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Aim: There is increasing recognition that plant–soil feedbacks drive species co‐existence; therefore the below‐ground compartment should be better considered in ecological restoration. Addition of carbon is a restoration measure that relies on indirect plant–soil relationships by immobilizing soil nitrogen in microbial biomass to manipulate competitive hierarchies between plant species. The aim of the present work was to evaluate the impact of six years of carbon amendment in old‐field restoration in the long term, 20 years after the first application. Location: Old‐fields and reference sand steppe on dry calcareous sandy soil in Fülöpháza, Kiskun LTER site (46°53′ N, 19°24′ E), Hungary, Pannonian biogeographic region, Europe. Methods: We applied carbon amendment in the form of sucrose and sawdust on three abandoned agricultural fields between 1998 and 2003. Vegetation was surveyed in 1998–2006 and re‐sampled in 2008, 2010 and 2018 on permanent 2 m × 2 m coenological relevés for carbon‐amended, control and reference plots (n = 144). We used principal response curves (PRC) to describe vegetation development trajectories and linear mixed‐effects models to test changes in cover of vascular plants and cryptogams, nitrogen requirement groups and restoration species groups with time and treatment. Results: Carbon amendment resulted in lower soil nitrogen availability, and lower cover of vascular plants and cryptogams compared to control, but these differences became visible only four to five years after the first application and disappeared three years after the cessation of treatment. Minor treatment effects on the cover of oligotrophic, mesotrophic and target species were found. Conclusions: Carbon amendment did not speed up the recovery of sand grasslands; however, the reduction of cover (vegetation and cryptogam) can be a window of opportunity for other species to colonize that can be used as a complement to other treatments. Long‐term monitoring is especially important in evaluating restoration interventions that focus on indirect above‐ground–below‐ground linkages. [ABSTRACT FROM AUTHOR]

Community assembly theory is suggested as a guiding principle for ecological restoration to help understand the mechanisms that structure biological communities and identify where restoration interventions are needed. We studied three hypotheses related to propagule limitation, stress‐dominance, and limiting similarity concepts in community assembly in a restoration field experiment with a trait‐based null model approach. The experiment aimed to assist the recovery of sand grassland on former arable land in the Kiskunság, Pannonian biogeographic region, Europe. Treatments included initial seeding of five grassland species, carbon amendment, low‐intensity mowing, and combinations in 1 m by 1 m plots in three old fields from 2003 to 2008. The distribution of 10 individual plant traits was compared to the null model and the effect of time and treatments were tested with linear mixed effect models. Initial seeding had the most visible impact on species and trait composition confirming propagule limitation in grassland recovery. Reducing nutrient availability through carbon amendment strengthened trait convergence for length of flowering as expected based on the stress‐dominance hypothesis. Mowing changed trait divergence to convergence for plant height with a strengthening impact with time, supporting our hypothesis of increasing dominance of limiting similarity with time. Our results support the idea that community assembly is simultaneously influenced by propagule limitation and multiple trait‐based processes that act through different traits. The limited impact of manipulating environmental filtering and limiting similarity compared to seeding, however, supports the view that only targeting the dispersal and environmental filters in parallel would improve restoration outcome. [ABSTRACT FROM AUTHOR]

The application of a filter-based assembly model is a promising concept to support ecological restoration. The filters illustrate dispersal, abiotic and biotic constraints that narrow down the species pool and determine the realized species composition. The aim of restoration interventions is the manipulation of filters to achieve a restoration target., We tested the applicability of a filter-based assembly model in the restoration of endemic Pannonian sandy grassland on old-fields. We report the results of a six-year experiment to identify key filters and find possible interactions. Treatments included the following: (i) seeding (dispersal filter), (ii) carbon amendment to lower nitrogen availability (abiotic filter) and (iii) mowing (biotic filter). Treatments were carried out in 1 m2 plots following the same Latin square design in three old-fields., Seeding was the most important treatment increasing species richness, vascular cover and enhancing target species composition. Mowing played a secondary role, acting primarily in interaction with the other treatments. Carbon amendment significantly reduced soil mineral nitrogen, but played a subordinate role in determining vegetation composition. Significant interactions were found between the biotic and dispersal and the biotic and abiotic filters determining primarily the structural characteristics of the vegetation in terms of vascular cover, moss cover, litter and bare ground. Regarding species composition, synergistic links between the dispersal and biotic filters for seeded species and long-lived forbs, and antagonistic interaction between the dispersal and abiotic filters for seeded species were found., Synthesis and applications. Based on the impact and interactions of filters uncovered in this experiment, we propose a scheme for Pannonian grassland restoration, which can be validated and also used for other oligotrophic systems. In the proposed conceptual model, the dispersal filter plays a key role in determining the outcome of restoration measures followed by the biotic filter if manipulated by mowing, while the abiotic filter altered by nitrogen immobilization proved to have a weak effect. Based on our results, targeting several filters in parallel would improve restoration outcome. We propose that filter interactions should form an integral part of filter-based assembly models and should be taken into account in restoration decisions. [ABSTRACT FROM AUTHOR]

Soil nitrogen immobilization by carbon amendment is a management technique used for conservation purposes to increase the competitive ability of late-seral plant species over early-seral species based on their different tolerance of low soil nitrogen content., We immobilized nitrogen over six growing seasons on three ex-arable fields with poor sandy soils in the Hungarian lowland in order to restore endemic sandy grassland. Sucrose and sawdust were applied at rates based on previous laboratory experiments using local soils. We tested the efficacy of long-term carbon amendment for lowering soil nitrogen availability and favouring late-seral native species over early-seral weed species., Carbon amendments resulted in significant increase in soil microbial biomass C and reduced soil nitrogen availability after 2 years., Total vegetation cover was reduced by reducing soil nitrogen availability, but total species richness was not impacted. Cover of early-seral species decreased, and species richness and cover of late-seral species increased irrespective of nitrogen immobilization. However, after 4-6 years reducing soil nitrogen availability hampered the spread of moss under vascular vegetation., Synthesis and applications. This study supports the efficacy of carbon amendment as a tool to immobilize available soil nitrogen in the upper soil layers. However, the desired impact on vegetation was not fully achieved despite application over several years. Nitrogen immobilization was most relevant to bryophytes, lacking deep root systems, which may explain the responsiveness of this group to N limitation. The different impact of N availability on the complex of early-seral, late-seral vascular species and that of the bryophyte layer provides opportunity for directing state transformations in arid grasslands. Bryophyte cover can be suppressed through carbon amendments in order to enhance the germination and establishment of grassland species. The advantage of the method is that it opens bryophyte cover gradually without disturbing the soil surface, possibly avoiding the establishment of invasive species. However, further studies are required for deeper insight. [ABSTRACT FROM AUTHOR]

In order to find the best and most reliable practices for ecological restoration of degraded lands, longer time scales should be considered when evaluating restoration efforts. We assessed the long-term (16 years) effects of different grassland restoration treatments – seeding, mowing, and carbon amendment – in the Pannonian sand grasslands. After re-plowing in 1 m × 1 m plots, treatments were carried out in two abandoned croplands. Seeding was applied only initially (2002) while mowing and carbon amendment were carried out for six years (2003–2008). Vegetation was surveyed yearly from 2003 to 2008 and re-sampled in 2019 in each permanent treatment plot. We used principal coordinates analysis to describe the trajectories of vegetation development and linear mixed-effects models to test changes in the relative cover of native sand grassland (target) and invasive (neophyte) species with time and treatments. Relative cover of target species increased while neophyte species decreased with time in both sites. There was a higher relative cover of target species from the first or third year on and a lower relative cover of neophyte species from the third year on in one site in seeded plots compared to other treatments. Seeded species also spread into non-seeded plots by 2019, obscuring the differences between treatments 16 years after sowing. Carbon amendment proved to be beneficial in the early and mowing in the later phases of restoration. Based on the long-term results, initial seeding is the best method for restoring sand grasslands in old fields by favoring the establishment of target species and controlling non-native invasion. As a supplement to seeding, carbon amendment can be suggested in the initial phases and/or low-intensity mowing in the later phases of the restoration after land abandonment. Although the spread from seeded plots obscured the long-term differences between treatments, it optimized the restoration process, suggesting that the use of small seed introduction units can be enough to restore the whole degraded area. • Long-term monitoring is needed to better interpret restoration interventions. • Early seeding favors target species and controls invasion in grassland restoration. • Small introduction plots can trigger restoration of larger areas. • Low intensity mowing can be applied in the later phases of the restoration process. • Carbon amendment is suggested in combination with seeding right after abandonment. [ABSTRACT FROM AUTHOR]