Micromorphology of Surface Crusts in the Knersvlakte,South Africa

Soils in the Knersvlakte are particularly prone to crusting and have lower inherent infiltrability than other soils across western southern Africa. Micromorphological techniques were used to examine the structure and porosity of soil crusts in the Knersvlakte to ascertain why crusting is so intense in this region. Quantile regression using boundary lines was employed to examine the relationships between infiltrability and soil properties for all samples （n = 67）. This analysis showed that infiltrability is potentially maximal at low water- dispersible ＇clay plus silt＇ content and low silt content （r^2 = 0.72 and 0.64; respectively, n = 67） （Figure 2）. The strength of crusts, pH, EC, clay mineralogy, and water-dispersible clay, silt and ＇clay plus silt＇ content were compared, and a pore analysis using optical microscopy was undertaken on images of six soil thin sections （n = 6） （circular and parallel polarizers）. Pore analysis was further undertaken on five horizontal slices of equal dimensions taken through each soil thin section. The porosity samples with low infiltrability （〈 100 mm·hr^-1, n = 4） had greater crust strength, lower porosity （both total and in the least porous slice） and greater water-dispersible ＇day plus silt＇ and silt content than the porosity samples with high infiltrability （〉 100mm·hr^-1, n = 2）. The porosity samples with low infiltrability showed a trend of lower pH and greater water dispersible clay percentage. Porosity varied within the porosity samples due to the presence of dense clay/silt bands （〈 0.5 mm in width） with relatively few air vesicles. The porosity samples with horizontal slices of low porosity （but large numbers of air vesicles） had low infiltrability, while those without slices of low porosity （and relatively few air vesicles） had high infiltrability. We conclude that the intense crusting and resultant low infiltrability of soils in the Knersvlakte appears to be related to the formation of thin, dense clay/silt bands in the pedoderm.

Aggregate Development and Organic Matter Storage in Mediterranean Mountain Soils

Soil aggregation and organic matter of soils from the pre-Pyrenean range in Catalonia (NE Spain) were studied,in order to assess their quality as carbon sinks and also to select the best soil management practices to preserve their quality.Aggregate stability,organic carbon and micromorphology were investigated.The highest amount of organic carbon was found in alluvial,deep soils (228 Mg C ha -1 ),and the lowest was in a shallow,stony soil with a low plant cover (78 Mg C ha -1 ).Subsurface horizons of degraded soils under pastures were the ones with smaller and less-stable aggregates.Fresh residues of organic matter (OM) were found mostly in interaggregate spaces.Within the aggregates there were some organic remains that were beginning to decompose,and also impregnative nodules of amorphous OM.Although OM was evenly distributed among the aggregate fractions,the larger blocky peds had more specific surface,contained less decomposed OM and had a lower organic/mineral interphase than smaller crumb aggregates,which were also more stable.Soil carbon storage was affected primarily by the OM inputs in the surface horizons.In order to store organic carbon over the mid- and long-term periods,the mechanisms favouring structuration through biological activity and creating small aggregates with intrapedal stable microporosities seemed to be the most effective.

Soil mineral nitrogen dynamics in fallow periods in a rainfed semiarid Mediterranean agricultural system

Rainfed agricultural systems in semiarid Mediterranean environments are subject to erratic but often heavy rainfall events.As an agronomic practice,fallow periods can be included even within the existing European Union common policy for crop diversification.This study aimed to quantify the effects of previous mineral fertilization on soil mineral nitrogen(N_(min))content and potential nitrate leaching during no-till fallow periods of crop rotation.The Leaching Estimation and Chemistry Model(LEACHM)was used for soil N_(min)prediction after fallow.During fallow periods,N_(min)measured in the soil profile increased by an average of 125 kg N ha^(-1),whereas the model showed an average increase of 95 kg N ha^(-1).The amount of nitrogen(N)leached was estimated to be 11–38 kg N ha^(-1).The N balance simulated using LEACHM might differ from the actual situation.The calibrated LEACHM underestimated soil N_(min)after a drought period followed by soil water replenishment,probably due to a pulse in soil C and N mineralization caused by the wetting of dry soil,but overestimated soil N_(min)after occasional rainy spells when soil quickly became saturated,probably due to occasional nitrous oxide emissions not being fully accounted for by the model and specific preferential water flow,which might lead to greater nitrate leaching than that simulated by LEACHM using the convection-dispersion equation.The results show that soil N_(min)measurements after fallow periods cannot be well predicted by LEACHM.The findings of this study provide a support for fertilization planning for crop rotation systems including a fallow period to avoid overfertilization in the following cropping season and reduce N environmental impacts.