Long-term impact of domestic ungulates versus the local controls of the litter decomposition process in arid steppes
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Abstract Aims Soil processes in arid ecosystems are strongly controlled by resource scarcity. Grazing intensification can induce changes in ecosystem processes through multiple pathways, adding new constraints to those of local conditions. We focus on grazing-induced changes in litter traits and soil environment that may affect litter decomposition and N dynamics in temperate grass-shrub steppes. Methods We performed three litterbag decomposition experiments to evaluate: i) the effect of litter traits in a common garden (ex situ), ii) the effect of soil environment using a foreign common litter substrate (in situ), and iii) the interactive effects of litter traits and soil environment (grazed vs.vs. exclosure communities, reciprocal transplants in situ). Field experiments were replicated in three blocks with paired plots under a long-term exclosure (> 25 years) and under year-round sheep grazing. Local litter included mixtures of species of grasses and shrubs, separately. Results Grazing exclusion did not alter litter decomposition rates, either through changes in litter traits or in soil environment. Nevertheless, N released during grass litter decomposition was 286% higher in exclosures than in grazing communities. The difference was associated to changes in litter C:N ratio. The effects were maintained when results were integrated to the entire litter community. Conclusion Our study suggests that litter decomposition rates in arid steppes are strongly controlled by local drivers. Ungulate grazing does not have an important influence on litter mass loss, but it can exert a strong control on N flux during decomposition, by changing grass litter traits. Published in: Piazza, MV., Oñatibia, G.R., Aguiar, M.R., Chaneton E.J. Long-term impact of domestic ungulates versus the local controls of the litter decomposition process in arid steppes. Plant Soil (2021). https://doi.org/10.1007/s11104-021-05088-4
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The study was conducted in the experimental field station of INTA (Instituto Nacional de Tecnología Agropecuaria) located in Río Mayo, Chubut Province, Argentina (45° 41´ S, 70° 16´ W). Mean annual precipitation is 132 mm, 60 % of which falls between April and September; mean annual temperature is 9.3 °C. Soils are coarse-textured Aridisols. Vegetation corresponds to the Occidental district of Patagonian arid steppes. Sheep stocking rates in the study field have been ~ 0.2 sheep ha-1 during the last 50 years. Three litterbag decomposition experiments were performed to test our model. Field design consisted in three blocks (replicates) with paired plots of areas under a long-term sheep year-round grazing (320-880 ha), adjacent areas under long-term grazing exclosure (1-8 ha). Grazing plots were in three different paddocks. Sheep exclosures were established 28, 39 and 57 years before the start of the experiment, age was part of the block design. Inside each plot (grazing or exclosure), four experimental units were randomly selected, at a minimum distance of 50 m from each other, where all experimental determinations were performed. The experimental sampling and determinations were performed in the field in the period between 2010 and 2013. We sorted the steppe vegetation community into two groups: grasses and shrubs. Grass composition was different between grazing and exclosure conditions. Litter was placed in 15 x 20 cm fiberglass bags with 2 mm mesh, containing 3 g air-dry weight with different litter species proportions. Litter quality at the beginning of the experiments was evaluated through five fresh litter samples per litter type. We estimated initial handling losses (weight pre-post establishment), water content (60°C oven-dried, 48 hours), ash content (ground samples 4 hour at 500°C) and total C and N concentration (LECO-CR12 dry combustion analyser and semi-micro Kjeldahl method, respectively). Total N content (semi-micro Kjeldahl method) was estimated in all samples from the latest harvest date (25 months for experiments 1 and 3, and 12 months for experiment 2). Site environmental measured variables were soil water gravimetric content (100°C oven), soil bulk density, and soil total C and N concentration (LECO-CR12 dry combustion analyser and semi-micro Kjeldahl method, respectively). In each sampling area, 2-3 sub-samples were collected using a 4 cm diameter and 10 cm height cylinder (4 groups per site and grazing condition). Nutrient (C-N) stock (ton/ha) was estimated from concentration and bulk density data. Soil temperature was measured with a digital thermometer buried 2-4 cm from soil surface (3 sub-samples). Data were analysed with linear models (lm) and mixed-effect models (lme) using R software. In the different models, fixed effects was litter type, grazing environment, and their interaction, and the random factor was the nested structure of data. See more details on methods in the published article.