Agroforestry for biomass production and carbon sequestration: an overview

Ever since the Kyoto Protocol, agroforestry has gained increased attention as a strategy to sequester carbon (C) and mitigate global climate change. Agroforestry has been recognized as having the greatest potential for C sequestration of all the land uses analyzed in the Land-Use, Land-Use Change and Forestry report of the IPCC; however, our understanding of C sequestration in specific agroforestry practices from around the world is rudimentary at best. Similarly, while agroforestry is well recognized as a land use practice capable of producing biomass for biopower and biofuels, very little information is available on this topic. This thematic issue is an attempt to bring together a collection of articles on C sequestration and biomass for energy, two topics that are inextricably interlinked and of great importance to the agroforestry community the world over. These papers not only address the aboveground C sequestration, but also the belowground C and the role of decomposition and nutrient cycling in determining the size of soil C pool using specific case studies. In addition to providing allometric methods for quantifying biomass production, the biological and economic realities of producing biomass in agroforestry practices are also discussed.     

Soil carbon balance of rice-based cropping systems of the Indo-Gangetic Plains

An agricultural land use system centred on rice-based cropping systems as common in the Indo-Gangetic Plains (IGP), with its annual cycles of wet and dry, puddling and ploughing, is unique and exerts a specific influence on soil organic matter (SOM) dynamics. Reports of yield ‘stagnation’ in some parts of the IGP with a decline in SOM quantity and quality raises concerns about the sustainability of the rice-wheat system in the region. Proper understanding of the soil carbon balance and of measures required to build up or maintain the soil carbon status of such a production system is therefore important for its sustainable production. Long-term experiments conducted in this region are especially useful in gaining understanding of soil carbon dynamics, since the processes affecting carbon dynamics are slow in nature. We used a simple analytical model—Yang's model—to calculate carbon balances in the rice-based cropping systems of the IGP in India. We used eight data sets from rice-based cropping systems from different sub-regions in the IGP, with different crop managements applied to rice, wheat or a third crop. Carbon input into the soil from crop biomass was calculated using data on crop yield and Harvest Index (HI). The values of soil organic carbon content predicted by the model were comparable to the observed values (r = 0.91). The model performs well in situations with porous soils (low clay content), with a pH values in the neutral range (7-7.5) and low annual rainfall as in the situation of Ludhiana-1 and 2. However, it underperforms in situations with heavy clay soils with high rainfall, causing severe anaerobic conditions. The model projections for the long-term (by 2080) show a decline in SOC at all sites in the IGP. Hence, the yield stagnation in the IGP, which has been attributed to a decline in SOC and the associated reduction in nutrient supply, could lead to further decreases in SOC levels, aggravated by climate change-induced higher temperatures.     

Nitrogen mineralization in a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

Information on temporal and spatial patterns of N mineralization is critical in designing tree-crop mixed systems that could maximize N uptake while minimizing N loss. We quantified N mineralization rates in a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alley cropping system in northwestern Florida with (non-barrier) and without tree-crop belowground interactions (barrier separating the root systems of pecan and cotton). Monthly rates of mineralization were estimated using buried bag incubations over a 15-month period. In addition, seasonal mineralization rates and cotton lint yield on soils supplied with two sources of N—inorganic fertilizer and organic poultry litter—were assessed. Results indicated that temporal variations in net NH₄ and NO₃ accumulation and mineralization rates were driven primarily by environmental factors and to a lesser degree by initial soil NH₄ and NO₃ levels. Mineralization varied by belowground interaction treatment during the initial growing season, when the non-barrier treatment exhibited a higher mineralization rate than the barrier treatment, likely due to reduced nutrient uptake by cotton in the non-barrier or a higher degree of immobilization in the barrier treatment. Mineralization during the second growing season was similar for both treatments. Source of N had no effects on N transformation in the soil. Lint yield reductions were observed in the non-barrier treatment during both years compared to the barrier treatment, likely due to interspecific competition for water. Yield differences between treatments in the second growing season were likely compounded by a diminishing pre-study fallow effect. Source of N was found to have a significant effect on cotton yield, with inorganic fertilizer resulting in 39% higher lint compared to poultry litter in the barrier treatment.     

Homegarden agroforestry systems: an intermediary for biodiversity conservation in Bangladesh

Biodiversity conservation is one of the important ecosystem services that has been negatively impacted by anthropogenic activities. Natural forests (NF) harbor some of the highest species diversity around the world. However, deforestation and degradation have resulted in reduced forest land cover and loss of diversity. Homegarden agroforestry (AF) systems have been proven to be an intermediary for biodiversity conservation. In this study, we evaluate the effectiveness of home garden AF practices to conserve tree species diversity in Bangladesh and compare them with tree species diversity in NF. A total of nine locations were selected for this synthesis from published literature which comprised of five AF sites and four NFs. Shannon?CWeiner Diversity Index (H) was similar for home-garden AF (3.50) and NF (2.99), with no statistical difference between them. Based on non-metric multi-dimensional scaling (NMDS) ordination analysis, the AF and NF plots showed distinct separation. However, Bray?CCurtis dissimilarity index ranged from 0.95 to 0.70 indicating nearly no overlap in species composition to significant overlap between AF and NF. Based on our results, we conclude that AF can serve as an important ecological tool in conserving tree species diversity, particularly on landscapes where NF fragments represent only a small fraction of the total land area. Creating and maintaining AF habitats in such human dominated landscapes should be part of the biodiversity conservation strategy.     

Temperate agroforestry research: considering multifunctional woody polycultures and the design of long-term field trials

The many benefits of agroforestry are well-documented, from ecological functions such as biodiversity conservation and water quality improvement, to cultural functions including aesthetic value. In North American agroforestry, however, little emphasis has been placed on production capacity of the woody plants themselves, taking into account their ability to transform portions of the landscape from annual monoculture systems to diversified perennial systems capable of producing fruits, nuts, and timber products. In this paper, we introduce the concept of multifunctional woody polycultures (MWPs) and consider the design of long-term experimental trials for supporting research on agroforestry emphasizing tree crops. Critical aspects of long-term agroforestry experiments are summarized, and two existing well-documented research sites are presented as case studies. A new long-term agroforestry trial at the University of Illinois, “Agroforestry for Food,” is introduced as an experiment designed to test the performance of increasingly complex woody plant combinations in an alley cropping system with productive tree crops. This trial intends to address important themes of food security, climate change, multifunctionality, and applied solutions. The challenges of establishing, maintaining, and funding long-term agroforestry research trials are discussed.     

LINTUL3, a simulation model for nitrogen-limited situations: Application to rice

LINTUL3 is a crop model that calculates biomass production based on intercepted photosynthetically active radiation (PAR) and light use efficiency (LUE). It is an adapted version of LINTUL2 (that simulates potential and water-limited crop growth), including nitrogen limitation. Nitrogen stress in the model is defined through the nitrogen nutrition index (NNI): the ratio of actual nitrogen concentration and critical nitrogen concentration in the plant. The effect of nitrogen stress on crop growth is tested in the model either through a reduction in LUE or leaf area (LA) or a combination of these two and further evaluated with independent datasets. However, water limitation is not considered in the present study as the crop is paddy rice. This paper describes the model for the case of rice, test the hypotheses of N stress on crop growth and details of model calibration and testing using independent data sets of nitrogen treatments (with fertilizer rates of 0–400 kg N ha^?1) under varying environmental conditions in Asia. Results of calibration and testing are compared graphically, through Root Mean Square Deviation (RMSD), and by Average Absolute Deviation (AAD). Overall average absolute deviation values for calibration and testing of total aboveground biomass show less than 26% mean deviation from the observations though the values for individual experiments show a higher deviation up to 41%. In general, the model responded well to nitrogen stress in all the treatments without fertilizer application as observed, but between fertilized treatments the response was varying.     

The ecology and ecosystem services of native trees: Implications for reforestation and land restoration in Mesoamerica

Tropical forests provide a variety of goods and services to humanity. Although efforts to protect and manage these forests have increased in recent decades, forest loss continues. The last decade and a half has seen considerable research on reforestation with native species in Mesoamerica, including both biophysical and social aspects. Advances in knowledge have led to increasingly sophisticated reforestation treatments as well as novel ways of deriving forest goods and services from human dominated landscapes.This Special Issue includes articles produced as the result of a conference held in Panama City, Panama in January 2010 with the goal to summarize the state of knowledge of native species reforestation and associated ecosystems services in Mesoamerica. The introduction concludes with a call for continued research, including a mechanistic understanding of tree interactions with the biophysical environment in order to advance or knowledge of ecosystem services and their interactions. Results from these and other studies aimed at socioeconomic aspects of reforestation are critical to land use planning.     

Production physiology of three native shrubs intercropped in a young longleaf pine plantation

The objective of this study was to evaluate how competition would affect the physiology, and thus productivity of American beautyberry (Callicarpa americana L.), wax myrtle [Morella cerifera (L.) Small] and inkberry [Ilex glabra (L.) A. Gray] when intercropped in a longleaf pine (Pinus palustris Mill.) plantation in the southeastern United States. The effect of competition was assessed via comparisons of mortality, biomass, light transmittance, gas exchange and soil moisture between intercropping and monoculture (treeless) treatments. Overall, shrubs in the intercropping treatment performed worse than those in the monoculture, with higher mortality, and reductions in biomass of 75.5, 50.6, and 68.7% for C. americana, M. cerifera and I. glabra, respectively. Root–shoot ratios for all species were significantly higher and soil moisture during dry periods was significantly lower in the intercropping treatment. Light transmittance below the pine canopy was high (57.7%) and I. glabra was the only species that exhibited reduced A _max when belowground resources were not limiting. These results suggest that the effect of shading is minimal and belowground competition is likely the most important determinant of productivity in this system.     

Agroforestry for soil health

Healthy soil is one of the most critical resources for the health and sustainability of ecosystems, including agroecosystems. Although the agroforestry community has long been convinced of the soil health benefits of agroforestry practices, many of such practices remain to be fully accepted by the mainstream agriculture community. Agroforestry, as a sustainable land management practice, has shown solid evidence of its role in improving soil quality and health based on at least four decades of data gathered from the world over. This thematic issue presents 28 papers that add further to the body of knowledge to reaffirm that agroforestry can improve the major measurable soil metrics that define soil health. Collectively, these papers show that agroforestry has the ability to (1) enrich soil organic carbon better than monocropping systems, (2) improve soil nutrient availability and soil fertility due to the presence of trees in the system, and (3) enhance soil microbial dynamics, which would positively influence soil health. It is imperative that agroforestry, as part of a multifunctional land-use strategy, should receive increased attention in our policy discussion for the future of soil and soil health.