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The effects of cultivating different crops or applying different management practices on water erosion have been widely evaluated in plot or field experiments. While these experiments have focused on the direct effects of a certain crop, there is comparably little information on how crops influence soil loss during the following years. Our objectives were to evaluate the extent to which water erosion differs between potato (Solanum tuberosum L.) and maize (Zea mays L.), and how these crops influence soil loss of a following winter wheat (Triticum aestivum L.) crop. Soil erosion was measured in four small neighboring watersheds (0.8-4.2 ha in size) during 198 rainfall-runoffevents (1994-2001). Each watershed included one field with a crop rotation of winter wheat, potato, winter wheat, and maize. This rotation was shifted by 1 yr for each field, and hence a comparison between the fields as well as a comparison over two crop rotations was possible. Runoff and soil loss from potato and maize differed only slightly because the better protection by cover during maize years was compensated by a better protection by contouring with potato ridges. Both effects were adequately described by the cover management and support practice (C and P) factors of the Revised Universal Soil Loss Equation. A clear difference in soil loss depending on the preceding crop occurred in the succeeding winter wheat fields. Especially in October, November, and February, soil loss after the potato crop was significantly higher. This could be explained by little residue cover, disintegration of large aggregates, and low stability of small aggregates following the potato crop. Carryover effects should be taken into account, optimizing crop rotations with respect to soil conservation. Moreover, they are highly relevant for modeling of water erosion from agricultural areas. Abbreviations: RFR, roughness index; RUSLE, Revised Universal Soil Loss Equation; USLE, Universal Soil Loss Equation.

We studied the adsorption characteristics of P in altered peat soils of the Hula Valley, Israel, which has undergone repeated drying and rewetting cycles. As a result, the water quality of Lake Kinneret, the only freshwater lake in Israel, may have been adversely affected. Peat sample collection was based on pedogenic evaluation of the wetland's history and on the redox potential of aerobic (Eh = 450 mV) and anaerobic (Eh < -220 mV) conditions. Extractable citrate-bicarbonate--dithionite iron ([Fe.sub.CBD]) was a dominant mineral phase in the aerobic layers (29 [+ or -] 5 g [kg.sup.1]). Mossbauer spectra suggested thai hematite, goethite, ferrihydrite, and magnetite are the main Fe minerals in these peat soils. The sorption maximum ([S.sub.max] of the aerobic layers ranged from 670 to 1750 mg P [kg.sup.-1], with a mean value of 1250 mg P [kg.sup.-1], whereas the anaerobic layers ranged from 625 to 975 mg P [kg.sup.-1] with a mean value of 775 mg P [kg.sup.-1]. The equilibrium phosphorus concentration ([EPC.sub.0] values in the most anaerobic peat layers were several orders of magnitude higher (0.31 mg [L.sup.-1]) than in the aerobic layers (0.01 mg [L.sup.-1). Only a weak to moderate correlation was found between the sorption attributes and the Fe content due to precipitation of Ca-P phases. The main snurce of Ca in these soils is gypsum. Rewetting of the peat soils leads to a decrease in [S.sub.max] and the buffer capacity, and an increase in [EPC.sub.o], which could lead to higher P mobility. The increased potential of P mobility declined with a concurrent increase in Ca-P precipitation due to enhanced dissolution of gypsum.

An understanding of P transformations in altered wetlands has mainly developed from temperate and humid regions with neutral to acidic soils. Little is known regarding downstream water quality impact of P transformations in semiarid wetlands that undergone repeated cycles of drying and rewetting. The P geochemistry was studied using the Hedley fractionation scheme in the altered peat soils of the Hula Valley, Israel. The peat soils were sampled according to the peat depth and redox potential characteristics. The mean total P concentration ([P.sub.t]) in the surface peat horizons (1190 [+ or -] 300 mg [kg.sup.-1]) was significantly higher than in the anaerobic ([E.sub.H] < -220 mV) peat layers (650 [+ or -] 260 mg [kg.sup.-1]). The concentrations of P in all fractions except the most labile P were significantly higher in the aerobic ([E.sub.H] = 400 mV) peat layers. The predominant extractable fractions in the surface peat horizons were Ca-P extracted by 1 M HCl (21-60% of [P.sub.t]), Iron-P extracted by 0.5 M NaOH (11-41% of [P.sub.t]), and residual P extracted by [H.sub.2]S[O.sub.4] (20-40% of [P.sub.t]). The source of the Ca in the Ca-P fraction was mainly from gypsum dissolution following the rewetting cycles. The results clearly showed that the drainage of these wetlands facilitated rapid organic matter (OM) oxidation, release of organically bound metals and P followed by sesquioxides and gypsum precipitation. These geochemical transformations enhanced the [P.sub.t] concentration per mass of altered peat soil and changed the P distribution among the different pools. Most of the P is currently associated with Fe oxides and hydroxides and/or coprecipitated with Ca.