On the temperate coast of central Japan, a unique agroforestry system exists, wherein red turnips (Brassica rapa L. var. glabra Kitam.) are grown on slash-and-burn sites following the harvesting of mature Japanese cedar (Cryptomeria japonica D. Don.) plantations. We studied the effect of fire on nutrient dynamics, soil CO2 efflux, and microbial biomass at two slash-and-burned sites. Burning, besides raising soil pH from acid to neutral, had an ash-fertilizing effect, reflected by a large increase in soluble P (15.6-fold) and exchangeable cations (Ca, Mg, and K) in the topsoils (0-5 cm). Burning resulted in a 4.8-fold rise in soil NH4-N because moderate heating of the topsoil induced high microbial activities and, therefore, greater mineralization of soil organic N. On the other hand, there was a pronounced increase in soil CO2 efflux after burning, which may have resulted from the fire-induced high microbial activity. Thus a significant amount of C was released not only by combustion of litter, but also by decomposition of soil organic matter (SOM). Carbon and nitrogen fluxes during the growth of the turnip crop, however, were small compared to the total stocks of SOM accumulated in the mature cedar plantation (CP) pre-fire. Moreover, soluble P content and pH of the topsoils remained at post-fire levels until the end of the turnip growing season, while the initial increases in soil inorganic N and exchangeable cations returned to pre-fire levels after a single cropping season. We highlight the probable existence of a positive feedback response in carbon sequestration during the fallow due to a higher NPP arising from the modified soil conditions. When estimating the total carbon budget of slash-and-burn agricultural ecosystems, this positive feedback needs to be balanced against the short-term increases in carbon dioxide emissions from litter combustion and fire-induced CO2 efflux from soils.
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