J. Causapé

Irrigation agriculture affects organic matter decomposition in semi-arid terrestrial and aquatic ecosystems

Many dryland areas are being converted into intensively managed irrigation crops, what can disrupt the hydrological regime, degrade soil and water quality, enhance siltation, erosion and bank instability, and affect biological communities. Still, the impacts of irrigation schemes on the functioning of terrestrial and aquatic ecosystems are poorly understood. Here we assess the effects of irrigation agriculture on breakdown of coarse organic matter in soil and water. We measured breakdown rates of alder and holm oak leaves, and of poplar sticks in terrestrial and aquatic sites following a gradient of increasing irrigation agriculture in a semi-arid Mediterranean basin transformed into irrigation agriculture in 50% of its surface. Spatial patterns of stick breakdown paralleled those of leaf breakdown. In soil, stick breakdown rates were extremely low in non-irrigated sites (0.0001-0.0003 day(-1)), and increased with the intensity of agriculture (0.0018-0.0044 day(-1)). In water, stick breakdown rates ranged from 0.0005 to 0.001 day(-1), and increased with the area of the basin subject to irrigation agriculture. Results showed that irrigation agriculture affects functioning of both terrestrial and aquatic ecosystems, accelerating decomposition of organic matter, especially in soil. These changes can have important consequences for global carbon budgets.

Impacts of agricultural irrigation on nearby freshwater ecosystems: the seasonal influence of triazine herbicides in benthic algal communities.

A small hydrological basin (Lerma, NE Spain), transformed from its natural state (steppe) to rain-fed agriculture and recently to irrigation agriculture, has been monitored across four seasons of an agricultural year. The goal of this study was to assess how and whether agricultural activities impacted the nearby freshwater ecosystems via runoff. Specifically, we assessed the toxicity of three triazine herbicides, terbuthylazine, atrazine and simazine on the photosynthetic efficiency and structure of algal benthic biofilms (i.e., phototropic periphyton) in the small creek draining the basin. It was expected that the seasonal runoff of the herbicides in the creek affected the sensitivity of the periphyton in accord with the rationale of the Pollution Induced Community Tolerance (PICT): the exposure of the community to pollutants result in the replacement of sensitive species by more tolerant ones. In this way, PICT can serve to establish causal linkages between pollutants and the observed biological impacts. The periphyton presented significantly different sensitivities against terbuthylazine through the year in accord with the seasonal application of this herbicide in the crops nowadays. The sensitivity of already banned herbicides, atrazine and simazine does not display a clear seasonality. The different sensitivities to herbicides were in agreement with the expected exposures scenarios, according to the agricultural calendar, but not with the concentrations measured in water, which altogether indicates that the use of PICT approach may serve for long-term monitoring purposes. That will provide not only causal links between the occurrence of chemicals and their impacts on natural communities, but also information about the occurrence of chemicals that may escape from traditional sampling methods (water analysis). In addition, the EC50 and EC10 of periphyton for terbuthylazine or simazine are the first to be published and can be used for impact assessments.

Effect of Wetlands on Water Quality of an Agricultural Catchment in a Semi-Arid Area Under Land Use Transformation

The reduction of nutrients and sediments from agriculturalrunoff by natural wetlands has been commonly accepted, buttheirrole in water quality improvement at the catchmentscale has been seldom studied, especially in irrigated catchments. This study aims to elucidate the effect of natural and recently created wetlands on stream water quality after the conversion of a catchment for irrigation purposes.Water quality andmorphometrical andvegetation-related variables weremeasured in 19 wetlands on a 750-ha agricultural catchment under semi-arid conditions in the Ebro basin(NE Spain). A pollution gradient was found, increasing from the wetlands located in the upper catchment to those in the lower catchment.Wetlands with the lowest degree of artificiality, measured asthe amount of human createdstructures(e.g., channel excavation, dikes), and higher plant richness hadthepoorest water quality, probablybecause they were in the lower catchment and their water contained more pollutants carried from agricultural and salinesoils upstream. Some of these wetlands also had the highest rates of sediment and N-NO3 retention, incontrast to more artificial wetlands, which exported nutrients and sediments. Less artificial wetlands could also provide ancillary benefits such as biodiversity enhancement or landscape heterogeneity improvement.

Nitrate and salt water contamination associated with the transition of an agrarian basin into an irrigated area.

The introduction of irrigation to agrarian areas involves several environmental changes that can be aggravated by physical and agronomic factors. The aim of this study is to analyze the spatiotemporal dynamics of the environmental impact on water resources that result from the transition of an agrarian basin into an irrigated area. During five hydrological years, a spatiotemporal comparison was carried out on the quantity and quality of drainage from the four zones in which the study area was subdivided. Introduction of irrigation activities incorporated flows that lowered salinity and increased nitrate concentration in the basin. The zones/years with most irrigation showed the greatest exports of salts and nitrates, with temporal variations related to rainfall patterns and spatial variations related to soil salinity and nitrogenous fertilization. The agro-environmental impacts of the gradual introduction of irrigation into a previously non-irrigated area were congruent to that of established irrigation areas with similar characteristics.

Watershed Monitoring for the Assessment of Irrigation Water Use and Irrigation Contamination

One of the main current questions on the sustainability of life in our planet is if in the next years there will be sufficient water to satisfy the necessities of agriculture and of the other users of this important resource (urban, industrial, touristic and ecological uses). Irrigation activities allow for the increase of agrarian yields, also allowing for a greater stability in food supply, mainly in those regions where the development of crops is limited by rain. In this way, agriculture consumes 70% of all water extracted from natural courses, being considered the main responsible factor for global fresh water shortage (FAO, 2002). Nevertheless, although the volumes employed by the agrarian sector are high, at a global level it is estimated that only 50% of the water extracted is finally utilized by plants; the remaining share ends up in drainage and irrigation return flows in rivers and aquifers (FAO, 2003). These volumes returned to water systems could contribute to a reduction in the impact generated by the extraction of resources if the water quality was not very distant from that of the original water extracted, due to the transport of salts and agrochemicals from the soil profile. Regarding the presence of agrochemicals, nitrate is a very important issue for water quality, and above all, is associated with notable changes implemented in agriculture in the last decades (OMS, 2004). The problem of nitrate with respect to other agrochemicals is its effect on human health by the simple fact of being present in high concentrations in potable water. The consumption of water with high concentrations of nitrate causes the development of methemoglobinemia in the blood, making the blood stream incapable of transporting enough oxygen through the organism and leading to death of the individual (OMS, 2004). On the other hand, the occurrence of high concentrations of nitrate in rivers and oceans is causing serious environmental effects on aquatic plants and animals, leading to the occurrence of anoxic zones and eutrophication of water resources (Diaz, 2001), as is evidenced on the coast of the United States (Scavia and Bricker, 2006) and China (Wang, 2006). The impacts generated by irrigation can be aggravated by physical (geology and climate) and agronomic (management of irrigation and fertilization) factors. For example, the natural salinity of the area in which irrigation is implemented can contribute significantly to the