In this study, changes in the spatial and temporal patterns of climate extreme indices were analyzed. Daily maximum and minimum air temperature, precipitation, and their association with climate change were used as the basis for tracking changes at 50 meteorological stations in Iran over the period . Sixteen indices of extreme temperature and 11 indices of extreme precipitation, which have been quality controlled and tested for homogeneity and missing data, are examined. Temperature extremes show a warming trend, with a large proportion of stations having statistically significant trends for all temperature indices. Over the last 15 years (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), the annual frequency of warm days and nights has increased by 12 and 14 days/decade, respectively. The number of cold days and nights has decreased by 4 and 3 days/decade, respectively. The annual mean maximum and minimum temperatures averaged across Iran both increased by 0.031 and 0.059°C/decade. The probability of cold nights has gradually decreased from more than 20 % in 1975-1986 to less than 15 % in 1999-2010, whereas the mean frequency of warm days has increased abruptly between the first 12-year period (1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)) and the recent 12-year period (1999-2010) from 18 to 40 %, respectively. There are no systematic regional trends over the study period in total precipitation or in the frequency and duration of extreme precipitation events. Statistically significant trends in extreme precipitation events are observed at less than 15 % of all weather stations, with no spatially coherent pattern of change, whereas statistically significant changes in extreme temperature events have occurred at more than 85 % of all weather stations, forming strongly coherent spatial patterns.

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The aim of this study was assessment of ammonium (NH 4(+) ) adsorption isotherms in some agricultural calcareous soils and modeling of that by using the mechanistic exchange model. Ten surface soils (0-30 cm) were collected from areas covered with different land uses in Hamedan, western Iran. Isotherm experiments were carried out by concentrations of NH 4(+) prepared from NH4Cl salt (0, 10, 20, 30, 40, 50, 100, and 150 mg NH 4(+)  l(-1)) in presence of 0.01 M CaCl2 solution. The empirical models including simple adsorption isotherm and Freundlich equations were fitted well to the experimental data. The average amounts of adsorbed NH 4(+) in studied soils varied from 8.95 to 35.23 %. Adsorption percentage indicated positive correlation with pH, cation-exchange capacity (CEC), equivalent calcium carbonate, and clay content and had negative correlation with sand content. In order to predict and model NH 4(+) adsorption, cation-exchange model in PHREEQC program was used. The model could simulate the NH 4(+) adsorption very well in all studied soils. The values of CEC played the major role in modeling of NH 4(+) adsorption in this study indicating that cation-exchange process was the major mechanism controlling NH 4(+) adsorption in studied soils.

The use of organic residues might be appropriate in maintaining long-term P requirement of crops. Eight plant residues and two manures in a wide range of C : N ratios were incubated for 12 weeks at 25C, during which the processes of decomposition and formation of acid-extractable P (P release) were assessed. Residues were incubated moist in bags and inoculated with a soil suspension. The exponential model was found to be suitable to describe decomposition and P-release rates from residues. There were two distinct parts of P release in each time curve, which gave two rate constants and were calculated as the slope of each part. The rates of P release of the residues were considerably higher during the first 4 weeks of incubation than during the second phase of incubation (weeks 5-12). Phosphorus release by residues was similar to the decomposition pattern. The residue P content was correlated with P release, but not with decomposition rate. Mass loss and release of P were greater from sunflower and wheat residues than from sheep manure, suggesting that a high C : N ratio does not necessarily retard decomposition. The model parameters of P release were derived which are suitable to estimate the Pfertilizer effect of organic residues. Our results suggest that the use of organic residues as a source for short-and long-term P supply for crops should be encouraged.