Dear, soil biological activity and declining earthworm numbers are also associated with a fall in soil pH.

Please follow the below given link and PDF attachments

https://www.hindawi.com/journals/aess/2012/616383/

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Applied and Environmental Soil Science

Volume 2012 (2012), Article ID 616383, 11 pages

http://dx.doi.org/10.1155/2012/616383

Review Article

Agroforestry and the Improvement of Soil Fertility: A View from Amazonia

Rachel C. Pinho,1 Robert P. Miller,2 and Sonia S. Alfaia1

1National Institute for Research in Amazon, INPA/CPCA, Agrarian Sciences Research Center, Avenue André Araújo, 69038-000 Manaus, AM, Brazil

2United Nations Development Program, (PNUD), Cj SHIS QI 25 Cj 03 C, 71640-220 Brasília, DF, Brazil

Received 16 October 2011; Revised 28 January 2012; Accepted 28 January 2012

Academic Editor: Robert L. Bradley

Copyright © 2012 Rachel C. Pinho et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This paper discusses the effects of trees on soil fertility, with a focus on agricultural systems in Amazonia. Relevant literature concerning the effects of trees on soil physical and chemical properties in tropical, subtropical, and temperate regions is reviewed, covering both natural ecosystems and agroecosystems. Soil carbon, in the form of organic matter, is considered as an indicator of biological activity as well as in relation to policy issues such as carbon sequestration and climate change. In the case of tropical soils and Amazonia, information on the effects of trees on soils is discussed in the context of traditional agriculture systems, as well as in regard to the development of more sustainable agricultural alternatives for the region. Lastly, attention is given to a case study in the savanna region of Roraima, northern Brazil, where a chronosequence of indigenous homegarden agroforestry systems showed clear effects of management practices involving trees on soil fertility. The use of diverse tree species and other practices employed in agroforestry systems can represent alternative forms of increasing soil fertility and maintaining agricultural production, with important practical applications for the sustainability of tropical agriculture.

1. Introduction

According to a study by the World Agroforestry Centre, ICRAF, 43% of the planet’s agricultural lands (more than a billion hectares) has more than 10% tree cover [1]. A lesser but still significant area of agricultural land, 160 million hectares, has more than 50% tree cover. The potential of trees to bring improvements in nutrition, income, housing, health, energy needs, and environmental sustainability in the agricultural landscape has guided ICRAF’s mission, with the presence of trees being the principal component of an “evergreen agriculture” [2]. Within the array of benefits brought by trees, an important element is the positive effect of trees on soil properties and consequently benefits for crops. This paper explores current knowledge as to this relation between trees and soil, based on agroforestry systems research, as well as studies innon-agricultural or natural environments that demonstrate effects of trees on soil. Although we consider information from various ecosystems and biomes, the focus will be on Amazonia, where the authors have most of their experience. This focus on Amazonia is also due to the strong policy demands for the development of more sustainable agricultural systems in the region, as alternatives to forms of land use that have shown significant and negative impacts on natural resources and ecosystem services, such as deforestation for extensive cattle ranching. In this scenario, agroforestry systems have been indicated as one of the more promising alternatives to achieve a more sustainable agriculture, in greater equilibrium with the environment [3].

The presence of trees in farming systems, although an ancient practice, began to gain institutional attention during the 1970s and 1980s, with the beginning of studies on “agroforestry systems”. One of the principal definitions employed in this context was that proposed by Lundgren and Raintree in 1982: “Agroforestry is a collective name for land-use systems and technologies where woody perennials (trees, shrubs, palms, bamboos, etc.) are deliberately used on the same land-management units as agricultural crops and/or animals, in some form of spatial arrangement or temporal sequence. In agroforestry systems there are both ecological and economical interactions between the different components” [4] (Figures 1, 2, and 3).

Figure 1: Agroforestry system with rubber (Hevea brasiliensis), cacao (Theobroma cacao), and açaí (Euterpe oleracea) in Tomé-açu, Pará, showing the litter layer that is typically found in such multistrata systems.

Figure 2: Agroforestry system in initial phase, with black pepper (Piper nigrum) as principal cash crop, interplanted with cupuaçu (Theobroma grandiflorum) and açaí (Euterpe oleracea) for future fruit production, as well as timber trees (mahogany—Swietenia macrophylla and ipê—Tabebuia sp.) and Brazil nut (Bertholletia excelsa) as long-term products.

Figure 3: Multistrata agroforestry system in Tomé-açu, Pará, with harvest of hogplum (Spondias mombin) grown as the upper canopy over cacao and açaí.

While trees in general can provide a number of environmental benefits in both rural and urban landscapes, and play key roles in ecosystem services provided by natural areas, in this paper we will restrict our focus to the effects of trees on soil fertility, in the specific context of agricultural systems. Although the benefits that trees can provide on rural properties such as food security, household income, economic stability, and thermal comfort (shade) are most often associated with their products, such as fruit, timber, or other items, the inclusion of trees in agricultural systems can also optimize nutrient cycling and have positive effects on soil chemical and physical properties. This process is especially important in tropical soils, where a high degree of weathering has created deep, leached soils that are poor in plant nutrients [5, 6]. Although poor in nutrients, tropical soils are very rich in biodiversity, with higher diversity and biomass of microorganisms than temperate soils, with these being the principal agents mediating the supply of nutrients to the soil by means of the decomposition of organic matter, derived from the vegetation [7–9].

In the humid tropics, the removal of surface litter or organic matter generally results in the depletion of soil fertility in a few years [10, 11]. In agricultural systems practiced by traditional peoples, this limitation is circumvented by using the land for a short period (generally 2-3 years), after which the cultivated areas are left to fallow with natural regeneration of secondary vegetation. The associated ecological interactions reestablish nutrient cycling and recuperate soil qualities, after which the area can once again be used for agriculture [12, 13]. This is the basis for shifting cultivation in Amazonia, a system that has permitted native populations to manage their natural resources over centuries, with small-scale environmental impacts that do not exceed the support capacity and resilience of ecosystems. However, the present-day situation of population growth and increasing pressure on agricultural lands lead to situations where there is demand for more intensive land use. This most often implies in repeated burning, the cheapest way to prepare land for planting, which can interrupt processes of nutrient cycling and accumulation, leading to loss of soil fertility and consequently slowing the recuperation of natural vegetation during fallow cycles [14].

In light of the present-day situation of Amazonia, where there are now good reasons and policy demands to balance conservation with development, it is necessary to think in terms of agricultural systems that optimize nutrient cycling and permit permanent or semipermanent production, as well as minimize dependence on external inputs and have low environmental impact. The inclusion of diverse tree species is a key element in maintaining the production of organic matter and generating other positive benefits, as well as allowing the diversification of products. However, before we discuss topics specific to tropical soils and Amazonia, the following sections will review general information about the influence of trees on soil and their role in accumulation of soil carbon stocks.

2. A General View of the Influence of Trees on Soil Fertility

One of the pioneer studies to measure the effects of individual trees on soils was that by Zinke [15], who looked at pines growing on dunes in northern California, USA. His study found that under trees, certain soil properties exhibited a pattern of radial symmetry, with changes in pH, nitrogen, cations, and cation exchange capacity varying according to distance from the tree trunk, with a peak in these characteristics at a certain distance.

Subsequent studies also demonstrated patterns in the variation of soil characteristics as influenced by trees, such as in tropical savannas [16, 17], deserts [18], and areas of temperate forests [19–23]. In analyzing soil characteristics under individual tree crowns in Kenyan savannas, Belsky et al. [16] found greater levels of mineralizable N, microbial biomass, P, K, and Ca underneath the crowns when compared to open savanna. Burke et al. [17] explain that in dry savannas the strong limitation on water availability permits only punctuated establishment of trees and shrubs but that under crowns cycling occurs in a different form than in open grasslands, with the possibility of soil enrichment in a scale of decades. However, such soil changes can be reverted with the death of the tree or by fires. Belsky et al. [16] also point out the effect of nutrients deposited in dung by birds and large mammals that utilize trees as resting places or roosts.

Such patterns form what have been called “islands of fertility” or “resource islands” created by trees or bushes, generally in savannas or desert areas. The microenvironment of these “islands” can also influence the composition of the herb stratum [16, 19], soil density [19, 20], and earthworm activity [20, 23] among other factors, allowing the creation of positive feedbacks that favor plant establishment and productivity [23, 24]. At the same time, these patterns can be important indicators of stability or risk of desertification in such areas [17, 18].

Studies of forests in temperate climates indicate variations in soil that can be related to individual tree species. Besides the expected correlations, such as greater levels of N under legumes [20] or lower pH under species that produce acidifying litter, such as Pinus spp. [20, 23], other interesting interactions show that different species can alter soil in distinct ways, with variations in the increment of soil carbon [20], exchangeable Ca and Mg and per cent base saturation [21, 23].

In a study of 14 tree species in Poland, Reich et al. [23] found varied effects on soil characteristics; however, these effects were significantly related to the level of Ca in litter, independent of the species. Trees producing litter rich in Ca were associated with soils with greater pH, exchangeable Ca, and per cent base saturation, as well as greater rates of forest floor turnover and greater diversity and abundance of earthworms. Dijkstra [22] emphasizes that the rate of mineralization of organic Ca is a fundamental factor in this process, since it determines the immediate availability of this nutrient in the soil and can vary between species.

The study of vertical patterns of the distribution of nutrients in soil can indicate other phenomena that are not detected when only the horizontal distribution of nutrients is examined. In an evaluation of more than 20,000 globally distributed soil profiles, the greater part in temperate climates, Jobbágy and Jackson [25] found that cycling mediated by plants exerts a marked influence on the vertical distribution of nutrients in the soil, especially in the case of more limiting nutrients such as P and K. Patterns of greater concentration of these nutrients in surface layers (0–20 cm) were attributed to the fact that since these are more important to plants, they are subject to greater uptake and cycling, being absorbed from deeper layers and returned to the soil surface through litterfall and rain water throughfall. This process of uptake functions in opposition to leaching, which moves nutrients downward and acts more strongly on those nutrients that are in lessdemand by plants. If a nutrient is not limiting, its movement in the soil profile will be more influenced by leaching than by cycling and it will present higher concentrations at greater depth, as occurs with Na, Cl, and Mg [25]. In Poland, Ulery et al. [20] found this sort of pattern in soils influenced by the presence of four planted tree species, with increments of almost 3 times as much K in the surface layer in relation to the original soil before planting, while below 20 cm this increment was absent or negative. Their study also showed a high degree of leaching of Na, which is less in demand by plants.

Associated with biological cycling and leaching, other processes that influence the vertical distribution of nutrients in soil are atmospheric deposition and weathering (Trudgill, 1988 as cited by [25]). However, atmospheric deposition is considered minor when compared to the annual uptake by plant communities and generally has little influence on the vertical distribution of nutrients [25, 26]. The degree of weathering, however, appears to have a marked influence on the vertical distribution of nutrients, such that in more weathered soils the pattern of concentration in the surface layer is accentuated [25]. This emphasizes the importance of biological cycling in supplying nutrients in weathered soils, as is found in the greater part of the tropics and will be discussed in a subsequent section.

3. Trees and Soil Carbon

Trees add organic matter to the soil system in various manners, whether in the form of roots or litterfall or as root exudates in the rhizosphere [27]. These additions are the chief substrate for a vast range of organisms involved in soil biological activity and interactions, with important effects on soil nutrients and fertility. In participating in these complex processes, trees contribute to carbon accumulation in soils, a topic that is increasingly present in discussions on the mitigation of greenhouse gases associated with global warming and climate change. Although carbon (C) constitutes almost 50% of the dry weight of branches and 30% of foliage, the greater part of C sequestration (around 2/3) occurs belowground, involving living biomass such as roots and other belowground plant parts, soil organisms, and C stored in various soil horizons [28].

In a study that gathered information from sites around the world, Nair et al. [29] found values for soil organic C stocks ranging from 6.9 to 302 Mg ha−1. Despite the great amplitude of these values, attributed to the variation between systems, ecological regions, and soil types, the study revealed a general trend of increasing soil C sequestration in agroforestry when compared to other land-use practices, with the exception of forests.

Although the ability of soils to accumulate C is generally related to characteristics that are little influenced by management, such as texture (clay soils typically accumulate more C than sandy soils), some management practices can influence soil C sequestration, particularly the insertion of trees in agricultural systems. Soils in various sites studied by Takimoto et al. [30] in the African Sahel were not markedly different among each other in terms of their characteristics such as pH, bulk density, and particle size, such that variations in their C contents seemed to be related to the influence of trees. In 8-year-old alley-cropping systems with five different species, for example, the authors found that greater C content is nearer to the trees. However, the greater part of this C was found in the form of particles of size between 250–2000 μm, fractions that are considered to be large and less stable. In systems where trees where present for more than 30 years (parklands), there was a predominance of soil C in smaller fractions (

Kindly look here too..

http://www.crida.in/DRM1-Winter%20School/KLS.pdf

Trees have nutrients concentrated  principally in the leaves, small branches and bark.  If you remove these from the forest the result is nutrient depletion

 Agreed with Dr Paul, there is a need to evaluate the nurient and water balance in forestry and agro-forestry systems, crop coefficients for water use and nutrients uptake efficiencies for forest eco-systems need to be collated from the scattered studies in various eco-regions

regards

Agroforestry can increase organic matter in the soil. The pH changes could be explained as a function of the concentration of the organic matter. Soil acidification can be attributed to degradation of organic matter in soils. Soil organic matter increases the capacity of water retention. It is therefore necessary to assess the water level and the water balance in forest and agroforestry systems.

Kindly opne the attached PDF..

Thanks to all researchers for their valuable comments.

Can anyone have the published research papers on soil pH in different agroforestry systems at the different regions of India and in different countries? 

Some fascinating answers. Let  us also consider the greater ability of forest trees to divert a comparatively  greater concentration of  carbon to their root system , besides locking up significant quantity of nutrients ( As Paul has rightly pointed out), while debating the chanages in pH...

Dear Sarvade,

In agroforestry systems, there is a good amount of cycling of basic cations. The cycling of bases in tree litter can assist in i) ameliorating soil acidity or checking acidification and ii) reclaiming saline or alkaline soils. Trees have been successfully incorporated in the reclamation of saline and alkaline soils with associated cereal intercropping. Agroforestry systems have shown beneficial effects on soil fauna with consequent improvements in soil fertility. On the other hand, it is possible that trees, whether intimately mixed with crops or planted in rows will improve the total water supply

by reducing evaporation.

I attach herewith PDF copy of a research paper for you, and believe you'll find it relevant and interesting.

The tree litter,  leaves twigs etc increase the organic matter in the soil through the process of decomposition, which releases organic acids in soil and subsequently lowers down the pH. 

Dear, soil biological activity and declining earthworm numbers are also associated with a fall in soil pH.