Phosphorus cycling in silvo-pastoral systems established on highly weathered tropical soils

 

Background
Low soil phosphorus availability of highly weathered tropical soils can strongly limit both plant and animal productivity and also several other ecosystem functions. In the ANR (Agence Nationale de la Recherche, Paris, France) funded AMAZ project carried out in the colombian Amazon arc of deforestation, the relationships among socio economic parameters, landscapes, biodiversity, production of food and other commodities and soil ecosystem services were studied on farms distributed among landscape windows with different ages and histories of colonization. Based on the results of the AMAZ project an eco-efficiency index was calculated, which defines how efficiently natural resources are used to produce more food, enable family farms to be economically competitive, and deliver sustainable increases in productivity, while avoiding natural resource degradation. The main conclusion from the AMAZ project was that most of the land use systems in this region are low in their eco-efficiency. While incomes per unit of used surface and labor are low in most livestock based systems, soil degradation and loss of biodiversity are strong. The role of the soil phosphorus status in eco-efficiency, however, was not yet studied.

Objectives
In this MSc project we studied soil phosphorus status and phosphorus dynamics on a selection of fields located on farms in the silvo-pastoral landscape in the department of Caquetá, Colombia. Soil samples were taken in i) degraded grass pastures, ii) productive grass alone pastures and iii) productive mixed grass-legume pastures to test the hypotheses that i) degraded pastures suffer from phosphorus deficiency and that ii) the inclusion of legumes improves phosphorus use by the plants through enhanced microbiological phosphorus cycling in the soil.

Research
The total and the plant available phosphorus content of the soil and the different inorganic and organic phosphorus forms in the soil, including phosphorus contained in soil microbes, were determined to characterize the phosphorus status of the soil. Moreover, the biomass and the phosphorus content of living and dead plant material was analyzed to evaluate the phosphorus nutritional state of the plants and the importance of plant residues in phosphorus cycling.

Results
Compared to the productive grass pastures, the degraded grass pastures were characterized by higher soil compaction, lower water retention capacity and they produced lower amounts of plant biomass which in turn led to lower amounts of plant residues returned to the soil. The concentration of organic phosphorus forms in the soil was also lower and soil microbes seemed to be less active. Comparing the two productive pasture types, no significant differences could be found in any of the measured soil and plant parameters. Biological P cycling was clearly reduced in the degraded grass pastures compared to the productive grass pastures while it was not substantially affected by the legumes in in the productive grass-legume pastures.

Based on the results obtained for plant available phosphorus in the soil and phosphorus concentration in the plant biomass we could not confirm our hypothesis about lower phosphorus availability in the degraded grass pastures compared to the productive grass pastures. This may partly be due to the selected phosphorus availability indicators, but also to interactions between phosphorus availability with other nutrients such as nitrogen or soil carbon content. This is why in a following MSc study mentioned below more emphasis is put on soil nitrogen and carbon. Also we could not find any effect of the legumes on soil biological activity, which is probably due to the fact that most grass-legume pastures only showed a very low proportion of legumes in the plant stand. Because we could not find grass-legume associations with satisfactory legume proportions, this pasture type will be excluded in the future MSc thesis.

Finally it has to be considered, that this study was performed on farmers' fields without controlled experimental conditions, i.e. on completely unaltered agricultural production systems. Despite high variation in the obtained data we obtained very interesting results. Therefore we encourage researchers to adopt on-farm research in their studies.

Collaborations
All field work was realized in close collaboration with the Universidad de la Amazonia, Florencia, Colombia. Analytical work was carried out at the International Center for Tropical Agriculture (CIAT), Cali, Colombia and at ETH Zurich, Switzerland. All results obtained from this study were made available to all participants of this project. Partners from the local Universidad de la Amazonia will spread and implement the acquired knowledge among local researchers and farmers. An apposite document was created to inform all the farmers which participated in this study about the most important results concerning the fertility of their soils. At CIAT, results will feed into on-going and future activities related to silvo-pastoral systems. At ETH, a new MSc thesis was already built on the knowledge acquired from this study. The MSc project entitled “Phosphorus, carbon and nitrogen interactions in highly weathered tropical soils: elements distribution in soil aggregates” is beeing performed on the same sites analyzed in this project. The results of both MSc thesis will fit together to provide a detailed understanding of phosphorus, carbon and nitrogen status in the pasture soils of Caquetá.

Project duration
Field work and analytical work in Colombia: September 2010 – December 2010

Analytical work, data analysis and writing of the thesis in Switzerland: January 2011 – April 2011

 

Contact

ETH Zurich
Group of Plant Nutrition
Institute of Plant, Animal and Agroecosystem Sciences
Eschikon 33, 8315 Lindau, Switzerland
Django Hegglin: dhegglin@ethz.ch
Dr. Astrid Oberson: astrid.oberson@ipw.agrl.ethz.ch
www.plantnutrition.ethz.ch

International Center of Tropical Agriculture CIAT
Apartado Aereo 6713, Cali, Colombia
Pfof. Dr. Patrick Lavelle: p.lavelle@cgiar.org
Dr. Idupulapati Rao: i.rao@cgiar.org
www.ciat.cgiar.org

Universidad de la Amazonia
Calle 17 Diagonal 17 con Carrera 3F, Barrio Porvenir, Florencia, Colombia
Prof. Dr. Bertha Ramirez: belerapa@yahoo.com
Prof. Dr. Jaime Velasquez: jaimevere@yahoo.com
www.uniamazonia.edu.co/v8/