Controlling Litter Decomposition Research Articles

Differential responses of litter decomposition to increased soil nutrients and water between two contrasting grassland plant species of Inner Mongolia, China

Soil chemistry and physical conditions are the key factors controlling litter decomposition. We studied the effects of increased soil nitrogen (N), phosphorus (P), and water on the decomposition rates and associated nutrient dynamics of two dominant grassland plant species (i.e. Allium bidentatum Fisch. ex Prokh. & Ikonn.-Gal. and Stipa krylovii Roshev.) with contrasting life forms and tissue chemistry in a typical steppe of Inner Mongolia, China. The treatments included addition of urea at the rates equivalent to 0, 8, 16, and 32 gN/m 2, and additions of mixed urea and triple superphosphate at the rates equivalent to 0, 8 gN/4 gP, 16 gN/8 gP, and 32 gN/16 gP/m 2, with and without water added. We found marked differences between the two species in the rates, as well as in the responses to water and addition rates of N and P, of litter decomposition. Additions of N alone or in mixture with P stimulated the rate of litter decomposition in both species. Adding water significantly increased the values of decay constant, k, in A. bidentatum, but not in S. krylovii. N and P concentrations in litters of A. bidentatum and S. krylovii all increased corresponding to increases in the rates of N or mixed N and P additions. Our results clearly indicate that the decomposition of high quality litter is more likely to be limited by soil moisture regimes, while that of low quality litter is more sensitive to nutrient availability. Our findings suggest that plant species with different litter qualities should be taken into consideration when we are to model the carbon cycle and nutrient dynamics in grassland ecosystems and that A. bidentatum is expected to contribute more than S. krylovii to the carbon cycle and nutrient dynamics of the semi-arid grasslands of Inner Mongolia.

Lignin degradation controls the production of dissolved organic matter in decomposing foliar litter

SummaryLignin is considered to be a crucial component controlling litter decomposition but its role in the production of dissolved organic matter (DOM) from litter is not well understood. Our main objective therefore was to examine the amounts and properties of DOM produced in decomposing litter, with special emphasis on the role of lignin degradation. We exposed litter of five different tree species (Sycamore maple, Mountain ash, European beech, Norway spruce, Scots pine) in litterbags at the soil surface of two neighbouring sites to degradation under field conditions. Litterbags were sampled eight times during 27 months of exposure in the field. We determined mass loss and characterized the lignin fraction by two different methods (van Soest procedure, acid‐detergent lignin: ADL, CuO oxidation). Litter was irrigated in the laboratory and leachates were analysed for dissolved organic carbon (DOC) and characterized by UV and fluorescence spectroscopy. Litter decomposition followed a two‐stage model characterized by initially rapid and then decreasing degradation with time. In the initial phase of litter decomposition, leached amounts of DOM decreased with time and no effects of lignin degradation were found. The contents of ADL in the litter residues and CuO oxidation products suggest larger degradation and oxidation of lignin in beech, spruce and pine litter than in maple and ash litter. The production of DOM from litter with larger lignin degradation increased in the second phase of decomposition, when mass loss exceeded 10–20%. In contrast, DOM produced from litter showing weak lignin degradation (maple, ash) did not increase further in the second phase of decomposition. In the leachates of litter with large lignin degradation (beech, spruce, pine), UV absorbance and fluorescence spectroscopy indicated a larger increase in the contribution of lignin‐derived compounds to DOM with increasing mass loss than for litter species with relatively stable lignin. We conclude that degradation of lignin is an important control on DOM production during the second phase of litter decomposition.

Litter decomposition and faunal activity in Mediterranean forest soils: effects of N content and the moss layer

Accumulation of soil carbon is mainly controlled by the balance between litter production and litter decomposition. Usually In Mediterranean forests there are contrasting conditions in the distribution of faunal activity and the moss layer that may have different effects on litter decomposition. Decomposition and faunal activity were studied by exposing litter of contrasting quality (Pinus halepensis Mill. and Quercus ilex L.) for 3.5 yr in three Mediterranean pine forests of the eastern Iberian Peninsula. The effects of mosses on decomposition and on faunal activity were studied by exposing P. halepensis litter either on moss patches or directly on the forest floor. Faecal pellet production was used as an indication of faunal activity. Water availability or soil characteristics seem to limit faunal activities in the drier sites. Faecal pellets were not found during the first stages of decomposition and in all sites they appeared when about a 30% of the initial litter had decomposed. Under wet conditions faecal pellet production was very high and a mass balance suggested that soil faunal activity may result in a net flow of organic matter from the lower organic horizons to the surface Oi horizon. Mosses slightly increased mass loss of pine litter probably as a consequence of high potentially mineralizable nitrogen in the Oa horizon of moss patches and also, perhaps, as a consequence of the higher moisture content measured in the Oi horizon needles sampled among the mosses. In contrast, moss patches reduced faunal activity. The effect of litter quality on mass loss was not always significant, suggesting an interaction between litter quality and site conditions. During the first stages of decomposition there was N immobilisation in P. halepensis litter (poorer in N) and N release from Q. ilex litter (richer in N). In conclusion, in these forests soil microclimate and/or N availability appear to be more important controlling litter decomposition than the distribution of faunal activity.

Litter decomposition and faunal activity in Mediterranean forest soils: effects of N content and the moss layer

Accumulation of soil carbon is mainly controlled by the balance between litter production and litter decomposition. Usually In Mediterranean forests there are contrasting conditions in the distribution of faunal activity and the moss layer that may have different effects on litter decomposition. Decomposition and faunal activity were studied by exposing litter of contrasting quality ( Pinus halepensis Mill. and Quercus ilex L.) for 3.5 yr in three Mediterranean pine forests of the eastern Iberian Peninsula. The effects of mosses on decomposition and on faunal activity were studied by exposing P. halepensis litter either on moss patches or directly on the forest floor. Faecal pellet production was used as an indication of faunal activity. Water availability or soil characteristics seem to limit faunal activities in the drier sites. Faecal pellets were not found during the first stages of decomposition and in all sites they appeared when about a 30% of the initial litter had decomposed. Under wet conditions faecal pellet production was very high and a mass balance suggested that soil faunal activity may result in a net flow of organic matter from the lower organic horizons to the surface Oi horizon. Mosses slightly increased mass loss of pine litter probably as a consequence of high potentially mineralizable nitrogen in the Oa horizon of moss patches and also, perhaps, as a consequence of the higher moisture content measured in the Oi horizon needles sampled among the mosses. In contrast, moss patches reduced faunal activity. The effect of litter quality on mass loss was not always significant, suggesting an interaction between litter quality and site conditions. During the first stages of decomposition there was N immobilisation in P. halepensis litter (poorer in N) and N release from Q. ilex litter (richer in N). In conclusion, in these forests soil microclimate and/or N availability appear to be more important controlling litter decomposition than the distribution of faunal activity.

Microbiota, fauna, and mesh size interactions in litter decomposition

Plant litter decomposition is a key process in carbon and nutrient cycling. The critical role of soil‐faunal community composition in decomposition has been demonstrated using different mesh size litterbags to control exposure of litter to different faunal size classes. However, the faunal community surrounding the litterbags has not been manipulated despite potentially large indirect effects of their activity on biotic and abiotic processes that control litter decomposition at the habitat‐scale. We combined microcosm and litterbag techniques to facilitate a more comprehensive understanding of the role of direct and indirect effects of soil‐faunal community composition on litter decomposition. We placed litterbags of three mesh sizes across model grassland miniecosystems manipulated to enable communities containing 1) microfauna; 2) micro‐ and meso‐fauna; 3) micro‐, meso‐ and macro‐fauna. All communities contained bacteria and fungi. The approach permitted correction of mesh size artefacts inherent to field studies. Indirect effects have been divided into two separate terms, direct‐indirect effects and indirect effects. Decomposition in micromesh litterbags was significantly decreased by the indirect effects of meso‐ and macro‐fauna. In macrofauna communities, increased mesh size significantly increased decomposition through mesh size per se and faunal effects. Relative effects of manipulated faunal community composition on litter mass loss and C:N ratio were equivalent for green and senesced litter. The presence of meso‐ and macro‐fauna increased litter decomposition rate overall despite inhibiting decomposition by microfauna, bacteria and fungi through indirect effects.

Leaf structure and defence control litter decomposition rate across species and life forms in regional floras on two continents

There is some evidence that traits of fresh leaves that provide structural or chemical protection (‘defence’) remain operational in the leaf litter and control interspecific variation in decomposition rate in or on the soil. We tested experimentally whether the negative relationship between foliar defence and litter decomposition rate is fundamental, i.e. whether it is seen consistently across higher plant species and life forms, and whether it is repeated in the floras of geographically and climatically distinct areas separated by an ocean. We employed the published results of two outdoor litter bag experiments, in which we simultaneously compared the relative mass losses (‘decomposibility’) of leaf litters of a wide range of plant species. One experiment was in Córdoba, Argentina, and included 48 Argentine species typical of the dry, subtropical landscapes along a steep altitudinal gradient. The other was in Sheffield, UK, and hosted 72 British species typical of the temperate–Atlantic landscape there. We linked the two experiments through a similar experiment in Sheffield that hosted litters of subsets of both the Argentine and British species. We also tested fresh leaves of all species from the same areas for tensile strength (‘toughness’) and relative palatability to generalist herbivorous snails in multi‐species ‘cafeteria’ experiments. Both in Argentina and in Great Britain there were highly significant correlations between leaf palatability (r=0.61; 0.73) or leaf tensile strength (r=−0.60; −0.60) and litter mass loss across all species. These relationships could be explained by variation both between and within broad life‐form groups. Specific leaf area (area∶dry mass) of fresh leaves was consistently correlated only with litter mass loss within British life form groups. We illustrated the possible ecosystem consequences of these relationships by comparing functional traits of British species differing in leaf habit. In comparison with deciduous species, evergreens generally had innately slow growth, which corresponded to their longer‐lived leaves of lower specific leaf area, higher tensile strength and lower palatability to generalist invertebrate herbivores. Correspondingly, evergreens produced more resistant leaf litter. Thus, slow‐growing evergreens might maintain their position in infertile ecosystems through leaf traits that help them to conserve their nutrients efficiently and to keep nutrient mineralization low, thereby not allowing potentially fast‐growing deciduous species to outcompete them.

13C nuclear magnetic resonance spectroscopy with cross-polarization and magic-angle spinning investigation of the proximate-analysis fractions used to assess litter quality in decomposition studies

Proximate analysis is often used in decomposition studies to characterize the organic components of foliar litter. The percent weight residue remaining after extraction by nonpolar and polar solvents and H3SO4 hydrolysis (Klason lignin, KL) is commonly used as a measure of litter quality and a modelling parameter. While KL is associated with resistance to decay, its nature is not well understood and it has long been suspected that it incorporates nonlignin components. We used solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to characterize litter, extracted residue, and the KL fraction of five species. NMR shows that cutin and condensed tannin are both significant components of litter and its extraction residues, in addition to lignin and carbohydrate. Hydrolysis with H2SO4 removes carbohydrates and amino acids, leaving the KL fraction derived from cutin, tannin, and lignin. Tannin retention in KL was also demonstrated by a hydrolysis study of purified tannins and a brown-rot lignin, using both NMR and the proanthocyanidin assay for condensed tannins. Although the NMR results are qualitative at this stage, it is clear that KL has limited use as a parameter controlling litter decomposition, and that other biopolymers should not be ignored in conceptual models, chemical analysis, and experimental design. Key words: Klason lignin, 13C CPMAS NMR, proximate analysis, litter quality, decomposition, tannin.

The Dynamics of Chemical Elements in Forest Litter

Litter bags with natural mixed litter were incubated until °60—70% mass loss in two oak—hornbeam and two pine—beech forest stands in southern Poland. At the same stands the input of chemical elements with throughfall was followed. Decomposition constants k for the oak—hornbeam litters were —0.57 and —0.55, and for the pine—beech litters —0.30 and —0.27. Chemical elements (except for Cu and Mn) revealed similar relative mobility in the four litters. On average the elements could be ordered by decreasing mobility as follows: K > Mg > Ca > S > Cu > Na > Mn = N > Cd > Pb = Zn > Fe. Instead of the two presupposed factors controlling litter decomposition, biological and chemical, three factors were specified: (1) biological, dominating the decay of organic matter and the dynamics of N, Ca, Mg, Mn, and S; (2) physical, dominated by leaching and atmospheric deposition, and controlling the dynamics of organic matter, K, Na, Pb, Cd, and Zn; and (3) chemical, determining the dynamics of Fe, Zn, Pb, and Cd through the fixation of metal ions to humic substances. Potassium was the only element that decreased in concentration in all litters, while the concentrations of N, Na, Fe, Zn, Pb, and Cd increased in all litters. S. Ca, Mg, and Mn concentrations revealed different patterns in different litters, presumably due to the differences in initial concentrations and soil acidity. No clear trend was found for Cu. In all litter types, Fe, Zn, Pb, and Cd significantly increased in absolute amounts at the end of litter—bag incubation. In all four stands the input with throughfall was high enough to explain the increases in amount of elements, with the exception of Fe in the oak—hornbeam litters.

Leaf-litter decomposition in the Picea/tsuga forests of Olympic National Park, Washington, U.S.A.

The factors controlling litter decomposition of eleven species of leaf litter were examined in the Hoh Rain Forest, Olympic National Park, Washington, using the litter-bag method. Leaching of litter decreased the decay rate-constant as compared to unleached litter. This decrease was in proportion to the readily leachable fraction ( rlf) in the litter. Decay of both leached and unleached litter was highly correlated ( P <0.001) with lignin: nitrogen ratio, although the regressions differed significantly between leached and unleached litter. Of the unleached litters used, Cornus nutallii decayed fastest ( k=2.35−2.46/year), and Pinus monticola decayed slowest ( k=0.38/year). A double-exponential regression was used to fit decay time-series from four species. This analysis indicated that species differed markedly in the proportion of fast and slow components and that the fast component was correlated with rlf for three of four species tested. Fast components decayed an order of magnitude faster than slow components. The rate at which these components decayed varied between species, indicating that both the amount and decay rate-constant of fast and slow components must be estimated to use the double-exponential equation.

Interactions among soil biota in desert ecosystems

In desert systems, the availability of soil nitrogen can be the single most limiting factor controlling plant growth during periods of adequate soil moisture. Decomposition and nitrogen-mineralization rates prior to or during peak periods of plant growth can therefore be critical. Decomposition rates in deserts are generally high, and are relatively independent of rainfall. These high rates can be attributed to a combination of abiotic weathering, the types of microflora and microfauna associated with plant litter, and the adaptations of the soil biota to moisture pulses. The factors controlling litter decomposition and subsequent nitrogen mineralization are dependent upon the location of the organic matter in the habitat. The breakdown of surface litter occurs primarily through either abiotic weathering or by termite consumption. Microarthropods do not appear to have any role in the decomposition of surface litter owing to abiotic constraints on activity. The decomposition of roots, and buried litter, however, has been shown to be dependent on the presence of soil microarthropods. Microbial-microfaunal activity in the root region of desert plants may also have a significant effect on the short-term cycling of nitrogen in arid systems.