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.

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).

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

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.

C
NUMERICAL AND STATISTICAL ANALYSES
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.
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).
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.

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+, NO3and 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 the S-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).
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