Nutrient and Organic Carbon Losses, Enrichment Rate, and Cost of Water Erosion

ABSTRACT Soil erosion from water causes loss of nutrients and organic carbon, enriches the environment outside the erosion site, and results in costs. The no-tillage system generates increased nutrient and C content in the topsoil and, although it controls erosion, it can produce a more enriched runoff than in the conventional tillage system. This study was conducted in a Humic Cambisol in natural rainfall from 1997 to 2012 to quantify the contents and total losses of nutrients and organic C in soil runoff, and to calculate the enrichment rates and the cost of these losses. The treatments evaluated were: a) soil with a crop, consisting of conventional tillage with one plowing + two harrowings (CT), minimum tillage with one chisel plowing + one harrowing (MT), and no tillage (NT); and b) bare soil: one plowing + two harrowings (BS). In CT, MT, and NT, black oat, soybean, vetch, corn, turnip, and black beans were cultivated. Over the 15 years, 15.5 Mg ha-1 of limestone, 525 kg ha-1 of N (urea), 1,302 kg ha-1 of P2O5 (triple superphosphate), and 1,075 kg ha-1 of K2O (potassium chloride) were used in the soil. The P, K, Ca, Mg, and organic C contents in the soil were determined and also the P, K, Ca, and Mg sediments in the runoff water. From these contents, the total losses, the enrichment rates (ER), and financial losses were calculated. The NT increased the P, K, and organic C contents in the topsoil. The nutrients and organic C content in the runoff from NT was greater than from CT, showing that NT was not a fully conservationist practice for soil. The linear model y = a + bx fit the data within the level of significance (p≤0.01) when the values of P, K, and organic C in the sediments from erosion were related to those values in the soil surface layer. The nutrient and organic C contents were higher in the sediments from erosion than in the soil where the erosion originated, generating values of ER>1 for P, K, and organic C. The value of the total losses of nutrients in the form of triple superphosphate fertilizer, potassium chloride, and urea and limestone by water erosion was higher in CT than in NT. For triple superphosphate, the cost of erosion losses was 29 % higher in NT than in CT, while in urea and limestone, the effectiveness of NT in reducing costs was 65 and 50 %, respectively