Cindy E. Prescott

Does nitrogen availability control rates of litter decomposition in forests

The effects of increased exogenous N availability on rates of litter decomposition were assessed in several field fertilization trials. In a jack pine (Pinus banksiana Lamb.) forest, needle litter decomposed at the same rate in control plots and in plots fertilized with urea and ammonium nitrate (1350 kg N ha-1) with or without P and K. Mixed needle litter of western hemlock (Tsuga heterophylla (Raf.) Sarg.), western red cedar (Thuja plicata Donn) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) incubated in plots recently amended with sewage sludge (500 kg N ha-1) lost less weight during 3 years than did litter in control plots. Forest floor material also decomposed more slowly in plots amended with sewage sludge. Paper birch (Betula papyrifera Marsh.) leaf litter placed on sewage sludge (1000 kg N ha-1), pulp sludge, or sewage-pulp sludge mixtures decomposed at the same rate as leaf litter in control plots. These experiments demonstrate little effect of exogenous N availability on rates of litter decomposition.The influence of endogenous N availability on rates of litter decomposition was examined in a microcosm experiment. Lodgepole pine (Pinus contorta var. latifolia Engelm.) needle litter collected from N-fertilized trees (525 kg N ha-1 in ammonium nitrate) were 5 times richer in N than needles from control trees (1.56% N versus 0.33% N in control trees), but decomposed at the same rate. Green needles from fertilized trees contained twice as much N as needles from control trees (1.91% N versus 0.88% N), but decomposed at the same rate. These experiments suggest that N availability alone, either exogenous or endogenous, does not control rates of litter decomposition. Increased N availability, through fertilization or deposition, in the absence of changes in vegetation composition, will not alter rates of litter decomposition in forests.

Humus in northern forests: friend or foe?

Abstract Organic matter is of primary importance to the sustainability of long-term site productivity in forest ecosystems. In boreal forests, organic matter accumulates at the surface as mor humus. This may represent a substantial portion of the total nutrient capital of a site, and its decomposition is essential for the short-term availability of nutrients for tree growth and long-term site fertility. However, organic matter accumulation at the soil surface can also effect the forest ecosystem by immobilizing nutrients making them unavailable for plant uptake, and by creating physical and environmental conditions that can impede seedling establishment and survival. Therefore, it is necessary to understand the processes of humus formation and decomposition in order to manage these soils in a manner that will maintain or improve site productivity. This paper provides an overview of (i) the composition of humus, (ii) the conditions in the boreal forest that result in the surface accumulation of humus, (iii) decomposition processes and (iv) the effects of humus on nutrient (especially nitrogen) availability. Questions relating to the detrimental role of surface organic matter accumulation, the effects of natural disturbances (e.g., fire) and harvesting disturbances on humus loss and accumulation and management practices that can maintain long-term site productivity will also be discussed.

Effects of clearcutting and alternative silvicultural systems on rates of decomposition and nitrogen mineralization in a coastal montane coniferous forest

Abstract Rates of litter decomposition and N mineralization were measured in an old growth forest and in adjacent areas harvested by clearcut, patch cut, shelterwood and green tree retention systems. The site was a montane forest of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and amabilis fir (Abies amabilis Dougl.) on Vancouver Island, in British Columbia, Canada. During the first two years after harvesting, weight loss of needle litter was fastest in the old growth forest, possibly owing to higher moisture in surface layers in the uncut forest during the summer. Forest floor material lost about 10% of its initial weight during the two years in all systems. In-situ rates of net N mineralization in the forest floor were greatest in the clearcut and least in the old growth. Concentrations of nitrate were greater in the clearcut than in the other systems or the old growth. The results indicated that alternative silvicultural systems affected N mineralization less than clearcutting, and that the increase in N mineralization and nitrification after clearcutting was not the result of faster decomposition of organic matter. Reduced input of fresh litter and the resulting decline in C availability and immobilization of N into microbial biomass may better explain the increase in N availability alter clearcutting in this ecosystem.

Patterns of Carbon, Nitrogen and Phosphorus Dynamics in Decomposing Foliar Litter in Canadian Forests

We examined the patterns of nitrogen (N) and phosphorus (P) gain, retention or loss in ten foliar tissues in a litterbag experiment over 6 years at 18 upland forest sites in Canada, ranging from subarctic to cool temperate. N was usually retained in the decomposing litter until about 50% of the original C remained. The peak N content in the litter was observed at between 72 and 99% of the original C remaining, with C:N mass quotients between 37 and 71 (mean 55). The rate of N release from the litters was not related to the original N concentration, which may be associated with the generally narrow range (0.59–1.28% N) in the litters. P was immediately lost from all litters, except beech leaves, with critical litter C:P mass quotients for P release being in the range 700–900. The rate of P loss was inversely correlated with the original litter P concentration, which ranged from 0.02 to 0.13%. The soil underlying the litterbags influenced the pattern of N and P dynamics in the litters; there were weak correlations between the N and P remaining at 60% C remaining in the litters and the C:N and C:P quotients of the surface layer of the soil. There was a trend for higher N and P retention in the litter at sites with lower soil C:N and N:P quotients, respectively. Although there was a large variation in C:N, C:P and N:P quotients in the original litters (29–83, 369–2122 and 5–26, respectively), and some variation in the retention or loss of N and P in the early stages of decomposition, litters converged on C:N, C:P and N:P quotients of 30, 450 and 16, when the C remaining fell below 30%. These quotients are similar to that found in the surface organic matter of these ecosystems.

Characterization of Humus Microbial Communities in Adjacent Forest Types That Differ in Nitrogen Availability

To address the link between soil microbial community composition and soil processes, we investigated the microbial communities in forest floors of two forest types that differ substantially in nitrogen availability. Cedar-hemlock (CH) and hemlock-amabilis fir (HA) forests are both common on northern Vancouver Island, B.C., occurring adjacently across the landscape. CH forest floors have low nitrogen availability and HA high nitrogen availability. Total microbial biomass was assessed using chloroform fumigation-extraction and community composition was assessed using several cultivation-independent approaches: denaturing gradient gel electrophoresis (DGGE) of the bacterial communities, ribosomal intergenic spacer analysis (RISA) of the bacterial and fungal communities, and phospholipid fatty acid (PLFA) profiles of the whole microbial community. We did not detect differences in the bacterial communities of each forest type using DGGE and RISA, but differences in the fungal communities were detected using RISA. PLFA analysis detected subtle differences in overall composition of the microbial community between the forest types, as well as in particular groups of organisms. Fungal PLFAs were more abundant in the nitrogen-poor CH forests. Bacteria were proportionally more abundant in HA forests than CH in the lower humus layer, and Gram-positive bacteria were proportionally more abundant in HA forests irrespective of layer. Bacterial and fungal communities were distinct in the F, upper humus, and lower humus layers of the forest floor and total biomass decreased in deeper layers. These results indicate that there are distinct patterns in forest floor microbial community composition at the landscape scale, which may be important for understanding nutrient availability to forest vegetation.

Influences of evergreen gymnosperm and deciduous angiosperm tree species on the functioning of temperate and boreal forests

It has been recognized for a long time that the overstorey composition of a forest partly determines its biological and physical–chemical functioning. Here, we review evidence of the influence of evergreen gymnosperm (EG) tree species and deciduous angiosperm (DA) tree species on the water balance, physical–chemical soil properties and biogeochemical cycling of carbon and nutrients. We used scientific publications based on experimental designs where all species grew on the same parent material and initial soil, and were similar in stage of stand development, former land use and current management. We present the current state of the art, define knowledge gaps, and briefly discuss how selection of tree species can be used to mitigate pollution or enhance accumulation of stable organic carbon in the soil. The presence of EGs generally induces a lower rate of precipitation input into the soil than DAs, resulting in drier soil conditions and lower water discharge. Soil temperature is generally not different, or slightly lower, under an EG canopy compared to a DA canopy. Chemical properties, such as soil pH, can also be significantly modified by taxonomic groups of tree species. Biomass production is usually similar or lower in DA stands than in stands of EGs. Aboveground production of dead organic matter appears to be of the same order of magnitude between tree species groups growing on the same site. Some DAs induce more rapid decomposition of litter than EGs because of the chemical properties of their tissues, higher soil moisture and favourable conditions for earthworms. Forest floors consequently tend to be thicker in EG forests compared to DA forests. Many factors, such as litter lignin content, influence litter decomposition and it is difficult to identify specific litter‐quality parameters that distinguish litter decomposition rates of EGs from DAs. Although it has been suggested that DAs can result in higher accumulation of soil carbon stocks, evidence from field studies does not show any obvious trend. Further research is required to clarify if accumulation of carbon in soils (i.e. forest floor + mineral soil) is different between the two types of trees. Production of belowground dead organic matter appears to be of similar magnitude in DA and EG forests, and root decomposition rate lower under EGs than DAs. However there are some discrepancies and still are insufficient data about belowground pools and processes that require further research. Relatively larger amounts of nutrients enter the soil–plant biogeochemical cycle under the influence of EGs than DAs, but recycling of nutrients appears to be slightly enhanced by DAs. Understanding the mechanisms underlying forest ecosystem functioning is essential to predicting the consequences of the expected tree species migration under global change. This knowledge can also be used as a mitigation tool regarding carbon sequestration or management of surface waters because the type of tree species affects forest growth, carbon, water and nutrient cycling.

Recreating a Functioning Forest Soil in Reclaimed Oil Sands in Northern Alberta: An Approach for Measuring Success in Ecological Restoration

During oil-sands mining all vegetation, soil, overburden, and oil sand is removed, leaving pits several kilometers wide and up to 100 m deep. These pits are reclaimed through a variety of treatments using subsoil or a mixed peat-mineral soil cap. Using nonmetric multidimensional scaling and cluster analysis of measurements of ecosystem function, reclamation treatments of several age classes were compared with a range of natural forest ecotypes to discover which treatments had created ecosystems similar to natural forest ecotypes and at what age this occurred. Ecosystem function was estimated from bioavailable nutrients, plant community composition, litter decomposition rate, and development of a surface organic layer. On the reclamation treatments, availability of nitrate, calcium, magnesium, and sulfur were generally higher than in the natural forest ecotypes, while ammonium, P, K, and Mn were generally lower. Reclamation treatments tended to have more bare ground, grasses, and forbs but less moss, lichen, shrubs, trees, or woody debris than natural forests. Rates of litter decomposition were lower on all reclamation treatments. Development of an organic layer appeared to be facilitated by the presence of shrubs. With repeated applications of fertilizers, measured variables for the peat-mineral amendments fell within the range of natural variability at about 20 yr. An intermediate subsoil layer reduced the need for fertilizer and conditions resembling natural forests were reached about 15 yr after a single fertilizer application. Treatments over tailings sand receiving only one application of fertilizer appeared to be on a different trajectory to a novel ecosystem.

Litter Quality and Its Potential Effect on Decay Rates of Materials from Canadian Forests

Decomposition is influenced by a wide array of factors including macroclimate, microclimate, soil biota, soil nutrients, substrate piece size and substrate quality. To separate the influence of some of these factors a 10-year study, the Canadian Intersite Decomposition Experiment, was established in 1992 to measure the decay of 11 standard litter types on a range of forest types at 21 sites across Canada. As part of the study we analysed the initial elemental contents (N, P, S, K, Ca, Mg) and carbon (C) fractions (extractables, cellulose, hemicellulose, lignin) by13C NMR and wet chemical proximate analysis in a total of 37 primarily foliar litter types representative of the range of species found at the different CIDET sites. Litter types especially non-conifer species varied greatly in their qualities. Principal component analyses showed that the litter types could be distinguished by the elemental macronutrient contents through the ratio of N+P+K:S, by proximate chemical analyses through the ratio of water soluble:acid fractions, and by NMR through the ratio of O-alkyl:alkyl C. Litter quality data was used in three simple models of litter decay to predict how the mass loss of the different litter types could vary. Two models using a linear or single exponential decay equation and litter lignin and N content predicted a 2–5 fold difference in total mass loss for the different litter types. A third model using a summed exponential decay equation for three chemical fractions and a ligno-cellulose index predicted that for all but one litter type, variation in mass loss between types would be less than a 20%.

NITROGEN TURNOVER IN FOREST FLOORS OF COASTAL DOUGLAS‐FIR AT SITES DIFFERING IN SOIL NITROGEN CAPITAL

Nitrogen cycling is generally considered to be more rapid on sites with high availability of N; this is usually associated with differences in tree species composition. We tested whether N cycling in stands of a single tree species increased with increasing mineral soil nitrogen capital. Rates of N cycling in nine stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) were estimated by measuring annual N input in litter, N content of the forest floor, and net N mineralization rate in the forest floor. Rates of C and N turnover were estimated from the litterfall:forest floor ratio. The amount of N returned in litter increased as soil N capital increased. The increase in litter N content resulted from both increased litter mass (C content) and increased N concentration in litter. Despite the greater litter input, forest floor mass was smaller at N-rich sites, indicating more rapid turnover of the forest floor on N-rich sites. There was a positive relationship between fractional annual loss of N from the forest floor and soil N capital. Therefore, even without changes in tree species composition, sites with greater soil N capital returned more N in annual litterfall and had faster turnover of N in the forest floor. Fractional annual loss of C also increased with increasing soil N capital, indicating faster decomposition on N-rich sites. The rate of net N mineralization during laboratory incubations of the forest floor was not correlated with soil N capital or N concentrations of litter, but was related to the C:N ratio of the forest floor. Net N mineralization was appreciable only at two sites where forest floor C:N ratios were <35. The rate of net N mineralization or C:N ratio of the forest floor were not good indicators of N availability at these sites. The results of this study are consistent with the hypothesis that rates of N cycling are faster on N-rich sites, and that this effect can occur in the absence of changes in tree species composition.

Influence of forest floor type on rates of litter decomposition in microcosms

Abstract Rates of mass loss of leaf litter of paper birch were measured during a 70 wk laboratory incubation in microcosms containing forest floor material from red alder (C-to-N ratio = 27), Douglas fir (C-to-N, 32), and lodgepole pine (C-to-N, 59) forests. Mass loss from leaf litter was fastest in alder forest floors and slowest in pine, and so was related to the C-to-N ratio of the forest floors. The decomposition rate was related to the amount of KCl-extractable NH 4 + N and NO 3 − N in each microcosm, but not to the concentrations of extractable N (mg g −1 ), which were greater in Douglas fir forest floors than in alder for most of the incubation. There were larger amounts of faecal material from soil fauna in the litter incubated in the alder forest floors, indicating greater faunal activity in these floors. The relationship between rates of decomposition and N concentrations may be due to the influence of soil fauna activity on both factors, rather than to a direct influence of N availability on rates of litter decomposition.