From 1996 to 1999, an European project called WELSONS was led to better understand the impacts of changes in climate and land-use on soil degradation by wind erosion for agricultural soils in a semiarid region of north-east Spain (Central Aragon). This paper presents the main results obtained from the measurement activities of the WELSONS project aimed to generate an original data base to study wind erosion processes. The first objective was to evaluate the effects of climate and anthropogenic processes on surface characteristics of agricultural soils. During the whole WELSONS experiment, a physical dynamics characterization of the soil surface and of the atmosphere near the ground were determined for each field plot studied. Results from the characterisation of soil surface shows that reduced tillage (RT) lower soil erodibility by wind compared to conventional tillage (CT).Soil crusting, which is a common feature of soils in the area, can also be considered as a protecting element against wind erosion. The second objective was to study the mobilisation processes of soil-particles into the saltation layer. Strong and erosive Cierzo winds occurred frequently in the area of the experimental field, but because a soil surface crust was present, only a limited supply of material was available for wind erosion. However, a measured saltation transport flux could be obtained for each sampled period from the measurement of the mass of saltating particles transported during each erosion event. The third objective was to assess the atmospheric emission of fine dust into the suspension layer. Significant vertical dust flux of PM20 was observed only in the CT plot because, as the erosion events are supply-limited, a lower amount of material lying on the crust was transported by the wind and sandblasted to provide suspended dust. Finally, the sandblasting efficiency calculated in the present experiment could be interpreted as the result of a higher binding energy for silt loam soil particles compared to sandy loam, loamy sand and sandy textured soils.

The wind-erodible fraction of the soil (EF) (percentage of aggregates b 0.84 mm in diameter) is a key parameter to estimate the soil susceptibility to wind erosion. The standard method for EF determination is the dry sieving by means of a rotary sieve. Flat sieving with a set of sieves and the use of the equation EF = (29.09 + 0.31 sand + 0.17 silt + 0.33 sand/clay -2.59 organic matter -0.95 CaCO 3 ) / 100, R 2 = 0.67, [Fryrear, D.W., Krammes, C.A., Williamson, D.L., Zobeck, T.M., 1994. Computing the wind erodible fraction of soils. J. Soil Water Conserv. 49: 183-188] are two alternative ways of determining EF. As the flat sieving has still not been contrasted against the standard rotary sieve method nor the Fryrear et al. equation tested for soils other than US soils, we estimated EF with both dry sieving methods and tested the equation for soils of semiarid regions of Central Aragon (NE Spain) and the Semiarid Pampas (centre of Argentina), two regions prone to wind erosion. Results showed that EF values obtained with the flat sieve were comparable with those obtained using the standard rotary sieve, indicating that the flat sieving technique is suitable for EF determinations. The estimation of EF with the model proposed by Fryrear et al. [Fryrear, D.W., Krammes, C.A., Williamson, D.L., Zobeck, T.M., 1994. Computing the wind erodible fraction of soils. J. did not fit with the measured EF values, indicating that this model is not useful for predicting EF in Spanish and Argentinian soils. This was attributed to the high CaCO 3 contents of Spanish soils and the low sand/clay ratios and high organic matter contents of some Argentinean soils. The equation EF = 9.98 + 6.91 sand/clay + 14.1/organic matter (r = 0.933; P b 0.001) was proposed to predict EF in the studied soils.

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1In Central Aragón (NE Spain), where strong and dry winds are frequent all year round, fallow 2 lands are susceptible to wind erosion due to insufficient crop residues on the surface and loose, 3 finely divided soils by multiple tillage operations. Effects of conventional tillage (mouldboard 4 ploughing followed by a compacting roller) and reduced tillage (chisel ploughing) on soil surface 5 properties affecting wind erosion were studied during three experimental campaigns in a dryland 6 field of Central Aragón. Reduced tillage provided higher soil protection than conventional tillage 7 through a lower wind erodible fraction of soil surface (on average, 10% less) and a significantly 8 higher percentage of soil cover with crop residues and clods (30% higher). Random roughness 9 was also higher after reduced tillage than after conventional tillage (15% vs. 4%). These results 10 indicate that reduced tillage can be an effective soil management practice for wind erosion 11 prevention during the fallow period in semiarid Aragón. The study shows, likewise, that 12 significant changes in soil aggregate size distribution associated with wind erosion processes may 13 occur in short periods of time. Thus, temporal variability of soil surface properties, including crust 14 and clods stability, needs to be considered in wind erosion research in agricultural soils.

Soil organic carbon (SOC) plays an essential role in the sustainability of natural and agricultural systems. The identification of sensitive SOC fractions can be crucial for an understanding of SOC dynamics and stabilization. The objective of this study was to assess the effect of long-term no-tillage (NT) on SOC content and its distribution between particulate organic matter (POM) and mineralassociated organic matter (Min) fractions in five different cereal production areas of Aragon (northeast Spain). The study was conducted under on-farm conditions where pairs of adjacent fields under NT and conventional tillage (CT) were compared. An undisturbed soil nearby under native vegetation (NAT) was included. The results indicate that SOC was significantly affected by tillage in the first 5 cm with the greatest concentrations found in NT (1.5-43% more than in CT). Below 40 cm, SOC under NT decreased (20-40%) to values similar or less than those under CT. However, the stratification ratio (SR) never reached the threshold value of 2. The POM-C fraction, disproportionate to its small contribution to total SOC (10-30%), was greatly affected by soil management. The pronounced stratification in this fraction (SR>2 in NT) and its usefulness for differentiating the study sites in terms of response to NT make POM-C a good indicator of changes in soil management under the study conditions. Results from this on-farm study indicate that NT can be recommended as an alternative strategy to increase organic carbon at the soil surface in the cereal production areas of Aragon and in other analogous areas.

A B S T R A C TThe inherent complexity of soil organic matter (SOM) and its stabilization processes make suitable the identification of SOM fractions that reflect the management-induced changes in soil organic carbon (SOC) dynamics. This is of special interest in semiarid regions where the capacity of soil for agricultural production is limited. This study aims to evaluate the effect of different tillage and soil management practices on the distribution of C among SOM fractions and determine the influence of soil texture on the protection of SOC in a semiarid Mediterranean region (Aragon, NE Spain). Under on-farm conditions, pairs of adjacent fields under long-term no tillage (NT) and conventional tillage (CT) were compared in five different cereal production areas. In all cases, a nearby undisturbed soil under native vegetation (NAT) was included. Results indicate that the two isolated mineral-associated OM (Min) fractions, d-Min and magg-Min (outside and within stable microaggregates, respectively), constituted the main part of total SOC (mean contributions of 54 and 26%, respectively) and were not consistently affected by soil management. Soil clay was a determinant factor for d-Min-C and total SOC (r 2 = 0.60-0.70; P < 0.001), indicating that chemical stabilization, through clay-organic complexes, seems to be a main preservation mechanism in the studied soils. Physical protection seems to be another SOC stabilization process in these soils due to strong correlation found between magg-Min-C and the mass of water-stable microaggregates (r = 0.900; P < 0.0001). With smaller contributions to total SOC, the two labile fractions, coarse and fine particulate OM (cPOM and fPOM) were sensitive to soil management and their concentrations decreased as soil disturbance increased (NAT > NT > CT). The highest differences between NT and CT corresponded to fPOM at the soil surface where this fraction was 1.2-3 times higher under NT. Higher soil stratification ratios in NT, always >2 for the POM fractions, indicate an improvement in soil quality with long-term NT adoption in this semiarid Mediterranean region.

1Winter barley is the major crop on semiarid drylands in central Aragon (NE Spain). In this 2 study we compared, under both continuous cropping (BC) (5-6 month fallow) and a crop-3 fallow rotation (BF) (16-18 month fallow), the effects of three fallow management treatments 4 (conventional tillage, CT; reduced tillage, RT; and no-tillage, NT) on the growth, yield and 5 water use efficiency (WUE) of winter barley during three consecutive growing seasons in the 6 1999 to 2002 period. Daily precipitation measurements and monthly measurements of soil 7 water storage to a depth of 0.7 m were used to calculate crop water use (ET) and its 8 components. The average growing season precipitation was 195 mm. Above-ground dry matter 9 (DM) and corresponding WUE were high in years with high effective rainfalls (>10 mm day -1 ) 10 either in autumn or spring. However, the highest values of WUE for grain yield were mainly 11 produced by effective rainfalls during the time from stem elongation to harvest. Despite the 12 similarity in ET for the three tillage treatments, NT provided the lowest DM production, 13 corresponding to a higher soil water loss by evaporation and lower crop transpiration (T), 14 indicated by the lowest T/ET ratio values found under this treatment. No clear differences in 15 crop yield were observed among the tillage treatments in the study period. On average, and 16 regardless of the type of tillage, BF provided the highest values of DM and WUE and yielded 17 49% more grain than BC. These differences between cropping systems increased when water-18 limiting conditions occurred in the early stages of crop growth, probably due to the additional 19 soil water storage under BF at sowing. Although no significant differences in precipitation use 20 efficiency (PUE) were observed between BC and BF, PUE was higher under the BC system, 21 which yielded 34% more grain than the BF rotation when yields were adjusted to an annual 22 basis including the length of the fallow. The crop yield under BF was not dependent on the 23 increase in soil water storage at the end of the long fallow. In conclusion, this study has shown 24 that although conventional tillage can be substituted by reduced or no-tillage systems for 25 3 fallow management in semiarid dryland cereal production areas in central Aragon, the practice 1 of long-fallowing to increase the cereal crop yields is not longer sustainable 2 . 3 4