Restoring and Sustaining the Soil We Farm

The sight of brown water running down farmland during heavy rains immediately brings to mind the question: how much soil is being lost through erosion? What is less obvious is that the top layer of soil being eroded contains a lot of soil carbon. This is because the top soil has higher soil organic matter content and associated living soil organisms than deeper soil layers.

Soil tillage also generates large losses of soil carbon. This is because soil is clumped together, in lumps that contain organic matter that is protected from the action of soil microbes responsible for decomposition. Tillage operations break up these soil aggregates and the microbial decomposition of the newly exposed organic matter results in gaseous losses of carbon. Furthermore, the same decomposition process also results in the production of soil nitrate, which is a very mobile form of nitrogen that can easily contribute to leaching or gaseous losses that negatively affect the environment.

Soil carbon loss is a central aspect of soil degradation and restoring these stocks is a key global priority. Realizing the full benefits from recovering soil organic carbon stocks during restoration hinges on nurturing a community of soil organisms that are able to perform a diverse set of key ecological functions. In many degraded soils, its capacity to function normally is impaired and crop yields do not respond to mineral fertilizer inputs. This ‘non-responsiveness’ is often, at least partly, the result of a reduction in the diversity of organisms during degradation causing the loss of critical soil functions. These degraded soils occupy up to 60 percent of arable land in densely populated smallholder communities, removing any incentive farmers might have had to apply fertilizer.^1,2

The low use of fertilizer on farms is a widespread problem in Africa and has resulted in ‘mining’ soil nutrients as successive crops receive little or no nutrient input, but nutrients are removed in harvested products. Cropping without sufficient nutrient replenishment is unsustainable and leads to soil degradation. The soil can be conceived of as a savings account in a bank. If money is taken out continuously, without putting any money back in, the account will eventually run out.

Integrated Soil Fertility Management (ISFM) involves using fertilizers, organic inputs, and improved germplasm, combined with the knowledge to adapt these practices to local conditions to increase crop productivity and biomass production.³ Providing the continuous supply of organic inputs required to restore soil carbon stocks and soil health is often a challenge. This is particularly so when relying on residues of annual crops that are often in high demand for other on-farm uses like animal feed or as fuel for cooking.

Agroforestry is a diverse set of land management practices that involve the introduction or selective retention of trees within agricultural systems.⁴ The continuous supply of organic materials to the soil through aboveground and belowground organic inputs by trees is one key benefit of agroforestry. But, trees differ in the quantity and quality of organic inputs that they supply, which in turn influences soil organic matter dynamics and soil carbon storage. How trees are managed affects other plant characteristics, like canopy size, that affect their impact on microclimatic conditions near trees, which in turn affects the abundance and activity of soil organisms.⁵ We are building our capacity to understand how the chemical characteristics of organic inputs derived from trees (leaves, leaf litter, and roots) affects their speed of decomposition, through the development of an organic resource database for agroforestry tree species. The total nitrogen, lignin, and polyphenols are plant tissue chemical parameters that have been the most reliable indicators of the speed of decomposition and nutrient release from organic inputs.^6,7 These indicators relate the quality of organic inputs derived from different trees to their residence time on the soil surface when applied as mulch, which is of particular importance for soil moisture conservation in drier environments. The decomposition patterns of organic inputs also allow us to predict the timing of nitrogen release so that additional measures can be put in place to efficiently utilize the nitrogen as it is released, such as early crop planting, or the selection of crops or crop varieties with appropriate duration. A greater predictive capacity fosters a more efficient use of nutrient resources through land management that increase synchrony of nutrient supply and demand, and thus minimize nutrient losses.⁸

The study of the way different soil organisms are distributed in relation to different tree species; that is, how many are found close to rather than further away from trees, and how active they are in these different locations, is helpful in understanding how trees influence important soil functions and hence their role in sustaining soil functions in farmer’s fields.⁹ A recent review showed that agroforestry consistently generated important increases in the mean number of soil organisms of different sizes when compared to adjacent continuous cropping without trees.  Further, it also showed that soil biological activity was greater near rather than away from trees, but the magnitude of the response varied with tree species.⁵ This study and other evidence in the literature support the idea that trees in farms and agricultural landscapes constitute resource islands that provide shelter to soil organisms.^10,11 By protecting soil organisms, particularly during periods of environmental stress (e.g., drought and floods), trees also protect the functions performed by such organisms. Given the expected increase in frequency and intensity of extreme climatic events in the next decades resulting from climate change, the sheltering role of trees for soil organisms becomes increasingly important for sustaining critical soil functions in agricultural landscapes that generate benefits for society.

A number of studies have highlighted the wealth of soil knowledge and experience held by local farmers as an essential complement to scientific knowledge.^12–14 We use research tools that encourage knowledge sharing and thus promote co-learning among farmers, agricultural professionals, and researchers to guide the selection of study trees in agricultural farms and landscapes.^15,16 This approach not only strengthens the relevance, credibility, and legitimacy of our research but also provides a solid basis for the selection of tree species to be studied from the large pool of tree species usually found in tropical agricultural landscapes. We are systematically characterizing and linking selected tree characteristics with the abundance, diversity, and activity of soil organisms that drive ecosystem functions and services mediated by the soil. For instance, by studying the way in which soil aggregation processes occur, assessing soil carbon stocks, and the abundance, diversity, and activity of key soil organisms (e.g., earthworms) under and away from native and exotic trees, we evaluate the net effect of tree species on soil.  We use soil’s concentration of carbon and its stability when in contact with water to indicate overall health, as these characteristics reflect the influence of trees on the regulation of carbon storage.¹⁷ Tree characteristics mentioned earlier are systematically compared to indicators of soil health to enhance our understanding of how above and below ground biodiversity influence each other.¹⁸ We aim to address the central question of what tree densities, arrangements, and species are needed to maintain essential ecosystem functions provided by soil organisms in agricultural landscapes. Furthermore, the combined use of modern technologies, such as molecular tools, are increasing our ability to identify and characterize the role of trees in fostering “hotspots” of biological activity across gradients of agricultural intensification.¹⁹

Beyond sustaining soil function, agroforestry has been increasingly recognized and practiced as a restorative land management option that can simultaneously contribute to income, food security, and the conservation of biodiversity, including important pollinators, that underpin a range of ecosystem services.²⁰ It has also been identified as a climate change mitigation and adaptation tool for agriculture.²¹ Here we have highlighted evidence that trees are multifunctional entities critical for sustaining beneficial impacts of ISFM implementation, given their capacity for continuous supply of organic inputs, their deep rooting ability that tightens nutrient cycles and minimizes nutrient losses, and their role in sustaining soil biodiversity and function while simultaneously buffering agroecosystems against climatic and economic changes.

References

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