Tree Roots and Plant Nutrition in Simultaneous Agroforestry Practices in the Humid Tropics: Implications for Guyana’s Agricultural Expansion

By Patrick Chesney

📃• World Soil Day • 

Published: December 5, 2022 | 

The unavoidable conclusion is to place agroforestry at the centre of an integrated landscape approach.

Patrick Chesney ✉️, PC Consultants.

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The universally accepted definition of agroforestry is that it is a collective name for land management systems and technologies where woody perennials (typically trees and shrubs) are grown on the same land-management units as agricultural crops and/or livestock, in some form of spatial arrangement or temporal sequence (FAO, 2015; King, 1987; Nair et al., 2021). When agroforestry systems combine crops and trees they are known as agri-silvicultural systems. Alley cropping is an example of a simultaneous agri-silvicultural practice where crops and trees are spatially separated. It is the growing of annual crops in alleys between rows of planted fast growing nitrogen fixing trees (Kang & Wilson, 1987).

A key implication of the agroforestry definition is the potential for both ecological and economical interactions between the different components in the land management system. Two-way complementary resource capture is a key objective of spatially arranged components in agroforestry systems (Ong et al., 1996) that often requires careful selection of the woody components. In the humid tropics, trees compete with associated crops for growth factors such as light and nutrients. Rao et al. (1998) have shown that trees are managed by periodic shoot pruning to prevent shading of the associated crop and the nutrient-containing pruned materials ae usually incorporated as green manure or applied as much to the soil to support crop nutrition or to protect the soil. Chesney (2012) reviewed the biophysical science of shoot pruning of the woody components in simultaneous agroforestry practices as an essential management practice to avoid competition for growth factors, such as light and nutrients, and to provide the service attributes (mainly biomass and nutrients) of the trees to the associated crops and soil. In a pruned-managed agroforestry practice, partial pruning preserves the service attributes of trees and minimises interspecific completion for growth factors.

Chesney (2008) found that biomass yield from partial shoot pruning of Erythrina poeppigiana (Walp.) O.F. Cook in an alley cropping practice with tomato (Lycopersicon esculentum Mill.) in the humid tropics has the potential to contribute between 4.4 and 9 Mg ha-1 of aboveground biomass containing between 123 to 256 kg N ha-1. Partial shoot pruning conserved fine root length for nutrient capture and uptake. The nitrogen yield could meet the elemental nutrient requirement for most annual crops grown in the tropics (IICA, 1989). Marschner (1997) indicated that root growth is enhanced at sites with high nutrient supply. In annual crop species, rooting density rapidly increases several fold in zones of higher nutrient concentrations, especially of nitrogen. The implication for crop nutrition is the return of nutrient rich-tree biomass to the soil and the release of nutrients for uptake by crops to produce the economic yield. The use of fast growing nitrogen fixing trees like Erythrina allows for the rapid decomposition of the biomass, as the level of biochemical barriers to decomposition such as lignin and polyphenols is low (Palm & Sanchez, 1991).

The use of agroforestry is an attractive land management system for low fertility soils in the hinterland of Guyana, the present frontier for agricultural expansion given the current and projected impacts of climate change (sea level rise, salt water intrusion, and extreme weather events) on the ecosystems of the Low Coastal Plain natural region, and the recent policy decisions to shift commercial agriculture inland (Government of Guyana, 2018; 2022). The 2005 floods caused economic damage equivalent to 60% of gross domestic product, and the 2021 floods were declared a national disaster and affected over 52,000 hectares and 1.3 million animals (Government of Guyana, 2022).

Low fertility soils are associated with poor agricultural land (Land Capability Classification Class III), which accounts for 8,227,247 ha or 39% of Guyana’s landmass found in ‘hotspots’ (areas with high potential for development), such as Matthews Ridge/Port Kaituma, Bartica-Linden, the Intermediate Savannahs-Canje Basin, and Northern Rupununi (GLSC, 2013). These areas are candidate sites for sustainable agriculture under the Low Carbon Development Strategy (Government of Guyana, 2022). There are central government plans to invest significantly in the commercial development of corn (Zea mays L.) and soya beans (Glycine max L.) in the Intermediate Savannahs (Department of Public Information, 2021). Hopefully, the investment becomes fully compliant with the provisions of the Seed Act No. 8 of 2011 and the National Biosafety Framework (EPA, 2007), given that Guyana is a signatory to the Cartagena Protocol on Biosafety to the Convention on Biological Diversity.

Agroforestry research (Chesney et al., 2010; Simpson & Wickham 2000) has been carried out in the Intermediate Savannahs that show promise for cowpea (Vigna unguiculata L.), corn and sorghum (Sorghum bicolor (L.) Moench.). These publications build on more than 50 years of research on food and agriculture in the Intermediate Savannahs and other hotspots. The unavoidable conclusion is to place agroforestry at the centre of an integrated landscape approach (Reed et al., 2021) to food and agriculture that integrates ecology. Such an approach will allow Guyana to leverage its high biocapacity, which represents the productivity of its ecological assets (including cropland, grazing land, forest land, fishing grounds, and built-up land) for food and agriculture. This approach also addresses issues related to biodiversity loss, climate change adaptation, and the provision of environmental services.



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