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.     

Fine root turnover of irrigated hedgerow intercropping in Northern Kenya

Fine root turnover (<2 mm) was determined from repeated measurements of root distribution up to 120 cm soil depth by core sampling in four month intervals. Sole cropped Sorghum bicolor and Acacia saligna were compared with the agroforestry combination in an alley cropping system in semiarid Northern Kenya. Three methods for the calculation of root production were used: the max-min, balancing-transfer and compartment-flow method. The highest root biomass was found in the topsoil for all cropping systems, though trees had a deeper root system. Trees and crops had a similar amount of below-ground biomass during the vegetation period (0.3 and 0.4 Mg DM ha^-1 120 cm^-1), but in the agroforestry combination root biomass was more than the sum of the sole cropped systems (1.1 Mg DM ha^-1 120 cm^-1). The tree system showed a very static root development with little fluctuation between seasons, whereas root biomasses were very dynamic in the crop and tree + crop systems. Root production was highest in the tree + crop combination with 2.1 Mg DM ha^-1 a^-1, with about 50% less in sole cropped trees and crops. Root N input to soil decreased in the order tree + crop>tree>crop system with 13.5, 11.0 and 3.2 kg N ha^-1 a^-1, and cannot be estimated from total below-ground biomass or carbon turnover, as N is accumulated in senescing roots. Such low N input to soil stresses the need for investigating other processes of nutrient input from roots to soil. Areas of highest N input were identified in the topsoil under the tree row in the tree system. Resource utilisation and C and N input to soil were highest with a combination of annual and perennial crops.     

Switchgrass growth and pine–switchgrass interactions in established intercropping systems

Intercropping switchgrass (Panicum virgatum L.) with loblolly pine (Pinus taeda L.) has been proposed for producing bioenergy feedstock in the southeastern United States. This study investigated switchgrass growth and pine–switchgrass interactions at two established experimental fields (7‐year‐old Lenoir site and 5‐year‐old Carteret site) located on the coastal plain of eastern United States. Position effects (edge and center of switchgrass alley in intercropping plots) and treatment effects (intercropping vs. grass‐only) on aboveground switchgrass growth were evaluated. Interspecific interactions with respect to capturing resources (light, soil water, and nitrogen) were investigated by measuring photosynthetically active radiation (PAR) above grass canopy, soil moisture, and soil mineral nitrogen contents. Switchgrass growth was significantly (P = 0.001) affected by treatments in Lenoir and by position (P < 0.0001) in both study sites. Relative to the center, PAR above grass canopy at edge in both sites was about 48% less during the growing season. Soil water content during the growing season at the edge of grass alley was significantly (P = 0.0001) lower by 23% than at the center in Lenoir, while no significant (P = 0.42) difference was observed in Carteret, in spite of more grass growth at center at both sites. Soil mineral nitrogen content at the center of intercropping plots in Lenoir (no fertilization during 2015) was significantly (P < 0.07) lower than at the edge during the peak of growing season (June, July, and August), but not during early and late parts of growing season (May, September, and November). Position effects on soil water and mineral nitrogen were less evident under conditions with higher external inputs (rainfall and fertilization) and lower plant uptake during nongrowing seasons. Results from this study contributed to a better understanding of above‐ and belowground pine–switchgrass interactions which is necessary to properly manage this new cultivation system for bioenergy production in the southeastern United States.     

Population and agricultural intensity in the humid tropics

A review of models of agricultural intensification and their application to the agricultural systems of the humid tropics is presented. Taken into account are the distributions of these systems at various population densities, available data on labor efficiencies, the costs of establishing continuous cropping, and data regarding soils under cultivation and various types of fallow. The findings that fallows much longer than 10 or 15 years serve no known agronomic function, that given preindustrial technology, grass fallows are disadvantageous, even environmentally destructive, and that continuous cropping usually entails a considerable amount of environmental modification support the interpretations that agricultural intensification in the humid tropics is best understood in terms of ecologically optimal strategies at different population densities. Points needing further investigation are highlighted: the reasons for very long fallows, and the comparative labor efficiencies of fallow and continuous cropping systems where crops and environments are similar.     

Nitrogen use efficiency. 1. Uptake of nitrogen from the soil

The nitrogen use efficiency (NUE) of crop plants can be expressed very simply as the yield of nitrogen per unit of available nitrogen in the soil. This NUE can be divided into two processes: uptake efficiency, the ability of the plant to remove N from the soil normally present as nitrate or ammonium ions, and the utilisation efficiency, the ability of the plant to transfer the N to the grain, predominantly present as protein. In this article, we have highlighted the latest developments in the isolation and characterisation of the genes involved in the uptake of nitrogen from the soil.     

Access and excess problems in plant nutrition

As plant nutrition issues are redefined by society, new applications emerge for a basic understanding of nutrient use efficiency in soil-plant processes to avoid excess on rich soils as commonly found in the temperate zone and make the best of it under access-limited conditions common in the tropics. The main challenge of plant nutrition may be to increase the width of the domain between the access and excess frontiers, rather than to define a single `economic optimum' point. Two approaches are discussed to widen this domain: the technical paradigm of precision farming and the ecological analogue approach based on filter functions and complementarity of components in mixed plant systems. Current understanding of plant nutrition, largely focused on monocultural situations, needs to be augmented by the interactions that occur in more complex systems, including agroforestry and intercropping as these may form part of the answer in both the excess and shortage type of situation. Simulations with the WaNuLCAS model to explore the concepts of a 'safety-net' for mobile nutrients by deep rooted plants suggested a limited but real opportunity to intercept nutrients on their way out of the system and thus increase nutrient use-efficiency at the system level. The impacts of rhizosphere modification to mobilize nutrients in mixed-species systems were shown to depend on the degree of synlocation of roots of the various plant components, as well as on the long-term replenishment of the nutrient resources accessed. In conclusion, the concepts and tools to help farmers navigate between the scylla of access and the charibdis of excess problems in plant nutrition certainly exist, but their use requires an appreciation of the site-specific interactions and various levels of internal regulation, rather than a reliance alone on genetic modification of plants aimed at transferring specific mechanisms out of context.     

Potential importance of the subsoil for the P and Mg nutrition of wheat

A method is described which allowed the quantification of the potential uptake of P and Mg from the subsoil (>30cm) by spring wheat. Wheat was grown on an artificial topsoil (sand with no plant available P or Mg) which was superimposed on loess subsoils in N. Germany. The supply of P and Mg in the topsoil was varied by application of different quantities of P and Mg fertilizer. Uptake of P and Mg from the subsoil was calculated as the difference between total plant uptake (determined by plant analysis) and the quantities of P and Mg removed from the topsoil (determined by soil analysis). P uptake from the subsoil increased from 37% to 85% of total P uptake, with decreasing P supply in the topsoil. Calculations of potential supply by diffusion showed that, with a CAL-extractable P₂O₅ content in the subsoil of 9 mg 100g^-1, supply from the subsoil was only possible if the influence of root hairs was considered. The method also showed that the total demand for Mg by spring wheat could be satisfield from the supply of Mg from the subsoil of typical loess soils. Mg uptake from the subsoil decreased to 33% of total uptake with increasing Mg supply in the topsoil.     

Assessment and attempt to explain the high performance of Azolla in subdesertic tropics versus humid tropics

Two similar experiments aimed at selecting performingAzolla strains at Richard Toll, Senegal (subdesertic tropics) and Los Baños, Philippines (humid tropics) show that annual mean productivity of the tested strains is higher in the first site than in the second, the best ecotypes in Senegal producing more than twice as much as in the Philippines.Attempts to explain these differences by climatic or interaction of climatic variables (in the range explored) are unsuccessful. Further analyses are thus needed to understand the reasons, climatic or other, responsible for the different behaviour ofAzolla under the two studied ecologies.     

Using competitive and facilitative interactions in intercropping systems enhances crop productivity and nutrient-use efficiency

This paper reviews recent research on the processes involved in the yield advantage in wheat (Triticum aestivum L.)/maize (Zea mays L.), wheat/soybean [Glycine max (L.) Merr.], faba bean (Vicia faba L.)/maize, peanut (Arachis hypogaea L.)/maize and water convolvulus (Ipomoea aquatica Forsk.)/maize intercropping. In wheat/maize and wheat/soybean intercropping systems, a significant yield increase of intercropped wheat over sole wheat was observed, which resulted from positive effects of the border row and inner rows of intercropped wheat. The border row effect was due to interspecific competition for nutrients as wheat had a higher competitive ability than either maize or soybean had. There was also compensatory growth, or a recovery process, of subordinate species such as maize and soybean, offsetting the impairment of early growth of the subordinate species. Finally, both dominant and subordinate species in intercropping obtain higher yields than that in corresponding sole wheat, maize or soybean. We summarized these processes as the `competition-recovery production principle'. We observed interspecific facilitation, where maize improves iron nutrition in intercropped peanut, faba bean enhances nitrogen and phosphorus uptake by intercropped maize, and chickpea facilitates P uptake by associated wheat from phytate-P. Furthermore, intercropping reduced the nitrate content in the soil profile as intercropping uses soil nutrients more efficiently than sole cropping.     

Nitrogen management in a maize-groundnut crop rotation of humid tropics: effect on N2O emission

Development of appropriate land management techniques to attain sustainability and increase the N use efficiency of crops in the tropics has been gaining momentum. The nitrous oxides (N2Os) affect global climate change and its contribution from N and C management systems is of great significance. Thus, N transformations and N2O emission during maize-groundnut crop rotation managed with various N sources were studied. Accumulation of nitrate (NO3- ) and its disappearance happened immediately after addition of various N sources, showing liming effect. The mineral N retained for 2-4 weeks depending on the type and amount of N application. The chicken manure showed rapid nitrification in the first week after application during the fallow period, leading to a maximum N2O flux of 9889 g N2O-N m(-2) day(-1). The same plots showed a residual effect by emitting the highest N2O (4053 microg N2O-N m(-2) day(-1)) during maize cultivation supplied with a half-rate of N fertilizer. Application of N fertilizer only or in combination with crop residues exhibited either lowered fluxes or caused a sink during the groundnut and fallow periods due to small availability of substrates and/or low water-filled pore space (<40%). The annual N2O emission ranged from 1.41 to 3.94 kg N2O-N ha(-1); the highest was estimated from the chicken manure plus crop residues and half-rate of inorganic N-amended plots. Results indicates a greater influence of chicken manure on the N transformations and thereby N2O emission.     

华北石质山区核桃-绿豆复合系统氘同位素变化及其水分利用

通过对比核桃枝条和绿豆茎内δD值差异来分析核桃和绿豆水分来源和利用。结果表明,核桃-绿豆农林复合系统的根系在表层土壤(0—30cm)中交叉存在,生态位重叠。旱季中表层土壤含水量与δD值之间存在显著的负相关关系(R2=0.77,P=0.02),雨季相关关系不显著(R2=0.03,P=0.73)。δD值分析表明,旱季中核桃利用深层土壤(30—80cm)水分占总水分来源的51%以上,雨季中则主要利用浅层土壤水分,间作绿豆和单作绿豆主要利用表层土壤水分。雨季中表层土壤水分能同时满足核桃和绿豆生长需要,但复合系统中光能竞争导致间作绿豆光合速率显著地低于单作绿豆。旱季间作绿豆0—20cm土壤水分含量、凌晨叶片水势和光合速率明显高于单作绿豆,显示间作绿豆体内水分状况好于单作绿豆。线性模型分析结果显示间作绿豆体内约有1.58%—5.39%的水分来核桃夜晚水力提升,表明复合系统在旱季一定程度上缓冲季节性水分胁迫对农作物生长的影响。     

Heavy metal absorption status of five plant species in monoculture and intercropping

Absorption ability for heavy metals varies among plant species. This study is to evaluate the absorption characteristics of different plant species and planting patterns for heavy metals. Five plant species (tomato, maize, greengrocery, cabbage, and Japan clover herb) were cultivated in monoculture and in intercropping in soil contaminated with heavy metals (Cd, Pb, Cr, Cu, and Fe), to determine the absorption status. Tomato absorbs greater amounts of heavy metals (especially Cd). Furthermore, accumulation of heavy metals increased when tomato was intercropped with other plant species. Maize accumulates greater amounts of Cr, Cu, and Fe. The heavy metal concentrations were reduced when maize was intercropped. Cd and Pb accumulated more in roots of Japan Clover Herb, and the levels of all five heavy metals decreased when intercropped. Tomato intercropping is a feasible method for phytoremediation of heavy metal-contaminated soil, and maize intercropping is feasible for obtaining safe harvest which can be eaten securely.     

间套作提高农田水分利用效率的节水机理

综合国内外多学科的研究成果,从地表水向土壤水的转化效率、农田水分的有效性、植物冠层结构、灌溉用水量和作物产量等方面,论述了间套作提高农田水分利用效率的节水机理.结果表明:间套作能够促进植物根系对农田水分的充分利用,有利于增加根层土壤的贮水量;间套作一方面减小棵间蒸发、抑制无效蒸腾,另一方面优化作物系统的源-库关系,创造出有利于植物生长发育的小气候,为资源在时间和空间上的集约利用和高产打好基础,在不增加农田灌溉水的同时大幅度提高单位面积产量,促进作物水分利用效率明显提高.     

Nitrification and mineralization potentials in a limed Ultisol in the humid tropics

Summary We studied the effect of liming on the rates of mineralization and nitrification in a coarse-textured kaolinitic Ultisol. Soil samples were taken from field plots which received lime rates from 0 to 4mt/ha three years prior to the study. The pH of the soil samples varied from 4.2 to 6.1. Ammonification of soil organic N and added urea source proceeded readily and was not affected by lime rate. Nitrification occurred in both limed and unlimed soils but the rate of nitrification depended upon the rate of lime application. Soil pH, exchangeable Ca and exchangeable A1 were significantly correlated with the amount of NO₃-N accumulated at the end of the 65 days incubation period. Nitrification of NH₄-N from ammonium sulfate was absent in soils receiving lower rates of lime which gave pH values ranging from 4.2 to 4.8. Added ammonium source was nitrified readily after a 3-week delay period in the soil (pH 6.1) which received a higher rate of lime (4 mt/ha).     

Nitrogen uptake and transfer in a soybean/maize intercropping system in the karst region of southwest China

Nitrogen (N) deficiency occurs in over 80% of karst soil of southwest China, which restricts regional agricultural production. To test whether N fixed by legumes becomes available to nonfixing companion species, N fluxes between soybean and maize under no, partial, and total restriction of root contact were measured on a karst site in southwest China. N content and its transfer between soybean and maize intercrops were explored in a 2‐year plot experiment, with N movement between crops monitored using ¹⁵N isotopes. Mesh barrier (30 μm) and no restrictions barrier root separation increased N uptake of maize by 1.28%–3.45% and 3.2%–3.45%, respectively. N uptake by soybean with no restrictions root separation was 1.23 and 1.56 times higher than that by mesh and solid barriers, respectively. In the unrestricted root condition, N transfer from soybean to maize in no restrictions barrier was 2.34–3.02 mg higher than that of mesh barrier. Therefore, it was implied that soybean/maize intercropping could improve N uptake and transfer efficiently in the karst region of southwest China.     

Biomass production, symbiotic nitrogen fixation and inorganic N use in dual and tri-component annual intercrops

The interspecific complementary and competitive interactions between pea (Pisum sativum L.), barley (Hordeum vulgare L.) and oilseed rape (Brassica napus L.), grown as dual and tri-component intercrops were assessed in a field study in Denmark. Total biomass production and N use at two levels of N fertilisation (0.5 and 4.0 g N/m²), were measured at five harvests throughout a growing season. All intercrops displayed land equivalent ratio values close to or exceeding unity, indicating complementary use of growth resources. Whereas both rape and barley responded positively to increased N fertilisation, irrespective of whether they were grown as sole- or intercrops, pea was strongly suppressed when grown in intercrop. Of the three crops barley was the strongest competitor for both soil and fertiliser N, rape intermediate and pea the weakest. Faster initial growth of barley than pea and rape gave barley an initial competitive advantage, an advantage that in the two dual intercrops was strengthened by the addition of N. Apparently the competitive superiority of barley was less strong in the tri-component intercrop, indicating that the impact of the dominantmay, through improved growth of both rape and pea, have been diminished through indirect facilitation. Interspecific competition had a promoting effect on the percent of nitrogen derived from N₂ fixation of pea, and most so at the low N fertilisation level. Results indicate that the benefits achieved from the association of a legume and nonlegume, in terms of N₂ fixed were greatest when pea was grown in association with rape as opposed to barley which could indicate that the benefits achieved from the association of a legume and nonlegume are partly lost if the nonlegume is too strong a competitor.     

Reproductive response in supplemented heifers in the humid tropics of Costa Rica

To evaluate the effect of two supplementary diets to determine the consequence on productive and reproductive performance in heifers (Bos indicus x Bos taurus) averaging between 24 and 36 months of age and grazing tropical pastures, two trials were conducted. Thirty animals (initial BW 325.1+/-33.6 kg) were divided in two groups in the initial study: supplemented (SG) and control (CG); SG received a concentrate (5.5% CP and 2.85 Mcal/kg of DE dry matter basis) at 1% of body weight (BW). In the second study, 45 heifers (initial BW 332.6+/-29.3 kg) were assigned in two treatments, with the same amount of supplement (1% BW) but with a greater nutrient content (13% CP and 3.15 Mcal/kg of DE). The proportion of animals with a corpus luteum at the end of each study was greater in the supplemented groups (P<0.05). Ovarian follicular dynamics was similar between groups in the first study, but in the second study there were more heifers in the SG group with follicles larger than 9 mm in diameter (P<0.05). Pregnancy rate was similar for SG and CG (P>0.05). The response to a regimen of estrous synchronization in both trials was numerically superior in the SG group. No differences were observed in the length of estrus. Daily gain and body condition score were similar for supplemented and control groups (0.27 compared with -0.06 in the first study and 0.90 kg compared with 0.60 in the second study, respectively). Dietary supplementation improved the number of animals initiating estrous cycles and the expression of estrus when compared with unsupplemented control heifers. The dietary regimens imposed in these studies appear to be an adequate for the management of growing heifers destined to a reproductive program.     

Influence of subsoil zinc on dry matter production,seed yield and distribution of zinc in oilseed rape genotypes differing in zinc efficiency

The effects of Zn supply (+Zn: 1 mg kg^–1 soil, -Zn: no Zn added) in subsoil were examined in three genotypes of Brassica napus (Zhongyou 821, Xinza 2, Narendra) and one genotype of Brassica juncea (CSIRO-1) in a glasshouse experiment in pots (100 cm long, 10.5 cm diameter). The topsoil (upper 20 cm soil in pots) was supplied with Zn in all treatments whereas Zn was either supplied or omitted from the subsoil. Supplying Zn to subsoil significantly increased the root growth in the lower zone, markedly decreased the number of aborted and unfilled pods plant^–1 and significantly increased the number of developed pods plant^–1, number of seeds pod^–1, individual seed weight and overall seed yield. Subsoil Zn also significantly increased the Zn concentration and Zn content of seed and improved the ratio of Zn uptake by seed to total Zn uptake by seed and shoots. These effects of subsoil Zn were more pronounced in Zhongyou 821, Xinza 2 and Narendra compared with CSIRO-1. CSIRO-I had 92% Zn efficiency (ratio of -Zn subsoil seed yield to +Zn subsoil seed yield expressed in percentage) compared with 63% for Zhongyou 821. Among the four genotypes, CSIRO-1 had the lowest Zn concentration in roots and shoots but highest Zn concentration and content in seed, suggesting it has a superior Zn transport mechanism from source (roots) to sink (seed). CSIRO-1 also significantly decreased the rhizosphere pH in lower rooting zone (20-93 cm) in -Zn subsoil treatment compared with +Zn treatment.     

Influence of spatial arrangement in maize-soybean intercropping on root growth and water use efficiency

Background and aims

The relationship between transpiration and root distribution under different spatial arrangements of intercropping is poorly understood. The effects of three spatial arrangements in the maize (Zea mays L.) - soybean (Glycine max L.) intercropping on root distribution, transpiration, water use efficiency (WUE) and grain yield were examined.

Methods

Two-year field experiments were conducted using three spatial arrangements of 2 rows maize × 4 rows soybean (M2S4), 2 rows maize × 2 rows soybean (M2S2) and 4 rows maize × 2 rows soybean (M4S2), with their respective sole crops (monocrop) for comparison.

Results

The grain yield of maize in intercrops was higher than its monocrop and that of soybean in intercrops was lower than its monocrop. Except for M2S2 in 2014, there were yield advantages in intercropping due to improvement in the land use efficiency. Transpiration in maize was higher than in soybean regardless of the spatial arrangements. Transpiration of both maize and soybean was influenced by the spatial arrangements of the intercropping with M4S2 or M2S4 tending to have higher daily transpiration than monocrops and other spatial arrangements. Intercropping enhanced root length density (RLD) in both maize and soybean compared to the corresponding monocrop. RLD was higher and land equivalent ratio (LER) was lower under M2S2 than under other spatial arrangements of intercropping, WUE was higher in M4S2 than in other spatial arrangements.

Conclusions

Intercropping was more efficient in using the environmental resources than monocropping. The M4S2 spatial arrangement in the maize-soybean intercropping could be selected because of its sustainability and greater land and water use efficiency.