Papers by Marina Gonzalez Polo
Soil Biology and Biochemistry, 2009
In arid and semi-arid ecosystems that are frequently classified as water limited, it is unclear h... more In arid and semi-arid ecosystems that are frequently classified as water limited, it is unclear how spatial and temporal variability of vegetation and climate could affect microbially-mediated soil processes. Our objective was to determine how aboveground spatial heterogeneity creates characteristic soil conditions that modulate microbial growth and activity in a semi-arid Patagonian steppe. In particular, we explored how micro-environmental and biogeochemical soil characteristics generated by the native vegetation could control soil b-glucosidase activity. Both life-form (shrubs, grasses, mosses and bare soil) and season exerted strong controls on all measured abiotic (soil temperature and gravimetric soil water content, inorganic nitrogen, pH and total C and N) and biotic (microbial biomass C and b-glucosidase activity) soil characteristics. Partial correlation between b-glucosidase activity and extracellular organic C (EOC) was high across seasons (r ¼ 0.5; P < 0.001) while soil water content did not correlate with soil enzymatic activity (r ¼ 0.09; P > 0.05). We postulate that labile soil carbon rather than water availability functions as a principal limitation of microbial activity in this semi-arid ecosystem, and the distribution of this carbon is, in large part, determined by the patchy distribution of vegetation.
Ecosystems, 2013
In most temperate forest ecosystems, tree mortality over time generates downed logs that accumula... more In most temperate forest ecosystems, tree mortality over time generates downed logs that accumulate as coarse woody debris (CWD) on the forest floor. These downed logs and trunks have important recognized ecosystem functions including habitat for different organisms and long-term organic C storage. Due to its recalcitrant chemical composition and slow decomposition, CWD can also have direct effects on ecosystem carbon and nutrient turnover. CWD could also cause changes indirectly through the physical and chemical alterations that it generates, although it is not well-understood how important these indirect effects could be for ecosystem processes and soil biogeochemistry. We hypothesized that in an old-growth mature forest, CWD affects carbon and nutrient cycles through its ''proximity effects'', meaning that the forest floor near CWD would have altered soil biotic activity due to the environmental and biogeochemical effects of the presence of CWD. We conducted our study in an old-growth southern beech temperate forest in Patagonia, Argentina, where we estimated and classified the distribution and mass, nutrient pools and decay stage of CWD on the forest floor, and evaluated its impact on litter decomposition, soil mites and soil enzymatic activity of carbon and phosphorus-degrading enzymes. We demonstrate here that CWD in this ecosystem represents an important organic carbon reservoir (85 Mg ha -1 ) and nitrogen pool (0.42 Mg ha -1 ), similar in magnitude to other old-growth forests of the Northern Hemisphere. In addition, we found significant proximity effects of CWD, with increased C-degrading soil enzyme activity, decreased mite abundance, and more rapid litter decomposition beneath highly decayed CWD. Considered at the ecosystem scale in this forest, the removal of CWD could cause a decrease of 6% in soil enzyme activity, particularly in the summer dry season, and nearly 15% in annual litter decomposition. We conclude that beyond the established importance of CWD as a long-term carbon reservoir and habitat, CWD contributes functionally to the forest floor by influencing the spatial heterogeneity of microbial activity and carbon and nutrient turnover. These proximity effects demonstrate the importance of maintenance of this ecosystem component and should be taken into consideration for management decisions pertaining to carbon sequestration
Ecosystems, 2009
We explored the net effects of grazing on soil C and N pools in a Patagonian shrub-grass steppe (... more We explored the net effects of grazing on soil C and N pools in a Patagonian shrub-grass steppe (temperate South America). Net effects result from the combination of direct impacts of grazing on biogeochemical characteristics of microsites with indirect effects on relative cover of vegetated and unvegetated microsites. Within five independent areas, we sampled surface soils in sites subjected to three grazing intensities: (1) ungrazed sites inside grazing exclosures, (2) moderately grazed sites adjacent to them, and (3) intensely grazed sites within the same paddock. Grazing significantly reduced soil C and N pools, although this pattern was clearest in intensely grazed sites. This net effect was due to the combination of a direct reduction of soil N content in bare soil patches, and indirect effects mediated by the increase of the cover of bare soil microsites, with lower C and N content than either grass or shrub microsites. This increase in bare soil cover was accompanied by a reduction in cover of preferred grass species and standing dead material. Finally, stable isotope signatures varied significantly among grazed and ungrazed sites, with d 15 N and d 13 C significantly depleted in intensely grazed sites, suggesting reduced mineralization with increased grazing intensity. In the Patagonian steppe, grazing appears to exert a negative effect on soil C and N cycles; sound management practices must incorporate the importance of species shifts within life form, and the critical role of standing dead material in maintaining soil C and N stocks and biogeochemical processes.
Biogeochemistry, 2006
Human impact on nitrogen cycling, in particular the introduction of reactive nitrogen in terrestr... more Human impact on nitrogen cycling, in particular the introduction of reactive nitrogen in terrestrial and aquatic ecosystems, can be examined at multiple scales, from the global impact on atmospheric chemistry to the impact of human activities on soil organic matter and fertility at the scale of square meters. Nevertheless, anthropogenic loading of nitrogen cycling in natural and managed ecosystems can be seen most directly at the regional scale, where concentrated human activity results in disruption of the nitrogen balance, with consequences for biogeochemical cycling and their interactions. Differences in land-use and agricultural practices between North and South America, and the importance of economic drivers that determine the fate of new reactive nitrogen demonstrate a contrasting picture of human impact on N cycling when the consequences are considered at the global vs. the regional scale. In particular, in the Pampa region of Argentina, the central agricultural zone of the country, the expansion of soybean cultivation in the last 20 years and the use of synthetic fertilizers have resulted in an influx of reactive nitrogen into these systems, with unexpected consequences for the nitrogen balance. A mass balance of nitrogen for soybean demonstrates that increased nitrogen inputs from biological fixation do not compensate for losses due to seed export, such that most areas under soybean cultivation are currently experiencing a substantive net loss of nitrogen. In addition, other crops that are currently being fertilized still show a net loss of nitrogen also due to the effect of primary exports from these agroecosystems. These simple models demonstrate that socioeconomic factors in large part drive the contrasting effects of anthropogenic impact on nitrogen cycling at global vs. regional scales. The future impact on nitrogen cycling in the Americas requires an integration of both ecological factors and socioeconomic drivers that will ultimately determine human disruption of the nitrogen cycle.
Biodiversity across scenarios
Chapter 10 Biodiversity across Scenarios Coordinating Lead Author: Osvaldo E. Sala Lead Authors: ... more Chapter 10 Biodiversity across Scenarios Coordinating Lead Author: Osvaldo E. Sala Lead Authors: Detlef van Vuuren, Henrique Miguel Pereira, David Lodge, Jacqueline Alder, Graeme Cumming, Andrew Dobson, Volkmar Wolters, Marguerite A. Xenopoulos Contributing ...
Agroforestry Systems, 2018
The conversion of native vegetation to tree plantation (afforestation) implies a drastic change i... more The conversion of native vegetation to tree plantation (afforestation) implies a drastic change in life forms and as a consequence, changes in the microenvironmental conditions, and the quantity and quality of organic matter entering the soil. This could affect soil microbial communities and the processes catalyzed by them. In Patagonia, afforestation with exotic, fast-growing tree species was a common practice but the consequences on the ecosystem remain poorly quantified. The objective was to study the effects of pine afforestation on litter decomposition, soil organic matter, soil microbial activity and associated biogeochemical functions in a semiarid area of NW Patagonia. We hypothesized that afforestation would decrease litter decomposition rate and soil biological activity including net N mineralization, due to changes of environmental conditions and organic matter quality. We measured in situ and potential soil net N mineralization, soil microbial biomass-C, soil enzyme activities (βglucosidase, acid phosphatase and leucin-aminopeptidase) and litter decomposition rate. We also characterized soil pH, electrical conductivity, extractable P and total C and N. Pine plantations clearly affected decomposition rates of native grass vegetation, which was 10% lower under pine canopy cover, and decreased soil microbial biomass. Acid phosphatase activity and leucin-aminopeptidase activities were also marginally reduced. On the other hand, we did not find any significant effects of pines on soil chemical properties and N transformations after 13 years of plantation. Because effects depend strongly on time, the decrease of soil microbial biomass, acid phosphatase activity and grass decomposition rate (and the trend to lower enzyme activities related to P and N) under pine cover could be an evidence of possible changes on the long-term.
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Papers by Marina Gonzalez Polo