Estimating net primary production from measurements made on soil organic-matter

A model for the turnover of organic matter in soil, ROTHC-26.3, can be used to calculate how much organic C needs to enter a soil annually in order to maintain a specified stock of soil organic C. The annual return of organic C thus calculated, plus the amount of organic C removed annually from the site by harvesting, burning, etc., provides an estimate of the Net Primary Production (NPP) of that site, averaged over many years. The new method was used to calculate NPP for two adjacent savanna sites in the Nairobi Nation al Park in Kenya, one grazed and one not, and for a dry Miombo woodland sit e in Zambia. Both the Kenyan and Zambian sites are taken to be at equilibri um, with soil organic C levels at steady stale. Soils from the three sites were analyzed by layer for organic C, delta(14)C, delta(13)C, soil microbia l biomass C, total N, pH, and clay content. Radiocarbon measurements were > 100% modern in the surface layers (0-15 cm) of the Kenyan soils (both Verti sols) and in all three layers (0-15, 15-30 and 30-50 cm) of the Zambian soi l tan Oxisol), presumably because of C-14 coming from the testing of thermo nuclear bombs. The 15-30 cm layer of the Kenyan soils dated at similar to 5 00 yr and the 30-50 cm layer at similar to 900 yr. The C-14 data were consi stent with the presence of a small inert fraction of organic C that account ed for an increasing proportion of total organic C with increasing soil dep th. The C-13 data indicated that the Kenyan soils had developed under C, ve getation, whereas the Zambian soils had developed under vegetation dominate d by C, plants. From these results the annual input of C to soil from the u ngrazed Kenyan site was calculated to be 388 g C . m(-2) . yr(-1), to the g razed site 380 g C . m(-2) . yr(-1), and to the Zambian soil 373 g C . m(-2 ) . yr(-1). Taking the loss of C from the Kenyan sites by burning to be 40 g C . m(-2) . yr(-1), the mean NPP for both Kenyan sites is 424 g C , m(-2) . yr(-1). This value for NPP is compatible with earlier estimates of NPP b y botanical methods from the same site in Kenya. Wood-taking is thought to be minimal in the protected Zambian woodland, so that here the annual input of C to the soil can be taken as the NPP without great error. This new met hod provides a long-term, integrated measure of NPP that should complement and enhance productivity measurements made by harvest methods over shorter periods.