Natural Regrowth Of Vegetation Research Articles

Comparing the climate change mitigation potentials of alternative land uses: Crops for biofuels or biochar vs. natural regrowth

Using biomass from dedicated crops for energy production and natural vegetation regrowth are key elements in future climate change mitigation scenarios. However, there are still uncertainties about the mitigation potentials that can be achieved by the different land-based systems and how they perform relative to each other. In this study, we use harmonized future land use datasets to identify global land areas dedicated to second generation bioenergy crop production in 2050 under different climate scenarios. We then assess the global climate change mitigation potentials of using biomass for producing bioethanol with (BECCS) or without carbon capture and storage, biochar, or a synthetic fuel (e-methanol). For the latter, the electricity required to produce hydrogen for e-methanol synthesis is sourced from either wind power or the projected average electricity mix in 2050. Mitigation potential from natural regrowth on the identified land is also quantified. For all the cases, we modelled emissions of greenhouse gases from the life-cycle stages and use parameterized models to estimate local biomass growth rates. The identified land areas range from 1.95 to 13.8 million hectares and can provide from 30 to 178 mega ton (Mt) dry biomass annually from dedicated crops. Climate change mitigation potentials range from 11 to 257 MtCO2-eq. yr−1, depending on technological option and land availability. The largest mitigation is delivered by BECCS, but e-methanol can achieve similar findings when hydrogen is sourced from wind power. If hydrogen is produced from grid electricity, e-methanol can result in net positive emissions. E-methanol can also deliver more final energy than bioethanol (4.04 vs. 1.27 EJ yr−1). Natural vegetation regrowth can generally achieve higher mitigation than bioethanol, but less than biochar. An optimal combination of BECCS and natural vegetation regrowth can achieve a larger mitigation, up to 281 MtCO2-eq. yr−1, indicating that integrated solutions can help to achieve successful land management strategies for climate change mitigation.

Climate change mitigation potentials of biofuels produced from perennial crops and natural regrowth on abandoned and degraded cropland in Nordic countries

Bioenergy expansion is present in most climate change mitigation scenarios. The associated large land use changes have led to concerns on how bioenergy can be sustainably deployed. Promising win-win strategies include the production of perennial bioenergy crops on recently abandoned cropland or on cropland prone to land degradation, as perennial crops typically reduce soil erosion rates. Natural vegetation regrowth is an alternative nature-based solution that can also co-deliver negative emissions and other environmental benefits. In this study, we explore the potential to deploy bioenergy crops in Nordic countries (Norway, Sweden, Finland, and Denmark) on abandoned cropland and on cropland threatened by soil erosion and compare the achievable climate change mitigation benefits with natural regrowth. We found 186 thousand hectares (kha) of abandoned cropland and 995 kha of cropland threatened by soil erosion suitable for bioenergy crop cultivation. The primary bioenergy potential in the region is 151 PJ (PJ) per year, corresponding to 67–110 PJ per year of liquid biofuels depending on biorefinery technology. This has a climate change mitigation potential from −6.0 to −17 megatons of carbon dioxide equivalents (MtCO2eq) per year over the first 20 years (equivalent to 14–40% of annual road transport emissions), with high-end estimates relying on bioenergy coupled to carbon capture and storage (BECCS). On the same area, natural regrowth can deliver negative emissions of −10 MtCO2eq per year. Biofuel production outperforms natural regrowth on 46% of abandoned cropland with currently available biorefinery technologies, 83% with improved energy conversion efficiency, and nearly everywhere with BECCS. For willow windbreaks, improved biorefinery technology or BECCS is necessary to ensure the delivery of larger negative emissions than natural regrowth. Biofuel production is preferable to natural regrowth on 16% of croplands threatened by soil erosion with the current biorefinery technology and on 87% of the land area with BECCS. Without BECCS, liquid biofuels achieve larger climate benefits than natural regrowth only when bioenergy yields are high. Underutilized land and land affected by degradation processes are an opportunity for a gradual and more sustainable bioenergy deployment, and local considerations are needed to identify case-specific solutions that can co-deliver multiple environmental benefits.

Carbon, Nitrogen, and Phosphorus Stocks from Fallow of Forage Legumes on Alfisols of Guinea Savanna Nigeria

The aim of this study was to assess soil carbon (C), nitrogen (N), and phosphorus (P) stocks from fallow of two forage legumes: Centrosema pascuorum (Cp) and Macrotyloma uniflorum (Mu) on Alfisols of Guinea Savanna, Nigeria. The study was conducted at the Institute for Agricultural Research (IAR) experimental field Samaru Zaria, Nigeria (2008 to 2009). Treatment consisted of 3 plots (Cp planted, Mu planted, and control - natural vegetation regrowth). Plot size was 5 m * 3 m = 15 m2 replicated three times for each treatment. Pre-experimental composite soil samples were taken with an auger at 0-15 depth. The soil samples collected were air-dried, grounded, sieved with a 2 mm sieve, and the less than 2 mm fraction was analysed for C, N, and P. The results of the analysis of soil organic C, total N, and available P were 6.1 gkg-1, 0.53 gkg-1, and 8.75 mgkg-1 respectively. Soil pH was 5.9. The plots were left fallow for one year and again soil samples were collected at 0-15 cm depth and analysed. The results showed that Cp significantly improved and had higher soil available P (13.74 mgkg-1) after one year followed by Mu (7.68 mgkg-1) and control (4.6 mgkg-1). On the other hand, the control plots significantly had highest soil organic C (5.9 gkg-1) compared to 5.2 and 3.7 gkg-1 from Cp and Mu. Similarly, higher total N (2.9 gkg-1) was recorded from control plots compared to 1.4 and 0.5 gkg-1 from Mu and Cp respectively. Results from this study indicate that one year fallow of cultivated Cp has potential to improve soil available P compared to Mu and natural vegetation regrowth. In terms of plant nutrient uptake, the N and P content of Mu was the highest (4.28 and 2.65 % respectively). The natural vegetation from the control plots had the lowest (1.95 %) N concentration. There was no significant difference in the P content of the natural vegetation from the control plots and Cp (1.92 and 1.84 % respectively). Fallow periods of more than a year of cultivated Centrosema pascuorum and Macrotyloma uniflorum or their incorporation into soils are suggested for further studies.

Variability of soil surface characteristics in a mountainous watershed in Valle del Cauca, Colombia: Implications for runoff, erosion, and conservation

Understanding catchment sediment or solute transport frequently relies on understanding of soil nutrient conditions and physical properties. This study investigates hydropedological patterns in a tropical catchment by understanding soil nutrient and soil surface changes. Soil nutrient concentrations and hydraulic properties were measured from the La Vega micro watershed in the southwestern Colombian Andes at 16 distributed locations in four elevation ranges (between 1450 and 1700 m a.s.l.). The site is a part of a conservation partnerships which implements programs and monitor impacts. Soil samples were analyzed for total nitrogen (TN), Bray II-available phosphorus, exchangeable cations, pH, organic matter, and texture. Soil hydraulic conductivities at two depths (0–5 cm and 5–10 cm) were determined in conservation implementation areas (enclosures and natural regrowth). In the upper elevation range, regrowth of natural vegetation was found on deep soils (∼3 m) with moderate infiltration (26 cm hr−1), the lowest bulk density (0.92 g cm−3), and the highest TN (0.4%). The lowest elevation (mixed land use of grazing and riparian forests with deep profiles) had the lowest infiltration (4 cm hr−1), highest bulk density (1.02 g cm−3), and the lowest TN (0.26%). In the middle elevation ranges, conserved tropical forest vegetation were located on shallow soil depths with high organic matter (∼6%) and high infiltration (86 cm hr−1). The lowest infiltration rate average (2.3 cm hr−1) exceeded the estimated erosive regional precipitation intensity (∼2.5 cm hr−1) about 60% of the time, while the median infiltration rate (10 cm hr−1) exceeded rainfall intensities 94% of the time, indicating that infiltration excess and saturation excess runoff mechanisms are both present. Coupling data with sediment concentration and solute concentration patterns can help discern correlations between scales and will help to monitor effectiveness of conservation programs aimed at sustaining ecosystem services.

A comparison of the effects of natural vegetation regrowth with a plantation scheme on soil structure in a geological hazard‐prone region

The effects of natural vegetation regrowth and plantation schemes on soil aggregate structure and total soil porosity were examined at a typical landslide hazard site in a semi‐humid region in northwest China. The schemes were started in 2008 using four plots, two of which were kept bare; in one natural local vegetation was allowed to grow and in the other planting was managed. Analysis of soil samples (0–100 cm) taken in 2016 indicated that the proportion of soil aggregates <0.25 mm was 43.22 and 29.01% under natural vegetation and plantation, respectively, which was significantly greater than that in the plots kept bare (23%). Soil tensile stress increased from approximately 12 kPa in the bare plots to 18.34 and 16.45 kPa under natural vegetation and plantation, respectively. Total soil porosity, water retention and rate of infiltration increased, whereas bulk density decreased considerably, after 8 years under natural vegetation and plantation in comparison with those properties in the bare plots. Furthermore, natural vegetation had a greater effect than the plantation on soil structure in terms of aggregation and total porosity. The findings of this study provide some insight into the effects of native vegetation and a plantation on soil structure in semi‐humid regions as a prerequisite to the control of both soil erosion and slope stability in regions prone to seismic activity.Highlights Soil structure was analysed after 8 years of natural vegetation regrowth and plantation. Eight years of revegetation improved aggregation and total porosity compared with bare land. Natural vegetation affected soil structure more than the plantation did. Our results provide some insight into a prerequisite to control soil erosion and slope stability in unstable regions.

A riverbank erosion control method with environmental value

Between 1992 and 1998, 55km of riverbank on the Ottawa River was protected from erosion using rounded granular material of glaciofluvial origin from local gravel pits. The method did not require slope reprofiling or the installation of a key or geotextile. It allows for the slopes of the structure and the banks to reach an equilibrium state and relies on natural vegetation regrowth on the banks and embankments rather than bioengineering revegetation techniques. A follow-up was done in 2011 to assess its effectiveness. After fifteen years, the structures have halted erosion, blended into the local landscape and created a riparian ecotone. They quickly acquired a natural, gently sloping profile and recreated a sinuous shoreline dotted with sandy beaches. The follow-up also shows that the banks and structures are 80–90% covered by indigenous vegetation. Furthermore, this vegetation is as diverse as the natural vegetation seen on the unstabilized sections of riverbank. The work minimized damage to the riverbanks as it was done in winter so that shore ice could be used for access. This approach helped avoid interventions on the banks themselves and protected the existing vegetation. The availability of granular sources near the work site was a key criterion. Riverbank protection work of this scale is uncommon. This article discusses the effectiveness of the protection method and its environmental advantages and limitations. This method could prove to be an alternative to traditional rock-fill methods or bioengineering techniques, which are often quite complex and costly.

Effects of improved fallow with Sesbania sesban on maize productivity and Striga hermonthica infestation in Western Kenya

Striga hermonthica is a major constraint to smallholder subsistence agriculture production in the sub-Saharan African region. Low soil fertility and overall environmental degradation has contributed to the build-up of the parasitic weed infestation. Improved cropping systems have to be introduced to address the interrelated problems of S. hermonthica and soil fertility decline. Thus, the effects of improved fallow with leguminous shrub Sesbania sesban on maize yields and levels of S. hermonthica infestation on farm land in the bimodal highlands of western Kenya were investigated. The experimental treatments were arranged in a phased entry, and randomized complete block scheme were six months Sesbania fallow, 18 months Sesbania fallow, six months natural fallow consisting of regrowth of natural vegetation without cultivation, 18 months natural fallow, continuous maize cropping without fertilizer application, and continuous maize cropping with P and N fertilization. Results show that Sesbania fallows significantly (p<0.05) increase maize yield relative to continuous unfertilized maize. S. hermonthica plant populations decrease in continuous maize between the first season (mean = 428 000 ± 63 000 ha−1) and second season (mean=51 000 ± 15 000 ha−1), presumably in response to good weed management. S. hermonthica seed populations in the soil decrease throughout the duration of the experiment in the continuous maize treatments. Short-duration Sesbania fallows can provide modest yield improvements relative to continuous unfertilized maize, but short-duration weedy fallows are ineffective. Continuous maize cultivation with good weed control may provide more effective S. hermonthica control than fallowing.

Possibilities of discriminating tropical secondary succession in Amazônia using hyperspectral and multiangular CHRIS/PROBA data

CHRIS/PROBA data collected in the Brazilian Amazônia in 4 view angles (−36°, nadir, +36°, +55°) and 62 bands (410–1000nm range) were evaluated for the discrimination between primary forest and 3 stages of secondary succession after deforestation: initial (SS1; <5 years), intermediate (SS2; 5–15 years) and advanced (SS3; >15 years). Single view angle and multiangular approaches (nadir plus anisotropic information derived from reflectance ratios between view angles) were tested for discrimination. Both approaches used principal components analysis (PCA) applied to pixel spectra representative of each class in order to reduce data dimensionality at each dataset, to enhance separability between the classes, and to provide input variables for multiple discriminant analysis (MDA). The results showed that the off-nadir viewing improved discrimination between the successional stages. Discrimination between SS2 and SS3 was enhanced with PCA at +36° view angle. Primary forest and SS3 presented a more anisotropic behavior than SS2 and SS1, especially in the backward scattering direction (positive view angles) in which great amounts of sunlit canopy components were viewed by the sensor. MDA classification results showed that the multiangular approach produced an overall improvement in the discrimination. From the single (nadir) to the multiangular approach, classification accuracy using a separate set of pixels increased from 83.3% to 98.3% for SS1, 53.3% to 70.0% for SS2, and 58.3% to 76.7% for SS3. The nadir and multiangular classifications were statistically different at a 0.05% level of significance. Kappa statistics increased from 0.63 to 0.82. The results showed that multiangular data can improve the differentiation between primary forest and old stages of natural vegetation regrowth, which have been reported in the literature as the most difficult classes to be mapped in the Amazonian environment.

Soil properties and physiographic factors controlling the natural vegetation re-growth in a disturbed catchment of the Central Spanish Pyrenees

Large changes in land use have occurred in the Central Spanish Pyrenees during the twentieth century. This mountain area supported an intense human use since the Middle Ages, that greatly transformed the natural landscape. The land uses changed dramatically during the first half of the twentieth century, due to major socioeconomic forces which lead to depopulation and land abandonment. Since then, a process of natural vegetation recovery has taken place. The anthropogenic impact during centuries deteriorated the soil quality, threatening the sustainability of agroecosystems and the natural vegetation re-growth. In this study, a soil survey was performed to identify the soil types and the physico-chemical properties of the soil that are relevant for maintaining the soil function for the recovery of vegetation after land abandonment. The study was conducted in the Arnas River catchment, which is representative of the region. The highest cation exchange capacity (CEC), nutrients, water retention, water availability and infiltration rates, as well as the lowest pH and carbonates, occurred in Lithic Hapludolls and Calcic Haploxerolls at the shady aspect, under a forest cover. Lithic Ustochrepts and Lithic Xerorthents on the sunny aspect, which were covered by shrubs, were of lesser quality than were the deeper and better-developed soils on the shady aspect. This is seen as an evidence of different patterns of soil degradation, which conditioned the re-growth of natural vegetation after land abandonment. The results provide insights into the main soil factors that have affected the re-establishment of natural vegetation in recent years. Recommendations are given for designing effective strategies for soil conservation after land abandonment in similar mountain environments.

Bedload transport under different flow conditions in a human-disturbed catchment in the Central Spanish Pyrenees

This paper reports on bedload transport in a small catchment in the Flysh area of the Central Spanish Pyrenees. The Arnás experimental catchment, located in the headwaters of the Aragón River, is affected by natural vegetation regrowth, resulting from farmland abandonment. The main stream is totally covered with coarse and tabular-shaped particles ( D 50 = 100), a legacy of intense erosive activity decades ago. Discharge and suspended sediment transport were continuously measured at the outlet of the catchment. Bedload was retained in a trap located upstream of the flume. During most floods, this trap overfilled and the volume of the accumulated bed material was obtained from measurements made by means of a 3 m long profilometer. Twelve floods of different characteristics were studied, making it possible to estimate a critical discharge around 0.4 m 3 s − 1 . The occurrence of an “exceptional” flood event showed the important role of low-frequency floods in bedload transport. Results show that bedload reasonably depends on peak discharge and, to a lesser extent, on effective runoff. In the Arnás catchment, bedload is never more than 30% of the total sediment yield. This low value can be related to the characteristics of the bed particles and those of the channel slope, but also to a reduction in stream torrentiality, as a consequence of an increase in plant cover.

Impacts of constructing a rural cycle way on suspended sediment transport processes

This study reports on the short-term impact of constructing a rural cycle way on the suspended sediment load (SSL) of Golborne Brook, Cheshire, UK between August 2003 and February 2004. A float switch activated automatic water sampler retrieved 275 water samples during the 5 months following ground disturbance, which were analysed for their suspended sediment concentration (SSC). SSC vs. discharge ( Q) rating curves established for three phases of the disturbance were combined with a 10-min Q record to estimate SSLs before, during and after the work. A three-fold increase in SSL was observed 5 m downstream of where drainage ditches created alongside the cycle way entered Golborne Brook. There is strong evidence that the SSLs decreased to near their pre-disturbance levels within 5 months. Re-growth of natural vegetation on the roadside drainage ditches cleared during the construction work along with the build up of natural deciduous organic debris dams (consisting mainly of leaves, twigs and small branches from hedgerows alongside the ditches) at the ends of the drainage ditches some 1–2 m before they discharged into Golborne Brook contributed to the reduced SSLs observed downstream. Further research into the precise role played by these deciduous organic debris dams in trapping sediment is needed.

Soil amendments reduce trace element solubility in a contaminated soil and allow regrowth of natural vegetation

We tested the effects of three amendments (a biosolid compost, a sugar beet lime, and a combination of leonardite plus sugar beet lime) on trace element stabilisation and spontaneous revegetation of a trace element contaminated soil. Soil properties were analysed before and after amendment application. Spontaneous vegetation growing on the experimental plot was studied by three surveys in terms of number of taxa colonising, percentage vegetation cover and plant biomass. Macronutrients and trace element concentrations of the five most frequent species were analysed. The results showed a positive effect of the amendments both on soil chemical properties and vegetation. All amendments increased soil pH and TOC content and reduced CaCl 2-soluble-trace element concentrations. Colonisation by wild plants was enhanced in all amended treatments. The nutritional status of the five species studied was improved in some cases, while a general reduction in trace element concentrations of the aboveground parts was observed in all treated plots. The results obtained show that natural assisted remediation has potential for success on a field scale reducing trace element entry in the food chain.

Nd isotope signature of Holocene Baltic Mn/Fe precipitates as monitor of climate change during the Little Ice Age

Neodymium (Nd) isotope profiles were analyzed on two Baltic Mn/Fe precipitates (99/2 and TL1) from shallow water (20 m) of the Mecklenburg Bay. The age range of these Mn/Fe precipitates determined by 226Ra ex/Ba dating reaches from recent growth back to ∼4300 and 1000 yr BP, respectively. Over this time range, the Nd isotope composition varies from ε Nd (0) = −13.1 to −17.5 in the selected Baltic precipitates indicating substantial changes in the Nd isotope composition of the Baltic Sea. The lowest ε Nd values were recorded during the time interval of the Little Ice Age (LIA, AD ∼1350 to 1850). These minimum values indicate either an increase of the input of less radiogenic Nd from Scandinavian Archean-Proterozoic sources (ε Nd about −22) to the Baltic Sea or a decrease of the input of more radiogenic Nd from continental European sources (ε Nd about −12) and/or North Sea water (ε Nd about −10). Variations of both, erosive continental input and North Sea inflow may indicate a direct response of the Nd isotope signal in the Baltic Sea to climate changes during the LIA and be related to cyclic shifts in the atmospheric circulation triggered by the North Atlantic Oscillation (NAO). Another aspect that possibly influenced the input of trace elements and Nd isotopes into the Baltic Sea is the population development in the circum Baltic area during the LIA. The lowest ε Nd values also correspond to the medieval demographic crises that led to a significant decrease of agricultural activity and farmland. The reduction of soil erosion and enhanced regrowth of natural vegetation may have changed the amount and proportions of dissolved and suspended particulate matter transported into the Baltic Sea by rivers which in turn may have resulted in a change of the Nd isotope composition of Baltic Sea water.

Surface moisture and energy exchange from a restored peatland, Québec, Canada

Measurements of micrometeorological variables were made for two snow-free periods at a Restored vacuum harvested and non-restored (Comparison) section of a peatland in eastern Québec, Canada. Measurements of evapotranspiration and surface heat fluxes were obtained using an eddy correlation energy balance system at the Restored site and a combination Priestley–Taylor and lysimeter approach at the Comparison site. At the ecosystem scale, the energy, water and gas exchange processes are strongly coupled. Through harvesting, a peatland may lose most of its surface vegetation cover, altering the thermal regime of the peat, while the drier conditions required for the harvesting drastically alters the system's hydrology. The measurements indicate that the adopted restoration practices (blockage of drainage ditches and the spreading of a surface mulch layer) reduce the loss of water from the peat leading to the regrowth of natural vegetation at the Restored site. The Restored site lost approximately 13 and 8% less water to evapotranspiration than the Comparison site in 2000 and 2001, respectively.

Surface moisture and energy exchange from a restored peatland, Qu�bec, Canada

Measurements of micrometeorological variables were made for two snow-free periods at a Restored vacuum harvested and non-restored (Comparison) section of a peatland in eastern Québec, Canada. Measurements of evapotranspiration and surface heat fluxes were obtained using an eddy correlation energy balance system at the Restored site and a combination Priestley–Taylor and lysimeter approach at the Comparison site. At the ecosystem scale, the energy, water and gas exchange processes are strongly coupled. Through harvesting, a peatland may lose most of its surface vegetation cover, altering the thermal regime of the peat, while the drier conditions required for the harvesting drastically alters the system's hydrology. The measurements indicate that the adopted restoration practices (blockage of drainage ditches and the spreading of a surface mulch layer) reduce the loss of water from the peat leading to the regrowth of natural vegetation at the Restored site. The Restored site lost approximately 13 and 8% less water to evapotranspiration than the Comparison site in 2000 and 2001, respectively.

Effects of planted tree fallows on soil nitrogen dynamics, above-ground and root biomass, N2-fixation and subsequent maize crop productivity in Kenya

The effects of planted fallows of Sesbania sesban (L.) Merr. and Calliandra calothyrsus (Meissner) on soil inorganic nitrogen dynamics and two subsequent maize crops were evaluated under field conditions in the highlands of eastern Kenya. Continuous unfertilised maize, maize/bean rotation and natural regrowth of vegetation (weed fallow) were used as control treatments. The proportion of symbiotic N2-fixation was estimated by measuring both leaf 15N enrichment and whole-plant 15N enrichment by the 15N dilution technique for Sesbania and Calliandra, using Eucalyptus saligna (Sm.) and Grevillea robusta (A. Cunn) as reference species. Above- and below-ground biomass and N contents were examined in Sesbania, Calliandra, Eucalyptus and Grevillea 22 months after planting. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. Compared to the continuous unfertilised maize treatment, both residual crop yields were significantly higher when mineral N (one application of 60 kg N ha−1) was added. Furthermore, the second crop following the Sesbania fallow was significantly higher than the continuous maize crop. The above-ground biomass of the trees at final harvest were 31.5, 24.5, 32.5 and 43.5 Mg ha−1 for the Sesbania, Calliandra, Grevillea and Eucalyptus, respectively. For the total below-ground biomass the values for these same tree species were 11.1, 15.5, 17.7, and 19.1 Mg ha−1, respectively, of which coarse roots (>2 mm), including tap roots, amounted to 70–90%. About 70–90% of the N in Sesbania, and 50–70% in Calliandra, was derived from N2-fixation. Estimates based on leaf 15N enrichment and whole-plant 15N enrichment were strongly correlated. The N added by N2-fixation amounted to 280–360 kg N ha−1 for Sesbania and 120–170 kg N ha−1 for Calliandra, resulting in a positive N balance after two maize cropping seasons of 170–250 kg N ha−1 and 90–140 kg N ha−1, for Sesbania and Calliandra, respectively. All the other treatments gave negative N balances after two cropping seasons. We conclude that Sesbania sesban is a tree species well suited for short duration fallows due to its fast growth, high nutrient content, high litter quality and its ability to fix large amounts of N2 from the atmosphere.

Inorganic nitrogen dynamics in fallows and maize on an Oxisol and Alfisol in the highlands of Kenya

Fallows with naturally regenerated or planted vegetation are important in many subsistence agricultural systems of tropical regions, but the underlying soil processes in fallows are not properly understood. We investigated N dynamics under different fallow vegetation on a Kandiudalfic Eutrudox (2372-mm rain in 16 months) and a Kandic Paleustalf (1266-mm rain in 15 months) in the Kenyan highlands. The treatments, which extended for three cropping seasons (15–16 months), were Zea mays (maize), natural regrowth of vegetation (natural fallow), planted Sesbania sesban (sesbania fallow) and uncultivated soil without vegetation (bare fallow). Inorganic N (nitrate+ammonium-N) to 2-m depth under bare fallow increased by 242 kg N ha −1 year −1 on the Oxisol and 54 kg N ha −1 year −1 on the Alfisol, indicating that N mineralization exceeded N losses. Subsoil inorganic N (0.5–2.0 m) remained relatively unchanged after three crops of unfertilized maize, which produced limited total biomass because of P deficiency. Inorganic N decreased during natural and sesbania fallows, and both fallows similarly depleted subsoil inorganic N. The fallows depleted inorganic N at 0.5–2.0 m by 75–125 kg N ha −1 year −1 down to a minimum N content between 40 and 80 kg N ha −1. After slashing sesbania and incorporating the above-ground biomass with 154–164 kg N ha −1, soil inorganic N increased within 2 months by 136 kg N ha −1 on the Oxisol and 148 kg N ha −1 on the Alfisol. Inorganic N decreased after cropping the bare fallow on the Oxisol with maize, indicating that inorganic N was prone to leaching during heavy rains when the maize was small. A considerable part of the N in biomass of the natural fallow was recycled. Much of the total N accumulated by the sesbania fallow was removed with the wood and the amount of N recycled was similar on the Oxisol and Alfisol. We conclude that sesbania fallows can retrieve considerable subsoil inorganic N on deep soils with high subsoil N and effectively cycle this N through its rapidly decomposable biomass to subsequent crops.

Soil Phosphorus Fractions in Unfertilized Fallow‐Maize Systems on Two Tropical Soils

AbstractThe rotation of crops with either legumes or natural regrowth of native vegetation is known to supply N to crops in tropical land‐use systems, but little is known about the effect of such rotations on P supply to crops. An experiment was conducted on two soils in Kenya, an Oxisol (very fine, isohyperthermic Kandiudalfic Eutrudox), and an Alfisol (mixed, isothermic Kandic Paleustalf), to determine the effect of unfertilized, organic‐based land‐use systems on fractions of soil inorganic P (Pi) and organic P (Po). The land‐use systems involved growth of three maize (Zea mays L.) crops (18 mo total duration) after 17 mo of either (i) Sesbania sesban (L.) Merr. tree growth (sesbania fallow), (ii) natural regrowth of vegetation without cultivation (natural fallow), (iii) three crops of unfertilized maize (maize monoculture), or (iv) bare uncultivated soil (bare fallow). Soil (0–15 cm deep) was collected before post‐fallow maize and after the three post‐fallow maize crops. Sesbania and natural fallows increased the amount of P in light fraction soil organic matter (OM) (&gt;150 µm, &lt;1.13 Mg m‐3) and macroorganic matter (250–2000 µm) on each soil and increased chloroform extractable P on the Oxisol. Land‐use systems had no effect on extractable Pi (bicarbonate, Bray‐1, iron‐oxide impregnated paper), except for resin P on the Oxisol. Unfertilized sesbania and natural fallows cycled soil P through plant residues, resulting in increased P in soil microbial biomass immediately after the fallows and increased P in light fraction soil OM for three post‐fallow cropping seasons.

Soil microarthropod populations under natural and planted fallows in southwestern Nigeria

Microarthropods, such as soil mites (Acari) and springtails (Collembola), with body width between 0.08 mm and 0.5 mm play important roles in soil fertility maintenance through their regulatory activities in decomposition and nutrient turnover. Observations were made at IITA, Ibadan, southwestern Nigeria to evaluate the effects of natural regrowth of vegetation — mainly the shrub Chromolaena odorata — and three planted woody fallow species (Acacia leptocarpa, Senna siamea, and Leucaena leucocephala) on soil microarthropods in a degraded Alfisol. Populations of soil microarthropods were higher in the rainy season than the dry season, and populations were greater under natural fallow than for continuous cropping with maize (Zea mays) and cassava (Manihot esculenta). Populations of soil microarthropods were comparable under leucaena and natural fallow, but populations in the rainy season were 38% higher under senna than natural fallow and 36% higher under acacia than natural fallow. Regression analysis indicated that soil microarthropod population under fallow species was positively correlated with the lignin contents of leaf litter.

Relating Preseason Soil Nitrogen to Maize Yield in Tree Legume‐Maize Rotations

AbstractLegumes can be an important source of N for cereals in tropical, subsistence farming systems that use little or no fertilizers. Our objective was to identify measures of soil N availability, following growth of diverse legumes, that correlated with yield of a subsequent unfertilized maize (Zea mays L.) crop. Unfertilized maize followed nine 3‐yr‐old systems (six monocultures of planted trees legumes, a groundnut [Arachis hypogea L.]‐maize‐soybean [Glycine max (L.) Merr.] rotation, an uncultivated fallow with natural regrowth of vegetation, and unfertilized maize monoculture) in two experiments on a Ustic Rhodustalf in eastern Zambia. Total soil C and N before maize planting were not related to grain yield of maize. Preseason soil inorganic N (NO3 + NH4), aerobic N mineralization, and light‐fraction N (N in macroorganic matter, 150–2000 µm and &lt;1.37 Mg m‐3) at 0‐ to 15‐ cm depth correlated (P &lt; 0.01) with maize grain yield. Preseason inorganic N combined with light‐fraction N accounted for 59% of the variance in maize grain yield. The predictability of maize yield was slightly improved (64% of the variance) by including the population of the parasitic weed striga [Striga asiatica (L.) Kuntze] with preseason inorganic N and light‐fraction N. The results suggest that yield of unfertilized, rain‐fed maize following different rotational systems on a N‐limiting soil in an area with monomodal rainfall was strongly related to the additive effects of preseason soil inorganic N and a soil N fraction related to N mineralization.