José Manuel Mirás Avalos

Crop Residue Effects on Organic Carbon, Nitrogen, and Phosphorus Concentrations and Loads in Runoff Water

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‐m2 plots using a rainfall simulator with constant 65 mmh−1 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−1 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−1, N from 16.65 to 88.82 kgha−1, TP from 1.72 to 7.87 kgha−1, and TDP 57 to 87 gha−1. Total losses of the studied nutrients were correlated with soil losses.

Phosphorus Contents and Loads at the Outlet of an Agroforestry Catchment in Northwestern Spain

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 km2 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 µg/L, and TDP contents ranged up to 259 µg/L. However, in most of the study events particulate P was less than 300 µg/L and during base flow less than 50 µg/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.

Crop Residue Effects on Calcium, Magnesium, Potassium, and Sodium Runoff Losses from a Soil Prone to Crusting

In Galicia (northwestern Spain), cultivated soils developed on schists from the Ordenes series are susceptible to surface degradation, mostly when soil organic content decreases. Therefore, management systems that protect the soil and increase its organic-matter content should also improve its quality. However, tillage practices may cause crusting. Degraded soil surface conditions favor surface runoff, thus enhancing nutrient losses. This study examined the effect of applying crop residues to the soil surface on the basic cation nutrient [calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na)] losses by runoff from a tilled soil with relatively low organic-matter content. Runoff and sediment yield were measured on 1-m2 plots using a rainfall simulator with constant 65 mm h−1 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−1 in the five treatments studied. Total and dissolved concentrations of the elements studied showed a tendency to decrease due to the effect of corn straw on soil losses. After 140 mm cumulative rainfall, total nutrient losses were as follows: Ca from 12.32 to 28.94 mg L−1, Mg from 20.81 to 148.90 mg L−1, K from 14.20 to 35.17 mg L−1, and Na from 14.99 to 23.41 mg L−1. The relative contribution of the dissolved fraction to the total nutrient content loss was highly variable, being up to 90% for Na. The results confirm that corn residues applied to a degraded soil, with low structural stability, prevent cation nutrient losses.

Seasonal Evolution of Soil Dehydrogenase Activity at Two Different Depths in an Eucalyptus Stand and a Cultivated Field

Dehydrogenase activity is a good indicator of changes in fertility, quality and microbiological status of the soil, which may vary seasonally. The aim of this study was to evaluate the seasonal evolution of this activity in two different soils (a cultivated field and an Eucalyptus stand) located in a periurban area of Coruna (NW Spain). From April 2004 to April 2005, a total of 38 soil samples were collected in the forest site and 152 samples in the cultivated field (two replicates at the top and the bottom of the hillslope at two depths: 0–5 cm and 5–10 cm). General soil properties were analyzed by routine methods, soil respiration under laboratory conditions and dehydrogenase activity using soluble tetrazolium salts. This activity ranged from 5 10−5 to 0.033 μg TPF g−1 soil 24 h−1 among individual samples in the forest, whereas it varied from 0.005 to 0.023 μg TPF g−1 soil 24 h−1 in the cultivated field. No differences with depth were observed in the cultivated field; whereas in the forest soil, higher activities were found in the top layer, which combined better conditions for microbial development. Dehydrogenase activity was affected by position and season in the cultivated field; higher values were detected in the top of the hillslope. In the forest site, higher values were observed in the top layer during spring and fall; due to favourable temperature and moisture conditions. In the cultivated field, dehydrogenase activity was correlated to soil basal respiration and soil pH, though, in the forest soil, no significant correlations were observed.

ESTIMATIVA DAS PERDAS DE SOLO E DEPOSIÇÃO DE SEDIMENTOS EM UMA SUB-BACIA HIDROGRÁFICA SOB PROCESSO DE DEGRADAÇÃO AMBIENTAL

As perdas de solo e a deposicao de sedimentos tem sido a principal causa da degradacao ambiental em bacias hidrograficas. Este trabalho teve por objetivo estimar as perdas de solo e a deposicao de sedimentos na sub-bacia Jardim Novo Horizonte em diferentes eventos de precipitacao, com o intuito de identificar as areas com maior suscetibilidade a erosao utilizando como ferramenta o modelo LISEM. Os solos da sub-bacia sao o Latossolo Vermelho e o Argissolo Vermelho-Amarelo, ambos de textura media. Para as simulacoes foi utilizado o modelo LISEM, sendo coletados dados de intensidade de precipitacao dos anos de 2009 a 2012, o qual foram selecionados 10 eventos com maiores intensidades de precipitacao para o periodo avaliado. Para obtencao dos parâmetros de entrada necessarios, foram coletas amostras de solos. Estas coletas foram realizadas em dez locais ao longo da sub-bacia, sob as diferentes formas de uso e ocupacao do solo, sendo seis localizados no Latossolo e quatro no Argissolo. As perdas de solo e a deposicao de sedimentos estao relacionadas com o escoamento superficial em todos os eventos analisados, observando-se maiores valores para os eventos com maiores intensidades de precipitacao. Os locais apontados pelos mapas gerados pelo modelo como mais suscetiveis a processos erosivos correspondem as areas cultivadas com pastagens, e isso ocorre devido a degradacao do solo e manejo inadequado destas areas. O modelo LISEM se mostrou eficiente para a localizacao de areas suscetiveis aos processos erosivos na sub-bacia estudada.

Temporal Oscillations of Calcium, Magnesium, Potassium, and Sodium Dissolved Contents in an Agroforestry Catchment from the Atlantic Galicia

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-km2 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.

Oscillation of Three Phosphorus Forms and Suspended Solids Content from 1999 to 2007 in a Spanish Agroforestry Catchment under Atlantic Climate

Phosphorus (P) transfers may accelerate water eutrophication to waters. Increasing awareness of the role of diffuse P sources motivates land managers in different regions of Europe to undertake conservation programs that place emphasis not only on soil conservation but also on water quality and eutrophication risk. Fertilizer applications and wastes are the main sources of dissolved P in Galicia and in the Atlantic regions of Spain. The aim of this study was to assess the temporal changes in concentration of total P (TP), sedimentary P (SP), and total dissolved P (TDP) and suspended solids at the outlet of an agroforestry catchment located in northwestern Spain. The study datasets range from January 1999 to December 2007, with 992 water samples collected. The water-collection strategy was a stratified point sampling involving more frequent collections when flow was high. Phosphorus contents were assessed by inductively coupled plasma (ICP)–mass spectroscopy (MS), and suspended solids were measured by filtration. The content ranges of the different studied P forms and suspended solids were as follows: TDP between 1 and 672 mg L−1, SP between 1 and 1064 mg L−1, TP between 1 and 1111 mg L−1, and suspended solids between 1 and 1044 mg L−1. A few events of intense precipitation with peaks of TP greater than 200 mg L−1 and in some cases even more than 1000 mg L−1 were responsible for most of the P losses in the catchment studied. TP and SP, TP and suspended solids, and SP and suspended solids showed highly significant correlations during the entire study period, evidencing the erosive origin of P in this catchment.

On Geostatistical Analysis of Rainfall Using Data from Boundary Sites

This study examines the effect of considering data from rain gauges nearby the boundaries of Galicia (NW Spain) in order to minimize the border effect. Two datasets were considered: the first one comprised 232 climatic stations within Galicia and the second one consisted of 322 rain gauges including the former 232 from Galicia and adding 90 stations from boundary provinces (42 from Asturias, 31 from Leon and 17 from Zamora). Total monthly rainfall data from 2006 was analyzed and descriptive statistics demonstrated slight differences between both datasets. Theoretical structures were described for all the studied monthly datasets. Spatial dependence analysis showed that the best-fitting semivariogram model structure was the same for both datasets in most of the cases, even though the model parameters showed great differences. Similarly, cross-validation parameter values were clearly distinct among datasets; mostly, the ones corresponding to the 322 stations dataset were closer to the ideal values. Ordinary kriging was performed for both datasets and resulting variance maps showed improvements when the information from boundary regions was taken into account. These improvements can reach up to 25% of the maximum variance value and they were observed in wet months such as January whereas, in dry months such as July, no improvement was observed. Minimum error values were usually lower when extra information was used in the interpolations. In conclusion, a better mapping of the rainfall within a region can be achieved using data from boundary areas, reducing the variance of the estimates.