Does nitrogen availability control rates of litter decomposition in forests?

Plant and Soil 168-169: 83-88, 1995. © 1995 Kluwer Academic Publishers. Does nitrogen availability control rates of litter decomposition in forests? C.E. Prescott Faculty of Forestry, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4 Key words: dp-composition, fertilization, forests, litter, nitrogen Abstract 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 -I) 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 -I) 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 papyrijera Marsh.) leaf litter placed on sewage sludge (1000 kg N ha -I), 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. latijolia Engelm.) needle litter collected from N-fertilized trees (525 kg N ha- I in ammonium nitrate) were 5 times richer in N than needles from control trees (l.56% N versus 0.3~ 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 (l.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. Introduction It is widely considered that the availability of nitrogen controls rates of litter decomposition, especially in the early stages of decay. This is largely based on three common observations. First, as described by Gosz (1981), litter quality and rates of decomposition are more rapid on N-rich sites than on N-poor sites. Secondly, there is often a good correlation between the initial N concentration in litter and the rate of decay (Taylor et ai., 1989). Thirdly, N often accumulates in litter during decomposition (Berg and Staaf, 1981), from which it has been interpreted that concentrations in fresh litter are sub-optimal for microbial activity. In spite of this indirect evidence for N limitation of decomposition, the results of experiments that test this hypothesis by adding N and measuring changes in litter decomposition rates, have been inconsistent. Rates of litter decomposition in N-fertilized plots have been reported to be faster (Hunt et ai., 1988; Prescott et aI., 1992), slower (Nohrstedt et aI., 1989; Titus and Malcolm, 1987) or the same (Theodoru and Bowen 1990; Will et aI., 1984) as those in control plots. Berg et al. (1987) found that higher N concentrations in pine needle litter stimulated decomposition during the first year, but depressed rates of decay in later stages. In this study the availability of N in the forest floor (exogenous N) or in the litter (endogenous N) was increased by adding organic wastes or chemical fertilizers to forests. Rates of weight loss of several types of litter in treated and control plots were compared to test the hypothesis that greater N availability results in more rapid decomposition of litter in fertilized forests. 84 Methods Fiberglass mesh bags with pore size of about 1.5 mm were used for all experiments. A known dry weight (70°C) of litter was placed in the bags and incubated for various periods of time, after which the dry weight of material remaining in each bag was determined. Each bag was kept in an envelope until placed in the field and the weight of spillage was subtracted from the original weight. Experiment 1: Effects of N-fertilization on decomposition rate of needle litter Needle litter oflodgepole pine was collected during the autumn of 1989 from fifteen 0.25 m2 wooden trays in an unfertilized 90-year-old pine forest in the Kananaskis Valley of Alberta (Prescott et aI., 1989). The litter was incubated in mesh bags in fertilized and control plots (0.02 ha) in a 65-year-old jack pine forest in Mont Tremblant Park, Quebec. The soil was deep outwash sand with an orthohumo-ferric podzol profile and a 6 cm deep mor humus layer. The plots were part of an optimum nutrition trial described by Weetman and Fournier (1984). Plots had been fertilized 6 times between 1971 and 1979 with a total of 672 kg N ha- I and 336 kg ha -I of P and of K. Measurements of N mineralization in humus samples collected in June 1991 indicated higher available N in fertilized plots at the time of this study (unpublished data). In August 1990, 11 mesh bags containing 1.0 g of pine needle litter were pinned on the litter surface in each of two control (NO) plots, two N-fertilized plots (N2), and two plots fertilized with N, P and K (N2PK). Five bags were retrieved from each plot in July 1991 and 6 were retrieved in May 1993, and the dry weight of litter remaining in each bag was determined. Experiment 2: Effects of sewage sludge on decomposition rate of needle litter Needle litter of western hemlock, western red cedar and Douglas-fir was collected in plastic trays in an unfertilized 130-year-old mixed forest at the University of British Columbia Research Forest near Maple Ridge, B.c., during the autumn of 1989. The soil was a ferro-humic podzol over ablation till, with a 6 cm deep duff mull humus (DeCatanzaro and Kimmins, 1985, mesic site). In May 1990, 25 mesh bags containing 1.0 g of mixed species needle litter were pinned on the litter surface in 3 control plots and in 3 plots that had received about 500 kg N ha- I of municipal sewage sludge (3.0% N) 5 months earlier. All plots were 0.01 ha and were located in a nearby 25-year-old mixed stand of hemlock, cedar and Douglas-fir. Five bags were collected from each plot after 6, 12, 18, 24 and 36 months, and the dry weight of litter remaining in each bag was determined. Experiment 3: Effects of sewage sludge on decomposition rate offorestfloor material Partially decomposed "F" layer material was collected from the unfertilized forest described in Experiment 2, after removing the litter layer. Roots and woody material were removed and 2.0 g dry weight portions were put into mesh bags that were double-layered to reduce spillage. In June 1990, 15 bags were buried in the forest floor in each of the 3 control plots and 3 sewage sludge treated plots. Five bags were collected from each plot in February 1991, April 1992 and May 1993. Roots that had grown into the bags were removed, and the remaining dry weight of material in each bag was determined. Experiment 4: Effects of sewage sludge and pulp and paper sludge on decomposition rate of litter Freshly fallen leaf litter of paper birch was collected from the forest floor in a mixed forest near Clearwater, B.C. in October 1991 and stored at 4°C until January 1992. The moisture content of the leaves was measured (11.8%) and 1.12 g of moist leaves (1.0 g dry weight equivalent) was placed in mesh bags. Leaves were kept moist to minimize fragmentation during processing. In February 1992, 20 bags were pinned on the surface of 3 control plots, 3 plots treated with municipal sewage sludge (3.0% N) at about 1000 kg N ha -I, 3 plots treated with an equivalent mass of pulp and paper sludge (0.3% N), and 3 plots that received a mixture of sewage and pulp sludge (1:1 by volume). The organic amendments had been applied to the 0.01 ha plots in December 1991. The plots were all in a plantation of hybrid poplar (Populus trichocarpa x deltoides) on a small island (#6) in the Fraser River of southwestern B.C. The soil was sandy flood plain deposits, classified as a gleyed regosol, with no humus accumulation. Five bags were collected from each plot after 3, 6, 9 and 12 months, and the dry weight of birch leaf litter remaining in each bag was measured. 85 Experiment 5: Effects of greater N concentration in litter on decomposition rate of pine needles 1.0 ---N 0.9 Senescent (brown) needle litter and current year (green) needles were collected from a 20-year-old stand of lodgepole pine near Okanagan Falls, B.c. in October and May 1991, respectively. Green and brown needles were collected from the branches of at least 10 trees in 3 N-fertilized (N3) and 3 control plots in the optimum nutrition trial described by Whynot and Weetman (1991). The soil was a orthic dystric brunisol over glaciofluvial deposits, with a 1 cm deep hemimor humus layer. The N-fertilized plots received a total of 525 kg N ha -1 in ammonium nitrate in 4 applications between 1982 and 1988. Initial N concentrations (in 1991) in brown needles were 0.33% in control plots and 1.56% in fertilized plots (5 times richer). Initial N concentrations in green needles were 0.88% in control plots and 1.91 % in fertilized plots (2 times richer). 1.0 g dry weight portions of each litter type were put in 35 mesh bags. One bag of each type (green or brown, control or fertilized) was buried vertically in 1.0 L plastic tubs containing homogenized forest floor material from an untreated mature stand of lodgepole pine near the optimum nutrition trial. The forest floor material in the microcosms was maintained at 70% water (wet weight basis) and about 18°C during the 70 week incubation in the laboratory. At 10 week intervals, all bags in 5 microcosms were collected and the dry weight of needles remaining in each bag was determined. Statistical analysis At each sampling time, the mean values for weight of litter remaining in the different treatments were compared using analysis of variance. Scheffe multiple range tests were applied where more than 2 treatments were being compared. Two-way ANaYA was used for experiments with 3 replicate plots per treatment (# 2, 3, 4 ), to test effects of treatments, plots and interactions. One-way ANaYA was applied in experiments 1 and 5. The level of significance in all tests was p < 0.05. ··········NPK -control ~ Cfl ......, 0.8 Cfl C c 0.7 0 E 1:' ~ '"'0" 0.6 0.5 0.4 2 0 3 Yeors Fig. 1. Mass loss of unfertilized lodgepole pine needle litter in plots fertilized with N (672 kg ha - I) with and without P and K (386 kg ha- 1), and in control plots at the Mont Tremblant optimum nutrition trial, in the boreal forest of Quebec. Mean values (± I sd) of 10 samples per treatment are shown. There were no significant differences (p < 0.05) between treatments at either sampling time, based on one-way ANOVA. Results Experiment 1: Effects of N fertilization on decomposition rate of needle litter Mass loss from lodgepole pine needle litter was similar in control and N- or NPK-fertilized plots in the first year (Fig. 1). After 33 months, there were still no significant differences between fertilized and control plots, although the average mass remaining was slightly larger in the fertilized plots. Experiment 2: Effects of sewage sludge on decomposition rate of needle litter Mixed species needle litter lost mass at the same rate in control and sludge-treated plots during the first 1.5 years (Fig. 2). Thereafter, less mass was lost from litter in sludged plots, resulting in significantly greater masses of litter remaining in sludged plots after 3 years. Experiment 3: Effects of sewage sludge on decomposition rate offorestfloor material F-Iayer material lost mass at the same rate in control and sludged plots during the first 8 months (Fig. 3). Thereafter, very little mass loss occurred in sludged plots, resulting in significantly greater masses of forest floor material remaining in sludged plots after 22 and 35 months. 86 1.0 -control ·----·sludged 0.9 ......Cl 0.9 0.8 '-' :~ Cl 0.7 0 0.6 Cl :~ c a E '"'0" ::l: 0.5 b E 0.7 ''o"" 0.6 ::l: 0.4 0.3 L---I._--'-_-'-_"----'_-'----' 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.8 o ~ Q) '- c 0.5L-~ Fig. 2. Mass loss of unfertilized mixed needle litter of western hemlock, western red cedar and Douglas-fir in plots treated with municipal sewage sludge (500 kg N ha- 1) and control plots at the V.B.C. Research Forest in coastal British Columbia. Mean values (± I sd) of 15 samples per treatment are shown. At each sampling time, mean values for the two treatments with different letters are significantly different (p < 0.05), based on two-way ANOVA. 2.0 -control ·----·sludged 1.8 ......Cl 1.7 Cl 1.6 '-' c 'c 0 E Q) '- ''"0" ::l: I ------------- ------------- a a 1.5 b 1.4 b 1.3 1.2 1.1 0 o 3 6 9 12 Months Years 1.9 ·----·sewage . - - - -sewage+pulp ........_·pulp -control 24 12 36 Months Fig. 3. Mass loss of unfertilized forest floor material in plots treated with municipal sewage sludge (500 kg N ha- 1) and control plots at the V.B.C. Research Forest in British Columbia. Mean values (± I sd) of 15 samples per treatment are shown. At each sampling time, mean values for the two treatments with different letters are significantly different (p < 0.05), based on two-way ANOVA. Experiment 4: Effects of sewage sludge and pulp sludge on decomposition rate of leaf litter Paper birch leaf litter lost mass more slowly in plots treated with sewage sludge (alone or mixed with pulp sludge) than in control plots during the first 3 months (Fig. 4). Thereafter, there were no significant differences in the mass of material remaining in control and treated plots. Fig. 4. Mass loss of unfertilized paper birch leaf litter in plots treated with sewage sludge, pulp and paper sludge, mixed sewage and pulp sludge, and in control plots at Island 6 in coastal British Columbia. Mean values of 15 samples per treatment are shown. At each sampling time, mean values for the four treatments with different letters are significantly different (p < 0.05), based on two-way ANOVA. Experiment 5.- Effects of greater N concentration in litter on decomposition rate of pine needles Brown needle litter from N-fertilized trees lost mass at the same rate as brown needles from control trees, despite being 5 times richer in N (Fig. 5). Green needles from N-fertilized trees also lost mass at the same rate as green needles from control trees, despite being twice as rich in N. More mass was lost from green needles than from brown needles during the first 10 weeks, and this difference was maintained until 70 weeks. Green needles from control trees contained less N (0.88%) initially than brown needles from fertilized trees (1.56%), but loss mass more rapidly during the first 10 weeks. Discussion The experiments in this study provide no evidence that greater N availability results in more rapid litter decomposition. In fact, the only significant difference between control and fertilized plots was the slower decay of litter and forest floor material in plots treated with municipal sewage sludge during the second half of the 3 year incubation. Depression of decomposition rates by high N availability has been reported in other studies (Fogg, 1988), especially during the late stages of decomposition, when lignin is degraded (Berg and Tamm, 1991). Slower decomposition in sludged plots 87 90 -brown control ......... brown fertilized 80 -green control - - - green fertilized 0> c c o E ~ 70 Vl ~ 60 E o c 0> '': a IX 50 1 40 ± 30 o 10 20 30 40 50 60 70 Weeks Fig. 5. Mass loss of needle litter and green needles of lodgepole pine from plots fertilized with N (525 kg N ha -1) and from control plots during incubation in laboratory microcosms. Mean values (± 1 sd) of 5 samples per treatment are shown. There were no significant differences (p < 0.05) between control and fertilized needles of either type at any sampling time, based on one-way ANOVA. in this study cannot be attributed to N availability with certainty, since many other factors could have been changed by sludge application. Sewage sludge provided other nutrients and carbon, and small amounts of heavy metals, and also formed an artificial barrier between the forest floor and the decomposing litter. There was no evidence of altered moisture contents or greater contamination of decomposing litter in sludged plots. Surface temperatures may have been lower in sludged plots as a result of increased mass of foliage. It can be concluded, nevertheless, that increasing N availability by applying sewage sludge, or decreasing N availability by applying pulp and paper sludge, did not alter rates of decomposition sufficiently to affect rates of mass loss. Together with the similar rates of decay of needle litter in N-fertilized plots and control plots (Experiment 1) these results suggest that exogenous N availability has little control over rates of litter decomposition. Why, then, are rates of litter decomposition (i.e. k-values) greater on N rich sites, as described by Gosz (1981 )? This observation was based on comparisons of sites with different species and microclimates in which the species that grow on more favourable sites have higher litter quality and decompose more rapidly and completely. Increased availability of N alone (i.e. in the absence of differences in the species composition of vegetation or climate) may not increase rates oflitter decay. It was also surprising that a five-fold enrichment of N concentrations in pine needle litter had no effect on the rate of mass loss, given the general correlation between the initial N concentration and rate of decay (Edmonds, 1980). This observation is also based on comparisons of different litter types, in which those litter types that are richer in N decompose more rapidly. Within a single litter type (e.g. pine needles), increased N concentrations do not appear to affect decay rates. Berg et aI. (1987) also found no difference in mass loss of N-enriched pine needles, except during the earliest phase. Green needles of lodgepole pine decomposed more rapidly than brown needle litter, at least during the first 10 weeks of the lab incubation. Faster decay of green needles was also reported by Baker et ai. (1989) and by Berg et ai. (1982) during the early stages of decay, and was attributed to their being richer in N. In the present study, higher N concentrations were not responsible for the faster decay of green needles, since the brown needles from fertilized trees were richer in N but still decayed more slowly than green needles from control trees. Perhaps larger amounts of N must be provided in conjunction with greater amounts of microbially-metabolizable C (as in green needles) to stimulate rates of decay. This relationship may also explain the correlation between initial N concentration and decay rates of different litter types, since N concentrations are often positively correlated with concentrations of labile materials and negatively correlated with lignin content (Taylor et aI., 1991). Since the faster decay of green needles occurred only during the first 10 weeks, it can probably be attributed to larger amounts of labile material in green needles, which would rapidly be metabolized or leached. Taylor et ai. (1991) reported a good correlation between initial labile content and decay rate of several litter types. The final apparently contradictory evidence that must be reconciled is the tendency for N to accumulate in litter during the early stages of decay. This is usually considered as evidence that additional N is required for microbial activity. However, as pointed out by Fogg (1988), there is little evidence that the accumulated N is used by microbes; rather, it appears to be "luxury uptake". During the breakdown of substrates rich in N, microbes immobilize more N per unit CO 2 released, and so accumulate N, until the supply of easily decomposed C has been exhausted. Some of the N imported by microbes may be precipitated by tannins and immobilized in the lignin or acid-insoluble fractions (Berg, 88 1988; Gallardo and Merino, 1992), rather than being maintained in microbial biomass. The inability of increased availability of N, alone, to increase rates of litter decomposition, may also explain the lack of long term response of forests to N fertilization. In theory, if N availability limits decomposition rates, N-fertilization will result in higher N concentrations in litter, which will decompose and mineralize nutrients more rapidly, thereby prolonging the fertilization effect (Gosz, 1981; Vitousek et aI., 1982). The greater availability ofN in soils and the growth responses of trees in fertilized plots are, however, usually short-lived (Prescott et aI., 1993). The lack ofN control of litter decomposition rates demonstrated in this study and others may explain this paradox, if increases in N availability alone (i.e. in the absence of changes in other litter quality parameters) do not increase decomposition rates. In conclusion, N availability alone, either exogenous or endogenous, does not appear to control rates of litter decomposition. It is therefore unlikely that increases in N availability, through fertilization or deposition, will result in increased rates oflitter decomposition in forests. Acknowledgements The fertilization trials used in this study were established by G F Weetman and J P Kimmins. I thank T Aarnio, S Gonzales, B Kishchuk, S Morin, C Staley, K Thomas, M McDonald and D MacCarthy for assistance, and H R MacCarthy for providing the laboratory for the microcosm experiment. References Baker T G, Will G M and Oliver G R 1989 Nutrient release from silvicultural slash: leaching and decomposition of Pinus radiata needles. For. Ecol. Manage. 27, 53-60 Berg B 1988 Dynamics of nitrogen (15N) in decomposing Scots pine (Pinus sylvestris) needle litter. Long-term decomposition in a Scots pine forest. VI. Can. J. For. Res. 66, 1539-1546. Berg Band Staaf H 1981 Leaching, accumulation and release of nitrogen in decomposing forest litter. In Terrestrial Nitrogen Cycles. Eds. FE Clark and T Rosswall. Ecol. Bull. (Stockholm) 33,163-178 Berg B, Staaf Hand Wessen B 1987 Decomposition and nutrient release in needle litter from nitrogen-fertilized Scots pine (Pinus sylvestris) stands. Scand. J. For. Res. 2,399-415. Berg B, Wesson B and Ekbohm G 1982 Nitrogen level and decomposition in Scots pine needle litter. Oikos 38, 291-296. Berg B and Tamm C 0 1991 Decomposition and nutrient dynamics of litter in long-term optimum nutrition experiments. I. Organic matter decomposition in Picea abies needle litter. Scand. J. For. Res. 6, 305-321. DeCatanzaro J and Kimmins J P 1985 Changes in the weight and nutrient composition of litterfall in three forest ecosystem types on Coastal British Columbia. Can. J. Bot. 63, 1046--1056. Edmonds R L 1980 Litter decomposition and nutrient release in Douglas-fir, red alder, westem hemlock and Pacific silver fir ecosystems in western Washington. Can. J. For. Res. 10, 327337. Fogg K 1988 The effect of added nitrogen on the rate of decomposition of organic matter. BioI. Rev. 63, 433-462. Gallardo A and Merino J 1992 Nitrogen immobilization in leaf litter at two Mediterranean ecosystems of SW Spain. Biogeochemistry 15,213-228. Gosz J R 1981 Nitrogen cycling in coniferous ecosystems. In Terrestrial Nitrogen Cycles. Eds. F E Clark and T Rosswall. Ecol. Bull. (Stockholm) 33, 405-426. Hunt H W, Ingham E R, Coleman D C, Elliot E T and Reid C P P 1988 Nitrogen limitation of production and decomposition in prairie, mountain meadow and pine forest. Ecology 69, 1009-1016. Nohrstedt H 0, Amebrant K, Baath E and Soderstrom B 1989 Changes in carbon content, respiration rate, ATP content and microbial biomass in nitrogen-fertilized pine forest soils in Sweden. Can. J. For. Res. 19,323-328. Prescott C E, Corbin J P and Parkinson D 1989 Input, accumulation and residence times of carbon, nitrogen and phosphorus in four Rocky Mountain coniferous forests. Can. J. For. Res. 19, 489498. Prescott C E, Corbin J P and Parkinson D 1992 Immobilization and availability of Nand P in the forest floors of fertilized Rocky Mountain coniferous forests. Plant and Soil 143, 1-10. Prescott C E, McDonald M, Gessel S P and Kimmins J P 1993 Long-term effects of sewage sludge and inorganic fertilizers on nutrient turnover in litter in a coastal Douglas-fir forest. For. Ecol. Manage. 59, 149-164. Taylor B R, Parkinson D and Parsons W F J 1989 Nitrogen and lignin content as predictors of litter decay rates: a microcosm test. Ecology 70, 97-104. Taylor B R, Prescott C E, Parsons W J F and Parkinson D 1991 Substrate control of litter decomposition in four Rocky Mountain coniferous forests. Can. J. Bot. 69, 2242-2250. Theodoru C and Bowen G D 1990 Effects of fertilizer on litterfall and N and P release from decomposing litter on a Pinus radiata plantation. For. Ecol. Manage. 32, 87-102. Titus B D and Malcolm D C 1987 The effect of fertilization on litter decomposition in c1earfelled spruce stands. Plant and Soil 100, 297-322. Vitousek P M, Gosz J R, Grier C C, Melillo J M and Reiniers W A 1982 A comparative analysis of potential nitrification and nitrate mobility in forest ecosystems. Ecol. Monogr. 52, 155-177. Weetman G F and Fournier R M 1984 Ten-year growth and nutrition effects of a straw treatment and of repeated fertilization on jack pine. Can. J. For. Res. 14,416-423. Whynot T W and Weetman G F 1991 Repeated fertilization effects on nitrogen fluxes measured by sequential coring. Soil. Sci. Soc. Am. 1. 55, 1l01-11i1. Will G M, Hodgkiss P D and Madgwick H A I 1984 Nutrient losses from litter bags containing Pinus radiata litter: Influences of thinning, clearfelling, and urea fertilizer. N. Z. J. For. Sci. 13, 291-304.