BIOLOGICAL SOIL CRUSTS CONTROL N DYNAMICS AND MICROBIAL FUNCTIONAL DIVERSITY IN

1. BIOLOGICAL SOIL CRUSTS CONTROL N DYNAMICS AND MICROBIAL FUNCTIONAL DIVERSITY IN RESPONSE TO NUTRIENT ADDITIONS Manuel Delgado-Baquerizo (mdelbaq@upo.es)1, Lourdes Morillas (lmorvin@upo.es)1, Fernando T. Maestre (fernando.maestre@urjc.es)2 & Antonio Gallardo (agallardo@upo.es)1. 1 Área de ecología, Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Carretera de Utrera km. 1, 41013 Sevilla, Spain. 2 Área de Biodiversidad y Conservación, Departamento de Biología y Geología, Escuela Superior de Ciencias Experimentales y Tecnología, Universidad Rey Juan Carlos, 28933 Móstoles, Spain INTRODUCTION STUDY SITE AND SAMPLING DESIGN Human activities are changing carbon (C), nitrogen (N), and phosphorus (P) ratios in soil ecosystems 1. • In drylands, biological soil crusts (BSCs) play a prevalent role in the Nitrogen (N) cycle 2-3, however its influence on the phosphorus (P) cycle is poorly known. • We evaluated how BSCs and different N, C and P ratios affect to the N dynamics and microbial functional diversity • This study was conducted in a semi-arid grassland at the Aranjuez experimental station (Fig 1). • Soil sampling was carried at Spring (2010). Five soil samples (top 4 cm) were taken under two microsites: bare soil (BS) and BSC (Fig 1). • Soil were incubated using different N (100 mg N kg-1), C (2.323 mg C kg-1) and P (20 mg P kg-1) treatments (Control, N, C, P, C+N, N+P, C+P, and C+N+P)4 at 20ºC during 2 different incubation periods (1-week and 3-week) in a factorial design. LAB ANALISYS A B • The functional diversity of soil heterotrophic microbial communities was analyzed with the MicroResp technique5 • NH4+, N03- and dissolved organic nitrogen (DON) were estimated by colorimetry from K2SO4 0.5 M extracts in the incubated and initial (air-dried) soil samples2. The potential net N trasnformation rates (depolymerization, ammonification and nitrification; [NTRs]) were calculated for each nutrient treatment3. NUMERICAL AND STATISTICAL ANALYSES C D The Shannon-Weaver (S-W) Diversity Index (H') was calculated by using the CO2 responses to the different C sources. Pi is the % of response of a each C substrate in regard to the total6. • We calculated the increment in the relative dominance of N-forms (NH4+, N03- and DON), total available N, potential net N transformation rates and SW-diversity index in regard to control. • The effects of incubation period, microsite and nutrient treatments on the increment of all these variables in regard to control were tested by using PERMANOVAs7. Figure 1. Study site. A: Localization of our study site at the Iberian peninsula map; B: semi-arid grassland (Stipa tenacissima L); C: BS microsite (Dominate by Diploschistes diacapsis; ratio C:N: 11.24; pH: 7.2); D: BSC microsite (Ratio C:N: 10.11; pH: 7.4). DISCUSSION • In general, when C and P was added, BSC showed a higher increase in DON, depolymerization and the S-W diversity index than bare soil microsite (Fig 1-3). N treatment decrease the microbial functional diversity in both microsites (p<0.01). Complex processes as depolymerization may require a larger and diverse group of heterotrophic microorganisms. Besides, P may be an essential nutrient in BSC communities where N fixation. N addition derived from human activities may have an important impact on the N dynamics and microbial functional diversity8. • Changes in the labile C:N ratio more than N availability modulate the N-forms and NTRs in both microsites (Fig 1 and 3). Thus, in the 3-week period of incubation, NO3- and nitrification dominate when N treatments (N and N+P), NH4+ and amonification in the C+N+P treatments and DON and depolymerization when C treatments (C and C+P; p<0.01)9-10. CONCLUDING REMARKS • The increase in depolymerization, DON and microbial functional diversity in the BSC microsite with regard to bare soil, when C or P were added, suggest that BSC may confer resistance to this variables to changes in nutrient ratios derived from human activities. • Changes in the ratios of labile C:N more than N availability seems to modulate the N dominance form and N transformation processes. Figure 1. Increment in the relative dominance of N-forms (NH4+, NO3- and DON) under the different nutrient treatments along the periods of incubation, in the BSC and BS microsites. Means ± SE (n = 5). Figure 2.Increment in theS-W diversity index for the different nutrient treatments along the periods of incubation, in the BSC and BS microsites Means ± SE (n = 5). Figure 3. Increment in the NTRs for the different nutrient treatments, in the BSC and BS microsites. Means ± SE (n = 5). REFERENCES: 1. 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