Analía L. Carrera

Relationship between plant nitrogen conservation strategies and the dynamics of soil nitrogen in the arid Patagonian Monte, Argentina

During three consecutive years with contrasting precipitation, we analysed the relationship between strategies of N conservation in the dominant plant functional groups (perennial grasses and evergreen shrubs) of the Patagonian Monte and the main components of N cycling in soil. We hypothesised that the different patterns of N conservation in perennial grasses and evergreen shrubs would have direct consequences for soil-N, inorganic-N release and microbial-N flush in soil. In autumn and late spring of 1999, 2000, and 2001, we assessed N and C concentration in green and senesced leaves, N-resorption efficiency and C/N ratio in senesced leaves of three dominant species of each plant functional group. In the soil associated with species of each plant functional group, we determined N and C concentration, potential-N mineralisation, and the associated microbial-N flush. Slow-growing evergreen shrubs exhibited low N-concentration in green leaves, high N-concentration in senesced leaves and low N-resorption from senescing leaves. In contrast, fast-growing perennial grasses showed high N-concentration in green leaves, low N-concentration in senesced leaves, and high N-resorption from senescing leaves. In evergreen shrubs, the maintenance of long-lasting green leaves with low N-concentration was the most important mechanism of N conservation. In contrast, perennial grasses conserved N through high N-resorption from senescing leaves. Soil-N concentration, potential N-mineralisation, and microbial-N flush in the soil were higher underneath evergreen shrubs than beneath perennial grasses. Observed differences, however, were lower than expected considering the quality of the organic matter supplied by each plant fuctional group to the soil. A possible reason for this relatively weak trend may be the capacity of evergreen shrubs to slow down N cycling through low leaf turnover and the presence of secondary compounds in leaves. Alternatively or simultaneously, the weak relationship between plant and soil N could result from shrubs being able to colonise N-poor soils while grasses may preferably occupy fertile microsites previously influenced by the decomposition pathway of evergreen shrubs. Differences between evergreen shrubs and perennial grasses in the mechanisms of plant N-conservation and in components of N cycling in the underlying soil were consistent over the three years of the study with differing precipitation. Inter-annual differences in N concentration in green leaves and in the microbial-N flush in soil indicate that during the wettest year fast-growing perennial grasses would outcompete slow-growing evergreen shrubs and microorganisms for N uptake.

Patterns of nitrogen conservation in shrubs and grasses in the Patagonian Monte, Argentina.

We analysed the main plant strategies to conserve nitrogen in the Patagonian Monte. We hypothesized that the two main plant functional groups (xerophytic evergreen shrubs and mesophytic perennial grasses) display different mechanisms of nitrogen conservation related to their structural and functional characteristics. Evergreen shrubs are deep-rooted species, which develop vegetative and reproductive growth from spring to late summer coupled with high temperatures, independently from water inputs. In contrast, perennial grasses are shallow-rooted species with high leaf turnover, which display vegetative growth from autumn to spring and reproductive activity from mid-spring to early-summer, coupled with precipitation inputs. We selected three evergreen shrubs (Larrea divaricata Cav., Atriplex lampa Gill. ex Moq. and Junellia seriphioides (Gilles and Hook.) Moldenke) and three perennial grasses (Stipa tenuis Phil., S. speciosa Trin. and Rupr. and Poa ligularis Nees ex Steud.), characteristic of undisturbed and disturbed areas of the Patagonian Monte. N concentration in expanded green and senesced leaves was estimated in December 1997 (late spring) and June 1998 (late autumn). Deep-rooted evergreen shrubs displayed small differences in N concentration between green and senesced leaves (low N-resorption efficiency), having high N concentration in senesced leaves (low N-resorption proficiency). Shallow-rooted perennial grasses, conversely, showed high N-resorption efficiency and high N-resorption proficiency (large differences in N concentration between green and senesced leaves and very low N concentration in senesced leaves, respectively). The lack of a strong mechanism of N resorption in evergreen shrubs apparently does not agree with their ability to colonize N-poor soils. These results, however, may be explained by lower N requirements in evergreen shrubs resulting from lower growth rates, lower N concentrations in green leaves, and lower leaf turnover as compared with perennial grasses. Long-lasting N-poor green tissues may, therefore, be considered an efficient mechanism to conserve N in evergreen shrubs in contrast with the mechanism of strong N resorption from transient N-rich tissues displayed by perennial grasses. Evergreen shrubs with low N-resorption efficiency provide a more N-rich substrate, with probably higher capability of N mineralization than that of perennial grasses, which may eventually enhance N fertility and N availability in N-poor soils.

Leaf strategies and soil N across a regional humidity gradient in Patagonia

We analyzed leaf traits related to carbon-fixation, nutrient conservation strategies, and decomposability and their relationships with potential N-mineralization and microbial N in soil in 19 species of 5 dominant life forms growing in 40 sites across a regional humidity gradient in northern Patagonia. We hypothesized that (1) the shifting of species and life forms across the humidity gradient is related to a shifting in traits of green and senesced leaves with some overlapping among life forms, and (2) leaf traits associated with litter decomposition are related to the potential dynamics of soil-N across the humidity gradient. LMA in green leaves and P-resorption efficiency decreased with humidity while concentrations of lignin and total phenolics in green and senesced leaves and P concentration in senesced leaves increased with humidity. Soil C and N concentrations were positively correlated to humidity. Increasing soil N concentration was related to increasing rates of absolute (per unit soil mass) potential net N-mineralization and microbial-N flush. Relative (per unit N mass) potential net N-mineralization and microbial-N flush decreased with soil N and were inversely correlated to lignin concentration and C/N ratio in senesced leaves. We found overlapping in N concentration and C/N ratio in green and senesced leaves, P concentration in green leaves, and N resorption among species and life forms across the humidity gradient. We concluded that (1) leaf traits related to carbon fixation and the decomposition pathway significantly varied with humidity and were not overlapped between plant life forms from dry and humid habitats, (2) the largest overlapping among species and plant life forms across the gradient occurred in those leaf traits related to N conservation in the plant, and (3) life forms from humid habitats produce more recalcitrant litter that induce lower rates of relative potential net N mineralization (per unit N) than those of dry habitats.

Soil-associated lichens in rangelands of north-eastern Patagonia. Lichen groups and species with potential as bioindicators of grazing disturbance

Soil-associated lichen species characteristic of north-eastern Patagonia are classified by cluster analysis into six groups using ecological and morphological characters. The constancy of species, the total number of species, the number of species per crust and the relative frequency of species are analysed at sites with different grazing levels: three non-grazed, three regulary grazed, and three heavily grazed. Using the results, the potential use of lichen groups as bioindicators of rangeland conservation and degradation are explored. Species of three lichen groups (group A: lichens growing on calcareous gravels; group C: terricolous lichens with pale, crustose non-areolate thalli; group F: terricolous lichens with pale, areolate thalli) are identified as sensitive to grazing, and most of the species forming these groups are suggested as potential bioindicators of grazing disturbance. Thus, Rinodina bischoffii, Caloplaca holocarpa, Catillaria lenticularis, Acarospora heppii (group A); Lecanora dispersa and Rinodina mucronatula (group C); and Psora decipiens (group F) are the species most sensitive to grazing disturbance. Conversely, species of group D (terricolous lichens with dark, foliose thallus: Collema coccophorum), and group B (lichens growing on siliceous gravels: Aspicilia contorta) may be indicated as the most resistant to grazing disturbance.

Effect of Fine‐Scale Spatial Variation of Soil Nitrogen on the Performance of the Sexes of Poa ligularis in Patchy Ecosystems of Northern Patagonia

In patchy environments of arid Patagonia, males of the dioecious grass Poa ligularis dominate in N‐poor microsites, while females are more common in N‐rich microsites. In order to explore functional differences related to spatial segregation of the sexes, we analyzed biomass allocation and tissue N concentration in males and females of P. ligularis growing in a range of soil N concentrations (sN). Based on the general patterns of responses described for plants from N‐rich and N‐poor habitats, we hypothesized that (1) females, which dominate in N‐rich microsites, would increase biomass allocation with increasing sN, while males, frequent in N‐poor microsites, would show a limited response and (2) tissue N concentration would display a wider variation in males than females in response to changes in sN. At three sites in northern Patagonia, we randomly selected 15 plants of each sex of P. ligularis growing inside shrub patches and 15 in the interpatch areas and evaluated the biomass and N concentration of aboveground (vegetative and reproductive) and belowground structures. Biomass allocation to belowground structures and N concentration in roots increased with increasing sN for both males and females. Aboveground biomass increased with increasing sN only in females. In the N‐poorest sites, we found higher N allocation to tiller crowns with increasing sN in males relative to females. In both sexes, biomass allocation to sexual reproductive structures (panicles) did not change significantly with variations in sN. These results provide partial evidence on morphological and functional dimorphism in a dioecious species with spatial segregation of the sexes.

Relationships Among Plant Litter, Fine Roots, and Soil Organic C and N Across an Aridity Gradient in Northern Patagonia, Argentina

Abstract: We analyzed the relationship between organic C and N in the upper soil and the quantity (mass) and quality (N, soluble phenolic, and lignin concentrations) of plant aboveground litter and upper fine roots at contrasting microsites (plant patch and bare soil) in 15 study sites across an aridity gradient in Patagonia. At each site, we estimated the total, grass, and shrub cover and randomly selected 10 plant patches of modal size (height and crown diameter) and species composition. We extracted an upper soil core (0–10 cm depth) and collected the aboveground litter underneath each plant patch canopy and at the contiguous bare soil. We separated the fine roots (< 2 mm in diameter) from the soil and assessed the biomass and the concentrations of organic C and N in soil and the lignin, soluble phenolic, and N in shrub and grass components of aboveground litter and in fine roots. Total plant and grass cover decreased with increasing aridity. Total litter mass did not vary across the aridity gradient, but the proportion of shrub components in litter increased and litter quality decreased with increasing aridity. The mass of fine roots was positively correlated to soil organic C and N and decreased with increasing aridity. All these trends were consistent at plant patch and bare soil microsites. We conclude that the decrease in soil organic C and N across the aridity gradient was the outcome of different above- and belowground controls resulting from the replacement of grasses by shrubs. Accordingly, the main controls of soil organic C and N could be the quality of aboveground litter and the quantity of the organic matter input to soil through fine-root turnover from belowground.

Production and turnover rates of shallow fine roots in rangelands of the Patagonian Monte, Argentina

Selective sheep grazing in arid rangelands induces a decrease in total cover and grass cover and an increase in the dominance of shrubs. Both life forms differ in aboveground and belowground traits. We hypothesized that grazing disturbance leads to the replacement of grass by shrub fine roots in the upper soil, and this is reflected in changes in the seasonal dynamics of shallow fine roots at the community level. In two sites representative of non-grazed and grazed vegetation states in the Patagonian Monte, we assessed the canopy structure, and the fine root biomass, N concentration, production, and turnover during two consecutive years. The non-grazed site exhibited higher total, grass, and shrub cover than the grazed site. The grazed site had larger or equal fine root biomass than the non-grazed site except for late spring of the second year. This could be associated with the ability of shrubs to develop dimorphic-root systems occupying the soil freed by grasses at the grazed site, and with the larger contribution of grass than shrub fine roots in relation to an extraordinary precipitation event at the non-grazed site. This was consistent with the N concentration in fine roots. Fine root production was positively correlated to temperature at the grazed site and with precipitation at the non-grazed site. Fine root turnover did not differ between sites. Our results indicate that grazing leads to a shifting in the seasonality and main climatic controls of fine root production, while fine root turnover is mostly affected by changes in soil water conditions.

Do soil enzymes respond to long-term grazing in an arid ecosystem?

Background and aimsOur objective was to assess the effects of long-term continuous grazing on soil enzyme activities in relation to shifts in plant litter attributes and soil resources in an arid ecosystem, considering both spatial and temporal variations.MethodsWe randomly extracted soil samples with the respective litter cover at 5 modal size plant-covered patches (PCP) and the nearest inter-canopy areas (IC) at Patagonian Monte sites with low, medium and high grazing intensity in winter and summer from 2007 to 2009. We analyzed enzyme activities (dehydrogenase, ß-glucosidase, protease, alkaline and acid phosphatase), microbial biomass-C, organic-C, total soil-N, and moisture in soil and mass and quality in plant litter. We assessed faeces density and plant cover in the field.Results and conclusionsGrazing led to reduced grass cover, decreasing plant litter mass with increasing soluble phenolics, and reduced phosphatases, ß-glucosidase and microbial biomass-C at PCP. A localized nutrient input from animal excreta seems to promote microbial biomass-C, alkaline phosphatase and dehydrogenase activities but only at IC from the site with high grazing intensity. Plant heterogeneous distribution, plant litter quantity and quality, nutrient inputs from grazers and seasonal variation in soil moisture, also affecting soil resources and microbial biomass, modulate soil enzyme responses to long-term grazing in the arid Patagonian Monte.

Aboveground Vegetation and Perennial Grass Seed Bank in Arid Rangelands Disturbed by Grazing

ABSTRACT Recruitment by seeds can be an important mechanism for recovery of plant communities following disturbance. Our objective was to assess the density and spatial patterning of perennial grass (highly preferred by herbivores) seeds in litter patches at locations with different aboveground vegetation structure in sites with different grazing history characteristic of the Patagonian Monte (Argentina). We asked whether structural differences in aboveground vegetation are reflected in the density and spatial patterning of perennial grass seeds in litter patches. We selected two study sites characteristic of the Patagonian Monte and within them three locations representing different vegetation states, resulting from different combinations of grazing and/or release from grazing history. At each location, we assessed the density of perennial grass seeds in litter patches at microsites beneath plant patches (canopy) and in interpatch areas without or with scattered vegetation (bare soil) at three dates during the reproductive and seed dispersal periods. The density of perennial grass seeds in litter patches was greater at canopy than at bare soil microsites, and the number of litter patches without seeds increased with decreasing total plant cover at both microsites. The density of perennial grass seeds in litter patches did not vary with differences in total plant cover or litter patch attributes at canopy microsites, while it was reduced with decreasing total plant cover at bare soil microsites. We concluded that differences in aboveground plant cover differentially affected the density of perennial grass seeds in litter patches at contrasting soil microsites. Thus potential microsites for perennial grass recruitment by seeds would increase from litter patches at bare soil microsites to litter patches at canopy microsites at locations with high and low aboveground plant cover, respectively. These issues should be considered for the sustainable management of these rangelands.

Diversity of phenolic compounds and plant traits in coexisting Patagonian desert shrub species of Argentina

Desert shrubs often accumulate different types of phenolic compounds but what determines the amount and diversity of these compounds is an issue scarcely explored. The aim of this study was to assess differences in the amount and diversity of phenolic compounds in leaves among coexisting shrub species differing in rooting depth and leaf turnover. We hypothesized that the diversity and amount of phenolic compounds in leaves of desert shrubs are related to access to soil water through rooting depth, and to leaf turnover. The study was carried out in the Patagonian Monte of Argentina. We collected green leaves of six species representing the dominant shrub morphotypes (tall evergreen, tall deciduous, and medium evergreen shrubs) and assessed lignin concentration and groups of soluble phenols obtained by sequential extraction with ethyl ether, ethyl acetate, and amyl alcohol. We also assessed nitrogen concentration in leaves and leaf mass per unit area (LMA) as traits related to leaf lifespan. The diversity of phenolic compounds was higher in green leaves of tall shrubs with deep rooting depth than in those of medium evergreen shrubs with shallow rooting depth. Diversity of phenolic compounds in green leaves was negatively related to lignin concentration. Evergreen shrubs had higher amount of phenolic compounds in green leaves than deciduous ones and the total amount of phenolic compounds in green leaves was positively related to LMA. We concluded that access to soil water sources and leaf turnover were related to the amount and diversity of phenolic compounds in green leaves of desert shrub species and these results are consistent with those predicted by the resource availability theory for plants from resource-rich and resource-poor habitats.