Impact of the conversion of Brazilian woodland savanna (cerradão) to pasture and Eucalyptus plantations on soil nitrogen mineralization.

The Brazilian savanna (Cerrado) has been extensively converted to croplands, pastures and forestry plantations, and the deforestation frontier continues expanding. Land conversion may cause critical changes in soil functioning, yet very little is still known about the impact of Cerrado conversion on nutrient cycling and soil fertility. Here, we addressed this knowledge gap by investigating the effects of the woodland cerrado (cerradão) conversion into pastures and Eucalyptus plantations on nitrogen availability and mineralization potential, considering a wide range of spatial and temporal variability due to soil depth, site conditions, and seasonal variation. For three sites in São Paulo state and each of the target land cover types, we assessed the total N and inorganic N (NH4-N and NO3-N) pools, potentially mineralizable nitrogen (PMN) and soil urease activity in the first 2 m of the soil profile. Cerrado conversion to either pastures or Eucalyptus plantations significantly reduced NH4-N, while NO3-N showed similar values in Cerrado and Eucalyptus and lower values in pastures. We found a consistent pattern of lower N mineralization in the uppermost soil layers associated to Cerrado conversion, with decreases in PMN rate and urease activity. The soil below 30 cm depth showed no relevant changes. Considering the first 30 cm of the soil profile, the reduction in the stocks of inorganic N (NH4-N + NO3-N) ranged from ~14% for the conversion to Eucalyptus to ∽20% for the conversion to pasture. The impact of land conversion on N cycling surpassed the influence of the spatial (between-site) and seasonal variation. Overall, the results indicate a decline in available N and overall soil fertility due to Cerrado conversion, which could further increase N limitation in the Cerrado region, increase fertilization needs for future exploitation, and compromise the recovery of Cerrado in case of land abandonment or restoration.

Watering, fertilization, and slurry inoculation promote recovery of biological crust function in degraded soils.

Biological soil crusts are very sensitive to human-induced disturbances and are in a degraded state in many areas throughout their range. Given their importance in the functioning of arid and semiarid ecosystems, restoring these crusts may contribute to the recovery of ecosystem functionality in degraded areas. We conducted a factorial microcosm experiment to evaluate the effects of inoculation type (discrete fragments vs slurry), fertilization (control vs addition of composted sewage sludge), and watering frequency (two vs five times per week) on the cyanobacterial composition, nitrogen fixation, chlorophyll content, and net CO2 exchange rate of biological soil crusts inoculated on a semiarid degraded soil from SE Spain. Six months after the inoculation, the highest rates of nitrogen fixation and chlorophyll a content were found when the biological crusts were inoculated as slurry, composted sewage sludge was added, and the microcosms were watered five times per week. Net CO2 exchange rate increased when biological crusts were inoculated as slurry and the microcosms were watered five times per week. Denaturing gradient gel electrophoresis fingerprints and phylogenetic analyses indicated that most of the cyanobacterial species already present in the inoculated crust had the capability to spread and colonize the surface of the surrounding soil. These analyses showed that cyanobacterial communities were less diverse when the microcosms were watered five times per week, and that watering frequency (followed in importance by the addition of composted sewage sludge and inoculation type) was the treatment that most strongly influenced their composition. Our results suggest that the inoculation of biological soil crusts in the form of slurry combined with the addition of composted sewage sludge could be a suitable technique to accelerate the recovery of the composition and functioning of biological soil crusts in drylands.