Anders Malmer

Optimum soil water for soil respiration before and after amendment with glucose in humid tropical acrisols and a boreal mor layer

Abstract The measurements of water content most often used by soil microbiologists are gravimetric water content (Wg), volumetric water content (Wv), percentage of water holding capacity (%WHC), percentage of water filled porosity (%WFP), and water potential (Wp). This study investigated the relationship of these water measurements to soil respiration, before and after substrate addition (glucose, nitrogen and phosphorus), focusing on the water contents at which the exponential respiration rates after amendments (μ) were at their maximum. Three tropical Haplic Acrisols and one boreal mor layer were used. One Acrisol had high loss of ignition (LOI 17.4% w/w) and high clay content. The second Acrisol had high clay content and low LOI (3.9%), and the third had low clay content and low LOI (5.7%). The mor layer had a LOI of 95.6%. The maximum μ was found at −15 kPa, (50% WHC), irrespectively of the soil type. The decrease in μ above −15 kPa was not as pronounced for the Swedish mor layer as for the Acrisols. At water potentials above −2 kPa the respiration rate was no longer exponential for the Acrisols, making it impossible to define μ. For Wg, WFP, and Wv the maximum differed greatly between the soils. The maximum for basal respiration was found to be at higher water content than the maximum for μ. However, the water contents used were not high enough to specify optimum water content for basal respiration except for the humus rich Acrisol which was at 66% of the WHC or about −5 kPa. It is suggested that respiration measurements of the Acrisols, after substrate addition, should not be made using water contents exceeding −15 kPa or 50% WHC. Because of the dependency of method, WHC must be defined and complemented by water potential. The use of WFP, Wv, and Wg is not recommended for adjusting the water contents of these soils.

Soil disturbance and loss of infiltrability caused by mechanized and manual extraction of tropical rainforest in Sabah, Malaysia.

Abstract The rate of infiltration, or infiltrability, is known to change due to soil disturbance. The infiltrability is normally higher in the initial stages of infiltration before wetting of the soil, when pressure gradients predominate over the gravitational forces. This is described by the term ‘sorptivity’ which is the cumulative infiltration as a function of the square root of time, which often is constant during early stages of infiltration. As gravitation starts to dominate, infiltrability decreases to a constant-rate, steady-state infiltrability. Drastic changes in infiltrability may lead to overland flow causing erosion and nutrient leaching. Dry bulk density, steady-state infiltrability and sorptivity were measured in reference forest and after manual and crawler-tractor timber extraction in different watersheds on Orthic Acrisol (40% clay, 40% sand) and Gleyic Podsol (10% clay, 80% sand). Six-year-old tractor tracks after selective logging were also investigated. New tractor tracks covered 24% of the mechanically extracted area, while skid rails covered 4% of the manually extracted area. In the top 10 cm, mean dry bulk density increased after all treatments, but was significant only in the top 5 cm on clay soil after tractor extraction, where it increased from 0.82 to 1.28 g cm−3. Means of steady-state infiltrability and sorptivity were lower on treated areas than in the forest. Differences of mean steady-state infiltrability were significant for all tractor treatments compared with reference forest on both soil types and also with manual extraction on clay soil. On clay soil (and sand) mean stead state infiltrabilities were 154 (48.7), 36.7 (11.6), 0.28 (1.26) and 0.63 (−) mm h−1 in forest, on manual and on new and old tractor tracks respectively.

Changes in soil chemical and microbial properties after a wildfire in a tropical rainforest in Sabah, Malaysia

Abstract Changes in soil caused by drought and wildfire in a Dipterocarp rainforest in Sabah, Malaysia were assessed by phosphorus fractionation, extractable nitrogen and nutrient limited respiration kinetics (after addition of glucose+N or P). Fire increased the concentration of total phosphorus (P) in the litter layer (per ha and per dry soil) by raising the 0.2 M NaOH extractable-P. In the soil organic layer, membrane exchangeable P was reduced by fire while 1.0 M HCl extractable-P, and 0.5 M NaHCO 3 extractable-P increased. Microbially available P increased after the fire and was most closely related to NaOH extractable-P that has been considered available to plants only over long time-scales. Total nitrogen (N) increased in the litter layer (per ha and per dry soil) due to post-fire litter fall, while the NO 3 − increased up to 10-fold down to the 10 cm mineral soil. In contrast, the microbially available N decreased by 50%. Basal respiration and substrate-induced respiration increased in the litter layer and decreased in the organic horizon (per dry soil and per organic matter). P limited microbial growth resulted in a slow and non-exponential increase in respiration, presumably reflecting the P-fixing nature of the soils, while N limitation resulted in a fast exponential increase. However, higher respiration rates were eventually achieved under P limitation than under N limitation.

The effect of trees on preferential flow and soil infiltrability in an agroforestry parkland in semiarid Burkina Faso.

Water scarcity constrains the livelihoods of millions of people in tropical drylands. Tree planting in these environments is generally discouraged due to the large water consumption by trees, but this view may neglect their potential positive impacts on water availability. The effect of trees on soil hydraulic properties linked to groundwater recharge is poorly understood. In this study, we performed 18 rainfall simulations and tracer experiments in an agroforestry parkland in Burkina Faso to investigate the effect of trees and associated termite mounds on soil infiltrability and preferential flow. The sampling points were distributed in transects each consisting of three positions: (i) under a single tree, (ii) in the middle of an open area, and (iii) under a tree associated with a termite mound. The degree of preferential flow was quantified through parameters based on the dye infiltration patterns, which were analyzed using image analysis of photographs. Our results show that the degree of preferential flow was highest under trees associated with termite mounds, intermediate under single trees, and minimal in the open areas. Tree density also had an influence on the degree of preferential flow, with small open areas having more preferential flow than large ones. Soil infiltrability was higher under single trees than in the open areas or under trees associated with a termite mound. The findings from this study demonstrate that trees have a positive impact on soil hydraulic properties influencing groundwater recharge, and thus such effects must be considered when evaluating the impact of trees on water resources in drylands. Key Points Trees in dryland landscapes increase soil infiltrability and preferential flow Termite mounds in association with trees further enhance preferential flow

Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics

Water scarcity contributes to the poverty of around one-third of the world’s people. Despite many benefits, tree planting in dry regions is often discouraged by concerns that trees reduce water availability. Yet relevant studies from the tropics are scarce, and the impacts of intermediate tree cover remain unexplored. We developed and tested an optimum tree cover theory in which groundwater recharge is maximized at an intermediate tree density. Below this optimal tree density the benefits from any additional trees on water percolation exceed their extra water use, leading to increased groundwater recharge, while above the optimum the opposite occurs. Our results, based on groundwater budgets calibrated with measurements of drainage and transpiration in a cultivated woodland in West Africa, demonstrate that groundwater recharge was maximised at intermediate tree densities. In contrast to the prevailing view, we therefore find that moderate tree cover can increase groundwater recharge, and that tree planting and various tree management options can improve groundwater resources. We evaluate the necessary conditions for these results to hold and suggest that they are likely to be common in the seasonally dry tropics, offering potential for widespread tree establishment and increased benefits for hundreds of millions of people.

Tending for Cattle: Traditional Fire Management in Ethiopian Montane Heathlands

Fire has long been a principal tool for manipulating ecosystems, notably for pastoralist cultures, but in modern times, fire use has often been a source of conflicts with state bureaucracies. Despite this, traditional fire management practices have rarely been examined from a perspective of fire behavior and fire effects, which hampers dialogue on management options. In order to analyze the rationale for fire use, its practical handling, and ecological effects in high-elevation ericaceous heathlands in Ethiopia, we used three different information sources: interviews with pastoralists, field observations of fires, and analysis of vegetation age structure at the landscape level. The interviews revealed three primary reasons for burning: increasing the grazing value, controlling a toxic caterpillar, and reducing predator attacks. Informants were well aware of critical factors governing fire behavior, such as slope, wind, vertical and horizontal fuel structure, and fuel moisture. Recent burns (1-4 years since fire) were used as firebreaks to control the size of individual burns, which resulted in a mosaic of vegetation of different ages. The age structure indicated an average fire return interval of ~10 years. At these elevations (> 3500 m), the dry period is unreliable, with occasional rains. Of all observed fires, 83% were ignited during very high Fire Weather Index levels, reached during only 11% of all days of the year. Burning is illegal, but if this ban was respected, our data suggest that the Erica shrubs would grow out of reach of cattle within a few years only, creating a dense and continuous canopy. This would also create a risk of large high-intensity wildfires since the landscape is virtually devoid of natural fuel breaks. Under the present management regime, this heathland ecosystem should be quite resilient to degradation by fire due to a relatively slow fuel buildup (limiting fire intervals) and an effective regrowth of Erica shoots. Nevertheless, if burning is done during severe drought, there may be a risk of smoldering fires killing the lignotubers. Given the intimate knowledge of fire behavior and fire effects among these pastoralists, it should be possible to develop a fire management plan that can sustain the present land use and ecosystem, and be sanctioned by both authorities and the local community.

Trees in agricultural landscapes enhance provision of ecosystem services in Sub-Saharan Africa

ABSTRACT The recent interest in multi-functional agricultural landscapes has not been matched with formal assessment of the roles that trees play across the spectrum of ecosystem services (ESs) provided in Sub-Saharan Africa (SSA). A structured literature review (1995–2014) assessed 350 journal articles about provision of one or more ESs by trees on farms and in agricultural landscapes in SSA. This revealed information on 15 ESs from studies in 23 countries covering arid (1% of studies), semi-arid (49%), sub-humid (26%) and humid (24%) agro-ecological zones. The majority of the studies reported provisioning (39%) and supporting (35%) followed by regulating (26%) ESs while studies on cultural services were scarce. Beneficial impacts of trees were dominating (58%), in particular in semi-arid zones where they were associated with enhancing water and nutrient cycling. A decline in some ESs was reported in 15% of the studies, while 28% found no effect of trees. Although the effects of trees were mainly positive, a decline in crop production was noted as a key trade-off against the provisions of ESs, such as modification of microclimate. This highlights the need to manage trade-offs among impacts of trees on ES provision to reduce competition and increase complementarity between trees and crops.

Phosphorus Loading to Tropical Rain Forest Streams After Clear-Felling and Burning in Sabah, Malaysia

Most estimates of P export from natural or disturbed humid tropical ecosystems by streams have been based only on export of dissolved P, even though P often is limiting and can be expected to be strongly associated to particles. Therefore loss of ignition (LOI) and particulate P (Ppart) analyses were made on organic and inorganic detritus resulting from surface erosion and on stream-suspended sediments in an undisturbed rain forest (control), as well as during and after conversion of rain forest into forest plantation. Control forest surface erosion and stream sediments consisted mainly of organics, and dissolved P (Pdiss) dominated over Ppart in stream water. The same relation was found after conversion, with a maximum mean Pdiss/Ppart ratio of up to 10 after burning, compared with 2–2.5 for control forests. This larger difference was assumed to depend on PO4 dissolved from ashes to larger concentrations than could be adsorbed during the short time (<1 hour) to reach peak flow during rainstorms.

Performances of Cotton–Maize Rotation System as Affected by Ploughing Frequency and Soil Fertility Management in Burkina Faso

On-farm experiments were conducted on two soil types (Lixisol and Luvisol) in the western cotton area of Burkina Faso with the objective to develop sustainable water and soil fertility management techniques that improve cotton–maize productivity. The hypothesis that reducing ploughing frequency with addition of organic and mineral fertilizers may improve cotton (Gossypium hirsutum) and maize (Zea mays L.) productions was tested. The treatments were combination of two tillage regimes (annual oxen ploughing, AP and ploughing/manual scarifying, RT) with compost (Co) and without compost (nCo) application. The treatment annual ploughing with compost addition (AP + Co) had the highest soil water content (WC) on both the Lixisol and the Luvisol. The cotton yield increase was 46 and 36% on reduced tillage plot with compost additions (RT + Co) compared to the control in the Lixisol and the Luvisol, respectively. In the Lixisol the highest maize grain yield was recorded in the annual ploughing plot with the additional amount of nitrogen equivalent to the compost nitrogen content. Reduced tillage together with compost additions had the highest maize yield in the Luvisol. These results confirmed the hypothesis that reduced tillage with organic and mineral fertilization improved cotton and maize productions.

Canopy cover effects on local soil water dynamics in a tropical agroforestry system: Evaporation drives soil water isotopic enrichment

Despite the widely held assumption that trees negatively affect the local water budget in densely planted tree plantations, we still lack a clear understanding of the underlying processes by which canopy cover influences local soil water dynamics in more open, humid tropical ecosystems. In this study, we propose a new conceptual model that uses a combination of stable isotope and soil moisture measurements throughout the soil profile to assess potential mechanisms by which evaporation (of surface soil water and of canopy‐intercepted rainfall) affects the relationship between surface soil water isotopic enrichment (lc‐excess) and soil water content. Our conceptual model was derived from soil water data collected under deciduous and evergreen plants in a shade grown coffee agroforestry system in Costa Rica. Reduced soil moisture under shade trees during the “drier” season, coinciding when these trees were defoliated, was largely the result of increase soil water evaporation as indicated by the positive relationship between soil water content and lc‐excess of surface soil water. In contrast, the evergreen coffee shrubs had a higher leaf area index during the “drier” season, leading to enhanced rainfall interception and a negative relationship between lc‐excess and soil water content. During the wet season, there was no clear relationship between soil water content and between lc‐excess of surface soil water. Greater surface soil water under coffee during the dry season may, in part, explain greater preferential flow under coffee compared with under trees in conditions of low rainfall intensities. However, with increasing rainfall intensities during the wet season, there was no obvious difference in preferential flow between the two canopy covers. Results from this study indicate that our new conceptual model can be used to help disentangling the relative influence of canopy cover on local soil water isotopic composition and dynamics, yet also stresses the need for additional measurements to better resolve the underlying processes by which canopy structure influences local water dynamics.