Juan Diego León

Role of Litter Turnover in Soil Quality in Tropical Degraded Lands of Colombia

Land degradation is the result of soil mismanagement that reduces soil productivity and environmental services. An alternative to improve degraded soils through reactivation of biogeochemical nutrient cycles (via litter production and decomposition) is the establishment of active restoration models using new forestry plantations, agroforestry, and silvopastoral systems. On the other hand, passive models of restoration consist of promoting natural successional processes with native plants. The objective in this review is to discuss the role of litter production and decomposition as a key strategy to reactivate biogeochemical nutrient cycles and thus improve soil quality in degraded land of the tropics. For this purpose the results of different projects of land restoration in Colombia are presented based on the dynamics of litter production, nutrient content, and decomposition. The results indicate that in only 6–13 years it is possible to detect soil properties improvements due to litter fall and decomposition. Despite that, low soil nutrient availability, particularly of N and P, seems to be major constraint to reclamation of these fragile ecosystems.

Silvopastoral Systems Enhance Soil Quality in Grasslands of Colombia

In the tropical drylands of Colombia, the soils subjected to traditional systems of livestock production are severely degraded and depleted of plant nutrients. Multistrata silvopastoral systems are viable alternatives to improve livestock production; however, it is unknown whether these systems can reduce the negative environmental impacts of traditional systems on soil quality. The objective of this study was to evaluate the effects of 13-year-old multistrata silvopastoral systems on soil quality parameters in degraded soils of the Sinu River Valley, Colombia. The results show that the trees in the silvopastoral systems increased or maintained soil pH values and nutrient availability (phosphorus, potassium, and calcium) with respect to the pastures with only grasses. The effects were significantly controlled by the types of plant species, particularly Guazuma ulmifolia and Cassia grandis.

Passive and Active Restoration Strategies to Activate Soil Biogeochemical Nutrient Cycles in a Degraded Tropical Dry Land

The potential use of two restoration strategies to activate biogeochemical nutrient cycles in degraded soils in Colombia was studied. The active model was represented by forest plantations of neem (Azadirachta indica) (FPN), while the passive model by successional patches of native plant species was dominated by mosquero (Croton leptostachyus) (SPM). In the field plots fine-litter traps and litter-bags were established; samples of standing litter and surface soil samples (0–10 cm) were collected for chemical analyses during a year. The results indicated that the annual contributions of fine litterfall in FPN and SPM were 557.5 and 902.2 kg ha−1, respectively. The annual constant of decomposition of fine litter (k) was 1.58 for neem and 3.40 for mosquero. Consequently, the annual real returns of organic material and carbon into the soil from the leaf litterfall decomposition were 146 and 36 kg ha−1 yr−1 for FPN and 462 and 111 kg ha−1 yr−1 for SPM, respectively. Although both strategies showed potential to activate soil biogeochemical cycles with respect to control sites (without vegetation), the superiority of the passive strategy to supply fine litter and improve soil properties was reflected in higher values of soil organic matter content and cation exchange capacity.

Vegetation cover and rainfall seasonality impact nutrient loss via runoff and erosion in the Colombian Andes

Mountain ecosystems provide key services to a large portion of the population in the tropics. However, they are particularly vulnerable to regional environmental changes such as soil degradation, via soil erosion and associated nutrient loss, both dissolved in runoff and suspended in sediment. Current trends in land use conversion combined with projections of intensified hydrological extremes potentially amplify these threats. We analyze the interactive effects of rainfall characteristics (at three time scales) and vegetation cover on the runoff–erosion–nutrient loss progression for a group of vegetation cover types that represent different land use conversion stages. After a year of observations we found, as expected, that natural forests have the highest potential for regulating precipitation–runoff–erosion–nutrient loss. The highest amounts of runoff occurred in pasturelands, and croplands had the highest erosion losses. Croplands showed the highest concentrations of soluble nutrients in runoff and in sediment. However, due to higher runoff amounts, pasturelands had the greatest loss of dissolved nutrients. Precipitation seasonality significantly influenced both erosion and nutrient loss. This is particularly critical in managed agricultural and pasture systems where increased runoff and erosion rates, combined with unsustainable management practices, may lead to alterations in soil and water quality. Our results indicate how agricultural practices need to adapt fertilization scheduling to rainfall seasonality to minimize potential environmental impacts. Collectively, our results highlight a fundamental management need in tropical mountains where the combination of rapid land use change and altered climate threatens ecosystem integrity and ecosystem services.

Effectiveness of Single and Combined Ectomycorrhizal Inocula on Three Species of Pinus at Nursery

The objective of this study was to determine individual and combined inoculation effects with ectomycorrhizal fungi (EMF) (Amanita muscaria, AM; Suillus luteus, SL; and Amanita sp., AS) on seedling growth and P uptake of Pinus patula, P. oocarpa, and P. tecunumanii. Beneficial effects of ectomycorrhizal inoculation depended on the plant species × inoculum combinations. In the case of P. patula, the benefit was evident only in P uptake and this occurred only when the three fungi were concomitantly applied. In P. tecunumanii, the effects were associated not only with P uptake but also with plant growth and not only with the triple inoculation but also with the individual (AS) and dual (AM + AS) inoculations. In P. oocarpa, AS inoculation enhanced both plant growth and P uptake, while AM inoculation only improved plant growth; triple and dual inoculation (SL+AM and SL+AS) also increased plant growth but not P uptake.

Assessment of the Effectiveness of Ectomycorrhizal Inocula to Promote Growth and Root Ectomycorrhizal Colonization in Pinus patula Seedlings Using the Most Probable Number Technique

The aim of this study was to evaluate the response of Pinus patula seedlings to two inocula types: soil from a Pinus plantation (ES) and an in vitro produced inoculum (EM). The most probable number method (MPN) was used to quantify ectomycorrhizal propagule density (EPD) in both inocula in a 7-order dilution series ranging from 100 (undiluted inoculum) to 10−6 (the most diluted inoculum). The MPN method allowed establishing differences in the number of infective ectomycorrhizal propagules’ density (EPD) ( per g; per g). The results suggest that the EPD of an inoculum may be a key factor that influences the successfulness of the inoculation. The low EPD of the ES inoculum suggests that soil extracted from forest plantations had very low effectiveness for promoting root colonization and plant growth. In contrast, the high EPD found in the formulated inoculum (EM) reinforced the idea that it is better to use proven high quality inocula for forest nurseries than using soil from a forestry plantation.

Land cover effects on water balance partitioning in the Colombian Andes: improved water availability in early stages of natural vegetation recovery

Vegetation actively affects different components of the water budget in multiple spatial and temporal scales. Changes in vegetation cover and structure—such as those resulting from land use—alter natural ecohydrological dynamics, leading to changes in natural hydrologic regimes. In tropical mountain ecosystems, such as the Colombian Andes, significant areas of native forests have been converted to agro-ecosystems that include pasturelands and croplands, to supply societal demands for other ecosystem services. Yet, services such as water provision and hydropower generation that depend on the regulation of hydrologic fluxes are also demanded from these ecosystems, potentially generating conflicting societal demands. In this study, we assess the effect of vegetation cover type and rainfall seasonality on the dynamics of hydrological partitioning—an indicator of hydrologic regulation—at three temporal scales, in a simulated gradient of human disturbance characterized by seven types of vegetation cover. Overall, vegetation cover effects on hydrologic partitioning are more pronounced in shorter, weekly to seasonal, timescales than in annual timescales. Natural vegetation cover types have a higher potential for maintaining water availability, as evidenced by lower variability of soil moisture storage and hydrological fluxes both within and between seasons. Notably, among all cover types, early stages of natural vegetation recovery appear to be more effective in maintaining higher levels of soil moisture while decreasing potential overland flow and other water losses, therefore more effectively contributing to deep drainage and potentially to groundwater recharge, which relate to hydrologic regulation and, ultimately, water availability. Collectively, our results provide insights for decision-making in land management, particualrly when provisioning and regulatory ecosystem services are demanded from these strategic ecosystems.

Plant-Microbe Interactions for Phosphate Management in Tropical Soils

Low soil phosphate (P) availability is a major limiting factor for tropical agriculture and forestry because this severely limits the plant productivity. A viable alternative consists of using arbuscular mycorrhizal fungi (AMF) that form a symbiotic association with plant roots, the AMF hyphae are by far most efficient than roots alone in taking up water and low-mobile nutrient from soils, particularly P. Also, there are phosphate-solubilizing microorganisms (PSM) capable of dissolving native and applied P compounds. With both microbial groups, plants enhance its capability of taking up P and thus overcome this limiting factor. The objective in this chapter was to discuss the effectiveness of both AMF and PSM to enhance plant P uptake in tropical soils.