Full text
| |
| Author(s): |
Luiz Felippe Salemi
Total Authors: 1
|
| Document type: | Doctoral Thesis |
| Press: | Piracicaba. |
| Institution: | Universidade de São Paulo (USP). Centro de Energia Nuclear na Agricultura (CENA/STB) |
| Defense date: | 2014-03-20 |
| Examining board members: |
Luiz Antonio Martinelli;
Sílvio Frosini de Barros Ferraz;
Ricardo de Oliveira Figueiredo;
Quirijn de Jong van Lier;
Humberto Ribeiro da Rocha
|
| Advisor: | Luiz Antonio Martinelli; Jorge Marcos de Moraes |
| Abstract | |
Nitrogen (N) is a nutrient that might limit agricultural yields in most of agroecosytems. On the other hand, its unwise use might promote substantial alterations to the functioning and structure of terrestrial and aquatic ecosystems. Eutrophication is only one visible aspect of its pernicious effects on the environment. N from terrestrial ecosystems can reach water bodies through many hydrological flowpaths including overland flow (OV). This hydrological process occurs for two reasons: (1) rainfall intensity exceeds the infiltration capacity and (ii) the soil moisture storage capacity is exceeded by the amout of rain entering the soil, producing OV due to soil saturation. The soil saturation zones expand and shrink during a rainfall event and/or the rainy season following the variable source area model. It is expected that saturated areas act as biogeochemical hotspots of N reducing processes due to their generally high water content. That is, nitrates are reduced to N gases like nitrous oxide (N2O). In the present study, we describe the soil water dynamics of soil saturation zones focusing on its spatial variability. Moreover, the N dynamics is also focused bringing more light into the N2O fluxes, their spatial variability and controls. To do so, three flow convergence zones (FCZ1, FCZ2 and FCZ3) covered by sugarcane and riparian forest were instrumented by using tensiometers, tension lysimeters, static chambers, water table wells and overland flow sensors. Each of the FCZs were divided into three parts (downslope, middleslope and upslope being FCZ1 and FCZ2: downslope under riparian forest, middleslope and upslope under sugarcane and FCZ3: downslope and middleslope both under riparian forest and upslope under sugarcane) following a gradient of increasing contribuition area. All of these FCZs parts were instrumented with the outlined equipment. The hydrological monitoring period occurred on a weekly basis from April-2011 to March-2012. The biogeochemical samplings took place eleven times throughout the course of the same period. The soil saturation zones might be formed by two main processes: bottom-up and top-down. The existence of a wider riparian forests buffer within FCZ3 avoided the development of erosion features that have been observed under FCZ1 and FCZ2. As for N, significant differences were found only for FCZ3 during the dry season, where the downslope part presented higher N2O fluxes compared to the other two parts of the hillslope. The clayey soil under FCZ3 downslope may have been the main factor promoting such differences mainly because of its higher water and carbon retention that, in conjunction, favour N2O production. At FCZ1 and FCZ2 the likely lack of factors that promote production and emission of N2O might be the reason for the lack of differences. Our results showed that the dynamics of soil saturation within tropical regions is similar to the ones presented for temperate regions. The soil type (texture) might have decisive importance in controlling N2O emissions. This is the case especially for the dry season when the contrast of soil moisture contents between different soil types became more apparent (AU) | |