The soil microbial community in a sewage-sludge-amended semi-arid grassland

Summary The value of sewage sludge for improving the fertility and productivity of a degraded semi-arid grassland soil was tested by quantifying and describing the effects of surface application of sewage sludge on soil chemical properties and the soil microbial community. Three surface application rates (22.5, 45, and 90 Mg sludge ha^–1) were tested over the course of two growing seasons. Most nutrient levels, including N, P, and K, increased linearly with increasing sludge application rates. Soil pH, however, declined linearly, from 7.8 to 7.4, with increasing sludge application rates. With the exception of Zn, heavy metals, including Cd, did not increase with the small decrease in pH or with increasing sludge application rates. Soil bacterial, fungal, and ammonium oxidizer populations increased linearly with increasing sludge application rates, and Streptomyces spp. populations remained relatively unchanged. The diversity of fungal groups declined initially with increasing sewage sludge rates but rebounded to near pretreatment levels under the low and intermediate application rates within 1 year. High fungal populations and low fungal diversity were related to the high nutrient contents provided by sludge amendment. Mucor spp. and Penicillium chrysogenum dominated the sludge-amended soils, and their densities in the treated soils in the first growing season were almost directly proportional to the sludge application rates. The improvement in soil fertility of a degraded semi-arid grassland due to sludge application was reflected in populations, diversity, and composition of the soil microbial community.The research reported here was conducted in cooperation with the USDI Bureau of Land Management which furnished funds and field study locations     

Nitrogen mineralization in coal mine spoil and topsoil

Summary N mineralization was compared in fresh topsoil, stockpiled topsoil, coal-mine spoil, and in various mixtures of soil and mine spoil, with and without sludge amendment. N mineralization was slightly lower in stockpiled topsoil than in fresh topsoil or mine spoil. Differences between stockpiled and fresh topsoil were small and were attributed to changes in the relative proportions of readily degradable versus slowly degradable organic fractions. Sludge amendment increased N mineralization, but straw amendment immobilized N through 12 weeks of incubation. More N was leached from mixtures of mine spoil and soil than from soil or mine spoil alone, but net N mineralization decreased with increasing mine-spoil-to-soil ratios, probably as a result of greater denitrification losses. Mixing mine spoil with soil can increase plant available N more than soil or spoil alone, because spoil contributes mineralizable organic N while soil improves the physical and chemical environment of the mine spoil.     

Heavy metal uptake by blue grama growing in a degraded semiarid soil amended with sewage sludge

Sewage sludge application to semiarid grassland may represent a beneficial means of utilizing this waste product for restoration of degraded sites. Consequently, dried municipal sewage sludge was applied at three rates (22.5, 45, and 90 Mg ha^–1) to a degraded semiarid grassland soil in order to determine the effects of sludge amendments on forage productivity, soil heavy metal content, and metal uptake by blue grama (Bouteloua gracilis).Soil and plant properties in control and amended plots were measured after 1, 2, and 5 growing seasons.Soil nutrients increased linearly with increased sludge application in the first two growing seasons. Consequently, forage quality and total production of blue grama improved significantly over the unamended control as the tissue levels of N, P, K, and crude protein increased. Cadmium and Pb in the sludge-treated plots did not increase significantly over the control after 1 and 2 growing seasons. Levels of DTPA-extractable soil micronutrients (Cu, Fe, Mn, Zn) increased linearly with increased sludge application rate to soil concentrations recommended for adequate plant growth. Soil N, P, and K concentrations remained higher in the sludge-amended soils after 5 growing seasons, while Cu and Cd increased to slightly above desireable limits as the soil pH decreased to 7.4 and 7.0 in the 45 and 90 Mg ha^–1 treatments, respectively. However, with the exception of Mn which remained within desirable limits, metal concentrations (including Cu and Cd) in blue grama tissue were not significantly different from the control treatment after five growing seasons. Based on soil and plant tissue metal concentrations, it appears that sludge applied at rates between 22.5 and 45 Mg ha^–1 will maintain the most favorable nutrient levels coupled with significant improvements in forage production in this semiarid grassland environment.