The challenge of HIV/AIDS: Where does agroforestry fit in? - World ...

Chapter 5: Confronting land degradation in Africa47Below the farm: harnessingthe biosphereThe community of organisms in the soilperform many essential processes. They actas the primary driving agents of nutrientcycling; regulate the dynamics of soil organicmatter, soil carbon sequestration andgreenhouse gas emission; modify soil physicalstructure and water regimes; enhancethe amount and efficiency of nutrient acquisitionby the vegetation through mycorrhizaand nitrogen-fixing bacteria; and influenceplant health through the interactionof pathogens and pests with their naturalpredators and parasites. These processesalso provide a range of services to humans,including maintaining the availability andquality of water resources, erosion control,biological control of pests, climate regulationand, of course, food production.The status and activity of the soil communityhas only rarely been taken into accountin modern approaches to soil management.Any form of integrated nutrient managementnonetheless relies on the capacity ofthe decomposer community to process theorganic inputs. Similarly, the benefits ofconservation tillage rely on regulation ofthe soil’s physical condition by earthwormsand other biological ploughs. There is thusincreased awareness of the need to understandthe functioning of the living soilcommunity, as expressed in the ‘secondparadigm’ for soil fertility managementproposed by Sanchez (1994) (see also Swift1998), i.e. for improved crop growth, ‘relyon biological processes by adapting germplasmto adverse soil conditions, enhancingsoil biological activity and optimizingnutrient cycling to minimize external inputsand maximize the efficiency of theiruse’ (our emphasis). The second challengedirectly addresses the central, but largelyunrealized, clause in this paradigm.Challenge 2: How can the functionsof the soil community be optimizedwith respect to different ecosystemservices?One of the major reasons for the slowprogress in understanding the functionalbiology of the soil biota has been the lackof sensitive methods for investigating soilmicro-organisms. This problem has comecloser to solution since the advent of molecularmethods for identifying and trackingspecific soil organisms and for assessingchanges in overall biodiversity (Ammanand Kuhl 1998; Amman and Ludwig 2000).The Centre has already embarked on innovativestudies of this kind (e.g. Bossio etal. 2005). Combining such work with continuingstudies of the keystone role of treesin agroecosystems (see following section)offers great promise as a means of linkingthe driving functions of ecosystem servicesacross scales from below the plot to thelandscape.Beyond the farm: landscapesand institutionsThe biggest challenges for the managementof natural resources probably lie at thebroadest scales and can be placed underthe inexact term of ‘the landscape’. Atthese scales, predictions of effects derivenot so much from specific biological processes,but from their aggregate and interactiveeffects. The main driver of these effectsis the nature and location of different landusesystems on the landscape, includingtheir history and management. Trees influencelandscape scale dynamics more thanany other organisms (although, of course,humans have now appropriated this claim).Investigation of this keystone role mustremain not just a major part of the Centre’sresearch agenda, but at the very heart of it,because of the huge number of secondaryinteractions that flow from the incorporationof trees within any land use system.One specific feature that is strongly influencedby the presence of trees is the abundanceand quality of soil organic matter.This, in its turn, influences both soil fertilityand all the other ecosystem services derivedfrom soil. It is therefore both a majorresource and an indicator of soil status. Thework of Shepherd and Walsh in developingmethods for remote sensing and mappingof soil carbon status and its linkages toother soil properties at different scales hasprovided extremely powerful tools for bothassessing and predicting the environmentaleffects of land use change (Shepherd andWalsh 2002).Soil carbon is now of global interest becauseof the opportunity to utilize sequestrationas a mechanism for correcting theimbalances in emissions of carbonic gasesthat are believed to be driving climatechange at an unacceptable rate. However,organic matter is not an inert componentof soil. It is the substrate for many soilorganisms and thence contributes energyfor many of the essential biological processesthat support plant production andsoil structure maintenance. There is thusa major challenge to assess (for differenttypes of land use) how to optimize the useof soil carbon. This is fundamentally an issueof balancing the needs of the farmer (atthe scale of the plot) to exploit soil organicmatter energy for crop production with theneeds of society in general (at the scale ofthe landscape) to conserve carbon (Tomichet al. 2005). It is possible to hypothesizethat these two goals may be mutually incompatibleor have significant quantitative