Results from experiments with natural variation of litter Mn (Berg, 2014 Berg et al., 2015) and external Mn addition (Trum et al., 2015) both showed positive effects of Mn on litter decomposition, which is interpreted with the essential contribution of Mn to the production and activity of lignin-degrading enzymes (Perez & Jeffries, 1992 Trum et al., 2015). For example, higher Ca and Mg concentrations led to faster litter decomposition across different ecosystems and biomes (García-Palacios et al., 2016). Although the role of N and phosphorus (P) in driving litter decomposition is reasonably well-studied, some other nutrients, including calcium (Ca), magnesium (Mg) or manganese (Mn), are less frequently assessed in litter decomposition studies, but may actually predict decomposition rates well (Makkonen et al., 2012 Berg et al., 2015). Commonly, higher litter nutrient concentrations are followed by more rapid decomposition (Davey et al., 2007 Cornwell et al., 2008). In addition to C quality, the concentrations of different nutrients in plant litter also are affected by N deposition, often with higher nutrient concentrations, in particular N, when N deposition rates rise (Berg & Matzner, 1997). A more detailed description of how litter C quality varies among species in response to N addition and its consequences on decomposition would facilitate a more mechanistic understanding of plant-mediated biogeochemical cycling with continued N deposition in many of the worlds’ ecosystems. However, the impacts of N addition on litter C chemistry are highly species-specific and also may vary among different ecosystem types (Liu et al., 2016). Likewise, N addition may reduce cellulose content that may be followed by faster decomposition, as was reported for litter of spruce needles (Sjöberg et al., 2004). For example, long-term N addition appears to decrease litter lignin content in a semi-arid grassland of northern China (Hou et al., 2018). Nitrogen deposition can affect litter quality through altering litter C chemistry (Dias et al., 2013 Liu et al., 2016). soil chemical properties) interactively regulate the responses of litter decomposition to N deposition is fundamental for a mechanistic understanding and the prediction of N deposition effects on C and nutrient cycling in terrestrial ecosystems.

litter quality and soil microbial communities) and abiotic factors (e.g. Therefore, understanding how the biotic (e.g. Most of these control factors are affected by atmospheric nitrogen (N) deposition (Vitousek et al., 1997 Li et al., 2017, 2019), with consequences for biodiversity and ecosystem functioning (Sala et al., 2000). Litter decomposition is a fundamental process driving carbon (C) and nutrient cycling in terrestrial ecosystems (Parton et al., 2007 Waring, 2013) and is controlled mainly by litter quality, environmental conditions such as soil properties or climate, and microbial communities (Swift et al., 1979 Bradford et al., 2016). The soil-driven effect on decomposition reported here may have long-lasting impacts on nutrient cycling, soil organic matter dynamics and ecosystem functioning. Shifts in plant species composition and litter quality played a minor role compared to N-driven reductions in soil pH and C : N, which increased soil Mn availability and altered microbial community structure.

Several factors contributed jointly to higher rates of litter decomposition in response to N deposition.

Soil C : N ratios were lower with N addition that subsequently led to markedly higher bacterial to fungal ratios, which also stimulated litter decomposition.

Higher soil manganese (Mn) availability, which apparently was a consequence of N addition-induced lower soil pH, was the most important factor for faster decomposition. Decomposition increased consistently with increasing rates of N addition in all litter types.We assessed these different mechanisms with a decomposition experiment using litter from four abundant species ( Achnatherum sibiricum, Agropyron cristatum, Leymus chinensis and Stipa grandis) and litter mixtures representing treatment-specific community composition in a semi-arid grassland under long-term simulation of six different rates of N deposition.Litter decomposition is a key process contributing to these changes, but the numerous mechanisms for altered decomposition remain poorly identified. The continuing nitrogen (N) deposition observed worldwide alters ecosystem nutrient cycling and ecosystem functioning.

Ecotone vs continuum