Effect of soil moisture and plant litterfall quality on the nitrifying activity of arid soils from the Patagonian Monte

MARCOS, M.S.; BERTILLER, M.B.; OLIVERA, N.L.

Mar del Plata

Congreso; X Congreso Argentino de Microbiología General (SAMIGE); 2014

Sociedad Argentina de Microbiología General (SAMIGE)

Water availability has a major influence on nutrient cycling; particularly in arid and semiarid ecosystems were rainfall events are scarce. In addition, the composition of plant cover through the quantity and chemical quality of its litterfall, which is altered by selective grazing, influences the input of nutrients into the soil and the nutrient cycling. The aim of this study was to analyze the effect of both soil moisture and plant litter quality from sites with different grazing disturbances on the nitrifying activity of arid soils from the Patagonian Monte. Upper soil samples associated to vegetated patches were collected at two sites with different signs of grazing disturbance (DS: disturbed site; CS: conserved site) within the Wildlife Refuge "La Esperanza" (42°12´S, 64°58´W). Then, samples of each site were pooled in a composite sample. Aliquots of the composite sample were used to assess soil moisture, pH and texture. Also, the concentration of organic C was measured by wet combustion (8.69 ± 0.79 mg/g dry soil) and the concentration of total N by the semi-micro Kjeldahl technique (0.71 ± 0.03 mg/g dry soil). In addition, plant litterfall samples were collected at each site. Both litter samples differed significantly (p < 0.01) in their concentrations of total N (DS: 7.95 ± 0.36; CS: 10.02 ± 0.52 mg/g dry soil) and soluble phenolics determined by the Folin-Ciocalteu method (DS: 5.20 ± 0.26; CS: 3.64 ± 0.24 mg/g dry soil). One hundred and forty four soil microcosms containing 100 g composite soil covered by plant litter from DS or CS and control microcosms without litterfall (treatments DL, CL and CTRL, respectively) were subjected to two different moisture conditions (5 and 15% humidity). Microcosms were incubated at room temperature and three replicates of each treatment (combination of soil moisture and litter quality) were withdrawn at different times to analyze the evolution of nitrifying activity in soil. During the first week of incubation, the highest nitrification activity was in the 5% soil moisture treatments, suggesting that the slow growing-nitrifying community could be in a lag phase and would still have not responded to the watering treatment. However, this effect was reversed in the following weeks, were nitrification was in average 16% higher at the highest soil moisture. Nitrification in CL was higher or equal to DL, but never lower. The chemical composition of plant litter from DS (high phenolic concentration) may be negatively affecting the rate of nitrification in soils from this site. Finally, at the end of the incubation period there was interaction between the effects of soil moisture and litter amendment. In this study, high soil moisture and plant litterfall quality exerted a positive effect over the nitrifying activity. Further studies are being performed to estimate the abundance of the nitrifying microbial populations in these microcosms by means of qPCR.