Soil Organic Matter Stabilization and Associated Degradation Threshold Dynamics.
James Kinyangi1, Johannes Lehmann1, Alice Pell1, Janice Thies1, Solomon Ngoze1, Susan Riha1, David M. Mbugua2, and Louis Verchot2. (1) Cornell Univ, Crop and Soil Sciences, Ithaca, NY 14853, (2) World Agroforestry Center, P.O. Box 30677, Nairobi, Kenya
Models that describe Soil Organic Carbon (SOC) dynamics ignore functional relationships that occur during land use which may confer degradation thresholds. We investigated the dynamics of soil organic matter degradation on an Oxisol following conversion of native forest vegetation to agriculture along a 100 year cultivation chronosequence in western Kenya. Seven broad categories of agricultural conversion sites were identified, representing < 3, 3-9, 10-22, 23-34, 35-57, 58-83 and > 83 years following cultivation. Soil samples were collected from randomly selected agricultural fields where maize was historically cultivated with little or no fertilizer/manure inputs. Twelve undisturbed forest reference plots were included in a paired combination with the cultivated plots. Soil samples were treated to a two step organic matter fractionation process involving density separation with sodium iodide and physical energy disruption through sonication to yield three functional organic C pools that were related to seasonal C dynamics, aggregate C protection and long-term soil C stabilization. The pools were sequentially separated from soil as light-fraction C, recovered following density flotation at 1.8 g cm-3, aggregate-protected C, isolated after density separation following high energy sonication and organo-mineral associated C from the residue. Mass spectrometry evaluations of δ13C signature were used to partition resident C stocks into native forest C3 C and contemporary C4 crop C sources. SOC declined rapidly and reached a new equilibrium beyond 40 years of maize cultivation at which time the soil C (19 g C kg-1) and N (2.0 g kg-1) contents were less than 20% of the original forest C (101 g C kg-1) and N (25.3 g kg-1). At this new stable equilibrium point, the δ13 C values revealed that one half of the remaining C is of C3 native forest origin while the other half is due to C4 cultivated crop inputs. In addition, biogeochemical transformations of C and N within functionally specific fractions will be reported.