SEWAGE-SLUDGE EFFECTS ON SOIL AND PLANT-QUALITY IN A DEGRADED, SEMIARID GRASSLAND

A major problem affecting grassland productivity in the semiarid southwestern USA is the low quantity of soil organic matter and plant-available N. In this study, dried, anaerobically digested sewage sludge was applied at three rates (22.5, 45, and 90 Mg ha-1) to a degraded semiarid grassland site to determine the effects of sludge on soil chemical and heavy metal properties, and vegetative yields and quality over two growing seasons. Most nutrients, including soil N, P, and K, increased linearly as a result of sludge amendment. Soil pH decreased linearly from 7.8 to 7.4 with the application of sludge after the second growing season, but did not significantly increase the solubility of soil heavy metals (Pb and Cd). On the other hand, the levels of DTPA-extractable soil micronutrients (i.e., Cu, Mn, Zn) increased linearly with sludge rates to levels recommended for adequate plant growth. Total herbaceous plant yields increased significantly with sludge amendment, particularly at the 45 Mg ha-1 sludge rate, as compared with the control treatment. Blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Steud.] yields increased two- to threefold in amended plots over the unamended control after two growing seasons. The forage quality of blue grama, galleta [Hilaria jamesii (Torr.) Benth.], and bottlebrush squirreltail [Sitanion hystrix (Nutt.) J.G. Sm.], improved significantly as the levels of tissue N, P, and K, and crude protein increased linearly with sludge application. As expected, levels of Cd and Pb in all plant tissues did not increased significantly as a result of sludge amendment. All micronutrients increased to acceptable levels in deficient plant tissues. Sewage sluge applied at the rate of 45 Mg ha-1 gave the most favorable soil and vegetaion results.