Long-term outcome of nitrogen immobilization to restore endemic sand grassland in Hungary.

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]