Phosphate (Pi) is considered to be one of the least available plant nutrients in the soil. High-affinity Pi transporters are generally accepted as entry points for Pi in the roots. The physiological, genetic, molecular and biochemical analysis of phosphate starvation response mechanisms highlight the ability of plants to adapt and thrive under phosphate limiting conditions. These responses help them enhance the availability of Pi, increase its uptake and improve the use-efficiency of Pi within a plant. Enhanced ability to acquire Pi appears to be regulated at the level of transcription of high-affinity phosphate transporters. These transporters are encoded by a family of small number of genes having characteristic tissue and organ associated expression patterns. Many of them are strongly induced during phosphate deficiency thus providing plants with enhanced ability to acquire and transfer phosphate. In addition, plants also activate biochemical mechanisms that could lead to increased acquisition of phosphate from both inorganic and organic phosphorus sources in the soil. Furthermore, altered root morphology and mycorrhizal symbiosis further enhance the ability of plants to acquire Pi. Interestingly most of these responses appear to be coordinated by changes in cellular phosphate levels. It is becoming apparent that phosphate acquisition and utilization are associated with activation or inactivation of a host of genes in plants. In this article we describe molecular, biochemical and physiological factors associated with phosphate acquisition by plants.

Phosphate Acquisition

Acid soils significantly limit crop production worldwide because approximately 50% of the world's potentially arable soils are acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring tolerance to acid soil stress has been a focus of intense research interest over the past decade. The primary limitations on acid soils are toxic levels of aluminum (Al) and manganese (Mn), as well as suboptimal levels of phosphorous (P). This review examines our current understanding of the physiological, genetic, and molecular basis for crop Al tolerance, as well as reviews the emerging area of P efficiency, which involves the genetically based ability of some crop genotypes to tolerate P deficiency stress on acid soils. These are interesting times for this field because researchers are on the verge of identifying some of the genes that confer Al tolerance in crop plants; these discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of these tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving crop Al tolerance via both molecular-assisted breeding and biotechnology.

How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorous efficiency.

Climate change mitigation: a spatial analysis of global land suitability for Clean Development Mechanism afforestation and reforestation

Soil biota, ecosystem services and land productivity

Agriculture and fishing are the primary source of almost all our food, as well as of many other products, so remain vital activities even though their share in the economy is small and in continuous decline. Partly for this reason public policy in these areas is almost entirely integrated at European level in the Common Agricultural Policy (CAP) and Common Fisheries Policy (CFP). Forestry policies are also heavily affected by EU-level decision-making. As agriculture dominates in terms of direct contribution to GDP and numbers of people engaged in it, as well as accounting for the largest amount of public support expenditure, it is agriculture that will receive the greatest attention here.

Agriculture, Forestry and Fisheries

Soil fertility depletion in smallholder farms is the fundamental biophysical root cause for declining per-capita food production in Africa, and soil fertility replenishment should be considered as an investment in natural resource capital. An average of 660 kg N ha-1, 75kg P ha-1, and 450 kg K ha-1 has been lost during the last 30 years from about 200 million ha of cultivated land in 37 African countries. The consequences are felt at the farm, watershed, national, and global levels. There is an exact congruence between the concepts of capital stocks and service flows in economics and that of nutrient pools and fluxes in soil science. Phosphorus-replenishment strategies are mainly fertilizerbased with biological supplementation, while N replenishment strategies are mainly biological with chemical supplementation. Africa has ample phosphate rock deposits that could be used directly or as superphosphates to reverse P depletion. Decomposing organic inputs produce organic acids that help solubilize phosphate rocks. Agroforestry trees and herbaceous leguminous green manures play a major role in N capture and internal cycling. Accompanying technologies, such as soil conservation are needed to make recapitalization operational. Policy improvements are needed to provide the appropriate fertilizers at a reasonable cost and at the right time; better infrastructure; access to micro credit; timely access to markets; adaptive research and extension education. Soil fertility replenishment was found to be profitable at the farm level in three contrasting case studies, but resource poor farmers lack the capital and access to credit to make the initial investment. The issue of who should pay for this recapitalization is based on the principle that those who benefit from a course of action should incur the costs of its implementation. Progress to date on soil fertility initiatives throughout Africa will be summarized.

Meredith J. Soule

Papers

Soil Fertility Replenishment in Africa: An Investment in Natural Resource Capital