Soil net nitrogen mineralisation across global grasslands

Soil nitrogen mineralisation (Nmin), the conversion of organic into inorganic N, is important for productivity and nutrient cycling. The balance between mineralisation and immobilisation (net Nmin) varies with soil properties and climate. However, because most global-scale assessments of net Nmin are laboratory-based, its regulation under field-conditions and implications for real-world soil functioning remain uncertain. Here, we explore the drivers of realised (field) and potential (laboratory) soil net Nmin across 30 grasslands worldwide. We find that realised Nmin is largely explained by temperature of the wettest quarter, microbial biomass, clay content and bulk density. Potential Nmin only weakly correlates with realised Nmin, but contributes to explain realised net Nmin when combined with soil and climatic variables. We provide novel insights of global realised soil net Nmin and show that potential soil net Nmin data available in the literature could be parameterised with soil and climate data to better predict realised NNational Science Foundation Research Coordination Network; Long-Term Ecological Research; Institute on the Environment at the University of Minnesota.http://www.nature.com/ncommspm2020Mammal Research InstituteZoology and Entomolog

Primer of Ecological Restoration - Karen D Holl, published in cooperation with the Society for Ecological Restoration

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Optimisation or satiation, testing diet selection rules in goats

Several hypotheses have been formulated to explain diet selection by herbivores, focusing on the maximization of nutrient intake, the minimization of plant secondary compounds, or the satiety hypothesis. This research aimed at studying diet selection revealing which chemical characteristics of plants form the bases for dietary preferences of goats. This was done by setting up a feeding experiment with three different combinations of tree species Acacia karroo, A. nilotica and A. sieberana. The chemical characteristics of these three Acacia species were used to predict diet selection. To test the validity of the satiety hypothesis, goats were placed on a conditioning diet of one of the three species. We found a clear preference for A. karroo and an avoidance of A. nilotica when these two were offered to the goats. In trials where A. nilotica was present, tannin minimization was the best explaining diet selection rule. In trials where A. nilotica was not present, however, tannin minimization was not the best explanation. Our findings suggest that tannins are not avoided but kept below a certain threshold. Below this threshold, goats based their dietary choices on other chemical characteristics of the Acacia species. Acid detergent fibre (ADF) minimization could then best explain preferences in trials with Acacia karroo and A. sieberana that have generally low tannin content. Goats did not maximize nutrient intake or digestibility, and we found no support for the satiety hypothesis

Patch density determines movement patterns and foraging efficiency of large herbivores

Few experimental studies have tested theoretical predictions regarding the movement strategies of large herbivores and their consequences for foraging efficiency. We therefore analyze how the movement and foraging behavior of goats are related to patch density, with patches being trees and bushes. We show that their movements become slower and more tortuous when patch density increases, resulting in shorter steps, more acute turns, and a lower net displacement. Furthermore, the movements of the goats can be well described by Lévy walks (LWs). In agreement with hypotheses generated by LW models, the goats move with μ ≈ 2 at low patch density but with μ ≈ 3 when patches are abundant. However, simplified statistical descriptors of movement patterns like the shape of the step/flight length and turn angle distributions become insufficient in predicting foraging efficiency when patch density is high because then the sequence of steps and turns becomes an important determinant of foraging efficiency. By changing their movements and behavior with increasing patch density, the goats intensify their utilization of resources and consequently are able to raise the efficiency of the foraging process more than proportional to the increase in patch density. This resembles the concept of area-restricted search, stating that animals concentrate their foraging effort in areas with high reward, thereby increasing the efficiency of foraging. The findings as presented in this paper provide support for theoretical expectations on the movement and foraging behavior of large herbivores in relation to resource density. Copyright 2007, Oxford University Press.

Competition with trees does not influence root characteristics of perennial grasses in semi-arid and arid savannas in South Africa

Savannas support mixed tree-grass communities and interactions between these are typically viewed as being competitive based on studies that focused on grass aboveground production. However, an important plant response to competition and resource limitation is an increase in root reserves. We investigated root characteristics of perennial grasses in the presence and absence of trees as a proxy of competition in South African savannas in three sites that differed in rainfall. We based our study on the hypothesis that competition from trees and water limitation will result in increased storage in roots of grasses under trees. Results indicate no significant effect of variation in rainfall of the different study locations on root characteristics of grasses. Furthermore, trees did not significantly influence most grass root characteristics that we measured. The only exception was nitrogen-content that showed an increase with rainfall and tree presence through potentially higher mineralization rates and nitrogen availability in the under-tree canopy environment. As the study sites are in the drier rainfall range in South Africa, it is likely that trees and grasses in these dry savannas may have a positive relationship conforming to the stress-gradient hypothesis. Alternatively, grasses and trees may be using complementary water and nutritional resources

Nitrogen but not phosphorus addition affects symbiotic N2 fixation by legumes in natural and semi-natural grasslands located on four continents

Background and aims The amount of nitrogen (N) derived from symbiotic N2 fixation by legumes in grasslands might be affected by anthropogenic N and phosphorus (P) inputs, but the underlying mechanisms are not known. Methods We evaluated symbiotic N2 fixation in 17 natural and semi-natural grasslands on four continents that are subjected to the same full-factorial N and P addition experiment, using the 15N natural abundance method. Results N as well as combined N and P (NP) addition reduced aboveground legume biomass by 65% and 45%, respectively, compared to the control, whereas P addition had no significant impact. Addition of N and/or P had no significant effect on the symbiotic N2 fixation per unit legume biomass. In consequence, the amount of N fixed annually per grassland area was less than half in the N addition treatments compared to control and P addition, irrespective of whether the dominant legumes were annuals or perennials. Conclusion Our results reveal that N addition mainly impacts symbiotic N2 fixation via reduced biomass of legumes rather than changes in N2 fixation per unit legume biomass. The results show that soil N enrichment by anthropogenic activities significantly reduces N2 fixation in grasslands, and these effects cannot be reversed by additional P amendment

Temporal rarity is a better predictor of local extinction risk than spatial rarity

Spatial rarity is often used to predict extinction risk, but rarity can also occur temporally. Perhaps more relevant in the context of global change is whether a species is core to a community (persistent) or transient (intermittently present), with transient species often susceptible to human activities that reduce niche space. Using 5–12 yr of data on 1,447 plant species from 49 grasslands on five continents, we show that local abundance and species persistence under ambient conditions are both effective predictors of local extinction risk following experimental exclusion of grazers or addition of nutrients; persistence was a more powerful predictor than local abundance. While perturbations increased the risk of exclusion for low persistence and abundance species, transient but abundant species were also highly likely to be excluded from a perturbed plot relative to ambient conditions. Moreover, low persistence and low abundance species that were not excluded from perturbed plots tended to have a modest increase in abundance following perturbance. Last, even core species with high abundances had large decreases in persistence and increased losses in perturbed plots, threatening the long-term stability of these grasslands. Our results demonstrate that expanding the concept of rarity to include temporal dynamics, in addition to local abundance, more effectively predicts extinction risk in response to environmental change than either rarity axis predicts alone