Degraded soil surface conditions favor surface runoff, thus enhancing sediment and nutrient losses. This study examined the effect of crop residues on carbon (C), nitrogen (N), total phosphorus (TP), and total dissolved P (TDP) losses by surface runoff in an agricultural loamy soil with degraded structure. Runoff and sediment yield were made over 1-m 2 plots using a rainfall simulator with constant 65 mmh 21 intensity. Four successive rainfall applications were performed, the first three ones 25 mm each and the last one 65 mm. Added corn straw varied between 0 and 4 tha 21 in the five studied treatments. Organic C, N, and TP concentrations in runoff showed a trend to decrease because of the effect of corn straw on soil losses. However, TDP concentration increased with increasing crop residue in the lower two rainfall events. Sedimentary P constituted the major proportion of P in runoff. After 140 mm of cumulative rainfall, nutrient losses were as follows: organic C from 137.7 to 792.7 kgha 21 , N from 16.65 to 88.82 kgha 21 , TP from 1.72 to 7.87 kgha 21 , and TDP 57 to 87 gha 21 . Total losses of the studied nutrients were correlated with soil losses.
Phosphorus (P) export in watershed runoff can accelerate the eutrophication of fresh waters. Intensification of crops and animal farming in many areas has created regional imbalances in P inputs and P outputs in farm practice. The aim of this study was to assess the temporal changes in concentration and losses of total P (TP), total sedimentary P (TSP), and total dissolved P (TDP) at the outlet of an agroforestry catchment 36.3 km 2 in size located at northwestern Spain. The study data sets range from January 1999 to December 2005. Phosphorus load of surface water sampled at the catchment outlet was strongly affected by agricultural practices. Most particulate SP was exported during storm flow events. The TSP contents ranged up to 1063 mg/L, and TDP contents ranged up to 259 mg/L. However, in most of the study events particulate P was less than 300 mg/L and during base flow less than 50 mg/L. Total and particulate P showed a very close relationship, but there was no consistent relationship between water discharge and total P or dissolved P. A few events of intensive precipitation are responsible for the transport of suspended sediments and particles, mobilizing sediments from different source areas. The mean yearly TP export was 0.80 kg/ha and annual dissolved TDP export was approximately 0.30 kg/ha.
Tillage practices may reduce the organic matter content in near-surface soil horizons causing crust formation. Surface conditions may cause an increase in surface run-off, thus enhancing contaminant transfer of heavy metals or an acceleration in nutrient loss. This study examines the effect of applying crop residues to the surface of tilled soils on heavy metal losses by run-off. Losses in iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu) were analysed. Run-off and sediment yield were measured on 1 m 2 plots using a rainfall simulator with a constant 65 mm ⁄ h intensity. Four successive rainfall applications were performed, the first three at 25 mm each and the last at 65 mm. Added corn straw varied between 0 and 4 t ⁄ ha in the five studied treatments. After 140 mm cumulative rainfall, total heavy metal losses were as follows: Fe from 137 to 950 mg ⁄ L, Mn from 2.3 to 12.83 mg ⁄ L, Cu from 0.09 to 0.72 mg ⁄ L and Zn from 0.31 to 2.46 mg ⁄ L. Dissolved fractions were as follows: Fe from 0.014 to 0.229 mg ⁄ L, Mn from 0.034 to 1.45 mg ⁄ L, Cu from 0.002 to 0.013 mg ⁄ L and Zn from 0.02 to 0.12 mg ⁄ L. Total concentrations of the studied elements decreased exponentially due to the effect of corn straw on soil loss. However, dissolved contents of Fe and Cu scarcely varied. Significant positive linear correlations were observed between total heavy metal content and soil and sediment loss by run-off. It is concluded that the addition of straw to a soil of low fertility prevents heavy metal loss.
There is an increasing awareness worldwide of the conveyance of sediment-associated nutrients across eroding soil surfaces into receiving waters. The objective of this study was to analyze the temporal oscillations of concentrations as well as the exportation of calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) at the outlet of an agroforestry catchment. These macronutrients were measured in the drainage waters of a 36.3-km 2 catchment located at the Valiñas River (Coruña, northwest Spain), where 560 samples were taken from 2003 to 2007. The water collection strategy was a stratified point sampling involving more frequent collections when flow was high. Element contents in the water were analyzed by inductively coupled argon plasma-mass spectrometry (ICP-MS). The dissolved content ranges were as follows: Ca between 4.47 and 20.7 mg L -1 , Mg between 2.4 and 7.3 mg L -1 , K between 1.13 and 8.6 mg L -1 , and Na between 8.9 and 27.2 mg L -1 . Significant correlations among all the analyzed elements were observed during the entire study period. Mostly, nutrient losses were related to degradation induced by soil erosion on the cultivated land portion of the catchment.
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