Tree Diversity, Initial Litter Quality, and Site Conditions Drive Early-Stage Fine-Root Decomposition in European Forests

Decomposition of dead fine roots contributes significantly to nutrient cycling and soil organic matter stabilization. Most knowledge of tree fine-root decomposition stems from studies in monospecific stands or single-species litter, although most forests are mixed. Therefore, we assessed how tree species mixing affects fine-root litter mass loss and which role initial litter quality and environmental factors play. For this purpose, we determined fine-root decomposition of 13 common tree species in four European forest types ranging from boreal to Mediterranean climates. Litter incubations in 315 tree neighborhoods allowed for separating the effects of litter species from environmental influences and litter mixing (direct) from tree diversity (indirect). On average, mass loss of mixed-species litter was higher than those of single-species litter in monospecific neighborhoods. This was mainly attributable to indirect diversity effects, that is, alterations in microenvironmental conditions as a result of tree species mixing, rather than direct diversity effects, that is, litter mixing itself. Tree species mixing effects were relatively weak, and initial litter quality and environmental conditions were more important predictors of fine-root litter mass loss than tree diversity. We showed that tree species mixing can alter fine-root litter mass loss across large environmental gradients, but these effects are context-dependent and of moderate importance compared to environmental influences. Interactions between species identity and site conditions need to be considered to explain diversity effects on fine-root decomposition.

Decomposition of Fine Roots and Aboveground Agroforestry Litter as Plant-soil Feedback After Volcanic Ash Deposition

Background and PurposeAbove- and belowground organic inputs feed decomposer communities in the soil enhancing soil organic matter (Corg) formation, depending on the vegetation, soil, contextual factors and human management of (agro)ecosystems. Plant-soil feedback in volcanic ash rapidly increases Corg during transformation to Andisols. We quantified fine root turnover in agroforestry systems, including the ash-adapted native tree Parasponia rigida, as part of the C accumulation process. MethodsFine root (<2 mm) decomposition was quantified with a total of 1440 litterbag samples, testing the effects of six tree species (Coffea canephora, Persea americana, Durio zibethinus, Gliricidia sepium, Falcataria moluccana and Parasponia rigida), three distances to the nearest coffee trees, two seasons (rainy and dry), two sites (with and without recent ash deposits), four time intervals (2, 4, 6 and 8 weeks) and five replicates. Soil temperature around the litterbags was used to derive equivalent decomposition rates at 20oC. The ratio of lignin plus phenolics over nitrogen was used as main litter quality indicator.ResultsDecomposition of fine tree roots was up to three times faster than that of aboveground litter with the same quality index measured in the same habitat. Root decomposition was slower in topsoils with recent volcanic ash, with a mean residence time extended by, on average, two weeks. Decomposition of roots of the ash-adapted native tree Parasponia rigida was especially rapid. ConclusionsFine root turnover contributes to the Corg accumulation that turns low-C volcanic ash into high-carbon andic soil and has relatively short necromass residence times.

Degradation-driven changes in fine root carbon stocks, productivity, mortality, and decomposition rates in a palm swamp peat forest of the Peruvian Amazon

Background Amazon palm swamp peatlands are major carbon (C) sinks and reservoirs. In Peru, this ecosystem is widely threatened owing to the recurrent practice of cutting Mauritia flexuosa palms for fruit harvesting. Such degradation could significantly damage peat deposits by altering C fluxes through fine root productivity, mortality, and decomposition rates which contribute to and regulate peat accumulation. Along a same peat formation, we studied an undegraded site (Intact), a moderately degraded site (mDeg) and a heavily degraded site (hDeg) over 11 months. Fine root C stocks and fluxes were monthly sampled by sequential coring. Concomitantly, fine root decomposition was investigated using litter bags. In the experimental design, fine root stocks and dynamics were assessed separately according to vegetation type (M. flexuosa palm and other tree species) and M. flexuosa age class. Furthermore, results obtained from individual palms and trees were site-scaled by using forest composition and structure. Results At the scale of individuals, fine root C biomass in M. flexuosa adults was higher at the mDeg site than at the Intact and hDeg sites, while in trees it was lowest at the hDeg site. Site-scale fine root biomass (Mg C ha−1) was higher at the mDeg site (0.58 ± 0.05) than at the Intact (0.48 ± 0.05) and hDeg sites (0.32 ± 0.03). Site-scale annual fine root mortality rate was not significantly different between sites (3.4 ± 1.3, 2.0 ± 0.8, 1.5 ± 0.7 Mg C ha−1 yr−1 at the Intact, mDeg, and hDeg sites) while productivity (same unit) was lower at the hDeg site (1.5 ± 0.8) than at the Intact site (3.7 ± 1.2), the mDeg site being intermediate (2.3 ± 0.9). Decomposition was slow with 63.5−74.4% of mass remaining after 300 days and it was similar among sites and vegetation types. Conclusions The significant lower fine root C stock and annual productivity rate at the hDeg site than at the Intact site suggests a potential for strong degradation to disrupt peat accretion. These results stress the need for a sustainable management of these forests to maintain their C sink function.

Effect of Root and Mycelia on Fine Root Decomposition and Release of Carbon and Nitrogen Under Artemisia halodendron in a Semi-arid Sandy Grassland in China

Plant fine root turnover is a continuous process both spatially and temporally, and fine root decomposition is affected by many biotic and abiotic factors. However, the effect of the living roots and the associated mycorrhizal fungal mycelia on fine root decomposition remains unclear. The objective of this study is to explore the influence of these biotic factors on fine root decomposition in a semi-arid ecosystem. In this study, we investigated the effect of fine roots and mycelia on fine root decomposition of a pioneer shrub (Artemisia halodendron) in Horqin sandy land, northeast China, by the ingrowth core method combined with the litterbag method. Litterbags were installed in cores. Results showed that core a allowed the growth of both fine roots and mycelia (treatment R + M), core b only allowed the growth of mycelia (treatment M), and in core c the fine root and mycelia growth were restricted and only bulk soil was present (treatment S). These findings suggest that the process of root decomposition was significantly affected by the living roots and mycelia, and carbon (C) and nitrogen (N) concentration dynamics during root decomposition differed among treatments. Mycelia significantly stimulated the mass loss and C and N release during root decomposition. Treatment R + M significantly stimulated the accumulation of soil total C, total N, and organic N under litterbags. The mycelia significantly stimulated the accumulation of the inorganic N (ammonium-N and nitrate-N) but the presence of fine roots weakened nitrate-N accumulation. The presence of living roots and associated mycelia strongly affected the process of root decomposition and matter release in the litter-soil system. The results of this study should strengthen the understanding of root-soil interactions.

Benefits to Plant Health and Productivity From Enhancing Plant Microbial Symbionts

Plants exist in close association with uncountable numbers of microorganisms around, on, and within them. Some of these endophytically colonize plant roots. The colonization of roots by certain symbiotic strains of plant-associated bacteria and fungi results in these plants performing better than plants whose roots are colonized by only the wild populations of microbes. We consider here crop plants whose roots are inhabited by introduced organisms, referring to them as Enhanced Plant Holobionts (EPHs). EPHs frequently exhibit resistance to specific plant diseases and pests (biotic stresses); resistance to abiotic stresses such as drought, cold, salinity, and flooding; enhanced nutrient acquisition and nutrient use efficiency; increased photosynthetic capability; and enhanced ability to maintain efficient internal cellular functioning. The microbes described here generate effects in part through their production of Symbiont-Associated Molecular Patterns (SAMPs) that interact with receptors in plant cell membranes. Such interaction results in the transduction of systemic signals that cause plant-wide changes in the plants’ gene expression and physiology. EPH effects arise not only from plant-microbe interactions, but also from microbe-microbe interactions like competition, mycoparasitism, and antibiotic production. When root and shoot growth are enhanced as a consequence of these root endophytes, this increases the yield from EPH plants. An additional benefit from growing larger root systems and having greater photosynthetic capability is greater sequestration of atmospheric CO2. This is transferred to roots where sequestered C, through exudation or root decomposition, becomes part of the total soil carbon, which reduces global warming potential in the atmosphere. Forming EPHs requires selection and introduction of appropriate strains of microorganisms, with EPH performance affected also by the delivery and management practices.

The influence of root decomposition on N2O fluxes and N2O microbial production pathways in soil with contrasting pore characteristics

&lt;p&gt;An understanding of the drivers of hotspot/hot moments of N&lt;sub&gt;2&lt;/sub&gt;O production is required to better constrain the global N&lt;sub&gt;2&lt;/sub&gt;O budget and to plan the mitigation strategies. Hot spots are areas with very high N&lt;sub&gt;2&lt;/sub&gt;O emission rates relative to the surrounding area, while hot moments are short periods of time with very high emission rates. As the decomposition of fresh organic matter is transitory in nature, it may have a strong influence on hotspot and hot moment N&lt;sub&gt;2&lt;/sub&gt;O production. Roots are well known to be hotspots for microbial activity but roots direct contribution to N&lt;sub&gt;2&lt;/sub&gt;O production and emissions in soil remain poorly understood.&lt;/p&gt;&lt;p&gt;In this study, we evaluated the role of root decomposition on N&lt;sub&gt;2&lt;/sub&gt;O production and emissions, as a function of soil pore size and water content. We hypothesized that (i) the greatest N&lt;sub&gt;2&lt;/sub&gt;O emissions will be observed from root decomposition in the soil dominated by large (&gt;30 &amp;#181;m &amp;#216;) pores due to their high connectivity and (ii) enhanced N&lt;sub&gt;2&lt;/sub&gt;O production by denitrification will be observed due to local anaerobic conditions, generated by O&lt;sub&gt;2&lt;/sub&gt; consumption by decomposers.&lt;/p&gt;&lt;p&gt;To evaluate the role of root decomposition on N&lt;sub&gt;2&lt;/sub&gt;O production we used soil microcosms cultivated with switchgrass (Panicum virgatum L. variety Cave-in-rock). From the same composite soil samples we created two soil materials with contrasting pore architectures, namely soil with prevalence of large pores (&amp;#8805; 35 &amp;#956;m &amp;#216;) and small pores (&amp;#8804; 10 &amp;#956;m &amp;#216;). After four months of growing in a greenhouse, plants were cut and soil microcosms with roots were incubated in the dark at room T for 21 days, at two contrasting soil moisture conditions: 40% and 70% water filled pore space (WFPS). Gas headspace samples were collected at different time points during incubation for N&lt;sub&gt;2&lt;/sub&gt;O and CO&lt;sub&gt;2&lt;/sub&gt; concentration analysis and isotopic characterization of N&lt;sub&gt;2&lt;/sub&gt;O (&amp;#948;&lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;bulk&lt;/sup&gt;, site preference (&lt;em&gt;S&lt;sub&gt;P&lt;/sub&gt;&lt;/em&gt;), and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O).&lt;/p&gt;&lt;p&gt;The daily emissions of N&lt;sub&gt;2&lt;/sub&gt;O and CO&lt;sub&gt;2&lt;/sub&gt; from soil microcosms with grown roots showed the same trend during the incubation period and were significantly higher compared to soil microcosms without roots (control) (p &lt; 0.05). Microcosm with large pores soil had significantly higher N&lt;sub&gt;2&lt;/sub&gt;O flux rates compared to the microcosms with small pore soil for both soil moisture treatments (p &lt; 0.001). The relationship between &lt;em&gt;S&lt;sub&gt;P&lt;/sub&gt;&lt;sub&gt;&amp;#160; &lt;/sub&gt;&lt;/em&gt;and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O (isotope mapping) indicated that heterotrophic bacterial denitrification strongly dominated N&lt;sub&gt;2&lt;/sub&gt;O production between day 1 to 7 of the incubation (&amp;#8805; 97%) and N&lt;sub&gt;2&lt;/sub&gt;O reduction was higher during this period (40 &amp;#8211; 60%) in soil microcosms with both pore size and moisture treatment. Later on, N&lt;sub&gt;2&lt;/sub&gt;O reduction decreased (1 &amp;#8211; 35%) while the share of nitrification/fungal sources increased for soil microcosms with large pores.&lt;/p&gt;&lt;p&gt;Our results indicated that decomposing roots acted as hotspots enhancing N&lt;sub&gt;2&lt;/sub&gt;O emissions and N&lt;sub&gt;2&lt;/sub&gt;O hotspots occurring during root decomposition are strongly influenced by soil pore architecture. While differences in soil pore architecture did not cause differences in N&lt;sub&gt;2&lt;/sub&gt;O production process at the initial phase of decomposition, it might influence the relative contribution of N&lt;sub&gt;2&lt;/sub&gt;O microbial production pathways in later stage of decomposition.&lt;/p&gt;