The patterns of primary succession were studied in soil—island communities on a granite outcrop in Georgia. The island communities increase in area and depth over time and the soil becomes more organic. The strong moisture and temperature fluctuations that occur in shallow pioneer soils are significantly reduced in the later stages. Plant biomass and vertical stratification increase throughout succession as larger plant species invade the deeper communities. A small winter annual is the dominant pioneer species. Lichens, annuals, and eventually perennial species invade as succession progresses. Interspecific competition for moisture and nutrients regulates plant species composition at successive stages. Macroarthropod populations increase in density, biomass, and diversity throughout succession. The few soil microarthropod species that occur in the pioneer stages often exhibit rapid density oscillations in the shallow substrates. The deeper and more environmentally constant substrate of later stages contains a greater variety of microarthropods. Biotic diversity generally increases during primary succession on the outcrops. Plant diversity peaks at intermediate stages while microarthropod and macroarthropod diversity increase from pioneer through later stages. The strong physical factors on the outcrops determine the rate and extent of community development in particular soil—islands. However, as the many soil—islands undergo succession they converge in community characteristics such as total density, biomass, and diversity.

Data are assembled from the Rothamsted classical field experiments on the effects of long-continued cropping and manuring on the amount of organic matter in soil, on the age of this soil organic matter, on the amount of microbial biomass in the soil, and on the rate at which plant residues decompose in these soils. These data were then fitted to a model in which soil organic matter was separated into five compartments: decomposable plant material (DPM, half-life 0.165 years); resistant plant material (RPM, 2.31 years); soil biomass (BIO, 1.69 years); physically stabilized organic matter (POM, 49.5 years) and chemically stabilized organic matter (COM, 1980 years) For unitary input of plant material (1 t fresh plant C ha year) under steady-state conditions, after 10,000 years, the model predicts that the soil will contain 0.01 t C in DPM, 0.47 t in RPM, 0.28 t in BIO, 11.3 t in POM, and 12.2 t in COM. The predicted radiocarbon age is 1240 years (equivalent age). The fit between predicted and measured data is sufficiently good to suggest that the model is a useful representation of the turnover of organic matter in cropped soils.

A simulation model of the phosphorus cycle in semiarid grasslands was developed and tested. When used with data sets for biotic and abiotic driving variables of the Pawnee (Colorado) and Matador (Saskatchewan) Sites, this model predicted plant and decomposer uptakes and turnover rates of the principal phosphorus compartments. Daily P requirements for plant and decomposer uptake are taken from pools of labile inorganic P in each soil layer. These pools are replenished mainly by mineralization of labile organic P as well as leaching of water—soluble forms from standing dead biomass and litter. Phosphorus solubility (recalculated daily from relationships to the labile inorganic pools), soil H₂O content, and rates of diffusion of P through soil are the major controls on rates of uptake by the active fraction of the live root biomass. Model operation was found to be more sensitive to soil parameters than to plant parameters. Simulation results indicated rates of decomposer uptake 4—4x greater than plant uptake in semiarid grasslands. Simulated P concentrations in live plant tops were highly responsive to the pattern of seasonal rainfall which agreed well with published data. The most critical informational needs revealed by model development and operation were in the areas of activity and morphology of roots and the rates of mineralization of organic P as affected by soil depth.

51 reserved fields were studied with the harvest method in Central Finland in 1974. 107 vascular plant taxa were identified, having a total oven-dry green biomass of 273.5 g/m2 on the average, and a total mean biomass of 1458.1 g/m2. The amount of the above-ground biomass stays about the same at least for three years after 23 years of increase, whereas the underground biomass increases strongly at least during the first six years, if succession starts after open cultivation. The general tendency in succession at the species level is for the typical weed species of open cultivations to reduce in a few years, and for the species of meadow vegetation to increase both in frequency and abundance. Five vegetation types were distinguished: I) Galeopsis-type, 2) Phleum-type, 3) Anthoxanthum-type, 4) Deschampsia-type, and 5) Elytrigia-type. They can all be placed into a certain succession scheme that is mainly determined by the age, soil and moisture conditions of the reserved field.

Vast hectarages of potentially valuable pasture land in Colorado and Wyoming are dominated by saltgrass (Distichlis stricta (Torr.) Rydb.), an undesirable pasture component. Saltgrass meadows could possibly be revegetated with a more valuable forage species such as tall wheatgrass (Agropyron elongatum (Host) (Beauv.). A greenhouse study was conducted to evaluate the effects of the A, B, and C horizons of a solonetz soil on herbage yield of tall wheatgrass; the effect of N application was also investigated. Yields of plants grown in soils from the A horizon were highest, those from the B horizon where somewhat lower, and those from the C horizon were very low. Mixtures of A and B horizons usually produced higher yields than the A or B alone. Plants grown in any soil mixture which included the C horizon showed reduced yields. The deleterious effect of the C horizon was attributed to its high Na-salt content. Nitrogen, applied at rates equivalent to 56 to 448 kg N/ha, significantly increased herbage yield. Compared to no N application, the highest rate resulted in a three-fold increase in yield. The highest N efficiences in terms of total biomass produced per unit of N applied were obtained from rates of 112 and 224 kg N/ha which produced 39 and 37 kg of additional biomass per kg of N applied, respectively.