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About half of the applied nitrogen (N) is not consumed by crops, causing environmental and economic costs. This N can be lost as ammonia (NH3) volatilization, nitrous oxide (N2O) emission or leaching, among others. This work aimed to compare the amount of gaseous N losses using three different fertilizers on two consecutive experiments: one summer crop (maize) and one winter crop (wheat) in the Rolling Pampa, Argentina. The fertilizers used were urea ammonium nitrate (UAN), calcium ammonium nitrate (CAN) and AN+DMPP (ammonium nitrate‐based NPK fertilizer with DMPP nitrification inhibitor). NH3 emissions were estimated using a semi open‐static absorption system during the first month after fertilization for each experiment. N2O emissions were estimated using vented static chambers during the growing season of each crop. Results show that CAN or AN+DMPP fertilizers used instead of UAN helped to reduce NH3 volatilization by 45–50% and 62–63% on maize and wheat experiments respectively, but failed to reduce N2O emissions. In addition, contrary to the expected, AN+DMPP increased N2O emissions during the maize experiment. The majority of the gaseous N losses occurred at specific moments of the crop cycle (after N fertilization and around leaf senescence). Losses as NH3 volatilization were higher than N2O emissions in the maize experiment, as expected because of the warmer temperature during this summer crop. However, N2O emissions were higher during the wheat crop, emphasizing the importance of factors such as meteorological conditions, previous land‐use, residual soil nitrate and stubble quality on the soil. [ABSTRACT FROM AUTHOR]

Absorbed solar radiation and radiation use efficiency ( RUE) can be used to estimate net primary productivity of terrestrial ecosystems. In ecosystems dominated by grasses, belowground productivity cannot be neglected in terms of carbon balance because of the high proportion of biomass allocated to roots. The objective of this study was to quantify total RUE ( tRUE), which includes both below and aboveground biomass of two C3 ( Lolium perenne and Dactylis glomerata) and one C4 ( Cynodon dactylon) grass species, under four treatments with contrasting water and nitrogen availabilities. The ratios between tRUE and aboveground RUE ( aRUE) for species and treatments were analyzed. The tRUE was calculated from measurements of incoming photosynthetically active radiation ( PAR), the fraction of PAR intercepted and shoots and roots productivity in pot experiments. The highest tRUE values in the three species were found in the treatment without growth limitations (4.32-6.93 g MJ−1), while the lowest tRUE values were observed under water and nutrient deficits conditions (2.62-2.85 g MJ−1). Contrary to predictions from the optimization theory, one of the C3 grass species allocated relatively high biomass to the roots when water availability was high while for the C4 grass species the shoot:root ratios and the root mass fraction did not change under resource limitation conditions compared to ample resource availability. tRUE exhibited small changes in the C4 species with variations in resource availability while it did decrease substantially for the C3 species when at least one of the resources was limited. These results highlight belowground biomass importance in calculating RUE of grasses. [ABSTRACT FROM AUTHOR]

The radiation use efficiency (RUE) model is one of the most used tools to generate large spatial and temporal scale net primary productivity (NPP) estimations by remote sensing. It involves two key issues to make accurate estimations of NPP: the estimation of the fraction of photosynthetically active radiation (PAR) intercepted by vegetation (fPAR) and the estimation of the plant RUE. The objectives of this work were to quantify the above-ground RUE under optimal water and nutrient conditions in two C3 and one C4 grass species and to analyse the effect of restrictions in these factors upon RUE by comparing both metabolic pathways. Grasses were cultivated from seeds and four treatments combining contrasting availabilities of water and nutrients were applied. RUE values were calculated from measurements of the incoming PAR, fPAR and productivity. In each of the species, plants with sufficient water and nutrients showed the highest RUE (2.61-3.52 g MJ−1), whereas those with deficiencies in both resources presented the lowest RUE (1.15-2.39 g MJ−1). Cynodon dactylon (C4) was the species with higher value of RUE and no significant differences were detected between treatments. However, no significant differences were detected between C. dactylon and D. glomerata under no stress treatment (N1W1) and between C. dactylon and L. perenne under water stress treatment (N1W0). RUE values of Dactylis glomerata (C3) diminished if only one of the two stress factors was presented, while Lolium perenne (C3) only when both stress factors were present. The decreases under stress treatments were between 35% and 60% compared with the no stress treatment. When regional NPP is estimated it is therefore important to take into account the decrease in the RUE, especially in areas under severe stress. [ABSTRACT FROM AUTHOR]

Question: How do meteorological variations at seasonal, interannual scales differentially affect the canopy dynamics of four contrasting landscape units within a region? Location: Flooding Pampa, Buenos Aires, Argentina. 5000 km². Central point: 35°15' S, 57°45' W. Methods: We used a 19-year series of the normalized difference vegetation index (NDVI) derived from NOAA-AVHRR PAL (Pathfinder AVHRR Land) images and meteorological data provided by a nearby weather station. The NDVI was used as surrogate of canopy photosynthetic status. The relationship between annually integrated NDVI and meteorological conditions was explored by stepwise multiple regressions for each defined unit. PCA was performed to compare units and growing seasons on a multivariate basis. Results: Mean seasonal NDVI curve was similarly shaped among landscapes. However, the absolute values differed widely. There was high interannual variation so that the mean seasonal pattern was seldom observed in any particular year. Annually integrated NDVI of all landscapes was negatively associated with summer temperature and positively with previous year precipitation. It was also directly related with current year winter precipitation in two landscapes and with summer precipitation in the others. NDVI response to September and March precipitation accounted for some of the differences in interannual variation among landscapes. Conclusions: Our results revealed a strong intra-regional variation of canopy dynamics, closely linked to landscape (vegetation-soil) and water availability (mainly in summer and during the previous year). These links may be used to predict forage production rates for livestock. [ABSTRACT FROM AUTHOR]

1. In some ecosystems there is a positive feedback between forage quality and grazing intensity. This involves three components of plant tolerance to grazing: functional traits, herbivore selectivity and response to grazing. We analysed the relationships between these components at species and community levels in Patagonian steppe grasslands.2. We measured plant functional traits[height, specific leaf area (SLA) and foliar toughness] and estimated sheep selectivity and grazing response indices for 35 plant species. Sheep selectivity indices were obtained from microhistological and species’ availability data, and grazing response indices from species’ abundances in sites with contrasting grazing intensities. We performed correlations and multiple regressions among the three types of variables across the pool of 35 species.3. To analyse data at the community level, we computed weighted averages of traits and sheep selectivity indices for 34 floristic samples taken from each side of 17 fence lines with contrasting grazing intensities. Correlations between mean trait values and sheep selectivity across the 34 samples, and paired comparisons of those variables between sides of the fences, were performed.4. Taller plants had leaves with lower SLA and/or higher toughness. Short species of intermediate toughness were selected more often by sheep, while SLA was not related to sheep selectivity. Short species with high SLA increased with grazing, while toughness and sheep selectivity were unrelated to grazing response.5. At the community level, short swards with high average SLA had high selectivity indices and were more abundant on the most intensively grazed sides of fence lines. Leaf toughness was unrelated to other traits or to sheep selectivity, and showed no significant response to grazing.6. Synthesis and applications. Intensive grazing can increase the forage value of grasslands by the creation of lawns dominated by tolerant species. However, results from this study showed that some plant species that were avoided by grazers also increased, indicating a potential risk of a shift in composition of grazing lawns towards states of low forage value. This suggests that periodic resting of lawns could be a good management strategy to favour palatable species, thereby minimizing the risk of undesirable shifts in the overall species composition.Journal of Applied Ecology(2005) doi: 10.1111/j.1365-2664.2004.00978.x [ABSTRACT FROM AUTHOR]

Question: How well does the use of NDVI predict secondary productivity at landscape scales? What is the influence of vegetation quality and phenology over secondary productivity? Location: Magellanic steppe in Tierra del Fuego, Argentina. (52°45′ to 54° S, 68°15′ to 67°30′ W). Methods: Monthly and yearly integrated NDVI (NDVI-I) were calculated from AVHRR/NOAA 14, as estimators of phenology and aerial net primary productivity respectively. From a vegetation map we obtained the proportional cover of different physiognomic types and calculated the palatable fraction (forage) productivity that were used as estimators of vegetation quality. Data were analysed through correlations and regressions. Results: NDVI-I was not related with secondary productivity indices, while December and annual maximum NDVI, proportion of lawns and tussock grasslands and forage productivity were positively related with secondary productivity. A negative relationship was found between the proportion of heathlands and secondary productivity, but a positive relationship between heathland's proportion and NDVI-I was found. Conclusions: NDVI-I is not a good predictor of secondary productivity at the scale of our study. These results could be due to: (1) NDVI-I is not related to primary productivity and (2) primary productivity is not related to secondary productivity. [ABSTRACT FROM AUTHOR]

. We analysed vegetation dynamics in Tierra del Fuego steppes using Normalized Difference Vegetation Index (NDVI) data provided by advanced very high-resolution radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration (NOAA) polar satellite. Our objective, at a regional scale, was to analyse the spatial variability of NDVI dynamics in relation to parent material and geographic location, representing the fertility and climate gradients respectively; at a local scale, it was to analyse the inter-annual variability associated with climate and its relation with sheep production indices. The general pattern of NDVI dynamics was analysed with Principal Component Analysis. We found that the geographic location was more important than landscape type in explaining NDVI dynamics despite the fact that the variation in landscape type reflects a fertility gradient strongly associated with floristic composition and secondary productivity. Discriminant Analysis was performed to identify the variables that better distinguish geographic units. The Northern region (with the lowest precipitation and the highest temperatures) had lower NDVI values over the year. In the Central region, NDVI reached the highest value of the season, surpassing both other regions. The Southern region (the coldest and moistest) had its growth pattern displaced towards the summer. For the Central region we analysed 10 years of monthly NDVI data with PCA. We found that precipitation from August to December and winter temperature are the most important determinants of overall NDVI values. Lamb production was correlated with spring and early summer NDVI values. Sheep mortality is affected by low NDVI values in late summer and high annual amplitude. Satellite information allowed us to characterize the vegetation dynamics of three ecological areas across the Fuegian steppe. [ABSTRACT FROM AUTHOR]

In the Festuca gracillima tussock steppe of northern Tierra del Fuego there is a physiognomic pattern at the community scale composed of a matrix of tussocks, scrub patches of Chiliotrichum diffusum and lawn patches. In this paper we compared floristic composition, soils, microclimate and sheep use of these three types. Species composition greatly differed among physiognomic types. Lawn patches showed the highest diversity and maximum cover of exotics, grazing escape growth forms and species of high-fertility habitats. Soil profiles of tussock steppe and lawn were similar but the A-horizon of the latter containedmore clay and canons. Radiation and temperature during daylight at the soil surface in summer were highest in the lawn patches. Soil impedance, a good indicator of trampling, was highest on the lawn patches. According to dung pellet density, lawn patches received much heavier use than the tussock matrix. Our results suggest that lawn patches supported the highest grazing impact and undergo a eutrophication process favored by high mineral inputs, high mineralization rates in thesoil during the warm season and restricted leaching through the profile. They also support the idea that a feedback mechanism between vegetation heterogeneity and sheep grazing behaviour maintains the patches. Although grazing behaviour suggests that the lawn patches favor animal production, their expansion would result in a system less protected against the impact of water deficits and temperature variations and more vulnerable to erosion and to the loss of important species. [ABSTRACT FROM AUTHOR]