Effects of <Emphasis Type="Italic">Faidherbia albida</Emphasis> canopy and leaf litter on soil microbial communities and nitrogen mineralization in selected Zambian soils

The nitrogen status of most Zambian soils is inherently low. Nitrogen-fixing trees such as Faidherbia albida (F. albida) could have the potential to restore soil fertility. We conducted a study to examine the role of mature F. albida trees on the soil microbial communities and overall N fertility status in Zambia. Soil samples were collected under and outside the canopies of F. albida trees in representative fields from two sites namely; Chongwe (loamy sand) and Monze (sandy loam). To assess the long term canopy effects; total N, mineral N and soil organic carbon (C_org) content were directly measured from soils collected under and outside the canopy. Short term litter effects were assessed by subtracting concentrations of biochemical properties of non-amended controls from amended soils with F. albida litter during an 8 week incubation experiment. We also determined N mineralization rates, microbial community structure—Phospholipid fatty acids, microbial biomass carbon, and labile organic carbon (\({\text{C}}_{{{\text{org[K}}_{ 2} {\text{SO}}_{ 4} ]}}\)) during incubation. For the long term canopy effect, average N mineralization rate, C_org, total N and mineral N content of non-amended soils under the canopy were (all significant at p < 0.05) greater than soils outside the canopy on both sites. In the short term, amending soils with litter significantly increased N mineralization rates by an average of 0.52 mg N kg^?1 soil day^?1 on soil from Monze. Microbial biomass carbon measured after 4 weeks of incubation was on average significantly higher on amended soils by 193 and 334 mg C kg^?1 soil compared with non-amended soils in Chongwe and Monze soils, respectively. After 6 weeks of incubation, the concentration of all selected biomarkers for major microbial groups concentrations in non-amended soils were significantly higher (all p < 0.05) under the canopy than outside in Monze soil. Using principal component analysis, we found that the segregation of the samples under and outside the canopy by the first principal component (PC1) could be attributed to a proportional increase in abundances of all microbial groups. Uniform loadings on PC1 indicated that no single microbial group dominated the microbial community. The second principal component separated samples based on incubation time and location. It was mainly loaded with G-positive bacteria, and partly with G-negative bacteria, indicating that microbial composition was dominated by these bacterial groups probably at the beginning of the incubation on Monze soils. Our results show that the improvement of soil fertility status by F. albida could be attributed to a combination of both long term modifications of the soil biological and chemical properties under the canopy as well as short term litter fall addition.     

Comparison of soil nitrogen dynamics under beech,Norway spruce and Scots pine in central Germany

To investigate the effect of tree species on soil N dynamics in temperate forest ecosystems, total N (Nt), microbial N (Nmic), net N mineralization, net nitrification, and other soil chemical properties were comparatively examined in beech (64–68 years old) and Norway spruce (53–55 years old) on sites 1 and 2, and beech and Scots pine (45 years old) on site 3. The initial soil conditions of the two corresponding stands at each site were similar; soil types were dystric Planosol (site 1), stagnic Gleysols (site 2), and Podzols (site 3). In organic layers (LOf₁, Of₂, Oh), Nmic and Nmic/Nt, averaged over three sampling times (Aug., Nov., Apr.), were higher under the beech stands than under the corresponding coniferous ones. However, the Nmic in the organic layers under beech had a greater temporal variation. Incubation (10 weeks, 22 °C, samples from November) results showed that the net N mineralization rates in organic layers were relatively high with values of 8.1 to 24.8 mg N kg^–1 d^–1. Between the two corresponding stands, the differences in net N mineralization rates in most of the organic layers were very small. In contrast, initial net nitrification rates (0.2–17.1 mg N kg^–1 day^–1) were considerably lower in most of the organic layers under the conifer than under the beech. In the mineral soil (0–10 cm), Nmic values ranged from 4.1–72.7 mg kg^–1, following a clear sequence: August>November>April. Nmic values under the beech stands were significantly higher than those under the corresponding coniferous stands for samples from August and April, but not from November. The net N mineralization rates were very low in all the mineral soils studied (0.05–0.33 mg N kg^–1 day^–1), and no significant difference appeared between the two contrasting tree species.     

Bioamelioration of a sodic soil by silvopastoral systems in northwestern India

In sodic soils, excessive amounts of salts have an adverse effect on soil biological activity and stability of soil organic matter. The study analyzes the role of silvopastoral systems to improve soil organic matter and microbial activity with a view for effective management of soil fertility. The silvopastoral systems for the present study (located at Saraswati Reserved Forest, Kurukshetra; 29°4′ to 30°15′ N and 75°15′ to 77°16prime; E) are characterized by tree species of Acacia nilotica, Dalbergia sissoo and Prosopis juliflora along with grass species of Desmostachya bipinnata and Sporobolus marginatus. Soil microbial biomass carbon was measured using the fumigation extraction technique and nitrogen mineralization rates using aerobic incubation method. The microbial biomass carbon in the soils of D. bipinnata and S. marginatus treatments were low. In silvopastoral systems, microbial biomass carbon increased due to increase in the carbon content in the soil – plant system. A significant relationship was found between microbial biomass carbon and plant biomass carbon (r = 0.83) as well as the flux of carbon in net primary productivity (r = 0.92). Nitrogen mineralization rates were found greater in silvopastoral systems compared to 'grass-only' system. Soil organic matter was linearly related to microbial biomass carbon, soil N and nitrogen mineralization rates (r = 0.95 to 0.98, p < 0.01). On the basis of improvement in soil organic matter, enlarged soil microbial biomass pool and greater soil N availability in the tree + grass systems, agroforestry could be adopted for improving the fertility of highly sodic soil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil nitrogen as affected by Gliricidia sepium in a silvopastoral system in Guadeloupe, French Antilles

Knowledge of the status and dynamics of soil N is essential to improving the production and management of silvopastoral system in the tropics. Soil N status and dynamics were analyzed as key factors affecting productivity and sustainability of a cut-and-carry silvopastoral system. The total soil N and N mineralization as affected by soil moisture and temperature were studied in a Gliricidia sepium (Jacq.) Walp – Dichanthium aristatum (Poir) C.E. Hubbard grassland association and in an adjacent open grassland located in a subhumid tropical region. The plot was installed in 1989 and the pruning residues and cut grass were removed from the site. No N fertilizer was applied. Total soil N increased at an average rate of 180 kg N ha⁻¹ in the 0–0.2 m soil layer of the silvopastoral plot. Only a third of this value could be explained by the litter, nodule and root turnover. Nitrogen mineralization in both soils varied as a function of temperature but it was not affected by soil moisture. Even if mean soil temperature was 1 °C to 2 °C greater in the open grassland, the estimated daily rate of in-situ N mineralization was 20% greater in the silvopastoral system. Our results indicated that greater N mineralization in the silvopastoral system was due to greater soil biological activity associated with higher soil organic matter rather than due to more favorable soil temperature and water conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Litter decomposition in a subtropical plantation in Qianyanzhou,China

A long-term (20 months) bulk litter decomposition experiment was conducted in a subtropical plantation in southern China in order to test the hypothesis that stable isotope discrimination occurs during litter decomposition and that litter decomposition increases concentrations of nutrients and organic matter in soil. This was achieved by a litter bag technique. Carbon (C), nitrogen (N) and phosphorus (P) concentrations in the remaining litter as well as δ¹³C and δ¹⁵N during the experimental period were measured. Meanwhile, organic C, alkali-soluble N and available P concentrations were determined in the soils beneath litter bags and in the soils at the control plots. The dry mass remaining (as % of the initial mass) during litter decomposition exponentially declined (y = 0.9362 e^−0.0365x , R ² = 0.93, P < 0.0001), but total C in the remaining litter did not decrease significantly with decomposition process during a 20-month period. By comparison, total N in the remaining litter significantly increased from 5.8 ± 1.7 g kg⁻¹ dw litter in the first month to 10.1 ± 1.4 g kg⁻¹ dw litter in the 20th month. During the decomposition, δ¹³C values of the remaining litter showed an insignificant enrichment, while δ¹⁵N signatures exhibited a different pattern. It significantly depleted ¹⁵N (y = −0.66x + 0.82, R ² = 0.57, P < 0.0001) during the initial 7 months while showing ¹⁵N enrichments in the remaining 13 months (y = 0.10x − 4.23, R ² = 0.32, P < 0.0001). Statistically, litter decomposition has little impact on concentrations of soil organic C and alkali-soluble N and available P in the top soil. This indicates that nutrient return to the topsoil through litter decomposition is limited and that C cycling decoupled from N cycling during decomposition in this subtropical plantation in southern China.     

Assessment of repeated application of poultry litter on phosphorus and nitrogen dynamics in loblolly pine: Implications for water quality

The Southeastern United States has a robust broiler industry that generates substantial quantities of poultry litter as waste. It has historically been applied to pastures close to poultry production facilities, but pollution of watersheds with litter-derived phosphorus and to a lesser extent nitrogen have led to voluntary and in some areas regulatory restrictions on application rates to pastures. Loblolly pine (Pinus taeda L.) forests are often located in close proximity to broiler production facilities, and these forests often benefit from improved nutrition. Accordingly, loblolly pine forests may serve as alternative land for litter application. However, information on the influence of repeated litter applications on loblolly pine forest N and P dynamics is lacking. Results from three individual ongoing studies were summarized to understand the effects of repeated litter applications, litter application rates, and land use types (loblolly pine forest and pasture) on N and P dynamics in soil and soil water. Each individual study was established at one of three locations in the Western Gulf Coastal Plain region. Annual applications of poultry litter increased soil test P accumulation of surface soils in all three studies, and the magnitude of increase was positively and linearly correlated with application rates and frequencies. In one study that was established at a site with relatively high soil test P concentrations prior to poultry litter application, five annual litter applications of 5 Mg ha⁻¹ and 20 Mg ha⁻¹ also increased soil test P accumulation in subsurface soils to a depth of up to 45 cm. Soil test P accumulations were greater in surface soils of loblolly pine stands than in pastures when both land use types received similar rates of litter application. In one study which monitored N dynamics, lower soil organic N, potential net N mineralization, potential net nitrification, and soil water N was found in loblolly pine stands than pastures after two annual litter applications. However, increases in potential net N mineralization, net nitrification, and soil water N with litter application were more pronounced in loblolly pine than in pasture soils. Loblolly pine plantations can be a viable land use alternative to pastures for poultry litter application, but litter application rate and frequency as well as differences in nutrient cycling dynamics between pine plantations and pastures are important considerations for environmentally sound nutrient management decisions.     

Changes in soil chemical and physical characteristics in Japanese cypress (Chamaecyparis obtusa Endl.) stands by mixture of deciduous broad-leaved trees in the northern Kanto region of Japan

In order to clarify the effects of a mixture of deciduous broad-leaved trees on soil fertility, we investigated litter biomass accumulation, mineral soil chemical and physical characteristics, characteristics of nitrogen mineralization, and the mutual relationships between them in Japanese cypress (Chamaecyparis obtusa) stands mixed with deciduous broad-leaved trees at different ratios (mixture ratio; MR = 0, 16, 33, 43, 100% by basal area) in the northern Kanto region of Japan. Litter biomass in the forest floor and mineral soil was 19.1 Mg ha⁻¹ in MR 0% and decreased approximately 60 % in MR 33%, MR 43% and MR100%. The permeability at 0–5 cm soil depth in MR100% was twice as much as that in MR 0%. Increases in soil permeability were likely due to larger soil pores in the higher MR with much accumulated deciduous broad-leaves. At 0–5 cm soil depth, the differences in carbon concentration among the plots were not clear. On the other hand, carbon concentrations at 5–10 cm depth increased from 90 g kg⁻¹ to 147 g kg⁻¹ with increases in MR from 0% to 100%. Concentrations of exchangeable bases increased two to four times with increases in MR from 0 to 100% at 0–10 cm depth. Soil pH (H₂O) generally increased with increases in MR at each depth. The rates of net nitrogen mineralization at 0–5 cm depthin vitro increased from 25 to 87 mg kg⁻¹ 2 weeks⁻¹ with increases in MR from 0 to 100%. However, increases in nitrification with increases in MR were not clear compared with nitrogen mineralization. These results indicated that a mixture of deciduous broad-leaved trees in a Japanese cypress stand was effective in preventing soil fertility decline. This study was supported by a grant from the Showa Shell Sekiyu Fundation for Promotion of Environmental Research. A part of this study was presented at the 7th International Congress of Ecology (1998).     

Nutrition and growth of wheat–sorghum rotation in soils amended with leaf litter of trees before planting of wheat

Nutrient release from plant residues can be manipulated as per crop demand through several approaches. A pot study was conducted to study the influence of incorporation of leaf litter of poplar (Populus deltoides), eucalypt (Eucalyptus hybrid) and dek (Melia azedarach) inoculated with cellulolytic fungus culture (Aspergillus awamori) on the nutrition and biomass of wheat (Triticum aestivum, cv. PBW 343) in loamy sand and sandy loam soils. The residual effect of leaf litter after wheat harvest was studied on sorghum (Sorghum bicolor, cv. Punjab Sudax Chari 1). The treatments consisted of a control (no leaf litter) and three uninoculated as well as inoculated leaf litter levels of tree species–0.15%, 0.30% and 0.45% (w/w, dry weight basis). A uniform dose of N, P and K @ 50, 11 and 10 mg kg⁻¹ soil, respectively from inorganic fertilizers was applied to all the treated pots. Straw and grain yield, and nutrient content of wheat increased with increasing level of uninoculated or inoculated leaf litter in both the soils. The inoculated leaf litter augmented the yield and nutrient content of crop significantly (P < 0.05) as compared to the corresponding uninoculated treatments. Poplar and dek leaf litter produced higher wheat yield, plant nutrient content and available nutrients in soil after wheat harvest than eucalypt leaf litter. Dry matter yield of sorghum raised on residual fertility increased significantly with increasing levels of leaf litter application. The comparative responses in yield and nutrient content of crops were higher in loamy sand than in the sandy loam soil. The study shows the beneficial influence of use of cellulolytic microorganisms on enhancement in decomposition and nutrient release from litterfall of tree species.     

Soil and litter fauna of cacao agroforestry systems in Bahia,Brazil

Agroforestry systems deposit great amounts of plant residues on soil and this leads to high levels of soil organic matter content and has increased soil biodiversity and improved its conservation. This study compares the distribution of meso and macrofaunal communities in soil and litter under cacao agroforestry systems and in a natural forest in the southern Bahia state of Brazil. Soil and litter samples were obtained in September 2003, February 2004, and August 2004 in five cacao agroforestry systems. The systems evaluated included: cacao renewed under Erythrina sp. (Erythrina poeppigiana) (CRE); cacao renewed under natural forest (Cabruca, CRF); an old cacao system under Erythrina sp. (OCE); an old cacao system under a natural forest system (Cabruca, OCF) and a cacao germplasm collection area (CGC). As a reference soil and litter under a natural forest (NF) was included. Organisms were collected over a 15-day period with a Berlese–Tullgren apparatus. The density and richness of total fauna varied distinctly according to sites, sampling time and material sampled (soil and litter). 16,409 of fauna were recovered from soil and litter samples and the density of total fauna was 2,094 individuals m⁻² in the litter and 641 individuals m⁻² in the soil. The richness was 11.8 in the litter and 7.5 in the soil. The cacao agroforestry systems adopted for growing cacao in the southern Bahia region of Brazil have beneficial effects on the soil and litter faunal communities, and such systems of cacao cultivation could be considered as a conservation system for soil fauna. The development of a litter layer resulted in higher abundance and diversity of soil fauna.     

Soil animals and nitrogen mineralization under sand-fixation plantations in Zhanggutai region,China

The effects of soil animals on soil nitrogen （N） mineralization and its availability were studied by investigating soil animal groups and their amounts of macro-faunas sorted by hand, and middle and microfaunas distinguished with Tullgren and Baermann methods under three Pinus sylvestris var. mongolica Litv. plantations in Zhanggutai sandy land, China. In addition, soil N mineralization rate was also measured with PVC closed-top tube in situ incubation method. The soil animals collected during growing season belonged to 13 orders, 5 groups, 4 phyla, whose average density was 86 249.17 individuals-m^-2. There were significant differences in soil animal species, densities, diversities and evenness among three plantations. Permanent grazing resulted in decrease of soil animal species and diversity. The average ammonification, nitrification and mineralization rates were 0.48 g：m^-2·a^-1, 3.68 g·m^-2·a^-1 and 4.16 g·m^-2·a^-1, respectively. The ammonification rate in near-mature forest was higher than that in middle-age forests, while the order of nitrification and net mineralization rates was： middle-age forest without grazing 〈 middle-age forest with grazing 〈 near-mature forest with grazing （P〈0.05）. Soil N mineralization rate increased with soil animal amounts, but no significant relationship with diversity. The contribution of soil animals to N mineralization was different for different ecosystems due to influences of complex factors including grazing, soil characteristics, the quality and amount of litter on N mineralization.     

Indicators of nitrogen conservation in Austrocedrus chilensis forests along a moisture gradient in Argentina

Plant nitrogen conservation which may affect, for instance, rates of litter decomposition, soil N mineralization and N availability is thought to vary along gradients of soil fertility. Since Austrocedrus chilensis is adapted to a wide moisture gradient, we hypothesed that different intensities of N conservation would be found depending on site characteristics. We studied four sites along a moisture gradient in the Andean–Patagonian Region of Argentina, representative of the three A. chilensis forest-types (marginal, compact and mixed forests), and measured the following indicators of N conservation: (i) carbon, nitrogen and C/N ratio in young, mature and senescent leaves, total soil litter and soil; (ii) lignin concentration and lignin/N ratio in senescent leaves and total litter, and (iii) potential soil N mineralization during a 16-week incubation. A. chilensis showed a strong capacity to conserve N: (i) low N concentration in both young and mature leaves (10 and 6.5 g kg⁻¹, respectively); (ii) high N resorption proficiency (5.1 g N kg⁻¹ in senescent leaves) and N use efficiency (200), and (iii) high values of C/N, lignin and lignin/N in senescent leaves (107, 250 g kg⁻¹ and 50, respectively), and total litter (36, 420 g kg⁻¹ and 33, respectively). Some indicators (resorption proficiency, C/N in senescent leaves and lignin/N in total litter) were independent of site characteristics, while others (N and C/N in green leaves and lignin in litter) showed significant differences, suggesting a higher capacity to conserve N in the intermediate sites of the gradient (compact forests). Contrary to expectations, the marginal forest (drier, less fertile soils) showed the lowest values of lignin in litter, the highest N concentrations in green leaves and the highest rates of potential N mineralization.     

Nutrient stocks of short-term fallows on a high base status soil in the humid tropics of Papua New Guinea

In order to understand nutrient dynamics in tropical farming systems with fallows, it is necessary to assess changes in nutrient stocks in plants, litter and soils. Nutrient stocks (soil, above ground biomass, litter) were assessed of one-year old fallows with Piper aduncum, Gliricidia sepium and Imperata cylindrica in the humid lowlands of Papua New Guinea. The experiment was conducted on a high base status soil (Typic Eutropepts), and in Papua New Guinea such soils are intensively used for agriculture. Soil samples were taken prior to fallow establisment and after one year when the fallows were slashed and above ground biomass and nutrients measured. The above ground and litter biomass of piper was 13.7 Mg dry matter ha^-1, compared to 23.3 Mg ha^-1 of gliricidia and 14.9 Mg ha^-1 of imperata. Gliricidia produced almost 7 Mg ha^-1 wood. Total above ground biomass returned to the soil when the fallows were slashed was the same for piper and gliricidia (8 Mg ha^-1). Gliricidia accumulated the largest amounts of all major nutrients except for K, which was highest in the above ground piper biomass. Imperata biomass contained the lowest amount of nutrients. The largest stocks of C, N, Ca and Mg were found in the soil, whereas the majority of P was found in the above ground biomass and litter. Almost half of the total K stock of piper and gliricidia was in the biomass. During the fallow period, soil organic C significantly increased under gliricidia fallow whereas no net changes occurred in piper and imperata fallows. The study has shown large differences in biomass and nutrient stocks between the two woody fallows (piper, gliricidia) and between the woody fallows and the non-woody fallow (imperata). Short-term woody fallows are to be preferred above grass (imperata) fallows in the humid lowlands of Papua New Guinea because of higher nutrient stocks.     

Litter dynamics and soil properties under different tree species in a semi-arid region of Rajasthan,India

Litterfall and decomposition are the two main processes accounting for soil enrichment in agroforestry. The extent of enrichment in soil properties depends on the tree species, management practices and the quantity and quality of litter. A field investigation was carried out to study litterfall production, decay rates, release of nutrients and consequent changes in soil physicochemical properties under crowns of four multipurpose tree species (MPTs) in irrigated conditions in farm fields. The species were Prosopis cineraria (L.), Dalbergia sissoo (Roxb.) ex DC, Acacia nilotica (L.) Del. and Acacia leucophloea (Roxb.) Willd. Annual accretion of litter ranged from 36 to 54 kg tree⁻¹ year⁻¹ and was highest under D. sissoo and lowest under A. nilotica. Total litterfall production was in the order: P. cineraria > A. leucophloea > A. nilotica > D. sissoo. P. cineraria showed the highest NPK concentration in litter. For all MPTs, a large pulse of litterfall coincided with the winter season (November to February). Litter of P. cineraria decomposed fastest while that of A. nilotica was slowest. More than 95% of the leaf litter of P. cineraria decomposed in 6 months, of D. sissoo in 7 months and A. leucophloea and A. nilotica in 9 months. Decomposition rate of litter was highly correlated with neutral detergent fibre (NDF) (r = −0.94) and P (r = −0.91) concentration. N, P and K release were best correlated with NDF, acid detergent fibre (ADF), P, lignin, lignin/N and C/P ratios and NDF alone explained 88% to 94% of the variability in litter decomposition and nutrient release rates. There was significant build up of soil organic carbon and available NPK in the agrisilvicultural systems but also a decrease in soil pH. Build up in soil fertility was significantly correlated with litterfall and soil improvement was greatest under P. cineraria.     

Effects of soil moisture and soil depth on nitrogen mineralization process under Mongolian pine plantations in Zhanggutai sandy land, P. R. China

选择章古台地区三块典型樟子松(Pinussylvestrisvar.mongolica)人工固沙林为研究对象,采用实验室好氧培养法测定了不同土壤层次和在不同水分条件下的N矿化过程。结果表明:土壤0-60cm层N净矿化速率垂直变化范围为1.06–7.52mg·kg-1·month-1;土壤层次和含水量及其交互作用对土壤N净矿化速率的影响均达到差异显著(P<0.05);净矿化速率随着土壤层次的加深而明显下降,0-15cm层占总净矿化量的60.52%;半饱和与饱和含水量处理差异不显著,但均高于不加水处理。为此,在半干旱地区必须进一步加强开展调控生态系统N矿化、循环及其收支平衡影响因素的研究。图1表4参20。     

Nitrogen and fine root length dynamics in a tropical agroforestry system with periodically pruned Erythrina poeppigiana

The effect of pruning all branches (complete pruning) or retaining one branch (partial pruning) on the dynamics of nitrogen cycling in aboveground biomass, nitrogen supplying power of an amended Eutric Cambisol, and fine root length, was studied in an Erythrina poeppigiana (Walp.) O.F. Cook—tomato (Lycopersicon esculentum Mill.) alley cropping practice in Turrialba, Costa Rica during 1999–2000. Over the 1 year pruning cycle, in which trees were completely or partially pruned four times, respective aboveground biomass production was 4.4 Mg or 7 Mg ha⁻¹ (2-year-old trees) and 5.5 Mg or 9 Mg ha⁻¹ (8-year-old trees); N cycled in aboveground biomass was 123 kg or 187 kg ha⁻¹ (2-year-old trees) and 160 kg or 256 kg N ha⁻¹ (8-year-old trees); mean fine root length was 489 or 821 m (2-year-old-trees), 184 or 364 m per tree (8-year-old-trees). Pruning intensity did not significantly affect net N mineralisation and net nitrification rates during the tomato-cropping season. For the tomato crop, pre-plant mean net N mineralisation rate of 2.5 mg N kg⁻¹ soil day⁻¹ was significantly lower than 16.7 or 11.6 mg N kg⁻¹ soil day⁻¹ at the end of vegetative development and flowering, respectively. Mean net nitrification rates of 3.5, and 4.3 mg N kg⁻¹ soil day⁻¹, at pre-plant and end of vegetative development, respectively, were significantly higher than 0.3 mg N kg⁻¹ soil day⁻¹ at end of flowering. In humid tropical low-input agroforestry practices that depend on organic inputs from trees for crop nutrition, retention of a branch on the pruned tree stump appears to be a good alternative to removal of all branches for reducing N losses through higher N cycling in aboveground biomass, and for conserving fine root length for higher N uptake, although it might enhance competition for associated crops.     

Response of nitrogen mineralization dynamics and biochemical properties to litter amendments to soils of a poplar plantation

Understanding the impact of plant litters on soil nitrogen(N) dynamics could facilitate development of management strategies that promote plantation ecosystem function.Our objective was to evaluate the effects of different litter types on N mineralization and availability,microbial biomass, and activities of L-asparaginase and odiphenol oxidase(o-DPO) in soils of a poplar(Populus deltoides) plantation through 24 weeks of incubation experiments.The tested litters included foliage(F), branch(B), or root(R) of poplar trees, and understory vegetation(U) or a mixture of F, B, and U(M).Litter amendments led to rapid N immobilization during the first 4 weeks of incubation, while net N mineralization was detected in all tested soils from 6 to 24 weeks of incubation, with zeroorder reaction rate constants(k) ranging from 7.7 to9.6 mg N released kg~(-1) soil wk~(-1).Moreover, litter addition led to increased microbial biomass carbon(C) 49–128% and increased MBC:MBN ratio by 5–92%,strengthened activities of L-asparaginase and o-DPO by14–74%; Up to about 37 kg N ha~(-1) net increase in mineralized N in litter added soils during 24 weeks of incubation suggests that adequate poplar and understory litter management could lead to reduced inputs while facilitate sustainable and economic viable plantation production.     

Inorganic and organic soil phosphorus and sulfur pools in an Amazonian multistrata agroforestry system

In the central Amazon basin, the effects of secondary vegetation and primary forest on inorganic and organic P and S pools were compared with those of different fruit and timber tree species in a multistrata agroforestry system. The soils (Xanthic Ferralsols) were low in readily available P and S. Fertilizer applications increased the less accessible nutrient pools more than the plant available pools. For example, dilute-acid extractable P increased substantially (from 2 to 76 mg P kg⁻¹), whereas Mehlich P (plant available) increased less (from 3 to 19 mg P kg⁻¹). In contrast, the recalcitrant soil P pools, such as the residual P, did not increase on the short term, but only after more than six years following application. The proportion of less available ester-sulfate S was significantly higher in fertilized sites than in unfertilized sites, in contrast to soluble inorganic sulfate S or carbon-bonded S. The marked increase of successively available soil P and S pools through fertilization was advantageous with respect to the long-term effect of nutrient applications. Soil nutrient availability was not only related to the amount of nutrients applied but was also influenced by tree species. Nutrient return by litterfall and litter quality played an important role in soil P and S dynamics. Incorporation of applied nutrients into successively available organic nutrient pools will decrease potential P fixation and S losses by leaching and increase long-term nutrient availability. Therefore, tree species with rapid above-ground nutrient cycling and high quality litter (such as annato [Bixa orellana] and peach palm [Bactris gasipaes]) should constitute the majority of crops in multistrata agroforestry systems on infertile soils to ensure adequate medium to long term availability of P and S. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of coarse wood and pine saplings on nitrogen mineralization and microbial communities in young post-fire Pinus contorta

Nitrogen (N) limits productivity in many coniferous forests of the western US, but the influence of post-fire structure on N cycling rates in early successional stands is not well understood. We asked if the heterogeneity created by downed wood and regenerating pine saplings affected N mineralization and microbial community composition in 15-yr old lodgepole pine (Pinus contorta var. latifolia) stands established after the 1988 fires in Yellowstone National Park (Wyoming, USA). In three 0.25-ha plots, we measured annual in situ net N mineralization in mineral soil using resin cores (n = 100 per plot) under pine saplings, downed wood (legacy logs that survived the fire, and fire-killed trees that had fallen and were contacting or elevated above the ground), and in bare mineral soil. Annual in situ net N mineralization and net nitrification rates were both greater in bare mineral soil (8.4 ± 0.6 and 3.6 ± 0.3 mg N kg_soil⁻¹ yr⁻¹, respectively) than under pine saplings, contact logs, or elevated logs (ca. 3.9 ± 0.5 and 0.8 ± 0.1 mg N kg_soil⁻¹ yr⁻¹, respectively). Net nitrification was positively related to net N mineralization under all treatments except for elevated logs. In laboratory incubations using ¹⁵N pool dilution, NH₄⁺ consumption exceeded gross production by a factor of two in all treatments, but consumption and gross production were similar among treatments. Contrary to our initial hypothesis, microbial community composition also did not vary among treatments. Thus, two- to three-fold differences in in situ net N mineralization rates occurred despite the similarity in microbial communities and laboratory measures of gross production and consumption of NH₄⁺ among treatments. These results suggest the importance of microclimate on in situ annual soil N transformations, and differences among sites suggest that broader scale landscape conditions may also be important.     

Soil nitrogen mineralization at various levels of canopy cover in red pine (Pinus resinosa) plantations

The objective of this study was to determine the rate of nitrogen (N) mineralization in response to various levels of canopy cover in red pine (Pinus resinosa Ait.) stands. Experimental plots consisted of various levels of canopy cover,i.e., clearcut, 25% (50% during first sampling year), 75%, and uncut in red pine plantations in northern Lower Michigan, USA. Net N mineralization and nitrification in the top 15 cm of mineral soil were examined during the first two growing seasons (1991–1992) following the canopy cover manipulations, using anin situ buried bag technique. Mean net N mineralization over the course of both growing seasons (May–October) ranged from 26.9 kg ha⁻¹ per growing season in the clearcut treatment to 13.4 kg ha⁻¹ per growing season in the uncut stand. Net N mineralization and nitrification increased significantly in the clearcut treatment compared to the uncut treatment during the second growing season only. However, net N mineralization and nitrification did not differ significantly between the partial canopy cover treatments and the uncut stand. Increased N mineralization and nitrification in the clearcut during the second growing season may be associated with increased soil temperature and changes of organic matter quality with time since canopy removal. This study was supported in part by the USDA Forest Service and Michigan Technological University.     

The influence of trees on soil fertility on two contrasting semi-arid soil types at Matopos,Zimbabwe

An investigation into the influence of indigenous trees on soil fertility was conducted in an area of semi-arid tropical savanna in Zimbabwe on two contrasting soil types: dystrophic savanna soils (sandy soils) and eutrophic savanna soils (fine-textured soils). The study adds further support to the growing literature showing that trees have a positive influence on soil fertility. The study suggests that tree clearance, as advocated in these agropastoral systems, may not necessarily result in long-term benefits. It is argued that the primary mechanism by which soil fertility is improved is through increased litter and soil organic matter compartments under trees. The influence of trees on cation levels is greater on sandy soils than fine-textured soils because the exchange capacity of fine-textured soils is determined largely by soil texture whereas organic matter is the prime determinant of exchange capacity in sandy soils. The present study demonstrates that fertility improvement under trees is not at the expense of fertility decline in the surface soils of the zone around the tree. Leaf quality, as reflected simply in leaf C:N ratio, influences decomposition rates but the activities of termites probably confound any simple relationship. Litter quality of tree species is probably important in determining levels of soil organic matter under canopies, with higher levels under species with lower leaf quality.