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11 Pag., 7 Tabl., 8 Fig., The response of soil CO2 flux to long-term tillage practices (no-tillage, NT; minimum tillage, MT; conventional tillage, CT) and N fertilization level (zero; medium, 60 kg N ha−1; high, 120 kg N ha−1) was studied during three growing season in a rainfed Mediterranean agroecosystem. Soil CO2 flux was related to the crop growth, with the highest flux during spring (i.e., March–May). Tillage and N fertilization effects on soil CO2 flux during growing seasons depended on weather conditions: greater soil CO2 flux under MT and NT on dry years, greater under CT and MT on a wet year. Nitrogen fertilization affected soil CO2 flux during this wet growing season: flux with N fertilizer additions was higher than flux on the unfertilized plots. Thirteen years after establishment of the experiment, the soil organic carbon (SOC) stock under long-term NT was 3.9 Mg C ha−1 greater than under CT and 4.3 Mg C ha−1 greater than under MT. The SOC stocks with N fertilizer additions were 4 Mg C ha−1 greater than the stock on unfertilized plots. The increase of C inputs with N fertilization was more pronounced under NT than under MT or CT. For this reason the increased response of SOC stock to N fertilization is expected under NT in a longer period of time. The product between soil water content and soil temperature explained between 75 and 94% of the seasonal variability of soil CO2 flux. However, soil CO2 flux and SOC stock were hardly related., This work was supported by the Comision Interministerial de Ciencia y Tecnologia of Spain (Grants AGL 2004-07763-C02-02 and AGL2007-66320-CO2-02/AGR). F.J. Morell received a grant for his doctorate studies from the Spanish Ministry of Science and Innovation.

13 Pags., 6 Figs., 4 Tabls., Aims Root respiration is a major contributor to soil CO2 flux, and its response to management practices needs to be evaluated. The aim was to determine the effect of management practices (tillage systems and nitrogen fertilization levels) on root respiration and to develop a model able to simulate root respiration and its components. Methods The study was carried out during two contrasting growing seasons (2007–2008 and 2008–2009). Root respiration, including root tissue respiration (R ts ) and rhizomicrobial respiration of exudates (R rz ), was estimated as the difference between the soil CO2 flux of cropped and bare soil (the so-called root exclusion technique). Additionally a novel sub-model of R ts , was used to simulate root respiration based on root growth and specific root respiration rates. Results Root respiration was reduced under no-tillage. The model agreed well with the patterns and the amounts of the observed values of root respiration, although prior calibration was needed. Conclusions Root respiration was reduced by the long-term adoption of no-tillage, but was increased by N fertilizer. The root exclusion technique and the model were useful means to estimate root respiration on cropland under semiarid Mediterranean conditions. Additionally the model successfully separated out the theoretical contributions of R ts and R rz to root respiration., This work was supported with by the Comision Interministerial de Ciencia y Tecnologia of Spain (Grants AGL 2004-07763-C02-02 and AGL2007-66320-CO2-02/AGR). F.J. Morell received a grant for his doctorate studies from the Spanish Ministry of Science and Innovation, and wishes to acknowledge the Soil Science Department at Rothamsted Research for hosting during a research period in autumn 2009. Rothamsted Research is an institute of the UK Biotechnology and Biological Sciences Research Council. A.P. Whitmore acknowledges the support of the Institute Strategic Programme grant on Sustainable Soil Function.

36 Pags., 5 Tabls., 5 Figs. The definitive version is available at: http://www.sciencedirect.com/science/journal/01671987, Agricultural management plays an important role in global warming mitigation due to its effects on soil organic carbon (SOC) dynamics. In Mediterranean agroecosystems, the interactive effects of tillage and N fertilization on SOC storage have scarcely been studied. Hence, we here present a modelling study in which the effects of both tillage and N fertilization on SOC dynamics are investigated. We used SOC and C input data from a long-term (13 years) field study located in northeast Spain, firstly to validate both the Century model and the Rothamsted Carbon (RothC) model and secondly to predict future SOC dynamics until the year 2030. Tillage and N fertilization affected SOC stocks in the 0–30 cm soil layer. However, the interaction of the two factors was not significant. Averaged over the three N fertilization rates, the observed mean SOC stocks in conventional tillage (CT) and no-tillage (NT) were 29.8 and 36.8 Mg C ha−1, respectively. In addition, the observed SOC stocks, averaged for both tillage systems, increased with increasing N rates, with 30.6, 33.5 and 35.8 Mg C ha−1 for the 0, 60 and 120 kg N ha−1 rates, respectively. In general, both the Century model and the RothC model performed well in predicting SOC dynamics. Model predictions showed that in Mediterranean dryland agroecosystems SOC dynamics in the next 20 years would be variable according to the tillage and N fertilization applied. According to these predictions, scenarios with NT and high fertilization rates (e.g., 60–120 kg N ha−1) could lead to significant SOC sequestration and associated CO2 emission offsetting. However, these scenarios with high SOC sequestration rates also showed high mineral N accumulation in the soil profile with its associated environmental side effects., This work was supported by the Comisión Interministerial de Ciencia y Tecnología of Spain (Grants AGL2010-22050-C03-01/02) and the European Union (FEDER funds). We acknowledge the Consejo Superior de Investigaciones Cientificas (CSIC) for the contract granted to Jorge Álvaro-Fuentes within the “Junta para la Ampliación de Estudios” (JAE-DOC) programme co-financed by the European Social Fund. Furthermore, Daniel Plaza-Bonilla was awarded a FPU fellowship by the Spanish Ministry of Education.

38 Pags., 6 Tabls., 4 Figs. The definitive version is available at: http://www.sciencedirect.com/science/journal/01671987, Conservation tillage systems (no-tillage, NT; and minimum tillage, MT) are being adopted in rainfed agroecosystems of the Mediterranean basin where water availability is the main limiting factor for crop productivity. We hypothesized that long-term adoption of conservation tillage systems would increase water use efficiency (WUE) and its response to N fertilizer additions due to improved soil water content. A field experiment was established in 1996 on a loamy Xerofluvent Typic in the Ebro river valley (NE Spain). The experiment compared three nitrogen (N) fertilization levels (zero, 0 kg N ha−1, medium, 60 kg N ha−1, and high, 120 kg N ha−1), under three tillage systems (CT, conventional tillage; MT and NT), annually cropped to barley (Hordeum vulgare, L.) as is usual in the region. Ten years after the experiment establishment, during four consecutive growing seasons, 2005–2006 to 2008–2009, we evaluated the response of soil water content, soil nitrate, above-ground dry matter, grain yield and yield components to long-term (>10 years) tillage and N fertilization treatments. The long-term sustainability of NT and MT was confirmed. Mean yield and WUE under long-term conservation tillage systems were 66% and 57% higher than under CT, respectively. This improvement was mainly attributed to improved soil water usage under conservation tillage, mainly due to reduced water use during the pre-anthesis period. However, in a wet year yield did not significantly differ among tillage systems. The improvement of WUE with N fertilization was confirmed under NT, which medium and high N fertilizer level increased 98% mean grain yield and 77% mean WUE compared to CT. The increased response of crop and yield to N fertilization under NT was due to improved soil water content. Soil N accumulation together with the lower water accumulation explained the lack of response to N fertilization under CT, even on a wet growing season (i.e., 2008–2009). Long-term NT adoption was a sustainable practice for barley monoculture in the region, allowing for reduced costs and yield increase with N fertilizer additions. N fertilizer rates on rainfed Mediterranean croplands should be adjusted depending on the reduction of tillage intensity and rainfall of the year. In our system and as an example for this agroecosystems, N fertilizer rates should be kept at or below 60 kg N ha−1, and should be further reduced on intensively cultivated soils., This work was supported with by the Comision Interministerial de Ciencia y Tecnologia of Spain (Grants AGL 2004-07763-C02-02 and AGL2007-66320-CO2-02/AGR). We thank Carlos Cortés and Silvia Martí for their technical assistance. We acknowledge the Spanish Ministry of Science and Innovation for funding the doctorate studies of F.J. Morell and the Consejo Superior de Investigaciones Científicas (CSIC) for the contract of Jorge Álvaro-Fuentes within the Junta Ampliación Estudios (JAE-DOC) program co-founded by the FSE. We acknowledge Alexander Pswarayi for language correction.