Experimental evidence for positive interactions, or facilitation, among plants has increased markedly during the last 10 years. Experiments documenting facilitation have been conducted in many diverse ecological systems, which suggests that positive interactions may be fundamental processes in plant communities. Here, I review the evidence for facilitation, the mechanisms by which facilitation operates, and the effects facilitation has on community structure. Facilitative mechanisms may act simultaneously with resource competition or allelopathy, and the overall effect of one species on another may be the product of multiple, complex interactions. Positive interactions may also determine community spatial patterns, permit coexistence, enhance diversity and productivity, and drive community dynamics. Once viewed as anecdotal and idiosyncratic, facilitation is now contributing to a more complete understanding of community structure and dynamics.

Positive interactions among plants

Soils act as sources and sinks for greenhouse gases (GHG) such as carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O). Since both storage and emission capacities may be large, precise quantifications are needed to obtain reliable global budgets that are necessary for land-use management (agriculture, forestry), global change and for climate research. This paper discusses exclusively the soil emission-related processes and their influencing parameters. It reviews soil emission studies involving the most important land-cover types and climate zones and introduces important measuring systems for soil emissions. It addresses current shortcomings and the obvious bias towards northern hemispheric data. When using a conservative average of 300 mg CO 2 e m −2  h −1 (based on our literature review), this leads to global annual net soil emissions of ≥350 Pg CO 2 e (CO 2 e = CO 2 equivalents = total effect of all GHG normalized to CO 2 ). This corresponds to roughly 21% of the global soil C and N pools. For comparison, 33.4 Pg CO 2 are being emitted annually by fossil fuel combustion and the cement industry.

Greenhouse gas emissions from soils—A review

MANY tropical soils are poor in inorganic nutrients and rely on the recycling of nutrients from soil organic matter to maintain fertility. In undisturbed rainforests such nutrients are recycled via the litter1; 'slash-and-burn' agriculture, meanwhile, depends on the mineralization of organic nutrients from the plant remains2,3 or on (short-lived) inputs from ash4. This dependence on organic nutrients in tropical soils has the result that tests of soil quality which only give isolated measures of inorganic nutrient status are unreliable5, and that the effects of fertilization can be inconsistent because of leaching or fixation of inorganic nutrients. Here we attempt to quantify the role of organic matter in sustaining the fertility of soils from three different climate zones. We estimate rates of carbon turnover from ecological measurements and 14C dating, and determine its relation to the soil carbon and nutrient budgets. We find that agriculture without supplementary fertilization was economical for 65 years on temperate prairie and for six years in a tropical semi-arid thorn forest. An extremely nutrient-poor Amazonian soil showed no potential for agriculture beyond the three-year lifespan of the forest litter mat, once biological nutrient cycles were interrupted by slash-burning. These observations suggest that quantification of organic-matter cycling may provide an important guide to the agricultural potential of soils.

The Role of Soil Organic-Matter in Sustaining Soil Fertility

. Agricultural soils, having been depleted of much of their native carbon stocks, have a significant CO2 sink capacity. Global estimates of this sink capacity are in the order of 20-30 Pg C over the next 50-100 years. Management practices to build up soil C must increase the input of organic matter to soil and/or decrease soil organic matter decomposition rates. The most appropriate management practices to increase soil C vary regionally, dependent on both environmental and socioeconomic factors.

In temperate regions, key strategies involve increasing cropping frequency and reducing bare fallow, increasing the use of perennial forages (including N-fixing species) in crop rotations, retaining crop residues and reducing or eliminating tillage (i.e. no-till). In North America and Europe, conversion of marginal arable land to permanent perennial vegetation, to protect fragile soils and landscapes and/or reduce agricultural surpluses, provides additional opportunities for C sequestration.

In the tropics, increasing C inputs to soil through improving the fertility and productivity of cropland and pastures is essential. In extensive systems with vegetated fallow periods (e.g. shifting cultivation), planted fallows and cover crops can increase C levels over the cropping cycle. Use of no-till, green manures and agroforestry are other beneficial practices. Overall, improving the productivity and sustainability of existing agricultural lands is crucial to help reduce the rate of new land clearing, from which large amounts of CO2 from biomass and soil are emitted to the atmosphere.

Some regional analyses of soil C sequestration and sequestration potential have been performed, mainly for temperate industrialized countries. More are needed, especially for the tropics, to capture region-specific interactions between climate, soil and management resources that are lost in global level assessments.

By itself, C sequestration in agricultural soils can make only modest contributions (e.g. 3-6% of total fossil C emissions) to mitigating greenhouse gas emissions. However, effective mitigation policies will not be based on any single ‘magic bullet’ solutions, but rather on many modest reductions which are economically efficient and which confer additional benefits to society. In this context, soil C sequestration is a significant mitigation option. Additional advantages of pursuing strategies to increase soil C are the added benefits of improved soil quality for improving agricultural productivity and sustainability.

Agricultural soils as a sink to mitigate CO2 emissions

Factors controlling humification and mineralization of soil organic matter in the tropics

Mineral Nutrients in Tropical Forest and Savanna Ecosystems.@@@Special Publications Series of the British Ecological Society.

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Mineral Nutrients in Tropical Forest and Savanna Ecosystems.@@@Special Publications Series of the British Ecological Society.