Factors affecting nitrogen dynamics in a semiarid woodland (Dry Chaco, Argentina)

Abstract

In an effort to elucidate the factors affecting soil N dynamics in the Dry Chaco ecosystem, soil respiration and microbial biomass N were measured for one year underneath 5 vegetation types: a leguminous tree (Prosopis flexuosa DC), a non-leguminous tree (Aspidosperma quebracho-blanco Schlecht.), a non leguminous shrub (Larrea spp.), the open interspaces, and a pure grassland.Ammonifier and nitrifier densities and N content in litter were also measured in some cases. Results were compared with previously reported N mineralization rates and soil fertility.During the dry season microbial biomass N and net N mineralization were low, while accretion of easily mineralizable C occurred (estimated through soil respiration rates in lab under controlled temperature and moisture). With the onset of rain, microbial biomass N and N mineralization increased markedly, resulting in a decrease in easily mineralizable C. Throughout the wet season N mineralization varied with soil moisture while microbial biomass N remained consistently high. Mean values of immobilized N in this ecosystem were high (20-140 mg kg-~), of about the same order of magnitude as accumulated net N mineralization (50-150 mg kg -~ yr -~). Microbial decay in the dry season, considered as a source of easily mineralizable N, accounted for only 40% of gross N mineralization increase at the beginning of the wet season. Ammonifier densities correlated significantly with soil moisture and N mineralization, but nitrifiers did not.The highest values of total N, N mineralization, inorganic N, microbial biomass N, nitrifier densities, N content in litter, total organic C and easily mineralizable C were found under Prosopis and the lowest values under shrubs and the interspaces. The main differences between tree species were in N mineralization at the beginning of the wet season, in total and inorganic N pools, and in nitrifier densities; all of which were significantly lower under Aspidosperma than under Prosopis. N mineralization in the pure grassland was very low despite high values of total N and C sources. Although N immobilized in microbial biomass was similarly high under Aspidosperma, Prosopis and the pure grassland, net N mineralization rates were quite different.

Carbon and nitrogen limitations of soil microbial biomass in desert ecosystems

“…Differences in microbial biomass under and between shrubs were also reported by Mazzarino et al (1991). In the shrub interspaces, microbial biomass-N ranged from 14.5 to 29 ,ug g-l (again, using k,, = 0.69), which is significantly higher than the range found in the Chihuahuan desert (3.7 to 8.8 pg g-l).…”

“…If we had used the same k,,, our values would be very similar to his values (2.39 g rnp2 in grasslands, 2.98 g me2 under shrubs and 1.3 1 g me2 between shrubs). Mazzarino et al (1991) studied soil microbial biomass-N in a semiarid woodland under individuals of Larrea spp. in northwestern Argentina and found values ranging between 10 ,ug g-l in the dry season to 43 ,ug g-* in the wet season.…”

Patterns of flowering and fruiting in populations of Larrea divaricata in dry Chaco (Argentina)

“…As a consequence of the greater soil water availability (Mazzarino et al, 1991;PaH ez, unpublished data), there is increased nutrient cycling, and greater nitrogen availability under Prosopis in the study area (Mazzarino et al, 1991). Nutrient concentration underneath shrub and tree canopies is a common phenomenon in arid environments (e.g.…”

Growth and litter decomposition of woody species inoculated with rhizobia and arbuscular mycorrhizal fungi in Semiarid Brazil

“…A nurse plant phenomenon, wherein one plant facilitates establishment of another, has been described in arid and semiarid regions throughout the world [10]. The most commonly mechanism of facilitation involves woody plants [16] such as leguminous trees that are more efficient than non-leguminous in maintaining N and P availability [30] due to their ability to develop symbiotic associations with both rhizobial bacteria and arbuscular mycorrhizal fungi (AMF). Bacteria of the genus Rhizobium or Bradyrhizobium, capable of fixing the atmospheric nitrogen in root nodules, can guarantee nitrogen supply to the leguminous tree, making it independent of the nitrogen in the soil.…”