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NOVOSÁDOVÁ, I., ZÁHORA, J., RUIZ-SINOGA, J. D.: The availability of mineral nitrogen in Mediterranean open steppe dominated by Stipa tenacissima L. Acta univ. agric. et silvic. Mendel. Brun., 2011, LIX, No. 5, pp. 187–192 The area of interest is located in the Sierra de los Filabres in semi-arid steppe of the province of Almeria in Spain. The amount of water in the soil is a limiting factor and its availability aff ects the structure and species composition of ecosystem. On the other hand, the type of vegetation aff ects the water loss via evapotranspiration and thus the soil microclimate. It has a great infl uence on the growth and activity of soil microbial communities and hence the dynamics of decomposition of organic matter and nutrient availability. The aim of this study was to assess the intensity of microbial transformations of soil organic nitrogen and describe changes in the content of nitrogen mineral forms at diff erent depths in the semi-arid climate soil in the Mediterranean region. Availability and movement of nitrogen was monitored by capturing the mineral nitrogen into the structures of ion exchange resin applied to the soil in three diff erent variants (control variant, a variant with the addition of cellulose, and the variant with the addition of raw silk). Ion exchange resins have been installed into soil profi le in 2008, 2009, and 2010. A er the in situ exposure the ion exchange resins were removed from the soil profi le and the quantity of captured mineral N was determined by distillation titration method. The availability of ammonia-nitrogen was signifi cantly aff ected by the addition of diff erent substrates mainly by the additions of the raw silk, where the availability was regularly the highest. However, the availability of ammonia-nitrogen form was generally higher than the availability of nitrate form. Mediterranean soil, mineral nitrogen, microbial transformations, ion exchange resin The microbial activity plays an important and key role in the soil processes; fi rst of all in the transformation of diff erent organic compounds entering the soil milieu, in the formation of persistent soil organic matter and in the sustainable releasing of the nutrients. Also soil fauna may substantially aff ect the living conditions of microfl ora and thus indirectly aff ect microbial activity (Frouz and Nováková, 2005). The soil microbial biomass is a labile pool of organic matter and comprises 1–3% of total soil organic matter. The soil microbial biomass acts as a source and sinks of the plant nutrient and regulates the functioning of the soil system (Kaur et al., 2000). These source-sink dynamics depend on complex interactions between climate, topography, vegetation and soil surface properties (Maestre, 2006). Plant cover through its eff ects on the quantity and quality of organic matter inputs infl uences the levels of soil microbial biomass (Kaur et al., 2000). The nutrient release processes have a fundamental role in ecosystem functioning, particularly in Mediterranean areas, where nutrient availability, mainly nitrogen, represents a limiting factor (Rutigliano et al., 2009). However, in soil with less nitrogen the nutrient cycle is subject of strict biological control. Plant litter is attacked by decomposers, and almost all the nitrogen it releases is immediately immobilized by microorganisms. In Mediterranean areas the nutrient availability is also closely linked to water availability in the soil. 188 I. Novosádová, J. Záhora, J. D. R. Sinoga This strong link between N availability and water use effi ciency makes particularly important the understanding of factors aff ecting soil N availability in Mediterranean ecosystem in view of the future predicted increasing drought in this area (Rutigliano et al., 2009). Open steppes dominated by tussock grass Stipa tenacissima constitute one of the major vegetation types in the driest areas of the Mediterranean basin. Stipa tenacissima steppes are good model systems in arid land ecology, as the vegetation patterns and the processes governing their functioning resemble those described in arid and semiarid regions throughout the world (Maestre, 2006). The aim of this study was to assess the intensity of microbial transformations of soil organic nitrogen and describe changes in the content of nitrogen mineral forms in diff erent depths of the Mediterranean soil which is infl uenced by tu s of grass Stipa tenacissima. MATERIALS AND METHODS The experimental plots were situated between 1090–1165m a.s.l in Almería which is province of Autonomous Community of Andalucía, Spain. These area extent of 82 000 m2 and is located in Sierra de los Filabres Mountains. The mean annual rainfall of the region is of about 240 mm mostly concentrated in autumn and spring which is common in semiarid Mediterranean climate. The mean annual temperature is 13.9 °C. The studied soil, classifi ed as Lithosol, has a loam to sandy clay texture (FAO-ISRIC and ISSS, 1998) and pH 6.5 and content of organic matter 1.6%. The vegetation cover of these areas is an open steppe dominated by Stipa tenacissima L. (coverage of up to 75%). The availability (more or less in the case of ammonia-nitrogen) and movement of percolated nitrogen (mainly in the case of nitrate-nitrogen) was estimated in situ according to Binkley at Matson (1982) by the trapping of mineral N into the ion exchange resin (IER) inserted into two diff erent types of special cover. Ion exchange resin stockings were prepared by placing ion exchange resin (CER, cation exchange resin No. Purolite C100E, and AER, anion exchange resin no. Purolite A520E) in cylindrical stockings (0.9 cm diameter by 21.3 cm long), made of fi ne nylon mesh (grid size of 42 μm) which were then stapled shut. The stockings contained either 11.2 g moist weight cation exchange resin (13.7 mL, 5.4 g dry weight) or 9.3 g moist weight anion exchange resin (13.7 ml, 4.9 g dry weight). Exchange sites of IER were saturated with Cl− and Na+ ions. The fi rst set of CERand AERstockings were prepared for insertion into the soil as a “control” variant. Second set of stockings were coated with pure cellulose as the source of carbon and third set were coated with raw silk as the source of both, carbon and nitrogen to support microbial activities. The fi rst types of prepared IER stockings (bags) were randomly inserted into cylindrical holes (1.0 cm diameter) in the experimental plots. The holes were prepared by pushing a metal rode into a soil profi le at a 45 ° angle to a soil depth of 15 cm (Fig. 1). This ion exchange resin bags can be placed into soil with minimal soil disturbance. This type of IER stocking were inserted to the soil on the research place in 12 repetitions. Another set of special annular fl at cover (disc) for estimation of the movement of inorganic nitrogen was made from PVC tubes (diameter 7 cm, thickness 4 cm). Nylon mesh (grid size of 0.1 mm) was stuck on the PVC ring. Mixed IER (CER and AER in ratio 1:1) were then placed into the inner space of annular fl at cover (Fig. 2). The annular fl at IER cover was inserted into the soil in the depth of 30 cm in the experimental plot in 30 repetitions. Accumulation of N took 24-week over the 36 month period and it included wet season (October – April) and dry season (May – September). For the quantifi cation of NH4 +-N and NO3 −-N trapped by the resin, the IER were allowed to dry at room temperature. Absorbed NH4 +-N and NO3 −N were evaluated from IER using 100 ml 1.7 M NaCl and determined by distillation and titration method (Peoples et al., 1989). Results from the ion exchange resin bags were expressed as mg of NH4 +-N.10 ml−1 IER and NO3 −-N.10 ml−1 IER. The results from the ion exchange resin discs were expressed as mg NH4 +-N.m−2 and NO3 −-N.m−2. Statistical evaluation 1: Scheme of application of ion exchange resin (IER) in the first type of special cover, in fine nylon stockings 2: Second type of special cover annular flat ion exchange resin cover which were inserted into the soil in the depth of 30 The availability of mineral nitrogen in Mediterranean open steppe dominated by Stipa tenacissima L. 189 was performed by means of analysis of variance (ANOVA P < 0.05). When comparing the means, the Tukey’s HSD procedure was used as a multiple range tests. RESULTS AND DISCUSSION Presented experiments are based on the estimation of nitrogen ion mobility and transport using ion capture into the structure of resin bags. The sensitivity of the resin bag method to ion mobility and transport may be useful for assessing N availability to plant roots, because the driving forces which aff ected the captured amount into the resin bags are nearly the same as in the case of nutrient uptake by roots (Binkley, 1984). There exist several experiments based on using ion exchange resin in laboratory conditions but the laboratory incubations cannot refl ect changes in temperature and moisture that occur in the fi eld, which are crucial for the nutrient availability in arid and semiarid soils. The sum of captured ammonium and nitrate ions in ion exchange resins is directly proportional to the availability of given key nutrient in its own environment. In other words, this is amount proportional to the quantity which was released (or lost) from the enclosed internal cycles. The size of standard deviation illustrates the great variability of results which is typical for fi eld studies of nitrogen transformations. The ammonium ions are less mobile in the soil profi le and they are derived mainly from microbial transformation of organic nitrogen. Therefore the ion exchange resins applied into cylindrical holes situated in the upper soil layer (to a depth of 15 cm) have a higher amount of trapped ammonium forms of nitrogen than nitrate forms (Fig. 3 and 4). The availability of soil ammonia-nitrogen in control stockings, and a er the addition of raw silk was not statistically diff erent between studied years. Signifi cant diff e