Root distributions partially explain 15N uptake patterns in Gliricidia and Peltophorum hedgerow intercropping systems

The relative distributions of tree and crop roots in agroforestry associations may affect the degree of complementarity which can be achieved in their capture of below ground resources. Trees which root more deeply than crops may intercept leaching nitrogen and thus improve nitrogen use efficiency. This hypothesis was tested by injection of small doses of (¹⁵NH₄)₂SO₄ at 21.8 atom% ¹⁵N at different soil depths within established hedgerow intercropping systems on an Ultisol in Lampung, Indonesia. In the top 10 cm of soil in intercrops of maize and trees, root length density (L_rv) of maize was greater than that of Gliricidia sepium trees, which had greater L_rv in this topsoil layer than Peltophorum dasyrrachis trees. Peltophorum trees had a greater proportion of their roots in deeper soil layers than Gliricidia or maize. These vertical root distributions were related to the pattern of recovery of ¹⁵N placed at different soil depths; more ¹⁵N was recovered by maize and Gliricidia from placements at 5 cm depth than from placements at 45 or 65 cm depth. Peltophorum recovered similar amounts of ¹⁵N from placements at each of these depths, and hence had a deeper N uptake distribution than Gliricidiaor maize. Differences in tree L_rv across the cropping alley were comparatively small, and there was no significant difference (P<0.05) in the uptake of ¹⁵N placed in topsoil at different distances from hedgerows. A greater proportion of the ¹⁵N recovered by maize was found in grain following ¹⁵N placement at 45 cm or 65 cm depth than following placement at 5 cm depth, reflecting the later arrival of maize roots in these deeper soil layers. Thus trees have an important role in preventing N leaching from subsoil during early crop establishment, although they themselves showed a lag phase in ¹⁵N uptake after pruning. Residual ¹⁵N enrichment in soil was strongly related to application depth even 406 days after ¹⁵N placement, demonstrating the validity of this approach to mapping root activity distributions.     

Root competition for phosphorus between the tree and herbaceous components of silvopastoral systems in Kerala, India

Root competition in polyculture systems involving combinations of four tree species and four grass species was evaluated based on ³²P recovery by each species in mixed and sole crop situations. The tree species were: Leucaena leucocephala, Casuarina equisetifolia, Acacia auriculiformis and Ailanthus triphysa, and the grass species were: Pennisetum purpureum (hybrid napier), Brachiaria ruziziensis (congo signal), Panicum maximum (guinea grass) and Zea mexicana (teosinte). Four lateral distance (25 and 50 cm) and depth (15 and 50 cm) treatments were included in the study to characterize the relative fine root distribution of trees. Absorption of ³²P was monitored through radioassay of leaves. Regardless of the species, ³²P uptake from 50 cm soil depth was lower than that of 15 cm depth. Absorption of ³²P from 50 cm lateral distance was also less than that of 25 cm distance in Acacia and Casuarina. Grass species in sole crop situations absorbed more ³²P than in mixed systems. None of the grass species when grown in association with tree components affected the absorption of ³²P by trees. All grass species exerted a complementary effect on ³²P absorption by Casuarina. Leucaena also benefited in the same way when grown in association with congo signal and/or teosinte. Of the tree species, Acacia and Leucaena adversely affected the ³²P uptake by grass species.     

The vertical distribution of N and K uptake in relation to root distribution and root uptake capacity in mature Quercus robur, Fagus sylvatica and Picea abies stands

We have measured the uptake capacity of nitrogen (N) and potassium (K) from different soil depths by injecting ¹⁵N and caesium (Cs; as an analogue to K) at 5 and 50 cm soil depth and analysing the recovery of these markers in foliage and buds. The study was performed in monocultures of 40-year-old pedunculate oak (Quercus robur), European beech (Fagus sylvatica) and Norway spruce (Picea abies (L.) Karst.) located at an experimental site in Palsgård, Denmark. The markers were injected as a solution through plastic tubes around 20 trees of each species at either 5 or 50 cm soil depth in June 2003. After 65 days foliage and buds were harvested and the concentrations of ¹⁵N and Cs analysed. The recovery of ¹⁵N in the foliage and buds tended to be higher from 5 than 50 cm soil depth in oak whereas they where similar in spruce and beech after compensation for differences in immobilization of ¹⁵N in the soil. In oak more Cs was recovered from 5 than from 50 cm soil depth whereas in beech and spruce no difference could be detected. Out of the three investigated tree species, oak was found to have the lowest capacity to take up Cs at 50 cm soil depth compared to 5 cm soil depth also after compensating for differences in discrimination against Cs by the roots. The uptake capacity from 50 cm soil depth compared with 5 cm was higher than expected from the root distribution except for K in oak, which can probably be explained by a considerable overlap of the uptake zones around the roots and mycorrhizal hyphae in the topsoil. The study also shows that fine roots at different soil depths with different physiological properties can influence the nutrient uptake of trees. Estimates of fine root distribution alone may thus not reflect the nutrient uptake capacity of trees with sufficient accuracy. Our study shows that deep-rooted trees such as oak may have lower nutrient uptake capacity at deeper soil layers than more shallow-rooted trees such as spruce, as we found no evidence that deep-rooted trees obtained proportionally more nutrients from deeper soil layers. This has implications for models of nutrient cycling in forest ecosystems that use the distribution of roots as the sole criterion for predicting uptake of nutrients from different soil depths.     

Authors index

In a runoff irrigation system in Northern Kenya, we studied the nutrient interactions of sole cropped and alley cropped Sorghum bicolor (L.) Moench and Acacia saligna (Labill.) H.L. Wendl. The trees were pruned once before the cropping season and the biomass was used as fodder for animals. The nutrient contents in leaf tissue, soil and soil solution were monitored and the uptake of applied tracers (¹⁵N, Sr) was followed. The grain yield of alley cropped sorghum was similar to or slightly higher than in monoculture and did not decrease near the tree-crop interface. Foliar N and Ca contents of the crop were higher in the agroforestry combination than in monoculture, corresponding to higher soil N and Ca contents. Soil solution and soil mineral N dynamics indicate an increase of N under the tree row and unused soil N at the topsoil in the alley of the sole cropped trees as well as below 60 cm depth in the crop monoculture. The N use efficiency of the tree+crop combination was higher than the sole cropped trees or crops. Competition was observed for Zn and Mn of both tree and crop whereas for Ca only the tree contents decreased. P, K, Mg and Fe dynamics were not affected by alley cropping at our site. The lower uptake of applied Sr by trees in alley cropping compared to those of the monoculture stand suggested a lower competitiveness of the acacia than sorghum, which did not show lower Sr contents when intercropped. The study showed the usefulness of combining soil and plant analyses together with tracer techniques identifying nutrient competition, nutrient transfer processes and the complementary use of soil nutrients, as the main features of the tree-crop combination. This revised version was published online in June 2006 with corrections to the Cover Date.     

Varietal differences in uptake of 32P labelled phosphate in clover plus ryegrass swards and monocultures

Lolium perenne cv. S.23, L. multiflorum cv. RvP, and Trifolium repens cvs S.184 and Olwen, were grown in mixed sward and monoculture during 1979. Whereas in mixtures grass roots absorbed more ³²P than clover roots, in monoculture clover generally absorbed more ³²P than grass roots. This showed that grass was a very strong competitor for uptake in mixed swards. Clover and grass monocultures absorbed most ³²P from 10 or 15 cm depth in the soil, while grass in mixtures absorbed most ³²P at 22.5 cm depth. Comparing varieties, in monocultures in June, Olwen was most active in absorbing ³²P at 15 cm. In August, Olwen absorbed more at 15 cm and 22.5 cm than S.184 or the grass varieties. Differences in absorption depth between varieties were less in mixtures than in monocultures. S.23 absorbed more ³²P in the late season than RvP, both in monoculture and in mixtures. Thus Olwen differed from S. 184 in depth and timing of uptake, whilst S.23 differed from RvP in time of uptake. Such varietal differences could be exploited by manipulation of depth and timing of fertiliser application to increase the precision of sward management.     

Nutrient acquisition from different soil depths by pedunculate oak

Eight oak trees (Quercus robur L.) received ³²P at a soil depth of 50 cm and ³³P at a soil depth of 15 cm at the end of June 2002 through plastic tubes inserted into the mineral soil. The phosphorus uptake from different soil depths was estimated by analysing the concentration of ³²P and ³³P in the foliage of oak growing in a mixed stand in southern Sweden. ³²P and ³³P were recovered in the leaves/needles after 21 and 39 days. The recovery of labelled P in oak was higher from 15 cm soil depth than from 50 cm, however, more than 4% of the total amount of labelled P was taken up from 50 cm. This indicates that oak can utilize deep soil layers for nutrient uptake. A study on the uptake of Cs (as an analogue to K) and ¹⁵N into the leaves was performed on the same trees and detectable amounts of ¹⁵N and Cs were recovered in leaves and buds. This indicates that ¹⁵N and Cs can be used to study nutrient uptake of mature trees from the mineral soil.     

Nitrogen cycling in leucaena (Leucaena leucocephala) alley cropping in semi-arid tropics

In an alley cropping system, prunings from the hedgerow legume are expected to supply nitrogen (N) to the associated cereal. However, this may not be sufficient to achieve maximum crop yield. Three field experiments with alley-cropped maize were conducted in a semi-arid environment in northern Australia to determine: (1) the effect of N fertilizer on maize growth in the presence of fresh leucaena prunings; (2) the effect of incorporation of leucaena and maize residues on maize yield and the fate of plant residue¹⁵N in the alley cropping system; and (3) the¹⁵N recovery by maize from¹⁵N-labelled leucaena, maize residues and ammonium sulphate fertilizer.Leucaena residues increased maize crop yield and N uptake although they did not entirely satisfy the N requirement of the alley crop. Additional N fertilizer further increased the maize yield and N uptake in the presence of leucaena residues. Placement of leucaena residues had little effect on the availability of N to maize plants over a 2 month period. The incorporation of leucaena residues in the soil did not increase the recovery of leucaena¹⁵N by maize compared with placement of the residues on the soil surface. After 2 months, similar proportions of the residue¹⁵N were recovered by maize from mulched leucaena (6.3%), incorporated leucaena (6.1%) and incorporated maize (7.6%). By the end of one cropping season (3 months after application) about 9% of the added¹⁵N was taken up by maize from either¹⁵N-labelled leucaena as mulch or¹⁵N-labelled maize residues applied together with unlabelled fresh leucaena prunings as mulch. The recovery of the added¹⁵N was much higher (42.7%) from the¹⁵N-labelled ammonium sulphate fertilizer at 40 kg N ha^-1 in the presence of unlabelled leucaena prunings. Most of the added¹⁵N recovered in the 200 cm soil profile was distributed in the top 25 cm soil with little leached below that. About 27–41% of the leucaena¹⁵N was apparently lost, largely through denitrification from the soil and plant system, in one cropping season. This compared with 35% of the fertilizer¹⁵N lost when the N fertilizer was applied in the presence of prunings. ei]H Lambers     

减量施氮对玉米-大豆套作系统下作物氮素吸收和利用效率的影响

为探索玉米-大豆套作系统中作物对N素吸收的差异特性,揭示减量施N对玉米-大豆套作系统的N高效利用机理。利用15N同位素示踪技术,结合小区套微区多年定位试验,研究了玉米单作(MM)、大豆单作(SS)、玉米-大豆套作(IMS)及不施N(NN)、减量施N(RN:180 kg N/hm2)、常量施N(CN:240 kg N/hm2)下玉米、大豆的生物量、吸N量、N肥利用率及土壤N素含量变化。结果表明,与MM(SS)相比,IMS下玉米茎叶及籽粒的生物量、吸N量降低,15N%丰度及15N吸收量增加,大豆籽粒及植株的生物量、吸N量及15N吸收量显著提高;IMS下玉米、大豆植株的N肥利用率、土壤N贡献率、土壤15N%丰度降低,15N回收率显著增加。施N与不施N相比,显著提高了单、套作下玉米、大豆植株的生物量、吸N量、15N丰度及15N吸收量;RN与CN相比,IMS下,RN的玉米、大豆植株总吸N量提高13.4%和12.4%,N肥利用率提高213.0%和117.5%,土壤总N含量提高12.2%和11.6%,土壤N贡献率降低12.0%和11.2%,玉米植株15N吸收量与15N回收率提高14.4%和52.5%,大豆的则降低57.1%和42.8%,单作与套作的变化规律一致。玉米-大豆套作系统中作物对N素吸收存在数量及形态差异,减量施N有利于玉米-大豆套作系统对N肥的高效吸收与利用,实现作物持续增产与土壤培肥。     

Alley cropping with Senna siamea in South-western Nigeria: II. Dry matter,total N,and urea-derived N dynamics of the Senna and maize roots

Crop and tree roots are crucial in the nutrient recycling hypotheses related to alley cropping systems. At the same time, they are the least understood components of these systems. The biomass, total N content and urea-derived N content of the Senna and maize roots in a Senna-maize alley cropping system were followed for a period of 1.5 years (1 maize-cowpea rotation followed by 1 maize season) to a depth of 90 cm, after the application of ¹⁵N labeled urea. The highest maize root biomass was found in the 0–10 cm layer and this biomass peaked at 38 and 67 days after planting the 1994 maize (DAP) between the maize rows (112 kg ha^–1, on average) and at 38, 67 and 107 DAP under the maize plants (4101 kg ha^–1, on average). Almost no maize roots were found below 60 cm at any sampling date. Senna root biomass decreased with time in all soil layers (from 512 to 68 kg ha^–1 for the 0–10 cm layer between 0 and 480 DAP). Below 10 cm, at least 62% of the total root biomass consisted of Senna roots and this value increased to 87% between 60 and 90 cm. Although these observations support the existence of a Senna root `safety net' between the alleys which could reduce nutrient leaching losses, the depth of such a net may be limited as the root biomass of the Senna trees in the 60–90 cm layer was below 100 kg ha^–1, equivalent to a root length density of only < 0.05 cm cm^–3. The proportion of maize root N derived from the applied urea (%Ndfu) decreased significantly with time (from 21% at 21 DAP to 8% at 107 DAP), while %Ndfu of the maize roots at the second harvest (480 DAP) was only 0.6%. The %Ndfu of the Senna roots never exceeded 4% at any depth or sampling time, but decreased less rapidly compared to the %Ndfu of the maize roots. The higher %Ndfu of the maize roots indicates that maize is more efficient in retrieving urea-derived N. The differences in dynamics of the %Ndfu also indicate that the turnover of N through the maize roots is much faster than the turnover of N through the Senna roots. The recovery of applied urea-N by the maize roots was highest in the top 0–10 cm of soil and never exceeded 0.4% (at 38 DAP) between the rows and 7.1% (at 67 DAP) under the rows. Total urea N recovery by the maize roots increased from 1.8 to 3.2% during the 1994 maize season, while the Senna roots never recovered more than 0.8% of the applied urea-N at any time during the experimental period. These values are low and signify that the roots of both plants will only marginally affect the total recovery of the applied urea-N. Measurement of the dynamics of the biomass and N content of the maize and Senna roots helps to explain the observed recovery of applied urea-N in the aboveground compartments of the alley cropping system.     

Effect of white clover cultivar on apparent transfer of nitrogen from clover to grass and estimation of relative turnover rates of nitrogen in roots

The apparent transfer of N from clover to associated grass was evaluated over a four year period both on the basis of harvested herbage and by taking account of changes in N in stubble and root (to 10 cm depth) in swards with perennial ryegrass and three different white clover cultivars differing in leaf size. The large leaved Aran transferred 15% of its nitrogen while Huia transferred 24% and the small leaved Kent Wild White transferred 34%. When changes in stubble and root N were taken into account the percentage of N transferred was calculated to be 5% less than in harvested herbage only, as the small leaved types had proportionately more N in the roots and stolons, but the large leaved type was probably more competitive towards the grass.Loss of N from clover roots from July to October was compared to that from grass roots in a grass/white clover sward continuously stocked with steers using a method which incorporated tissue turnover and ¹⁵N dilution techniques. Less than 1 mg N m^-2 d^-1 was lost from the grass roots. In contrast 8 mg m^-2 d^-1 were estimated to be lost from clover roots while 12 mg N m^-2 d^-1 were assimilated.It is concluded that clover cultivar and competitive ability on grass have to be taken into account together with the relationship between N turnover in roots and N available for grass growth when modelling N transfer in grass/clover associations.     

Absorption of 15NH3 volatilized from urea by Citrus trees

Background and aims

Gaseous losses of ammonia (NH₃) have been observed in citrus orchards when urea is surface-applied to the soils, and this loss might significantly limit the effectiveness of the nitrogen (N) fertilizer. However, a portion of the volatilized NH₃ might be absorbed by the plants through the leaves. To quantify the contribution of the leaf absorption of ¹⁵NH₃, a study with sweet oranges was conducted in two field areas where trees were grown at standard (480 trees ha^?1) and high densities (617 trees ha^?1).

Methods

Plastic trays were filled with soil, covered with mown grass to simulate field management conditions, fertilized with ¹⁵N labeled urea (12 atom % excess) and placed under each of three trees in the orchards. This experimental procedure prevented the uptake of N from the labeled urea by the roots. Two weeks after ¹⁵N fertilization, the trays were removed from the field, and the soil was homogenized and sampled for chemical analyses. The citrus trees under which the trays were placed were destructively harvested, and the total N concentrations and ¹⁵N/¹⁴N ratios were determined.

Results

After urea application, the NH₃ losses peaked within three days and subsequently decreased to negligible amounts after 10 days. The total NH₃ losses accounted for 55–82 % of the applied N. Although the NH₃ absorption by the citrus leaves was proportional to the tree density in the field, only 3–7 % of the ¹⁵NH₃ volatilized from the soil was recovered by the citrus trees, and the NH₃ absorption was also influenced by the proximity of citrus trees to the site of urea application and the leaf areas of the trees.

Conclusions

The citrus trees can absorb the NH₃ volatilized from urea, even though, the amount recovered by the trees is small and does not represent a significant proportion of total gaseous N losses, what demonstrates the importance of enhanced N use efficiency practices in field to reduce losses of NH₃ when urea is applied to soil surfaces.     

Competition for 15N-labeled fertilizer in a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

The degree of tree-crop competition for nitrogen (N) and its effect on fertilizer-use efficiency and N movement were examined in a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system. Assessment of competition was accomplished via the installation of a belowground polyethylene root barrier in half the number of plots in order to provide two treatments–barrier and non-barrier. The percentage of N derived from fertilizer (NDF) and fertilizer-use efficiency (UFN) were determined using ¹⁵N-enriched ammonium sulfate (5% atom enrichment) applied at 89.6 kg N ha^–1. In cotton, the barrier treatment resulted in higher leaf (38%), stem (66%), seed cotton (55%) and total (58%) biomass compared to the non-barrier treatment. Total N content in leaf, stem and seed cotton was 67% higher in barrier compared to non-barrier treatment. Percentage of NDF in cotton leaf and stem was significantly lower in barrier (15.8% and 17.3%, respectively) compared to non-barrier treatment (20.4% and 21.2%, respectively). For UFN, this trend was reversed, with plants in barrier treatment having a higher percentage of UFN. Root trenching did not affect pecan foliar N concentration, canopy N content, NDF or UFN. In soil, N recovery at 90–120 cm depth was lower in non-barrier treatment, indicating tree root uptake of fertilizer N. Although tree roots in non-barrier treatment had access to fertilizer N, competition was mainly for N already in the soil, since fertilizer was applied after major seasonal nutrient demands of the trees had been met. Overall, the alley cropping system in this study exhibits potential for efficient N cycling, given the apparent ability of pecan trees to intercept and uptake N fertilizer from deeper soil layers and return to surface soil via litterfall.     

Impacts of land‐use change on nitrogen status and mineralization in the Mulga Lands of Southern Queensland

Abstract: In the semiarid Mulga Lands of southern Queensland soil nitrogen (N) levels have declined after clearance of the native mulga (Acacia aneura F. Muell. ex Benth.) and conversion to grazed buffel grass (Cenchrus ciliaris) pasture. At three mulga sites, declines in soil total N ranged from 14% to 28% in the surface 10 cm of soil. In situ net N mineralization from December 2003 until November 2004 in the surface 10 cm was 49.5 kg N ha^?1 year^?1 in the mulga woodland, 48.2 kg N ha^?1 year^?1 in the young (<5 years old) buffel pasture (previously sown to wheat (Triticum aestivum L.) and 34.6 kg N ha^?1 year^?1 in the old buffel pasture (>20 years). Ammonium‐N was the dominant N pool under mulga in the top 30 cm, while nitrate‐N was dominant under the buffel pastures. Although ammonium‐N under mulga was significantly different to that for 21‐year‐old buffel pasture at all depths, nitrification and net N mineralization were not different between the three land uses at any depth or in the entire 90 cm profile. The Soil Nitrogen Availability Predictor model was used to predict field N mineralization rates for the mulga woodland and 21‐year‐old buffel pasture by using a medium‐term (6‐week) laboratory incubation to establish basal rates of N mineralization. The Soil Nitrogen Availability Predictor overestimated annual net N mineralization in the 0–30 cm depth of mulga by 9% and underestimated it by 28% for the old buffel pasture. The Soil Nitrogen Availability Predictor could be modified further to accurately predict net N mineralization for the mulga woodlands.     

Capture and allocation of nitrogen byQuercus douglasii seedlings in competition with annual and perennial grasses

Summary The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting¹⁵N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating¹⁵N uptake rates, pool sizes and¹⁵N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of¹⁵N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.     

Tree and grass rooting patterns in an African humid savanna

Abstract. Spatial and temporal soil partitioning between roots of the two savanna plant components, i.e. trees and grasses, were investigated in a West African humid savanna. Vertical root phytomass distribution was described for grass roots, large (> 2 mm) and fine (< 2 mm) tree roots, in open sites and beneath tree canopies. These profiles were established monthly over one year of vegetation growth. Natural ¹³C abundance measurement was used to determine the woody/herbaceous phytomass ratio in root samples. Tree and grass root distributions widely overlapped and both were mostly located in the top 20 cm of the soil. Grass root phytomass decreased with depth whereas woody root phytomass peaked at about 10 cm depth. No time partitioning was detected. These structural results do not support the hypothesis of soil resource partitioning between trees and grasses and are thus consistent with functional results previously reported.     

Imprint of oaks on nitrogen availability and δ<Superscript>15</Superscript>N in California grassland-savanna: a case of enhanced N inputs?

Woody vegetation is distributed patchily in many arid and semi-arid ecosystems, where it is often associated with elevated nitrogen (N) pools and availability in islands of fertility. We measured N availability and δ¹⁵N in paired blue-oak versus annual grass dominated patches to characterize the causes and consequences of spatial variation in N dynamics of grassland-savanna in Sequoia-Kings Canyon National Park. We found significantly greater surface soil N pools (0–20 cm) in oak patches compared to adjacent grass areas across a 700 m elevation gradient from foothills to the savanna-forest boundary. N accumulation under oaks was associated with a 0.6‰ depletion in soil δ¹⁵N relative to grass patches. Results from a simple δ¹⁵N mass balance simulation model, constrained by surface soil N and δ¹⁵N measured in the field, suggest that the development of islands of N fertility under oaks can be traced primarily to enhanced N inputs. Net N mineralization and percent nitrification in laboratory incubations were consistently higher under oaks across a range of experimental soil moisture regimes, suggesting a scenario whereby greater N inputs to oak patches result in net N accumulation and enhanced N cycling, with a potential for greater nitrate loss as well. N concentrations of three common herbaceous annual plants were nearly 50% greater under oak than in adjacent grass patches, with community composition shifted towards more N-demanding species under oaks. We find that oaks imprint distinct N-rich islands of fertility that foster local feedback between soil N cycling, plant N uptake, and herbaceous community composition. Such patch-scale differences in N inputs and plant–soil interactions increase biogeochemical heterogeneity in grassland-savanna ecosystems and may shape watershed-level responses to chronic N deposition.     

The distribution of tree and grass roots in savannas in relation to soil nitrogen and water

Here we describe the fine root distribution of trees and grasses relative to soil nitrogen and water profiles. The primary objective is to improve our understanding of edaphic processes influencing the relative abundance of trees and grasses in savanna systems. We do this at both a mesic (737 mm MAP) site on sandy-loam soils and at an arid (547 mm MAP) site on clay rich soils in the Kruger National Park in South Africa. The proportion of tree and grass fine roots at each soil depth were estimated using the δ¹³C values of fine roots and the δ¹³C end members of the fine roots of the dominant trees and grasses at our study sites. Changes in soil nitrogen concentrations with depth were indexed using total soil nitrogen concentrations and soil δ¹⁵N values. Soil water content was measured at different depths using capacitance probes. We show that most tree and grass roots are located in the upper layers of the soil and that both tree and grass roots are present at the bottom of the profile. We demonstrate that root density is positively related to the distribution of soil nitrogen and negatively related to soil moisture. We attribute the negative correlation with soil moisture to evaporation from the soil surface and uptake by roots. Our data is a snapshot of a dynamic process, here the picture it provides is potentially misleading. To understand whether roots in this system are primarily foraging for water or for nitrogen future studies need to include a dynamic component.     

A comparison of the effects of subsoiling on plant uptake and leaching losses of sulphur and nitrogen from a simulated urine patch

Synthetic cow urine labelled with ³⁵S and ¹⁵N was applied to large, undisturbed, monolith lysimeters sampled from subsoiled and non-subsoiled areas of a grass/clover pasture. For one year following the urine application, the lysimeters were subjected to a combination of natural rainfall, simulated rainfall and simulated flood irrigations. Drainage from the lysimeters was sampled regularly and monthly (approx.) pasture cuts taken. At the end of the year, the lysimeters were destructively sampled in 50 mm depth increments for soil analysis. Leachates, plant samples and soil samples were analysed for ³⁵S and ¹⁵N.There were no significant differences in plant uptake of ³⁵S and ¹⁵N between the subsoiled and nonsubsoiled lysimeters. Initially grass showed a higher degree of labelling than clover. Total amounts of ³⁵S and ¹⁵N leached from the subsoiled lysimeters were approximately twice that leached from the nonsubsoiled ones. Leaching patterns differed substantially between the two nutrients.Total recoveries of ³⁵S (in plants, leachates and soil extracts) accounted for 82% of the applied ³⁵S for the subsoiled lysimeters and 72% for non-subsoiled ones. The unrecovered ³⁵S is considered to have been incorporated into soil organic matter. Total recoveries of ¹⁵N (in plants, soil and leachates) were similar to those for ³⁵S, but unrecovered ¹⁵N is attributed to loss by denitrification.     

Effects of N application frequency on nitrogen uptake efficiency in Citrus trees

Two irrigation systems were used to compare nitrogen uptake efficiency in citrus trees and to evaluate the NO₃⁻ runoff in «Navelina» orange trees [Citrus sinensis (L.) Osbeck] on Carrizo citrange rootstock (Citrus sinensis × Poncirus trifoliata Raf.). These were fertilized with 125 g N as labelled K¹⁵NO₃ and grown outdoors in containers filled with a sand-loamy soil. Two groups of 3 trees received this N dose either in five equally split applications by a flooding irrigation system or in 66 applications by drip. Trees were harvested at the end of the vegetative cycle (December) and the isotopic ratios of ¹⁵N/¹⁴N were measured in the soil-plant system. The N uptake efficiency of the whole tree was higher with drip irrigation (75 percnt;) than with flooding system (64 percnt;). In the 0-90 cm soil profile, the N immobilized in the organic fraction was similar for both irrigation methods (around 13 percnt;), whereas the N retained as NO₃⁻ was 1 percnt; of the N applied under drip and 10 percnt; under flooding. In the last case, most of NO₃⁻ remained under root system and it could be lost to leaching either by heavy rainfalls or excessive water applications. These results showed that a drip irrigation system was more efficient for improving water use and N uptake from fertilizer, in addition to potentially reduced leaching losses.     

Carbon, nitrogen and phosphorus in volcanic soils following afforestation with native birch (Betula pubescens) and introduced larch (Larix sibirica) in Iceland

Afforestation has become an important tool for soil protection and land reclamation in Iceland. Nevertheless, the harsh climate and degraded soils are growth-limiting for trees, and little is know about changes in soil nutrients in maturing forests planted on the volcanic soils. In the present chronosequence study, changes in C, N and total P in soil (0–10 and 10–20 cm depth) and C and N in foliar tissue were investigated in stands of native Downy birch (Betula pubescens Enrh.) and the in Iceland introduced Siberian larch (Larix sibirica Ledeb.). The forest stands were between 14 and 97 years old and were established on heath land that had been treeless for centuries. Soils were Andosols derived from basaltic material and rhyolitic volcanic ash. A significant effect of tree species was only found for the N content in foliar tissue. Foliar N concentrations were significantly higher and foliar C/N ratios significantly lower in larch needles than in birch leaves. There was no effect of stand age. Changes in soil C and the soil nutrient status with time after afforestation were little significant. Soil C concentrations in 0–10 cm depth in forest stands older than 30 years were significantly higher than in heath land and forest stands younger than 30 years. This was attributed to a slow accumulation of organic matter. Soil N concentrations and soil P_tot were not affected by stand age. Nutrient pools in the two soil layers were calculated for an average weight of soil material (400 Mg soil ha⁻¹ in 0–10 cm depth and 600 Mg soil ha⁻¹ in 10–20 cm depth, respectively). Soil nutrient pools did not change significantly with time. Soil C pools were in average 23.6 Mg ha⁻¹ in the upper soil layer and 16.9 Mg ha⁻¹ in the lower soil layer. The highest annual increase in soil C under forest compared to heath land was 0.23 Mg C ha⁻¹ year⁻¹ in 0–10 cm depth calculated for the 53-year-old larch stand. Soil N pools were in average 1.0 Mg N ha⁻¹ in both soil layers and did not decrease with time despite a low N deposition and the uptake and accumulation of N in biomass of the growing trees. Soil P_tot pools were in average 220 and 320 kg P ha⁻¹ in the upper and lower soil layer, respectively. It was assumed that mycorrhizal fungi present in the stands had an influence on the availability of N and P to the trees. Responsible Editor: Hans Lambers.