Effect of earthworm addition on soil nitrogen availability,microbial biomass and litter decomposition in mesocosms

The aim of the study was to determine the effect of adding two tropical earthworm species, Rhinodrilus contortus and Pontoscolex corethrurus, to mesocosms on the availability of mineral N (NH₄ ⁺ and NO₃ ^– concentrations), soil microbial biomass (bio-N), and the decomposition rates of three contrasting leaf litter species, in a glasshouse experiment. The mesocosms were filled with forest soil and covered with a layer of leaf litter differing in nutritional quality: (1) Hevea brasiliensis (C/N=27); (2) Carapa guianensis (C/N=32); (3) Vismia sp., the dominant tree species in the second growth forest (control, C/N= 42); and, (4) a mixture of the former three leaf species, in equal proportions (C/N=34). At the end of the 97-day experiment, the soil mineral N concentrations, bio-N, and leaf litter weight loss were determined. Both earthworm species showed significant effects on the concentrations of soil NO₃ ^– (p<0.01) and NH₄ ⁺ (p<0.05). Bio-N was always greater in the mesocosms with earthworms (especially with R. contortus) and in the mesocosms with leaf litter of H. brasiliensis (6 µg N g^–1 soil), the faster decomposing species, than in the other treatments (0.1–1.6 µg N g^–1). Thus, earthworm activity increased soil mineral-N concentrations, possibly due to the consumption of soil microbial biomass, which can speed turnover and mineralization of microbial tissues. No significant differences in decomposition rate were found between the mesocosms with and without earthworms, suggesting that experiments lasting longer are needed to determine the effect of earthworms on litter decomposition rates.     

Microbial biomass dynamics during the decomposition of leaf litter of poplar and eucalyptus in a sandy loam

Soil microbiological properties during decomposition of leaf litter of poplar (Populus deltoides) and eucalyptus (Eucalyptus tereticornis) were studied under laboratory conditions. Microbial biomass C and ninhydrin-N were measured at different intervals up to 90 days following incorporation of poplar and eucalyptus leaves separately @ 20 and 100t ha^-1. In general, the net increase in total biomass C or ninhydrin N following amendment was larger in the soils which received poplar leaves than in the soils that received eucalyptus leaves. The amounts of biomass C, at day 90, in the soils which received eucalyptus leaves @ 20 and 100 t ha^-1 was about half and one-third, respectively, that of the soils that received poplar leaves at the same rates. Similarly, the field soils naturally receiving eucalyptus leaf litter contained about half the amounts of biomass C or ninhydrin N of the soils that received poplar leaf litter. In contrast, the amounts of organic C and total N were more in soils which received eucalyptus leaves both in the laboratory experiment and under field conditions than in the soils that received poplar leaves, indicating that the decomposition of eucalyptus leaves in soils was slower than that of poplar leaves. The ratio of biomass C/soil organic C in soils receiving eucalyptus leaves was about 2–4 times lower than those in soils with no admendment or soils receiving poplar leaves. These results, therefore, suggest that the allelochemicals released into soil during decomposition of eucalyptus leaves had a toxic effect on soil microorganisms and may thus affect the nutrient cycling and hence soil fertility.     

Effects of tree harvesting, forest floor removal, and compaction on soil microbial biomass, microbial respiration, and N availability in a boreal aspen forest in British Columbia

The effects of timber harvesting and the resultant soil disturbances (compaction and forest floor removal) on relative soil water content, microbial biomass C and N contents (C_mic and N_mic), microbial biomass C:N ratio (C_mic-to-N_mic), microbial respiration, metabolic quotient (qCO₂), and available N content in the forest floor and the uppermost mineral soil (0-3 cm) were assessed in a long-term soil productivity (LTSP) site and adjacent mature forest stands in northeastern British Columbia (Canada). A combination of principal component analysis and redundancy analysis was used to test the effects of stem-only harvest, whole tree harvest plus forest floor removal, and soil compaction on the studied variables. Those properties in the forest floor were not affected by timber harvesting or soil compaction. In the mineral soil, compaction increased soil total C and N contents, relative water content, and N_mic by 45%, 40%, 34% and 72%, respectively, and decreased C_mic-to-N_mic ratio by 29%. However, these parameters were not affected by stem only harvesting or whole tree harvesting plus forest floor removal, contrasting the reduction of white spruce and aspen growth following forest floor removal and soil compaction reported in an earlier study. Those results suggest that at the study site the short-term effects of timber harvesting, forest floor removal, and soil compaction are rather complex and that microbial populations might not be affected by the perturbations in the same way as trees, at least not in the short term.     

Effects of sulfuric,nitric, and mixed acid rain on litter decomposition,soil microbial biomass,and enzyme activities in subtropical forests of China

Acid rain pollution is changing gradually from sulfuric acid rain (SAR) to mixed acid rain (MAR) and then to nitric acid rain (NAR) with the rapidly growing number of motor vehicles. The influences of changed acid rain types on ecosystem functions, particularly on litter decomposition, remain unclear. Two dominant litter types from a coniferous forest and a broad-leaved forest were incubated in microcosms with original forest soils and treated by five types of acid rain with different SO₄²⁻ to NO₃⁻ ratios (1:0, 5:1, 1:1, 1:5, and 0:1). During a six-month incubation period, litter mass losses, soil microbial biomass, and enzyme activities were investigated. Results showed that various acid treatments inhibited litter decomposition, soil microbial biomass, and most enzyme activities, and the inhibitory effects of NAR were more significant than those of SAR and MAR. The resistance to external acid of microbial communities in broad-leaved forest was higher than that in coniferous forest. NAR and MAR treatments slowed down soil carbon (C), nitrogen (N), and phosphorus (P) mineralization by attenuating the correlations between litter mass losses and the enzymes involved in C, N, and P cycling. Results reveal that the ratio of SO₄²⁻ to NO₃⁻ in acid rain is an important factor which profoundly influences litter decomposition process. In the future, a decreasing ratio of SO₄²⁻ to NO₃⁻ in acid rain will be observed in subtropical forests. Thus, soil C would accumulate as a consequence of future acid precipitation, and this may seriously affect the balance of ecosystem C, N flux.     

Soil respiration is not limited by reductions in microbial biomass during long-term soil incubations

Declining rates of soil respiration are reliably observed during long-term laboratory incubations. However, the cause of this decline is uncertain. We explored different controls on soil respiration to elucidate the drivers of respiration rate declines during long-term soil incubations. Following a long-term (707 day) incubation (30 °C) of soils from two sites (a cultivated and a forested plot at Kellogg Biological Station, Hickory Corners, MI, USA), soils were significantly depleted of both soil carbon and microbial biomass. To test the ability of these carbon- and biomass-depleted (“incubation-depleted”) soils to respire labile organic matter, we exposed soils to a second, 42 day incubation (30 °C) with and without an addition of plant residues. We controlled for soil carbon and microbial biomass depletion by incubating field fresh (“fresh”) soils with and without an amendment of wheat and corn residues. Although respiration was consistently higher in the fresh versus incubation-depleted soil (2 and 1.2 times higher in the fresh cultivated and fresh forested soil, respectively), the ability to respire substrate did not differ between the fresh and incubation-depleted soils. Further, at the completion of the 42 day incubation, levels of microbial biomass in the incubation-depleted soils remained unchanged, while levels of microbial biomass in the field-fresh soil declined to levels similar to that of the incubation-depleted soils. Extra-cellular enzyme pools in the incubation-depleted soils were sometimes slightly reduced and did not respond to addition of labile substrate and did not limit soil respiration. Our results support the idea that available soil organic matter, rather than a lack microbial biomass and extracellular enzymes, limits soil respiration over the course of long-term incubations. That decomposition of both wheat and corn straw residues did not change after major changes in the soil biomass during extended incubation supports the omission of biomass values from biogeochemical models.     

Soil compaction and forest floor removal reduced microbial biomass and enzyme activities in a boreal aspen forest soil

Soil enzymes are linked to microbial functions and nutrient cycling in forest ecosystems and are considered sensitive to soil disturbances. We investigated the effects of severe soil compaction and whole-tree harvesting plus forest floor removal (referred to as FFR below, compared with stem-only harvesting) on available N, microbial biomass C (MBC), microbial biomass N (MBN), and microbial biomass P (MBP), and dehydrogenase, protease, and phosphatase activities in the forest floor and 0–10 cm mineral soil in a boreal aspen (Populus tremuloides Michx.) forest soil near Dawson Creek, British Columbia, Canada. In the forest floor, no soil compaction effects were observed for any of the soil microbial or enzyme activity parameters measured. In the mineral soil, compaction reduced available N, MBP, and acid phosphatase by 53, 47, and 48%, respectively, when forest floor was intact, and protease and alkaline phosphatase activities by 28 and 27%, respectively, regardless of FFR. Forest floor removal reduced available P, MBC, MBN, and protease and alkaline phosphatase activities by 38, 46, 49, 25, and 45%, respectively, regardless of soil compaction, and available N, MBP, and acid phosphatase activity by 52, 50, and 39%, respectively, in the noncompacted soil. Neither soil compaction nor FFR affected dehydrogenase activities. Reductions in microbial biomass and protease and phosphatase activities after compaction and FFR likely led to the reduced N and P availabilities in the soil. Our results indicate that microbial biomass and enzyme activities were sensitive to soil compaction and FFR and that such disturbances had negative consequences for forest soil N and P cycling and fertility.     

Influence of temperature on CO2 evolution,microbial biomass C and metabolic quotient during the decomposition of two humic forest horizons

The decomposition of the litter layer and the humic mineral horizon from a beech forest site was studied at temperatures of 5, 12, and 22°C for both substrates and additionally at 32°C for beech litter. Weight losses, basal and substrate-induced CO₂ production, and the extractable biomass C were monitored periodically during a 2-year incubation period. Weight losses and microbial activity were controlled by substrate quality and temperature. No significant differences were found between 5 and 12°C in decomposition, biomass C, and the metabolic quotient in the humic mineral horizon. The decay of beech litter and the humic mineral horizon was highest at 22°C but was faster in the litter material by a factor of 2.9 on average. In the glucose-amended samples, the relationship among the CO₂-C fluxes was 1:1:2:3 at temperatures of 5, 12, 22, and 32°C in the litter layer, and 1: 2: 2.4 at 5, 12, and 22°C in the A horizon, respectively. The microbial activity in the humic mineral horizon was only 2–11% of that in the litter layer. The level of biomass C remained constant over 1 year and no significant differences were obtained from the 12 and 22°C treatments in the litter layer.     

Decomposition and nutrient content of litter in a fertilized eucalypt forest

The decomposition and nutrient content of litter was studied for 2 years in regrowth Eucalyptus diversicolor forest to which N (0, 200 kg ha^-1 year^-1) and P (0, 30, 200 kg ha^-1) had been applied. The P addition increased, and the N addition decreased, the rate of dry weight loss of decomposing litter. Analysis of the coefficients of a double exponential decay model with components describing the release of labile and resistant fractions indicated that decomposition of the resistant component of litter was most affected by the fertilizer additions. Treatment with N reduced the rate of loss of this component and increased its half-life by approximately 30%, whereas P treatment increased its rate of decay and decreased its half-life by approximately 30%. P accumulated in litter during decomposition. P uptake and retention was greater in P-treated than untreated plots. The application of N reduced P accumulation in litter. An accumulation of N also occurred during decomposition, the amount of N imported into litter being greater on plots treated with N fertilizer. Treatment with N affected the amount of S in decomposing litter. Litter on N-treated plots either accumulated more S or released it more slowly than litter on plots not treated with N. The application of N as NH₄NO₃ decreased forest-floor litter pH, increased litter layer mass (by 15%), and increased the amount of N (by 34%) and S (by 32%) stored in the forest floor. Treatment with P reduced the amount of N (by 22%) stored in the litter layer. The application of 200 kg P ha^-1 in the absence of N increased the store of P in the litter layer by 80%, but when N and P were applied together the amount of P in the litter was not significantly different between P treatments.     

森林凋落物分解研究进展

 森林凋落物分解是森林生态系统养分生物循环的重要环节,而分解过程中所释放的CO2是全球碳素收支的重要组分,开展森林凋落物分解研究是充分认识森林生态系统结构和功能的基础。研究认为:凋落物分解的预测指标可分为3类,即环境指标(如实际蒸散量)、凋落物物理质量(如叶抗张强度)和化学质量指标(如C/N比、木质素/N比和C/P比等);凋落物分解过程中养分释放机制极其复杂,养分动态模式主要有淋溶—释放、淋溶—富集—释放和富集—释放3种,并因凋落物种类、分解阶段和元素本身性质的不同而异;凋落物混合分解并非单一树种分解的简单叠加,因树种组成和比例不同,基质的化学组成会发生变化,从而影响分解者的多样性、丰富度和生理活性,进而直接和间接地影响其分解速率;凋落物混合分解中可能存在无效应、促进效应和抑制效应;现有的研究结果显示,凋落物混合分解的适宜比例应与群落中不同树种的种群比例相一致;CO2浓度升高不仅影响凋落物的化学性质,而且与分解环境中土壤的生物活性密切相关,但CO2浓度升高并不改变凋落物质量与分解速率之间的关系;越来越多的研究显示,CO2浓度升高的环境下,植物群落的物种组成会产生变化,这种变化对养分循环速率的影响远大于单纯大气CO2浓度变化的影响。     

The influence of slug (Arion rufus) mucus and cast material addition on microbial biomass,respiration, and nutrient cycling in beech leaf litter

We investigated the effects of slug (Arion rufus L.) mucus and cast material on litter decomposition, nutrient mobilization, and microbial activity in two laboratory experiments: (1) Slug mucus and cast material was added to beech leaf litter (Fagus sylvatica L.), and leaching of N and P and CO₂ production in microcosm systems were measured during 77 days of incubation; (2) mucus was added to beech leaf litter, and basal respiration, microbial biomass (substrate-induced respiration), specific respiration (qO₂), microbial growth ability after C, CN, CP, and CNP amendment, and lag time (time between CNP addition and start of exponential increase in respiration rate) were measured during 120 days of incubation. Leaching of N and P from beech leaf litter was significantly increased in treatments with mucus or faecal material of A. rufus. Following day 3, slug mucus increased nitrification processes. Mucus addition to beech leaf litter also increased basal respiration and microbial biomass significantly. In contrast, specific respiration was not significantly affected by mucus addition, and generally declined until day 60 but then increased until day 120. Nutrient amendments indicated that between days 1 and 30, N was available for microbial growth in litter with mucus but not in control litter. Generally, the lag time in beech leaf litter with added mucus was shorter than in control litter. Lag times generally increased with age, indicating dominance of slow-growing microbial populations at later stages as a consequence of depletion of easily available C resources and nutrients. We conclude that C, N, and P cycling is accelerated by slug activity.     

Grazing and plant-canopy effects on semiarid soil microbial biomass and respiration

The major objectives of this study were to determine the influence of grazing on the soil microbial biomass and activity in semiarid grassland and shrubland areas and to quantify the canopy effect (the differences in soil microbial biomass and activities between soils under plant canopies and soils in the open between plants). We also quantified changes in microbial biomass and activity during seasonal transition from dry to moist conditions. Chronosequences of sites withdrawn from grazing for 0, 11, and 16 years were sampled in a grassland (Bouteloua spp.) area and a shrubland (Atriplex canescens) area on and near the Sevilleta National Wildlife Reguge in central New Mexico, USA. Samples were obtained from beneath the canopies of plants (Yucca glauca in the grassland and A. canescens in the shrubland) and from open soils; they were collected three times during the spring and summer of a single growing season. Organic C, soil microbial biomass C, and basal respiration rates (collectively called the soil C triangle) were measured. We also calculated the microbial: organic C ratio and the metabolic quotient (ratio of respiration to microbial C) as measures of soil organic C stability and turnover. Although we had hypothesized that individual values of the soil C triangle would increase and that the ratios would decrease with time since grazing, differences in microbial parameters between sites located along the chronosequences were generally not significant. Grazing did not have a consistion effect on organic C, microbial C, and basal respiration in our chronosequences. The microbial: organic C ratio and the metabolic quotient generally increased with time since grazing on the shrubland chronosequence. The microbial: organic C ratio decreased with time since grazing and the metabolic quotient increased with time since grazing on the grassland chronosequence. The canopy effect was observed at all sites in nearly all parameters including organic C, microbial C, basal respiration, the microbial: organic C ratio, and the metabolic quotient which were predominantly higher in soils under the canopies of plants than in the open at all sites. Microbial biomass and activity did not increase during the experiment, even though the availability of moisture increased dramatically. The canopy effects were approximately equal on the shrubland and grassland sites. The microbial: organic C ratios and the metabolic quotients were generally higher in the shrubland soils than in the grassland soils.     

Microbial biomass nitrogen pool in soils from a warm temperate grassland,and from deciduous and evergreen forests in Chiba,central Japan

We evaluated the status of the microbial biomass N pool in grassland, and in deciduous and evergreen forest soils in Chiba, central Japan. Microbial biomass N, a labile fraction of total N in the soil, ranged from 6.96 g N m^-2 (15 cm depth) in the grassland to 24.8 g in the deciduous and 20.7 g in the evergreen soils, on a landscape basis. Thus the pattern in the grassland and in the forest soils differed. The N flush measured by a fumigation-incubation method indicated that in the grassland soil microbial biomass N was underestimated by a factor of 2.6 compared with the results from a fumigation-extraction method, because of heavy N immobilization in the microbial biomass. This was in contrast to results from the forest soils, which did not immobilize N. Thus, the forest soils were in a steady-state condition compared with the grassland which formed a seral phase in the ecological succession. Simple correlation coefficients indicated a significant positive relationship between biomass N and organic C in the soil and the N concentration in the litter, the main component of organic matter in the soils of the three ecosystems.     

Fumigation-extraction and substrate-induced respiration derived microbial biomass C,and respiration rate in limed soil of Scots pine sapling stands

The effect of liming on microbial biomass C and respiration activity was studied in four liming experiments on young pine plantations. One of the experimental sites had been limed and planted 12 years before, two 5 years before, and one a year before soil sampling. The youngest experimental site was also treated with ash fertilizer. Liming raised the pH_KCl of the humus layer by 1.5 units or less. Microbial biomass was measured using the fumigation-extraction and substrate-induced respiration methods. Liming did not significantly affect microbial biomass C, except in the experiment which had been limed 11 years ago, where there was a slight biomass increase. Basal respiration, which was measured by the evolution of CO₂, increased in the limed soils, except for the youngest experiment, where there was no effect. Ash fertilization raised the soil pH_KCl by about 0.5 unit, but did not influence microbial biomass C or basal respiration. Fumigation-extraction and substrate-induced respiration derived microbial biomass C values were correlated positively with each other (r=0.65), but substrate-induced respiration gave approximately 1.3 times higher results. In addition, the effect of storing the soil samples at +6 and -18°C was evaluated. The effects were variable but, generally, the substrate-induced respiration derived microbial biomass C decreased, and the fumigation-extraction derived microbial biomass C and basal respiration decreased or were not affected by storage.     

Effect of novaluron on microbial biomass,respiration, and fluorescein diacetate-hydrolyzing activity in tropical soils

The aim of this study was to assess the potential harmful effects of novaluron on soil microbiological parameters in clay loam alluvial soil (Typic udifluvent) and coastal saline soil (Typic endoaquept) under controlled laboratory tests. The applications of novaluron were made at or above the recommended rates, which includes field rate (FR), two times (2FR), and ten times (10FR) the FR. The laboratory incubation study was carried out at 60% of maximum water holding capacity of soils and at 30°C. Novaluron application rate even up to 10FR resulted in a short-lived and transitory toxic effect on soil microbial biomass C and fluorescein diacetate-hydrolyzing activity. Microbial metabolic quotient changed but for a short period. It can be concluded that novaluron had a transient and negligible harmful effect on the soil microbiological parameters studied at higher rates than those usually used in the field.     

Dynamics of soil microbial biomass C, N and P in disturbed and undisturbed stands of a tropical wet-evergreen forest

To understand the spatial and temporal dynamics of soil microbial biomass and its role in soil organic matter and nutrient flux in disturbed tropical wet-evergreen forests, we determined soil microbial biomass C, N and P at two soil depths (0–15 and 15–30 cm), along a disturbance gradient in Arunachal Pradesh, northeastern India. Disturbance resulted in considerable increase in air temperature and light intensity in the forest and decline in the soil nutrients concentration, which affected the growth of microbial populations and soil microbial biomass. There were significant correlations between bacterial and fungal populations and microbial biomass C, N and P. Soil microbial population was higher in the undisturbed (UD) forest stand than the disturbed forest stands during post-monsoon and less during rainy season due to heavy rainfall. Greater demand for nutrients by plants during rainy season limited the availability of nutrients to soil microbes and therefore, low microbial biomass C, N and P. Microbial biomass was negatively correlated with soil temperature and pH in all the forest stands. However, there were significant positive relationships among microbial biomass C, N and P. Percentage contribution of microbial C to soil organic C was higher in UD forest, whereas percentage contribution of microbial biomass N and P to total N and total P was higher in the moderately disturbed site than in the highly disturbed (HD) site. These results reveal that the nutrient retention by soil microbial biomass was greater in the selective logged stand and would help in the regeneration of the forest upon protection. On the other hand, the cultivated site (HD) that had the lowest labile fractions of soil organic matter may recover at a slower phase. Further, minimum and maximum microbial biomass C, N and P during rainy and winter seasons suggest the synchronization between nutrient demand for plant growth and nutrient retention in microbial biomass that would help in ecosystem recovery following disturbance.     

Relationships of soil respiration to microbial biomass, substrate availability and clay content

The roles of microbial biomass (MBC) and substrate supply as well as their interaction with clay content in determining soil respiration rate were studied using a range of soils with contrasting properties. Total organic C (TOC), water-soluble organic carbon, 0.5 M K₂SO₄-extractable organic C and 33.3 mM KMnO₄-oxidisable organic carbon were determined as C availability indices. For air-dried soils, these indices showed close relationship with flush of CO₂ production following rewetting of the soils. In comparison, MBC determined with the chloroform fumigation-extraction technique had relatively weaker correlation with soil respiration rate. After 7 d pre-incubation, soil respiration was still closely correlated with the C availability indices in the pre-incubated soils, but poorly correlated with MBC determined with three different techniques—chloroform fumigation extraction, substrate-induced respiration, and chloroform fumigation-incubation methods. Results of multiple regression analyses, together with the above observations, suggested that soil respiration under favourable temperature and moisture conditions was principally determined by substrate supply rather than by the pool size of MBC. The specific respiratory activity of microorganisms (CO₂-C/MBC) following rewetting of air-dried soils or after 7 d pre-incubation was positively correlated with substrate availability, but negatively correlated with microbial pool size. Clay content had no significant effect on CO₂ production rate, relative C mineralization rate (CO₂-C/TOC) and specific respiratory activity of MBC during the first week incubation of rewetted dry soils. However, significant protective effect of clay on C mineralization was shown for the pre-incubated soils. These results suggested that the protective effect of clay on soil organic matter decomposition became significant as the substrate supply and microbial demand approached to an equilibrium state. Thereafter, soil respiration would be dependent on the replenishment of the labile substrate from the bulk organic C pool.     

Near-infrared characteristics of forest humus are correlated with soil respiration and microbial biomass in burnt soil

Near-infrared spectroscopy and soil physicochemical determinations (pH_H2O, organic matter content, total C content, NH _inf4 ^sup+ , total N content, cation-exchange capacity, and base saturation) were used to characterize fire-or wood ash-treated humus samples. The spectroscopic and the soil physicochemical analysis data from the humus samples were used separately to explain observed variations in soil respiration and microbial biomass C by partial least-square regression. The first regression component obtained from the physicochemical and spectroscopic characterization explained 10–12% and 60–80% of the biological variation, respectively. This suggests that information on organic material collected from near-infrared spectra is very useful for explaining biological variations in forest humus.     

Effects of addition of maize litter and earthworms on C mineralization and aggregate formation in single and mixed soils differing in soil organic carbon and clay content

C mineralization and aggregate stability directly depend upon organic matter and clay content, and both processes are influenced by the activity of microorganisms and soil fauna. However, quantitative data are scarce. To achieve a gradient in C and clay content, a topsoil was mixed with a subsoil. Single soils and the soil mixture were amended with 1.0 mg maize litter C g soil⁻¹ with and without endogeic earthworms (Aporrectodea caliginosa). The differently treated soils were incubated for 49 days at 15 °C and 40% water holding capacity. Cumulative C mineralization, microbial biomass, ergosterol content and aggregate fractions were investigated and litter derived C in bulk soil and aggregates were determined using isotope analyses. Results from the soil mixture were compared with the calculated mean values of the two single soils. Mixing of soil horizons differing in carbon and clay content stimulated C mineralization of added maize residues as well as of soil organic matter. Mixing also increased contents of macro-aggregate C and decreased contents of micro-aggregate C. Although A. caliginosa had a stimulating effect on C mineralization in all soils, decomposition of added litter by A. caliginosa was higher in the subsoil, whereas A. caliginosa decreased litter decomposition in the soil mixture and the topsoil. Litter derived C in macro-aggregates was higher with A. caliginosa than with litter only. In the C poor subsoil amended with litter, A. caliginosa stimulated the microbial community as indicated by the increase in microbial biomass. Furthermore, the decrease of ergosterol in the earthworm treated soils showed the influence of A. caliginosa on the microbial community, by reducing saprotrophic fungi. Overall, our data suggest both a decrease of saprotrophic fungi by selective grazing, burrowing and casting activity as well as a stimulation of the microbial community by A. caliginosa.     

Influence of leaf litter types on microbial functions and nutrient status of soil: Ecological suitability of forest trees for afforestation in tropical laterite wastelands

The impact of forest tree leaf litters on microbial activity and nutrient status of red laterite soil was tested for the ecological suitability of Cassia siamea, Shorea robusta, Acacia auriculiformes and Dalbergia sissoo, which are typically used for afforestation of wastelands in eastern India. The objectives were to compare seasonal variation in soil enzyme activity in 30-years old afforested sites, and to study nutrient status and microbial biomass and function during short-term in-situ incubation of litter in decomposition pits. In afforested soils, enzyme activities significantly varied between litters and seasons. All enzyme activity except invertase dominated in the soils containing Dalbergia and Cassia litters compared to the others. The seasonal effect was enzyme-dependent, with amylase and cellulase reaching peaks during the rainy season but invertase activity showed a reverse trend with lowest values in rainy season, except in Acacia soil, and protease activity was lowest in the soil containing Cassia and Dalbergia during the rainy season. Dehydrogenase activity was negligible in the soils containing Shorea and Acacia, but remained high with respect to Dalbergia and Cassia during all seasons. The decomposition pit study showed significant increase of soil nutrients with respect to litter types and intervals, except with respect to electrical conductivity. Cassia and Dalbergia litters enabled notable increase of soil nutrients than Shorea and Acacia. The soil enzyme activity, in general, increased with duration of litter decay, but microbial biomass C (MBC) decreased over time except in Shorea. Therefore, the enzyme rates normalized to the MBC indicated inverse relations for all enzymes, except in the soil containing Shorea. A positive relationship existed between MBC and soil respiration in Cassia, Acacia and Dalbergia. Analysis of variance revealed main effects of litter types for increasing protease, MBC and CO₂ output, and a main effect of intervals for enhancing enzymes other than cellulase. Rates of soil respiration were greater in soils contain Cassia and Dalbergia, and showed significant differences between litters and between intervals. All enzymes were significantly correlated with electrical conductivity, organic carbon and available phosphorus contents, and all enzymes except invertase were correlated with nitrate concentrations. The acidic soil pH did not affect enzyme activities, and soil nutrients exerted only weak effect on MBC and respiration. Our study showed that leaf litters of Cassia and Dalbergia trees improved the nutrient status and microbial activity in soil more so than Shorea and Acacia litters, and therefore, afforestation using Cassia and Dalbergia trees may be particularly suitable for soil restoration in tropical laterite wastelands.     

Carbon and phosphorus transformations during decomposition of pine forest floor with different phosphorus status

Summary Information on the mineralization of inorganic phosphate (P_i) from organically bound P (Po) during decomposition of forest floor and soil organic matter is vital for understanding P supply in forest ecosystems. Carbon (C) and phosphorus (P) fluxes were determined for forest floor samples from three Pinus radiata plots which had received no P (Control), 62.5 kg P ha^–1 (Low P) and 125 kg P ha^–1 (High P) 20 years before sampling. The P concentration of the forest floor samples had increased with fertilizer application, and the C:P ratio ranged between 585 and 1465. During a 9-week laboratory incubation 8.2–19.0% of the forest floor C was evolved as CO₂-C. The amount of CO₂ evolved from the forest floor of the Control plot was more than twice the amounts from the Low P and High P plots. There was little change in net P mineralization in the Control and Low P treatments throughout the incubation, but it increased slightly for the High P samples, suggesting a critical forest floor C:P ratio of 550 for net P mineralization. Changes in the ³²P-specific activities of the P_i and microbial P pools during incubation, and concurrent changes in microbial-³²P and ³²P_i, indicated internal P cycling between these pools. The rate of internal P cycling varied with forest floor quality, and was highest in the High P forest floor. The High P samples had microbial C:P ratios of 22 : 1 which remained constant during the incubation, suggesting the microorganisms had adequate P levels. Received: 2 July 1997