Soil bio-functioning under <Emphasis Type="Italic">Acacia nilotica</Emphasis> var. <Emphasis Type="Italic">tomentosa</Emphasis> protected forest along the Senegal River

Acacia nilotica var. tomentosa trees from the Diarra protected forest located in the Senegal River valley were identified for the assessment of both biological nitrogen fixation, using the natural abundance method, and soil bio-functioning parameters (nodulation, root biomass, total microbial biomass, and potential N mineralization). The presence and the genetic diversity of indigenous rhizobia nodulating A. nilotica var. tomentosa was also investigated, taking into account distance from the trunk (0, 1, 2, and 3 m) and depth (0–25, 25–50, and 50–75 cm). Surprisingly, no nodules on the trees root systems were found, whereas under laboratory conditions the presence of indigenous rhizobia nodulating A. nilotica var. tomentosa was demonstrated in the analyzed soils (90% of the nodules harvested on the trapped plants were occupied by the same Inter-Genic Spacer (IGS) group, IGS1). There was no significant influence of trees and/or depth on total microbial biomass and potentials of nitrogen mineralization. Some assumptions were formulated on the possible combined effect of flooding, which usually occurs annually during 4–7 months, and the clayey soils in the Diara forests. Although a deeply natural nodulation of A. nilotica var. tomentosa trees by indigenous rhizobia is not excluded, but it still remains to be demonstrated.     

Productivity and economics of lowland rice as influenced by incorporation of N-fixing tree biomass in mid-altitude subtropical Meghalaya,North East India

The climatic conditions of North East India are favorable for trees to produce biomass in the form of foliage and twigs that are very rich in essential plant nutrients. Effective recycling of this biomass would help meet the nutritional requirement of crops. Field experiment was conducted in kharif (June–November) seasons for consecutive 3 years (2003, 2004, and 2005) at a lowland farm, subtropical Meghalaya (950 m asl), India, to study the effect of incorporating N-fixing tree biomass (leaves and twigs) on productivity and economics of rice (Oryza sativa L.). Fresh biomass from five tree species including erythrina (Erythrina indica), acacia (Acacia auriculiformis), alder (Alnus nepalensis), tree bean (Parkia roxburghii), and cassia (Cassia siamea) were applied at a rate of 10 t/ha. A plot with recommended NPK rate (80:60:40 kg/ha) and a control plot were also maintained for comparison. Among the tree species used, the biomass of E. indica was superior in terms of N (3.2%), P (0.47%), K (1.5%), and organic C (18.8%) contents. In the first and second year, productivity of rice was high with recommended NPK rate (4.82 t/ha in 2003 and 5.08 t/ha in 2004) followed by rice with incorporation of E. indica biomass. In the third year of the experiment, effects of tree biomass incorporation on growth, yield and yield attributes surpassed those of the recommended NPK rate, with the exception of A. nepalensis biomass. In that year, the maximum grain yield was recorded under E. indica treatments, exceeding yields under the recommended NPK rate and control by 10.5 and 69.3%, respectively. Incorporation of tree biomass significantly improved (14–19% N and 62–83% P over control) the stocks of soil available N and P. Treatment with A. auriculiformis and E. indica biomass resulted in significantly higher soil organic C content which exceeded that under the recommended NPK rate by 10.3 and 9.2% and that under the control by 15.2 and 14%, respectively higher by species. The highest net return was recorded with the recommended NPK rate (381/ha) followed by < i > E. indica < /i > ( 381/ha) followed by E. indica ( 303/ha). Since the local farmers are resource poor and rarely use chemical fertilizers, application of plant biomass, particularly that of E. indica, to lowland rice is a recommendable option to improve productivity and income, and to sustain soil health.     

Nitrogen dynamics in maize-based agroforestry systems as affected by landscape position in southern Malawi

In Malawi, agroforestry is very promising for N replenishment; however, there are still large variations in the performance of these agroforestry technologies on farmers’ fields. A study was conducted on-farm to determine the influence of three landscape positions on N dynamics in maize (Zea mays L.)-based agroforestry systems. The agroforestry systems were relay fallow using Sesbania sesban (L.) Merr or Tephrosia vogelii (Hook F.), simultaneous fallow using Gliricidia sepium(Jacq.) Walp., and maize without trees as a control. Sesbania was superior to other systems in the bottom slope, producing the highest tree biomass (1,861 kg ha⁻¹), whereas, gliricidia gave the highest tree biomass production in the mid-slope (2,147 kg ha⁻¹) and upper slope (1,690 kg ha⁻¹). Preseason inorganic N, maize flag leaf N concentration, maize total N uptake and maize yields followed a similar trend to tree biomass production with tree-based cropping systems exhibiting higher productivity (P < 0.05) than the cropping systems without trees. Nitrogen leaching from gliricidia agroforestry systems was lower than in the other agroforestry systems across all landscape positions as evidenced by 17% lower amounts (P < 0.05) of inorganic N adsorbed to ionic exchange resin membranes at 60 cm soil depth most likely due to the permanent root system of gliricidia. The difference between δ¹⁵N values of the trees and the soil did not change between landscape positions suggesting that if the leguminous trees fixed atmospheric N₂, the proportion of total N uptake was identical at all locations. We concluded that landscape positions have a significant effect on tree performance with sesbania remarkably adapted to the bottom slope, gliricidia to the mid-slopes and tephrosia fairing similar in both the bottom slope and mid-slopes.     

Relative contribution of trees and crops to soil carbon content in a parkland system in Burkina Faso using variations in natural 13C abundance

The origin of organic matter was studied in the soils of a parkland of karité (Vitallaria paradoxa C.F. Gaertn) and néré (Parkia biglobosa (Jacq.) Benth.), which is extensively cultivated without the use of fertilisers. In such systems, fertility (physical, chemical and biological) gradients around trees have been attributed by some authors to a priori differences in fertility, allowing for better tree establishment on richer sites. In reverse, other workers believed that these gradients are due to the contribution of trees to the formation of soil organic matter through litter and decay of roots. Measurements of the variations in the ¹³C isotopic composition allowed for a distinction between tree (C₃) derived C and crop and grass (C₄) derived C in the total soil organic C content. The organic carbon contents of the soils were recorded under the two species at two soil depths and at five distances going from tree trunk to the open area and their C isotopic signatures were analysed. The results showed that soil carbon contents under karité (6.43 ± 0.45 g kg⁻¹) and néré (5.65 ± 0.27 g kg⁻¹) were significantly higher (p<0.01) than in the open area (4.09 ± 0.26 g kg⁻¹). The δ¹³C of soil C was significantly higher (p<0.001) in the open area (−17.5 ± 0.3‰) compared with the values obtained on average with depth and distance from tree under karité (−20.2 ± 0.4‰) and néré (−20.1 ± 0.4‰). The C₄-derived soil C was approximately constant, and the differences in total soil C were fully explained by the C₃ (tree) contributions to soil carbon of 4.01 ± 0.71, 3.02 ± 0.53, 1.53 ± 0.10 g kg⁻¹, respectively under karité, néré and in the open area. These results show that trees in parklands have a directly positive contribution to soil carbon content, justifying the need to encourage the maintenance of trees in these systems in semi-arid environments where the carbon content of soil appears to be the first limiting factor for crop growth.     

Nutrient dynamics through fine litterfall in three plantations in Sabah,Malaysia, in relation to nutrient supply to surface soil

To investigate soil amelioration effects by older tropical fast-wood plantations, we examined the fine litterfall and accompanying nutrient flux of a 20-year-old Acacia mangium site over 3 years under a wet tropical climate in Sabah, Malaysia. The litterfall of a Swietenia macrophylla site and an Araucaria cunninghamii site was also examined for comparison. Annual nitrogen (N) flux through litterfall (kg N ha⁻¹) was larger in A. mangium (207–223) than in S. macrophylla (126–153) or A. cunninghamii (72–94), whereas annual phosphorus (P) flux through litterfall (kg P ha⁻¹) was considerably smaller in A. mangium (2.7–3.4) than in S. macrophylla (7.5–15.6) or A. cunninghamii (7.8–9.2). N flux through litterfall, forest floor N, and N concentration in topsoil (0–5 cm) were in the order of A. mangium > S. macrophylla > A. cunninghamii, but other element fluxes were not related to concentrations in soils. Our findings suggest that topsoil N increased because of a large N flux from litterfall. We conclude that these plantation trees, including A. mangium have the potential to produce a N flux in litterfall for the rapid return of organic N to soils larger than or equivalent to that in adjacent primary forests. However, the litterfall of a single species may lead to deficits of a particular element and cause nutrient imbalances. Using a mixture of fertilizer tree species or applying mixed litter might be a better solution.     

Litter decomposition, microbial biomass and activity of soil organisms in three agroforestry sites in central Amazonia

Soil organisms play a central role in the decomposition of organic matter. The activity of soil organisms was comparatively examined in three experimental sites in central Amazonia (Brazil): a peach palm monoculture (Bactris gasipaes) a, rubber tree plantation (Hevea sp.), and an agroforestry system (four tree species planted in rows, the space between covered by upcoming secondary vegetation). The overall decomposition rates in the systems and the role of different groups of soil organisms (macrofauna, mesofauna, microflora) were studied with leaf litter (Vismia guianensis) enclosed in litter bags. Microbial respiration and biomass (SIR method) in litter and soil were measured (IRGA). Microbial respiration in all sites decreased in the gradient litter > topsoil (0–5 cm) > soil at 5–15 cm. The highest decomposition rate was always observed in the litter bags of coarse mesh size, pointing to the crucial role of the macrofauna in maintaining a high decomposition rate of the organic material in all systems. The Hevea (k = 3.4) and the Bactris plantation (k=3.1) both showed the highest decomposition rates, followed by the polyculture system (k=1.9). The Bactris plantation also had the highest level of microbial respiration and biomass in litter and soil. We discuss these findings in the light of data on rainfall, pH and canopy closure. They suggest that microclimate is a more important factor than biomass in determining litter decomposition rates and activity of soil organisms at these sites.     

Shrubs affect soil nutrients availability with contrasting consequences for pasture understory and tree overstory production and nutrient status in Mediterranean grazed open woodlands

Increases of woody plant cover in former savannas and managed savanna-like systems are currently considered one of the major components of global environmental change. Besides, shrub encroachment is frequently recommended as a practical tool to ensure tree regeneration. However, there is little evidence that this process affects neighboring plants??tree and pasture??production and nutrient status in Mediterranean open woodlands through the modification of soil properties. The effect of two contrasting, in terms relevant to nutrient cycling, shrub species, Cistus ladanifer and Retama sphaerocarpa, were investigated. Cistus, a shallow rooted shrub, reduced top-soil water content and enhanced P and K but depleted N and Mg availability under its canopy. It worsened consistently both tree overstory and pasture understory production and nutrient content. By contrast, Retama, a deep-rooted, N₂-fixing shrub, increased top-soil water content and N and Mg availability, but diminished P availability. In addition, it affected distinctively to pasture (facilitation) and trees (competition) biomass production. Overall, pasture production, irrespective of the habitat, was mainly limited by P, whereas trees growing with Cistus were limited by Ca and N and those trees growing with Retama by P availability. Our results showed that shrubs are capable to affect consistently both pasture and tree production and nutrient status through the modification of soil nutrient availability. However, these effects depended on specific shrub-understory and shrub-overstory combinations, which have implications for managing grazed open woodlands.     

Nutrient recycling potential in rice–vegetable cropping sequences under in situ residue management at mid-altitude subtropical Meghalaya

Low native soil nitrogen (N) and very low phosphorus (P) coupled with apathy of farmers towards use of fertilizer are the major constraints limiting rice productivity in the North Eastern Hill (NEH) Region of India. Field experiment was undertaken in narrow valley wetland during Kharif and pre-Kharif (rainy) seasons of 2003, 2004, and 2005, respectively, at Umiam (950 m MSL), Meghalaya to evaluate the nutrient recycling and production potential of rice (Oryza sativa L.)—vegetables cropping sequences under low input in situ residue management under rainfed conditions. After rice, five vegetable crops, viz. tomato (Lycopersicon esculentum Mill), potato (Solanum tuberosum L.), frenchbean (Phaseolus vulgaris), cabbage (Brassica oleracea L. var. capitata), and carrot (Daucas carota L.) were grown during pre-kharif season on temporary raised and sunken beds. Minimum tillage was given to both rice and vegetables. No external input including fertilizer, organic manure, pesticides, etc. were applied to either crops. Only the economic parts of the crops were harvested and the rest of the biomass of the entire crop and weed residues were chopped and incorporated into the soil. Among the five cropping sequences, rice–tomato gave the highest rice equivalent yield (214.40 q/ha), followed by rice–carrot (206.4 q/ha). Highest net return (rupees [Rs.] 66,635 ha⁻¹) was recorded in rice–carrot, closely followed by rice–tomato (Rs. 66,139 ha⁻¹). Rice–cabbage and rice–frenchbean were found to be on a par with each other in terms of yield and income. The highest amount of NPK recycling was recorded for the rice–potato sequence. Weed biomass production in the different sequences ranged from 37.5 q/ha for rice–tomato to as high as 50.6 q/ha in the case of rice–fallow. Soil fertility in terms of available NPK status was found to be stable in all the crop sequences except rice–cabbage, where fertility status declined slightly. The soil biological properties such as population of Rhizobium, bacteria, phosphorus-solubilizing microorganisms, and earthworm activity were all found to be remarkably higher in the experimental field compared to other adjacent plots that are managed inorganically. It was concluded that vegetables such as tomato, carrot, potato, etc. could be profitably grown after rice without the addition of fertilizer and manure, if residues are managed effectively under temporary raised beds without deteriorating soil fertility.     

Moisture conservation and nitrogen recycling through legume mulching in rainfed maize (<Emphasis Type="Italic">Zea mays</Emphasis>)–wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) cropping system

Mulching with vegetative materials is a highly beneficial and widely-investigated agro-technique in rainfed areas but the adoption of this practice has been constrained due to non-availability of mulch biomass locally. Live mulching with fast-growing annual green manure legumes like sunnhemp (Crotalaria juncea) or prunings of Leucaena leucocephala grown as hedge rows can be done for moisture conservation as well as nutrient cycling in the maize–wheat cropping system, which is predominantly followed in the high rainfall sub-mountainous region of north-western India. A field experiment was conducted at Selakui, Dehradun during 2000–2004 to study the effect of legume mulching, viz. in situ grown sunnhemp and Leucaena prunings, along with varying N levels, viz. 0, 30, 60 and 90 kg N ha⁻¹ (to maize), and 0, 40 and 80 kg N ha⁻¹ (to wheat) on productivity, soil moisture conservation and soil physico-chemical properties. Intercropped sunnhemp added 0.75–1.45 t dry matter and 21.6–41.3 kg N ha⁻¹ at 30–35 days, while Leucaena twigs added 1.89–4.15 t dry matter and 75.2–161.3 kg N ha⁻¹ at 60–65 days of maize growth. Live mulching with sunnhemp or Leucaena biomass improved soil moisture content at maize harvest (+1.15–1.57%) and crop productivity by 6.8–8.8% over no mulching. Combined use of both the mulching materials was more effective in improving the soil moisture content (+2.08–2.29%) and grain yield (15.1%) over their single application. Response of maize to N fertilizer application was significant up to 90 kg N ha⁻¹, and it was relatively more pronounced under the mulching treatments. Residual effect of mulching on wheat showed an increase in yield of 10.2% with sunnhemp or Leucaena, and 27.9% with sunnhemp + Leucaena. There was an improvement in organic C and total N status of soil, and a decrease in bulk density associated with an increase in infiltration rate due to mulching at the end of 4 cropping cycles. It was concluded that legume mulching is a highly beneficial practice for enhanced moisture and nutrient conservation, leading to increased productivity and soil health of maize–wheat cropping system under Doon valley conditions of north-western India.     

Forest understory biomass heterogeneity is “moisture complex” or associated litter the cause?

Understory biomass heterogeneity underPlatanus occidentalis L.,Quercus alba L.,Celtis occidentalis Pursh., andUlmus americana L. was studied. Soil pH, Ca, Mg, Mn, Cu, NH₄ nitrogen, total N, and soil moisture were significantly different under all dominant species. The pattern of soil moisture levels under different tree species was not consistent from one sampling period to the next. The aboveground biomass of understory vegetation also varied significantly under different tree species and was not correlated with variation in any of the soil properties. Maximum understory biomass gain (340%) from May to September occurred under oak trees, where soil moisture and most nutrients were the lowest. On the other hand, sycamore and hackberry had continuous release of allelochemics and the smallest understory biomass gain (103%) during the growing season, even though soil under both species had more moisture and nutrients than the soils under white oak. Eliminating all the factors studied leads to the conclusion that organic substances released in the immediate environment of dominant trees and their litter influenced soils and associated herb growth. In each dominant niche system, understory species may have to develop their own system (microniche) to minimize the stresses of competition and allelochemics.     

Legume cover crops and mulches: effects on nitrate leaching and nitrogen input in a pepper crop (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.)

There is not sufficient knowledge concerning the risks involved in NO₃–N leaching in relation to the use of cover crops and mulches. A 2 year field experiment was carried out in a pepper (Capsicum annuum L.) crop transplanted into different soil management treatments which involved the addition of mulch of three different types of winter cover crops (CC) [hairy vetch (Vicia villosa Roth.), subclover (Trifolium subterraneum L.), and a mixture of hairy vetch/oat (Avena sativa L.)], and an un-mulched plot. At the time of CC conversion into mulch, the hairy vetch/oat mixture accumulated the highest aboveground biomass (5.30 t ha⁻¹ of DM), while hairy vetch in pure stand accumulated the highest quantity of N (177 kg ha⁻¹) and showed the lowest C/N ratio (12). The marketable pepper yield was higher in mulched than in conventional (on average 33.5, 28.9, 27.7 and 22.2 t ha⁻¹ of FM for hairy vetch, subclover, hairy vetch/oat mixture, and conventional, respectively). Generally, the NO₃–N content of the soil was minimum at CC sowing, slightly higher at pepper transplanting and maximum at pepper harvesting (on average 15.2, 16.8, and 23.3 mg NO₃-N kg⁻¹ of dry soil, respectively). The cumulative leachate was higher during the CC period (from October to April) than the pepper crop period (from April to September), on average 102.1 vs 66.1 mm over the years, respectively. The cumulative NO₃–N leached greatly depended on the type of mulch and it was 102.3, 95.3, 94.7, and 48.2 kg ha⁻¹ in hairy vetch, subclover, hairy vetch/oat mixture, and conventional, respectively. A positive linear correlation was found between the N accumulated in the CC aboveground biomass and the NO₃–N leached during pepper cultivation (R ² = 0.87). This research shows that winter legume cover crops, especially hairy vetch in pure stand, converted into dead mulch in spring could be used successfully for adding N to the soil and increasing the yield of the following pepper crop although the risks of N losses via leaching could be increased compared to an un-mulched soil. Therefore when leguminous mulches are used in the cultivation of a summer crop, appropriate management practices of the system, such as a better control of the amount of irrigation water and the cultivation of a graminaceous or a cruciferous catch crop after the harvesting of the summer crop, should be adopted in order to avoid an increase in NO₃–N leaching.     

Population dynamics of mixed indigenous legume fallows and influence on subsequent maize following mineral P application in smallholder farming systems of Zimbabwe

Developing soil fertility management options for increasing productivity of staple food crops is a challenge in most parts of Sub-Saharan Africa, where soils are constrained by nitrogen (N) and phosphorus (P) deficiencies. A study was conducted to evaluate the response of indigenous legume populations to mineral P application, and subsequently their benefits to maize yield. Mineral P was applied at 26 kg P ha⁻¹ before legume species were sown in mixtures at 120 seeds m⁻² species⁻¹ and left to grow over two rainy seasons (2 years). Application of P increased overall biomass productivity by 20–60% within 6 months, significantly influencing the composition of non-leguminous species. Dinitrogen fixation, as determined by the N-difference method, was increased by 43–140% although legume biomass productivity was apparently limited by nutrients other than P and N. Crotalaria pallida and C. ochroleuca accounted for most of the fixed N. Improved N supply increases the abundance of non-leguminous species, particularly Conyza sumatrensis and Ageratum conyzoides. However, abundance of common weed species, Commelina benghalensis, Richardia scabra and Solanum aculeastrum, declined by up to18%. Application of P did not significantly influence productivity of those legume species that reached maturity within 3 months. There was increased N₂-fixation and biomass productivity of indifallows as influenced by specific legume species responding to P application. Compared with natural (grass) fallows, indigenous legume fallows (indifallows) increased subsequent maize grain yields by ~40%. Overall, 1- and 2-year indifallows gave maize grain yields of >2 and 3 t ha⁻¹, respectively, against <1 t ha⁻¹ under corresponding natural fallows. Two-year indifallows with P notably increased maize yields, but the second year gave low yields regardless of P treatment. Because of their low P requirement, indigenous legume fallows have potential to stimulate maize productivity under some of the most nutrient depleted soils.     

Effects of beef cow winter feeding systems, pen manure and compost on soil nitrogen and phosphorous amounts and distribution, soil density, and crop biomass

A field experiment was conducted on continuous barley to evaluate the effects of 3 beef cattle winter feeding systems (bale grazing (BG); swath grazing (SG); straw–chaff grazing (STCH)) and pen manure and compost application on soil N and P amounts and distribution, soil density and barley crop biomass. Cattle winter feeding systems were managed during the winter of 2005–2006. Effects of extensive winter feeding system on soil nutrients and soil density were determined in the spring of 2006 after winter feeding. Nitrate nitrogen (NO₃–N) amounts at the low slope position in the 0–15 cm depth were 53% higher (P < 0.10) on BG sites than STCH sites. This may be attributed to the larger concentration of feed, thus feed nutrients, in the BG wintering system. Phosphorus amounts on the BG wintering sites at high slope were 34% higher (P < 0.10) than amounts at the same slope on the SG or STCH sites. Soil density was 21% greater (P < 0.10) where cows BG compared to where cows grazed straw–chaff piles, indicating differences in soil strength and resistance to penetration by roots. Soil density decreased on compost and raw manure sites in comparison to where no manure was applied, thus validating the benefits of manure on soil structure. Crop biomass measured on BG sites was consistent with soil nutrients captured, resulting in a 15% greater (P < 0.10) total biomass compared to STCH and SG wintering sites. Soil nutrient and crop biomass distribution was consistent among winter feeding sites with the STCH sites having the most uniform distribution of nutrients and crop biomass, and the BG sites having the least. Managing manure nutrients from winter feeding systems can be beneficial when followed by an annual cropping system.     

Soil and vegetation carbon pools in a mountainous watershed of Nepal

Assessment of carbon stocks in vegetation and soil is a basic step in evaluating the carbon sequestration potential of an ecosystem. We collected soil (core and composite) samples from 0–10, 10–20, 20–40, and 40–70 cm depths, or down to the bed rock, in the soil profile of four types of forest (managed dense Shorea (DS), degraded forest (DF), pine mixed (PS), and Schima–Castanopsis (SC) forest) and two types of cultivated land (irrigated low land (Khet) and rain-fed upland (Bari)) in the Pokhare Khola watershed of Nepal. In addition to other essential properties, soil bulk density and carbon concentration were assessed. Fine roots were also collected from each sampling site. The biomass of standing trees and shrubs was estimated by using allometric relationships after measuring their diameter and height, while the biomass of grasses was estimated by a direct measurement of grass from a defined area. The carbon stocks in all forest vegetation (trees, shrubs, and ground grass) and in the soil profiles under different land uses were estimated. The vegetation carbon pool was largest in DS forest (219 ± 34 Mg ha⁻¹) and least in SC forest (36 ± 5 Mg ha⁻¹), while its order among forest types was DS > DF > PS > SC. The soil organic carbon (SOC) pool was largest in Bari land (15.7 ± 1.5 kg C m⁻²) and least in PS forest (6.2 ± 0.5 kg C m⁻²) but the overall order among land uses was Bari > DF > Khet > SC > DS > PS. The total SOC stock in the whole watershed was 59 815 Mg, of which 36, 32, and 32% were in the 0–20, 20–40, and >40 cm soil depths, respectively. In the surface layer (0–10 cm), SOC stock was highest in Bari (36%) followed by DS (31%), and least was in PS forest (3%). This distribution pattern can primarily be assigned to SOC concentration and area covered by these land uses.     

Phosphorus uptake during four years by different vegetative cover types in a riparian buffer

Vegetative buffers have been shown to reduce nutrient loss associated with the transport of detached soil particles and may through plant uptake offer a means to capture dissolved nutrients moving to surface waters through the soil solution. The objective of this 4-year study was to evaluate changes in the biomass and P content of the roots and shoots of plants growing in a multi-species versus a single species riparian buffer as an index of P capture potential. Periodic harvests of above ground vegetation were combined with root cores to estimate the total standing biomass and the pool of P in plant tissue in three vegetative cover types dominated by either switchgrass (Panicum virgatum L.), an alfalfa (Medicago sativa L.)-smooth bromegrass (Bromis inermis Leyss) mix, or a fast growing superior cottonwood (Populus deltoids Bartr., clone 42-7). An existing stand of smooth brome served as the single species control. Standing biomass increased in all three cover types during the 4 years of study, with the greatest increases observed in the cottonwood (2345 g m⁻²) and switchgrass (1818 g m⁻²). Biomass production in the smooth brome control did not change during the study period. Based on the 4th-year samples, standing pools of P closely paralleled total plant biomass and root surface area with cottonwood accumulating the greatest amount of P at 19.4 g m⁻² compared to 4.3 g m⁻² for the smooth brome control. Estimates of potential P export via biomass harvest from a mixed buffer over a 4-year interval were 101 kg ha⁻¹ compared to 62 kg ha⁻¹ for the smooth brome control; a 63% increase in export capacity due largely to the inclusion of cottonwood. Addition of a fast growing woody species combined with periodic biomass harvests has the potential to reduce P movement to surface waters.     

Effects of trees and understorey management on soil fertility and nutritional status of holm oaks in Spanish dehesas

A dehesa is a multipurpose agroforestry system with scattered oaks. This study focuses on the influence of trees on soil chemical fertility and on the consequences of dehesa land use for soil fertility and nutritional status of trees. To achieve that we have studied the content of several nutrients in soils and tree leaves in four Spanish dehesas, considering three types of land use: scattered holm oak with an understorey of either native grasses (G), shrubs (E) or crop (C: either fertilized or unfertilized). We randomly selected six oaks per farm and land use for soil and tree leaf sampling. Soil samples were taken at five distances from each tree trunk (2, 5, 10, 15, and 20 m) at a depth of 0–30 cm. SOM, CEC, total and mineral-N, available-P, and exchangeable Ca, Mg and K were analysed. Foliar samples were collected from every tree in two consecutive years (2002 and 2003) at leaf maturity. The total content of N, P, K, Ca, and Mg was determined. Soils of C plots showed significantly higher contents for most of the nutrients studied than G ones. Cropping also affected trees positively, with higher foliar contents of N and K in C plots than in G ones. Foliar content did not decrease for any nutrient, and tree-crop interaction is interpreted as a mechanism of complementarity in terms of nutrient use. Shrub encroachment led to a higher content of organic-N and exchangeable cations in soils, but to a decrease in mineral N and available-P. The tree foliar contents of N, Mg, and Ca were lower in E plots than in G plots, probably as a consequence of the tree-shrub competition for those nutrients. By contrast, P content of tree leaves increased significantly in E plots compared to G plots.     

Comparative short-term effects of different quality organic resources on maize productivity under two different environments in Zimbabwe

Major challenges for combined use of organic and mineral nutrient sources in smallholder agriculture include variable type and quality of the resources, their limited availability, timing of their relative application and the proportions at which the two should be combined. Short-term nutrient supply capacity of five different quality organic resources ranging from high to low quality, namely Crotalaria juncea, Calliandra calothyrsus, cattle manure, maize stover and Pinus patula sawdust were tested in the field using maize as a test crop. The study was conducted on two contrasting soil types at Makoholi and Domboshawa, which fall under different agro-ecological regions of Zimbabwe. Makoholi is a semi-arid area (<650 mm yr⁻¹) with predominantly coarse sandy soils containing approximately 90 g kg⁻¹ clay while Domboshawa (>750 mm yr⁻¹) soils are sandy-clay loams with 220 g kg⁻¹ clay. Each organic resource treatment was applied at low (2.5 t C ha⁻¹) and high (7.5 t C ha⁻¹) biomass rates at each site. Each plot was sub-divided into two with one half receiving 120 kg N ha⁻¹ against zero in the other. At Makoholi, there was a nine-fold increase in maize grain yield under high application rates of C. juncea over the unfertilized control, which yielded only 0.4 t ha⁻¹. Combinations of mineral N fertilizer with the leguminous resources and manure resulted in between 24% and 104% increase in grain yield against sole fertilizer, implying an increased nutrient recovery by maize under organic–mineral combinations. Maize biomass measured at 2 weeks after crop emergence already showed treatment differences, with biomass yields increasing linearly with soil mineral N availability (R ² = 0.75). This 2-week maize biomass in turn gave a positive linear relationship (R ² = 0.82) with grain yield suggesting that early season soil mineral N availability largely determined final yield. For low quality resources of maize stover and sawdust, application of mineral N fertilizer resulted in at least a seven-fold grain yield increase compared with sole application of the organic resources. Such nutrient combinations resulted in grain harvest indices of between 44% and 48%, up from a mean of 35% for sole application, suggesting the potential of increasing maize productivity from combinations of low quality resources with mineral fertilizer under depleted sandy soils. At Domboshawa, grain yields averaged 7 t ha⁻¹ and did not show any significant treatment differences. This was attributed to relatively high levels of fertility under the sandy-clay loams during this first year of the trial implementation. Differences in N supply by different resources were only revealed in grain and stover uptake. Grain N concentration from the high quality leguminous resources averaged 2% against 1.5% from sawdust treatments. We conclude that early season soil mineral N availability is the primary regulatory factor for maize productivity obtainable under poor sandy soils. Maize biomass at 2 weeks is a potential tool for early season assessment of potential yields under constrained environments. However, the likely impact on system productivity following repeated application of high N-containing organic materials on different soil types remains poorly understood.     

Effects of soil and crop management practices on yields,income and nutrients losses from upland farming systems in the Middle Mountains region of Nepal

On-farm runoff plots were established during 2004 and monitored for 4 years in the Pokhare Khola watershed (Nepal) in a completely randomized design with four replications of each three treatments: traditional Farmer Practice (FP) (Zea mays–Eleusine coracana), Reduced Tillage (RT; Z. mays–Vigna ungeuculata), and Commercial Vegetable with double dose of farm yard manure (CV; Z. mays–Capsicum species) to evaluate treatment effects on soil nutrient losses, nutrient balances and crop income on Bari land (rainfed terraces). Nutrient removal due to crop harvest was found to be significantly higher than nutrient loss through soil erosion, and CV treatment exhibited a significantly higher N uptake (123 kg ha⁻¹ year⁻¹) through crop harvest than other treatments. Moreover, the CV treatment produced significantly higher income per unit area of Bari land than the other treatments. Soil organic carbon and major nutrients losses (NPK) through soil erosion were minimal [25.5 kg ha⁻¹ year⁻¹ soil organic carbon (SOC) and 5.6:0.02:0.12 kg ha⁻¹ year⁻¹ nitrogen (N), phosphorus (P), potassium (K), respectively]. Result showed that no nutrients were lost through leaching. Nutrient losses due to soil erosion and runoff were lower than previously reported in the Middle Mountain region, indicating a need to re-evaluate the soil erosion and nutrient loss problems in this region. Interventions such as reduced tillage and double dose of FYM with vegetable production were found to be effective in maintaining soil fertility and increasing farm income compared to the traditional maize-millet production system. The nutrient balance calculations suggest that integrated nutrient management techniques such as residue incorporation and application of FYM with a minimum application of chemical fertilizer are potentially sustainable production approaches for the Mid-hills of Nepal.     

Chemical Nature and Diversity of Phosphorus in New Zealand Pasture Soils Using <Superscript>31</Superscript>P Nuclear Magnetic Resonance Spectroscopy and Sequential Fractionation

Information on the phosphorus (P) forms of pasture soils is central to agricultural and environmental sustainability. We used a combination of ³¹P nuclear magnetic resonance (NMR) spectroscopy of NaOH–EDTA extracts and sequential fractionation to investigate P forms, with an emphasis on organic P in relation to environmental and biophysical variables, in 24 diverse pasture soils taken from around New Zealand (organic C 19–102 g kg⁻¹, total P 116–2746 mg kg⁻¹, pH 5.2–7.0). Soils were separated by cluster analysis of soil physicochemical properties and sequentially extracted P pools into those either derived from volcanic-ash materials or not. This separation was also evident for organic P species in NaOH–EDTA extracts, which removed about 75% of total soil organic P. The major organic P compound class was monoesters (24–60% of extracted P), made up of 14 to 91% myo-inositol hexakisphosphate. The next largest organic P class was diesters (0–4% of P extracted), which were enriched in volcanic-ash soils (monoester to diester ratio = 14) compared to non volcanic-ash soils (ratio = 30). Correlation analysis indicated that mean annual temperature had a significant negative and positive effect on monoester and diester concentrations, respectively. This was attributed to better physical protection of monoesters (especially inositol phosphates) from mineralization, and increased diesters from microbial activity and biological turnover. The anomalous behaviour of volcanic-ash soils was attributed to slow mineralization and the sampling of soils at different times of year without full knowledge of the large and highly dynamic microbial biomass P pool which is predominantly diesters.     

Long-term residual effect of short-term fertilizer application on Ca,N and P concentrations in grasses <Emphasis Type="Italic">Nardus stricta</Emphasis> L. and <Emphasis Type="Italic">Avenella flexuosa</Emphasis> L.

The resilience (the ability of an ecosystem to recover after termination of perturbation) of plant species composition after short-term fertilizer application can take decades in sub-alpine grassland, but little attention has been paid to the resilience of nutrient concentrations in the biomass of individual plant species. In 2004, an abandoned experiment in which phosphorus, nitrogen and calcium had been applied from 1965 to 1967 was identified in the Giant Mts. (Krkonoše/Karkonosze, Czech Republic). The biomass of two dominant grasses, Nardus stricta and Avenella flexuosa, was analyzed for Ca, N and P concentrations 37 years after the last fertilizer application. In treatments with P application, the P concentration was still significantly increased in both species. The N concentration was higher in treatments with N or Ca application and the lowest in P treatments. The N:P ratio ranged from 7.7 to 16.6 and from 6.2 to 16.3 in N. stricta and A. flexuosa, respectively, and was lowest in P treatments where A. flexuosa predominated. N. stricta dominated in treatments where the biomass N:P ratio was higher than 13, whereas lower ratios were more favourable for A. flexuosa. In the case of N. stricta, the Ca concentration was increased in Ca treatments. Ca and P concentrations in N. stricta biomass were significantly positively correlated with soil plant available P and Ca concentrations, but this was not recorded for A. flexuosa. In environments with different P availability, the competitive ability of investigated species was predetermined by the N:P ratio in their biomass. As in the case of plant species composition and soil chemical properties, the resilience of Ca, N and P concentrations in the biomass of individual species can take decades in sub-alpine grassland.