Tuesday, August 4, 2009 - 4:00 PM

OOS 16-8: Dynamic Regimes in Agroecosystems: Evidence from the a tropical watershed in the Lake Victoria Basin

Alex Awiti, The Aga Khan University, Faculty of Arts and Sciences (East Africa), Cheryl Palm, Earth Institute at Columbia University, and Markus Walsh, Wold Agroforestry Center.

Widespread shifts in ecosystem structure due to land use/cover change have a strong potential to change key ecosystem processes that could alter C and N cycles, soil hydrological properties and plant productivity at ecosystem, regional and global scales.

However, a fundamental challenge to managing changes in ecosystem structure and function, especially in, is the lack of appropriate approaches and tools for rapid and reliable assessments would permit diagnostic screening for spatially explicit targeting of management interventions.

1. What is the suitability of near infrared reflectance spectroscopy (NIRS) for detecting changes in soil quality properties across a forest-cropland chronosequence?

2. Are carbon isotope ratios in soil organic carbon a reliable technique for tracing shifts in C/N input dynamics along a forest-cropland chronosequence?

3. Can biomass allocation optical reflectance patterns of leaf samples validate variations in soil fertility along a forest-cropland chronosequence?

The study was conducted in the and its cultivated margins in the western highlands. The experimental design was based on a chronosequence that represents a dynamic boundary between forest and cropland. Five sampling clusters, each measuring 64 ha, were placed randomly in three chronosequence age class.

Reflectance spectra were recorded for soil and ground maize leaf sample using a FieldSpec™ FR spectroradiometer (Analytical Spectral Devices Inc,,) at wavelengths from 0.35-2.5 μm at an interval of 0.001 μm. Leaf samples were obtained from maize grown in soils from the chronosequence under greenhouse conditions for 14 days. A sub sample of 300 soil samples weighing 50–80 mg were used for determination of natural abundance of d13C, d15N, SOC and TN.

A spectral based soil fertility classification of “high”, “medium” and “low” was developed. d13C and d15N enrichment, low levels SOC and TN concentrations were observed in soils that were designated as low fertility based on ordinal fertility classes defined by NIRS.

Biomass partitioning patterns and leaf reflectance characteristics of 14 day old maize were consistent with the spectrally defined fertility classes of high, medium and low. Log-transformed root-shoot ratios revealed that young maize plants grown in high nutrient soils had a low root/shoot ratio while high root: shoot ratio was observed among young maize plants grown in soils classified as low nutrient.

A combination of reflectance spectroscopy, stable isotope and plant bioassay techniques present a promising tool box for rapid assessment and monitoring of regime shifts biogeochemical and plant productivity potential in agroecosystems.