Carbon and nitrogen dynamics in a Brazilian soil-pasture system irrigated with treated sewage effluent

The use of treated sewage effluents for agricultural irrigation represents a promising agronomic-environmental-economic practice; however, little is known about its effects on chemical attributes of tropical soils. This research project aimed at evaluating the effects of irrigation with secondary-treated sewage effluent (STSE) and various application rates of mineral-N fertilizer (MNF) on total carbon (TC) and nitrogen (TN) contents, and ammonium (NH4+-N) and nitrate (NO3--N) concentrations in a tropical soil-pasture system. The experimental field was cropped with Tifton 85 bermudagrass, over two years. The treatments were: T1 (control), irrigation with potable water and addition of MNF, 520 kg ha(-1) year(-1); T2-T5, irrigation with STSE (31.9 mg L-1 of total-N) and addition of MNF: 0, 171.6, 343.2, and 520 kg ha-1 year(-1), respectively. Soil TC and TN concentrations normally decreased over the experimental period. In general, higher NO3--N concentrations in the soil solution were found for treatments receiving higher rates of MNF plus TSE irrigation. Compared to fresh water irrigation, STSE caused no changes in soil TC and TN concentrations, and it resulted in slightly higher mineral N concentrations in the soil solution, but without environmental risk. N concentrations in the soil solution were directly influenced by the high N uptake capacity of the Tifton 85 bermudagrass.