, such as tillage, on microbial activity.
Total coliform (TC), fecal coliform (FC), and fecal streptococcal (FS) numbers were monitored for 3 years to determine the effect of grazing on the presence of these organisms in runoff from a cattle‐grazed and a nongrazed watershed in the Pacific Northwest. The watersheds were characterized by winter precipitation and summer grazing. Weighted‐average numbers of TC and FS in runoff did not appear to be appreciably different between the two watersheds during the study. Numbers of TC in runoff from both watersheds routinely exceeded 10,000/100 mL. Prolonged absence of grazing animals did not seem to affect number of TC and FS in runoff from the check watershed. Each spring after a period of warm weather and prolonged absence of animals, there were increases in numbers of TC, FC, and FS in the runoff. There was some correlation between recentness of grazing and numbers of indicator bacteria in runoff. However, more than a year after animals were removed from the nongrazed check watershed FC numbers in runoff still exceeded 200/100 mL in many samples, and not until the following year did they drop to < 10/100 mL. Sampling at several locations within the grazed watershed showed that sources of indicator bacteria were well distributed, and as a result were nonpoint after the initial runoff events. Thus, present FC recommendations developed for point‐sources would not apply adequately to grazed land in the Pacific Northwest. Indicator bacteria as presently analyzed would not provide a basis for developing best management practices.
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.
The causative factors for corn (Zea mays L.) response to methyl bromide (CH3Br) fumigation, in the absence of known specific pathogens, are unknown. This study was conducted to determine if deleterious nonspecific rhizosphere microorganisms are the causative agents. Soil fumigation was postulated to increase yield of continuous corn but to have less effect on the yield of corn grown in rotation, because more deleterious rhizosphere microorganisms were suspected in monoculture than in rotation. The effects of fumigation and crop rotation on corn grain yield, plant height and P content, available soil N and P, mycorrhizal infection, and soil microbial biomass were investigated at two sites near Mead, NE, on a Sharpsburg silty clay loam soil (fine, montmorillonitic, mesic Typic Argiudoll). Methyl bromide treatments were applied prior to planting for 4 yr at one site and 2 yr at the other site. The sites differed in fertilization, pest management, and crop sequences, but both contained continuous corn. Fumigation resulted in an increase in grain yield under monoculture only once in six site years. Unexpectedly, however, fumigation resulted in a decrease in grain yield under rotation in 7 of 12 observations. Plants in fumigated soil were P‐deficient early in the growing season despite similar soil test P concentrations in control and fumigated plots. Fumigation reduced mycorrhizal infection and soil microbial biomass. Mycorrhizal infection of corn shortly after germination appears to be important to initial corn growth in this soil. Fumigating soil revealed a considerable biological influence on corn growth and yield by reducing both deleterious and beneficial microorganisms.
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