Decomposition disentangled: A test of the multiple mechanisms by which nitrogen enrichment alters litter decomposition

Abstract

1. Nitrogen (N) enrichment has direct effects on ecosystem functioning by altering soil abiotic conditions and indirect effects by reducing plant diversity and shifting plant functional composition from dominance by slow to fast growing species. Litter decomposition is a key ecosystem function and is affected by N enrichment either by a change in litter quality (the recalcitrance of the plant material) or through a change in soil quality (the abiotic and biotic components of the soil that affect decomposition). The relative importance of soil and litter quality and how the direct and effects of N alter them remains poorly known. 2. We designed a large grassland field experiment manipulating N enrichment, plant species richness and functional composition in a full factorial design. We used three complementary litter bag experiments and a novel structural equation modelling approach to quantify the relative effects of the treatments on litter and soil quality and their importance for total decomposition. 3. Our results indicate that total decomposition was mostly driven by changes in litter quality rather than soil quality. Litter quality was affected by the nutrient contents (N and calcium) and structural components of the litter (leaf dry matter content, fibres). N enrichment increased litter decomposition mostly indirectly through a shift in functional composition toward faster growing plant species producing higher quality litter. N enrichment also had effects on soil, by directly and indirectly affected vegetation cover, but this had relatively few consequences for the total decomposition rate. 4. Synthesis. Our approach provides a mechanistic tool to test the drivers of litter decomposition across different ecosystems. Our results show that functional composition is more important than richness or soil quality in determining litter decomposition and that N enrichment effects mainly occur via above-rather than belowground processes. This highlights the importance of considering shifts in plant species composition when assessing the effects of N enrichment 52 on decomposition. 53. CC-BY-NC-ND 4.0 International license It is made available under a (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.

Nitrogen availability and plant functional composition modify biodiversity-multifunctionality relationships

“…The sorted target biomass (without the weeds) was highly correlated with the estimated target biomass (R 2 = 0.994, data not shown). The mean weed percentage was around five percent and therefore removing the weeds made little difference to the total biomass 60 .…”

“…We set up a field experiment in October 2015 in Münchenbuchsee (Switzerland, 47°03’N, 7°46’E, 564 m a.s.l.) with factorial manipulations of plant species richness, plant functional composition, N enrichment and foliar fungal pathogens 60 . The site had been unfertilised for at least 10 years before the start of the experiment.…”

“…Further details about the site characteristics and experimental setup of the PaNDiv Experiment can be found under ref 60 .…”

“…The functions described in this study were measured between June 2017 and autumn 2018. We excluded June 2016 aboveground measurements as the experiment had just started to establish and the weed and bare ground cover were still large 60 . For each function measured more than once, we used the mean value per plot over the whole period (see Table S5 and Figure S3).…”

Simulated atmospheric nitrogen deposition inhibited the leaf litter decomposition of Cinnamomum migao H. W. Li in Southwest China

“…The effects of N deposition on litter decomposition are variable depending on the duration of N deposited, and the litter type 18 . Therefore, the reported responses of litter decomposition and nutrient release to N deposition are conflicting 8 and include enhancement 19 , 20 , suppression 21 , 22 , and no response 23 , 24 . Litter stoichiometry traits are important regulators of litter decomposition 25 that are affected by initial C, N, and phosphorus (P) contents as well as by their C/N and C/P ratios 26 .…”